Large eddy simulation of turbulent premixed combustion flow around bluff body

Large eddy simulation (LES) of turbulent premixed flame stabilized by the bluff body is developed by using sub-grid scale combustion model based on the G-equation describing the flame front propagation. The basic idea of LES modeling is to evaluate the filtered-front speed, which should be enhanced in the grid scale by the scale fluctuations. The dynamic sub-grid scale models newly introduced into the G-equation are validated by the premixed combustion flow behind the triangle flame holder. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model to a complex turbulent premixed combustion such as bluff body stabilized turbulent premixed flames for realistic engineering application. A new turbulent flame speed model, introduced by the sub-grid turbulent diffusivity and the flame thickness, is also proposed and is compared with the usual model using sub-grid turbulent intensity and with the experimental data. The calculated results can predict the velocity and temperature of the combustion flow in good agreement with the experiment data.     

Large eddy simulation of turbulent premixed flame in turbulent channel flow

Large eddy simulation of turbulent premixed flame in turbulent channel flow is studied by usingG-equation. A flamelet model for the premixed flame is combined with a dynamic subgrid combustion model for the filtered propagation flame speed. The objective of this work is to investigate the validity of the dynamic subgridG-equation model to a complex turbulent premixed flame. The effect of model parameters of the dynamic subgridG-equation on the turbulent flame speed is investigated. In order to consider quenching of laminar flames on the wall, wall-quenching damping function is employed in this calculation. In the present study, a constant density turbulent channel flow is used. The calculation results are evaluated by comparing with the DNS results of Bruneaux et al.     

Linear stability analysis of acoustically driven pressure oscillations in a lean premixed gas turbine combustor

The dynamic response of a turbulent premixed flame to acoustic velocity perturbations was experimentally determined in a swirl-stabilized lean-premixed gas turbine combustor. CH* chemiluminescence intensity and the twomicrophone method were used to measure heat release rates and inlet velocity fluctuations, respectively. Using the n-τ formulation, gain and phase of flame transfer functions were incorporated into an analytic thermoacoustic model to predict instability frequencies and modal structures. Self-excited instability measurements were performed to verify eigenfrequencies predicted by the thermoacoustic model. Instability frequency predicted by the model is supported by experimental results. Results show that the self-excited instability frequency of ～ 220 Hz results from the fact that the flames amplify flow perturbations with f = 150 ～ 250 Hz. The other instability frequency of ～ 350 Hz occurs because the whole combustion system has an eigenfrequency corresponding to the ¼-wave eigenmode of the mixing section.     

A relative assessment of sub grid scale models for large eddy simulation of co-axial combustor

Large eddy simulation (LES) of a co-axial combustor is reported. Accuracy of LES depends on the ability of the sub-grid scale (SGS) models to predict the turbulent viscosity. The sensitivity of LES results for different SGS models is established for a coaxial annular combustor. The radial, axial and tangential velocity distribution predicted by four sub-grid scale models is compared with the experimental results of Sommerfeld and Qiu. It is observed that the flow physics is captured more accurately by WALE model as compared to other SGS models. The predictions of WALE model are closer to the experimental results for all stations in the flow domain.     

贫油直喷燃烧室回火的数值研究

针对燃烧稳定性中的回火问题,对贫油直喷燃烧室的回火特性进行了研究.燃烧中的回火指的是火焰从燃烧室传入到了预混区中的这样一种现象,它包含了声波、湍流、燃烧之间的复杂的相互作用,是燃烧研究中的关键问题之一.利用Fluent中的混合分数/PDF平衡化学反应模型对贫油直喷燃烧室进行了大涡模拟,分别计算了冷态和热态下的流场,捕捉了贫油直喷燃烧室内回火的动态过程,其中亚格子模型采用WALE模型,燃油射流采用离散相模型.研究结果表明,大涡模拟能够较好地反映流场分布情况,贫油直喷燃烧室内存在中心回流区,模拟结果与实验结果吻合较好;贫油直喷燃烧室中的回火发生在中心流区域,回火时,中心回流区向上游移动,这促使了火焰的向上游传播,并最终导致了回火的发生.     

