Mechanism analysis of the buoyancy effects on heat transfer of supercritical nitrogen in horizontal serpentine tube

In this paper, the buoyancy effects on heat transfer performance of cryogenic supercritical fluid in the submerged combustion vaporizer (SCV) are studied experimentally and numerically. Supercritical nitrogen is selected to substitute LNG for the safety reason. A three-dimensional model is established to calculate the flow and heat transfer of supercritical nitrogen, and the maximum relative error between the simulation and experiment is 2.78%. Based on the simulation results, a non-uniform circumferential heat transfer coefficient distribution is found in the pseudo-critical region, and the mechanism of the non-uniformity is analyzed. The reason for the phenomenon that the top and bottom generatrices reach the peak value of the heat transfer coefficient at different positions is further revealed by analyzing the average thermo-physical properties in different regions of the boundary layer. The results showed that the secondary flow caused by buoyancy leads to a non-uniform distribution of the thermal boundary layer thickness on the radial section. Under the influence of buoyancy, the temperature of the buffer layer near the top and bottom generatrices reach the large specific heat region successively, leading the peak position of the heat transfer coefficient of them not appear synchronously. Overall, comparing to the no-buoyancy condition, the whole heat transfer coefficient increased, with the heat transfer enhancement at the bottom and side generatrices but deterioration at the top generatrix.