Enhancing supercritical carbon dioxide heat transfer in helical tube heat exchangers with twisted tape inserts: A computational investigation

Supercritical carbon dioxide (sCO2) has recently gained considerable attention due to its unique thermal properties that make it favorable as working fluid in various thermal system. It is particularly promising in cooling application of powerplant in combination with helical tube heat exchanger. One way to further enhance its heat transfer performance is by introducing twisted tape insert to the helical tube heat exchanger working with sCO2. Nonetheless, no study evaluating this idea has been reported, withholding further implementation of this innovative idea. Hence, this study aims to investigate the heat transfer performance of sCO2 in helical tubes with twisted tape inserts. A three-dimensional computational model, considering mass, momentum, and energy conservation, is developed and validated against experimental data. The heat transfer characteristics are evaluated in terms of wall and bulk fluid temperature as well as heat transfer coefficient. The impact of twisted tape width, twisting turns, and operating parameters (heat and mass fluxes) are evaluated to gain understanding on the contribution of these parameters on the heat transfer mechanism of the flow. The results reveal that introduction of twisted tape inserts enhanced heat exchange performance because of the intensified secondary flow within the helical tube. The most notable enhancement takes place when the temperature remains within the pseudo-critical region, driven by the higher thermal conductivity that dominates heat transfer near the wall and the higher specific heat capacity, enabling efficient heat absorption while restraining temperature rise. However, increasing the number of turns and tape width does not necessarily result in a greater heat transfer enhancement. Performance evaluation criterion (PEC) is utilized to evaluate overall performance. Further exploration using the Taguchi method underscores the dominance of operating parameters including mass flux and heat flux, over geometric factors like the number of turns and tape width. Moreover, strong interactions are only observed between geometric parameters (number of turns and tape width) and the temperature difference between the inlet and pseudo-critical points. Results from this study are expected to augment critical information required in developing high performance cooling mechanisms for various thermal systems, especially power plants.