Numerical study of supercritical-pressure fluid flows and heat transfer of methane in ribbed cooling tubes

Regenerative engine cooling, which generally occurs at supercritical pressures, plays a very important role in maintaining engine lifetime and performance in many propulsion and power-generation systems. In this paper, a numerical study of turbulent fluid flows and heat transfer of cryogenic methane in ribbed cooling tubes at a supercritical pressure of 8 MPa is conducted. The conservation equations of mass, momentum, and energy are properly solved, with accurate calculations of thermophysical properties, which undergo drastic variations and thus produce strong effects on fluid flows and heat transfer at supercritical pressures. The present study examines the effects of several key influential parameters on both heat transfer enhancement and pressure loss, including the rib geometry, rib height, wall thermal conductivity, and surface heat flux. Numerical results reveal that a ribbed tube surface leads to significant heat transfer enhancement, and in particular, the physical phenomenon of heat transfer deterioration at a supercritical pressure, which occurs in a smooth cooling tube, is drastically weakened in a ribbed cooling tube. A thermal performance factor is applied for combined evaluation of both heat transfer enhancement and pressure loss. Under the tested conditions, results indicate that an optimum rib height exists for achieving the best overall thermal performance.