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

Prediction of pressure drop for duct flow through heat sinks involves calculation of inlet and exit losses. These predictions are typically done using K_c and K_e 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 K_c and K_e from the Kays and London book will result in over-estimate of the actual K_c and K_e values. The authors have performed analysis that allows accurate calculation of K_c and K_e 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/D_hRe). These results will allow accurate estimate of K_c and K_e 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 K_c and K_e based on fully developed flow for parallel plates is small.