U型管中单相液氮流的压降与传热特性

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

采用两种单U型管和两种多U型管,管径4～8 mm,曲率比8～15,对管中单相液氮流的压降和传热特性进行了实验研究.结果发现:无论是单U型管还是多U型管,在180°弯头内的当地传热特性与直管不同,壁面温度尤其是弯头后半段管的壁面温度明显下降,换热增大.对于单U型管,在弯头出口处换热系数达到最大;对于多U型管,换热系数沿程曲折增大,且在最后一个弯头出口处达到最大.基于Idelshik压降关联式,提出了单相液氮流在180°弯头内的摩擦压降关联式.新的关联式引入了离心力项,和实验数据良好吻合.

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 muhi-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.