Heat Transfer in a Short Parallel-Plate Channel in the Transition Regime

Abstract

Heat transfer in a parallel-plate channel with uniform wall heat flux and refrigerated by water is investigated experimentally for the laminar and transition hydrodynamic regimes. The dimensions of the channel were chosen such that entry effects are relevant for a significant part of the channel length. The influence of the following parameters has been studied: inlet Reynolds number, inlet temperature and wall heat flux. Experimental results are compared with predictions based on different empirical correlations for convection in ducts. This analysis shows that the correlation of Gnielinski provides a good agreement with the experimental results for Reynolds numbers between 5,000 and 10,000 at 20°C and under relatively low heat fluxes. Experiments performed at relatively low heat fluxes show that, in the entry region previous to the laminar-turbulent transition, the behavior of the flow is well described by the combined hydrodynamic and thermal entry solution obtained by Hwang and Fan (Finite Difference Analysis of Forced-Convection Heat Transfer in Inlet Region of a Flat Rectangular Duct, Appl. Sci. Res., vol. 13, pp. 401–422, 1964). The hydrodynamic and thermal entry effects for the experimental conditions in the present work are relevant for most of the test section. The location of the transition point is affected in the first instance by the local bulk Reynolds. On the other hand, the wall heat flux affects the fluid properties, i.e., the fluid viscosity, and therefore, it also affects the transition. It was found that for the transition regime, an increase of the wall heat flux produces lower values of the convection coefficient than the values predicted by the correlation of Gnielinski. This behavior, which seems to be associated to the variation of the fluid properties, is opposite to the increase of the convection coefficient observed for heating in the laminar and turbulent regimes for water and oils. The case of an inlet Reynolds number of 5,000 was studied in detail, and the influence of heat flux on the Nusselt number is addressed.