The impact of heat exchanger type on the design and performance of optimized thermoacoustic refrigerators

Abstract

A basic thermoacoustic cooler design was optimized with three different types of heat exchangers to obtain high COP predictions in a 200-W refrigeration application. The heat exchanger designs employed: (1) a parallel array of copper-alloy plates (conduction); (2) a single-pass, single row of finned tubes, with water/ethylene glycol mixtures as the secondary fluids (finned tube); and (3) a parallel array of infinitely conductive plates (ideal). The optimization scheme utilized the Simplex algorithm, and the analysis was performed by DELTAE coupled with an original heat exchanger modeling tool which incorporated conventional straight fin analysis and semiempirical flow correlations. The external cooling and heat rejection temperatures were fixed at 260 and 310 K, for which the Carnot COP is 5.2. The best COP’s resulting from the optimization of around 25 design parameters for each case were approximately 2.0 (conduction), 2.5 (finned tube), and 3.0 (ideal). In the finned tube device, the COP was limited by the cold exchanger secondary fluid properties. The use of a phase change secondary fluid, in conjunction with a heat pipe, for example, may further improve the performance of finned tube heat exchangers. The ideal case performance represents a probable upper limit for the configuration considered.