Effects of different working gases on the heat transfer enhancement performance of a heating tube with spiral insert under oscillatory flow

Abstract

The working gas type had an important effect on the heat transfer performance of the turbulence device in the heater of Stirling engine, but the related researches were scarce, especially under the oscillatory flow. This work investigated the effects of four working gases (i.e., H2, He, N2, and CO2) on the heat transfer characteristics of a heating tube with spiral insert compared with a smooth tube under oscillating flow for a Stirling engine. The transient physical fields under different phase angles in an oscillatory cycle were analyzed, and the Nusselt (Nu) number, pressure loss and outlet temperature of working gas were studied. The results show that the cycle average Nu number of the enhanced tube with H2, He, N2, and CO2 as working gas increased to 1.67, 1.62, 1.61 and 1.72 times when comparing with those of the smooth tube. The cycle average friction coefficient increased to 1.96, 2.37, 2.36 and 2.62 times, respectively. Moreover, the performance evaluation criterion (PEC) values of the enhanced tube using the four types of working gas were all greater than 1 (1.21∼1.33). This implies that the comprehensive heat transfer performance of the heating tube with spiral insert was all improved with the four types of working gas. Moreover, the heat transfer enhancement effect was best when hydrogen was used. While considering the thermodynamic performance and safety reliability, the helium was more recommended. The findings are beneficial to enhance the operating efficiency of a Stirling engine.