Thermal Drag and Critical Heat Flux for Natural Convection of Air in Vertical Parallel Plates

Abstract

Variable property effects on vertical channel natural convection in air are studied systematically. Numerical solutions of the governing equations show that both the mass flow rate and heat transfer in the channel are not only lower than the constant property results, but also show a nonmonotonic variation with increasing wall temperature or wall heat flux. This phenomenon, which seemingly conflicts with the conventional knowledge, has also been identified by experiments. For a vertical channel with a uniform heat flux boundary condition, the wall may experience a sharp rise in temperature up to damage of the channel if the wall heat flux is greater than the critical heat flux. This implies that the crisis phenomenon (or burnout) may occur in channel natural convection in gas as well as in the boiling process.