Study on Backflow Characteristics of Smoke from Conveyor Belt Fires and Critical Reverse-Flow Stopping Wind Velocity

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Combining scaled-down experimental platforms with numerical simulation methods, experiments on the backflow characteristics of smoke from belt fires and numerical simulation studies on the critical reverse-flow stopping wind velocity were conducted. Experimental results indicate that the concentration of reverse-flow smoke exhibits an overall trend of rapid initial increase followed by a gradual decrease. As the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. A linear relationship exists between the length of the reverse-flow smoke layer and the wind velocity. The critical reverse-flow stopping wind velocity obtained by fitting experimental data is 1.70m/s, while the value obtained from numerical simulation analysis is 1.74m/s. The research can provide a theoretical basis for the prevention and control of mine belt fires, and also has positive significance for reducing toxic smoke emissions and improving the atmospheric environmental quality in mining areas. Physical sciences/Energy science and technology Physical sciences/Engineering Earth and environmental sciences/Environmental sciences Physical sciences/Mathematics and computing Conveyor belt fire Backflow characteristics Critical reverse-flow stopping wind velocity Numerical simulation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction With the continuous advancement of coal mining technology and increasing mining depth, safety issues in underground coal mines have become increasingly prominent. Among these, conveyor belt fires have become one of the common disasters in underground coal mines [1] . Conveyor belt fires not only damage equipment and interrupt production but, more fatally, the smoke they produce, containing toxic and harmful substances such as carbon monoxide, carbon dioxide, and hydrogen sulfide, seriously threatens the lives and safety of underground workers [2] - [3] . During mining operations, a large amount of CH 4 flows into the mine from coal seams and surrounding rocks, and is then discharged into the atmosphere through ventilation backflow. This is one of the main reasons for the increase in CH 4 levels in the mine area's atmosphere. [4] When a conveyor belt fire occurs in an underground coal mine, the high-temperature smoke, under the influence of thermal buoyancy and the ventilation system, exhibits variable flow direction and speed, affecting smoke distribution [5] - [6] . Therefore, conducting in-depth research on the characteristics of smoke backflow in mine belt fires and the critical wind speed for preventing backflow is of great significance for efficient rescue, fire extinguishment, preventing the recurrence of fires, improving mine safety levels, and reducing atmospheric pollutant emissions in mining areas [7] . Many domestic and foreign scholars have studied the flow and distribution patterns of smoke from mine conveyor belt fires [8] - [9] [10] . Si Junhong [11] et al. conducted full-scale fire experiments in roadways, discovering uneven distribution of smoke concentration and temperature related to roadway structure. They also discussed the influence of wind velocity on fire evolution in different roadway structures and proposed factors affecting fire evolution in confined spaces. Polish scholar WL Budryk [12] proposed the theory of excess smoke, which provides a scientific explanation for the throttling effect and smoke backflow phenomena in roadway fires. He derived a discriminant for smoke flow direction and studied the airflow disorder patterns in roadways during underground fires. Qiu Changhe and his team [13] used theoretical analysis to deeply investigate the occurrence conditions of smoke backflow during horizontal roadway fires. They proposed a dimensionless B k criterion and derived a simple form K -1 B k >1 to judge whether smoke backflow would occur. Wang Wencai et al. [14] , through in-depth theoretical research on axisymmetric thermal convection in thermally buoyant neutral stratification, derived a calculation formula for the critical wind velocity to suppress smoke backflow in horizontal roadway fires and experimentally verified this formula. Liu Jian, Li Yufu, et al. [15] - [16] [17] [18] used numerical simulation techniques to deeply study the variation patterns of reverse-flow smoke layers generated by fires in roadways. Liu Han and his team [19] used FDS to investigate how obstacles at different distances from the fire source affect smoke flow from roadway fires. Research shows that in roadways with obstacles, compared to unobstructed roadway fires, the length of smoke backflow is longer, and the closer the obstacle is to the fire source in the upwind direction, the longer the smoke backflow length. Zhou Fubao, Wang Deming [20] derived a general dimensionless expression that reflects the variation pattern of smoke roll-back distance during tunnel fires from the perspective of dimensional analysis. They analyzed the monotonicity between each parameter in the expression and the dimensionless roll-back distance. The research results lay a foundation for simulation experiments on smoke roll-back in tunnel fires. French researcher E. Simode [21] proposed a simulation program for airflow temperature during fire based on wind temperature calculations under transient and steady-state conditions in 1976 and 1979.Zhou Xinquan, Wang Haiyan, Zhao Hongze [22] studied the smoke roll-back pattern in horizontal tunnel fires. Through analyzing the heat exchange process between the rolling smoke and the tunnel wall, they derived a functional expression for the smoke backflow distance during roll-back, which was simplified for practical application .Zhou Yan, Wang Xingshen [23] To grasp the occurrence patterns of smoke roll-back, they employed theoretical analysis methods to study the smoke roll-back phenomenon in horizontal tunnel fires. They derived the conditional formula for roll-back occurrence and established two dimensionless criteria. In summary, domestic and foreign scholars have primarily conducted experimental studies or numerical simulations on the flow and distribution patterns of smoke from mine conveyor belt fires. However, research combining both experiments and numerical simulations on the backflow characteristics of smoke from belt fires and the critical reverse-flow stopping wind velocity is relatively scarce. This paper takes mine PVC conveyor belt fires as the research object, using a scaled-down experimental platform to study the backflow characteristics of smoke from belt fires under different wind velocities and their impact on the reverse-flow smoke layer. Combined with FDS software, a 1:1 experimental roadway model is established. Based on data from the scaled-down experimental platform, the relationship between different wind velocities, different inclination angles, and the reverse-flow smoke layer is studied. The ultimate goal is to obtain the characteristics of smoke backflow in belt fires and the critical wind speed for preventing smoke backflow, providing technical support for reducing smoke emissions in mine fires and protecting the atmospheric environment in mining areas. 2. Experimental System and Conditions As the core transport tool for coal, if accidents such as belt burning, melting, or fire occur on conveyor belts, large amounts of smoke are generated, endangering the lives and safety of underground personnel. Therefore, the research object of this paper is mine-used polyvinyl chloride (PVC) conveyor belts. 