Pressure drop and heat transfer of single phase liquid nitrogen flowing in U tubes

Abstract:

Two U tubes and two multi-U tubes are used to investigate the pressure drop and heat transfer characteristics of single-phase liquid nitrogen flowing in U-tubes. The diameter and curvature ratio of U tubes is from 4 to 8mm, and 8 to 15, respectively. It is found that the local heat transfer characteristics of both single U tubes and multi-U tubes in the 180° return bend are different from that of straight pipe. The wall temperature, especially the wall temperature of the latter part of the bend, decreases, and the heat transfer coefficient increases. For a single-U tube, the maximum heat transfer coefficient is at the exit of the return bend, for a multi-U tube, the heat transfer coefficient is sawtooth-type growing along the way, and reaches the maximum at the exit of the last bend. Based on Idelshik’s correlation, the new correlation for the 180°return bend friction pressure drop is proposed. The new correlation adds the centrifugal force item and agrees well with the experimental data.