Experimental and numerical investigation of heat transfer characteristics of spirally finned tube heat exchanger in a thermoacoustic stirling generator

Using spirally finned tubes is a promising method to enhance the heat transfer in oscillating flow heat exchangers. However, a general empirical correlation that enables predicting the heat transfer performance of spirally finned tubes under oscillating flow is lacking. Besides, existing research surrounding spirally finned tube heat exchangers mainly focuses on component-level evaluation and numerical simulations, while system-level evaluation and experimental study remain scarce. In response, this study investigates the heat transfer characteristics of a spirally finned tube heat exchanger under oscillating flow conditions, aiming to enhance the performance of a high-temperature heat exchanger in a free-piston Stirling generator. With preliminary thermoacoustic-based calculations, an experimental apparatus was built and tested. Experimental empirical correlations were derived, and the effects of acoustic power, operating frequency, and charge pressure on the flow and heat transfer characteristics of the heat exchanger were studied. Experimental results show that the heat transfer coefficient and Nusselt number of spirally finned tubes under oscillating flow are intensified by factors of 1.21 and 1.11, respectively, compared to smooth tubes. Subsequently, by virtue of the validated modelling, these results were extrapolated to a high temperature range. System-level simulations on the free-piston Stirling generator demonstrate that under the identical operational conditions, the spirally finned tube increases both the heat transfer coefficient and Nusselt number by 1.47 times compared to smooth tubes, improving generator output power by 64 W and thermal-to-electrical efficiency by 0.82 percentage points, respectively. This work provides valuable insight into the development of high-performance heat exchangers for Stirling/thermoacoustic systems.