Influence of Turbulence Schmidt Number on Exit Temperature Distribution of an Annular Gas Turbine Combustor using Flamelet Generated Manifold

The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS(Reynoldsaveraged Navier–Stokes)simulations,where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer.This concept,applied to a lean premixed combustion system,was assessed in this paper in terms of exit temperature distribution.Because of the isotropic assumption involved in this analogy,the prediction in some flow condition,such as jet cross flow mixing,would be inaccurate.In this study,using Flamelet Generated Manifold as reaction model,some of the numerical results,obtained from an annular combustor configuration with the turbulent Schmidt number varying from 0.85 to 0.2,were presented and compared with a benchmark atmospheric test results.It was found that the Schmidt numberσt in mean mass fraction f transport equation had significant effect on dilution air mixing process.The mixing between dilution air and reaction products from the primary zone obviously improved asσt decreased on the combustor exit surface.Meanwhile,the sensitivity ofσt in three turbulence models including Realizable k-ε,SST(Shear Stress Transport)and RSM(Reynolds Stress Model)has been compared as well.Since the calculation method of eddy viscosity was different within these three models,RSM was proved to be less sensitive than another two models and can guarantee the best prediction of mixing process condition.On the other hand,the results of dilution air mixing were almost independent of Schmidt number Sct in progress variable c transport equation.This study suggested that for accurate prediction of combustor exit temperature distribution in steady state reacting flow simulation,the turbulent Schmidt number in steady state simulation should be modified to cater to dilution air mixing process.     

湍流燃烧模型对某燃气轮机燃烧室性能计算的影响

针对某型燃气轮机环形燃烧室,通过数值模拟,对比分析了Standardk—ε、RNGk-ε、Realizable k-ε、Standard k-ω、雷诺应力五种湍流模型以及简单概率密度模型（PDF）、有限速率模型、涡耗散模型（ED）、涡耗散概念模型（EDC）四种燃烧模型对环形燃烧室性能计算的影响。结果表明：Realizable k-ε模型模拟的冷态流场与PIV试验测量结果最符合;EDC模型最能合理的模拟燃烧效果,但在预测NOx排放时不如有限速率模型精确。以上结论为后续该型号燃气轮机环形燃烧室的数值计算和设计提供了参考。     

Numerical simulation of 3D turbulent isothermal flow in a vortex combustor

Numerical simulations of strongly swirling turbulent flows in a vortex combustor (VC) are conducted. A comprehensive investigation of a three-dimensional isothermal VC flow using three first-order turbulence models: the standard k–ε turbulence model, Renormalized Group (RNG) k–ε model and shear stress transport (SST) k–ω model; and a second-order turbulence model, Reynolds stress model (RSM) together with a second-order numerical differencing scheme is conducted in the present work. The computation indicates that the RSM is superior to the other turbulence models in capturing the swirl flow effect in comparison with measurements. The numerical results for the VC flow provide the characteristics of the flow in terms of relevant parameters for the VC design and operation, composed of axial and tangential velocities, pressure fields, and turbulence kinetic energy.     

Investigation of the effect of air turbulence intensity on NOx emission in non-premixed hydrogen and hydrogen-hydrocarbon composite fuel combustion

In this paper, the effect of air turbulence intensity on NO formation in the combustion of mixed hydrogen-hydrocarbon fuel is numerically studied. The fuels used in this study are 100% H₂, 70% H₂ + 30% CH₄, 10% H₂ + 90% CH₄ and 100% CH₄. Finite volume method is utilized to solve the governing equations. The obtained results using realizable k-ε and β-PDF models show good agreement with other numerical and experimental results. The results show that increasing air turbulence intensity decreases NO concentration in the flame zone and at the combustor outlet. With increasing air turbulence intensity, maximum decreasing of NO at the combustor outlet is for the case of pure hydrogen fuel. It is also found that adding hydrogen to methane rises the peak temperature of the flame.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