2.1 Experimental System This experiment primarily studies the spread pattern of fire smoke without fire suppression measures. Ethanol was selected as the ignition source. By burning ethanol in an oil pan (size: 25cm×20cm×5cm) to generate radiant heat, the process of a conveyor belt (size: 15cm×10cm) being ignited by a high-temperature heat source in an actual underground roadway was simulated. The main roadway structure of this experimental platform is based on similarity theory using a scaled-down mine roadway test bench model (as shown in Figure 1), placed on a 0.5m high steel frame, with a total length of 12m (the roadway model occupies 8.4m), ensuring maximum geometric similarity. An axial flow fan is installed on one side of the roadway model to control airflow within the experimental roadway model by adjusting the air volume control plate. A smoke monitor is equipped for smoke component analysis and concentration monitoring. A laser sheet light source is installed at the roadway port to observe smoke flow. Figure 1 External view of the experimental platform Figure 2 Schematic diagram of measurement point layout inside the roadway 2.2 Experimental Measurement Point Layout and Procedure The experimental platform is equipped with smoke measurement points, wind velocity measurement points, and smoke visualization equipment. The specific layout of measurement points is shown in Figure 2. A smoke monitoring point, labeled S 1 , is set 1.5m from the air inlet, 1m from the fire source, and 0.1m below the roadway model arch crown. Wind velocity measurement points are arranged at the air inlet, upstream of the fire source, and downstream of the fire source, labeled V 1 to V 3 . Based on the physical conditions of the scaled-down experimental platform, five inlet wind velocities were designed: 0.60 m/s, 0.87 m/s, 1.14 m/s, 1.41 m/s, and 1.68 m/s. The specific experimental steps are as follows: (1) Place the oil pan and conveyor belt together into the roadway test bench, 2.5m from the air inlet. Ignite the ethanol in the oil pan (fire source power: 8.5 KW) and continuously heat the belt to maintain a sustained burning state, approximating the fire scenario of a conveyor belt in an actual roadway. (2) Seal the roadway and control the fan speed. When the experimental belt sample itself begins to burn, remove the ethanol. Use an anemometer to measure the wind velocity at the inlet and fire source positions to ensure stable wind velocity, thus guaranteeing the accuracy of experimental results. Observe and record the length of the reverse-flow smoke layer using smoke visualization equipment, and monitor changes in smoke concentration at various points in the roadway over time. (3) After completing one set of experiments, wait for a period before starting the next set of tests to avoid errors caused by excessive operation of the experimental equipment. 3. Results and Discussion 3.1 Experimental Study on Fire Smoke Backflow and Reverse-Flow Stopping Wind Velocity 3.1.1 Influence of Inlet Wind Velocity on Smoke Backflow Characteristics Understanding the spread pattern of smoke is key to understanding its backflow characteristics. When a conveyor belt burns, it releases harmful gases such as CO and HCl, posing a serious threat to the life and health of underground workers. As shown in Figure 3, when the PVC conveyor belt burns under different wind velocities, the concentration of backflow smoke (CO) overall shows a trend of rapid initial increase followed by a gradual decrease, reaching a peak at 150s and then gradually decreasing. Meanwhile, as the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. At a wind velocity of 0.6 m/s, the smoke concentration peaked at 206.86 ppm at 150s. At a wind velocity of 1.68 m/s, the peak decreased to 133.74 ppm, representing a significant reduction of 35.3% in peak smoke concentration. This is because in the early stages of combustion, the surface layer of the PVC belt decomposes under heat, releasing a large amount of volatile flammable gases, causing the smoke concentration to rise rapidly initially. At 150s, the accumulated volatile concentration and heat release rate in the combustion zone reach their peak, leading to the maximum smoke concentration. Simultaneously, the hot smoke gradually rises within the roadway due to thermal buoyancy. Upon collision with the roadway roof, a ceiling jet phenomenon occurs. Then, constrained by the roadway space, the smoke spreads horizontally under the ceiling. As the wind velocity gradually increases, smoke dilution intensifies, and the smoke concentration decreases accordingly. Figure 3 Changes in CO concentration of reverse-flow smoke from PVC conveyor belts under different wind velocities 3.1.2 Determination of Critical Reverse-Flow Stopping Wind Velocity The experiment observed and recorded the reverse-flow smoke situation inside the roadway using a laser sheet light source smoke visualization device. The experimental results for the PVC belt reverse-flow smoke layer length and roadway wind velocity are shown in Table 1. Due to thermal resistance from the flame causing local contraction of the roadway cross-sectional area (A), according to the continuity equation (Q=A·V), with constant air volume (Q), a decrease in cross-sectional area directly leads to an increase in wind velocity. Since the downstream airflow passes through the heat source, the contraction effect is more significant, resulting in a substantial increase in the wind velocity upstream of the fire source (V 2 ) compared to downstream (V 3 ), with an increase of approximately 8.5% to 18.5%. At an inlet wind velocity of 0.87 m/s, the downstream wind velocity increase peaked at 18.5%, while at an inlet wind velocity of 1.68 m/s, the increase dropped to a minimum of 8.5%. Table 1 PVC belt reverse-flow smoke layer length and roadway wind velocity Countercurrent layer length L/cm 92 76 44 23 4 Air speed at the air inlet V 1 /（m/s） 0.60 0.87 1.14 1.41 1.68 Wind speed on the upwind side of the fire source V 2 /（m/s） 0.60 0.64 0.77 0.83 0.89 Wind speed on the downwind side of the fire source V 3 /（m/s） 0.55 0.54 0.65 0.74 0.82 Figure 4 shows the measured reverse-flow layer length during the stable phase within the inlet wind velocity range of 0.6-1.68 m/s and its linear fitting. Analysis of experimental data indicates a significant negative correlation between the reverse-flow layer length (L) and the inlet wind velocity (V). Fitting using a linear regression model (R 2 =0.99) shows that for every 0.1 m/s increase in wind velocity, the reverse-flow layer length decreases by approximately 16.88 cm, quantitatively revealing a strict linear decreasing relationship between the two. Within the wind velocity range from 0.6 m/s to 1.68 m/s, the reverse-flow layer length sharply shortened from an initial 92 cm to 4 cm, a reduction of 95.7% (ΔL/L 0 =(92−4)/92×100%), fully verifying the significant inhibitory effect of wind velocity on the spatial expansion of the reverse-flow layer. Therefore, substituting L=0 (reverse-flow layer length is 0) into Equation (1), the critical reverse-flow stopping wind velocity is found to be 1.7 m/s. L=-84.81V+144.49 （eq1） Where: V is the inlet wind velocity (unit: m/s), L is the reverse-flow layer length (unit: cm). Figure 4 Relationship curve between inlet wind velocity and reverse-flow smoke layer length 3.2 Numerical Simulation of Smoke Spread Characteristics in Mine Belt Roadway Fires 3.2.1 Model Construction and Reliability Verification Due to experimental limitations (such as insufficient fan pressure), scaled-down roadway model experiments have certain limitations and cannot directly yield the critical reverse-flow stopping wind velocity. Therefore, the FDS numerical software was used to construct a roadway numerical model (as shown in Figure 5), with roadway dimensions, fire source location, and other conditions identical to the experiment. Parameters such as wind velocity (0.6 m/s, 1.0 m/s, 1.4 m/s, 1.8 m/s) were changed to further study the smoke spread characteristics of belt fires. Due to the large physical model volume, the mesh size in the belt region was set to 0.1×0.1×0.1m, and the mesh size in other regions was 0.2×0.2×0.2 m, meeting the computational requirements. Additionally, to make the simulation process and results manageable and focused, the following basic assumptions were made regarding potential practical issues that might arise during the simulation [24] : (1)The ventilation airflow through the full cross-section of the inlet and the multi-component fire smoke are considered ideal gases and incompressible. (2)The gases produced by the fire and the airflow gas components in the mine do not undergo secondary reactions. (3)Heat exchange effects with the roadway walls are not considered. (4)Other gases generated by other components of the conveyor belt system due to high temperatures are not considered. (5)High-temperature regions do not affect the roadway morphology; no deformation occurs. (6)The fire process primarily produces CO; other toxic and harmful gases are not considered. (7)During fire development, the airflow velocity and temperature at the inlet are unaffected by the fire and remain constant. Figure 5 Roadway model structure diagram To ensure the reliability of the constructed model, the