Mixed convection between horizontal plates—I. Entrance effects

Abstract

Entrance effects in mixed convection between horizontal, differentially heated plates were studied in nitrogen by laser Doppler anemometry in a range 1368 < Ra < 8300 and 15 < Re < 170. Two entrance lengths were deduced from velocity profiles: one for the onset of buoyancy-driven convective instability, and one for the full development of the mixed flow. Explicit expressions for both entrance lengths are given. In addition, unsteady longitudinal convection rolls were observed. These are discussed in terms of an admixture of transverse convection rolls and/or contributions from upstream turbulence. The experimental results show that the critical Ra for the transverse convection roils increases as Re increases.