Effects of thermal boundary non-uniformity on turbulent heat transfer of supercritical pressure fluids in a micro spiral tube

The heat transfer mechanism becomes exceptionally complex when the significant property variations of supercritical pressure fluids are coupled with the characteristic flow induced by spiral tubes. This study employs large eddy simulation to investigate the mechanisms by which uniform and non-uniform thermal boundary conditions affect turbulent heat transfer of supercritical fluids within fine spiral tubes. Under the coupled effects of centrifugal force, buoyancy, and thermal acceleration, the overall heat transfer performance under the uniform heat flux condition surpasses that under the coupled heat transfer condition. Both conditions exhibit upstream heat transfer deterioration (HTD) and downstream heat transfer enhancement (HTE) sequentially, in which the difference in thermal boundary uniformity leads to significant distinctions in the location and intensity of these heat transfer events. HTD is caused by the relaminarization of the turbulence near the outer edge of the inner wall, in which the key mechanism triggering localized relaminarization is the collapse of the self-sustaining cycle, resulting from a sharp reduction in the mean shear rate within the buffer layer induced by thermal acceleration. HTE is driven by the transverse secondary flow during the turbulent recovery stage, generated by the combined effects of centrifugal force and buoyancy, which transports the pseudo-critical core fluid into the buffer layer at the outer edge of the inner wall. The local fluid, exhibiting both strong turbulence fluctuations and high heat-carrying capacity, enhances turbulent heat transport through turbulence bursts. The findings provide fundamental understanding for the design and safe operation of air-to-supercritical-fluid heat exchangers.