Idealized gas turbine combustor for performance research and validation of large eddy simulations

This paper details the design of a premixed, swirl-stabilized combustor that was designed and built for the express purpose of obtaining validation-quality data for the development of large eddy simulations (LES) of gas turbine combustors. The combustor features nonambiguous boundary conditions, a geometrically simple design that retains the essential fluid dynamics and thermochemical processes that occur in actual gas turbine combustors, and unrestrictive access for laser and optical diagnostic measurements. After discussing the design detail, a preliminary investigation of the performance and operating envelope of the combustor is presented. With the combustor operating on premixed methane/air, both the equivalence ratio and the inlet velocity were systematically varied and the flame structure was recorded via digital photography. Interesting lean flame blowout and resonance characteristics were observed. In addition, the combustor exhibited a large region of stable, acoustically clean combustion that is suitable for preliminary validation of LES models.     

Eulerian particle flamelet modeling for combustion processes of bluff-body stabilized methanol-air turbulent nonpremixed flames

The present study is focused on the development of the RIF (Representative Interactive Flamelet) model which can overcome the shortcomings of conventional approach based on the steady flamelet library. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF model can effectively account for the detailed mechanisms of NOx formation including thermal NO path, prompt and nitrous NOx formation, and reburning process by hydrocarbon radical without any ad-hoc procedure. The flamelet time of RIFs within a stationary turbulent flame may be thought to be Lagrangian flight time. In context with the RIF approach, this study adopts the Eulerian Particle Flamelet Model (EPFM) with mutiple flamelets which can realistically account for the spatial inhomogeneity of scalar dissipation rate. In order to systematically evaluate the capability of Eulerian particle flamelet model to predict the precise flame structure and NO formation in the multi-dimensional elliptic flames, two methanol bluffbody flames with two different injection velocities are chosen as the validation cases. Numerical results suggest that the present EPFM model has the predicative capability to realistically capture the essential features of flame structure and NOx formation in the bluff-body stabilized flames.     

直喷式柴油发动机缸内三维温度场的计算与试验

为计算直喷式柴油发动机缸内燃烧温度,提出利用理想混合比质量分数空间来计算缸内温度场.通过引入混合比质量分数基本概念,推导出各种组分与混合比质量分数的关系.通过引入小火焰的假设,分析典型非预混合小火焰结构,推导出绝热状况下缸内温度与混合比质量分数的关系.在理想混合比质量分数空间坐标系下建立新的温度输运方程,提供一种将湍流化学反应场和湍流运动场分离开的方法.经过一系列参数的输入,计算出缸内温度与扩散尺度速率关系,缸内温度与混合比质量分数关系以及缸内火焰温度分布,试验验证了计算结果.为计算直喷式柴油发动机缸内燃烧温度以及污染物成分提供新的计算方法.     

A lagrangian based scalar pdf method for turbulent combustion models

In this paper, a new “presumed” Probability Density Function (PDF) approach coupled with a Lagrangian tracking method is proposed for turbulent combustion modeling. The test and the investigation of the model are conducted by comparing the model results with DNS data for a premixed flame subjected in a decaying turbulent field. The newly constructed PDF, which incorporates the instantaneous chemical reaction term, demonstrates consistent improvement over conventional assumed PDF models. It has been found that the time evolution of the mean scalar, the variance and the mean reaction rate are strongly influenced by a parameter deduced by a Lagrangian equation which takes into account explicitly the local reaction rate. Tests have been performed for a moderate Damköhler number, and it is expected the model may cover a broader range of Damköhler number. The comparison with the DNS data demonstrates that the proposed model may be promising and affordable for implementation in a moment-equation solver.     