burning material was set as the belt (PVC material), the fire source power was set to 8.5 KW, and the inlet wind velocity was set to 0.6 m/s for model reliability verification. The entire process lasted from the ignition source releasing thermal radiation until the belt was completely burned. As shown in Figure 6, the CO concentration trends at the experimental and simulation measurement points are consistent, with small errors (error not exceeding 3.5 ppm), indicating that the simulation results of the constructed model are highly consistent with the experimental data. Therefore, the model can be used for related simulations of smoke spread from belt fires. Figure 6 Comparison of experimental and simulation results 3.2.2 Precise Determination of Critical Reverse-Flow Stopping Wind Velocity Simulations yielded slices of smoke visibility during smoke spread within the roadway (as shown in Figure 7). The figure shows the visibility conditions of smoke inside the horizontal roadway during the heating and burning process of the conveyor belt under four different wind velocity conditions (blue to red indicates increasing visibility from turbid to clear). From the initial heating stage (t=15s) to t=150s, the smoke exhibits different characteristics. In the early combustion stage (t=15s), the lower the wind velocity inside the roadway, the more pronounced the smoke backflow. When the fire developed to 150s, significant smoke backflow phenomena occurred under wind velocities of 0.6 m/s, 1.0 m/s, and 1.4 m/s. This is mainly because, in the early stage of the fire, the smoke generation rate is not high. The smoke can flow downstream with the fresh airflow and settle due to heat exchange with the air and walls. As the belt burning progresses, the smoke generation rate increases rapidly. Moreover, after the air and wall temperatures rise, the smoke cannot settle quickly and accumulates towards the top. Additionally, due to the low wind velocity and small airflow, when the resistance between smoke molecules exceeds the inertial force of the wind, the smoke spreads towards the upstream inlet, and the backflow phenomenon gradually intensifies. Under the wind velocity condition of 1.8 m/s, due to the sufficiently high wind velocity and sufficient wind inertial force within the roadway, the smoke backflow phenomenon is not obvious. Therefore, by further simulating within the range of 1.4 m/s to 1.85 m/s, the critical reverse-flow stopping wind velocity for smoke was ultimately determined to be 1.74 m/s (as shown in Figure 8). The simulated value of the critical reverse-flow stopping wind velocity (1.74 m/s) is higher than the value fitted from experimental data (1.70 m/s), with a relative error of 2.35%. This discrepancy may result from the combined effects of model simplification, numerical discretization, and measurement uncertainty. Figure 8 Further determination of critical smoke reverse-flow stopping wind velocity According to Figure 9, the spread pattern of CO inside the roadway under different wind velocity conditions is basically consistent with that of smoke. The smoke first diffuses towards the top and then spreads outwards after reaching the top. Different degrees of backflow phenomena occurred under all wind velocity conditions. When the fire was in its early combustion stage, the lower the airflow velocity inside the roadway, the higher the CO concentration in the reverse-flow layer, leading to higher monitored CO concentrations in the backflow region. When the fire reached its peak intensity at t=150s, under wind velocities of 0.6 m/s and 1.0 m/s, the CO volume fraction in the reverse-flow smoke in each roadway exceeded 0.003 mol. Under the condition of 1.4 m/s, it also exceeded 0.002 mol. However, under the condition of 1.74 m/s wind velocity, the CO volume fraction in the reverse-flow layer did not exceed 0.0001 mol. This is because, under high wind velocity conditions, the delivery rate of fresh air increases. Simultaneously, the wind inertial force in the outlet direction is stronger compared to low-wind-velocity conditions, causing the reverse-flow layer smoke to dissipate significantly. Fresh air accounts for a relatively high proportion in the backflow region, exerting a significant dilution effect on the smoke in that region. 3.2.3 Smoke Backflow Characteristics under Different Inclination Angles As shown in Figure 10, when a belt fire occurs, a trend is observed where the smaller the roadway inclination angle, the more significant the smoke backflow. In the early stage of fire combustion, smoke spread near the fire source in roadways with different inclination angles showed varying degrees of offset in the direction of the ventilation flow. However, as the roadway inclination angle increased, the smoke backflow length decreased. Roadways with inclination angles of 0° and 10° exhibited obvious smoke backflow phenomena. At an inclination angle of 20°, the reverse-flow layer length at this stage was approximately 0. At an inclination angle of 30°, the smoke showed a very clear offset in the direction of the ventilation flow at this stage, with almost no smoke backflow phenomenon. As the belt fire developed further, the burning intensified. When the fire gradually reached its peak intensity, the smoke backflow phenomenon became more severe in roadways with inclination angles of 0° and 10°, and the smoke backflow length in the 0° roadway was longer than that in the 10° roadway. It can be inferred that the critical smoke backflow wind velocity for 0° and 10° inclined roadways at this time is below 1.4 m/s. For the roadway with a 20° inclination angle, the degree of smoke backflow at this stage was at a critical state, suggesting that the critical wind velocity for the 20° inclined roadway at this stage is approximately 1.4 m/s. By observing the smoke backflow situation in roadways with different inclination angles at different combustion stages, the reason may be analyzed as follows: When the roadway inclination angle is larger, the distance the smoke floats upwards along the roadway roof under buoyancy increases. The angle at which the smoke spreads in different directions after colliding with the roof changes, and the angle at which it is affected by the inlet airflow force also changes. Consequently, the larger the roadway inclination angle, the shorter the smoke backflow length. The distribution changes of CO gas volume fraction within the roadway are shown in Figure 11. Since CO is primarily generated from the smoke produced during belt combustion, the variation pattern of CO volume fraction is basically consistent with the pattern of smoke spread within the roadway. 4. Conclusion This study provides a theoretical basis for effectively controlling the smoke spread of mine belt fires by determining the critical anti-reversal wind speed. This, in turn, reduces the emission of toxic and harmful gases into the atmospheric environment outside the mine, positively impacting the air quality around the mining area and mitigating the environmental impact of coal mining activities. The main conclusions drawn are as follows: (1) When PVC conveyor belts burn under different wind velocities, the concentration of reverse-flow smoke overall shows a trend of rapid initial increase followed by a gradual decrease, reaching a peak at 150s and then gradually decreasing. As the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. When the wind velocity increased from 0.6 m/s to 1.68 m/s, the peak smoke concentration decreased significantly by 35.3%. (2) As the wind velocity gradually increases, the spread length of smoke against the airflow direction within the roadway gradually decreases. A linear relationship was found between the experimental reverse-flow layer length and wind velocity. By fitting the experimental data, the critical reverse-flow stopping wind velocity was determined to be 1.70 m/s. (3) Through numerical simulation using FDS software, the critical smoke reverse-flow wind velocity was found to be 1.74 m/s, verifying the experimental critical wind velocity. The smaller the roadway inclination angle, the more significant the smoke backflow, and the shorter the smoke backflow length (the reverse-flow length approaches zero at 20° inclination), while the CO volume fraction decreases. Declarations Author Information Corresponding Authors Li Peixuan. School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi'an, 710054, Shaanxi, China . Email: [email protected] Authors Li Peixuan. School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi'an, 710054, Shaanxi, China CRediT authorship contribution statement All the work was completed by Li Peixuan. Declaration of Competing Interest 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. Data availability Data is provided within the manuscript . Statement of Financial Support I have no financial support from any source and will cover all expenses myself. References Wang, W D; Ji, J. Analysis of conveyor belt transportation management from conveyor belt fire accident analysis. Coal Science & Technology Magazine, 2001(03):49-50. Zhang, S Z; Cheng, W M; Li, Q J. Simulation and analysis of airflow stability during fire in mine belt roadway. Journal of Coal Science and Engineering.2010.16(4):375-380. Li, S R; Deng, J; Chen, X K, et al. 