Huiqiang Zhang

Presumed joint probability density function model for turbulent combustion

Abstract A presumed joint probability density function (pdf) model of turbulent combustion is proposed in this paper. The turbulent fluctuations of reactant concentrations and temperature are described using a presumed joint pdf of three-dimensional Gaussian distribution based on first and second-order moments of reactant concentration and temperature. Mean reaction rates in both premixed and diffusion combustion are obtained by mean of integration under the presumed joint pdf. This model is applied to predict turbulent premixed combustion of sudden-expansion flow and turbulent jet diffusion methane/air flame. For turbulent premixed combustion, the predicted results of temperature distribution and maximum temperature using the proposed model agree better with the experiment than that using the conventional eddy-breakup (EBU)–Arrhenius model. For the turbulent jet diffusion methane/air flame, the predicted results of velocity, temperature and species concentrations using the proposed model, the Arrhenius, EBU–Arrhenius, and laminar flamelet models are compared with experiment data. Results obtained with the presumed pdf model and that obtained by the laminar flamelet model both agree well with experiments, while results using the other models have a significant difference. The presumed joint pdf model is used to predict the NO formation process, which also agrees well with the experiment data. A unified turbulent combustion model, in which both effects of turbulent diffusion and chemical dynamics are considered, is established for both premixed and diffusion combustion, especially for the process of NO formation.

Flow dynamics of a spiral-groove dry-gas seal

The dry-gas seal has been widely used in different industries. With increased spin speed of the rotator shaft, turbulence occurs in the gas film between the stator and rotor seal faces. For the micro-scale flow in the gas film and grooves, turbulence can change the pressure distribution of the gas film. Hence, the seal performance is influenced. However, turbulence effects and methods for their evaluation are not considered in the existing industrial designs of dry-gas seal. The present paper numerically obtains the turbulent flow fields of a spiral-groove dry-gas seal to analyze turbulence effects on seal performance. The direct numerical simulation (DNS) and Reynolds-averaged Navier-Stokes (RANS) methods are utilized to predict the velocity field properties in the grooves and gas film. The key performance parameter, open force, is obtained by integrating the pressure distribution, and the obtained result is in good agreement with the experimental data of other researchers. Very large velocity gradients are found in the sealing gas film because of the geometrical effects of the grooves. Considering turbulence effects, the calculation results show that both the gas film pressure and open force decrease. The RANS method underestimates the performance, compared with the DNS. The solution of the conventional Reynolds lubrication equation without turbulence effects suffers from significant calculation errors and a small application scope. The present study helps elucidate the physical mechanism of the hydrodynamic effects of grooves for improving and optimizing the industrial design or seal face pattern of a dry-gas seal.

Mixing Enhancement of Compressible Planar Mixing Layer Impinged by Oblique Shock Waves

This paper presents the mixing enhancements of a spatially developing Mc=0.3 planar mixing layer interacting with an oblique shock wave by means of large-eddy simulation. The large-scale coherent vortices are found to be modulated by the oblique shock, which results in enhanced vorticity of the vortices. The thickness of the mixing layer impinged by oblique shock waves first decreases due to the increased compressibility effects of the shock wave, but then it increases and finally exceeds that of the shock-free mixing layer because of an accelerating growth rate larger than 0.05. The fluctuating levels of velocities and turbulent kinetic energy are strengthened in the shock–mixing-layer flows. The production term in the Reynolds stress transport equation dominates the increase of the transverse component of the Reynolds normal stresses, whereas the pressure–strain term decreases them and redistributes the energy to the streamwise component in shock–mixing layers, which then leads to the mixing enhancement...

Particle modulations to turbulence in two-phase round jets

The particle modulations to turbulence in round jets were experimentally studied by means of two-phase velocity measurements with Phase Doppler Anemometer (PDA). Laden with very large particles, no significant attenuations of turbulence intensities were measured in the far-fields, due to small two-phase slip velocities and particle Reynolds number. The gas-phase turbulence is enhanced by particles in the near-fields, but it is significantly attenuated by the small particles in the far-fields. The smaller particles have a more profound effect on the attenuation of turbulence intensities. The enhancements or attenuations of turbulence intensities in the far-fields depends on the energy production, transport and dissipation mechanisms between the two phases, which are determined by the particle properties and two-phase velocity slips. The non-dimensional parameter CTI is introduced to represent the change of turbulence intensity.

Effects of Inflow Mach Number and Step Height on Supersonic Flows over a Backward-Facing Step

The backward-facing step is practically implicated in many devices, encountering the massive separation flows. In the present study, simulations of supersonic flow over a backward-facing step have been carried out employing both RANS and LES. The simulated results are validated against the experimental data. The results of RANS and LES show a good comparison with the experimental results. Different inflow Mach numbers and expansion ratios are also investigated. The reattachment length decreases with the increase of inflow Mach number. The duct height has a great effect on the flow patterns. The present conclusions are helpful to understand the physics in supersonic separation flows and also provide theory basis for engineering applications.

