Mathematical Model And Numerical Simulation Based On CFD Of Two-phase Flow In Heat Pipe

Because of the excellent heat transfer performance, heat pipes whose internal heat andmass transfer mechanism involves vapor-liquid two-phase flow, evaporation andcondensation, bubble flow, surface flow and other complex physical processes, are widelyused in many fields that contain heat exchange. Based on the entire CFD methods, the thesisstarted from a simple two-phase flow and went deeply into a comprehensive mathematicalmodel for the heat and mass transfer process inside thermosyphon. The main researchcontents and conclusions are as follows:(1) By using the VOF method to calculate free surface and CSF model to treat surfacetension, the numerical simulation of bubble motion in water was accomplished. The resultsshow that the bubble transforms and rotates during rising in the water, and its trajectorychanges from straight to sinusoidal with its velocity increasing, which coincide well with theexperimental observation. According to the Reynolds number and Weber number, two-phaseinterfacial tension must be considered in the simulation.(2) The process containing typical bubbles and two-phase flow with free surface wascalculated. The results show that in the absence of external force, it is the difference in liquidlevel between the inlet and outlet of the syphon drives the syphonic flow, and the basic causeof liquid rising against gravity which maintains syphonic process is the difference betweenatmospheric pressure and the negative pressure at the top of the syphon. A greater liquid leveldifference or a higher outside atmospheric pressure leads to a more obvious syphonic flow.(3) On the basis of the bubble flow and free surface flow model, the user-definedfunction (UDF) was programmed and supplemented to FLUENT to solve evaporation andcondensation source terms for the heat and mass transfer equations. The computation of thephase change process was completed to verify the feasibility of the mathematical modelpresented to simulate the evaporation and condensation processes.(4) The mathematical model of the thermosyphon, which has the characteristics of19mm outer diameter,17.5mm inner diameter, a400mm-long evaporator section, a200mm-long adiabatic section and a400mm-long condenser section, was presented to simulatethe heat and mass transfer therein. The simulation results were compared with the experimental ones and the relative error of the average simulated wall temperatures of eachsection was less than8%. The experiments find out that the optimum heat transferperformance of thermosyphon is achieved at the filling ratio of0.5and the input heat powerof500W. The analyses carried out according to the simulation results indicate that increasingthe filling ratio would give two opposite effects: on the one hand, increasing fluid volume andthe solid-liquid heat transfer area promotes the boiling heat transfer; on the other hand, if thefilling ratio was too high, the vapor film adjacent to the inner wall surface would decrease theheat transfer efficiency, and the inner pressure would rise excessively to hinder boiling heattransfer. Meanwhile the CFD simulations clarify that the wall temperature rise at the top ofthe condenser section is due to the returning of the high temperature vapor and the thinnerliquid film on the inner wall surface.