An experimental study of friction factors and mixed convection in the thermal entrance region of vertically narrow, horizontal rectangular channels for different heating conditions and aspect ratios.

摘要

Heat transfer and fluid mechanical behavior of water flowing horizontally in vertically-narrow rectangular channels was studied in this work. Friction factor and Nusselt number variations were determined experimentally for four different wall heating conditions, and for aspect ratios (height to width) of 5, 4, 3, 2, and 1. Wall heating conditions examined were: all surfaces heated; three surfaces heated, the top adiabatic; one side surface heated the others adiabatic; and the bottom surface heated, the others adiabatic.; Friction factors, in laminar flow, varied as predicted from analysis. The critical Reynolds number varied linearly with ln(Dh).; Local Nusselt numbers were determined, at each aspect ratio and heating condition, as functions of Reynolds and Rayleigh numbers. Mixed convection was the result of buoyancy-induced secondary flows. Local Nusselt numbers decreased in a manner common to pure forced convection, reaching minimum values some distance from the entrance, then increased due to the presence of the secondary flows. For given aspect ratio, local Nusselt numbers were found to increase and the thermal entrance lengths decreased, with increasing Rayleigh numbers. In the thermal entry region, for all heating conditions except the bottom-heating case, local Nusselt number behavior showed minor dependence on aspect ratio.