Influence of variable viscosity of mineral oil on laminar heat transfer in a 2:1 rectangular duct

Abstract

An experimental study of laminar flow heat transfer in a 2:1 rectangular duct to mineral oil is carried out. The Hl thermal boundary condition corresponding to axially constant heat flux and peripherally constant temperature is adopted for three different heating configurations : (1) top wall heated, other walls adiabatic ; (2) bottom wall heated, other walls adiabatic ; (3) top and bottom walls heated, side walls adiabatic. Corresponding experiments are carried out using water as the test fluid. The experimental results for water show that when the upper wall is heated the influence of buoyancy force is minimal. However, in the case of oil the temperature-sensitive viscosity results in enhanced 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 by 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 associated with the variable viscosity are more symmetric. Thus the secondary flows are limited to those associated with natural convection and the oil results are in good agreement with the values found in water.