Measurements of wall heat (mass) transfer for flow through blockages with round and square holes in a wide rectangular channel

Abstract

Naphthalene sublimation and pressure measurement experiments were conducted to study heat (mass) transfer enhancement by blockages with staggered round and square holes for turbulent air flows through the holes in the blockages in an 8:1 rectangular channel. The diameter of six round holes and the width and the height of six square holes in each of four blockages were 2/3 of the height of the channel. The distance between consecutive blockages was two times the channel height. Average and local heat (mass) transfer and overall pressure drop results were obtained for four Reynolds numbers, defined based on the channel hydraulic diameter, of between 7,000 and 28,000. The results showed that the blockages enhanced the average heat (mass) transfer on the channel wall by 4.7 to 6.3 times, and increased the pressure drop along the test channel by up to almost 490 times that for fully developed turbulent flow through a smooth channel at the same mass flow rates. The blockages with round holes enhanced more heat (mass) transfer on the channel wall but caused larger pressure drops than the blockages with square holes. For a given pumping power, the blockages with square holes enhanced more heat (mass) transfer than the blockages with round holes. Because of the very large pressure drops caused by both the blockages with round and square holes, the heat (mass) transfer per unit pumping power was lower with the blockages than without the blockages. The local heat (mass) transfer was very high immediately upstream of a blockage, but was quite low immediately downstream of a blockage. The streamwise variation of the heat (mass) transfer coefficient was smaller on the wall segment between two blockages with square holes than between two blockages with round holes. Flow reattachment on the wall segment downstream of a square hole increased the heat (mass) transfer over two symmetrical regions on the wall segment. In the case of the round holes, the air jets might reattach near the downstream end of the wall segment where the heat (mass) transfer was very high.