Zhaohui Liu

A novel coupled lattice Boltzmann model for low Mach number combustion simulation

Abstract A novel coupled lattice Boltzmann model is developed for two- and three-dimensional low Mach number combustion simulations, in which the fluid density can bear sharp changes. Different to the hybrid lattice Boltzmann scheme [O. Filippova, D. Hanel, A novel lattice BGK approach for low Mach number combustion, J. Comput. Phys. 158 (2000) 139], this scheme is strictly pure lattice Boltzmann style (i.e., we solve the flow, temperature, and concentration fields using the lattice Boltzmann method only); different to the non-coupled lattice Boltzmann scheme [K. Yamamoto, X. He, G.D. Doolen, Simulation of combustion field with lattice Boltzmann method, J. Stat. Phys. 107 (2002) 367], the fluid density in this model is coupled directly with the temperature. In this model the time step and the fluid particle speed can be adjusted dynamically, depending on the “particle characteristic temperature”. And the algorithm is still a simple process of hopping from one grid point to the next, the same as the standard lattice Boltzmann method. Therefore the outstanding advantages of the standard lattice Boltzmann method are retained in this model besides better numerical stability. Excellent agreement between the present results and other numerical or experimental data shows that this scheme is an efficient numerical method for practical combustion simulations.

Comparison of Different Global Combustion Mechanisms Under Hot and Diluted Oxidation Conditions

Moderate and intensive low-oxygen dilution (MILD) combustion is a new combustion technology. It features as combustion under a hot and diluted oxidation condition. The objective of this article is to optimize global mechanisms for predicting the major species concentration of CH4 combustion under MILD condition. For this purpose, six different global combustion mechanisms, including the four-step mechanism of Jones and Lindstedt (1988), the two-step mechanism of Westbrook and Dryer (1981), and several modified versions of them, were investigated. Reference calculations were also conducted with detailed chemical kinetic mechanism GRI-Mech 3.0. The interaction between turbulence and chemistry was modeled by eddy dissipation concept (EDC). All of these global mechanisms are validated first by a fictitious plug flow reactor and then by a non-premixed turbulent jet flame of a H2/CH4 fuel mixture (Dally et al., 2002). The results show that modified the Westbrook and Dryer mechanism (WD4), which includes both CO oxidation rate and H2 oxidation rate modification, shows the best agreement with experiment.

A simple lattice Boltzmann scheme for combustion simulation

A simple lattice Boltzmann model is developed for two-dimensional combustion simulations. In this model the time step and the fluid particle speed can be adjusted dynamically, depending on the particle characteristic temperature. The algorithm is still a simple process of hopping from one grid point to the next, the same as the standard lattice Boltzmann method. Therefore the outstanding advantages of the standard lattice Boltzmann method are retained in this model besides better numerical stability. Excellent agreement between the present results and other numerical or experimental data shows that this scheme is an efficient numerical method for practical combustion simulations.

A NEW NUMERICAL APPROACH FOR FIRE SIMULATION

It is an urgent task to adopt/develop new numerical methods in fire research because of the intrinsic disadvantages of traditional numerical methods, such as in complicated geometries treatment, computational efficiency on parallel computers. The outstanding features of the lattice Boltzmann method (LBM) has emerged which is a very promising alternative in this field. Unfortunately, until recently the LBM could not be employed in fire research due to its inherent defects in combustion simulation. In order to extend the LBM into fire research, a novel LB model for fire simulation is designed in this study. Besides the intrinsic advantages of the standard LBM, this model shows improved numerical stability and can cover temperature ratio of more than one order of magnitude. The model is validated through a benchmark test coflow methane-air diffusion flame. Furthermore, because little attention has been paid to the effect of the interaction between the inlet boundary and the interior of the flow field on stability of the computation and the quality of the solution, therefore in this study we also discuss this problem in detail to bridge this gap.

