Learning From Experiences With Compact Heat Exchangers PDF Download

An accurate computational method for the calculations of flow and heat transfer in compact heat exchangers is developed in collaboration with the National Center for Supercomputing Applications. In this method, the unsteady Navier-Stokes and energy equations are solved. A linearly scalable performance of the code is achieved on the massively parallel CM5, demonstrating the capability of this method to solve large scale heat transfer problems. The heat transfer enhancement mechanisms and performance of parallel-plate fin heat exchangers are studied extensively. Geometry effects such as finite fin thickness and different fin arrangements have been investigated. The roles of individual enhancement mechanisms and their attendant effects on frictional loss have been quantified. At sufficiently high Reynolds numbers, when the actual flow is three-dimensional, corresponding two-dimensional simulations overpredict overall heat transfer efficiency by as much as 25%, while the overprediction of frictional loss is much less. More importantly, the overprediction of fluctuations in heat transfer and frictional loss in two-dimensional simulations is much larger, where rms of the amplitude of fluctuations from the two-dimensional simulations can be as much as 5 times of that from corresponding three-dimensional simulations. These differences are attributed to the strong coherence of spanwise vortices in two-dimensional simulations and the weakening of spanwise vortices in corresponding three-dimensional simulations due to the presence of streamwise vortices. In two-dimensional simulations, the coherent spanwise vortices enhance mixing and result in higher heat transfer efficiency. These spanwise vortices at the same time lowers skin friction on the fin surface. On the other hand, two-dimensional simulations overpredict form drag due to higher Reynolds stresses in the wake. In current two-dimensional simulations, the overprediction of form drag is nearly counter-balanced by the underprediction of skin friction. In the simulations of flow and heat transfer in more complex louvered fin geometries, current numerical results clearly show the different flow regimes as the Reynolds number is increased, which are generally in agreement with those observed in experimental flow visualizations. However, at low Reynolds numbers, current interpretation of the flow characteristics is somewhat different.