Numerical Study On Effects Of Geometry And Multisource Heat Input On Flow And Heat Transfer In Single Closed-loop Pulsating Heat Pipe

The expectation of sustainable development of energy and environment has widely attracted attention,and the high-efficiency heat device is greatly needed.Due to its obvious advantages-simple structure,low fabrication cost and excellent heat transfer capability,as a new heat transfer device,closed-loop pulsating heat pipes have a bright prospect for solar water heater and cooling of electronic devices with high heat flux.In this thesis,single closed-loop pulsating heat pipes(CLPHP)with two ends in different shapes,round or right angled in shape,were proposed on the basis of the study of ordinary equal diameter pulsating heat pipe.The VOF model was applied to simulate gas-liquid two phase flow while the surface tension model(CSF)was used to consider the interface force.The two equation turbulence model was applied to consider the turbulence effect,and the user-defined function method(UDF)was introduced to consider the heat and mass transfer caused by phase change.The gas-liquid two phase behavior and its flow characteristics in all kinds of pulsating heat pipes are analyzed by using the gas volume distribution image of the working fluid.The effect of the different shapes on the starting time and the oscillation of the pulsating heat pipe also was analyzed according to the heat fluxes oscillation curve of the evaporation and condensation section of the all kinds of pulsating heat pipes.And the fast Fourier transform(FFT)technique was used to analyze the heat fluxes oscillation.The effect of multisource heat input on the heat fluxes oscillation of four kinds of pulsating heat pipe was also obtained,and the two phase behavior and heat transfer of all kinds of pulsating heat pipe were compared and analyzed.The simulation results show that the right-angled elbow of pulsating heat pipe has a great influence on the flow characteristics,such as the flow direction switch and the flow deformation.Due to condensation,the 'local flow direction switch' and 'stop-over'are caused by long liquid slug at the evaporation section inside CLPHP.In the process of bubble combination,there exists several flow deformations,and a thin liquid film is formed on the right wall of condensation section.Compared with round-angled bend,the closed-loop pulsating heat pipe with right-angled elbow has shorter start-up time and higher heat transfer coefficient of 1477W/(m2·K).For all of these geometries,the CLPHP shares the same peak frequency of 0.381 Hz in the evaporation end.Due to the influence of bubble combination and size change,the characteristic frequency of heat fluxes is changeable for condensation end,indicating that the structure of condensation end has a more significant influence on its flow patterns.Due to the multisource input of heat,the 'local flow direction switch flow' and 'stop-over' would rarely be seen inside CLPHPs with round-angled bend at evaporation section,implicating that the uniformity and fluctuation of heat transfer is enhanced.