INFLUENCE OF VARIABLE VISCOSITY OF MINERAL-OIL ON LAMINAR HEAT-TRANSFER IN A 2-1 RECTANGULAR DUCT

An experimental study of laminar flow heat transfer in a 2:1 rectangul ar duct to mineral oil is carried out. The H1 thermal boundary conditi on corresponding to axially constant heat flux and peripherally consta nt temperature is adopted for three different heating configurations: (1) top wall heated, other walls adiabatic; (2) bottom wall heated, ot her walls adiabatic; (3) top and bottom walls heated, side walls adiab atic. Corresponding experiments are carried out using water as the tes t fluid. The experimental results for water show that when the upper w all is heated the influence of buoyancy force is minimal. However, in the case of oil the temperature-sensitive viscosity results in enhance d heat transfer as compared to water due to the distortion of the oil velocity profile. When the lower wall is heated, the influence of both buoyancy force and the stress differences of the mineral oil caused b y the variable viscosity results in a substantial increase in the heat transfer as compared to the values found for water. When both top and bottom walls are heated, the local heat transfer enhancement for the mineral oil is smaller as compared with that when each wall is heated alone since the velocity profile and the stress distribution associate d with the variable viscosity are more symmetric. Thus the secondary f lows are limited to those associated with natural convection and the o il results are in good agreement with the values found in water.