Hydrothermal characteristics of fluid flow in a circular tube fitted with free rotating axial-turbine-type swirl generators: Design, swirl strength, and performance analyses

Abstract

The effect of inserting free rotating axial-turbine-types swirl generators (ATTSGs) in a circular tube is numerically investigated for convective heat transfer enhancement. Different types of ATTSGs with different blade designs are placed inside the tube, which are rotating freely around their axis depending on the velocity of the fluid flowing in the tube. The moving reference frame (MRF) method is used to model the fluid rotation in the zones containing the ATTSGs and the source terms arising from the transformation of stationary to rotating reference frame. Free rotational speed of the ATTSGs is estimated at different Reynolds numbers by systematically analysing the net torque applied on the ATTSGs in terms of their angular velocity. The local and average convective heat transfer and pressure drop are investigated for the cases with ATTSGs and the case of a plain tube. Considerable enhancement in the convective heat transfer is observed for all the ATTSGs, with the maximum of around 75%. To investigate the efficiency of the ATTSGs in terms of heat transfer enhancement, the intensity of the vortex structures generated by the ATTSGs is represented by the Q-criterion iso-surfaces. The ATTSGs which generate stronger swirling effect at the vicinity of the heated wall cause greater heat transfer enhancement. Inserting the ATTSGs leads to an increase in the pressure drop. Performance evaluation criterion (PEC) is introduced and evaluated for different ATTSGs to identify configurations that enhance the convective heat transfer at the lowest pumping power. The highest PEC of 1.4 is reported for the ATTSG with trapezoid twisted blade design.