Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flame

Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flame | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flame Shirong Xin, Yong He, Tao Liu, Yingchun Wu, Xuecheng Wu, Zhihua Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2068135/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Low temperature and low air pressure would cause low fuel inlet temperature and low air flowrate in aero-engine combustion chamber working in the upper atmosphere. A system including a burner normally used in actual aero engines is established to study swirl-stabilized kerosene spray combustion at conditions of low fuel inlet temperature (T < − 16 ℃) and low air pressure. According to analysis of OH-PLIF images, sub-zero Celsius significantly results in the reduction of the OH radical concentration and poor heat release rate. The strongest signals decrease by at least 5 times. With analysis of LIF spectroscopy and detuned images, fuel LIF particles in the OH-PLIF images are mainly liquid kerosene LIF. The LIF signals of flame structure are almost all from OH radicals. Using LOG operator blob detection, it is found that, when at sub-zero Celsius, the total number of fuel LIF particles slightly decreases and the number of large particles increases at any air pressure. PLIF analysis in this study provides insight into complex combustion deterioration of kerosene swirling spray combustion for the first time. swirling spray combustion combustion deterioration flame structure low fuel temperature OH-PLIF Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Aircrafts are very vulnerable to adverse external conditions when flying in the upper atmosphere, such as low temperature and low pressure. According to statistics of the literature of Idowu Innocent Abbas ( 2012 ), 98 aircraft accidents in total from year of 1950 to 2000 were caused by external environmental factors at upper atmosphere. Several factors may lead to the deterioration of combustion in engine chambers at low temperature, including poor atomization (Dafsari et al. 2019 ). slower fuel evaporation rate, insufficient gas supply, and soot formation. They will result in difficulty for re-ignition after flameout and combustion instability without conduciveness for safe flight. Therefore, study for deterioration of combustion at low fuel inlet temperature is critical, like liquid fuel distribution, heat release and key species distribution etc., which will contribute to improving understanding of flame structure of the gas-liquid two-phase flow at operational limits (Chterev et al. 2017 ). However, literature about aviation kerosene swirling spray combustion at conditions of low fuel inlet temperature and insufficient air feed are scarce. Except the complex setup for artificially created experimental conditions, challenges associated with liquid fuel combustion are also difficult to handle for online optical visualization measurement, such as interference from liquid fuel, hydrocarbon species during fuel decomposition (Chterev, et al. 2017 ) as well as abundant of soot. Available studies almost focus on spray experiments, seldom related to combustion. Dafsari, et al. ( 2019 ) examined the viscosity effect on the pressure swirl atomization of alternative aviation fuel at cold fuel temperature conditions, discovering the increased viscosity with the increscent fuel Reynolds number. Hwang et al. ( 2015 ) investigated the fuel temperature influence on spray and combustion characteristics of diesel in a constant volume combustion chamber under simulated engine operating conditions, revealing cold fuel temperature reduced flame luminosity and spray cone angle. Aleiferis et al. ( 2021 ) made an optical investigation into the effect of atomization of multi-hole injectors for iso-octane and ethanol. They found that cold temperatures suppressed atomization for fuel, causing the narrowing of the spray cone angle and large droplet diameter. This was linked to increased viscosity and higher losses. Dahlander P (2008) found a larger spray liquid penetration length at -30 ℃. Wu et al. ( 2021 ) used picosecond pulsed digital off-axis holography to measure the near-nozzle droplet size and 3D distribution of a swirl kerosene spray, with a summary that the droplet peak diameter decreases with the increase of gas feed. However, the above mentioned investigations are mainly focused on spray atomization, flame structure studies for aviation kerosene swirling spray combustion at conditions of cold fuel temperature are seldom reported. For analysis of heat release and distribution of intermediate radicals, the OH-planar laser induced fluorescence (OH-PLIF) is commonly utilized as a kind of efficient optical diagnostics. The OH-PLIF is suitable for complex premixed flame of aviation kerosene swirling spray. There are several studies focused on the lean premixed flame of swirl-stabilized kerosene spray flame using OH-PLIF (Malbois et al. 2017 , Malbois et al. 2019 , Malbois et al. 2019 , Seitzman and Hanson 1993 ). However, kerosene usually have complex fluorescence which comes from aromatic hydrocarbons or inter products after decomposition, with wide wavelength range in spectroscopy under UV laser excitation (Chterev, et al. 2017 , Orain et al. 2014 ). Especially in the deterioration of combustion, the unburned kerosene may require more complicated image processing. Accordingly, verification work should be done to ensure the flame structure should be represented by OH-PLIF signals, thus statistical analysis can be conducted further. In this paper, an optical measurement system as well as a combustion system including a burner, fuel supply, air supply, and fuel cooling, was established for swirl-stabilized kerosene spray combustion investigation. OH-PLIF measurements were performed at different conditions including low/normal fuel inlet temperature, and different air flowrate. LIF spectrum in the flame and spray, as well as detuned analysis, were then carried out to identify the main LIF species on the flame structure. With utilization of blob detection algorithm, number of the unburned fuel LIF particles was counted. The contribution of this paper is to fill the gap of optical diagnostics of kerosene swirling spray combustion at low fuel inlet temperature and study of the phenomena of kerosene combustion deterioration. 2. Experimental Section 2.1. Experimental Setup and Ignition Details The experimental apparatus to investigate RP-3 swirling spray combustion is illustrated in Fig. 1 , mainly comprises the air supply system, fuel supply system, and monitoring equipment. The air supplied from an air compressor firstly passed through a dryer and filters to ensure no water icing occurs at sub-zero Celsius, and then it was adjusted to a specific pressure by valve. The pressure was monitored by a single crystal silicon pressure transmitter, and the relationship between pressure and flowrate was calibrated in advance by a rotameter. The fuel supply system consisted of a fuel tank, a filter to remove solid impurities, a servo motor system for pumping the liquid kerosene, and a cooling system for cooling fuel to sub-zero Celsius. The refrigerating unit used ethanol to cool the flowing fuel to a certain temperature through a heat exchanger. In order to monitor the fuel flowrate and fuel temperature, a flowmeter (accuracy of 1 mL/min and full scale of 0 ~ 400 mL/min) and a thermocouple (accuracy of 0.1 ℃) were installed in the fuel supply pipeline close to the burner inlet. All the measurement data was collected at a frequency of 20 Hz. In the present work, the fuel injection system composes of a special Laval fuel nozzle and radial counter swirlers. The structure are illustrated in the literature of Wu, et al. ( 2021 ), and the specific explanation of the injection system is similar to the description in the literature of Malbois, et al. ( 2019 ). The RP-3 kerosene used in this paper is normally for civil aviation in China (Wu, et al. 2021 ). The measured chemical formula can be written as C 13.75 H 29.21 . Before ignition at normal fuel temperature, the cooling system was kept off and the three-way valve turned fully towards burner, and then ignite the RP-3 kerosene spray at a certain flowrate by controlling the servo motor. Procedures of ignition at sub-zero Celsius were somewhat different. Before ignition, the cooling system continued refrigeration until the detected fuel temperature was close to a set value, and the three-way valve was kept open partially for separating the refrigeration main line and fuel injection bypass. Besides, the cold fuel from the refrigeration main line run through a coiler next to the burner, availably diminish the cold loss of the injected fuel when combustion. Detailed experimental conditions are listed in Table 1 . Note that by maintaining a constant fuel flow rate and varying the air gauge pressure, the effect of pressure is related to the air flowrate actually. The air Reynolds numbers at the burner exit are also listed in Table 1 . Table 1 All Recorded Experiment Conditions. case Fuel flowrate (g/s) Fuel temperature (℃) Air pressure (kPa) \({\text{R}\text{e}}_{\text{a}\text{i}\text{r}}\) 1 NT 0.285 25.4 0.986 4587 2 0.291 24.6 2.023 6810 3 0.292 24.5 3.018 8485 4 LT 0.288 -18.4 1.037 4716 5 0.285 -17.7 2.063 6883 6 0.300 -16.6 3.035 8511 * NT denotes cases of combustion at normal kerosene inlet temperature condition. * LT denotes cases of combustion at low kerosene inlet temperature condition. 