Free and mixed convective flow of air in a heated cavity of variable rectangular cross section and orientation

Abstract

Free and mixed convection in a strongly heated cavity of rectangular cross section have been investigated experimentally, to observe the effects of cavity shape and inclination and of ambient wind on the velocity and temperature distributions. The long edges ( c = 0.533 m) of the cavity were horizontal and parallel to an axis around which the cavity could be rotated. The aperture plane was either vertical (a = 0°), or inclined facing downward at a = 20° or a = 45°. The height of the aperture, b , was always 0.0947 m, while the depth of the cavity, a , was set so that = 0.5, 1.0, or 1.46. The bottom and back walls were electrically heated but the top wall was indirectly heated by conduction and radiation. The absolute average temperature of the wall, T c , was 2.21 times the absolute temperature at infinity, T^, and the Grashof number based on the difference of these two temperatures, with physical properties evaluated at T"^, was 4.2 x 10 7 . The Prandtl number was that of air, Pr « 0.7. In the studies of mixed convection, the axis of rotation was horizontal and normal to the ambient wind, U ^. When the aperture faced directly upstream the inclination angle was a = 0°. The Reynolds number, Re = was bU / v , varied from Re = 120-1100, where free convection was dominant, to Re = 2000-8740, where forced convection was dominant. For both free and mixed convection, wall and gas temperatures were measured with thermocouples and shadowgraph pictures were taken. For pure free convection, three time-averaged velocity components, the corresponding normal Reynolds stress components and one off-diagonal Reynolds stress component were measured by using a two-colour laser-Doppler velocimeter with the assistance of a minicomputer for automated data collection and reduction. The experiments were largely exploratory and revealed a wealth of interesting phenomena. These include instabilities that have previously been seen in more simply bounded flows and periodic oscillations that seem unique to the open cavity. This application of laser-Doppler velocimetry is interesting in its own right, confronting large variations in index of refraction, extraordinarily high percentage fluctuations in velocity and density and great difficulty in assuring the uniform distribution of light-scattering particles.