A joint PDF model for turbulent spray evaporation/combustion

A new joint-probability density function (PDF) method is proposed for modeling turbulent gas-spray flows, with special effort in considering the influence of gas turbulence on droplet dispersion, evaporation, and combustion. By taking the droplet velocities, temperature, diameter, and droplet-seen gas velocity, temperature, and mass concentration as stochastic variables, a joint-PDF evolution equation for the properties of droplets and gas eddies seen by droplets is derived. To close the turbulent dispersion model of droplets, the Langevin equation proposed by Simonin for simulating the gas velocity seen by particles, is taken to account for the crossing trajectory effect and the continuity effect. The turbulent head and species transport were modeled accordingly by a Langevin equation model with isotropic drift coefficients. The PDF model combined with the second-order moment (SOM) model of gas turbulence was used to simulate well-specified spray evaporation in a sudden-expansion chamber. The predicted droplet mean velocities, velocity fluctuations, mean diameter, root-mean-square (rms) diameter, and droplet axial mass flux profiles are in good agreement with the measurement results. The PDF model can give good predicted droplet properties even in the recirculation zone, where the droplet concentration is low and the stochastic particle trajectory (SPT) method always fails to give good results. The proposed method provides a sound basis for simulating turbulent spray combustion.

A second-order-moment–Monte-Carlo model for simulating swirling gas–particle flows☆

Abstract A second-order moment (SOM) gas-phase turbulence model, combined with a Monte-Carlo (MC) simulation of stochastic particle motion using Langevin equation to simulate the gas velocity seen by particles, is called an SOM–MC two-phase turbulence model. The SOM–MC model was applied to simulate swirling gas–particle flows with a swirl number of 0.47. The prediction results are compared with the PDPA measurement data and those predicted using the Langevin-closed unified second-order moment (LUSM) model. The comparison shows that both models give the predicted time-averaged flow field of particle phase in general agreement with those measured, and there is only slight difference between the prediction results using these two models. In the near-inlet region, the SOM-MC model gives a more reasonable distribution of particle axial velocity with reverse flows due to free of particle numerical diffusion, but it needs much longer computation time. Both models underpredict the gas and particle fluctuation velocities, compared with those measured. This is possibly caused by the particle–wall and particle–particle interaction in the near-wall region, and the effect of particles on dissipation of gas turbulence, which is not taken into account in both models.

Lattice Boltzmann method in simulation of thermal micro-flow with temperature jump

We simulate the gas flow and heat transfer in micro-Couette flow by the lattice Boltzmann method (LBM). A new boundary treatment is adopted in the numerical experiment in order to capture the velocity slip and the temperature jump of the wall boundary. Velocity and temperature profiles are in good agreement with the analytic results, which exhibits the availability of this model and boundary treatment in describing thermal micro-flow with viscous heat dissipation. We also find the upper boundarys temperature jump is zero at the critical Ec, which is around 3.0 with different Kn.

Simulation of Swirling Gas-Particle Flows Using Different Time Scales for the Closure of Two-Phase Velocity Correlation in the Second-Order Moment Two-Phase Turbulence Model

Three different time scales - the gas turbulence integral time scale, the particle relaxation time, and the eddy interaction time - are used for closing the dissipation term in the transport equation of two-phase velocity correlation of the second-order moment two-phase turbulence model. The mass-weighted averaged second-order moment (MSM) model is used to simulate swirling turbulent gas-particle flows with a swirl number of 0.47. The prediction results are compared with the PDPA measurement results taking from references

Computation of gas–solid flows by finite difference Boltzmann equation

Abstract In this paper, we will discuss in detail how to use a finite difference lattice Boltzmann equation model in which an external force term is involved to simulate two-way coupling gas–solid flows. The numerical results are found to be in good agreement with analytical data and some other numerical results.

NUMERICAL AND EXPERIMENTAL INVESTIGATIONS ON THE PERFORMANCE OF A 300 MW PULVERIZED COAL FURNACE

In this paper, we first develop a new numerical scheme for a computational fluid dynamics code and then investigate the impact of the airflow configuration on the performance of a 300 MW tangentially fired furnace by both simulations and experiments. Four different secondary and two tertiary air arrangements are tested. It is shown that the newly developed 27-point discrete scheme enhances the performance of the code, offering a satisfactory prediction of the velocity field for the isothermal furnace flow. Further effort is made to examine the effects of airflow configuration on combustion performance, outlet temperature deviation, and heat loss due to combustibles in the bottom ash. Both simulations and operation trials suggest that, for the furnace investigated, when firing bituminous coal, the double V-shaped secondary air configuration has the best performance.