Pore-scale study on porous media flows with chemical reaction using lattice Boltzmann method

Porous media flows with chemical reaction are common in nature and widely exist in many scientific and industrial applications. However, due to the complexity of coupled mechanisms, numerical modelling and comprehensive understanding of such flows face significant challenges. Therefore, this thesis develops novel lattice Boltzmann (LB) models to undertake pore-scale simulations of porous media flows with chemical reaction. These models, with new reaction source terms and boundary schemes, can describe both homogeneous reaction between two fluids and heterogeneous reaction (dissolution or combustion) at the fluid-solid interface. Unlike previous studies, current models recast heat and mass transfer equations to correctly consider the thermal expansion effects and the conjugate heat transfer and species conservation conditions. Separate LB equations are also developed to include different species properties. Density fingering with homogeneous reaction is studied at the pore scale. By changing species contributions to density, diffusion coefficients, initial concentrations, and medium heterogeneities, results obtained demonstrate that reaction can enhance, suppress, or trigger fingering. Then, pore-scale simulations of viscous fingering with dissolution reaction are performed. Effects of fluid diffusion, chemical dissolution, and viscosity contrast are extensively assessed. Results illustrate four fingering regimes as stable, unstable, reactive stable, and reactive unstable. Finally, pore-scale coke combustion in porous media is studied. General combustion dynamics are correctly produced, verifying the superior performance of the present LB model over previous ones. A parametric study demonstrates that the inlet air temperature and the driving force are influential factors and should be constrained within certain ranges for stable combustion fronts. These pore-scale findings provide valuable insights, like temperature fluctuations at the fluid-solid interface, porous structure evolutions, exact reaction and diffusion rates, and medium heterogeneity effects, which are more precise and explicit than macroscopic results. Furthermore, detailed fingering and combustion dynamics under diverse conditions are helpful in scientific and industrial fields

Pore-scale study of coke formation and combustion in porous media using lattice Boltzmann method

In-situ combustion (ISC) has long been recognized as a promising technique for heavy oil recovery. However, ISC includes multiple physicochemical processes, which are still poorly understood and difficult to predict and control. This study establishes a lattice Boltzmann (LB) model to simulate the two important aspects of ISC at the pore scale: coke formation and combustion. The LB model includes thermal expansion effects and solves the reactive air-coke interface without iterations. Moreover, this model improves upon previous models by considering both coke formation and two-step coke combustion, as well as the growth of solid geometry. Results show that the LB model correctly captures coke combustion properties. Meanwhile, the newly introduced coke formation and two-step combustion yield important findings. As heat released from combustion transfers downstream, oil cracking and coke formation ahead of the combustion front are successfully tracked. The generated coke fuels the upstream combustion, making the system self-sustained. During coke formation and combustion, four coke transition states are identified. In addition, a parametric study demonstrates that the large inlet oxygen content and driving force are desirable, while too high a driving force should be avoided as it causes high burning temperature. Furthermore, it suggests that the inlet air temperature should be set appropriately. On one hand, a high temperature may promote coke formation and retard the front propagation. On the other hand, a low temperature may slow down the combustion of coke 2, even though it is high enough to ensure the ignition of coke 1. The decelerated coke 2 combustion may further cause the insufficient heat release and the failed coke formation, thus inducing the early termination of combustion. Such effects of the inlet temperature indicate the necessity of considering coke formation and two-step coke combustion. These results help to improve the understanding and facilitate the development of ISC

Lattice Boltzmann method with nonreflective boundary conditions for low Mach number combustion

The paper presents a lattice Boltzmann (LB) method for premixed and nonpremixed combustion simulations with nonreflective boundary conditions, in contrast to Navier–Stokes solvers or hybrid schemes. The current approach employs different sets of distribution functions for flow, temperature and species fields, which are fully coupled. The discrete equilibrium density distributions are obtained from the Hermite expansions thus thermal compressibility is included. The coupling among the momentum, energy and species transport enables the model to be applicable for reactive flows with chemical heat release. The characteristic boundary conditions are incorporated into the LB scheme to avoid numerical reflections. The multi-relaxation-time collision schemes are applied to all the LB solution procedures to improve numerical stability. With detailed thermodynamics and chemical mechanisms for hydrogen-air, the LB modelling framework is validated against both premixed flame propagation and nonpremixed counterflow diffusion flame benchmarks. Simulations of circular expanding premixed flames further demonstrate the capability of the new reactive LB method. The developed LB methodology retains the advantages of classic LB methods and extends the LB capability to low Mach number combustion with potential applications in mesoscale and microscale combustors, catalysis, fuel cells, batteries and so on

Pore-scale study of miscible density instability with viscosity contrast in porous media

