| Offset fins are frequently used in compact heat exchangers for heat transfer enhancement. In the most commonly used configuration, the fluid flows parallel to the fins. Experiments were conducted on several liquid-cooled cold plates with internal fluid flow through an offset fin structure. The cold plates differed in the geometry of the fins as well as the direction of the flow with respect to the fins. The results showed that when compared on the basis of equal pumping power and heat transfer surface area, the configuration with flow perpendicular to the fins yielded higher heat transfer rates compared to parallel flow.Detailed two-dimensional computations were carried out to study the heat transfer and pressure drop characteristics of flows perpendicular to an offset fin structure, for Reynolds numbers up to 600, and Prandtl numbers of 0.7, 5.8 and 90. The flow and temperature fields become periodic fully developed after 5-7 fins depending on the Reynolds and Prandtl numbers. Nusselt number and friction factor results are presented for both the entrance and periodic fully developed regions. The friction factor was found to be a strong function of the Reynolds number, and the Nusselt number was found to be a strong function of both the Reynolds and Prandtl numbers. The Nusselt numbers from the two-dimensional calculations were found to predict the measured temperature data well.The heat transfer enhancement effect in perpendicular flow is higher for larger values of the dimensionless fin spacing parameter, s/l. For smaller values of s/l, the flow does not impinge on the upstream face of the fins, and hence the fin surface areas are not effectively utilized. It was found that the fin spacing parameter, s/l, had the largest influence on the performance of the perpendicular flow configuration, while the fin thickness parameter, t/l, and the fin height parameter, h/l, had only a minor influence. An analytical model was also developed for the periodic fully developed Nusselt number and friction factor. The model predictions match the computed two-dimensional results well. |
