A numerical study on heat transfer and boiling crisis in twisted heat exchanger tubes

Abstract

To explore the heat transfer enhancement mechanism of swirling two-phase flow, we conducted a numerical simulation study for twisted tubes with 18 different cross-sectional sizes using the GENTOP modelling scheme. In this study, we analyzed the phase distribution and the influence of centrifugal force and wall shear on the boiling crisis. The results show that, under the given operating conditions, the performance comparison indices for all 18 helical tubes are greater than 1, with more than half falling within the range of 1.5 to 2.5, and swirling increases the heat transfer coefficient between 24.2% and 101%, which demonstrating significantly enhanced heat transfer performance. Furthermore, swirling flow modifies the boiling crisis nature and postpones its onset. Particularly, the centrifugal force affects the onset point of the boiling crisis, with the delay distance ranging between 0.05 and 0.2 m. The larger the centrifugal force is, the more delayed is the occurrence of dryout. Wall shear increases with increasing liquid velocity and its increase rate decreases due to the tearing of the continuous liquid film on the wall. We propose a correlation that relates the channel center area ratio and the volume fraction ratio of continuous gas, which yet needs further experimental validated.