Entrance and Exit Losses for Developing Flow in Plain Fin Heat Sinks

Abstract

Prediction of pressure drop for duct flow through heat sinks involves calculation of inlet and exit losses. These predictions are typically done using Kc and Ke for “parallel plate channels” from the Kays and London book, Compact Heat Exchangers. However, these equations assume fully developed flow at the exit and thus include the effect of full velocity profile development. Electronic heat sinks operate in the “developing flow” region. So, use of the published Kc and Ke from the Kays and London book will result in over-estimate of the actual Kc and Ke values. The authors have performed analysis that allows accurate calculation of Kc and Ke values with parallel plate channels for operation in the “developing flow” region. The results are presented in graphical form as a function of contraction ratio and x+ (= x/DhRe). These results will allow accurate estimate of Kc and Ke values for developing flow. Entrance and exit losses can account for as much as 30% of the total pressure drop in electronic heat sinks having short flow lengths. However, the error associated with evaluation of Kc and Ke based on fully developed flow for parallel plates is small.