Two different combustion models,the autoignition(AI)model and flamelet/progress variable(FPV)model,have been applied to study the auto-ignition process of methane/air jet flame in vitiated co-flow.A priori study was c...Two different combustion models,the autoignition(AI)model and flamelet/progress variable(FPV)model,have been applied to study the auto-ignition process of methane/air jet flame in vitiated co-flow.A priori study was conducted to test the validity of the two models.Results show that the different range of predicted reaction rates is mainly responsible for their different performances in large eddy simulation(LES)studies.In this paper,beta PDF was used to model the mixture fraction distribution,while two different shapes of PDF,delta function and beta function,were applied for the reaction progress.Compared to the FPV model,the AI model combined with beta function for reaction progress could capture the auto-ignition process and predict the exact lifted height.Also the results indicate that the variance of reaction progress plays an important role in predicting the flame lifted height.展开更多
Large eddy simulations(LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of...Large eddy simulations(LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core(PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition(POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure.展开更多
Large eddy simulations (LES) have been performed to investigate the flow and combustion fields in the scramjet of the German Aerospace Center (DLR). Turbulent combustion is mod- eled by the tabulated thermo-chemis...Large eddy simulations (LES) have been performed to investigate the flow and combustion fields in the scramjet of the German Aerospace Center (DLR). Turbulent combustion is mod- eled by the tabulated thermo-chemistry approach in combination with the presumed probability density function (PDF). A/3-function is used to model the distribution of the mixture fraction, while two different PDFs, g-function (Model I) and //-function (Model II), are applied to model the reaction progress. Temperature is obtained by solving filtered energy transport equation and the reaction rate of the progress variable is rescaled by pressure to consider the effects of compressibil- ity. The adaptive mesh refinement (AMR) technique is used to properly capture shock waves, boundary layers, shear layers and flame structures. Statistical results of temperature and velocity predicted by Model II show better accuracy than that predicted by Model I. The results of scatter points and mixture fraction-conditional variables indicate the significant differences between Model I and Model II. It is concluded that second moment information in the presumed PDF of the reaction progress is very important in the simulation of supersonic combustion. It is also found that an unstable flame with extinction and ignition develops in the shear layers of bluff body and a fuel- rich partially premixed flame stabilizes in the central recirculation bubble.展开更多
The CH4/air premixed gas combustion processes in porous media were numerically studied using the two-temperature reacting fluid model with dispersions and detailed chemical reaction mechanism GRI 3.0. The mechanisms o...The CH4/air premixed gas combustion processes in porous media were numerically studied using the two-temperature reacting fluid model with dispersions and detailed chemical reaction mechanism GRI 3.0. The mechanisms of the propagation and stabilization of submerge flames and surface flames in porous media were illuminated distinctly by considering the magnitude of the terms in the two energy equations, analyzing the sensibility of flame propagation speed to flame location, heat exchange coefficient between gas and solid, thermal conductivity and radiative extinction coefficient of porous media. It was concluded that the propagation mechanism of a submerged flame is similar to that of a free flame with an additional preheat zone and that the surface-flame propagation mechanism in porous media is similar to that of a free flame with heat loss in reaction zone.展开更多
In this paper, the low Mach number he- lium and nitrogen flows in micro-channels are inves- tigated numerically with variations of inlet to outlet pressure ratios, aspect ratios, out pressures and fluid mediums by usi...In this paper, the low Mach number he- lium and nitrogen flows in micro-channels are inves- tigated numerically with variations of inlet to outlet pressure ratios, aspect ratios, out pressures and fluid mediums by using different continuum-based slip models. Theoretical solutions based on perturbation expansions of the Navier-Stokes equations have been developed under different order slip conditions. The validity of slip models has been examined by the corresponding experiments and the DSMC method at different Knudsen numbers. Simulations have shown good predictions of the compressibility, rarefaction and thermal creep effects on micro-channel flows with the present slip models. The higher order slip models relatively decrease the rarefaction effects comparing with a first-order slip model. Both of the Knudsen number and the Reynolds number have been identified as key parameters, which govern the rarefaction effects and thermal creep effects, respec- tively. The present slip models have been also dem- onstrated to be appropriate for micro-channel nitro- gen flows with the Knudsen number less than 0.15, and the higher order slip conditions improve the Na- vier-Stokes predictions in the slip flow regime with Kn<0.08. However, the continuum-based slip models significantly under-predict the rarefaction effects in the transitional flow regime as the Knudsen number exceeds 0.2.展开更多
Following the idea of Speziale's Very Large Eddy Simulation(VLES) method,a new unified hybrid simulation approach was proposed which can change seamlessly from RANS(Reynolds-Averaged Navier-Stokes) to LES(Large Ed...Following the idea of Speziale's Very Large Eddy Simulation(VLES) method,a new unified hybrid simulation approach was proposed which can change seamlessly from RANS(Reynolds-Averaged Navier-Stokes) to LES(Large Eddy Simulation) method depending on the numerical resolution.The model constants were calibrated in accordance with other hybrid methods.Besides being able to approach the two limits of RANS and LES,the new model also provides a proper VLES mode between the two limits,and thus can be used for a wide range of mesh resolutions.Also RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation(DES) concept.This new methodology was implemented into Wilcox's k-ω model and applications were conducted for fully developed turbulent channel flow at Reτ = 395 and turbulent flow past a square cylinder at Re = 22000.Results were compared with LES predictions and other studies.The new method is found to be quite efficient in resolving large flow structures,and can predict satisfactory results on relative coarse mesh.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.51176178 and 50936005)
文摘Two different combustion models,the autoignition(AI)model and flamelet/progress variable(FPV)model,have been applied to study the auto-ignition process of methane/air jet flame in vitiated co-flow.A priori study was conducted to test the validity of the two models.Results show that the different range of predicted reaction rates is mainly responsible for their different performances in large eddy simulation(LES)studies.In this paper,beta PDF was used to model the mixture fraction distribution,while two different shapes of PDF,delta function and beta function,were applied for the reaction progress.Compared to the FPV model,the AI model combined with beta function for reaction progress could capture the auto-ignition process and predict the exact lifted height.Also the results indicate that the variance of reaction progress plays an important role in predicting the flame lifted height.
