HYDRODYNAMIC AND THERMAL EFFECTS IN MOVING WATER LAYERS UNIFORMLY HEATED FROM BELOW.

摘要

The purpose of this study is to obtain a better understanding of convective heat transfer processes in a water layer flowing over a heated plate and to specifically consider the effects of buoyancy on hydrodynamic and thermal conditions in the water. To this end, experimental and theoretical studies were conducted.;To simulate the foregoing experiments, a model was also developed and solved numerically by using the Patankar-Spalding finite-difference scheme. In this model, the effects of buoyancy are treated by including a buoyancy force term in the momentum equation and by employing existing correlations to account for the effects of buoyancy on the structure of the turbulence. A comparison between predicted and experimental results indicates that this model can be used to predict the Nusselt number to reasonable accuracy in the range Gr(,q,H)/Re(,H)('2) < 15.;The experimental apparatus consisted of a water channel with a test section that was uniformly heated from below. Three different means of flow visualization were used and included the hydrogen bubble, dye-injection and shadowgraph techniques. The results indicate that buoyancy forces induced by heating from below will disturb the flow to the point of hastening the occurrence of transition from laminar to turbulent conditions, and will enhance mixing in the turbulent flow regime. For a given flow condition, these effects increase with increasing bottom heat flux; for a given heat flux, the effects increase with decreasing mean flow velocity. Convective heat transfer coefficients were also inferred from temperature measurements, and the results indicate that heat transfer is significantly enhanced by the effects of buoyancy. The magnitude of the enhancement depends on the relative importance of the buoyancy and inertia forces. Based on comparisons between the experimental results and existing boundary layer heat transfer correlations, the regimes are identified for which existing heat transfer correlations are applicable.