| The performance of parallel-plate heat exchangers in oscillating flow is studied empirically and theoretically, with the goal of improving the models of heat exchangers used in the design of thermoacoustic engines and refrigerators. A novel style of thermoacoustic device, the “no-stack” device, provides the framework for study. In a no-stack engine or refrigerator, the porous medium (the stack or regenerator) that is found in other thermoacoustic devices is eliminated, replaced by a small “no-stack gap.” A no-stack device, then, consists of two heat exchangers strategically placed in an enclosure containing oscillating pressure and flow. Such devices lie beyond the boundaries of the acoustic theory that has been used to model and design thermoacoustic devices based on stacks and regenerators. The theory developed here to study no-stack devices combines original calculations and conventional thermoacoustic theory with results from fluids engineering, which are used on an ad hoc basis as required. Further studies are carried out on those parts of this semi-analytical model that are deemed least reliable. In particular, measurements are made of heat transfer between two identical parallel-plate heat exchangers under conditions of oscillating flow over a range of frequencies and amplitudes. The results are analyzed and summarized in terms of heat-exchanger effectiveness, the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. Measured results are compared to the DeltaE model that is often used in the design of conventional thermoacoustic devices, and possible improvements to the model are offered. The influence of nonuniform velocity profiles on minor losses at the exit from oscillating-flow heat exchangers is studied. Heat exchanger performance in an environment with both oscillating flow and oscillating pressure is examined with a time-stepping computational method. Combined with the heat transfer measurements, the time-stepping model indicates that extremely effective exchangers would be required for successful no-stack devices, and that pressure oscillations give rise to significant changes in performance of heat exchangers at more typical pressure amplitudes, enhancing heat transfer in stack-based refrigerators and degrading it in stack-based engines. |
