Experimental study on single-phase heat transfer performance of internal helically finned tubes covering low Reynolds number in turbulent flow

This paper experimentally investigated the heat transfer performance of two internal helically finned tubes covering low Reynolds number in turbulent flow. A water-ethylene glycol mixture flowed inside and R134a boiled outside. The numbers of fin, the helix angles, and the ratios of fin height to diameter for the two test tubes were 38 and 60, 60 degrees and 45 degrees, and 0.0534 and 0.0222, respectively. The experimental Reynolds number was lower to 5400, covering the critical Reynolds number Recr for turbulent flow. The data was divided into good and poor linearity regions by the modified Wilson plot method, which was much suitable for this case than that the data was treated as one region. The j factors showed ascending trend for Re below Recr and descending trend for Re above Recr. Enhancement factors of friction factor or j factor of the test tubes increased with increasing Reynolds number and eventually approached at the maximum for the Reynolds number above Recr. The test tubes exhibited maximum efficiency indexes of 2.1 and 1.88, indicating superior overall performance compared to the majority of existing tubes. By utilizing the experimental data, a correlation was established basing on the critical Reynolds number, resulting in over 96% of the predicted errors falling within the range of +/- 8%. This study has the potential to provide valuable insights for industrial implementation and the advancement of general correlation development.