Three-dimensional numerical investigation of film boiling by the lattice Boltzmann method

A three-dimensional lattice Boltzmann model is presented to simulate the film-boiling phenomenon. Single- and multimode film boilings are investigated. The flow and temperature fields around the vapor phase are obtained for various Jakob numbers. Furthermore, the effects of Jakob number on the Nusselt number and vapor tip velocity are investigated. The results show that on increasing the Jakob number, the bubble tip velocity increases while the Nusselt number decreases. Furthermore, it is found that in multimode film boiling, the peak and trough values of the local Nusselt number happen at the bubble position and the gap valleys between adjacent bubbles, respectively.     

The effects of bubble translation on vapor bubble growth in a superheated liquid

A theoretical analysis of vapor bubble growth in a uniformly superheated liquid has been carried out to determine the effects of translational motion of the bubble on the bubble growth rate. Assuming potential flow in the region surrounding the bubble the appropriate convective diffusion equation is solved by means of a new similarity transformation. The results of the theoretical analysis are compared with available experimental data and with analyses of the limiting cases of no bubble translation and quasi steady state bubble growth. The analysis is shown to reduce to the Plesset and Zwick or Scriven analysis for stationary growing bubbles. The effects of translation are found to be significant when the translational velocity is sufficiently high at moderate Jakob numbers, but for high Jakob numbers radial convection predominates and translation has little effect on the growth rates. The analysis predicts results in good agreement with experimental data available in the literature.     

Prediction of departure diameter and bubble frequency in nucleate boiling in uniformly superheated liquids

A theory is presented to describe the motion of vapour bubbles growing at a nucleation site in a uniformly superheated liquid. By incorporating the classical theory for spherical phase growth, the equations of translatory motion were solved, enabling the radius and position of the bubble to be calculated as a function of time.     

A thermal immiscible multiphase flow simulation by lattice Boltzmann method

The lattice Boltzmann (LB) method, as a mesoscopic approach based on the kinetic theory, has been significantly developed and applied in a variety of fields in the recent decades. Among all the LB community members, the pseudopotential LB plays an increasingly important role in multiphase flow and phase change problems simulation. The thermal immiscible multiphase flow simulation using pseudopotential LB method is studied in this work. The results show that it is difficult to achieve multi-bubble/droplet coexistence due to the unphysical mass transfer phenomenon of “the big eat the small” – the small bubbles/droplets disappear and the big ones getting bigger before a physical coalescence when using an internal energy based temperature equation for single-component multiphase (SCMP) pseudopotential models. In addition, this unphysical effect can be effectively impeded by coupling an entropy-based temperature field, and the influence on density fields with different energy equations are discussed. The findings are identified and reported in this paper for the first time. This work gives a significant inspiration for solving the unphysical mass transfer problem, which determines whether the SCMP LB model can be used for multi-bubble/droplet systems.     

On the bubble departure diameter and release frequency based on numerical simulation results

Heterogeneous boiling on a horizontal plate in stagnant and slowly flowing fluid is simulated using the lattice Boltzmann approach. The bubble departure diameter and release frequency are determined from the simulation results. It is found that the bubble departure diameter is proportional to g^?1/2 in a stagnant fluid and the release frequency scales with g^3/4, where g is the gravitational acceleration. Simulation results show no dependence between the bubble departure diameter and the static contact angle, but the bubble release frequency increases exponentially with the latter. Considering forced boiling, exponential relation is observed between the bubble departure diameter and the flow driving pressure gradient.     

