Multiphase Lattice Boltzmann Model Research Articles

Bubble dynamics and dry spot formation during boiling on a hierarchical structured surface: A lattice Boltzmann study

In this paper, the bubble dynamics and the mechanism of dry spot formation during boiling on a two-level hierarchical structured surface are numerically investigated using a three-dimensional thermal multiphase lattice Boltzmann model with liquid–vapor phase change. The hierarchical structured surface consists of three parts: a smooth surface basement, primary pillars on the basement, and secondary pillars overlaid on the primary pillars. It is found that the boiling heat transfer on the hierarchical structured surface is significantly dependent on the bubble departure frequency and the dry area fraction, which are in turn affected by the structural parameters of secondary pillars. Increasing the height or width of the secondary pillars is found to effectively increase the bubble departure frequency, but it may also enlarge the size of dry spots on the hierarchical structured surface. The numerical investigation shows that, in order to prevent the formation of dry spots on the hierarchical structured surface, an effective approach is to reduce the proportion of the contact line on the lateral walls of secondary pillars to the whole contact line, which can be realized by reducing the area of the lateral walls of secondary pillars or appropriately increasing the secondary pillar spacing. The optimum boiling performance on the hierarchical structured surface is found to be achieved under the situation that the bubble departure frequency is sufficiently high, but the dry spot area is as small as possible.

Enhancing dropwise condensation on downward-facing surfaces through the synergistic effects of surface structure and mixed wettability

In this paper, the condensation performance and the dynamic behavior of condensed droplets on a downward-facing structured surface with mixed wettability are numerically investigated using a thermal multiphase lattice Boltzmann model, with a focus being placed on exploring the enhancement mechanism of dropwise condensation on downward-facing structured surfaces. The numerical investigation shows that the downward-facing structured surface with mixed wettability exhibits much better condensation performance than those with homogeneous wettability owing to the synergistic effects of surface structure and mixed wettability, which increase the droplet departure frequency and prevent the flooding phenomenon. Furthermore, it is found that the dynamic behavior of condensed droplets on the downward-facing structured surface with mixed wettability can be divided into three stages, i.e., the nucleation-growth stage, the coalescence-slip stage, and the stick-departure stage. Particularly, there exists a competition between the time of the first stage and that of the third stage in terms of the contact angle of the pillar top (θtop). The former reduces but the latter increases with decreasing θtop, because the contact lines are always pinned at the edges of the pillar top during the third stage when θtop is small. An optimal θtop is therefore found, which provides the best droplet dripping rate by achieving a suitable balance between a large droplet departure volume and a relatively short condensation cycle time.

Effects of laser oscillation on fluid flow in weld pool and macrosegregation mitigation during laser welding of Al-Si coated press-hardened steels

For the mitigation of macrosegregations in the weld seams, an oscillating laser was applied in the laser welding of Al-Si coated press-handened steels (PHS) using a Ni foils as a interlayer. As captured by the high-speed camera, the stirring effect of the oscillating laser led to obvious fluctuations of the weld pool liquid, and the weld pool width increased with the oscillation diameter of the laser. The concentration distributions of elements Ni and Al were measured in the weld seams obtained from the laser linear and oscillationn welding. It is indicated that the macrosegregation was migitated through the laser oscillation welding with an increasing diameter. Numerical modeling of the weld pool fluid fields during laser welding was carried out by using a three-dimensional multiphase lattice Boltzmann model. The simulation results for the laser linear welding case showed that the velocity vectors with high magnitudes were symerically perpendicular to the keyhole boundary and pointed to the interior liquid phase in the weld pool. Thus, these forced convections transported the Ni and Al atoms from the weld pool center and surface to the lower part of the weld pool. In the laser oscillation welding case, however, the velocity vectors at the keyhole boundary were asymetrical as the keyhole moves, which were beneficial for efficiently mixing the weld pool liquid. The simulation results provided reasonable explanations for the different patterns of Ni and Al concentration distribuitons in the weld seams obtained from laser linear and oscillation welding.

