Heat transfer characteristics of near-pseudocritical nitrogen in vertical small tubes—a new empirical correlation

This experimental study aims to investigate the characteristics of supercritical nitrogen (N2) internal flow convective heat transfer. A series of heat transfer experiments were conducted with N2 flowing upwardly in a uniformly heated vertical circular bare stainless steel tube (4.57 mm ID) under pressures of 3.5 and 4 MPa that beyond its critical pressure (3.4 MPa), mass fluxes of 27.9, 38.1, 50.8 kg/m2-s, and heat fluxes of 8.1, 9.3, 11.2 kW/m2, respectively. Experimental results demonstrate that supercritical N2 behaves similarly as other supercritical fluids such as water and CO2, both thermally and hydrodynamically, when transforming from subcritical to supercritical conditions in a heated vertical tube. In the vicinity of pseudo-critical point, the local convective heat transfer coefficient of N2 first elevated to a maximum peak value and then deteriorated as the fluid temperature becomes higher than pseudo-critical temperature. In general, the convective heat transfer coefficient increases with mass flux due to enhanced turbulence but decreased with pressure and heat flux, which can be attributed to the unique thermo-physical property variations of N2 near its pseudo-critical point. Furthermore, a new heat transfer correlation has been developed in this study exclusively for internal flow convective heat transfer of supercritical N2 flowing upwardly in a small circular bare tube based on the experimental results. It has been shown that the newly proposed heat transfer correlation can well predict, within ±15%, the corresponding heat transfer data of supercritical N2 while the heat transfer correlations in the literature constructed for other supercritical fluids (e.g. water and CO2) fail to do so. The new correlation inherits the basic form of Dittus-Boelter correlation, but takes extra considerations of fluid thermo-physical property differences between the heated tube wall and bulk fluid core and uses Eckert number to divide the whole heat transfer process into three regions around the pseudo-critical point. The results of this study also suggest that the research on internal flow convective heat transfer of supercritical N2 (e.g. continue to perfect the heat transfer correlation) should not stop and a more complete database covering a wider range of experimental conditions is needed to prepare the future blossom of supercritical N2 as one of the primary cryogenic heat transfer fluids.