Influence of straight tube on serpentine tube heat exchanger with supercritical pressure fluid

Serpentine tube heat exchangers are widely applied in many industries, e.g. electronic equipment cooling, refrigeration, and spacecraft thermal control. These exchangers typically have curved and straight sections, however, the design of such heat exchangers with supercritical pressure fluid is challenging and not yet fully understood since complexity of convective heat transfer for supercritical pressure fluids and combined structure. In this paper, the convective heat transfer of supercritical pressure n-decane in a serpentine mini-tube with a straight section length of 16 mm and 80 mm, an inner diameter of 2 mm, and a curvature diameter of 16 mm was investigated experimentally and numerically, and the impact of secondary flow in the curved section on downstream heat transfer in the straight section was analysed. It was observed that when the straight section exceeds a specific impact length, the secondary flow effects from the upstream curved section become negligible and the convective heat transfer characteristics would transform into a fully developed feature in the straight tube. The impact length is concentrated in 1.5D-2.5D range and also in 3D-5D range when fluid properties change significantly. A correlation for the impact length has been proposed that distinguishes three regions with different heat transfer characteristics and allows the three regions to be treated separately when designing such heat exchangers. Finally, a feasible calculation of the Nu number for three regions has also been provided to design this type of heat exchangers.