Structure and isotropy of lattice pressure tensors for multirange potentials.

We systematically analyze the tensorial structure of the lattice pressure tensors for a class of multiphase lattice Boltzmann models (LBM) with multirange interactions. Due to lattice discrete effects, we show that the built-in isotropy properties of the lattice interaction forces are not necessarily mirrored in the corresponding lattice pressure tensor. This finding opens a different perspective for constructing forcing schemes, achieving the desired isotropy in the lattice pressure tensors via a suitable choice of multirange potentials. As an immediate application, the obtained LBM forcing schemes are tested via numerical simulations of nonideal equilibrium interfaces and are shown to yield weaker and less spatially extended spurious currents with respect to forcing schemes obtained by forcing isotropy requirements only. From a general perspective, the proposed analysis yields an approach for implementing forcing symmetries, never explored so far in the framework of the Shan-Chen method for LBM. We argue this will be beneficial for future studies of nonideal interfaces.

Multiphase lattice Boltzmann modeling of dielectrophoresis fractionation of soft particles

Dielectrophoresis-field flow fractionation (DEP-FFF) is a promising method of fractionating particles from a continuous flow and has considerable application potential in the fields of biomedical, chemical, and environmental engineering. Particle deformation is an important issue in DEP-FFF, having a critical influence on the fractionation accuracy and viability of bioparticles. However, this problem has been largely ignored in both theoretical and numerical investigations. In the present work, a hybrid lattice Boltzmann scheme is introduced to study the deformation of soft particles subjected to the coupled effects of hydrodynamics and electrokinetics in a DEP-FFF process. The interaction of the particles with the fluid medium is calculated using a multiphase lattice Boltzmann model. The dielectrophoretic effect on the flow is introduced through a DEP force, which is obtained from a finite-element solution of the electric field. The hybrid scheme avoids the need to solve a coupled multiphysics problem, making it very efficient. The proposed simulation framework is validated through a well-known model, and the particle deformation and its influence on DEP-based fractionation are discussed.

Nucleate boiling enhancement by structured surfaces with distributed wettability-modified regions: A lattice Boltzmann study

In this paper, we conceive a novel pillar-structured surface for enhancing nucleate boiling heat transfer, namely a pillar-structured surface with distributed wettability-modified regions on the top of each pillar. A three-dimensional thermal multiphase lattice Boltzmann model with liquid–vapor phase change is employed to investigate the boiling performance on the pillar-structured surface with distributed wettability-modified regions and the associated mechanism of nucleate boiling heat transfer enhancement. According to the distribution of the wettability-modified regions, the bubble dynamics on the newly conceived pillar-structured surface can be classified into three regimes, i.e., regimes I, II, and III, among which the regime II shows relatively better boiling performance than the other two regimes due to the synergistic effects of surface structure and mixed wettability. It is found that in the regime II the bubbles nucleated at the wettability-modified regions do not coalesce with each other, which therefore elongates the length of the triple-phase contact lines on the pillar top in comparison with the pillar-structured surface with a unified wettability-modified region. Meanwhile, in the regime II the bubbles on the pillar top receive a strong bubble-wake effect supplied by the bubbles generated at the bottom substrate, which shortens the bubble growth cycle and promotes the departure of the bubbles on the pillar top, and also reduces the area of dry spots on the pillar top. The influences of the pillar width and the width of the wettability-modified regions are also studied. It is shown that the best boiling performance is always achieved in the cases that fall into the regime II.

