Heat transference and pressure loss for air flowing in passages of small dimensions

Abstract

This investigation had as its primary object the experimental determination of the heat-transfer and pressure-loss characteristics for air flowing in small triangular, square, hexagonal and round passages. The heat interchanger models, with a frontage six inches square, each comprised from just under 150 to over 2,250 passages, according to their size and spacing. The hydraulic diameter of the smallest tubes was about 0.08 inch. Previously, little information of this kind had been available for any except round tubes of more than 0.5 inch diameter. The heat transfer in small smooth passages was found to be less than that usually measured for turbulent flow in tubes of larger size, and there was a tendency for a prolonged transition. The investigation was extended to determine the greater heat flow obtained in bulged or waved passages and outside a nest of hexagonal tubes. The influence of variation of passage length, pitch and end shape was also examined. A simplified theoretical analysis furnished a basis for separation of the components of pressure loss due to friction, increase of momentum, turbulence and end losses. Because of the uncertainty regarding conditions in transitional flow, a more precise theoretical treatment was considered to be unjustified. The interdependence of friction and heat transfer was emphasised by estimating the useful friction from the measured heat transfer coefficient, using the relationship deduced by von Karman on the hypothesis of the existence of a buffer layer between the laminar boundary flow and the turbulent core. The pressure losses measured in the experiments were found to be represented with good accuracy by coefficients, which may confidently be used to predict the air pressure drop in similar types of passage when the rate of heat transference is known.