Supersonic combustion of hydrogen using an improved strut injection scheme

Numerical investigation of mixing is performed at Mach 2.0 model Scramjet combustor employing parallel strut injection schemes for fuel. In the present investigation, basic strut injector is modified in such a way to produce additional vortices in streamwise direction and improve fuel-air mixing. Air is injected at Mach 2.0 at the combustor inlet and fuel is injected at sonic speed from the blunt end of the strut. The flow field involving high-speed turbulent mixing and heat addition was modeled by three-dimensional Reynolds averaged Navier-Stokes equations. A realizable k-ε model was chosen to close the turbulence problem with the default model constants. Non-premixed combustion of hydrogen and air is modeled using the mixture fraction β-pdf framework. Turbulence-chemistry interactions are handled by a strained flamelet model. Comparisons of numerical results with experimental results have demonstrated the accuracy and applicability of computational grid and a numerical scheme for hot and cold flow solutions. The shock-shear layer interaction present within the combustor increases the local turbulent intensity and has a positive effect on mixing. The mixing efficiency obtained with improved strut injector is compared with the basic strut. Improved strut injection scheme showed a mixing efficiency of >95% with a 45% reduction in length. Further combustion efficiency is calculated in the streamwise direction and plot follows the similar trend as the mixing efficiency. The proposed modification of strut geometry showed improved mixing and combustion performance.     

微热光电系统带环形翅片燃烧室的数值模拟

最大化提高微燃烧室的燃烧效率对于微热光电系统是非常关键的。建立了微燃烧室内的流动、传热和燃烧模型并进行了数值模拟。利用试验结果验证了模型的可靠性。在此基础上,模拟了带有环形翅片燃烧室的情况,表明环形翅片能增强微燃烧室内混合气体的湍流扰动,改善燃烧状况,有效地提高燃烧效率。     

Computations of shear driven vortex flow in a cylindrical cavity using a modified k-ε turbulence model

In this paper, a new variant of the k-ε turbulence model (Saqr et al., CFD Letters, 1(2) pp. 87–94) is used to compute the shear driven vortex flow in an open cylindrical cavity. The results are compared with published LDA measurements for such flow configuration. The modified turbulence model demonstrated good agreement with experimental results, which further supports its validity in computing vortex dominated flows.     

Test of turbulence models for natural convection in an open cubic tilted cavity

A comparison was made between six turbulence models and experimental temperature profiles for the turbulent natural convection in a tilted open cubic cavity. The experimental setup consists of a cubic cavity of 1 m by side with one vertical wall receiving a constant and uniform heat flux, whereas the remaining walls are thermally insulated. The thermal fluid is air and the aperture is facing the heated wall. The temperature profiles were obtained at different heights and depths and each one consists of 10 positions inside the cavity. A commercial computational fluid dynamic software was used for the simulation and different turbulence models of k-ε_t and k-ω families were evaluated against experimental data. The lowest absolute average percentage difference for the experimental and numerical temperature profiles was for the rk-ε_t model and the highest was for the sk-ω model.     

Effects of rectangular rib on exergy efficiency of a hydrogen-fueled micro combustor

In order to further optimize the working performance of micro-cylindrical combustor, the combustion chamber of the micro-cylindrical combustor is inserted in a rectangular rib. Extensive numerical investigations are conducted to compare the exergy efficiency of non-ribbed and rectangular-ribbed micro combustors under various hydrogen mass flow rates and hydrogen/air equivalence ratios. Moreover, the effects of dimensionless rib positions and heights on the exergy efficiency of micro-cylindrical combustor are also widely investigated. Results suggest that the exergy efficiency of the rectangular-ribbed micro combustor is significantly higher than that of the non-ribbed micro combustor under different inlet conditions. Moreover, the exergy efficiency of the rectangular-ribbed micro combustor is significantly affected by the dimensionless positions l. The optimum dimensionless l is increased with the increase of hydrogen mass flow rate. This work offers us significant reference for optimizing the micro combustor in energy utilization.     

Effect of variation of inlet boundary conditions on the combustion flow-field of a typical double cavity scramjet combustor

The present research work deals with the numerical simulation of double cavity scramjet combustor by using two equation standard k-$\epsilon$ turbulence model and finite-rate/eddy-dissipation reaction models which is again coupled with Reynolds-Averaged Navier-Stokes (RANS) equations to investigate the influence of variation of inlet boundary condition of air as well as H₂ fuel on the combustion flow-field of scramjet engine subsequently. At the same time, the validations of the current computational approach have been completed against a standard experimental data which is available in the literature. An acceptable similarity is observed between present numerical approach with the experimentally attained schlieren photograph and the pressure distribution curve. In the present work, 8 different cases are studied. Among them, first four cases are investigated for the variation of inlet boundary condition of air and the remaining four cases are studied for the variation of inlet boundary condition of H₂ fuel. The obtained results show that the formation of high-pressure region around the cavities for case 3 and case 4 actually helps to push the greater amount of air to the cavity region where it is mixed with adequate amount of H₂ for proper and stable combustion whereas for case 6, it is observed that most of the combustion phenomena closely fitted into a small space of the combustor and mainly occurs near the cavity region.     