Effects of wettability on relative permeability of rough-walled fracture at pore-scale: A lattice Boltzmann analysis

Abstract Two-phase flow in fractured geo-material plays a significant role in various subsurface processes. In this study, the realistic issues regarding two-phase flow in rough-walled fractures (RWFs) with different surface wettability are initially investigated with an improved approach based on the free energy multiphase lattice Boltzmann model and a novel rough-walled discrete fracture network model. After validation with analytical solutions, the proposed model is applied to investigate the co-current flow of two immiscible fluids in RWFs at different surface wettability conditions, and the impact of contact angle and fracture aperture on fluid flow distribution, velocity field, and relative permeability curves are further clarified. For fluid initially distributed near the surface, its relative permeability increases monotonically with its saturation. For fluid flowing as the bulk phase, when its saturation is low, its relative permeability decreases with the decrease of its saturation. However, when its saturation is high (about 0.6–0.9), its relative permeability in RWFs exposes instabilities and could be lower than that in smoothed wall fractures, meaning additional flow resistance exists in RWFs. According to our simulation results and theoretical analysis, this phenomenon is mainly caused by the wettability-related microscale fluid distribution. In RWFs with small aperture, the fluid–solid surface interaction dominates, the distribution and flow pattern of fluid flowing near the surface alters the “apparent roughness” of the fracture and further impact the relative permeability curves.

Simulation of binary droplet collision with different angles based on a pseudopotential multiple-relaxation-time lattice Boltzmann model

Understanding the physics of droplet collisions is of crucial importance in many industrial processes such as inkjet printing, spray cooling, and spray combustion. In this paper, a modified multiple-relaxation-time pseudopotential multiphase lattice Boltzmann model is proposed to investigate binary droplet collision with different collision angles. The surface tension can be adjusted independently under the condition of high Reynolds number and large density ratio using this model. Then the model validation is performed by simulating three benchmark cases including stationary droplet, oscillating droplet, and capillary wave instability, respectively. Finally, different regimes of binary droplet collision are presented, and collision features such as liquid filament and satellite droplet are investigated at collision angles of 30°, 60°, 90°, 120°, and 150°. The results show that there is no liquid filament or satellite droplet at the collision angle of 30°, yet both of them appear as the angle increases. When the collision angle is 60°, the percentage of satellite droplet to liquid phase reaches 30%, while the proportions are less than 10% at collision angles of 90°, 120°, and 150°. Besides, two break-up mechanisms of “end-pinching” and capillary wave instability are inspected. It is found out that the back-flow phenomena at the joints of droplets and the liquid filament are caused by the “end-pinching” mechanism, which further leads to the capillary wave instability of the liquid filament.

Multi-phase lattice Boltzmann (LB) simulation for convective transport of nanofluids in porous structures with phase interactions

PurposeA combination of highly conductive porous media and nanofluids is an efficient way for improving thermal performance of relevant applications. For precisely predicting the flow and thermal transport of nanofluids in porous media, the purpose of this paper is to explore the inter-phase coupling numerical methods.Design/methodology/approachBased on the lattice Boltzmann (LB) method, this study combines the convective flow, non-equilibrium thermal transport and phase interactions of nanofluids in porous matrix and proposes a new multi-phase LB model. The micro-scale momentum and heat interactions are especially analyzed for nanoparticles, base fluid and solid matrix. A set of three-phase LB equations for the flow/thermal coupling of base fluid, nanoparticles and solid matrix is established.FindingsDistributions of nanoparticles, velocities for nanoparticles and the base fluid, temperatures for three phases and interaction forces are analyzed in detail. Influences of parameters on the nanofluid convection in the porous matrix are examined. Thermal resistance of nanofluid convective transport in porous structures are comprehensively discussed with the models of multi-phases. Results show that the Rayleigh number and the Darcy number have significant influences on the convective characteristics. The result with the three-phase model is mildly larger than that with the local thermal non-equilibrium model.Originality/valueThis paper first creates the multi-phase theoretical model for the complex coupling process of nanofluids in porous structures, which is useful for researchers and technicians in fields of thermal science and computational fluid dynamics.

