L.X. Zhou

Simulation of swirling gas–particle flows using USM and k–ε–kp two-phase turbulence models

Abstract The turbulent swirling gas–particle flows with swirl numbers 0.47 and 1.5 are simulated using a unified second-order moment (USM) (two-phase Reynolds stress equations) and a k–e–kp two-phase turbulence models. The results are compared with experiments. Both two models can well predict the axial time-averaged two-phase velocities in case of s=0.47, but the USM model is better than the k–e–kp model in predicting the tangential time-averaged two-phase velocities of strongly swirling flows (S=1.5). The anisotropic two-phase turbulence can well be described only using the USM model. The results give the difference in flow behavior between weakly swirling and strongly swirling gas–particle flows.

Simulation of swirling gas-particle flows using an improved second-order moment two-phase turbulence model

Abstract Swirling gas–particle flows in a co-axial sudden-expansion chamber with different swirl numbers are simulated using an improved second-order-moment two-phase turbulence model. The particle Reynolds stress equations and the two-phase fluctuation velocity correlation terms are closed based on a Lagrangian analysis, accounting for the crossing-trajectory effect, inertial effect and continuity effect. Predictions give the gas and particle axial and tangential averaged and fluctuation velocities for swirl numbers of s=0, s=0.47 and s=0.94. The swirl number is defined as the ratio of tangential momentum to axial momentum. Prediction results are in good agreement with the PDPA measurement results for both mean and fluctuation velocities of single-phase swirling flows and two-phase mean velocities of swirling gas–particle flows. However, the normal components of Reynolds stresses or the fluctuation velocities of two phases for swirling gas–particle flows are still underpredicted. The results show that increasing swirl number changes the shape and sizes of recirculation zones, the size of the solid-body rotation zone, reduces the turbulent fluctuation of two phases in the upstream region and enhances it in the downstream region.

Simulation of 3-D gas-particle flows and coal combustion in a tangentially fired furnace using a two-fluid-trajectory model

A two-fluid-trajectory (TFT) model is used to simulate three-dimensional (3-D) gas-particle flows and coal combustion in a tangentially fired furnace. A grid system, rotated by an angle, is proposed to reduce the numerical diffusion caused by non-orthogonal crossing of the burner jets with the grid lines. The predictions of cold two-phase flows using the two-fluid (TF) model are compared with those using the stochastic trajectory model (ST model of particle phase) and the phase Doppler particle anemometer (PDPA) measurements. The predicted particle concentration distribution using the TF model is in agreement with PDPA measurements, while the ST model gives too high particle concentration in the near-wall region, not observed in experiments. The 3-D coal combustion predictions give the hot-flow field, coal concentration, gas temperature, species concentration, and heat fluxes on the walls. The prediction results for coal combustion show a local high-temperature zone near the exit of the furnace.

Studies on the effect of swirl numbers on strongly swirling turbulent gas-particle flows using a phase-Doppler particle anemometer

Abstract The effect of swirl numbers on the flow behavior of strongly swirling turbulent gas-particle flows with swirl numbers of s=0.47, 1.0, 1.5 and 2.1 in sudden-expansion and cyclone chambers is studied using a 2-D and a 3-D phase Doppler particle anemometers (PDPA). The axial and tangential time-averaged and the root mean square (RMS) fluctuation velocities of gas and particle phases and particle concentration are measured. The result shows that the swirl number has an obvious effect on the axial velocity profiles, the Rankine vortex structure of tangential velocity profiles, the relationship between two-phase velocities, the turbulence intensity level and the anisotropy of turbulence.

Simulation of swirling gas-particle flows using a DSM-PDF two-phase turbulence model

Abstract A DSM–PDF two-phase turbulence model, namely a Reynolds stress equation (DSM) model of gas turbulence combined with a probability density distribution function (PDF) equation model of particle turbulence, is proposed to simulate the swirling sudden-expansion gas–particle flows with swirl number of 0.47. The predicted axial and tangential gas and particle time-averaged velocities and RMS fluctuation velocities using the DSM–PDF and k–e–kp models are compared with measurements reported in references. The results show that for weakly swirling flows both models can reasonably predict the mean-flow behavior, but the DSM–PDF model can better predict the anisotropy of two-phase turbulence and turbulence interaction between two phases, and hence may have the potential superiority in predicting strongly swirling flows.

Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model☆

Abstract A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k – e – k p two-phase turbulence model, the EBU–Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references.

Studies on the effect of swirl on no formation in methane/air turbulent combustion

Most present studies on pollutant formation concentrate on chemical reaction kinetics. To understand the interaction between turbulence and chemistry in NO formation, the effect of swirl number on NO formation in methane/air turbulent combustion is studied by experiments, in which a small amount of ammonia is added to the fuel to simulate fuel nitrogen, and simultaneously by numerical simulation, using a second-order-moment PDF turbulence-chemistry model. The predicted results are in overall agreement with the measured results. Both predictions and experiments show that as the swirl number increases from 0 to 1, the thermal NO at first increases and then decreases. In contrast, the fuel NO at first decreases and then increases. The studies also show that the increase in swirl number first leads to a rapid decrease and then a slower increase in turbulence intensity, and first an increase and then a slight decrease of temperature near the exit. As the activation energy of thermal NO formation is much larger than that of fuel NO formation, these results imply that the thermal NO is predominantly affected by temperature, whereas the fuel NO is predominantly affected by species mixing via turbulence. The research results are expected to be used for developing low-NO x burners.

Mechanotransduction-modulated fibrotic microniches reveal the contribution of angiogenesis in liver fibrosis

The role of pathological angiogenesis on liver fibrogenesis is still unknown. Here, we developed fibrotic microniches (FμNs) that recapitulate the interaction of liver sinusoid endothelial cells (LSECs) and hepatic stellate cells (HSCs). We investigated how the mechanical properties of their substrates affect the formation of capillary-like structures and how they relate to the progression of angiogenesis during liver fibrosis. Differences in cell response in the FμNs were synonymous of the early and late stages of liver fibrosis. The stiffness of the early-stage FμNs was significantly elevated due to condensation of collagen fibrils induced by angiogenesis, and led to activation of HSCs by LSECs. We utilized these FμNs to understand the response to anti-angiogenic drugs, and it was evident that these drugs were effective only for early-stage liver fibrosis inxa0vitro and in an inxa0vivo mouse model of liver fibrosis. Late-stage liver fibrosis was not reversed following treatment with anti-angiogenic drugs but rather with inhibitors of collagen condensation. Our work reveals stage-specific angiogenesis-induced liver fibrogenesis via a previously unrevealed mechanotransduction mechanism which may offer precise intervention strategies targeting stage-specific disease progression.

Simulation of swirling gas–particle flows using a nonlinear k–ε–kp two-phase turbulence model

Abstract The linear k–e–kp two-phase turbulence model is rather simple, but it cannot predict the anisotropic turbulence of strongly swirling gas–particle flows. The second-order moment, two-phase turbulence model can better predict strongly swirling flows, but it is rather complex. Hence, the algebraic Reynolds stress expressions are derived based on two-phase Reynolds stress equations, and then the nonlinear relationships of two-phase Reynolds stresses and two-phase velocity correlation with the strain rates are obtained. These relationships, together with the transport equations of gas and particle turbulent kinetic energy and the two-phase correlation turbulent kinetic energy constitute the nonlinear k–e–kp turbulence model. The proposed model is applied to simulate swirling gas–particle flows. Predictions give the two-phase time-averaged velocities and Reynolds stresses. The prediction results are compared with phase Doppler particle anemometer (PDPA) measurements and those predicted using the second-order moment model. The results indicate that the nonlinear k–e–kp model has modeling capability nearly equal to that of the second-order moment model, but the former can save much computation time.

DNS-LES Validation of an Algebraic Second-Order-Moment Combustion Model

Direct numerical simulation (DNS) of three-dimensional turbulent reacting channel flows with buoyancy is carried out using a spectral method. Statistical results from the DNS database are used to validate an algebraic second-order-moment sub-grid-scale (ASOM-SGS) combustion model and show that the ASOM-SGS model is reasonable. Furthermore, a methane–air jet flame is simulated by large–eddy simulation (LES) using the ASOM-SGS model and indicates that the Reynolds–averaged Navier-Stokes ASOM combustion model is a reasonable model.