Numerical investigation of low Prandtl number effect on mixed convection in a horizontal channel by the lattice Boltzmann method

The main purpose of this study is to numerically investigate the Prandtl number effect on mixed convection in a horizontal channel heated from below using the thermal lattice Boltzmann method (TLBM). The double-population model with two different lattices is used, in particular, the D2Q9 for the velocity field and D2Q5 for the thermal field. The developed lattice Boltzmann method code to simulate the fluid flow and heat transfer in the channel was validated with available literature results based on classical numerical methods, especially the finite volume method for Pr = 6.4 and the finite difference method for Pr = 0.667. The results obtained with the TLBM have shown good agreement with the conventional methods cited. The dynamic and thermal characteristics of the fluid flow were examined in the field of low Prandtl number, such that 0.05 ≤ Pr ≤ 0.667, and also compared to Pr = 6.4; for Ra = 2420 and 7400, the Reynolds number was fixed at 1. The results showed that the influence is relatively significant for the dynamic structure of flow convection for Pr ≤ 0.3 and is little influential beyond this value.     

低雷诺数下翼型绕流的格子Boltzmann方法数值模拟

文章采用混合格子Boltzmann方法模拟NACA0012翼型流场分离,该方法是将标准格子Boltzmann方法与非结构化有限体积方程相结合的一种方法。首先,分析不同网格分辨率下的计算精度;然后,分析了在雷诺数等于103的情况下不同攻角下翼型的气动特性;最后,计算了不同雷诺数下攻角为0°时的翼型流场。结果证明,混合格子Boltzmann方法在固体壁面有较高的计算精度,可以准确地评估翼型绕流流场。     

Numerical analysis on supercritical natural convection by lattice Boltzmann method

Abstract

One of the main factors affecting the heat transfer efficiency of solar collector is that the ordinary fluid in it is in the state of natural convection. Supercritical fluid is expected to improve the heat transfer efficiency of solar collectors due to its dramatic changes in thermal properties, so it is necessary to carry out the research of natural convection of supercritical fluid. Although researchers have made lots of related experimental investigations, heat transfer mechanism of supercritical natural convection is still unclear. In order to clarify its heat transfer mechanism, this article conducts a numerical analysis for supercritical natural convection applying lattice Boltzmann method, which has been proved to be valid and convenient. Considering the influences of temperature difference and pressure on natural convective heat transfer of supercritical fluid are seldom studied, the relevant researches are carried out in the article. The results imply that pressure of supercritical fluid in solar collectors should be less than 11?MPa so that high heat transfer performance can be obtained. Finally, the correlations of average Nusselt number, Rayleigh number, and pressure are fitted for the convenience of heat transfer calculation.     

Numerical simulation of compressible flows by lattice Boltzmann method

In this article, we propose a numerical framework based on multiple relaxation time lattice Boltzmann (LB) model and novel discretization techniques for simulating compressible flows. Highly efficient finite difference lattice Boltzmann methods are employed to simulate one- and two-dimensional compressible flows. These numerical techniques are applied on the single- and multiple-relaxation-time on the 16-discrete-velocity (Kataoka and Tsutahara, Phys. Rev. E, 69(5):056702, 2004) compressible lattice Boltzmann model. The Boltzmann equation is discretized via modified Lax-Wendroff and modified total variation diminishing schemes which have ability to damps oscillations at discontinuities, effectively. The results of compressible models are compared and validated with the well-known inviscid compressible flow benchmark test cases, so called Riemann problems. The proposed method shows its superiority over available techniques when compared to the analytical solutions. It is then used to solve two-dimensional inviscid compressible flow benchmarks, including regular shock reflection and Richtmyer–Meshkov instability problems to ensure its applicability for more complex problems. It is found that, the applied discretization techniques improve the stability of original LB models and enhance the robustness of compressible flow problems by preventing the formation of oscillation.     

