Numerical Simulation Of Two-phase Flows About Droplets And Liquid Film By Using Lattice Boltzmann Method

Lattice Boltzmann Equation method is a kinetic theory based CFDmethod to simulate fluid flows. It has advantages in many aspects incomparison with conventional method, for example: a clear physicalbackground, simple to hand boundary condtions, excellent paralleism andso on. Specifically, lattice Boltzmann method has incomparableadvantages in handling liquid vapor two-phase flows, and it is nowbecoming a hot area of research all over the world.Falling liquid flows and droplet splashing onto liquid film arecommon flow phenomena inside the saturator, which is regareded as akey device in the humid air turbine (HAT) cycle. These flowphenomenona often result in complex interface deformation and energytransport, like deposition, splash and rebound. Futher understanding ofthese fundamental flow phenomena share value in mainly three aspects:1)design optimization and efficiency enhancement;2) A deeperunderstanding of the flow mechanisms with surface flow will further ourknowledge about more complex two-phase flow problems;3) Thisresearch is a basis for further research of the heat and mass transfermechanisms along with these flow phenomenona.The work of this disseration comprises following three aspects:1) An additional term is introduced into the forcing term ofShan-Chen model to achieve the adjustment the surface tension force inthe Shan-Chen. Numerical tests were performed to validate the proposedsurface tension force adjustable Shan-Chen model. Based on theimproved Shan-Chen, falling liquid film flows on vertical wall underdifferent Reynolds numbers are simulated, test results showed Shan-Chenmodel is capable to simulate falling liquid film flows and that the flow morphology is in good qualitative agreement with experimental data.Furthermore, we investigated the influence of external disturbance andsurface tension to the falling liquid flow by imposing sinusoidal pulsatinginlet velocity and by adjusting the surface tension coefficient in the LBEmodel respectively.2) New difference schemes are proposed to enhance the numericalstability of a recent proposed two-phase lattice Boltzmann (LB) model.Based on the improved two-phase LB model, the impingement of a liquiddroplet onto a rest liquid film was simulated, simulation results matcheswell with the experiment data. Then, numerical simulations were carriedout for the splashing of a droplet onto a flowing liquid film. Differentvelocity ratios and relative film thickness were considered and discussed.In comparison with the simulation results of a droplet impact on a restliquid film and experimental data, the rule of radial expansion and heightevolution of the crown’s rim is obtained as a function of time.3) In this work, the OBC schemes are studied based on thetwo-phase LBE method without parasitic current. It is found that theexisting OBC schemes, i.e., the Neumann boundary condition (NBC), theconvective boundary condition (CBC), and the extrapolation boundarycondition (EBC), cannot be applied directly to the LBE models understudy. Thus additional assumptions are proposed in this work andperformances of proposed OBC schemes are extensively investigated.Numerical tests confirm that the proposed CBC schemes can give outrather satisfactory predictions within a wide range of density andviscosity ratios. And the proposed EBC scheme can work well when largevelocity gradient exists at the outflow boundary.