Simulation On Characteristics Of Flow And Heat Transfer Of Pulsating Heat Pipe With Hydrogen

The pulsating heat pipe(PHP)is an efficient heat transfer element,which has the advantages of small heat transfer temperature difference,high heat transfer efficiency,simple structure,strong adaptability.It can be used in the cryogenic fields,such as refrigerator cooling,superconductivity,aerospace and so on.At present,the gas-liquid two-phase flow of the working fluid in the cryogenic PHP and its heat transfer mechanism have not been fully revealed.As visualization experiment is difficult to be carried out,the analysis of numerical simulation is helpful to understand the heat transfer characteristics of PHPs at cryogenic temperature range.In this study,the following two parts are considered: the interaction between flow and heat transfer in the PHP and the change of the gas-liquid interface caused by mass and heat transfer.Based on that,a two-dimensional model is established for a singleloop PHP with hydrogen.The VOF method and CSF model were used to track the gas-liquid interface and simulate the surface tension,respectively.Constant heat flux and constant temperature were used as the boundary conditions of the evaporation and the condensation section,respectively.The following three aspects are mainly performed in this study:(1)The flow and heat transfer characteristics of the PHP with filling ratio of 80% in different stages were analyzed.The behaviors of bubble generation,vapor plugs expansion and contraction during the PHP start-up stage were analyzed.The results show that the gravity plays an important role in the initial stage.The start-up of the PHP is guaranteed by the pressure drop between the evaporation and the condensation section and related to the pressure change in the pipe.In this study,there are three flow modes of the working fluid during the start-up at the filling ratio of 80%.(2)The flow pattern of the working fluid in the tube,and the relationship between the speed fluctuation and the temperature fluctuation in the steady stage were analyzed,as well as the flow and heat transfer mechanism of the PHP.Meanwhile,the effects of heating power(0.27 W,0.5W,0.75W?1W and 1.25W)and filling ratio(40%,50%,60%,70% and 80%)on the heat transfer performance of the PHP were analyzed.The thermal resistances of the PHP under different operating conditions were discussed.During the steady stage,the wall temperature of the evaporation section oscillates periodically,and the frequency of its fluctuation is same as that of the velocity fluctuation.The flow patterns of the working fluid vary with filling ratio.The flow pattern is mainly oscillation flow with the filling ratios,which are less than 80%,while it is circulation flow at the filling ratio of 80%.The thermal resistance of the PHP gradually decreases and then stabilizes with the heating power increasing.Under the same heating power,the thermal resistance of the PHP at the filling ratio of 40% is the largest.When the heating power is 0.75 W and 1W at the filling ratio of 80%,the thermal resistance reaches a minimum value of 2.13 K/W.(3)The experimental studies on a hydrogen PHP were performed by our research group.The experiments were briefly introduced in this study.The experimental results and the simulation results were compared.Under the condition with the filling ratio of 80%,the error between them is less than 15%.At the filling ratio of 60%,the errors are between 30% and 40%.