基金supported by the National Natural Science Foundation of China(Grant Nos.51176178&91441117)the Key Program of National Natural Science Foundation of China(Grant No.50936005)
文摘Large eddy simulations(LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core(PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition(POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure.
基金financial support by the National Natural Science Foundation of China (Nos. 51176178, 50936005)
文摘Large eddy simulations (LES) have been performed to investigate the flow and combustion fields in the scramjet of the German Aerospace Center (DLR). Turbulent combustion is mod- eled by the tabulated thermo-chemistry approach in combination with the presumed probability density function (PDF). A/3-function is used to model the distribution of the mixture fraction, while two different PDFs, g-function (Model I) and //-function (Model II), are applied to model the reaction progress. Temperature is obtained by solving filtered energy transport equation and the reaction rate of the progress variable is rescaled by pressure to consider the effects of compressibil- ity. The adaptive mesh refinement (AMR) technique is used to properly capture shock waves, boundary layers, shear layers and flame structures. Statistical results of temperature and velocity predicted by Model II show better accuracy than that predicted by Model I. The results of scatter points and mixture fraction-conditional variables indicate the significant differences between Model I and Model II. It is concluded that second moment information in the presumed PDF of the reaction progress is very important in the simulation of supersonic combustion. It is also found that an unstable flame with extinction and ignition develops in the shear layers of bluff body and a fuel- rich partially premixed flame stabilizes in the central recirculation bubble.
基金the National Natural Science Foundation of China (Grant No. 50376060)
文摘The CH4/air premixed gas combustion processes in porous media were numerically studied using the two-temperature reacting fluid model with dispersions and detailed chemical reaction mechanism GRI 3.0. The mechanisms of the propagation and stabilization of submerge flames and surface flames in porous media were illuminated distinctly by considering the magnitude of the terms in the two energy equations, analyzing the sensibility of flame propagation speed to flame location, heat exchange coefficient between gas and solid, thermal conductivity and radiative extinction coefficient of porous media. It was concluded that the propagation mechanism of a submerged flame is similar to that of a free flame with an additional preheat zone and that the surface-flame propagation mechanism in porous media is similar to that of a free flame with heat loss in reaction zone.
基金This work was supported by the National Natural Science Foundation of China (Grant Nos. 50376060 and 10372099).
文摘In this paper, the low Mach number he- lium and nitrogen flows in micro-channels are inves- tigated numerically with variations of inlet to outlet pressure ratios, aspect ratios, out pressures and fluid mediums by using different continuum-based slip models. Theoretical solutions based on perturbation expansions of the Navier-Stokes equations have been developed under different order slip conditions. The validity of slip models has been examined by the corresponding experiments and the DSMC method at different Knudsen numbers. Simulations have shown good predictions of the compressibility, rarefaction and thermal creep effects on micro-channel flows with the present slip models. The higher order slip models relatively decrease the rarefaction effects comparing with a first-order slip model. Both of the Knudsen number and the Reynolds number have been identified as key parameters, which govern the rarefaction effects and thermal creep effects, respec- tively. The present slip models have been also dem- onstrated to be appropriate for micro-channel nitro- gen flows with the Knudsen number less than 0.15, and the higher order slip conditions improve the Na- vier-Stokes predictions in the slip flow regime with Kn<0.08. However, the continuum-based slip models significantly under-predict the rarefaction effects in the transitional flow regime as the Knudsen number exceeds 0.2.
基金supported by the National Natural Science Foundation of China (Grant No. 50936005)the National Basic Research Program of China (Grant No. 2010CB227302)
文摘Following the idea of Speziale's Very Large Eddy Simulation(VLES) method,a new unified hybrid simulation approach was proposed which can change seamlessly from RANS(Reynolds-Averaged Navier-Stokes) to LES(Large Eddy Simulation) method depending on the numerical resolution.The model constants were calibrated in accordance with other hybrid methods.Besides being able to approach the two limits of RANS and LES,the new model also provides a proper VLES mode between the two limits,and thus can be used for a wide range of mesh resolutions.Also RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation(DES) concept.This new methodology was implemented into Wilcox's k-ω model and applications were conducted for fully developed turbulent channel flow at Reτ = 395 and turbulent flow past a square cylinder at Re = 22000.Results were compared with LES predictions and other studies.The new method is found to be quite efficient in resolving large flow structures,and can predict satisfactory results on relative coarse mesh.