TRANSIENT NATURAL CONVECTION FLOWS AND NATURAL CONVECTION FLOWS IN POROUS MEDIA

Abstract

An investigation of transient natural convection flows adjacent to a vertical surface and an analysis of external natural convection flows in porous media is presented. In Chapter 1, experimental results and interpretations are given for transient natural convection adjacent to a suddenly heated flat vertical surface in water. Flows were visualized and velocity and temperature measurements made at various downstream locations, after imposing a uniform internal energy generation rate within the surface. Laminar flow persisted into steady state, for short downstream distances. Further up, the flow became turbulent during the transient. Relaminarization at later times occurred only for lower flux inputs. In Chapter 2, vertical transient natural convection response following a change in surface heating has been determined. For short times, closed form solutions for the fluid velocity and temperature have been obtained, for all values of the Prandtl number. Thereafter, the transient boundary layer equations are solved numerically, for water, Pr = 6.2. Experiments verify both the short time solutions and the finite difference computations. Short time transient response of a yet different kind is obtained in Chapter 3. The ambient medium is cold water, giving rise to a nonlinear density variation with temperature. Closed form solutions for flow velocity have been obtained for two important surface conditions. These are: (a) an initial step in the surface energy generation rate and (b) a step in the surface temperature. Transient and steady flows in cold water are experimentally investigated in Chapter 4. The range of ambient temperatures studied is, 1.1 (LESSTHEQ) t(,(INFIN)) (LESSTHEQ) 6.9(DEGREES)C, for heating rates, q(,(INFIN))\u27\u27, in the range 587-1870 W/m(\u272). Flows were visualized and local surface temperatures measured. Details of flow and transport evolution starting from quiescence are studied. Chapter 5 considers a slightly different flow circumstance, where the ambient is a fluid saturated porous medium. Higher order corrections to the steady boundary layer solutions have been obtained for three flows. These are: (a) an isothermal surface, (b) a uniform flux surface and (c) a line plume. Effects of a externally imposed free stream on transport are investigated for (a)