2.2. Optical Diagnostics Figure 2 shows the arrangement of the OH-PLIF measurement system. The second harmonic of 532 nm laser from a 10Hz Nd:YAG laser (Powerlite DLS 8010, Continuum) was used to pump a dye laser (Vista, Continuum). The Rhodamine 590 dye diluted by 95% ethanol was excited to generate ~ 566 nm laser, and then tuned through a frequency doubling module (SHG) to generate the needed ~ 283 nm UV laser. The laser power at 283 nm was around ~ 10 mJ/pulse. The UV laser was reflected by two UV high reflectivity mirrors, and then passed through a concave lens (ƒ = -75mm) and a convex lens (ƒ = +500mm) to form a planar laser sheet with height of 30 mm and thickness of 200 µm. The OH fluorescence was captured by an ICCD camera (PI-max 4 emICCD, Princeton Instrument), which was equipped with a 105 mm, ƒ / 4.5 UV-NIKKOR lens, a 310 nm narrow-band filter (FWHM of 10 nm, peak transmittance of ~ 60%, Alluxa), a 305 nm long-pass filter (305FG01-50, Andover), and a UG11 filter (FGUV11, Thorlabs). These combined filters had an optical density over 6 at range of 200ཞ305 nm and 317ཞ400 nm, to effectively eliminate the interference of the UV laser elastic scattering and flame luminosity. The in-plane spatial resolution was 0.15 × 0.15 mm 2 , and the ICCD gate width was set as 50 ns. In addition, to obtain the characteristic OH excitation wavelength, the experiments were preceded by scanning fluorescence spectrum on the ethanol gasification flame, and 283.567nm was selected corresponding to the spectral location of the Q1(5) rotational transition of the A 2 Σ + ← X 2 Π (1,0) electronic band by comparing with LIFBASE (J. Luque ). Raw OH-PLIF images were firstly corrected from the distortion and spatial energy distribution of the laser sheet, and then were filtered with a two-dimensional 3 × 3 median filter after size calibration. For the non-excited UV laser wavelength detuned experiment, the dye laser wavelength was shifted by 0.05 nm and other conditions were kept constant. The LIF signal was sensitive to the excited laser wavelength. When the laser was shifted to a non-excited wavelength, the PLIF signal would disappear. By comparing tuned and detuned images, the signals from OH radicals can be judged. Figure 2 also shows the arrangement of the spectral analysis setup. The LIF spectral signal was focused by two convex lenses (ƒ = +100mm) into the slit of about 200 µm at the entrance of the spectrometer (SP2300, Princeton Instrument), and the 3 cm target field of view is narrowed to a height comparable to that of the slit. In addition, a high-pressure mercury lamp was used for calibration in the experiment. 3. Results And Discussion 3.1. Flame Structure Analysis of PILF images Figure 3 shows the actual flame pictures of the case1 to case6 taken by high speed camera. All kerosene flames have the bright yellow zones, which means abundant soot in these flames. The obvious reason is insufficient air supply for swirling combustion. The global equivalence ratios of the flames are from 1.6 to 3.1. But with the increscent air pressure, like case3, some blue zones appear in the flame. It seems that local premixed combustion of gaseous fuel happens at higher air pressure. Besides, when the inlet temperature falls below − 16 ℃, the brightness of the flame decreases visibly, but it is difficult to distinguish further in the pictures. So OH-PLIF is necessary for further study of the flame section. Although there are interferences in these soot flames, PLIF images are clear to present the flame reaction zones. Figure 4 shows instantaneous turbulent OH-PLIF images. Images on the left are for cases of NT (normal kerosene inlet temperature) at different air pressure, while the right images are for cases of LT (low inlet temperature below − 16 ℃). It can be seen that intensities of the transient OH signal for NT cases are much higher than that of LT cases, regardless of air pressure. The PLIF was carried on at the same laser pulse energy, so it can be approximated that the intensity is proportional to the OH radical concentration and flame temperature. Thus, it can be inferred that OH radicals in flames of sub-zero Celsius cases are scarce and flame temperature is not high enough. The combustion deterioration can’t ease even with the increscent air pressure. One direct reason is the poor fuel atomization at low temperature. In these PLIF images, there are apparent speckles from unburn liquid kerosene. Next section will explain these speckles. More quantitative information can be seen in Fig. 5 (a) for signal distribution at strong OH-LIF zone (x/D > 0.6, z/D = 0.75, D is the diameter of the burner outlet), extracted from averaging all the OH images. When the fuel inlet temperature is reduced from above 20 ℃ to below − 16 ℃, the strongest mean signal decreases by at least 5 times under the same air pressure, indicating that the sub-zero Celsius significantly reduces the combustion intensity, as concluded in the literature of Hwang, et al. ( 2015 ). Actually, the fuel is not unburnt due to the flames are still attached on the burner outlet plane, but the intensities of OH-LIF signals decrease to below 2000. This means the combustion reactions are weakened because OH radicals of hydrocarbons come from oxidation decomposition reaction (Chen et al. 2019 ). Besides, OH-LIF intensity gradient can represent the combustion heat release rate (Singh et al. 2020 , Yuan et al. 2015 ) and the x-direction gradient \(\left|\nabla \text{I}\right|\) can be calculated from derivative for OH-LIF intensity curve, seen in Fig. 5 (b). Sub-zero Celsius reduces the heat release rate severely on the flame front at 0.7 < x/D < 1.1. The normalized \(\left|\nabla \text{I}\right|\) is below 0.2 average for sub-zero Celsius cases, while over 0.4 average for normal temperature cases. It can be inferred that the heat release rate of LT cases is much lower than NT cases, and thus combustion reactions of hydrocarbons get weakened. However, heat release reactions are related to evaporative-diffusive mixed combustion of liquid fuel. Hence, spray combustion is affected mainly by fluid dynamics and atomization characteristics, including discharge coefficient, spray cone angle, mean droplet size (Dafsari, et al. 2019 , Davanlou et al. 2015 , Fajgenbaum and dos Santos 2016 , Park et al. 2004 , Park et al. 2007 , Yoon et al. 2008 ) and effect of fuel density and viscosity (Bruce RM). More in-depth researches about how sub-zero Celsius affect the heat release reaction need applications of digital off-axis holography, 3D-PIV and two-line PLIF to detect the flow field, droplet size or combustion intermediate product. But it is difficult to achieve these measurements in these soot flames at present. 3.2. Detuned PLIF images As shown in Fig. 6 , when using the same filters as OH-PLIF imaging to measure the instantaneous kerosene spray LIF without ignition, even in the OH-LIF detection wavelength range, liquid kerosene droplet LIF remains widespread. Note that the LIF signals (around 310 ± 5 nm) in Fig. 6 are all from kerosene LIF, not the laser scattering or OH-LIF. Similar phenomenon is found in n-heptane spray. In the literature of Yuan, et al. ( 2015 ), using a 310 nm bandpass filter, fuel LIFs from n-heptane is found in both spray and flame, and the n-heptane LIFs are all round speckles. Different from kerosene, n-heptane is pure substance without aromatic compound, but the round fuel LIFs are still detected. An approximate reason is the liquid fuel density is much larger than gaseous density, resulting in strong liquid LIF signals limited in round fuel droplets. All detuned images acquire 200 shots, showing the same phenomena as the image in Fig. 7 (b), not by accident. When the laser wavelength is shifted by 0.05 nm to non-excited wavelength, the OH LIF signals disappear, but spotted kerosene LIF still remains widespread. Kerosene LIF appears almost exclusively in small round spots. Although faint signals also exist at the OH-LIF locations, the signal intensity is over 20 times weaker than the OH LIF. These faint signals are probably soot LII signals, PAH-LIFs and gaseous