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1","display":"","copyAsset":false,"role":"figure","size":5713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExternal view of the experimental platform\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/d474dce36a0e0134ed40aceb.png"},{"id":94396563,"identity":"fc249486-5766-4c8e-a8d4-d1bcbac52000","added_by":"auto","created_at":"2025-10-27 13:56:04","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":20578,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic diagram of measurement point layout inside the roadway\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/a7473b38c40294f2fe38ebd7.jpg"},{"id":94398403,"identity":"64926ee2-8855-4de9-a43b-3466a4145449","added_by":"auto","created_at":"2025-10-27 13:57:04","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":5713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eChanges in CO concentration of reverse-flow smoke from PVC conveyor belts under different wind velocities\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/b6387e7078ae449d196b1e2b.png"},{"id":94395606,"identity":"ea4d4626-5092-4e86-89f4-beb9b1f9f2b9","added_by":"auto","created_at":"2025-10-27 13:55:22","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":11138,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRelationship curve between inlet wind velocity and reverse-flow smoke layer length\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/77bc40688e354d9d19658abd.jpg"},{"id":94397259,"identity":"94a0f93f-3bc3-4214-8b5f-65f165ec4017","added_by":"auto","created_at":"2025-10-27 13:56:34","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":22923,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRoadway model structure diagram\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/6323ec248b090ffe0f7f9e73.jpg"},{"id":94397307,"identity":"0afb3346-0ca6-4352-bfe2-5f9bf04d7e31","added_by":"auto","created_at":"2025-10-27 13:56:37","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":15186,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of experimental and simulation results\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/02119fc3e8b9441eb184eb10.jpg"},{"id":94398172,"identity":"8a040770-184e-4a01-b7a2-64e740fadd34","added_by":"auto","created_at":"2025-10-27 13:57:00","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":119058,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSmoke spread visibility in roadway under different wind velocity conditions\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/94ceef7fb38e9b6447a61d16.jpg"},{"id":94396440,"identity":"f3811265-1897-4a3c-b130-087dc4b6cd9d","added_by":"auto","created_at":"2025-10-27 13:56:00","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":28943,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFurther determination of critical smoke reverse-flow stopping wind velocity\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/2f8937f74e08b21263b3f99a.jpg"},{"id":94396907,"identity":"eed671f9-de5e-4c78-b6b1-6537e9d5283d","added_by":"auto","created_at":"2025-10-27 13:56:20","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":5713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCO volume fraction in horizontal roadway under different wind velocity conditions\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/c80e2f69cfc97317f29afe21.png"},{"id":94396994,"identity":"a18f4a8a-28cf-48bd-a63d-52ede18836fd","added_by":"auto","created_at":"2025-10-27 13:56:23","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":5713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSmoke spread in roadways with different inclination angles\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/8cf8cb08e90574c3b2ffcfb8.png"},{"id":94398408,"identity":"f6acff47-ec1a-43c5-a9ac-2a4a685aa9ff","added_by":"auto","created_at":"2025-10-27 13:57:04","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":5713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCO changes in roadways with different inclination angles\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/c791c83d77e832a9ba0b04c1.png"},{"id":98431226,"identity":"173fc152-3fe8-48fe-8f89-51fdeeb91947","added_by":"auto","created_at":"2025-12-17 16:47:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1145770,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7630803/v1/112c2e16-260b-4ca7-87da-d23f03df2d46.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Study on Backflow Characteristics of Smoke from Conveyor Belt Fires and Critical Reverse-Flow Stopping Wind Velocity","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the continuous advancement of coal mining technology and increasing mining depth, safety issues in underground coal mines have become increasingly prominent. Among these, conveyor belt fires have become one of the common disasters in underground coal mines\u003csup\u003e[1]\u003c/sup\u003e. Conveyor belt fires not only damage equipment and interrupt production but, more fatally, the smoke they produce, containing toxic and harmful substances such as carbon monoxide, carbon dioxide, and hydrogen sulfide, seriously threatens the lives and safety of underground workers\u003csup\u003e[2]\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e[3]\u003c/sup\u003e. During mining operations, a large amount of CH\u003csub\u003e4\u003c/sub\u003e flows into the mine from coal seams and surrounding rocks, and is then discharged into the atmosphere through ventilation backflow. This is one of the main reasons for the increase in CH\u003csub\u003e4\u003c/sub\u003e levels in the mine area's atmosphere.\u003csup\u003e[4]\u003c/sup\u003e When a conveyor belt fire occurs in an underground coal mine, the high-temperature smoke, under the influence of thermal buoyancy and the ventilation system, exhibits variable flow direction and speed, affecting smoke distribution\u003csup\u003e[5]\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e[6]\u003c/sup\u003e. Therefore, conducting in-depth research on the characteristics of smoke backflow in mine belt fires and the critical wind speed for preventing backflow is of great significance for efficient rescue, fire extinguishment, preventing the recurrence of fires, improving mine safety levels, and reducing atmospheric pollutant emissions in mining areas\u0026nbsp;\u003csup\u003e[7]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eMany domestic and foreign scholars have studied the flow and distribution patterns of smoke from mine conveyor belt fires\u003csup\u003e[8]\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e[9]\u003c/sup\u003e\u003csup\u003e[10]\u003c/sup\u003e. Si Junhong\u003csup\u003e[11]\u003c/sup\u003eet al. conducted full-scale fire experiments in roadways, discovering uneven distribution of smoke concentration and temperature related to roadway structure. They also discussed the influence of wind velocity on fire evolution in different roadway structures and proposed factors affecting fire evolution in confined spaces. Polish scholar WL Budryk\u003csup\u003e[12]\u003c/sup\u003eproposed the theory of excess smoke, which provides a scientific explanation for the throttling effect and smoke backflow phenomena in roadway fires. He derived a discriminant for smoke flow direction and studied the airflow disorder patterns in roadways during underground fires. Qiu Changhe and his team\u003csup\u003e[13]\u003c/sup\u003eused theoretical analysis to deeply investigate the occurrence conditions of smoke backflow during horizontal roadway fires. They proposed a dimensionless B\u003csub\u003ek\u003c/sub\u003e criterion and derived a simple form K\u003csup\u003e-1\u003c/sup\u003eB\u003csub\u003ek\u003c/sub\u003e\u0026gt;1 to judge whether smoke backflow would occur. Wang Wencai et al.