| As an efficient two-phase flow heat transfer equipment without moving parts,loop heat pipe(LHP)has many advantages,such as long life,high reliability,anti-gravity ability and flexible long-distance heat transmission.It is increasingly used in the thermal control system of space remote sensing detection equipment.The evaporator and compensation chamber are always the key points of LHP experimental and theoretical research,which closely affect the temperature fluctuation and operation mode of LHP.Combined with the background requirements of the project,this paper mainly studies the heat and mass transfer mechanism of LHP from two aspects of the influence of mass flow and heat load on the evaporator.The mass flow characteristics and heat transfer performance of LHPs with different working fluids and different inner diameters of vapor line were studied by introducing a high-precision Coriolis mass flowmeter.The research of different heat load application methods is mainly realized through the visualization experiment of evaporator compensation chamber and the numerical simulation of steady-state operation of the LHP.According to the latent heat of vaporization,saturation pressure and density at room temperature and dynamic viscosity of vapor,acetone,ethanol and propylene were selected as the experimental working fluids.The mass flow and heat transfer characteristics of LHP filled with different working fluids were compared and analyzed.The experimental results show that the mass flow measurements can reflect the transient changes of the working fluid flow in the LHP timelier and more truly compared with the temperature measurement.The fluctuation of mass flow rate is affected by the compressibility of the vapor working fluid and the heat flow in the capillary core,and the heat in the capillary core is dominant under high heat loads.Through spectrum analysis,it is found that the fluctuation of liquid mass flow rate of LHP is also affected by the position of two-phase region of condenser.The experimental comparsion of LHP-1,LHP-2 and LHP-3 with different inner diameter(2mm,3mm and 4mm)of vapor line was carried out under the same filling rate with ethanol as the work fluid.The effects of the inner diameter of vapor line on the LHP steady state performance under different heat loads were studied in terms of pressure drop of the LHP and the heat leakage from evaporator to compensation chamber.The change of the inner diameter of the vapor line changes the "critical" heat load for the LHP to enter the fixed thermal conductance zone by affecting the total pressure drop of the LHP,thus affecting the operation mode of the LHP.When LHP-1,LHP-2 and LHP-3 are in the variable thermal conductance zone,the proportion of the pressure drop of the vapor line decreases with the increase of the heat load,and the influence of the inner diameter of the vapor line on the thermal resistance of LHP decreases.The reciprocal number of heat leakage resistance of evaporator to compensation chamber through wick 1 / Rec-cc,wick is an important characterization parameter,which is closely related to the vapor-liquid interface of the compensation chamber ? the mass flow rate of the LHP and the vapor-liquid distribution in the wick,reflecting the heat leakage between the wick and the compensation chamber.In order to explore the influence of different evaporator heat load application methods on the working fluid flow and performance of LHP,three typical evaporator heat load application methods were designed: heat load on the top of evaporator,heat load on the bottom of evaporator,heat load on the upper and lower surfaces of evaporator.Based on a transparent quartz glass and the sealing of PTFE / metal O-ring,the visualized prototype LHP-4 and LHP-5 with R245 fa and propylene as working fluids were developed,and the visualition experimental system of normal temperature and low temperature based on high-speed camera was built.The experimental results of the start-up characteristics of LHP-4 under different heating methods show that the influence of different heating methods on the evaporator temperature first increases and then tends to be flatten.This is mainly because the relative position of the liquid level of the evaporator cavity and the evaporator outlet affects the compressibility of the gas working fluid.The LHP-4 with top heating has the longest start-up time.The bubble growth in the evaporator cavity during the start-up process indicates that the compression period of the vapor working fluid is on the order of 0.01 s,which indirectly eveals the reason for the disordered fluctuation of mass flow in the LHP with different working fluid under low heat loads.In the two-sided heating mode,the reciprocal thermal resistance of evaporator and compensation chamber increases with the increase of heat load,and has a linear relationship with the liquid level in the compensation chamber,which is consistent with the analysis of heat leakage resistance from evaporator to compensation chamber when the ethanol LHP is in the variable thermal conductance zone.Different heating methods affect the vapor-liquid distribution in the evaporator-compensation chamber,changing the heat loss of the environment and LHP-4 components under the same heat load,the evaporation efficiency of the working fluid in the wick of the evaporator,and the heat leakage from the evaporator to the compensation chamber,and finally affecting the performance of the LHP-4.The thermal resistance of the bottom heated LHP-4 is the smallest under the same heat load.Howerever,when the heat load is30 W,the thermal resistance of the bottom heated LHP-4 is close to that of the upper and lower heated LHP-4.The liquid level of cryogenic LHP-5 compensation chamber under different heating methods increased rapidly at first and then slows down.In the evaporation start-up mode,the compensation chamber liquid level rises most slowly,the temperature of the corresponding vapor line outlet and liquid line outlet change more slowly,and the start-up is also the slowest.The start-up process of R245 fa LHP-4with different heating methods verified each other.Similar to the fluctuation of mass flow rate of room temperature propylene LHP-1,under heat loads lower than 60 W,the fluctuation of liquid level of LHP-5 compensation chamber heated at the bottom first decreases,then increases and then decreases with the increase of heat load and disappears at 30 W and 50 W.In the steady-state heat and mass transfer model of loop heat pipe,the difference of heat transfer coefficient between the wick in evaporator and the liquid and vapor in compensation chamberis fully considered.The liquid height of the compensation chamber affects the heat transfer area of both,and then affects the heat leakage from the wick of evaporator to the compensation chamber.In the LHP simulation model,the working condition of the wick in the evaporator is divided into two cases: full of liquid working fluid and vapor invasion into the wick.The equivalent vapor invasion thickness of the wick is introduced,and the effects of the change of liquid phase heat transfer mode between the evaporator and the compensation chamber under high heat load and the change of equivalent heat transfer coefficient between the evaporator shell and the capillary core are considered.By comparing the experimental results and simulation results of LHP-5,the simulated and experimental values of evaporator temperature and compensation chamber liquid level both decrease first and then increase with the increase of heat load.The experimental results(evaporator shell temperature,compensation chamber liquid level)of LHP-5 heated at the top and bottom are basically consistent with the simulation results,which verifies the credibility of the model. |
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