The transport of miscible fluids in porous media is a prevalent phenomenon that occurs in various natural and industrial contexts. However, this fundamental phenomenon is usually coupled with interface instabilities (e.g., viscous/density fingering), which has yet to be thoroughly investigated. In this paper, a multiple-relaxation-time lattice Boltzmann method is applied to study the displacement between two miscible fluids in porous media at the pore scale, with the coexistence of density difference (Rayleigh number Ra), viscosity contrast (R), and injection velocity (Utop). A parametric study is conducted to evaluate the impact of Ra, R, and Utop on the flow stability. For a fixed Ra that can trigger density fingering, the increase in R or Utop is found to suppress density fingering. Consequently, under a large Utop and a moderate R, the density fingering is fully stabilized and the flow follows a stabile pattern. Furthermore, as both R and Utop grow to a sufficiently high level, they can jointly trigger viscous fingering. In addition, the increasing Ra shows an enhancing effect on both density fingering and viscous fingering. Finally, by quantitatively analyzing the fingering length (lm) and the fingering propagation time (te), five different flow patterns are classified as viscosity-suppressed (I), viscosity-enhanced (II), viscosity-unstable (III), displacement-suppressed (IV), and stable (V) regimes. In a three-dimensional parameter space spanned by Ra, R, and Utop, the parameter ranges of the five regimes are determined according to lm and te. These findings hold a significant value in providing guidance for controlling the flow stability by selecting appropriate operating conditions

Droplet impact on a heated porous plate above the Leidenfrost temperature: A lattice Boltzmann study

Recently a droplet was observed to form a pancake shape and bounce as it impacted nanotube or micropost surfaces above the Leidenfrost temperature. This led to a significant reduction in droplet contact time. However, this unique bouncing phenomenon is still not fully understood, such as the influence of the plate configuration and the relationship between the droplet rebound time and evaporation mass loss. In this study, we carry out a numerical study of the droplet impact dynamics on a heated porous plate above the Leidenfrost temperature, using a multiphase thermal lattice Boltzmann model. Our model is constructed within the unified lattice Boltzmann method (ULBM) framework and is firstly validated based on theoretical and experimental results. Then, a comprehensive parametric study is performed to investigate the effects of the impact Weber number, the plate temperature and the plate configurations on the droplet bouncing dynamics. Results show that higher plate temperature, larger Weber number, and smaller pore intervals can accelerate the droplet rebound and promote the droplet pancake bouncing. We demonstrate that the occurrence of the pancake bouncing is attributed to the additional lift force provided by the vapour pressure due to the evaporation of liquid inside the pores. Moreover, the droplet maximum spreading time and maximum spreading factor can be described by a power law function of the impact Weber number. The droplet evaporation mass loss increases linearly with the impingement Weber number and the plate opening fractions

A three-dimensional non-orthogonal multiple-relaxation-time phase-field lattice Boltzmann model for multiphase flows at large density ratios and high Reynolds numbers

This study proposes a three-dimensional non-orthogonal multiple-relaxation-time (NMRT) phase-field multiphase lattice Boltzmann (PFLB) model within a recently established unified lattice Boltzmann model (ULBM) framework [Luo et al., Phil. Trans. R. Soc. A 379, 20200397, 2021]. The conservative Allen-Cahn equation and the incompressible Navier-Stokes (NS) equations are solved. In addition, a local gradient calculation scheme for the order parameter of the Allen-Cahn equation is constructed with the non-equilibrium part of the distribution function. A series of benchmark cases are conducted to validate the proposed model, including the two-phase Poiseuille flow, Rayleigh-Taylor instability, binary liquid/metal droplet collision, and a bubble rise in water. The present simulation results are in good agreement with existing simulation and experimental data. In the simulation of the co-current two-phase Poiseuille flow, the present model is proven to resolve the discontinuity at the phase interface and provide accurate results at extremely high density ratios (i.e., up to ). Finally, the proposed model is adopted to simulate two challenging cases: (1) water droplet splashing during its impacting on a thin liquid film and (2) liquid jet breakup. The simulation results demonstrate an excellent agreement with previous experimental results, both qualitatively and quantitatively. In these simulations, the Weber number and Reynolds number reach 105 and 6000, respectively, and the viscosity can be as low as , in the lattice unit