Method of determining the cohesion and adhesion parameters in the Shan-Chen multicomponent multiphase lattice Boltzmann models

The cohesion and adhesion parameters are the two essential parameters in the Shan-Chen multicomponent and multiphase lattice Boltzmann models for determining the separation of fluids and the wettability of surfaces, and thus should be carefully chosen. In this study, we propose a systematic method of determining these parameters from a physical point of view. The cohesion parameters of different water/alkane systems are determined by matching the interfacial tensions, and the adhesion parameters between the water/alkane and the silicon dioxide are obtained by matching the contact angles as such that their physical meanings are preserved. Based on the linear relationship between the interfacial tension and the cohesion/adhesion forces, an improved equation for predicting the adhesion parameter is proposed and validated by comparing the simulated contact angles using the predicted adhesion parameters with experimental results. Further, the effect of cohesion and adhesion parameters on the advancing and receding angles of the oil slug in a capillary tube in the movement initialization process was also investigated.

Improved pseudopotential lattice Boltzmann model for liquid water transport inside gas diffusion layers

Liquid water transport inside the gas diffusion layers (GDLs) plays a vital role in water management of proton exchange membrane fuel cells (PEMFCs). In this study, an improved pseudopotential multiphase lattice Boltzmann model is firstly developed to realize the actual density and viscosity ratios in porous media. The proposed model is based on a non-orthogonal multiple-relaxation-time (MRT) LB model and a new improved wettability boundary condition. In terms of the relationship between capillary pressure Pc and saturation s, the proposed model shows a good agreement with the experimental data. Using the validated model, the effects of capillary pressures and contact angles of mixed wettability on the liquid water invasion process for Toray-090 GDLs with two Polytetrafluoroethylene (PTFE) contents (10 wt% and 20 wt%) are studied. It is found that the liquid water shows capillary fingering behaviors and the liquid water saturation profiles along the through-plane direction of the GDLs become more non-uniform with increasing contact angle of PTFE.

Spontaneous imbibition in tight porous media with different wettability: Pore-scale simulation

Spontaneous imbibition is significantly influenced by rock wettability, and it has been extensively studied in core-based experiments and numerical simulations owing to its important role in the development of oil/gas reservoir. Due to the fine pore structure and complex wettability of tight sandstone, an in-depth exploration of the effects of wettability on the pore-scale flow physics during spontaneous imbibition is of great value to complement traditional experimental studies and enhance the understanding of microscopic flow mechanisms during the development of tight oil reservoirs. Based on a X-ray computed tomography scanning experiment and a lattice Boltzmann multiphase model, in this work, we systematically investigate the effects of different hydrophilic strengths on the evolution of the imbibition fronts within the micropores and the degree of nonwetting fluid recovery during spontaneous imbibition of tight sandstone. The results show that the wettability significantly affects the morphological characteristics of the imbibition fronts. Under strong hydrophilic conditions, the wetting fluid preferentially invades the pore corner in the form of angular flow. As the contact angle increases, the hysteresis effect at the main terminal interface decreases, and the two-phase interface becomes regular and compact. Wettability also significantly affects the imbibition rate and the nonwetting fluid recovery degree. The smaller the contact angle, the faster the imbibition rate and the higher the recovery degree of nonwetting fluids during the cocurrent spontaneous imbibition.

Pore-scale investigation on the effect of gas-liquid phase separation on reactive flow in a horizontal rough fracture using the lattice Boltzmann method

In this study, the effect of the gas-liquid phase separation caused by seawater boiling on the carbonate crack dissolution process was investigated using the lattice Boltzmann (LB) method at the pore scale. The gas-liquid two-phase flow with phase change, heat transfer, and solute transport is described by a single-component multiphase LB model, thermal LB model, and mass transport LB model, respectively. In addition, the dissolved solid mass is calculated via the Volume of Pixel method. The main conclusions are as follows: the phase separation negatively affects dissolution, especially in the upper boundary of the crack. Moreover, the negative effect is strengthened with the increase in inlet velocity and the decrease in crack width while the influence of wettability on dissolution is non-linear and its degree depends on the phase distribution of the two phases. In addition, slug flow greatly hinders solute transport but may accelerate the dissolution reaction near the inlet.

Achieving thermodynamic consistency in a class of free-energy multiphase lattice Boltzmann models.

