| Loop heat pipe(LHP)is an efficient heat management device that uses the capillary core in the evaporation chamber as the driving force for system operation and transports heat using the phase change of the working mass.It has a broad application prospect in the fields of power electronics,aerospace and electric vehicles due to its advantages of strong heat transfer capability,uniform heat dissipation,high efficiency and stability,flexible arrangement and no moving parts.This paper firstly describes the current research status of loop heat pipe and discusses its advantages as a heat transfer device in high heat flow density sites;introduces the components of loop heat pipe and its working principle,and designs a new stainless steel-anhydrous ethanol small disc type loop heat pipe experimental system: the disc shape fits more closely to the radiator parts to reduce heat transfer thermal resistance;sets the steam channel to reduce steam flow resistance;the upper cover plate is set in a stepped shape The upper cover plate is set in a stepped form to help fix the relative position of the capillary core and reduce the heat leakage caused by the poor contact between the capillary core and the evaporator channel.Secondly,the experimental platform is built for the designed loop heat pipe system;the stainless steel wire mesh is used as the capillary core,and the experimental study is conducted.In the experiments,the loop heat pipe performance was affected by the high thermal resistance of the loop heat pipe due to the irregular shape of the stainless steel wire mesh and the existence of gaps between the sheets.To solve this problem,a nickel powder double-aperture capillary core was prepared,and the porosity,permeability,effective pore size and effective thermal conductivity of the capillary core were characterized and calculated;the start-up and variable load performance of the loop heat pipe with double-aperture nickel core was tested.Finally,a solution is proposed for the thermal leakage of the dual-aperture nickel-core loop heat pipe: a low thermal conductivity capillary core is used in the upper layer near the compensation chamber,and a dual-aperture nickel core is used in the lower layer to form a functional gradient capillary core.Experiments on the heat transfer characteristics of the functional gradient capillary core loop heat pipe operation were completed.The performance of different capillary cores was compared in terms of start-up time,temperature difference between evaporator and compensation chamber and thermal resistance of the loop heat pipe.The following conclusions were obtained from the experiments.(1)The non-condensable gas in the air directly determines whether the loop heat pipe can be started successfully,and the vacuum level of the loop heat pipe system should be ensured before the experiment.The initial state affects the start-up and operation of the loop heat pipe.When the steam channel is filled with liquid work and the compensation chamber is filled with gas,the loop heat pipe starts with difficulty,backflow or even fails.Successful loop heat pipe start-up can be roughly divided into two forms:(1)Fast stabilization,reaching stable operation,the temperature measurement point temperature is basically constant;(2)Loop heat pipe did not stabilize,each temperature point has regular fluctuations in a cycle.(2)With the increase of heat load,the start-up time of small disc type loop heat pipe was shortened accordingly.In the case of low power,part of the evaporator acts as a condenser;the larger the filling rate of the system,the longer the start-up time;low heat sink temperature can improve the start-up performance of the loop heat pipe,but it has little effect on the overall system operating temperature.The disc type loop heat pipe has good self-adaptive ability and thermal shock resistance,and the system can respond quickly and operate stably under variable load conditions.Factors such as filling rate and heating power affect the thermal resistance of the loop heat pipe.The thermal resistance of the loop heat pipe is inversely proportional to the power,and the thermal resistance decreases with the increase of thermal load;under the condition of low power,the filling rate has a greater influence on the thermal resistance of the system,and the influence is smaller under high power.However,the thermal resistance of the system under large filling rate is higher than that under small filling rate.(3)loop heat pipe temperature fluctuations can be divided into the following three categories:(1)tiny fluctuations(temperature fluctuations are small,the frequency of temperature fluctuations is fast)(2)small fluctuations(temperature fluctuations in about 2-3℃,temperature fluctuations in the cycle of about a few minutes)(3)large fluctuations(temperature fluctuations in more than 10℃,the cycle of more than ten minutes).Temperature fluctuation phenomenon is a sub-stable start phenomenon.Its not only related to the heating load,but also and the rate of work filling and start the initial position and other factors have some correlation.Under low load steam heating compensator bubble generation leads to compensation chamber and other locations of the temperature rise,the return fluid cooling leads to compensation chamber and other locations of the temperature drop.As the thermal load increases,there are fewer and fewer bubbles in the compensation chamber.When the thermal load reaches a certain size,the compensation chamber is completely occupied by the liquid,the bubbles disappear and the temperature fluctuations disappear;at the same starting power and heat sink temperature,the large filling rate leads to significant temperature fluctuations.(4)The functional gradient capillary core can effectively improve the start-up and operation characteristics of the loop heat pipe,and the functional gradient capillary core can effectively reduce the loop heat pipe start-up phase time and thermal leakage during operation. |
