Comparison of the thermofluidic behaviors of 2-m nitrogen-charged cryogenic loop heat pipe under anti-gravity and horizontal conditions

Abstract

One of the main advantages of a cryogenic loop heat pipe (CLHP) is its heat transfer capability over long distances and operability under anti-gravity conditions. However, there are only a few studies on the thermal characteristics of long-distance CLHPs. It is essential to investigate the effect of a hydraulic head on CLHP performance to enhance the utilization of CLHPs in various applications. This study investigated the thermofluidic behaviors of a 2-m nitrogen CLHP with a capillary starter pump (CSP) under horizontal and anti-gravity conditions where the evaporator was 350 mm higher than the condenser. The novelty of the study is to reveal the heat transfer characteristics and operating mechanisms under anti-gravity conditions based on comparisons with experimental results under horizontal conditions. In the CLHP, a fine stainless-steel porous wick with a pore radius of 1.0 μm and permeability of 1.3 × 10−13 m2 was used for an evaporator and the CSP. The lengths of the vapor line, condenser, and liquid line were 2000, 1500, and 2000 mm, respectively. When a heat load of 4 W was applied to the CSP and evaporator, the CLHP successfully started with an initial cooling condition called a supercritical startup under anti-gravity conditions. The startup temperature behaviors were compared under horizontal and anti-gravity conditions. The thermal resistance of the CLHP with a stepped-up evaporator heat load and various CSP heat loads was evaluated for two CLHP orientations. The CLHP stably operated under evaporator heat loads of 4–24 W (horizontal) and 4–20 W (anti-gravity) for three CSP heat loads of 0, 2, and 4 W. The effect of the CLHP orientation on the thermal resistance with various CSP heat loads is discussed. This study enhances the applicability of the long-distance CLHP to various applications with a high degree of postural freedom by revealing the operating mechanism and thermal characteristics of the long-distance CLHP under anti-gravity and horizontal conditions.