Numerical simulation of a 2D electrothermal pump by lattice Boltzmann method on GPU

ABSTRACT

Electrothermal flow in a microfluidic system is a fast-developing technology because of the advancement in micro-electro-mechanical systems. The motion is driven by the electrothermal force generated by the AC electric field and non-uniform temperature distribution inside the system. Electrothermal force can be explored for pumps in microfluidic systems. In this paper, the lattice Boltzmann method (LBM) is used to simulate a 2D electrothermal pump. As an alternative numerical method for fluid dynamics, LBM has many advantages compared with traditional CFD methods, such as its suitability for parallel computation. With its parallel characteristic, LBM is well fitted to the parallel hardware in graphic processor units (GPU). To save computational time in parametric studies, a CUDA code was developed for executing parallel computation. The comparison of computational time between CPU and GPU is presented to demonstrate the advantage of using GPU. The effects of the frequency, thermal boundary conditions, electrode size, and gap between electrodes on volumetric flow rate were investigated in this study. It was shown that LBM is an effective approach to studying 2D electrothermal pumps on a CUDA platform.     

Impact of PTFE distribution on the removal of liquid water from a PEMFC electrode by lattice Boltzmann method

It is believed by many that polymer electrolyte membrane fuel cells (PEMFCs) will have a widespread application since they offer important features such as low operating temperature, high power density, and easy scale up. However, operation of PEMFCs is faced with some technical challenges including water management which may lead to flooding of the electrodes. Treatment of the gas diffusion layer (GDL) with a highly hydrophobic material such as PTFE is a common strategy for mitigating this issue. Several investigations have been done to clarify solely the effect of PTFE content. However, effects of PTFE distribution, which can be achieved through different treatment methods, has not been well studied yet. Lattice Boltzmann method (LBM) is one of the best choices for such numerical studies due to its capability of modeling multiphase flow in the complicated microstructure of a porous GDL considering its non-homogeneous and anisotropic transport properties. In the present study, droplet removal from four GDLs with different PTFE distributions through an interdigitated flow field is investigated using LBM. The results demonstrate that regardless of PTFE distribution, the interfacial forces between any untreated carbon fiber and a water droplet will strongly dominate over other forces and hence will prevent its removal. Therefore, it is concluded that an effective water management may be achieved by a suitable treatment method such that no carbon fiber inside the GDL remains uncoated.     

Pore-scale simulations on relative permeabilities of porous media by lattice Boltzmann method

Pore-scale simulations of two phase flows in a packed-sphere bed and in a carbon paper gas diffusion layer (GDL) are carried out using the free energy multiphase lattice Boltzmann method (LBM). The simulations are performed based on the detailed microstructure of the porous media under periodic boundary conditions such that the average phase saturations in the porous medium remain constant. A comparison of the simulated and measured relative permeabilities for the packed sphere bed as a function of non-wetting phase saturation is performed, and effects of the wettability and the anisotropic characteristics of relative permeabilities of the GDL are investigated.     

A numerical study of fluid flow passes two heated/cooled square cylinders in a tandem arrangement via lattice Boltzmann method

In this paper, the fluid flow pass two heated/cooled square cylinders in a tandem arrangement is simulated via the Multiple-Relaxation-Time lattice Boltzmann method. The distance between the upstream and downstream cylinder varies from the rear of the upstream one to 5 times of the cylinder width. The numerical experiments are done under different Richardson numbers (Ri, represents the effect of the buoyancy force) for two typical Re = 100, 60. The buoyancy effect on the flow and heat transfer around the two cylinders is mainly investigated. As is shown, if the force is in the same direction of incoming flow, the vortex street is always suppressed and no critical spacing seems to exist. However, if the force is in the opposite direction of the incoming flow, the vortex street can always be generated and the critical spacing always seems to exist. Correspondingly, the heat transfer around the cylinders measured by the Nusselt number on the surfaces of the two cylinders also shows different characteristics for various Ri s.     

