Loop heat pipes and capillary pumped loops-an applications perspective

Abstract

Abstract. Capillary pumped loops (CPLs) and loop heat pipes (LHPs) are versatile two-phase heat transfer devices which have recently gained increasing acceptance in space applications. Both systems work based on the same principles and have very similar designs. Nevertheless, some differences exist in the construction of the evaporator and the hydro-accumulator, and these differences lead to very distinct operating characteristics for each loop. This paper presents comparisons of the two loops from an applications perspective, and addresses their impact on spacecraft design, integration, and test. Some technical challenges and issues for both loops are also addressed. _TRODUCTION

Experimental Investigation of New Flat-Plate-Type Capillary Pumped Loop

“…where is the coefficient of surface tension, and r eff is the effective radius of the capillary. Based on a fundamental principle to the CPL operation, the maximum capillary pumping head, P c max , must be larger than the total loop pressure drop P loop [6,16,17]; that is…”

“…To solve or partly solve the above problems, a CPL needs preconditioning to ensure that the wick is wetted before applying power to the evaporator for startup. This involves the reservoir preheating to "prime" the evaporator pump and collapse any vapor bubbles, and this process is tedious and sometimes time consuming [6,7]. Once the priming procedure is completed, the heat can be applied to the capillary evaporator to start the loop operation, and this startup procedure is called "fully flooded startup."…”

“…When the pressure difference required to perform this purge exceeds the capillary limit of the wick, vapor bubbles from the vapor side will certainly be injected through the wick into the liquid core and cause depriming, which is responsible for more than 90% of CPL depriming [8]. To enhance the possibility of startup success, the CPL system generally uses starter heaters and a starter pump for clearing liquid in the vapor line; nevertheless, sometimes the CPL still cannot start up successfully [6,7].…”

“…In addition, the compensation chamber (CC) is located directly in the path of the liquid flow. These differences with CPL allow the LHP to realize an easy startup process and robust operation, but the heat leak through the secondary wick to the CC makes controlling the loop temperature difficult [6].…”

Bio-inspired evaporation from shaped interfaces: an experimental study

“…Modern utilization of evaporation is broad scale, for processes as diverse as thermal management, drying, mixing and separation of chemical substances. Structured textiles in perspiring wearables [3], heat pipes in lighting [4] and looped two-phase heat sinks for consumer electronics [5] are some of the emerging technological applications. Building engineers are also considering evaporative solutions namely 'sweating facades' for macro-scale cooling in water-available climates.…”