kerosene-LIFs. However, these interferences are always exit, despite well optical filtering of the combined filters. But considering the obvious difference, it is undoubted that the LIF signals of flame structure in the OH-PLIF images are almost all from OH radicals. This means that the UV laser pulse energy, ICCD gate width and used optical filters are appropriate in this work. In addition, LIF particles are mainly kerosene LIFs, not laser scattering or other combustion products, comparing with kerosene LIFs in Fig. 6 . This indicates that the current combined filters are just enough to detect the fuel LIF particles and OH-LIF, as well as suppress other signals at the same time. Figure 8 shows curves of the normalized spectral statistics for over 50 regions of fuel LIF particles found in 200 image shots of case 2. Raw spectrum images are obtained within around 3 cm up from the burner outlet. In Fig. 8 (a), the 283 nm laser is located in the leftmost peak of the spectrum, and the kerosene LIF appears in the range of 308 to 410 nm, agreeing well with kerosene spectrum of the literature of Orain, et al. ( 2014 ). The OH-LIF is located in the middle peak of 308 to 317 nm and the detailed spectrum can be seen in Fig. 8 (b), coincident with the LIFBASE simulation. It is obvious that the OH LIF peak is very close to the kerosene LIF peak, resulting in a significant impact on OH fluorescence. And before ignition, comparing the curves in Fig. 8 (a), the UV laser scattering is stronger, due to more fuel droplets. However, the 310 ± 5 nm filter and 305 nm long-pass filter can work well to block these signals. The fuel spray LIF spectrum also indicates that fuel LIF under UV laser excitation exists even in liquid kerosene, while the OH-LIF only exists in the flame. 3.3. Analysis of fuel LIF particles. Kerosene LIF particles can reflect the fuel atomization in a sense. To analyze the effect of fuel fluorescence, LOG (Laplacian of Gaussian) operator blob detection (Kong et al. 2013 ) was adopted for counting the number of fuel LIF particles. The LOG kernel function can be described as follows $$\text{L}\text{O}\text{G}\left(\text{x},\text{y}\right)=\varDelta {\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)=\frac{{\text{x}}^{2}{+\text{y}}^{2}-2{{\sigma }}^{2}}{{{\sigma }}^{4}}\text{e}\text{x}\text{p}\left({-(\text{x}}^{2}{+\text{y}}^{2}\right)/\left(2{{\sigma }}^{2}\right))$$ 1 where \({\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)\) is Gaussian function, \(\text{L}\text{O}\text{G}\left(\text{x},\text{y}\right)\) is the second derivative of \({\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)\) , σ is a standard deviation parameter in \({\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)\) . The shape of LOG kernel function is similar to a spot, with strength strong in the middle and weak around. In a blob detection zone of a two-dimensional ICCD image \({\text{I}}_{0}\left(\text{x},\text{y}\right)\) , LOG response value of binarized circular speckle in the image can be written as \(\text{L}\text{O}\text{G}\text{*}{\text{I}}_{0}\left(\text{x},\text{y}\right)=\varDelta {\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)\text{*}{\text{I}}_{0}\left(\text{x},\text{y}\right)\) , the convolution of \(\varDelta {\text{G}}_{{\sigma }}\left(\text{x},\text{y}\right)\) and \({\text{I}}_{0}\left(\text{x},\text{y}\right)\) . When executing the iterative algorithm, the LOG operator will change its scale to calculate and compare the response value. At a certain characteristic scale, the LOG response value will reach maximum, indicating the speckle size in the image converges to the shape of the Gaussian Laplace function, at which time the size and location of the speckle are recorded. A typical example can be seen in Fig. 9 . Using LOG operator blob detection, the mean number of fuel LIF particles can be counted for the OH-PLIF images taken in different cases, and the results are shown in Fig. 10 . However, distribution of all droplets can’t be counted because the actual droplet cannot be distinguished under the limited resolution of our ICCD camera. Only when a droplet is large enough, the fuel LIF intensity can reach saturation value of the ICCD camera, and LOG algorithm can only work for particles with saturation value. To distinguish these round speckles from actual droplets, these particles are named after “fuel LIF particles”. In Fig. 10 (a), it seems that the number of total fuel LIF particles for LT cases is slightly lower than that for NT cases. But a careful comparison in Fig. 10 (b) shows that the number of fuel LIF particles with diameter over 1 mm for LT cases is larger than that for NT cases at any air pressure. 1mm droplets are hardly ever found even in swirling spray (Wu, et al. 2021 ). So fuel LIF particles over 1mm are regarded as large particles in this paper. In fact, LOG algorithm can only count particles of certain diameters, and only LIF of diameter over 0.7 mm can be counted. In spite of this disadvantage, the distributed fuel LIF particles can reflect the characteristic of big droplets and the counted number of fuel LIF particles can be regarded as a supplementary evaluation for fuel atomization. Take case 1&4 as an example. When at sub-zero Celsius, number of large particles is 2.55 and total number N KT is 15.48, while N KL is 1.31 and N KT is 16.86 for normal temperature cases. It can be inferred that part of the kerosene is not atomized at sub-zero Celsius and small droplets formation becomes difficult, thus the total number slightly decreases and the number of large fuel LIF particles increases instead. This indicates that it gets more difficult for air to interact with liquid kerosene. Because density and the dynamic viscosity of kerosene increase fast at sub-zero Celsius (Viorel PALEU 2007) and the atomization gets worse, like droplet mean diameter gets larger and spray cone angle gets smaller (Aleiferis, et al. 2021 ). Droplet size distribution measurement is difficult in soot flame of this work. But it can be achieved easy in spray without ignition. In the literature of Wu et al. ( 2021 ), picosecond pulsed digital off-axis holography is used to measure the near-nozzle droplet size distribution of the same burner nozzle of our work. The data is in Fig. 10 (c). In near-nozzle spray, droplets in the range of 210 ~ 690 µm are considered to be larger droplets and account for a relatively small proportion of the droplet size distribution. Distribution of these large droplet presents the same phenomenon as fuel LIF particles in this paper. When at sub-zero Celsius, the droplet number decreases at any air pressure, indicating the worse atomization. But the droplets over 700 µm are not counted in the literature of Wu, et al. ( 2021 ), due to the interference of large droplets in 3D holography measurement. So from another perspective, the counted fuel LIF particles data in this paper can be considered as a supplementary evaluation for large droplets over 700 µm in the soot flame. 4. Conclusions In this study, flame structures of RP-3 swirling spray combustion at different conditions including low/normal fuel inlet temperature and different air pressure are evaluated. According to analysis of OH-PLIF images, sub-zero Celsius significantly results in the reduction of the OH radical concentration and poor heat release rate. The strongest signals decrease by at least 5 times. With analysis of LIF spectroscopy and detuned images, fuel LIF particles in the OH-PLIF images are mainly liquid kerosene LIF. It is confirmed that the LIF signals of flame structure in the OH-PLIF images are almost all from OH radicals. This means that the UV laser pulse energy, ICCD gate width and used optical filters in this work are appropriate to detect the fuel LIF particles and OH-LIF, as well as suppress other signals at the same time. Kerosene LIF particles can reflect the fuel atomization in a sense. Using LOG operator blob detection, it is found that, when at sub-zero Celsius, the total number of fuel LIF particles slightly decreases and the number of large particles increases at any air pressure. Although the LIF diameter cannot represent the actual particle’s diameter due to the limited ICCD resolution, the counted number of fuel LIF particles can also be a supplementary evaluation for fuel atomization. This study probably needs other laser diagnostics to explain the flow field and atomization, but the PLIF analysis provides insight into complex combustion deterioration of kerosene swirling spray combustion for the first time. Problems in this study would create some challenges for the measurement of flow filed, droplet size or other atomization properties under worse combustion conditions. Declarations Ethical approval not applicable. Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper . Authors' contributions Shirong Xin: Data curation, Formal analysis, Writing - original draft. Yong He: Conceptualization, Investigation, Methodology, Validation, Writing - review & editing. Tao Liu: Writing - review & editing. Yingchun Wu: Writing - review & editing, Resources. Xuecheng Wu: Conceptualization, Software, Resources. Zhihua Wang: Funding acquisition, Investigation, Writing - review & editing. Funding This work was supported by National Natural Science Foundation of China (52125605). 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Proc Combust Inst 35:1649–1656 DOI https://doi.org/10.1016/j.proci.2014.06.012 Additional Declarations No competing interests reported. Supplementary Files GraphicalAbstract.jpg Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2068135","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":137998957,"identity":"b45fcbea-1616-4ee6-9255-1348e7749ddf","order_by":0,"name":"Shirong Xin","email":"","orcid":"","institution":"Zhejiang University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Shirong","middleName":"","lastName":"Xin","suffix":""},{"id":137998958,"identity":"ca0858a6-c4ac-45d7-8c7b-9b7f4dc1cb72","order_by":1,"name":"Yong He","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAz0lEQVRIiWNgGAWjYBACAwhlw8DYAKR4SNCSRrqWwxCKKC3m7D2Gjwt+nbdnnpHA+OBtG4O8OSEtlj1njI1n9t1mZpyRwGw4t43BcGcDIYfdyDGT5u25zQbUwibN28aQYHCAkJb7b0BazvEAtbD/Jk7LDR4zaZ4fByRAtjATp+VMWrExb0OyAWPPw2bJOeckDDcQ1HL88MbHPH/s7A3bkw9+eFNmI0/QFgYGDgMGxjYGBsMGcGRKEFQPBOwPGBj+MDDIE6N2FIyCUTAKRiYAAO83PXh75YX3AAAAAElFTkSuQmCC","orcid":"","institution":"Zhejiang University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Yong","middleName":"","lastName":"He","suffix":""},{"id":137998959,"identity":"86022615-61ac-482b-9f10-a22259275257","order_by":2,"name":"Tao Liu","email":"","orcid":"","institution":"Aero Engine Corp China","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Tao","middleName":"","lastName":"Liu","suffix":""},{"id":137998960,"identity":"c97a6fcd-0151-4a7c-a70c-628a28f7e08d","order_by":3,"name":"Yingchun Wu","email":"","orcid":"","institution":"Zhejiang University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yingchun","middleName":"","lastName":"Wu","suffix":""},{"id":137998961,"identity":"c4ffcd21-194d-4950-a12e-75786cc84d52","order_by":4,"name":"Xuecheng Wu","email":"","orcid":"","institution":"Zhejiang University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Xuecheng","middleName":"","lastName":"Wu","suffix":""},{"id":137998962,"identity":"ea26042e-860b-48e4-b827-25c9900d1481","order_by":5,"name":"Zhihua Wang","email":"","orcid":"","institution":"Zhejiang University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Zhihua","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2022-09-15 09:29:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2068135/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2068135/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":26846823,"identity":"df5d6ec4-044a-41f5-80ad-f53015c481ef","added_by":"auto","created_at":"2022-09-22 20:41:02","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":122551,"visible":true,"origin":"","legend":"\u003cp\u003eRP-3 swirling spray combustion experimental apparatus.\u003c/p\u003e","description":"","filename":"Fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/984645102e61b770d86ae2f1.jpg"},{"id":26845476,"identity":"51f7870f-8bc5-4fc3-9d3c-a3e52763e2df","added_by":"auto","created_at":"2022-09-22 20:36:02","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":83499,"visible":true,"origin":"","legend":"\u003cp\u003eOH-PLIF measurement system and spectral analysis setup.\u003c/p\u003e","description":"","filename":"Fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/4a5c67dddaa9705165759c65.jpg"},{"id":26845474,"identity":"0ac7ac8e-0d41-4c8c-b117-dc1466e53fb3","added_by":"auto","created_at":"2022-09-22 20:36:02","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":61219,"visible":true,"origin":"","legend":"\u003cp\u003eInstantaneous pictures of all cases taken with the same gate width\u003c/p\u003e","description":"","filename":"Fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/49dfa9779e7e52cd5fc33806.jpg"},{"id":26846825,"identity":"974ded51-fd25-4023-aa8d-250477717087","added_by":"auto","created_at":"2022-09-22 20:41:03","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":178381,"visible":true,"origin":"","legend":"\u003cp\u003eInstantaneous OH-PLIF images at different temperature and air pressure. (a) to (c) are images in case 1 to 3 (inlet temperature at around 25 ℃), (d) to (f) are images in case 4 to 6 (inlet temperature below - 16 ℃).\u003c/p\u003e","description":"","filename":"Fig4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/721ade6201f4acc199d96b1f.jpg"},{"id":26845482,"identity":"d1b16f48-f2e3-40a5-bb80-a637428c661a","added_by":"auto","created_at":"2022-09-22 20:36:03","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":159942,"visible":true,"origin":"","legend":"\u003cp\u003e(a) distribution of average LIF signals at strong OH-LIF zone (x/D \u0026gt; 0.6, z/D = 0.75) for case 1 to 6. (b) normalized absolute value of OH-LIF intensity derivative (|∇I|), which can represent the intensity of combustion heat release.\u003c/p\u003e","description":"","filename":"Fig5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/dfffc10705f5450b6ff3bec7.jpg"},{"id":26845478,"identity":"521b3f6d-2a22-4f7a-8a51-4eb5cb27b0b0","added_by":"auto","created_at":"2022-09-22 20:36:03","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":48560,"visible":true,"origin":"","legend":"\u003cp\u003eKerosene spray droplets fluorescence at approximately 310 nm ± 5 nm, before ignition. The LIF is all from liquid kerosene, not laser scattering or flame luminosity. The filters used are the same as OH-PLIF.\u003c/p\u003e","description":"","filename":"Fig6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/d550f0cedd19152fe089d2e9.jpg"},{"id":26846827,"identity":"0aa092f4-a1ac-41e9-95d0-8f736b8db1ff","added_by":"auto","created_at":"2022-09-22 20:41:03","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":52578,"visible":true,"origin":"","legend":"\u003cp\u003eInstantaneous tuned image (a) and detuned image (b) of case2. Kerosene LIF mostly appears only on fuel LIF particles. (Detuned process: the laser wavelength is shifted by 0.05 nm to prevent the production of OH-PLIF so that kerosene-LIF can be imaged without OH-LIF. Tuned process: the laser wavelength is on the OH excitation line to excite OH-PLIF.)\u003c/p\u003e","description":"","filename":"Fig7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/73dc972c4cecf4657e3ce99d.jpg"},{"id":26848103,"identity":"fb277c4c-bdff-41fe-bc07-484e003b1d6b","added_by":"auto","created_at":"2022-09-22 20:46:03","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":138543,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized LIF average spectrum of (a) 270-410 nm (solid blue line represents LIF of flame and dashed red line represents kerosene spray LIF before ignition). (b) 305-335 nm.\u003c/p\u003e","description":"","filename":"Fig8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/f01b87bf88aae30bd9cf7ebb.jpg"},{"id":26845479,"identity":"a95cc4c1-a154-4417-afd0-a7c3c35e8a87","added_by":"auto","created_at":"2022-09-22 20:36:03","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":131628,"visible":true,"origin":"","legend":"\u003cp\u003eLocation recognition of fuel LIF particles in OH-PLIF grayscale image using LOG operator blob detection. (a) Raw OH-PLIF image. (b) The same image with locations of fuel LIF particles marked by white dots.\u003c/p\u003e","description":"","filename":"Fig9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/62a27b9a3adc56077060ec2b.jpg"},{"id":26845484,"identity":"648e42d7-dbaf-4a0b-8227-7fa6c6d8863b","added_by":"auto","created_at":"2022-09-22 20:36:03","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":201628,"visible":true,"origin":"","legend":"\u003cp\u003eResults of LOG operator blob detection for the mean number of fuel LIF particles counted in the OH images at different conditions. (a) total fuel LIF particles average. (b) large fuel LIF particles average. (c) the mean number of near-nozzle spray droplets without ignition, with diameter in the range of 210 ~ 690 μm (\u003ca href=\"https://www.researchsquare.com/article/rs-2068135/admin/draft#_ENREF_22\" title=\"Wu, 2021 #871\"\u003eWu, et al. 2021\u003c/a\u003e). The burner nozzle of the literature of \u003ca href=\"https://www.researchsquare.com/article/rs-2068135/admin/draft#_ENREF_22\" title=\"Wu, 2021 #871\"\u003eWu, et al. (2021)\u003c/a\u003e is the same as this paper. Red bar charts are for normal temperature cases, and blue bar charts are for sub-zero Celsius cases. (NkTmeans the mean number of total fuel LIF particles. NkL means number of large fuel LIF particles).