Experimental Investigation of Mixed Convection Heat Transfer in the Passages between Vertical Plane Surfaces

摘要

Experimental data are presented for turbulent mixed convection in the passages formed by vertical plates. The electrically heated plates simulated the oil-filled heat exchangers of large transformers which are to be cooled by gentle upward air flows. Each plate was heated from the leading edge and the velocity profile of the cooling air at the inlet of the passages was uniform rather than developed. Heat transfer at axial distances from the leading edge greater than around 10 to 20 equivalent diameters is similar to mixed convection in pipe flows. As in pipe flows buoyancy forces can reduce heat transfer to levels substantially below those calculated for forced convection. Heat transfer in the entry region has been correlated using an expression based on the correlation for turbulent forced convection boundary layers. Some data are also presented for heated downward flows. In these cases enhancement of heat transfer by buoyancy is observed. The data points are limited, however, because of the difficulty in achieving stable conditions where forced and free flows are in opposition. (Copyright (c) The Electricity Council-September 1989.)