\u003csup\u003e[14]\u003c/sup\u003e, through in-depth theoretical research on axisymmetric thermal convection in thermally buoyant neutral stratification, derived a calculation formula for the critical wind velocity to suppress smoke backflow in horizontal roadway fires and experimentally verified this formula. Liu Jian, Li Yufu, et al.\u003csup\u003e[15]\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e[16]\u003c/sup\u003e\u003csup\u003e[17]\u003c/sup\u003e\u003csup\u003e[18]\u003c/sup\u003e used numerical simulation techniques to deeply study the variation patterns of reverse-flow smoke layers generated by fires in roadways. Liu Han and his team\u003csup\u003e[19]\u003c/sup\u003eused FDS to investigate how obstacles at different distances from the fire source affect smoke flow from roadway fires. Research shows that in roadways with obstacles, compared to unobstructed roadway fires, the length of smoke backflow is longer, and the closer the obstacle is to the fire source in the upwind direction, the longer the smoke backflow length.\u0026nbsp;Zhou Fubao, Wang Deming\u003csup\u003e[20]\u003c/sup\u003ederived a general dimensionless expression that reflects the variation pattern of smoke roll-back distance during tunnel fires from the perspective of dimensional analysis. They analyzed the monotonicity between each parameter in the expression and the dimensionless roll-back distance. The research results lay a foundation for simulation experiments on smoke roll-back in tunnel fires. French researcher E. Simode\u0026nbsp;\u003csup\u003e[21]\u003c/sup\u003eproposed a simulation program for airflow temperature during fire based on wind temperature calculations under transient and steady-state conditions in 1976 and 1979.Zhou Xinquan, Wang Haiyan, Zhao Hongze\u0026nbsp;\u003csup\u003e[22]\u003c/sup\u003estudied the smoke roll-back pattern in horizontal tunnel fires. Through analyzing the heat exchange process between the rolling smoke and the tunnel wall, they derived a functional expression for the smoke backflow distance during roll-back, which was simplified for practical application .Zhou Yan, Wang Xingshen\u0026nbsp;\u003csup\u003e[23]\u003c/sup\u003eTo grasp the occurrence patterns of smoke roll-back, they employed theoretical analysis methods to study the smoke roll-back phenomenon in horizontal tunnel fires. They derived the conditional formula for roll-back occurrence and established two dimensionless criteria.\u003c/p\u003e\n\u003cp\u003eIn summary, domestic and foreign scholars have primarily conducted experimental studies or numerical simulations on the flow and distribution patterns of smoke from mine conveyor belt fires. However, research combining both experiments and numerical simulations on the backflow characteristics of smoke from belt fires and the critical reverse-flow stopping wind velocity is relatively scarce. This paper takes mine PVC conveyor belt fires as the research object, using a scaled-down experimental platform to study the backflow characteristics of smoke from belt fires under different wind velocities and their impact on the reverse-flow smoke layer. Combined with FDS software, a 1:1 experimental roadway model is established. Based on data from the scaled-down experimental platform, the relationship between different wind velocities, different inclination angles, and the reverse-flow smoke layer is studied. The ultimate goal is to obtain the characteristics of smoke backflow in belt fires and the critical wind speed for preventing smoke backflow, providing technical support for reducing smoke emissions in mine fires and protecting the atmospheric environment in mining areas.\u003c/p\u003e"},{"header":"2. Experimental System and Conditions","content":"\u003cp\u003eAs the core transport tool for coal, if accidents such as belt burning, melting, or fire occur on conveyor belts, large amounts of smoke are generated, endangering the lives and safety of underground personnel. Therefore, the research object of this paper is mine-used polyvinyl chloride (PVC) conveyor belts.\u003c/p\u003e\n\u003cp\u003e2.1 Experimental System\u003c/p\u003e\n\u003cp\u003eThis experiment primarily studies the spread pattern of fire smoke without fire suppression measures. Ethanol was selected as the ignition source. By burning ethanol in an oil pan (size: 25cm\u0026times;20cm\u0026times;5cm) to generate radiant heat, the process of a conveyor belt (size: 15cm\u0026times;10cm) being ignited by a high-temperature heat source in an actual underground roadway was simulated. The main roadway structure of this experimental platform is based on similarity theory using a scaled-down mine roadway test bench model (as shown in Figure 1), placed on a 0.5m high steel frame, with a total length of 12m (the roadway model occupies 8.4m), ensuring maximum geometric similarity. An axial flow fan is installed on one side of the roadway model to control airflow within the experimental roadway model by adjusting the air volume control plate. A smoke monitor is equipped for smoke component analysis and concentration monitoring. A laser sheet light source is installed at the roadway port to observe smoke flow.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;External view of the experimental platform\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Schematic diagram of measurement point layout inside the roadway\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e2.2 Experimental Measurement Point Layout and Procedure\u003c/p\u003e\n\u003cp\u003eThe experimental platform is equipped with smoke measurement points, wind velocity measurement points, and smoke visualization equipment. The specific layout of measurement points is shown in Figure 2. A smoke monitoring point, labeled S\u003csub\u003e1\u003c/sub\u003e, is set 1.5m from the air inlet, 1m from the fire source, and 0.1m below the roadway model arch crown. Wind velocity measurement points are arranged at the air inlet, upstream of the fire source, and downstream of the fire source, labeled V\u003csub\u003e1\u003c/sub\u003e to V\u003csub\u003e3\u003c/sub\u003e. Based on the physical conditions of the scaled-down experimental platform, five inlet wind velocities were designed: 0.60 m/s, 0.87 m/s, 1.14 m/s, 1.41 m/s, and 1.68 m/s. The specific experimental steps are as follows:\u003c/p\u003e\n\u003cp\u003e(1)\u0026nbsp;Place the oil pan and conveyor belt together into the roadway test bench, 2.5m from the air inlet. Ignite the ethanol in the oil pan (fire source power: 8.5 KW) and continuously heat the belt to maintain a sustained burning state, approximating the fire scenario of a conveyor belt in an actual roadway.\u003c/p\u003e\n\u003cp\u003e(2) Seal the roadway and control the fan speed. When the experimental belt sample itself begins to burn, remove the ethanol. Use an anemometer to measure the wind velocity at the inlet and fire source positions to ensure stable wind velocity, thus guaranteeing the accuracy of experimental results. Observe and record the length of the reverse-flow smoke layer using smoke visualization equipment, and monitor changes in smoke concentration at various points in the roadway over time.\u003c/p\u003e\n\u003cp\u003e(3) After completing one set of experiments, wait for a period before starting the next set of tests to avoid errors caused by excessive operation of the experimental equipment.\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e3.1 Experimental Study on Fire Smoke Backflow and Reverse-Flow Stopping Wind Velocity\u003c/p\u003e\n\u003cp\u003e3.1.1 Influence of Inlet Wind Velocity on Smoke Backflow Characteristics\u003c/p\u003e\n\u003cp\u003eUnderstanding the spread pattern of smoke is key to understanding its backflow characteristics. When a conveyor belt burns, it releases harmful gases such as CO and HCl, posing a serious threat to the life and health of underground workers. As shown in Figure 3, when the PVC conveyor belt burns under different wind velocities, the concentration of backflow smoke (CO) overall shows a trend of rapid initial increase followed by a gradual decrease, reaching a peak at 150s and then gradually decreasing. Meanwhile, as the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. At a wind velocity of 0.6 m/s, the smoke concentration peaked at 206.86 ppm at 150s. At a wind velocity of 1.68 m/s, the peak decreased to 133.74 ppm, representing a significant reduction of 35.3% in peak smoke concentration. This is because in the early stages of combustion, the surface layer of the PVC belt decomposes under heat, releasing a large amount of volatile flammable gases, causing the smoke concentration to rise rapidly initially. At 150s, the accumulated volatile concentration and heat release rate in the combustion zone reach their peak, leading to the maximum smoke concentration. Simultaneously, the hot smoke gradually rises within the roadway due to thermal buoyancy. Upon collision with the roadway roof, a ceiling jet phenomenon occurs. Then, constrained by the roadway space, the smoke spreads horizontally under the ceiling. As the wind velocity gradually increases, smoke dilution intensifies, and the smoke concentration decreases accordingly.