\u003c/p\u003e","description":"","filename":"Fig10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/1139026968e8fd156751193e.jpg"},{"id":27210279,"identity":"027e2973-222f-40aa-86ef-5e0e39229feb","added_by":"auto","created_at":"2022-09-30 20:59:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1013198,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/f4954625-b1db-48c8-aff0-523a69448428.pdf"},{"id":26848102,"identity":"e9818078-64a6-4f69-be9f-ff9f8016732b","added_by":"auto","created_at":"2022-09-22 20:46:02","extension":"jpg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":265200,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2068135/v1/871c7b9fa5215da35c531309.jpg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flame","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAircrafts are very vulnerable to adverse external conditions when flying in the upper atmosphere, such as low temperature and low pressure. According to statistics of the literature of Idowu Innocent Abbas (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), 98 aircraft accidents in total from year of 1950 to 2000 were caused by external environmental factors at upper atmosphere. Several factors may lead to the deterioration of combustion in engine chambers at low temperature, including poor atomization (Dafsari et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). slower fuel evaporation rate, insufficient gas supply, and soot formation. They will result in difficulty for re-ignition after flameout and combustion instability without conduciveness for safe flight. Therefore, study for deterioration of combustion at low fuel inlet temperature is critical, like liquid fuel distribution, heat release and key species distribution etc., which will contribute to improving understanding of flame structure of the gas-liquid two-phase flow at operational limits (Chterev et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). However, literature about aviation kerosene swirling spray combustion at conditions of low fuel inlet temperature and insufficient air feed are scarce. Except the complex setup for artificially created experimental conditions, challenges associated with liquid fuel combustion are also difficult to handle for online optical visualization measurement, such as interference from liquid fuel, hydrocarbon species during fuel decomposition (Chterev, et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) as well as abundant of soot.\u003c/p\u003e \u003cp\u003eAvailable studies almost focus on spray experiments, seldom related to combustion. Dafsari, et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) examined the viscosity effect on the pressure swirl atomization of alternative aviation fuel at cold fuel temperature conditions, discovering the increased viscosity with the increscent fuel Reynolds number. Hwang et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) investigated the fuel temperature influence on spray and combustion characteristics of diesel in a constant volume combustion chamber under simulated engine operating conditions, revealing cold fuel temperature reduced flame luminosity and spray cone angle. Aleiferis et al. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) made an optical investigation into the effect of atomization of multi-hole injectors for iso-octane and ethanol. They found that cold temperatures suppressed atomization for fuel, causing the narrowing of the spray cone angle and large droplet diameter. This was linked to increased viscosity and higher losses. Dahlander P (2008) found a larger spray liquid penetration length at -30 ℃. Wu et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) used picosecond pulsed digital off-axis holography to measure the near-nozzle droplet size and 3D distribution of a swirl kerosene spray, with a summary that the droplet peak diameter decreases with the increase of gas feed. However, the above mentioned investigations are mainly focused on spray atomization, flame structure studies for aviation kerosene swirling spray combustion at conditions of cold fuel temperature are seldom reported.\u003c/p\u003e \u003cp\u003eFor analysis of heat release and distribution of intermediate radicals, the OH-planar laser induced fluorescence (OH-PLIF) is commonly utilized as a kind of efficient optical diagnostics. The OH-PLIF is suitable for complex premixed flame of aviation kerosene swirling spray. There are several studies focused on the lean premixed flame of swirl-stabilized kerosene spray flame using OH-PLIF (Malbois et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Malbois et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Malbois et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Seitzman and Hanson \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1993\u003c/span\u003e). However, kerosene usually have complex fluorescence which comes from aromatic hydrocarbons or inter products after decomposition, with wide wavelength range in spectroscopy under UV laser excitation (Chterev, et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Orain et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Especially in the deterioration of combustion, the unburned kerosene may require more complicated image processing. Accordingly, verification work should be done to ensure the flame structure should be represented by OH-PLIF signals, thus statistical analysis can be conducted further.\u003c/p\u003e \u003cp\u003eIn this paper, an optical measurement system as well as a combustion system including a burner, fuel supply, air supply, and fuel cooling, was established for swirl-stabilized kerosene spray combustion investigation. OH-PLIF measurements were performed at different conditions including low/normal fuel inlet temperature, and different air flowrate. LIF spectrum in the flame and spray, as well as detuned analysis, were then carried out to identify the main LIF species on the flame structure. With utilization of blob detection algorithm, number of the unburned fuel LIF particles was counted. The contribution of this paper is to fill the gap of optical diagnostics of kerosene swirling spray combustion at low fuel inlet temperature and study of the phenomena of kerosene combustion deterioration.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1. Experimental Setup and Ignition Details\u003c/h2\u003e\n\u003cp\u003eThe experimental apparatus to investigate RP-3 swirling spray combustion is illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, mainly comprises the air supply system, fuel supply system, and monitoring equipment. The air supplied from an air compressor firstly passed through a dryer and filters to ensure no water icing occurs at sub-zero Celsius, and then it was adjusted to a specific pressure by valve. The pressure was monitored by a single crystal silicon pressure transmitter, and the relationship between pressure and flowrate was calibrated in advance by a rotameter. The fuel supply system consisted of a fuel tank, a filter to remove solid impurities, a servo motor system for pumping the liquid kerosene, and a cooling system for cooling fuel to sub-zero Celsius. The refrigerating unit used ethanol to cool the flowing fuel to a certain temperature through a heat exchanger. In order to monitor the fuel flowrate and fuel temperature, a flowmeter (accuracy of 1 mL/min and full scale of 0\u0026thinsp;~\u0026thinsp;400 mL/min) and a thermocouple (accuracy of 0.1 ℃) were installed in the fuel supply pipeline close to the burner inlet. All the measurement data was collected at a frequency of 20 Hz. In the present work, the fuel injection system composes of a special Laval fuel nozzle and radial counter swirlers. The structure are illustrated in the literature of Wu, et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e), and the specific explanation of the injection system is similar to the description in the literature of Malbois, et al. (\u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). The RP-3 kerosene used in this paper is normally for civil aviation in China (Wu, et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). The measured chemical formula can be written as C\u003csub\u003e13.75\u003c/sub\u003e H\u003csub\u003e29.21\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eBefore ignition at normal fuel temperature, the cooling system was kept off and the three-way valve turned fully towards burner, and then ignite the RP-3 kerosene spray at a certain flowrate by controlling the servo motor. Procedures of ignition at sub-zero Celsius were somewhat different. Before ignition, the cooling system continued refrigeration until the detected fuel temperature was close to a set value, and the three-way valve was kept open partially for separating the refrigeration main line and fuel injection bypass. Besides, the cold fuel from the refrigeration main line run through a coiler next to the burner, availably diminish the cold loss of the injected fuel when combustion. Detailed experimental conditions are listed in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Note that by maintaining a constant fuel flow rate and varying the air gauge pressure, the effect of pressure is related to the air flowrate actually. The air Reynolds numbers at the burner exit are also listed in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eAll Recorded Experiment Conditions.