Heat Transfer and Pressure Drop of Liquid Cooled Offset Fin Heat Exchangers

Abstract

An experimental and modeling study of heat transfer and pressure drop in liquid-cooled offset fin compact heat exchangers (cold plates) is described. Liquid coolants used in the testing are water and PAO (polyalphaolefin), for which the Prandtl number ranges from 3 to 150. Attention was focused on the Reynolds number range 10 - 2000 which spans most liquid cooled applications. From the data and from comparisons with previous air-cooled data, it was found that the Prandtl number has a significant effect on the Colburn factor of the offset fin geometry but little effect on the friction factor. A numerical heat transfer analysis was performed to investigate the surface temperature distribution and uniformity of heat flux in the cold plates. The results demonstrate good agreement with surface temperature measurements. The model results were used to guide data reduction procedures. In particular, significant end effects are predicted. Through experience with the heat transfer model, these end effects were isolated. The numerical model predicts approximately uniform heat flux over the central section of the cold plates. Predictive models were developed based on a surface contribution analysis of energy and momentum balances in a unit cell of the offset fin geometry. The Prandtl number effects on heat transfer can be viewed from two perspectives: fin perspective and array perspective. The fin perspective allows explanation of the Prandtl number dependence of the periodic fully developed Nusselt number. The array perspective is analogous to the usual thermal entry region in duct flow. Thermal development from the array perspective yields higher Nusselt numbers in the entry region. The surface contribution model shows significant Prandtl number effects on offset fin heat transfer performance. The models have estimated uncertainty of ±20%. The models have been validated for heat transfer and pressure drop for Prandtl number ranging from 0.7 to 150 and Reynolds number from 10 - 2000.