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

A numerical investigation has been conducted in order to determine the effects of variable properties on the flow pattern and heat transfer performances in laminar developing ascending flow with mired convectio n for two cases: in case I the fluid is heated, and in case II it is c ooled. Calculations are performed for air at various Grashof numbers w ith a fixed entrance Reynolds number of 500 using both the Boussinesq approximation (constant-prop erty 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 l aws, while the density varies linearly with the temperature, and the h eat capacity is assumed to be constant. The comparison between constan t- and variable-property models shows a substantial difference in the temperature and velocity fields when the Grashof number \Gr\ is increa sed. The friction factor is seen to be underpredicted by the Boussines q approximation when the fluid is heated (case I), while it is overpre dicted for the cooling case (case II). However, the effects on the hea t transfer performance remain negligible except for cases with reverse flow. On the whole, the variable-property model predicts flow reversa l at lower values of \Gr\, especially for flows with opposing buoyancy forces. The deviation in results is associated to the difference betw een the fluid bulk and the wad temperature.