Numerical Research Of Several Effects On Pool Boiling Heat Transfter With Laticce Boltzmann Method

Pool boiling heat transfer is an effective heat transfer mode in many engineering applications.With the increasing cooling requrements of electronic devices,heat transfer enhancement of pool boiling has drawn more and more attention.Pool boiling mechanism is very complicated and affected by many factors and parameters,therefore,it is more effective to investigate this phase change phenomannon and heat transfer mechiasm with multi-scale simulation method.The Shan-Chen(S-C)mutil-phase lattice Boltzmann method(LBM)with phase change has some advantages on simulating the liquid-vapor phase change process including boiling,condensation and evaperation.In this thesis,the Gong-Cheng phase-change LB model for single component is improved and a new mutil-component mutil-phase(MCMP)phase-change LB model is proposed to simulate pool boiling process.Entire boiling curves with effects of subcooling,heater surface wettability and size,heating modes,saturated temperature as well as the non-condensable gas are present with this numerical method.The detailed contents are given as follows:Improvement and development of the phase-change LB model.A half-implicit scheme is proposed based on the traditional explicit scheme,which can increase the convergence speed of the caluculation for the two-phase flow.The source term in the energy equation is re-deduced to be preciser.Based on Gong-Cheng's single-component phase change model,a novel multi-component/multi-phase LBM with vapor-liquid phasechange heat transfer is proposed to simulate the liquid-vapor phase change in the presence of another gas component.Saturated pool boiling research.The pool boiling phenomenon and curves under saturated state are studied with the consideration including heater surface wettability,heater size,heating modes and saturated temperature.And boliling hysteresis at transition boiling regime is also studied.Heater size effects shows relatively small in the nucleate boiling regime but large in transition boiling and film boiling regimes.Its effects on saturated boiling curves can be divided into three regimes depending on the dimensionless heater size.The difference between the controlled heat flux and wall temperature focuses on the transition boiling regime.In the cooling process,boiling hysteresis exists on both hydrophilic and hydrophobic conditions under controlled wall heat flux conditions.Under the controlled wall temperature conditions,no boiling hysteresis exists on a hydrophilic surface.For different saturated temperature,the LB simulation results matches well with the Rohsenow's correlation equation in the nucleate boiling regime.Obtained CHF appears higher than Kandlikar's model on the hydrophobic surfaces.Subcooled pool boiling and Marangoni effect.With the phase change model,pool boiling under subcooled state is studied.In the subcooled pool boiling,bubble's departure diameter decreases and nucleate release period increases.For the boiling curve,subcooling increases the heat transfer coefficient in the natural convection and incipient nucleate boiling regimes,as well as the CHF.However,it has little influence on the ONB temperature,the fully-developed nucleate boiling and film boiling regimes.Marangoni effect on a single bubble and heat transfer under zero gravity and macrogravity are also studied.It is shown that the floating vapor bubble moves towards the hot wall because of the Marangoni effect and finally in contact with the hot wall at equilibrium.With the increasing of Marangoni number,the heat transfer coefficient also increases.Pool boiling and condensation in the presence of non-condensable gas.With the mutilcomponent multil-phase lattice Boltzmann method with phase change,pool boiling process in the presence of non-condensable gas is preliminarily studied.And to validate the phase change model,the case of convection condensation on a subcooled horizontal plate is simulated and compared with analytical results.It is shown that the exist of non-condensable gas has effects on the bubble's nucleation and departure process,and improves the heat transfer in the early regime of nucleate boiling.However,it affects little in the fully developed nucleate regime and the CHF.With the increase of inlet gas fraction,the condensation liquid film on the subcooled plate becomes thinner,and the interfacial temperature decreases.Their fitted relationship compares well with the analytical model by Sparrow,which validates the correctness and accuracy of the newly developed multicomponent/multi-phase vapor/liquid phase-change lattice Boltzmann model.Film evaperation and boiling on a microstructured surface.With the SCMP LB model,the heat and mass transfer coefficients are calculated to compare with the classical Kucherov-Rikenglaz equation and shows good agreement.The Gong-Cheng phase change model is used to simulate dynamic evaporation along the vapor/liquid interface and boiling in the liquid thin film on a microstructured surface consisting of micro pillars.It shows that with the increasing of the heat flux applied on the heater base,the liquid film recedes gradually into the wick structure.Fixed and periodical bubble nucleation appears sequentially before dry-out heat flux on the stronger hydrophilic surfaces.Considerable increase of effective heat transfer coefficient can be obtained because the receding of liquid film can sustain a higher CHF.