Entrance and Exit Losses for Developing Flow in Parallel Plate Channels

Abstract

An analysis is presented to calculate the entrance and exit losses for developing flow in parallel plate channels. Such an analysis is of value for air-cooled heat sinks with short flow length, such as electronic heat sinks. These predictions are typically done using inlet (K c ) and exit (K e ) loss coefficients for parallel plate channels from the Kays and London book, Compact Heat Exchangers. However, the information presented in Compact Heat Exchangers assumes fully developed flow at the exit. Electronic heat sinks operate in the developing flow region. Thus, the use of the published K c and K e from Compact Heat Exchangers will result in an overestimation of the actual K c and K e values. The present author has completed an analysis that allows the accurate calculation of K c and K e values with parallel plate channels in the developing flow region. The results are presented in graphical and equation form as a function of contraction ratio and 4x+ (= 4x/D h Re). Entrance and exit losses can account for as much as 30% of the total pressure drop in electronic heat sinks having short flow lengths. Fortunately, the error associated with evaluation of K c and K e based on fully developed flow for parallel plates is small and expected to be less than 2%.