Numerical Simulation Research On Bubble Growing With Lattice Boltzmann Method Based On Free Energy Theory

Multiphase flow is a common phenomenon which plays an important role in our daily life and industries.Using numerical simulation methods to research multiphase flow,especially those where bubble movement is involved is a practical way to enhance our understanding of multiphase flow and heat transfer in the boiling process.In this thesis,a multiphase Lattice Boltzmann model proposed by Lee and Lin is analyzed in order to solve the local mass non-conservation problem in the model.An improper definition of pressure distribution function in the model proposed by Luz is pointed out.Based on the analysis,a modified model with a well-defined pressure distribution function to include the gravity directly is proposed.Coupling with energy function,numerical simulations were carried out for stable bubble,bubble coalescence,bubble rising as well as bubble growth on a heated surface using the modified model.The main conclusions are as follows.The influence of the local mass non-conservation in Lee's model is alleviated in this thesis by expanding the computing area and reducing the ratio between the interface area and total computing area.Results of bubble coalescence simulation indicate that total free energy of the system decreases during the coalescence process.Surface tension has influences on the speed of free energy decreasing.With a larger surface tension,the free energy decreases faster.When the surface tension becomes too large,the model may lose its stability.An analysis of Luz's multiphase Lattice Boltzmann model with gravity considered directly was carried out.Results suggest that this model has an error speed in the bulk phase area when gravity was considered due to the improper definition of the pressure distribution function.The pressureintroduced by Lee and Lin was originally considered to reduce the spurious currents.However,based on our analysis,should also include the influence of gravity,which is the reason of the improper definition in Luz's work.According to our analysis,the model was modified by introducing a properly defined pressure distribution function.Certification of the model was carried out by simulating the stable flat interface.Results show that the modified model can establish the pressure gradient along the direction of gravity in the bulk phase area.The modified model was used to simulate the unhindered bubble rising process.Bubble shape and bubble terminal speed agree well with the experimental data given in articles.Additionally,the boundary condition of contact angle was derived.The model with the boundary condition can deal with the contact angle less than 90~o.The included angle between the counter line of=.and the wall should be measured as the contact angle in the numerical result.Finally,using the gravity model,coupled with energy evolution function and contact angle boundary condition,simulations of bubble growth on a heated surface was carried out.Result shows a well agreed process of bubble growing with the experimental data.During the bubble growing process,temperature at the bottom of the bubble is higher than that at the top of the bubble.After bubble departure,bubble will rise unhindered in the fluid,bringing high temperature fluid near the heated surface to the low temperature fluid area.The temperature distribution in the heated surface indicates that during the bubble growing process,heat transfer between the heated surface and the fluid is stronger than that between the surface and the gas in the bubble.Energy needed in the bubble growing process was mainly supplied by the superheated fluid around it.