A numerical study of the transient natural convective heat transfer from thin, horizontal, isothermal plates of different shapes in lower Rayleigh number regime

Natural convective heat transfer is of paramount importance in cooling of low power electronic devices or devices in a restricted space. In the current study, unsteady natural convective heat transfer from both sides of thin, isothermal, horizontal plates of simple and complex shapes have been numerically investigated. The plates at 400K were exposed to air at ambient conditions. The Boussinesq approximation was adopted, i.e., all fluid properties, except density, were assumed to be constant. The simulation models were solved using the commercial CFD software ANSYS FLUENT. Mean heat transfer rates from the upper and lower surfaces of the plate were calculated using three length scales namely; width of the plate, square root of single side surface area and 4*Total area/Total perimeter and were expressed in terms of the transient Nusselt number for the Rayleigh numbers ranging between 10 2 to 10 5 . At the lowest Rayleigh number, the heat transfer was found to be primarily through conduction. At higher Rayleigh numbers, the Nusselt number first decreased to a minimum and then increased to the steady state value, indicating a combined process of conduction and convection. Unlike width and root area, 4A/P as the characteristic length scale yielded transient Nusselt number variation, largely independent of plate shape and size. The minor variations of heat transfer amongst plates of different shapes at higher Rayleigh numbers has been explained in terms of the pressure coefficient.