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ecase\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eFuel flowrate (g/s)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eFuel temperature (℃)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAir pressure (kPa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{R}\\text{e}}_{\\text{a}\\text{i}\\text{r}}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"3\" align=\"left\"\u003e\n\u003cp\u003eNT\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.285\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e25.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.986\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4587\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.291\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e24.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.023\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e6810\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.292\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e24.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.018\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e8485\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"3\" align=\"left\"\u003e\n\u003cp\u003eLT\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.288\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e-18.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.037\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4716\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.285\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e-17.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.063\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e6883\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.300\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e-16.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.035\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e8511\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003ctfoot\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"6\"\u003e* NT denotes cases of combustion at normal kerosene inlet temperature condition.\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"6\"\u003e* LT denotes cases of combustion at low kerosene inlet temperature condition.\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tfoot\u003e\n\u003c/table\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2. Optical Diagnostics\u003c/h2\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e shows the arrangement of the OH-PLIF measurement system. The second harmonic of 532 nm laser from a 10Hz Nd:YAG laser (Powerlite DLS 8010, Continuum) was used to pump a dye laser (Vista, Continuum). The Rhodamine 590 dye diluted by 95% ethanol was excited to generate\u0026thinsp;~\u0026thinsp;566 nm laser, and then tuned through a frequency doubling module (SHG) to generate the needed\u0026thinsp;~\u0026thinsp;283 nm UV laser. The laser power at 283 nm was around ~\u0026thinsp;10 mJ/pulse. The UV laser was reflected by two UV high reflectivity mirrors, and then passed through a concave lens (\u0026fnof; = -75mm) and a convex lens (\u0026fnof; = +500mm) to form a planar laser sheet with height of 30 mm and thickness of 200 \u0026micro;m. The OH fluorescence was captured by an ICCD camera (PI-max 4 emICCD, Princeton Instrument), which was equipped with a 105 mm, \u0026fnof; / 4.5 UV-NIKKOR lens, a 310 nm narrow-band filter (FWHM of 10 nm, peak transmittance of ~\u0026thinsp;60%, Alluxa), a 305 nm long-pass filter (305FG01-50, Andover), and a UG11 filter (FGUV11, Thorlabs). These combined filters had an optical density over 6 at range of 200ཞ305 nm and 317ཞ400 nm, to effectively eliminate the interference of the UV laser elastic scattering and flame luminosity. The in-plane spatial resolution was 0.15 \u0026times; 0.15 mm\u003csup\u003e2\u003c/sup\u003e, and the ICCD gate width was set as 50 ns. In addition, to obtain the characteristic OH excitation wavelength, the experiments were preceded by scanning fluorescence spectrum on the ethanol gasification flame, and 283.567nm was selected corresponding to the spectral location of the Q1(5) rotational transition of the A\u003csup\u003e2\u003c/sup\u003e\u0026Sigma;\u003csup\u003e+\u003c/sup\u003e \u0026larr; X\u003csup\u003e2\u003c/sup\u003e\u0026Pi; (1,0) electronic band by comparing with LIFBASE (J. Luque ). Raw OH-PLIF images were firstly corrected from the distortion and spatial energy distribution of the laser sheet, and then were filtered with a two-dimensional 3 \u0026times; 3 median filter after size calibration.\u003c/p\u003e\n\u003cp\u003eFor the non-excited UV laser wavelength detuned experiment, the dye laser wavelength was shifted by 0.05 nm and other conditions were kept constant. The LIF signal was sensitive to the excited laser wavelength. When the laser was shifted to a non-excited wavelength, the PLIF signal would disappear. By comparing tuned and detuned images, the signals from OH radicals can be judged. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e also shows the arrangement of the spectral analysis setup. The LIF spectral signal was focused by two convex lenses (\u0026fnof; = +100mm) into the slit of about 200 \u0026micro;m at the entrance of the spectrometer (SP2300, Princeton Instrument), and the 3 cm target field of view is narrowed to a height comparable to that of the slit. In addition, a high-pressure mercury lamp was used for calibration in the experiment.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1. Flame Structure Analysis of PILF images\u003c/h2\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the actual flame pictures of the case1 to case6 taken by high speed camera. All kerosene flames have the bright yellow zones, which means abundant soot in these flames. The obvious reason is insufficient air supply for swirling combustion. The global equivalence ratios of the flames are from 1.6 to 3.1. But with the increscent air pressure, like case3, some blue zones appear in the flame. It seems that local premixed combustion of gaseous fuel happens at higher air pressure. Besides, when the inlet temperature falls below \u0026minus;\u0026thinsp;16 ℃, the brightness of the flame decreases visibly, but it is difficult to distinguish further in the pictures. So OH-PLIF is necessary for further study of the flame section. Although there are interferences in these soot flames, PLIF images are clear to present the flame reaction zones.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows instantaneous turbulent OH-PLIF images. Images on the left are for cases of NT (normal kerosene inlet temperature) at different air pressure, while the right images are for cases of LT (low inlet temperature below \u0026minus;\u0026thinsp;16 ℃). It can be seen that intensities of the transient OH signal for NT cases are much higher than that of LT cases, regardless of air pressure. The PLIF was carried on at the same laser pulse energy, so it can be approximated that the intensity is proportional to the OH radical concentration and flame temperature. Thus, it can be inferred that OH radicals in flames of sub-zero Celsius cases are scarce and flame temperature is not high enough. The combustion deterioration can\u0026rsquo;t ease even with the increscent air pressure. One direct reason is the poor fuel atomization at low temperature. In these PLIF images, there are apparent speckles from unburn liquid kerosene. Next section will explain these speckles.