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Changes in CO concentration of reverse-flow smoke from PVC conveyor belts under different wind velocities\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e3.1.2 Determination of Critical Reverse-Flow Stopping Wind Velocity\u003c/p\u003e\n\u003cp\u003eThe experiment observed and recorded the reverse-flow smoke situation inside the roadway using a laser sheet light source smoke visualization device. The experimental results for the PVC belt reverse-flow smoke layer length and roadway wind velocity are shown in Table 1. Due to thermal resistance from the flame causing local contraction of the roadway cross-sectional area (A), according to the continuity equation (Q=A\u0026middot;V), with constant air volume (Q), a decrease in cross-sectional area directly leads to an increase in wind velocity. Since the downstream airflow passes through the heat source, the contraction effect is more significant, resulting in a substantial increase in the wind velocity upstream of the fire source (V\u003csub\u003e2\u003c/sub\u003e) compared to downstream (V\u003csub\u003e3\u003c/sub\u003e), with an increase of approximately 8.5% to 18.5%. At an inlet wind velocity of 0.87 m/s, the downstream wind velocity increase peaked at 18.5%, while at an inlet wind velocity of 1.68 m/s, the increase dropped to a minimum of 8.5%.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;PVC belt reverse-flow smoke layer length and roadway wind velocity\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"482\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 151px;\"\u003e\n \u003cp\u003eCountercurrent layer length\u003c/p\u003e\n \u003cp\u003eL/cm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e92\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 67px;\"\u003e\n \u003cp\u003e23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 151px;\"\u003e\n \u003cp\u003eAir speed at the air inlet\u003c/p\u003e\n \u003cp\u003eV\u003csub\u003e1\u003c/sub\u003e/（m/s）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e1.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 67px;\"\u003e\n \u003cp\u003e1.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e1.68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 151px;\"\u003e\n \u003cp\u003eWind speed on the upwind side of the fire source\u003c/p\u003e\n \u003cp\u003eV\u003csub\u003e2\u003c/sub\u003e/（m/s）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.77\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 67px;\"\u003e\n \u003cp\u003e0.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.89\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 151px;\"\u003e\n \u003cp\u003eWind speed on the downwind side of the fire source\u003c/p\u003e\n \u003cp\u003eV\u003csub\u003e3\u003c/sub\u003e/（m/s）\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 67px;\"\u003e\n \u003cp\u003e0.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 66px;\"\u003e\n \u003cp\u003e0.82\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eFigure 4 shows the measured reverse-flow layer length during the stable phase within the inlet wind velocity range of 0.6-1.68 m/s and its linear fitting. Analysis of experimental data indicates a significant negative correlation between the reverse-flow layer length (L) and the inlet wind velocity (V). Fitting using a linear regression model (R\u003csup\u003e2\u003c/sup\u003e=0.99) shows that for every 0.1 m/s increase in wind velocity, the reverse-flow layer length decreases by approximately 16.88 cm, quantitatively revealing a strict linear decreasing relationship between the two. Within the wind velocity range from 0.6 m/s to 1.68 m/s, the reverse-flow layer length sharply shortened from an initial 92 cm to 4 cm, a reduction of 95.7% (\u0026Delta;L/L\u003csub\u003e0\u003c/sub\u003e=(92\u0026minus;4)/92\u0026times;100%), fully verifying the significant inhibitory effect of wind velocity on the spatial expansion of the reverse-flow layer. Therefore, substituting L=0 (reverse-flow layer length is 0) into Equation (1), the critical reverse-flow stopping wind velocity is found to be 1.7 m/s.\u003c/p\u003e\n\u003cp\u003eL=-84.81V+144.49 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;（eq1）\u003c/p\u003e\n\u003cp\u003eWhere: V is the inlet wind velocity (unit: m/s), L is the reverse-flow layer length (unit: cm).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Relationship curve between inlet wind velocity and reverse-flow smoke layer length\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e3.2 Numerical Simulation of Smoke Spread Characteristics in Mine Belt Roadway Fires\u003c/p\u003e\n\u003cp\u003e3.2.1 Model Construction and Reliability Verification\u003c/p\u003e\n\u003cp\u003eDue to experimental limitations (such as insufficient fan pressure), scaled-down roadway model experiments have certain limitations and cannot directly yield the critical reverse-flow stopping wind velocity. Therefore, the FDS numerical software was used to construct a roadway numerical model (as shown in Figure 5), with roadway dimensions, fire source location, and other conditions identical to the experiment. Parameters such as wind velocity (0.6 m/s, 1.0 m/s, 1.4 m/s, 1.8 m/s) were changed to further study the smoke spread characteristics of belt fires. Due to the large physical model volume, the mesh size in the belt region was set to 0.1\u0026times;0.1\u0026times;0.1m, and the mesh size in other regions was 0.2\u0026times;0.2\u0026times;0.2 m, meeting the computational requirements. Additionally, to make the simulation process and results manageable and focused, the following basic assumptions were made regarding potential practical issues that might arise during the simulation\u003csup\u003e[24]\u003c/sup\u003e:\u003c/p\u003e\n\u003cp\u003e(1)The ventilation airflow through the full cross-section of the inlet and the multi-component fire smoke are considered ideal gases and incompressible.\u003c/p\u003e\n\u003cp\u003e(2)The gases produced by the fire and the airflow gas components in the mine do not undergo secondary reactions.\u003c/p\u003e\n\u003cp\u003e(3)Heat exchange effects with the roadway walls are not considered.\u003c/p\u003e\n\u003cp\u003e(4)Other gases generated by other components of the conveyor belt system due to high temperatures are not considered.\u003c/p\u003e\n\u003cp\u003e(5)High-temperature regions do not affect the roadway morphology; no deformation occurs.\u003c/p\u003e\n\u003cp\u003e(6)The fire process primarily produces CO; other toxic and harmful gases are not considered.\u003c/p\u003e\n\u003cp\u003e(7)During fire development, the airflow velocity and temperature at the inlet are unaffected by the fire and remain constant.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Roadway model structure diagram\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo ensure the reliability of the constructed model, the burning material was set as the belt (PVC material), the fire source power was set to 8.5 KW, and the inlet wind velocity was set to 0.6 m/s for model reliability verification. The entire process lasted from the ignition source releasing thermal radiation until the belt was completely burned. As shown in Figure 6, the CO concentration trends at the experimental and simulation measurement points are consistent, with small errors (error not exceeding 3.5 ppm), indicating that the simulation results of the constructed model are highly consistent with the experimental data. Therefore, the model can be used for related simulations of smoke spread from belt fires.