Features of convective heat transfer in heated helium channel flow

The aim of the present work is to choose an optimal method for thermohydraulic calculation of the gas flow in channels with intense heating at the flow Reynolds number below 10,000. These conditions are typical of the cooling channels of the High-Flux-Test Module of the International-Fusion-Materials-Irradiation-Facility (IFMIF/HFTM). A low Reynolds number and a high heating rate can result in partial relaminarization of the initially turbulent flow, and hence in a decrease in the heat transfer. A number of turbulence models offered by the commercial STAR-CD code were tested on the basis of the comparison of the numerical predictions with experimental data. This comparison showed that the low-Reynolds-number k-ε turbulence models predict the heat transfer characteristics close to the experimental data. The k-ε linear low Reynolds number turbulence model of Lien was applied as more appropriate for the thermohydraulic analysis of the IFMIF high flux test module.     

Effect of free stream turbulence on NOx and soot formation in turbulent diffusion CH4-air flames

A two-dimensional axisymmetric RANS numerical model was solved to investigate the effect of increasing the turbulence intensity of the air stream on the NOx and soot formation in turbulent methane diffusion flames. The turbulence–combustion interaction in the flame field was modelled in a k − ε/EDM framework, while the NO and soot concentrations were predicted through implementing the extended Zildovich mechanism and two transport equations model, respectively. The predicted spatial temperature gradients showed acceptable agreement with published experimental measurements. It was found that the increase of free stream turbulence intensity of the air supply results in a significant reduction in the NO formation of the flame. Such phenomenon is discussed by depicting the spatial distribution of the NO concentration in the flame. An observable reduction of the soot formation was also found to be associated with the increase of inlet turbulence intensity of air stream.     

Computational and experimental investigations of turbulent asymmetric vortex flames

In the present article we present computational and experimental investigations of a turbulent asymmetric vortex flame. Such flame was created in a novel asymmetric combustor, which is described for the first time in this article. The three dimensional isothermal and reacting flow fields have been described using a computational methodology that impalements the R_ε/k − ε and the eddy dissipation turbulence and combustion models, respectively. The computational model is validated for both isothermal and reacting flows. Additionally, the visible flame structure was captured by direct photography at a wide range of equivalence ratios in order to emphasize the exceptional stability of such flame. The mechanism of flame stability and interaction with the forced vortex field is preliminarily discussed. Finally, the basic characteristics of the asymmetric vortex flames are concluded.     

Numerical simulation of the effect of multiple obstacles inside the tube on the spontaneous ignition of high-pressure hydrogen release

Self-ignition may occur during hydrogen storage and transportation if high-pressure hydrogen is suddenly released into the downstream pipelines, and the presence of obstacles inside the pipeline may affect the ignition mechanism of high-pressure hydrogen. In this work, the effects of multiple obstacles inside the tube on the shock wave propagation and self-ignition during high-pressure hydrogen release are investigated by numerical simulation. The RNG k-ε turbulence model, EDC combustion model, and 19-step detailed hydrogen combustion mechanism are employed. After verifying the reliability of the model with experimental data, the self-ignition process of high-pressure hydrogen release into tubes with obstacles with different locations, spacings, shapes, and blockage ratios is numerically investigated. The results show that obstacles with different locations, spacings, shapes and blockage ratios will generate reflected shock waves with different sizes and propagation trends. The closer the location of obstacles to the burst disk, the smaller the spacing, and the larger the blockage ratio will cause the greater the pressure of the reflected shock wave it produces. Compared with the tubes with rectangular-shaped, semi-circular-shaped and triangular-shaped obstacles, self-ignition is preferred to occur in tube with triangular-shaped obstacles.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

Modelling of the gas-turbine colorless distributed combustion: An application to hydrogen enriched – kerosene fuel

