VARIABLE-PROPERTY EFFECTS IN LAMINAR AIDING AND OPPOSING MIXED CONVECTION OF AIR IN VERTICAL TUBES

Abstract

A numerical investigation has been conducted in order to determine the effects of variable properties on the flow pattern and neat transfer performances in laminar developing ascending flow with mixed convection for two cases: in case J the fluid is heated, and in case II it is cooled. Calculations are performed for air at various Grashof numbers with a fixed entrance Reynolds number of 500 using both the Boussinesq approximation (constant-property model) and a variable-property model. In the latter case, the fluid viscosity and thermal conductivity are allowed to vary with absolute temperature according to simple power laws, while the density varies linearly with the temperature, and the heat capacity is assumed to be constant. The comparison between constant- and variable-property models shows a substantial difference in the temperature and velocity fields when the Grashof number Gr is increased. The friction factor is seen to be underpredicted by the Boussinesq approximation when the fluid is heated (case I), while it is overpredictedfor the cooling case (case H). However, the effects on the heat transfer performance remain negligible except for cases with reverse flow. On the whole, the variable-property model predicts flow reversal at lower values of Gr, especially for flows with opposing buoyancy forces. The deviation in results is associated to the difference between the fluid bulk and the wall temperature.