The free-energy lattice Boltzmann (LB) model is one of the major multiphase models in the LB community. The present study is focused on a class of free-energy LB models in which the divergence of thermodynamic pressure tensor or its equivalent form expressed by the chemical potential is incorporated into the LB equation via a forcing term. Although this class of free-energy LB models may be thermodynamically consistent at the continuum level, it suffers from thermodynamic inconsistency at the discrete lattice level owing to numerical errors [Guo et al., Phys. Rev. E 83, 036707 (2010)10.1103/PhysRevE.83.036707]. The numerical error term mainly includes two parts: one comes from the discrete gradient operator and the other can be identified in a high-order Chapman-Enskog analysis. In this paper, we propose an improved scheme to eliminate the thermodynamic inconsistency of the aforementioned class of free-energy LB models. The improved scheme is constructed by modifying the equation of state of the standard LB equation, through which the discretization of ∇(ρc_{s}^{2}) is no longer involved in the force calculation and then the numerical errors can be significantly reduced. Numerical simulations are subsequently performed to validate the proposed scheme. The numerical results show that the improved scheme is capable of eliminating the thermodynamic inconsistency and can significantly reduce the spurious currents in comparison with the standard forcing-based free-energy LB model.

Design of Continuous Transport of the Droplet by the Contact-Boiling Regime.

Joule-heat-driven directional transport of liquid droplets has comprehensive engineering applications in various water and thermal management, cooling systems, and self-cleaning. Generally, the driving force for the transport of liquid droplets was always observed at an extremely high Leidenfrost temperature, which limits the potential application between liquid boiling and Leidenfrost points. In this work, we design a new strategy to directionally drive the transport of droplets by blockading the vapor cushion at a temperature much lower than the Leidenfrost point. On the surface of the microhole arrays, we observed the continuous rebound behavior of ethanol droplets at Ts = 110 °C. Employing the thermal multiphase lattice Boltzmann model, the continuous rebound behavior was reproduced, verifying that the driving force was provided by the blockaded vapor pressure in microholes. By cooperating with the Laplace pressure difference, we directionally transport ethanol and water droplets on the horizontal asymmetrical concentric microridge surface. The horizontal velocity of water is 11.25 cm/s at Ts = 180 °C, similar to the traditional ratchets at the Leidenfrost point. The design of microtextures enriches the fundamental understanding of how to drive droplets at far below the Leidenfrost point and pushes the application in nongravity-driven self-cleaning and cooling systems.

An Improved Single-Relaxation-Time Multiphase Lattice Boltzmann Model for Multiphase Flows with Large Density Ratios and High Reynolds Numbers

An Improved Single-Relaxation-Time Multiphase Lattice Boltzmann Model for Multiphase Flows with Large Density Ratios and High Reynolds Numbers

Dissolution process of a single bubble under pressure with a large-density-ratio multicomponent multiphase lattice Boltzmann model.

A large-density-ratio and tunable-viscosity-ratio multicomponent multiphase pseudopotential lattice Boltzmann model is used to study the dissolution process of a bubble under pressure. The multi-relaxation-time collision operator, exact-difference-method external force scheme, and scaling coefficient k are applied to ensure the numerical stability of the model. The influence of k in the equation of state (EOS) and intermolecule interaction strength on the stationary bubble evolution process are discussed, and the effect of k on thermodynamic consistency is also analyzed. The results indicate that adjusting the scaling coefficient in the EOS changes the surface tension and interface thickness, and that the gas-liquid interface width w is proportional to 1/sqrt[k]. Considering the effect of k on the surface tension, interface thickness, and thermodynamic consistency, the scaling coefficient should be between 0.6 and 1. Furthermore, the dissolution process of a single bubble under pressure is studied using the developed model, and it is found that the dissolution mass and concentration of dissolved gas increase linearly with increases in the pressure difference, and that the concentration of dissolved gas is proportional to the gas pressure after the fluid system reaches equilibrium. These results are consistent with Henry's law.

3D-Printed Bioinspired Cassie-Baxter Wettability for Controllable Microdroplet Manipulation.