This study examines hydrogen-enriched kerosene combustion under distributed regime in a gas turbine combustion chamber. With hydrogen enrichment, it is aimed at increasing combustion performance of those fuels. However, in this circumstance, it is obvious to increase the flame temperature with taking place hydrogen enrichment. Thus colorless distributed combustion (CDC), which is one of the advanced combustion techniques, can be suggested to control flame temperature with slowing down the reaction rate, resulting in ultra-low NO_X emissions and more uniform temperature distribution with a broadened flame. For this purpose, the hydrogen-enriched kerosene fuels were examined by modeling a CFD code using the eddy dissipation concept, the radiation model (P-1) and the turbulence model (standard k-ε). In this way, the thermal fields and the NO_X distributions have been obtained. The results showed that hydrogen enrichment increased the flame temperatures from about 2490 K to 2605 K at air-combustion conditions until 30% H₂, resulting in the NO_X levels predicted increased in the combustor. With reducing oxygen percentage the flame started to be the broadened one. The flame temperatures decreased, for instance, from about 2605 K to 2230 K at 15% O₂ for the 30% H₂ containing fuel. As a result of this, the NO_X levels reduced from about 30 ppm to the values lower than 1 ppm in the combustor. It is concluded that increments in temperature and NO_X levels with hydrogen can be suppressed with distributed regime, which enables that gas turbines can be operated at wider flammability limits with hydrogen enrichment.     

Numerical investigations on the performance of a multistrut hydrogen injector in a scramjet combustor

This study aims at investigating the effect of a multistrut-based hydrogen injector in a scramjet combustor underreacting case. The numerical analysis is carried out using two-dimensional Reynolds-averaged Navier–Stokes equations with the Shear Stress Transport k − ω turbulence model in contention to comprehend the flow physics during scramjet combustion. The three major parameters, such as the shock wave pattern, wall pressures, and static temperature across the combustor, are validated with the reported experimental results. The results comply with the range, indicating that the adopted simulation method for single strut injection can be extended for other investigations. It is noticed that with multistrut injectors, as hydrogen jet pressure increases in the supersonic flow field, the jet penetration rate in the lateral direction of the flow and the total pressure loss as compared with the baseline injection pressure conditions has increased. The supersonic flow characteristics are determined based on the flow properties, combustion efficiency, mixing efficiency, and total pressure loss. Compared with the single-strut output of a scramjet combustor, multistruts inclusion increased the combustion efficiency by almost 18%, the mixing efficiency attained the maximum with 16% fewer lengths. The total pressure loss in single-strut is 14% lower than that of multistrut.     

Modelling turbulence with an Actuator Disk representing a tidal turbine

RANS turbulence models in association with the Actuator Disk theory is among the most popular concepts to predict the wake of tidal turbines. This methodology has the advantage of giving accurate results at affordable computing costs. In this paper, several turbulence models usually used to simulate the wake behind a turbine represented by a porous disk are compared. The tested models are the Standard and the Realizable k-ε models, the SST k-ω model and the Reynolds Stress Model (RSM). The results of the simulations are validated against experimental data measured behind a porous disk placed in a laboratory water flume. The experimental data consist of a set of turbulence intensity and velocity profiles. Without modifying the turbulence models, both velocities and turbulence intensities are well simulated by all the models in the far wake. However, in the near wake, the results strongly differ from one model to another. In the vicinity of the disk, the Standard k-ε model provides the most accurate results. The other models tend to underestimate the turbulence production as the fluid passes through the disk. To compensate this, another set of simulations are performed adding TKE (Turbulent Kinetic Energy) within the porous disk. The optimal amount of additional TKE differs from one model to another. Except for the Standard k-ε model, the correlation between the model and the experimental results is significantly improved when enhancing the TKE in the area occupied by the disk.     

Numerical investigation of turbulent combustion with hybrid enrichment by hydrogen and oxygen

In this study, the NG + H₂/air + O₂ turbulent flame is numerically investigated using the Computational Fluid Dynamics CFD code. The modulation of combustion and radiation is performed respectively by the Eddy Dissipation Model and the Discrete Ordinate Model. The turbulence modeling is carried out by Shear Stress Transport (SST/k-ω) turbulence model. The H₂ amount in the fuel mixture varies under constant volumetric fuel flow between 0 and 60% and the oxidant is composed by 80% air and 20% pure oxygen. The results obtained show the hydrogen addition to Natural Gas improves the mixing between the reactants, reduces their residence time and reduces the length and thickness of the flame. On the other hand, the hydrogen enrichment minimizes the CO₂ and CO production and increases the NO_x level.