Investigation On The Operation Mechanism With Phase Change For Capillary Pumped Loop And Loop Heat Pipe

The capillary pumped loop (CPL) and the loop heat pipe (LHP) are the two-phase thermal control devices with the latent heat of evaporation of a working fluid to transfer heat and the capillary action for fluid transport, which are capable of transport large heat density and passively transporting heat over large distances with minimal temperature losses, and contain no moving parts. As a result, CPL/LHP becomes more active and interesting in many engineering domains including thermal management of satellites and spacecrafts as well as cooling of electronic devices.A prompt, simple and reliable startup performance is the basic characteristics of advanced CPL/LHP system. Whereas, a number of difficult startup problems of CPL/LHP were found in present literatures due to various reasons. An overall two-dimensional numerical model was developed to address the heat and mass transfer characteristics in the evaporator of mLHP, the numerical results showed that 'side wall thermal conductivity' had a significant effect on the heat and mass transfer in the evaporator of a mLHP. The start-up characteristics of a flat miniature loop heat pipe (LHP) with different heat loads were experimentally investigated in this paper and the experimental results showed that the mLHP couldn't start up successfully under normal condition except that a bag of ice-water was located on the bottom of the evaporator with a heat flux of 1W/cm~2. Reasons for the difficulties in its start-up were analyzed and 'side wall effect' was deduced , hence, a numerical study was addressed to investigate the optimization design of mLHP in the geometrical structure. The numerical results showed that the performance of the flat miniature LHP could be enhanced by improving the geometrical structure in a limited working space. Effects of thickness of the side wall, height of the compensation chamber and the wick on system performance were discussed in detail.Pulsations of the operating temperature, which take place in stable conditions of heat supply and removal, are a characteristic process inherent in some types of closed heat-transfer devices operating on the evaporation-condensation cycle, but the theoretical reports are seldom seen, hence, a mathematical model based on the Lucas -Washburn equation has developed to address the relations of the capillary height, capillary radius and the heat flux in a capillary column in un- gravity, and the formulas deduced as a consequent is used to analyze the influence of the height of the capillary wick in the capillary force and stability in a capillary loop with phase change. According to the theoretical analysis, the height of the wick will lower the capillary force in the loop, and the stability of the loop by introduced a small disturbance into the height of the capillary wick is studied in detail. The result shows that the steady state of the capillary loop with phase change is over-damped in non-gravitational condition. The root of the temperature pulsation is in the condenser, the closer to the condenser, the larger of the amplitude it is, and resonance will occur in the condenser when the frequency of the pressure pulsation is equal to the inherent frequency of the liquid column in the liquid line , therefore, a appropriate heat flux is required to keep the frequency of the liquid column far away from the pressure pulsation frequency in the vapor line.An accessorial loop with a micro-pump and ejector is applied into the Capillary Pumped Loop(CPL) to enhance the heat transfer capability of CPL. A 3-D model is developed to investigate the operation performance of the ejector. The numerical results show that the primary loop will not only can operate normally but the mass flux can be enhanced with a accessorial loop. However, vapor can be found in the outlet of the ejector and it can flow into the evaporator, which is a big disadvantage for system, hence, a sub-cooler is necessary to locate between the outlet of the ejector and the inlet of the evaporator to condense the mixed fluid into liquid.A phase change driving mechanism in Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP) is pointed out in this paper . A mathematical model has been developed to describe this driving mechanism. The calculating examples presented in the paper with methanol as working fluid shows that flow resistance in vapor side is larger than that in liquid side. Especially in the case of high heat flux, flow resistance in vapor side is dominative part of the whole system pressure drop, so that a remarkable phase change pressure head is needed to drive vapor flow. The formulas reflecting relations between thermodynamics parameters and flow resistance have been developed to quantitatively evaluate characteristics of heat transfer and flow in the system by calculating working fluids' temperature, pressure and other parameters in the evaporating and condensing interfaces, and to predict thermal resistance and temperature uniformity of the system, thereby guiding the design of CPL and LHP. The model established can be extended to all heat pipes with capillary wick or micro-groovy channels.