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Comparison of experimental and simulation results\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e3.2.2 Precise Determination of Critical Reverse-Flow Stopping Wind Velocity\u003c/p\u003e\n\u003cp\u003eSimulations yielded slices of smoke visibility during smoke spread within the roadway (as shown in Figure 7). The figure shows the visibility conditions of smoke inside the horizontal roadway during the heating and burning process of the conveyor belt under four different wind velocity conditions (blue to red indicates increasing visibility from turbid to clear). From the initial heating stage (t=15s) to t=150s, the smoke exhibits different characteristics. In the early combustion stage (t=15s), the lower the wind velocity inside the roadway, the more pronounced the smoke backflow. When the fire developed to 150s, significant smoke backflow phenomena occurred under wind velocities of 0.6 m/s, 1.0 m/s, and 1.4 m/s. This is mainly because, in the early stage of the fire, the smoke generation rate is not high. The smoke can flow downstream with the fresh airflow and settle due to heat exchange with the air and walls. As the belt burning progresses, the smoke generation rate increases rapidly. Moreover, after the air and wall temperatures rise, the smoke cannot settle quickly and accumulates towards the top. Additionally, due to the low wind velocity and small airflow, when the resistance between smoke molecules exceeds the inertial force of the wind, the smoke spreads towards the upstream inlet, and the backflow phenomenon gradually intensifies. Under the wind velocity condition of 1.8 m/s, due to the sufficiently high wind velocity and sufficient wind inertial force within the roadway, the smoke backflow phenomenon is not obvious. Therefore, by further simulating within the range of 1.4 m/s to 1.85 m/s, the critical reverse-flow stopping wind velocity for smoke was ultimately determined to be 1.74 m/s (as shown in Figure 8). The simulated value of the critical reverse-flow stopping wind velocity (1.74 m/s) is higher than the value fitted from experimental data (1.70 m/s), with a relative error of 2.35%. This discrepancy may result from the combined effects of model simplification, numerical discretization, and measurement uncertainty.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e8\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Further determination of critical smoke reverse-flow stopping wind velocity\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAccording to Figure 9, the spread pattern of CO inside the roadway under different wind velocity conditions is basically consistent with that of smoke. The smoke first diffuses towards the top and then spreads outwards after reaching the top. Different degrees of backflow phenomena occurred under all wind velocity conditions. When the fire was in its early combustion stage, the lower the airflow velocity inside the roadway, the higher the CO concentration in the reverse-flow layer, leading to higher monitored CO concentrations in the backflow region. When the fire reached its peak intensity at t=150s, under wind velocities of 0.6 m/s and 1.0 m/s, the CO volume fraction in the reverse-flow smoke in each roadway exceeded 0.003 mol. Under the condition of 1.4 m/s, it also exceeded 0.002 mol. However, under the condition of 1.74 m/s wind velocity, the CO volume fraction in the reverse-flow layer did not exceed 0.0001 mol. This is because, under high wind velocity conditions, the delivery rate of fresh air increases. Simultaneously, the wind inertial force in the outlet direction is stronger compared to low-wind-velocity conditions, causing the reverse-flow layer smoke to dissipate significantly. Fresh air accounts for a relatively high proportion in the backflow region, exerting a significant dilution effect on the smoke in that region.\u003c/p\u003e\n\u003cp\u003e3.2.3 Smoke Backflow Characteristics under Different Inclination Angles\u003c/p\u003e\n\u003cp\u003eAs shown in Figure 10, when a belt fire occurs, a trend is observed where the smaller the roadway inclination angle, the more significant the smoke backflow. In the early stage of fire combustion, smoke spread near the fire source in roadways with different inclination angles showed varying degrees of offset in the direction of the ventilation flow. However, as the roadway inclination angle increased, the smoke backflow length decreased. Roadways with inclination angles of 0\u0026deg; and 10\u0026deg; exhibited obvious smoke backflow phenomena. At an inclination angle of 20\u0026deg;, the reverse-flow layer length at this stage was approximately 0. At an inclination angle of 30\u0026deg;, the smoke showed a very clear offset in the direction of the ventilation flow at this stage, with almost no smoke backflow phenomenon. As the belt fire developed further, the burning intensified. When the fire gradually reached its peak intensity, the smoke backflow phenomenon became more severe in roadways with inclination angles of 0\u0026deg; and 10\u0026deg;, and the smoke backflow length in the 0\u0026deg; roadway was longer than that in the 10\u0026deg; roadway. It can be inferred that the critical smoke backflow wind velocity for 0\u0026deg; and 10\u0026deg; inclined roadways at this time is below 1.4 m/s. For the roadway with a 20\u0026deg; inclination angle, the degree of smoke backflow at this stage was at a critical state, suggesting that the critical wind velocity for the 20\u0026deg; inclined roadway at this stage is approximately 1.4 m/s. By observing the smoke backflow situation in roadways with different inclination angles at different combustion stages, the reason may be analyzed as follows: When the roadway inclination angle is larger, the distance the smoke floats upwards along the roadway roof under buoyancy increases. The angle at which the smoke spreads in different directions after colliding with the roof changes, and the angle at which it is affected by the inlet airflow force also changes. Consequently, the larger the roadway inclination angle, the shorter the smoke backflow length. The distribution changes of CO gas volume fraction within the roadway are shown in Figure 11. Since CO is primarily generated from the smoke produced during belt combustion, the variation pattern of CO volume fraction is basically consistent with the pattern of smoke spread within the roadway.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study provides a theoretical basis for effectively controlling the smoke spread of mine belt fires by determining the critical anti-reversal wind speed. This, in turn, reduces the emission of toxic and harmful gases into the atmospheric environment outside the mine, positively impacting the air quality around the mining area and mitigating the environmental impact of coal mining activities. The main conclusions drawn are as follows:\u003c/p\u003e\n\u003cp\u003e(1) When PVC conveyor belts burn under different wind velocities, the concentration of reverse-flow smoke overall shows a trend of rapid initial increase followed by a gradual decrease, reaching a peak at 150s and then gradually decreasing. As the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. When the wind velocity increased from 0.6 m/s to 1.68 m/s, the peak smoke concentration decreased significantly by 35.3%.\u003c/p\u003e\n\u003cp\u003e(2) As the wind velocity gradually increases, the spread length of smoke against the airflow direction within the roadway gradually decreases. A linear relationship was found between the experimental reverse-flow layer length and wind velocity. By fitting the experimental data, the critical reverse-flow stopping wind velocity was determined to be 1.70 m/s.\u003c/p\u003e\n\u003cp\u003e(3) Through numerical simulation using FDS software, the critical smoke reverse-flow wind velocity was found to be 1.74 m/s, verifying the experimental critical wind velocity. The smaller the roadway inclination angle, the more significant the smoke backflow, and the shorter the smoke backflow length (the reverse-flow length approaches zero at 20° inclination), while the CO volume fraction decreases.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAuthor Information\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Authors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eLi Peixuan. School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi'an, 710054, Shaanxi, China\u003c/em\u003e\u003cem\u003e.