It is a great challenge to fabricate a surface with Cassie-Baxter wettability that can be continuously adjusted from hydrophilicity to superhydrophobicity by changing of geometric parameters. In this paper, we propose and demonstrate a bioinspired surface fabricated by using a projection micro-stereolithography (PμSL) based 3D printing technique to address the challenge. Independent of materials, the bioinspired textured surface has a maximum contact angle (CA) of 171°, which is even higher than that of the omniphobic springtail skin we try to imitate. Most significantly, we are able to control the CA of the bioinspired surface in the range of 55-171° and the adhesion force from 71 to 99 μN continuously by only changing the geometric parameters of the bioinspired microstructures. The underlying mechanisms of the CA control of our bioinspired surface are also revealed by using a multi-phase lattice Boltzmann model. Furthermore, we demonstrate potential applications in droplet-based microreactors, nonloss water transportation, and coalescence of water droplets by employing our 3D-printed bioinspired structures with their remarkable precise Cassie-Baxter wettability control and petal effects. The present results potentially pave a new way for designing next generation functional surfaces for microdroplet manipulation, droplet-based biodetection, antifouling surfaces, and cell culture.

Lattice Boltzmann simulation of near/supercritical CO2 flow featuring a crossover formulation of the equation of state

In this work, we have incorporated a crossover equation of state into the pseudopotential multiphase Lattice Boltzmann Model (LBM) to improve the prediction of thermodynamic properties of fluids and their flow in near-critical and supercritical regions. Modeling carbon dioxide (CO2) properties in these regions is of increasing interest for industrial processes such as CO2 storage and heat transfer where CO2 is used as a working fluid. Despite the importance of accurately modeling near-critical and supercritical fluids, popular classical cubic equations of state (EoS) are not accurate. The proposed crossover EoS is a proven hybrid equation which uses the original classical EoS far from the critical point where it is valid, and near the critical point, it asymptotically switches to using non-analytic scaling laws. It also transforms into the ideal gas EoS as the density approaches zero. In order to demonstrate the validity and versatility of the crossover approach in the prediction accuracy of LBM in near-critical flows, this formulation was incorporated into the Peng-Robinson (P-R) EoS. First, 2D static droplets of CO2 and water at vapor-liquid equilibrium were modeled using both P-R EoS and its crossover formulation, and the numerically predicted results were then compared against the experimental data. The results demonstrate that the model accuracy in representing the thermodynamic behavior of the fluid in near-critical region is improved after adopting the crossover formulation with respect to the classical analytical EoS. The crossover P-R EoS was further applied to study several two-phase CO2 flows in mini-channels at near-critical temperatures. The flow patterns showed a good qualitative agreement with the experimental data in the near-critical region. The study is further extended to multi-component multiphase systems, specifically to water droplet penetration into porous media filled with CO2.

Influence of thermal gradients on the invasion patterns during drying of porous media: A lattice Boltzmann method

Drying of porous media sounds simple yet complicated to study the multiphase flow counterparts in porous media with intricate pore geometries. In the past, we have discussed the Lattice Boltzmann Model (LBM) as a powerful multiphase solver for the drying of porous media. In this study, we extend our previous work on the Shan Chen representation of the multiphase LBM to drying of porous media with imposed thermal gradients. A linearly varied stationary temperature profile is imposed concerning the depth of the porous medium, i.e., free evolution of temperatures due to the phase change is neglected. The preferential heating is divided into two kinds of gradients: First, the positive thermal gradient where temperatures varies linearly on an increasing order from top to bottom (e.g., the contact heating mode of drying). Second, the negative thermal gradient opposite the former (e.g., convective mode of heating). It is observed that the thermal gradient can lead to stabilizing and destabilizing drying fronts, where the latter situation incurs two drying fronts in a later period of drying. The novelty of this work is the establishment of thermal aspects to the previously discussed LBM and introduces the concepts of evaporation–condensation of trapped clusters and liquid bridges. The characteristics of thermal drying for stabilized and destabilized drying fronts is re-established at a magnified level of study using the developed LBM.