Laminar mixed convection in a horizontal annular duct. Case of thermodependent non-Newtonian fluid
This paper deals with both numerical and experimental analysis of combined forced- and free-convective heat transfer by a non-Newtonian fluid in a horizontal duct, where two cylinders are heated at a constant heat flux rate. At the inlet of the annular duct, the flow is fully developed and the temperature profile is uniform. The fluid is described by a power-law variable-consistency model, where the variation of rheological properties with temperature is taken into account. After the description of experimental instrumentation, the authors solve the governing Boussinesc equations by using a finite difference scheme. An asymptotic analytical solution of the corresponding boundary layer equations is also presented. The authors show that the core flow is decelerated due to the decrease in the consistency parameter \(K\) as the temperature \(T\) increases near the heated walls. It is also found that in the thermal entrance region, the enhancement of the Nusselt number due to the decrease in \(K\) near the heated walls is more important as the shear thinning of the fluid increases. These results are in agreement with experimental findings. Another important result is that, far from the entrance region, the thermal stratification induced by buoyancy force combined with the variation of \(K\) with \(T\) leads to another flow situation. There is an acceleration of the flow in the upper half-part of the annular duct, and a deceleration in the lower half-part. However, the forced convection remains the dominant mechanism in the heat transfer up to a dimensionless axial position \(X^+_c\). Beyond \(X_c^+\) the natural convection becomes the dominant mechanism.
scientific article; zbMATH DE number 1501306