Laminar convective heat transfer in helical twisted multilobe tubes

Augmentation of heat transfer performance has long been the main interest in thermal fluid engineering sector. Some commonly adopted thermal enhancement methods are adoption of helical tubes, twisted tubes and multilobe cross-sections. Several studies reported further enhancement when combination of these methods were implemented. Despite their promising potential, no investigation on the performance of heat transfer in helical twisted multilobe tubes has been reported. Therefore, present study is conducted with the main objective to study the laminar convective heat transfer of Newtonian fluid in a helical twisted multilobe tube through computational fluid dynamics (CFD) simulations. A three-dimensional model was developed with accordance to principles of fundamental conservation. The model was validated against available experimental data for which a good agreement was achieved. The model was thus utilised to investigate the fluid flow and heat transfer in the studied tube within a range of certain parameters, such as, number of lobes, number of twists, inlet Reynolds number as well as the geometry of the pipe (straight and helical). The performances of heat transfer of the investigated configuration are evaluated using the typical Nusselt number, friction factor and performance index (PI), which is a ratio between average Nusselt number and friction factor under constant pumping work condition. The results reveal that adding the number of lobes alone has negligible effect on the performance. However, combination of number of lobes and tube twisting result in considerable changes on the heat transfer coefficient with marginal effect on the friction factor. This is especially pronounced for tube with even number of lobe (bilobe and quadrilobe). Overall, helical coiled tube performs better within the range of Re 500 to Re 2000 as the performance index averagely increase by 18.4% as compared to straight tube, whereas below Re 500, straight tube is slightly outperforming the helical tube (3.65%). Twisting of the tubes result in the opposite effect, i.e. it improves the performance of straight tube but deteriorate the performance of helical tube. Thus, twisting is recommended primarily for straight tubes. Highest performance index yielded is 7.01 by bilobe helical tube without twist at Re 2000. Lastly, correlations for friction factor and Nusselt number are developed for straight and helical multilobe twisted tube within studied range. This correlation can serve as quick design tools for non-conventional heat exchanger utilizing twisting multilobe tubes.