Study On The Characteristics Of R134a Flow Boiling In Power Type Heat Pipe

For the complex of two-phase flow boiling in pump-assisted separated heat pipe and the poor reproducibility of experiment, an experimental apparatus of pump-assisted separated heat pipe is built to study the pressure gradient and heat transfer coefficient in the progress of R134 a flow boiling in the horizontal tube and the results are compared with the values in the open literature. The experimental results clearly show that most pressure gradient predictions made by Muller-Steinhagen-Heck correlation are within an error band of ±10% of experimental values. Besides, for vapor quality higher than 0.1, Mohseni correlation predicts the experimental heat transfer coefficient in an error band of ±10%, while for lower than 0.1, some error is more than 30%. Therefore, a modification correlation is put forward.In order to study the boiling flow and heat transfer in a horizontal tube, the physical model is established and numerical simulation based on volume of fluid(VOF) was performed. Forthermore, the numerical results of pressure gradient and heat transfer coefficient are compared with correlations. The results reveal that when numerical simulations using the VOF in the Fluent software with a user-defined function(UDF) as an evaporation model(with Lee’s model) to study the process of saturated R134 a gas-liquid flow boiling in tubes, the value of r affected by the factors of working mediu and heat flux, influences the amount of vapor and liquid superheat, so it should be determined through experiment and numerical simulations.When using VOF model in the Fluent software simulating the heat transfer process of two-phase flow boiling, the simulated results are not exact and the windages can not be neglected for a single momentum equation solved throughout the domain and the resulting velocity field shared among the phases and the accuracy of the velocities in cases where large velocity differences exist between the phases computed near the interface adversely affected, so a modified VOF method which also reasonablely considers boiling heat transfer mechanism has been put forward. The modified VOF method can more accurately simulate the pressure gradient of gas-liquid two-phase flow boiling, and the calculation values of pressure gradient consist with MullerSteinhagen-Heck correlation estimated values in the error range of ±15%. Besides, the modified VOF method can more accurately simulate heat transfer coefficient in the process of R134 a gas-liquid two-phase flow boiling in horizontal tubes, and simulated heat transfer coefficient is in good agreement with the modified Mohseni correlation.The result can be used as reference standard for experimental study, numerical simulation and optimization design of heat pipe system.