Characteristics of Evaporation Heat Transfer in a Small-Scale Cryogenic Heat Exchange System for the Utilization of LNG Cold Energy

LNG 냉열활용을 위한 초저온 열교환시스템의 축소모형에서 증발 열전달 특성

The characteristics of evaporation heat transfer for the utilization of LNG cold energy was investigated experimentally using liquified nitrogen and a solution of ethylene-glycol and water under horizontal two-phase conditions in the small-scale equipment of a cryogenic heat exchange system. The inner tubes in the double pipe heat exchanger with 8 mm and 15 mm inner diameter and 6 m length were adopted as a smooth test tubes and enhanced tubes by means of wire-coil inserts. Heat transfer coefficients and Nusselt number for the test tube were calculated from measurements of temperatures, flowrates and pressures. The correlations in a power-law relationship of the Nusselt number, the Reynolds number and Prandtl number for heat transfer were proposed which can be available for design of cryogenic heat exchangers. The correlations showed heat transfer coefficients for the wire-coil inserts were much higher than those for the smooth tubes, increased by more than 2.5 ${\~}$ 5.5 times depending upon the equivalent Reynolds number. Form and length of cryogenic double pipe heat exchanger were proposed for applicable to the utilization of LNG cold energy.

본 연구는 LNG냉열활용을 위해 초저온 열교환시스템을 제작하고 액화질소와 에틸렌-글리콜 수용액을 작동유체로 사용하여 증발 열전달 특성실험을 수행하였다. 초저온 열교환기는 2중관식 열교환기이며, 내부관 지름이 8, 15 mm이며 길이는 6m이다. wire-coil inserts를 사용하여 열전달촉진 성능평가를 수행하였다. 액화질소와 에틸렌-글리콜 수용액의 출입구 온도, 벽면온도, 유량, 압력을 측정하였고, 이를 증발 열전달계수와 누셀트수를 계산하는데 사용하였다. 열전달상관식을 누셀트수, 프란틀수와 등가레이놀즈수의 멱법칙관계로 제안하였고, 그 결과 열전달촉진관이 평활관보다 2.5 ${\~}$ 5.5배정도 열전달이 증가되었다. 이 상관식을 이용하여 LNG냉열이용을 위한 초저온 2중관 열교환기를 설계하였고, 그 결과로 제시하였다.