Study On The Operating Characteristics Of Pulsating Heat Pipes Under The Condition Of Non-uniform Heat Flux

With the advantages of simple installation,convenient installation and high heat transfer coefficient,pulsating heat pipe has been widely concerned by scientists in solving the problem of high heat flux of electronic devices.Due to the complex heat dissipation environment of electronic devices,the problem of uneven heat flux distribution in different elbows of pulsating heat pipe is existed,which will lead to the degradation of heat transfer performance.The performances of pulsating heat pipe under uniform heat flux with different operating parameters are extensively focused and studied.But the discussions of non-uniform heat flux are insufficient.To further explore the influence of different operating parameters on the heat transfer characteristics of PHP under the condition of non-uniform heat flux,broaden the application scene of PHP,and provides theoretical support for the selection of PHP in practical application,the mainly research contents are conducted:(1)The experimental platform of pulsating heat pipe under the condition of non-uniform heating is designed and built.The pulsating heat pipe module,heating module,cooling water circulation module,data measurement and acquisition module,and filling and vacuum working medium module are mainly consisted.In order to ensure the reliability of the experimental results,the stability test is carried out.(2)Under the conditions of 50% and 70% liquid filling rate,the working mediums of ethanol and HFE-7100 are selected.The performance of heat transfer,evaporation temperature and internal pressure fluctuation characteristics are studied by varying the heating power from 40 to 140 W and the dimensionless heat difference from 0 to 0.56.The installation angles of the experimental device are 45°,70°,90° and D90°,respectively.The results show that the HFE-7100 has lower operating thermal resistance at low heating power under uniform heat flux.With the increase of heating power,the thermal resistances of the two refrigerants at50% and 70% filling rate are gradually close.The influence of the installation angle of devices on ethanol is significant than that of HFE-7100 under the low heating power.(3)Alone with the raise of dimensionless heat difference,the operating thermal resistance of the pulsating heat pipe is lower or close to the uniform heat flux at the non-uniform heat flux.And the evaporation temperature of the pulsating heat pipe with high liquid filling rate has better stability during start-up and operation.The influence of installation angle on the thermal resistance of PHP is less than that of uniform heat flux.A critical value of dimensionless heat difference(0.33)for pulsating heat pipe with 70% liquid filling rate of non-uniform heat flux is exist.When the value exceeds 0.33,the vapor-liquid working fluid at different heat flux sides can break the equilibrium rapidly,improve the circulating flow state,and reduce the operating thermal resistance.(4)At the term of uniform and non-uniform heat flux,the ethanol with 70% filling rate is the research object,the numerical simulation is carried out to investigate the internal flow characteristics of the pulsating heat pipe.Then the simulation results are compared to the experimental data.There are long liquid plugs in the pipe at high filling rate when the working fluid is heated with uniform heat flux,which is easy to cause stagnation of working fluid flow.For the lots of high-temperature steam generated by the high heat flux side in the state of non-uniform heat flux,the moving direction of working fluid has consistency.This situation reduces the stagnation and reversal in the internal fluid flow,decreases the operating thermal resistance of the pulsating heat pipe within a certain dimensionless range,and improves the heat transfer performance.