Measurement of three-dimensional flame structure by combined laser diagnostics

To investigate three-dimensional flame structures of turbulent premixed flame experimentally, dual-plane planar laser induced fluorescence (PLIF) of CH radical has been developed. This dual-plane CH PLIF system consists of two independent conventional CH PLIF measurement systems and laser beam from each laser system are led to parallel optical pass using the difference of polarization, and CH PLIF is conducted in two parallel two-dimensional cross sections. The newly-developed dual-plane CH PLIF is combined with single-plane OH PLIF and stereoscopic particle image velocimetry (PIV) to clarify the relation between flame geometry and turbulence characteristics. The laser sheets for single-plane OH PLIF and stereoscopic PIV measurement are located at the center of two planes for CH PLIF. The separation between these two planes is selected to 500 μm. The measurement was conducted in relatively high Reynolds number methane-air turbulent jet premixed flame. The experimental results show that various three-dimensional flame structures such as the handgrip structure, which has been shown by three-dimensional direct numerical simulations (DNS), are included in high Reynolds number turbulent premixed flame. It was shown that the simultaneous measurement containing newly-developed dual-plane CH PLIF is useful for investigating the three-dimensional flame structures. To analyze the flame structures quantitatively, the flame curvature was estimated by using the CH and OH PLIF images, and the probability density function (pdf) of the curvatures was compared with the results of DNS. It was revealed that the minimum radius of curvature of the flame front coincides with Kolmogorov length. However, the feature of pdf of the flame curvature is slightly different from result of DNS, if the curvature was estimated from experimental results in two-dimensional cross section. On the other hand, the feature of pdf of mean curvature that calculated from triple-plane PLIF results is similar to that obtained from three-dimensional DNS. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Takashi Ueda received his B.S. degree in Mechano-Aerospace Engineering from Tokyo Institute of Technology, Japan, in 2008. Mr. Ueda is currently in Master course at Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Mr. Ueda’s research interests include turbulent combustion. Masayasu Shimura received his B.S. degree in Mechano-Aerospace Engineering from Tokyo Institute of Technology, Japan, in 2005. He then received his M.S. from Tokyo Institute of Technology in 2006. Mr. Shimura is currently in Ph.D candidate at Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Mr. Shimura’s research interests include noise control of turbulent combustion. Mamoru Tanahashi received his B.S. degree in Mechanical Engineering, Science from Tokyo Institute of Technology, Japan, in 1990. He then received his M.S. and Dr. Eng. from Tokyo Institute of Technology in 1992 and 1996, respectively. Prof. Tanahashi is currently an Associate Professor of Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Prof. Tanahashi’s research interests include turbulence and combustion. Toshio Miyauchi received his B.S. degree in Mechanical Engineering from Tokyo Institute of Technology, Japan, in 1971. He then received his M.S. and Ph.D. degrees from Tokyo Institute of Technology in 1973 and 1981, respectively. Prof. Miyauchi is currently a Professor of Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan. Prof. Miyauchi’s research interests include turbulence and combustion.     

Transported PDF approach and direct-quadrature method of moment for modeling turbulent piloted jet flames

The direct-quadrature method of moment (DQMOM) and transported PDF (TPDF) approaches have been adopted to predict the precise structure of turbulent CH4-air piloted jet flame. To realistically account for precise turbulent flame structure of turbulent jet flame, the mathematical formations of the transported PDF method are based on the joint-composition PDF model together with IEM mixing process. To minimize computational burden, the steady flamelet library is utilized to account for non-equilibrium chemistry. For unconditional means and conditional scatters for species mass fraction and temperature, comparison between predictions and measurement are made. Numerical results obtained by DQMOM and TPDF are reasonably well agreed with experimental data in terms of the mean and rms of species mass fraction and probability density function.     

燃烧室结构对天然气转子发动机燃烧过程的影响

以一台由端面进气汽油转子发动机改装而来的预混天然气转子发动机为研究对象,在FLUENT软件的基础上通过编程实现转子发动机三维网格的偏心运动,并选择合适的湍流模型、燃烧模型以及详细的CHEMKIN化学反应机理,建立基于化学反应动力学的端面进气天然气转子发动机三维动态数值模拟模型。通过与试验数据进行对比和分析,验证模型的可靠性。在此基础上,研究燃烧室结构对端面进气天然气转子发动机的缸内流场、温度场和中间产物浓度场的影响。结果表明,当燃烧室凹坑布置于转子曲面长度方向的前端和转子曲面宽度方向的中心时,燃烧过程同时利用了燃烧室后部的滚流以及燃烧室中部高速流区对火焰的加速作用,缸内整体燃烧速率最大。同时,其缸内压力最大以及中间产物OH的生成量也最大,其压力峰值比中置凹坑燃烧室提高了19.9%,但其NO质量分数仍在0.5%以内。     

