Study On High Capacity Cryogenic Loop Heat Pipe Working Between 180K And 200K

With the rapid development of the space detection technique, the space/optical imaging detection system has more and more demands for refrigerating capacities, especially the application of the large-scale focal plane detection technology and cryogenical optical system, the demand for cooling power has reached dozens or even hundreds of watts, so the high capacity cryocooler should be needed. High spatial resolution of the optical system is very sensitive to relative position, the deformation and vibration, and the vibration or electromagnetic interference of the cryocooler may cause certain influence on the optical imaging system, thus we need a flexible thermal transport scheme to transmit the cooling power from the cold head of the cryocooler to probe and focal plane or the cryogenical optical. Cryogenic loop heat pipe(CLHP)is a effective and passive two-phase heat transfer device. Because the evaporator and condenser is conneted through a flexible steam line and a liquid line. They can meet the requirements of various spatial layout, high cooling capacity and long distance transmission, as well as decreasing the vibration from the cryocooler. In order to meet the future demand for large survey optical detection system of large cooling capacity, this paper has carried out cryogenic loop heat pipe research on the temperature range from 180 K to 200 K.Based on the actual demand for large survey optical detection system, we have down with the work fluid selection and component design of the cryogenic loop heat pipe. In the process of condenser design, according to high efficiency, light weight and compact characteristics of the micro-channel heat exchanger technology, we design and construct two kinds of microchannel condenser: parallel microchannel condenser and serpentine microchannel condenser. And we designed a new kind of liquid tube structure to connect the compensation chamber and the evaporator. Two prototypes of cryogenic loop heat pipes CLHP1 and CLHP2 have been designed and made, and ethane as the working fluid and nickel sintered porous material as the capillary wick. Experiments showed that two prototypes of CLHP could transport 52 W power when the evaporator temperature is under 205 K. After that we compared the temperature differences between inlet and outlet of the two microchannel condensers. The Parallel microchannel condenser’s temperature difference on both ends is very small, and the temperature distribution uniform, but the serpentine microchannel condenser import and export temperature difference is very large, the surface of this condenser have big temperature gradient, the reason of this phenomenon is the different structure have a different process of fluid flow in the condenser.In order to find the reason what caused the working temperature is higher than we expected, we analyzed the heat leakage effects on the performance of the CLHP1 and CLHP2. The result showed that the thermal conductivity of the wick is the key fact, the lager its thermal conductivity is, the larger radial heat leakage to the liquid core. We simulated the performance of the CLHP2 by using a mathematical model, and the result can meet the experiment data. And we also predicted the performance of the CLHP when the thermal conductivity of the wick is smaller, such as 5W/(m·K)、2W/(m·K)and 1W/(m·K).We also study on the influence of different wicks made by various materials. In chapter six, the silicon nitride(Si3N4) ceramic is used as the capillary wick, because of the relative low thermal conductivity, and compared with the nickel powder sintered wick core from aspects of the machining precision and installation. We made two sets of cryogenic loop heat pipe CLHP1 A and CLHP2 A use Si3N4 porous as the wick. An experimental study was carried out to CLHP1A; the CLHP1 A heat transfer capacity could reach to 192 K @ 52 W. However, because of the higher thermal diffusion coefficient of Si3N4, the working fluid inside of the evaporator core is easy to gasificate, cryogenic loop heat pipe CLHP1 A and CLHP2 A may fail to startup. And the high gas content of Si3N4, the degassing process is very complex, non-condensable gas inside of the Si3N4 wick terrible influence on CLHP2 A performance.