\u003c/p\u003e\n\u003cp\u003eMore quantitative information can be seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (a) for signal distribution at strong OH-LIF zone (x/D\u0026thinsp;\u0026gt;\u0026thinsp;0.6, z/D\u0026thinsp;=\u0026thinsp;0.75, D is the diameter of the burner outlet), extracted from averaging all the OH images. When the fuel inlet temperature is reduced from above 20 ℃ to below \u0026minus;\u0026thinsp;16 ℃, the strongest mean signal decreases by at least 5 times under the same air pressure, indicating that the sub-zero Celsius significantly reduces the combustion intensity, as concluded in the literature of Hwang, et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e). Actually, the fuel is not unburnt due to the flames are still attached on the burner outlet plane, but the intensities of OH-LIF signals decrease to below 2000. This means the combustion reactions are weakened because OH radicals of hydrocarbons come from oxidation decomposition reaction (Chen et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). Besides, OH-LIF intensity gradient can represent the combustion heat release rate (Singh et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e, Yuan et al. \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e) and the x-direction gradient \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left|\\nabla \\text{I}\\right|\\)\u003c/span\u003e\u003c/span\u003e can be calculated from derivative for OH-LIF intensity curve, seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (b). Sub-zero Celsius reduces the heat release rate severely on the flame front at 0.7\u0026thinsp;\u0026lt;\u0026thinsp;x/D\u0026thinsp;\u0026lt;\u0026thinsp;1.1. The normalized \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left|\\nabla \\text{I}\\right|\\)\u003c/span\u003e\u003c/span\u003e is below 0.2 average for sub-zero Celsius cases, while over 0.4 average for normal temperature cases. It can be inferred that the heat release rate of LT cases is much lower than NT cases, and thus combustion reactions of hydrocarbons get weakened. However, heat release reactions are related to evaporative-diffusive mixed combustion of liquid fuel. Hence, spray combustion is affected mainly by fluid dynamics and atomization characteristics, including discharge coefficient, spray cone angle, mean droplet size (Dafsari, et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e, Davanlou et al. \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e, Fajgenbaum and dos Santos \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e, Park et al. \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e, Park et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e, Yoon et al. \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e) and effect of fuel density and viscosity (Bruce RM). More in-depth researches about how sub-zero Celsius affect the heat release reaction need applications of digital off-axis holography, 3D-PIV and two-line PLIF to detect the flow field, droplet size or combustion intermediate product. But it is difficult to achieve these measurements in these soot flames at present.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2. Detuned PLIF images\u003c/h2\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e, when using the same filters as OH-PLIF imaging to measure the instantaneous kerosene spray LIF without ignition, even in the OH-LIF detection wavelength range, liquid kerosene droplet LIF remains widespread. Note that the LIF signals (around 310\u0026thinsp;\u0026plusmn;\u0026thinsp;5 nm) in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e are all from kerosene LIF, not the laser scattering or OH-LIF. Similar phenomenon is found in n-heptane spray. In the literature of Yuan, et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e), using a 310 nm bandpass filter, fuel LIFs from n-heptane is found in both spray and flame, and the n-heptane LIFs are all round speckles. Different from kerosene, n-heptane is pure substance without aromatic compound, but the round fuel LIFs are still detected. An approximate reason is the liquid fuel density is much larger than gaseous density, resulting in strong liquid LIF signals limited in round fuel droplets.\u003c/p\u003e\n\u003cp\u003eAll detuned images acquire 200 shots, showing the same phenomena as the image in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e (b), not by accident. When the laser wavelength is shifted by 0.05 nm to non-excited wavelength, the OH LIF signals disappear, but spotted kerosene LIF still remains widespread. Kerosene LIF appears almost exclusively in small round spots. Although faint signals also exist at the OH-LIF locations, the signal intensity is over 20 times weaker than the OH LIF. These faint signals are probably soot LII signals, PAH-LIFs and gaseous kerosene-LIFs. However, these interferences are always exit, despite well optical filtering of the combined filters. But considering the obvious difference, it is undoubted that the LIF signals of flame structure in the OH-PLIF images are almost all from OH radicals. This means that the UV laser pulse energy, ICCD gate width and used optical filters are appropriate in this work. In addition, LIF particles are mainly kerosene LIFs, not laser scattering or other combustion products, comparing with kerosene LIFs in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. This indicates that the current combined filters are just enough to detect the fuel LIF particles and OH-LIF, as well as suppress other signals at the same time.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e shows curves of the normalized spectral statistics for over 50 regions of fuel LIF particles found in 200 image shots of case 2. Raw spectrum images are obtained within around 3 cm up from the burner outlet. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e (a), the 283 nm laser is located in the leftmost peak of the spectrum, and the kerosene LIF appears in the range of 308 to 410 nm, agreeing well with kerosene spectrum of the literature of Orain, et al. (\u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). The OH-LIF is located in the middle peak of 308 to 317 nm and the detailed spectrum can be seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e (b), coincident with the LIFBASE simulation. It is obvious that the OH LIF peak is very close to the kerosene LIF peak, resulting in a significant impact on OH fluorescence. And before ignition, comparing the curves in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e (a), the UV laser scattering is stronger, due to more fuel droplets. However, the 310\u0026thinsp;\u0026plusmn;\u0026thinsp;5 nm filter and 305 nm long-pass filter can work well to block these signals. The fuel spray LIF spectrum also indicates that fuel LIF under UV laser excitation exists even in liquid kerosene, while the OH-LIF only exists in the flame.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3. Analysis of fuel LIF particles.\u003c/h2\u003e\n\u003cp\u003eKerosene LIF particles can reflect the fuel atomization in a sense. To analyze the effect of fuel fluorescence, LOG (Laplacian of Gaussian) operator blob detection (Kong et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e) was adopted for counting the number of fuel LIF particles. The LOG kernel function can be described as follows\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ1\" class=\"mathdisplay\"\u003e$$\\text{L}\\text{O}\\text{G}\\left(\\text{x},\\text{y}\\right)=\\varDelta {\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)=\\frac{{\\text{x}}^{2}{+\\text{y}}^{2}-2{{\\sigma }}^{2}}{{{\\sigma }}^{4}}\\text{e}\\text{x}\\text{p}\\left({-(\\text{x}}^{2}{+\\text{y}}^{2}\\right)/\\left(2{{\\sigma }}^{2}\\right))$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e is Gaussian function, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{L}\\text{O}\\text{G}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e is the second derivative of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e, \u0026sigma; is a standard deviation parameter in \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e. The shape of LOG kernel function is similar to a spot, with strength strong in the middle and weak around. In a blob detection zone of a two-dimensional ICCD image \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{I}}_{0}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e, LOG response value of binarized circular speckle in the image can be written as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{L}\\text{O}\\text{G}\\text{*}{\\text{I}}_{0}\\left(\\text{x},\\text{y}\\right)=\\varDelta {\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)\\text{*}{\\text{I}}_{0}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e, the convolution of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varDelta {\\text{G}}_{{\\sigma }}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{I}}_{0}\\left(\\text{x},\\text{y}\\right)\\)\u003c/span\u003e\u003c/span\u003e. When executing the iterative algorithm, the LOG operator will change its scale to calculate and compare the response value. At a certain characteristic scale, the LOG response value will reach maximum, indicating the speckle size in the image converges to the shape of the Gaussian Laplace function, at which time the size and location of the speckle are recorded. A typical example can be seen in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eUsing LOG operator blob detection, the mean number of fuel LIF particles can be counted for the OH-PLIF images taken in different cases, and the results are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. However, distribution of all droplets can\u0026rsquo;t be counted because the actual droplet cannot be distinguished under the limited resolution of our ICCD camera. Only when a droplet is large enough, the fuel LIF intensity can reach saturation value of the ICCD camera, and LOG algorithm can only work for particles with saturation value. To distinguish these round speckles from actual droplets, these particles are named after \u0026ldquo;fuel LIF particles\u0026rdquo;. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e (a), it seems that the number of total fuel LIF particles for LT cases is slightly lower than that for NT cases. But a careful comparison in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e (b) shows that the number of fuel LIF particles with diameter over 1 mm for LT cases is larger than that for NT cases at any air pressure. 1mm droplets are hardly ever found even in swirling spray (Wu, et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). So fuel LIF particles over 1mm are regarded as large particles in this paper. In fact, LOG algorithm can only count particles of certain diameters, and only LIF of diameter over 0.7 mm can be counted. In spite of this disadvantage, the distributed fuel LIF particles can reflect the characteristic of big droplets and the counted number of fuel LIF particles can be regarded as a supplementary evaluation for fuel atomization. Take case 1\u0026amp;4 as an example. When at sub-zero Celsius, number of large particles is 2.55 and total number N\u003csub\u003eKT\u003c/sub\u003e is 15.48, while N\u003csub\u003eKL\u003c/sub\u003e is 1.31 and N\u003csub\u003eKT\u003c/sub\u003e is 16.86 for normal temperature cases. It can be inferred that part of the kerosene is not atomized at sub-zero Celsius and small droplets formation becomes difficult, thus the total number slightly decreases and the number of large fuel LIF particles increases instead. This indicates that it gets more difficult for air to interact with liquid kerosene. Because density and the dynamic viscosity of kerosene increase fast at sub-zero Celsius (Viorel PALEU 2007) and the atomization gets worse, like droplet mean diameter gets larger and spray cone angle gets smaller (Aleiferis, et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Droplet size distribution measurement is difficult in soot flame of this work. But it can be achieved easy in spray without ignition. In the literature of Wu et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e), picosecond pulsed digital off-axis holography is used to measure the near-nozzle droplet size distribution of the same burner nozzle of our work. The data is in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e (c). In near-nozzle spray, droplets in the range of 210\u0026thinsp;~\u0026thinsp;690 \u0026micro;m are considered to be larger droplets and account for a relatively small proportion of the droplet size distribution. Distribution of these large droplet presents the same phenomenon as fuel LIF particles in this paper. When at sub-zero Celsius, the droplet number decreases at any air pressure, indicating the worse atomization. But the droplets over 700 \u0026micro;m are not counted in the literature of Wu, et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e), due to the interference of large droplets in 3D holography measurement. So from another perspective, the counted fuel LIF particles data in this paper can be considered as a supplementary evaluation for large droplets over 700 \u0026micro;m in the soot flame.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, flame structures of RP-3 swirling spray combustion at different conditions including low/normal fuel inlet temperature and different air pressure are evaluated. According to analysis of OH-PLIF images, sub-zero Celsius significantly results in the reduction of the OH radical concentration and poor heat release rate. The strongest signals decrease by at least 5 times. With analysis of LIF spectroscopy and detuned images, fuel LIF particles in the OH-PLIF images are mainly liquid kerosene LIF. It is confirmed that the LIF signals of flame structure in the OH-PLIF images are almost all from OH radicals. This means that the UV laser pulse energy, ICCD gate width and used optical filters in this work are appropriate to detect the fuel LIF particles and OH-LIF, as well as suppress other signals at the same time. Kerosene LIF particles can reflect the fuel atomization in a sense. Using LOG operator blob detection, it is found that, when at sub-zero Celsius, the total number of fuel LIF particles slightly decreases and the number of large particles increases at any air pressure. Although the LIF diameter cannot represent the actual particle\u0026rsquo;s diameter due to the limited ICCD resolution, the counted number of fuel LIF particles can also be a supplementary evaluation for fuel atomization. This study probably needs other laser diagnostics to explain the flow field and atomization, but the PLIF analysis provides insight into complex combustion deterioration of kerosene swirling spray combustion for the first time. Problems in this study would create some challenges for the measurement of flow filed, droplet size or other atomization properties under worse combustion conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003enot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eShirong Xin: Data curation, Formal analysis, Writing - original draft. Yong He: Conceptualization, Investigation, Methodology, Validation, Writing - review \u0026amp; editing. Tao Liu: Writing - review \u0026amp; editing. Yingchun Wu: Writing - review \u0026amp; editing, Resources. Xuecheng Wu: Conceptualization, Software, Resources. Zhihua Wang: Funding acquisition, Investigation, Writing - review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by National Natural Science Foundation of China (52125605). Authors also thank the support from the Fundamental Research Funds for the Central Universities (2021FZZX001-11).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003enot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAleiferis PG, Shukla J, Brewer M, Cracknell RF (2021) Spray development of iso-octane, ethanol, hydrous ethanol and water from a multi-hole injector under ultra cold fuel temperature conditions. 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Proc Combust Inst 35:1649\u0026ndash;1656 DOI \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.proci.2014.06.012\u003c/span\u003e\u003cspan address=\"10.1016/j.proci.2014.06.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"swirling spray combustion, combustion deterioration, flame structure, low fuel temperature, OH-PLIF","lastPublishedDoi":"10.21203/rs.3.rs-2068135/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2068135/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLow temperature and low air pressure would cause low fuel inlet temperature and low air flowrate in aero-engine combustion chamber working in the upper atmosphere. A system including a burner normally used in actual aero engines is established to study swirl-stabilized kerosene spray combustion at conditions of low fuel inlet temperature (T\u0026thinsp;\u0026lt;\u0026thinsp;\u0026minus;\u0026thinsp;16 ℃) and low air pressure. According to analysis of OH-PLIF images, sub-zero Celsius significantly results in the reduction of the OH radical concentration and poor heat release rate. The strongest signals decrease by at least 5 times. With analysis of LIF spectroscopy and detuned images, fuel LIF particles in the OH-PLIF images are mainly liquid kerosene LIF. The LIF signals of flame structure are almost all from OH radicals. Using LOG operator blob detection, it is found that, when at sub-zero Celsius, the total number of fuel LIF particles slightly decreases and the number of large particles increases at any air pressure. PLIF analysis in this study provides insight into complex combustion deterioration of kerosene swirling spray combustion for the first time.\u003c/p\u003e","manuscriptTitle":"Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flame","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-09-22 20:36:00","doi":"10.21203/rs.3.rs-2068135/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c804b022-6f27-407f-afb0-252e2b3a7a10","owner":[],"postedDate":"September 22nd, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2022-09-30T20:59:16+00:00","versionOfRecord":[],"versionCreatedAt":"2022-09-22 20:36:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2068135","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2068135","identity":"rs-2068135","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}