Forced convection heat transfer of subcooled liquid nitrogen in a vertical tube

Experimental research on forced convection heat transfer of subcooled liquid nitrogen ranging from the pressures of 0.3 MPa to its supercritical pressure is carried out for wide ranges of inlet temperature and flow velocity. A stainless steel tube heater with the inner diameter of 5.4 mm and the length of 100 mm is mounted vertically. The heat transfer coefficients in non-boiling region and the DNB (departure from nucleate boiling) heat fluxes are higher for higher flow velocity and higher subcooling. The trend of the heat transfer coefficients in the non-boiling region agree with those by the Dittus-Boelter correlation, although they are unaffected by the flow velocity for R_e < 8000. The lowest limits agree with those obtained for no forced flow. The correlation of DNB heat flux that can describe the experimental data is presented. On the other hand, forced convection heat transfer of supercritical nitrogen deteriorates when the heated surface temperature, T_w, exceeds its pseudo-critical temperature, T_c', although they are similar to those under subcritical pressure for T_w < T_c'. The heat transfer characteristics of supercritical nitrogen can be predicted by authors' correlation.