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Abstract: The development of unsteady free convective flow in a rectangular enclosure has been numerically studied. The enclosure considered has rectangular horizontal lower and upper surfaces and vertical side surfaces. The horizontal width of enclosure is twice the vertical height and longitudinal length of the enclosure. There are two square isothermal heated sections on the lower surface, the rest of this surface being adiabatic. The vertical side-walls of the enclosure are kept at a uniform low temperature. The horizontal rectangular upper surface of the enclosure is either (a) adiabatic or (b) isothermal and at the same low temperature as the vertical side-walls. The governing unsteady, three-dimensional flow equations, written in dimensionless form, have been solved using an iterative, semi-implicit finite-difference method. The solution has the following parameters: the Rayleigh number, Ra, the Prandtl number, Pr, the dimensionless size, w H, of the square heated sections and the dimensionless distance between the heated sections on the lower surface, w S. Results have been obtained for a Prandtl number of 0.7. Most of the results presented here are for w H = w S =1/3. The results indicate that for the flow situation considered the flow is steady at low Rayleigh numbers, becomes unsteady at intermediate Rayleigh numbers and then again becomes steady at higher Rayleigh numbers The conditions under which unsteady flow develops and the nature of the unsteady flow have been investigated and the variation of mean Nusselt number with Rayleigh number has been explored. [Copyright &y& Elsevier]

Natural convective flow over narrow plates induces an inward flow near the edges of the plate causing the flow to he three-dimensional near the edges of the plate. This influences the heat transfer rate near the edges of the plate and is referred to as the edge effect. The primary objective of this paper is to numerically study this edge effect and the interaction of the flows over two inclined vertically separated narrow heated plates of the same size embedded in a plane adiabatic surface. The cases where the plates and surrounding adiabatic surface are inclined at positive or negative angles to the vertical have been considered. Results were obtained by numerically solving the full three-dimensional form of governing equations using the commercial finite volume based software fluent©. Results have only been obtained for a Prandtl number of 0.7; this being the value existing in the application which involved airflow that originally motivated this study. The results presented here cover Rayleigh numbers between 103 and I07, at all values ofW considered, plate widtli-to-height ratios between 0.2 and 1.2, gap, at all values of W considered, to the plate height ratios of between 0 and 1.5, and, at all values ofW considered, angles of inclination of between --45 deg and +45 deg. The effects of the Rayleigh number, dimensionless plate width, dimensionless gap between plates, and inclination angle on the heat transfer rate have been studied in detail. Empirical correlations defining the effect of these parameters on the heat transfer rate have been derived. [ABSTRACT FROM AUTHOR]