| Performance characteristics of various state-of-the-art compact heat exchanger surfaces have been studied through flow visualization as well as heat transfer and pressure drop testing. Special consideration was given to the low Reynolds number regime. Flow visualization was first performed in scaled-up models of traditional types of enhanced surfaces; an offset-strip fin, a louvered fin, and a wavy fin. The flow visualization experiments were used to gain a phenomenological understanding of the transport mechanisms associated with these surfaces. The results indicated a degradation, and in some cases an absence, of the key enhancement mechanisms with reduced Reynolds numbers. Based on the observations, trends in the fin parameters which will suppress the low Reynolds number degradation were identified, and surfaces with these optimized parameters were constructed for performance testing.; Diminished flow performance in the traditional surfaces with reduced Reynolds numbers prompted the development of a new heat transfer surface. The resulting surface, a "pin fin," was specifically designed to provide high performance at low Reynolds numbers through sustained boundary layer disruption and formation of three-dimensional flow patterns. Visualization results for the new surface indicated substantial improvements and significant potential for high performance levels. Geometric parameters for this surface were varied, and optimal trends were identified. Based on the optimized configuration, the new surface was manufactured for performance testing.; The surfaces were tested to quantify their heat transfer and pressure drop performance characteristics. The results revealed performance levels for the new surface to be competitive with the best of the traditional surfaces, with sustained performance trends at low Reynolds numbers and heat transfer characteristics higher than any of the other surfaces in the upper Reynolds number region.; This thesis provides a description of the flow features and performance characteristics of various compact heat exchanger surfaces, including the novel surface which was developed. More importantly, a technique for the identification and development of high performance heat transfer surfaces is presented. This technique, based on flow characterization in inexpensive scaled-up models, precludes the expensive necessity of high volume building and testing which is typically used for surface development. |
