| Loop heat pipe is a highly efficient heat-transfer device which does not require any additional regulating actions from the outside for ensuring its serviceability. Thus, LHP offer many advantages over traditional heat pipe, such as operability against gravity, over large distances with minimal temperature losses and no moving parts for pumping the working fluid, etc. So LHP is widely used in the field of thermal control of spacecraft and cooling of electronics with high heat flux.The thesis introduced the theory and characteristics of LHP. With increasing power densities of electric devices, the LHP technology continues to be an important area of research. The LHP with cylindrical as well as flat evaporators have been developed and tested successfully. Compared with conventional cylindrical LHP, the LHP with flat evaporator have more advantages. Firstly, it is more convenient for the flat-plate type evaporator to connect with the electronic devices which should be cooled, because most objects to be cooled have a flat thermal contact surface; secondly, the angle of velocity vector and temperature gradient is smaller than that of LHP with cylindrical evaporator, that is, from the viewpoint of field synergy principle, the heat transfer efficiency of flat evaporator is better; So the capability of LHP with flat-plate evaporator for transferring high heat-flux achieve easily. The LHP with flat evaporator can be considered as an optimum design for compact enclosures as it provide relatively more scope for design miniaturization. The thesis aimed at the experimental and theoretical research for LHP with flat evaporator.A LHP with flat disk evaporator, which is with body material of stainless steel, wick material of sintered nickel and working fluid of ammonia, was experimental studied. The investigation of the start-up and variable heat load cycles showed that the LHP was reliable. Besides, the effects of temperature of condenser to the operation characteristics of LHP were researched by changing the temperature of cooling water from -10℃to -15℃. The results showed that decreasing temperature of condenser is little useful to decreases the temperature of evaporator. The main objective of the thesis was to design and investigate the start-up phenomenon, steady state performance, operating characteristic at variable heat load cycles of the flat evaporator miniature loop heat pipe with copper body, stainless mesh wick and methanol as working fluid. In this phase of experiments, the LHP was operated at the tilt angle of 10°, 50°and 90°, and the charging ratio of 50vol.%, 60vol.% and 70vol.% in the cold mode. The procedure of tests included measurements of temperatures at characteristic points of LHP with a successive stepwise 12W increase and decrease of the heat load. And the start-up tests with the heat load from 12W (heat-flux density 1W/cm~2) to 120W (heat-flux density 10W/cm~2) was operated. The effects of tilt angle and charging ratio to the LHP operation characteristics was detailed researched. Otherwise, the effects of working fluid to the LHP were studied by comparing methanol and acetone as working fluid.Temperature oscillation is a rather wide-spread phenomenon accompanying the LHP operation. The investigations conducted make it possible to differentiate three main types of the LHP operating temperature. The first of them is characterized by a low-amplitude and a high-frequency. The second type is also characterized by a low-amplitude of temperature oscillation, but their cycle is longer and reaches several minutes. The third type is distinguished by high amplitude of temperature oscillation, which reaches tens of degrees, and a still longer period, which may be equal to tens of minutes. The mechanism of temperature oscillation was complex; it was relation to the tilt angle, charging ratio, structure of LHP, heat load and the property of working fluid, and so on. The paper studied the temperature oscillation by analyzing the hydrodynamic instability of the two phase flow in the condenser inlet, which showed that the process of condensation is one of the sources of temperature oscillation. The experimental results showed that the tilt angle and charging ratio affect the power of temperature oscillation and the zone which appear temperature oscillation. Comparing with the temperature oscillation of LHP with acetone, it is more fiercely of LHP with methanol as working fluid.In order to analyze the mechanism of LHP and study the relation of different part of LHP, the dynamic simulation of LHP with flat evaporator has been established, and the mathematical models of flat plate evaporator, condenser, vapor liquid transport line and liquid transport line were developed based on the nodal method. MATLAB/Simulink was used to perform the simulation to obtain the operational parameters. The dynamic parameters are useful for the design of subsequent experiment of LHP.The thesis studied the LHP with flat evaporator by experiments, theoretical analyses and system simulation and researched the start-up and power cycle performance, the temperature oscillation, and the effects of charging ratio, the tilt angle and property of working fluid, and so on. The experimental and theoretical research is the fundament of developing the LHP with flat evaporator.
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