\u0026nbsp;\u003c/em\u003e\u003cem\u003eEmail:\u003c/em\u003e \u003cem\[email protected]\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eLi Peixuan. School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi'an, 710054, Shaanxi, China\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eCRediT authorship contribution statement\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAll the work was completed by Li Peixuan.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eDeclaration of Competing Interest\u003c/p\u003e\n\u003cp\u003e\u003cem\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.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eData availability\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eData is provided within the manuscript .\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eStatement of Financial Support\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eI have no financial support from any source and will cover all expenses myself.\u003c/em\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eWang, W D; Ji, J. Analysis of conveyor belt transportation management from conveyor belt fire accident analysis. Coal Science \u0026amp; Technology Magazine, 2001(03):49-50.\u003c/li\u003e\n \u003cli\u003eZhang, S Z; Cheng, W M; Li, Q J. Simulation and analysis of airflow stability during fire in mine belt roadway. Journal of Coal Science and Engineering.2010.16(4):375-380.\u003c/li\u003e\n \u003cli\u003eLi, S R; Deng, J; Chen, X K, et al. Distribution law of potential fire point caused by friction of conveyor belt in underground coal mines. Journal of Xi\u0026rsquo;an University of Science and Tenchnology. 2011, 31(06):679-683.\u003c/li\u003e\n \u003cli\u003eGu, J Y. Impact of coalfield exploration, development, and utilization on the environment and prevention measures. Northeast Coal Technology. 1998.10\u003c/li\u003e\n \u003cli\u003eTimm Gudehus, H K. Transport Systems. Comprehensive Logistics,2011:623-691.\u003c/li\u003e\n \u003cli\u003eZhang, Y T; Ma, T; Lin, J, et al. Analysis of fire incidents and characteristics of spatio-temporal distributions for serious fires from 2007 to 2016 in China. Journal of Xi\u0026apos;an University of Science and Technology, 2017, 37(06):829-836.\u003c/li\u003e\n \u003cli\u003eZheng, X Z; Hui, S; Wen, H, et al. Research Progress on Preventing and Controlling Technology and Disaster-forming Mechanism of Mine Fire. Safety in Coal Mines, 2017, 48(10):148-151+155.\u003c/li\u003e\n \u003cli\u003eZhou, Y; Wang, D M; Zhou, F B. Experimental Study on Length of Smoke Back-Flow Layer of Fire in Horizontal Tunnel. Journal of China University of Mining \u0026amp; Technology, 2001, (05):24-26.\u003c/li\u003e\n \u003cli\u003eYuan, J M; Zhang, F F. A Study on Smoke Spread Law in Tunnel Fire. Journal of Shanxi Datong University (Natural Science), 2016, 32(05):61-63.\u003c/li\u003e\n \u003cli\u003eDeng, H Y. Numerical Simulation of the Mining Area Mountain Fire Smoke Movement Rules and the Influence on the Ventilation System. Guangxi University, 2015.\u003c/li\u003e\n \u003cli\u003eLin, L; Junhong, S; Zongxiang, L. Characteristics of the spatial and temporal evolution of the environmental parameters for belt fire in underground coal mine roadway. Case Studies in Thermal Engineering,2023,49\u003c/li\u003e\n \u003cli\u003eSwaminathan S, Smidts C. The event sequence diagram framework for dynamic probabilistic risk assessment. Reliability Engineering \u0026amp; System Safety, 1999, 63(1): 73-90.\u003c/li\u003e\n \u003cli\u003eQiu, C H; Zhou, Y. Study on occurrence conditions of smoke backflow during horizontal roadway fire. Mining Safety \u0026amp; Environmental Protection, 2003, (06):14-16+1.\u003c/li\u003e\n \u003cli\u003eWang, W C; Gong, W G; Jiang, Y H, et al. Research on critical wind velocity to reverse the flow of smoke during a fire in a horizontal coal mine roadway. China Coal, 2013, 39(07):101-104.\u003c/li\u003e\n \u003cli\u003eLi, Y F. Numerical Simulation Study of Change Law of Fume Parameter During Mine Fires . Liaoning Technical University, 2012.\u003c/li\u003e\n \u003cli\u003eLi, J. Countercurrent Layer and the Resistance Effect Research of Downstream Ventilation Fire Roadway by Numerical Simulation. Liaoning Technical University, 2013.\u003c/li\u003e\n \u003cli\u003eLiu, J W; Li, Y F. CFD numerical simulation of the length of countercurrent layer in level tunnel. Journal of Liaoning Technical University (Natural Science), 2014, 33(08):1053-1057.\u003c/li\u003e\n \u003cli\u003eLiu, J; Wang, Y H; Li, J, et al. Numerical simulation for the countercurrent fire change regularity in the inclined tunnel. Journal of Safety and Environment, 2015, 15(04):94-97.\u003c/li\u003e\n \u003cli\u003eLiu, H; Xu, Q K, Huang, T. On Impact of Obstacles on Fire Smoke Flow in Longitudinally Ventilated Mine Roadway. Journal of Southwest University of Science and Technology, 2016, 31(03):55-58.\u003c/li\u003e\n \u003cli\u003eZhou, F B; Wang, D M. Theoretical study on the backflow distance of smoke and hot gasesin a roadway fire. \u003cem\u003eJ. XIANGTAN MIN.INST.\u003c/em\u003e 2003, (04): 22-24.\u003c/li\u003e\n \u003cli\u003eWu Y, Bakar M Z. Control of smoke flow in tunnel fires using longitudinal ventilation systems-a study of the critical velocity. Fire Safety Journal,2000.\u003c/li\u003e\n \u003cli\u003eZhou, X Q; Wang, Y; Zhao, H Z.\u0026nbsp;Rollback Law of Reverse Smoke Flow in a Horizontal Airway during Mine Fire.\u0026nbsp;Journal of University of Science and Technology Beijing, 2004, (02): 118-121.\u003c/li\u003e\n \u003cli\u003eZhou, Y; Wang, X S. Conditions for Formation of a Reverse Smoke Flow in a Horizantal Tunnel.\u0026nbsp;Journal of China Coal Society, 1998, (04):\u003c/li\u003e\n \u003cli\u003eTie, Y. Experimental analysis of the natural smoke filling in large underground space. Journal of Thermal Science and Technology, 2019, 18(03):234-242.\u003c/li\u003e\n\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":"Conveyor belt fire, Backflow characteristics, Critical reverse-flow stopping wind velocity, Numerical simulation","lastPublishedDoi":"10.21203/rs.3.rs-7630803/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7630803/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"To systematically investigate the backflow characteristics of smoke after conveyor belt fires, this study selected mine-used polyvinyl chloride (PVC) conveyor belts as the research object. Combining scaled-down experimental platforms with numerical simulation methods, experiments on the backflow characteristics of smoke from belt fires and numerical simulation studies on the critical reverse-flow stopping wind velocity were conducted. Experimental results indicate that the concentration of reverse-flow smoke exhibits an overall trend of rapid initial increase followed by a gradual decrease. As the wind velocity increases stepwise, the peak concentration of smoke also decreases progressively. A linear relationship exists between the length of the reverse-flow smoke layer and the wind velocity. The critical reverse-flow stopping wind velocity obtained by fitting experimental data is 1.70m/s, while the value obtained from numerical simulation analysis is 1.74m/s. The research can provide a theoretical basis for the prevention and control of mine belt fires, and also has positive significance for reducing toxic smoke emissions and improving the atmospheric environmental quality in mining areas.","manuscriptTitle":"Study on Backflow Characteristics of Smoke from Conveyor Belt Fires and Critical Reverse-Flow Stopping Wind Velocity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-25 23:23:04","doi":"10.21203/rs.3.rs-7630803/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":"15a5aaf5-8d82-4451-8e0b-810e7271206b","owner":[],"postedDate":"October 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":56608148,"name":"Physical sciences/Energy science and technology"},{"id":56608149,"name":"Physical sciences/Engineering"},{"id":56608150,"name":"Earth and environmental sciences/Environmental sciences"},{"id":56608151,"name":"Physical sciences/Mathematics and computing"}],"tags":[],"updatedAt":"2025-12-14T18:53:39+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-25 23:23:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7630803","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7630803","identity":"rs-7630803","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}