Influence of surface roughness on natural convection above a horizontal plate

Numerical and experimental analyses are carried out to study the free convection above a horizontal plate submitted to a uniform heat flux and placed in a semi-infinite medium (Pr = 0.7 or 7). The surface of the plate in contact with the fluid is described by a sinusoidal profile. The natural convection flow is considered laminar and two-dimensional. The numerical results show that the flow pattern is strongly dependent on operating heat flux and on parameters of the surface profile (amplitude and period). The isothermal lines spreads along the surface of the plate. So, the local Nusselt number is maximal above the top of the protuberances and minimal in the hollows of the profile. We also show that the local heat transfer decreases with the amplitude and increases with the period of the sinusoidal profile. Whatever parameters of the surface were used, the heat transfer above a sinusoidal plate is lower than those obtained from a plane plate of equal projected area. The numerical results are experimentally checked by using an experimental apparatus which allows us to measure the air temperature and the position of this measure above the plate. (C) 2000 Elsevier Science Ltd. All rights reserved.