Numerical study on unsteady flow transitions and heat transfer augmentations in supercritical CO2 helical heat exchangers

Supercritical helical heat exchangers are utilized in a range of advanced applications, including nuclear power plant recuperators, solar thermal systems, electronic cooling, liquefaction technologies, and propulsion systems. Supercritical helical heat exchangers operating under pulsating flow has not been explored which forms the central theme of this study. Flow through helical channels with supercritical fluids is characterized by intricate secondary flow phenomena, driven by the steep gradients in thermophysical properties and the interaction between Dean vortices and Lyne vortices, particularly under oscillating flow conditions. The simulations are conducted for typical operating conditions having a peak mass flow rate of 0.00894 kg/s, at 8 MPa. Multiple pulsating flow profiles such as square, cosine, and triangular waveforms at frequencies of 2, 4, and 8 Hz are applied to both circular and square cross-sectional geometries, with and without internal twists. A key finding is the emergence of Lyne vortices, which rotate in the opposite direction to Dean vortices during the deceleration phase of pulsating flow. This phenomenon is attributed to annular velocity distributions and contributes to enhanced radial mixing. Notably, a 4 Hz square-wave pulsating flow in a circular channel resulted in improved heat transfer compared to steady flow, albeit with a significant pressure drop. Among all scenarios, the 4 Hz cosine waveform in a circular channel achieved the highest heat transfer enhancement (Nu = 126.79) with minimal pressure drop. Additionally, the twisted square cross-sectional helical configuration facilitated effective redistribution of secondary flows and improved heat transfer uniformity.