Natural convective heat transfer finds importance in applications such as cooling of low power electronics, Printed Circuit Boards (PCBs) and of components in a restricted space. The variable load on such components also warrants the study of transient behaviour. In this investigation, unsteady natural convection was numerically computed around thin horizontal flat isothermal plates of different shapes using commercial CFD solver Ansys FLUENT. The plates were maintained at 400K and exposed to air at ambient temperature of 300K. Boussinesq approximation was adopted. Transient heat transfer from the top and bottom surface of the plates were presented in terms of the Nusselt number, Rayleigh number and dimensionless time, calculated using two length scales- 4*Total Area/Total Perimeter and square root area of single side surface area. Rayleigh numbers between 10 5-10 7 were considered. Analysis of the transient heat transfer profiles, calculated using the two length scales revealed that plate shapes had more pronounced effect on the heat transfer process compared to the length scale. Prior to reaching the steady state, the Nusselt number profile undergoes transition from initial conductive phase to convective flow development. Increasing the Rayleigh number tends to shorten the conduction and convective flow development phases. Further investigation revealed that unlike in laminar flow regime, the heat transfer in turbulent regime is significantly affected by fluid flow pattern, influenced by plate shape. Normalized Nusselt number was used to incorporate this shape effect, which enabled obtaining a singular transient heat transfer variation independent of plate shape and size. These profiles would assist the designers to compute the complete transient natural convective heat transfer characteristics of any plate shape using only the steady state heat transfer value.

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.