Optimising the Hydro-Thermal Performance of a Four-Lobed Swirl Tube by Changing the Post-Swirl Pipe Length

This study explored the heat transfer performance of a decaying swirl flow generated by a four-lobed swirl generator with simulations and experiments. In the experimental studies, the thermal performance of a swirl generator was compared with that of a circular tube, indicating that the printed swirl generator provided better heat transfer performance than the printed circular tube. Further experiments were performed over a range of Reynolds numbers with different lengths of post-swirl, circular pipe to assess the decay of swirl, downstream. Here, for shorter lengths of post-swirl, circular pipe, the thermal improvement was maintained, but this advantage was lost with longer circular pipe runs. In numerical studies, the effect of various pitch-to-diameter (PD) ratios of the swirl pipe, including the actual swirl pipe (in-swirl) as a part of the heat exchanger or excluding it (ex-swirl), and the effect of different post-swirl section lengths were investigated. The heat transfer rate and pressure drop increased with the reduction of the PD ratio, and the in-swirl arrangements gave higher thermal enhancement and similar pressure loss compared with the ex-swirl arrangements. The PD6 in-swirl arrangement gave the highest thermal enhancement for all the geometries. Applying the swirl intensity and the field synergy principle confirmed these results, showing that any thermal enhancement created by swirl generators disappeared rapidly after exiting the swirl tube. In addition, the field synergy principle was more suited to explain the thermal enhancement effect of the decaying swirl flow instead of swirl intensity. This study demonstrates that by optimising the post-swirl pipe length, overall heat transfer performance can be increased by around 5% with a pressure drop of less than 16%.