Natural convection in a nanofluid-filled eccentric annulus with constant heat flux wall: A lattice Boltzmann study with immersed boundary method

A numerical investigation of natural convection in a Cu–water nanofluid-filled eccentric annulus with constant heat flux wall is presented. The governing equations of the flow and temperature fields are solved by lattice Boltzmann method (LBM), and the Dirichlet and Neumann boundary conditions are treated using the immersed boundary method (IBM). Influences of the Rayleigh number (10³ ≤ Ra ≤ 10⁷), eccentricity (ε = −0.625,0 and 0.625), nanoparticles volume fraction (0 ≤ ϕ ≤ 0.03) and radial ratio (rr = 2.33,2.6 and 3) on the streamlines, isotherms and Nusselt number are studied. It is found that the inclusion of the nanoparticles into pure fluid changes the flow pattern. And the Nusselt number has a positive relationship with nanoparticle volume fraction, Rayleigh number and radial ratio. Also, it can be confirmed that Nusselt number in the case with negative eccentricity (ε = −0.625) is larger than the others.     

Three-dimensional numerical simulation of solid oxide fuel cell cathode based on lattice Boltzmann method with sub-grid scale models

In the present study, three-dimensional numerical simulations are carried out to predict the electrochemical characteristics of solid oxide fuel cell (SOFC) cathodes based on a coupled method using sub-grid scale (SGS) model in a lattice Boltzmann method (LBM). Lattice Boltzmann method is used to solve the governing equations. In each coarse computational grid, local gas diffusivities, ion and electron conductivities are obtained by directly solving diffusion equations using the original fine sub-grid scale information. Two types of SGS models are introduced in this study, i.e. isotropic and anisotropic local diffusivities and conductivities. Proposed SGS local diffusivities and conductivities can maintain detailed information of the original fine microstructure even when coarser grid is used in the calculation. In the anisotropic SGS model, weighted summation of particle distribution function is applied in the LBM to maintain the invariances of local concentrations and potentials. From the tortuosity factor and overpotential calculation results, it is concluded that the proposed SGS models can drastically improve the computational accuracy without costing additional calculation time. Moreover, SGS model considering geometric anisotropies inside the calculation grid presents more precise results than the isotropic SGS model.     

A thermodynamic analysis for heterogeneous boiling nucleation on a superheated wall

A thermodynamic model based on Gibbs free energy and availability is developed for onset of heterogeneous nucleation on heated surfaces with different wettabilities in pool boiling. Different from classical nucleation theory, this model takes into consideration the temperature gradient in the superheated liquid layer adjacent to the wall as well as the contact angle between the liquid and the wall. Using Gibbs free energy equilibrium condition, a closed form solution is obtained on the critical radius for onset of heterogeneous boiling nucleation on walls with different wettabilities. Effects of contact angles and wall temperatures on the critical radius, the wall temperature gradient of the superheated liquid layer and the heat flux at onset of heterogeneous nucleate boiling are illustrated. These effects on the change of availability during the heterogeneous nucleation process, representing the energy barrier for the occurrence of the first-order phase transition, are also discussed.     

基于LBM多孔介质复合腔体交界面热滑移效应的研究

本文采用格子Boltzmann方法对真实多孔介质复合腔体内的对流换热进行研究,分析了不同Ra数、多孔介质高度Y和厚度δ条件下交界面处的热滑移效应,并确定热滑移系数。利用X-CT技术对真实多孔介质材料进行断层扫描,获得实际材料内部结构图片,并进行图片处理,再导入格子Boltzmann模型中进行求解。计算结果表明：等效热滑移系数随高度Y的影响较大,靠近壁面或固体表面的系数偏大,而间隙处的系数偏小,但两处各自的值基本相同;Ra数和厚度δ的变化对等效热滑移系数的作用较小。     

Simulation of droplets impact on curved surfaces with lattice Boltzmann method

Several dimensionless parameters are studied to describe their effects on the deformation of a droplet after impact on a 2D round surface by using lattice Boltzmann implementation of pseudo-potential model. Four typical deformation process can be found: moving, spreading, nucleating and falling. In addition, in some special cases, part splashing is involved. It is observed that impact velocity of droplet has a significant influence on the droplet impacting dynamics. With the increasing of the impact velocity, different states have been found during the process. Moreover, when the surface is hydrophobic, splash occurs.