A Study On Correlation Moments Of Two-Phase Fluctuating Velocity Using Direct Numerical Simulation

Existing models of two-phase fluctuating velocity correlation moments are unsatisfactory because of their inability to clearly identify the dependency of two-phase velocity covariance on fluid- and particle-phase velocity second moments. This is especially true of wall-bounded turbulent flows. In this paper, the statistical fluctuating velocity of both phases in particle-laden turbulent channel flows were obtained numerically by means of direct numerical simulation (DNS) coupled to the Lagrangian particle trajectory method. The effects of particle Stokes number on the scaling of two-phase fluctuating velocity correlation moments were analyzed considering effects of flow inhomogeneity. An improved two-phase correlation closure model of exponential decay with emphasis on the particle-phase kinetic energy was then proposed based on the results of an evaluation of five existing models. This new model was found to be better than previous models, which used local equilibrium assumption. The present investigations may facilitate understanding of two-phase flow physics and the construction of models capable of predicting the movements of particle-laden turbulent flows accurately using Reynolds-averaged Navier–Stokes (RANS) methods.

Analysis of Inertial Particle Drift Dispersion by Direct Numerical Simulation of Two-Phase Wall-Bounded Turbulent Flows

Abstract This paper analyzes inertial particle drift dispersion based on the numerical database established by direct numerical simulation of a dilute particle-laden two-phase turbulent channel flow, in which the released particles are tracked by a Lagrangian trajectory method. Low-inertia particles tend to travel toward the walls at negative velocities. High-inertia particles however display very low drift in the near-wall regions. Effects of particle size and material density on particle drifts are also jointly studied. The statistical results show that particles drift dispersion depends significantly on particle Stokes number. Low-inertia particle drift dispersion causes a direct low-inertia particle deposition on the wall, and hence increases the particle concentration close to the walls. The results are valuable for understanding the mechanisms of particle dispersion by turbulence.

EVALUATION OF PARTICLE STOCHASTIC SEPARATED FLOW MODELS VIA LARGE EDDY SIMULATION

This paper evaluates three widely used particle stochastic separated flow (SSF) models through large eddy simulation (LES) of gas-particle two-phase turbulent flows over a backward-facing step. The ability of the models to predict mean velocities, fluctuating velocities, and spatial dispersion of particles are carefully examined in comparison with LES reference results. Evaluation shows that the improved time-series SSF model produces good predictions on mean and fluctuating velocities in the particle phase which highly agree with LES results. However, the time-series SSF model has higher computational cost. Further, compared with the two other models, the time-series SSF model predicts better results on the spatial dispersion of particles. It has an overall advantage in terms of accuracy and efficiency in predicting velocity moments and particle dispersion even without the presence of so many particles. The dependence of different SSF models on the number of computational particles in a converged flow field is also discussed. This paper is useful for the selection and application of SSF models in numerical simulations of practical two-phase turbulent flows.

Particle transport behavior in air channel flow with multi-group Lagrangian tracking*

The particle motions of dispersion and transport in air channel flow are investigated using a large eddy simulation (LES) and Lagrangian trajectory method. The mean and fluctuating velocities of the fluids and particles are obtained, and the results are in good agreement with the data in the literature. Particle clustering is observed in the near-wall and low-speed regions. To reveal the evolution process and mechanism of particle dispersion and transport in the turbulent boundary layer, a multi-group Lagrangian tracking is applied when the two-phase flow has become fully developed: the fluid fields are classified into four sub-regions based on the flow characteristics, and particles in the turbulent region are divided accordingly into four groups when the gas–particle flow is fully developed. The spatiotemporal transport of the four groups of particles is then tracked and analyzed. The detailed relationship between particle dispersion and turbulent motion is investigated and discussed.

Stochastic separated flow models with applications in numerical computations of supersonic particle-laden turbulent flows:

In this article, three stochastic separated flow models were applied to predict the dispersion of inertial fuel particles in the supersonic turbulent flows. The flow field of continuous phase was simulated by means of Reynolds-averaged Navier–Stokes method with a two-equation turbulence model. Clift’s expression was used to modify the drag force on the particle considering the compressibility effects. The particle-phase statistics were obtained by a secondary-order time-weighed Eulerian method. The ability of those stochastic separated flow models was then compared for predicting the mean particle velocity and the particle dispersion. For obtaining a statistically stationary solution, the stochastic separated flow model required the largest number of computational particles, whereas the improved stochastic separated flow model was found to need the least. The time-series stochastic separation flow model lay in-between. Compared with the other two models, the particle dispersion was over-predicted by the stochastic separated flow model in the supersonic particle-laden boundary layer flow, while the improved stochastic separated flow model was less predictable for the particle spatial distribution in the particle-laden strut-injection flow. Three models could well predict the mean velocities of the particle phase. This study is valuable for selecting a validated model used for predicting the particle dispersion in supersonic turbulent flows.