Numerical study of heat transfer and fluid flow of supercritical water in twisted spiral tubes

Supercritical water (SCW) as fluid with enough potential abilities such as high thermal efficiency can be replaced in water cooling systems. However, helical pipes are the most common type of swirl generators used for heat transfer enhancement, and their geometrical parameters contribute significantly to the compactness and efficiency of heat exchangers. In this study, for the Reynolds ranges of 8000-20000, a numerical study is performed to investigate SCW flow within circular tube, twisted oval tube and twisted multi-lobe tubes. Effects of such parameters as cross-section shapes and lobed numbers on heat transfer performance and flow physics of different spiral tubes were investigated. Results indicated SCW has a contradictory behavior with ordinary water. The results showed that twisted tri-lobed tubes had higher average Nusselt number than circular tube and twisted oval tube, which was 53% and 41%, respectively. Also in these tubes the larger radius of connecting arc and the smaller length of major axis conclude the higher amount like 4%-6% of heat flux due to the enhancement of swirling fluid flow in the pipe and the heat flux. By increasing the number of lobes, the swirling flow rate that is bounded by the lobes will increase.