Lattice Boltzmann modelling of salt precipitation during brine evaporation

Salt precipitation during brine evaporation in porous media is an important phenomenon in a variety of natural and engineering scenarios. This work establishes a multiphase multicomponent lattice Boltzmann (LB) method with phase change for simulating salt precipitation during brine evaporation. In the proposed LB models, the gas–brine multiphase flow, brine evaporation, salt concentration evolution, salt precipitate nucleation and growth are simultaneously considered. Simulations of the Stefan problem are first conducted to verify the proposed numerical models and determine the diffusion coefficient of brine vapour. Once the lattice Boltzmann models have been validated, salt precipitation during brine evaporation is simulated to investigate the competition mechanisms between salt precipitate nucleation and growth reaction. The results show that the typical salt precipitation patterns in existing experimental observation can be successfully reproduced, including the ring-like and pancake-like patterns. The difference in the salt precipitation patterns is explained by the competition mechanism between precipitate growth and nucleation according to the present study. Furthermore, the salt precipitation during gas injection into a microfluidic chip is investigated. The evolution of salt and brine saturation shows similar patterns to existing experimental results, and the effects of the gas injection rate on salt precipitation performance are clarified. The LB models in the present work can simulate salt precipitation with comprehensive consideration of multiphase brine evaporation, salt species mass transport, precipitate nucleation and growth, which have not been realized in previous studies. The numerical showcases demonstrate the excellent performance of the proposed models for the simulation of salt precipitation in porous media, which promise to guide practical engineering applications like CO2 sequestration

Study of CO2 desublimation during cryogenic carbon capture using the lattice Boltzmann method

Cryogenic carbon capture (CCC) can preferentially desublimate CO2 out of the flue gas. A widespread application of CCC requires a comprehensive understanding of CO2 desublimation properties. This is, however, highly challenging due to the multiphysics behind it. This study proposes a lattice Boltzmann (LB) model to study CO2 desublimation on a cooled cylinder surface during CCC. In two-dimensional (2-D) simulations, various CO2 desublimation and capture behaviours are produced in response to different operation conditions, namely, gas velocity (Péclet number Pe) and cylinder temperature (subcooling degree Tsub). As Pe increases or Tsub decreases, the desublimation rate gradually becomes insufficient compared with the CO2 supply via convection/diffusion. Correspondingly, the desublimated solid CO2 layer (SCL) transforms from a loose (i.e. cluster-like, dendritic or incomplete) structure to a dense one. Four desublimation regimes are thus classified as diffusion-controlled, joint-controlled, convection-controlled and desublimation-controlled regimes. The joint-controlled regime shows quantitatively a desirable CO2 capture performance: fast desublimation rate, high capture capacity, and full cylinder utilization. Regime distributions are summarized on a Pe–Tsub space to determine operation parameters for the joint-controlled regime. Moreover, three-dimensional simulations demonstrate four similar desublimation regimes, verifying the reliability of 2-D results. Under regimes with loose SCLs, however, the desublimation process shows an improved CO2 capture performance in three dimensions. This is attributed to the enhanced availability of gas–solid interface and flow paths. This work develops a reliable LB model to study CO2 desublimation, which can facilitate applications of CCC for mitigating climate change

Droplet impact on a heated porous plate above the Leidenfrost temperature: A lattice Boltzmann study

In the past few decades, the droplet impact on a heated plate above the Leidenfrost temperature has attracted immense research interest. The strong hydrophobicity caused by the Leidenfrost effect leads to the droplet bouncing from a flat plate at a given contact time predicted by the classical Rayleigh theory. Numerous investigations were conducted to break the theoretical Rayleigh's limit to reduce the interfacial contact time. Recently, a droplet was observed to form a pancake shape and bounce as it impacted nanotube or micropost surfaces above the Leidenfrost temperature. This led to a significant reduction in droplet contact time. However, this unique bouncing phenomenon is still not fully understood, such as the influence of the plate configuration and the relationship between the droplet rebound time and evaporation mass loss. In this study, we carry out a numerical study of the droplet impact dynamics on a heated porous plate above the Leidenfrost temperature, using a multiphase thermal lattice Boltzmann model. Our model is constructed within the unified lattice Boltzmann method framework and is first validated based on theoretical and experimental results. Then, a comprehensive parametric study is performed to investigate the effects of the impact Weber number, the plate temperature, and the plate configurations on the droplet bouncing dynamics. Results show that higher plate temperature, larger Weber number, and smaller pore intervals can accelerate the droplet rebound and promote the droplet pancake bouncing. We demonstrate that the occurrence of the pancake bouncing is attributed to the additional lift force provided by the vapor pressure due to the evaporation of liquid inside the pores. Moreover, the droplet maximum spreading time and maximum spreading factor can be described by a power law function of the impact Weber number. The droplet evaporation mass loss increases linearly with the impingement Weber number and the plate opening fractions. This study provides new insights into the Leidenfrost droplet impingement on porous plates, which may potentially facilitate the design of novel engineering surfaces and devices.ISSN:1070-6631ISSN:1089-7666ISSN:0031-917