Numerical modeling of turbulent nonpremixed lifted flames

The present study has focused on numerical investigation on the flame structure, flame lift-off and stabilization in the partially premixed turbulent lifted jet flames. Since the lifted jet flames have the partially premixed nature in the flow region between nozzle exit and flame base, level set approach is applied to simulate the partially premixed turbulent lifted jet flames for various fuel jet velocities and co-flow velocities. The flame stabilization mechanism and the flame structure near flame base are presented in detail. The predicted lift-off heights are compared with the measured ones.     

PSR-based microstructural modeling for turbulent combustion processes and pollutant formation in double swirler combustors

The present study numerically investigates the fuel-air mixing characteristics, flame structure, and pollutant emission inside a double-swirler combustor. A PSR (Perfectly Stirred Reactor) based microstructural model is employed to account for the effects of finite rate chemistry on the flame structure and NO formation. The turbulent combustion model is extended to nonadiabatic flame condition with radiation by introducing an enthalpy variable, and the radiative heat loss is calculated by a local, geometry-independent model. The effects of turbulent fluctuation are taken into account by the joint assumed PDFs. Numerical model is based on the non-orthogonal body-fitted coordinate system and the pressure/velocity coupling is handled by PISO algorithm in context with the finite volume formulation. The present PSR-based turbulent combustion model has been applied to analyze the highly intense turbulent nonpremixed flame field in the double swirler combustor. The detailed discussions were made for the flow structure, combustion effects on flow structure, flame structure, and emmission characteristics in the highly intense turbulent swirling flame of the double swirler burner.     

微热光电系统燃烧器的研究

介绍了一种新颖的微动力机电系统，即微热光电系统。该系统可以取代电池作为各种微型机械的动力，广泛应用于民用与军工部门。微燃烧器是微热光电系统的一个重要元件，其外表面必须产生一个高而均匀的温度分布，以尽可能高地输出辐射能。结合三维数值模拟与实验对微燃烧室进行了研究。研究结果表明，带有台阶的微火焰管燃烧室非常适合于微热光电系统的应用。     

Prediction of flame formation in highly preheated air combustion

Fundamental information about the ignition position and shape of a flame in highly preheated air combustion was obtained, and the suitability of the suggested reduced kinetic mechanism that reflects the characteristics of the highly preheated air combustion was demonstrated. Flame lift height and flame length with variations of premixed air temperature and oxygen concentration were measured by CH* chemiluminescence intensity, and were computed with a reduced kinetic mechanism. Flame attached near a fuel nozzle started to lift when preheated air temperature became close to auto-ignition temperature and/or oxygen concentration reduced. The flame lift height increased but the flame length decreased with decreasing preheated air temperature and flame length reversed after a minimum value. Calculated results showed good agreement with those of experiment within tolerable error. Flame shape shifted from diffusion flame shape to partial premixed flame shape with increasing lift height and this tendency was also observed in the computation results. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Gyung-Min Choi studied the areas of combustion engineering, heat recirculating combustion, and solid fuel gasification, receiving his Ph.D. degree in engineering from Osaka University in 2001. He served as a researcher at Japan Aerospace Exploration Agency and is now an associate professor in the School of Mechanical Engineering at Pusan National University.     

Experimental study on combustion instability and attenuation characteristics in the lab-scale gas turbine combustor with a sponge-like porous medium

The present study has experimentally investigated the combustion instability and its attenuation characteristics in the lab-scale swirlstabilized premixed combustor with a sponge-like porous medium. Unlike the conventional premixed burners, this model combustor has the unique features including a porous dump plane and an acoustic cavity, which was devised for attenuating the combustion instability. When replacing the dump plane with a non-porous medium, the burner becomes the conventional design. In order to evaluate the damping effects of the porous medium on the unstable flame dynamics, various acoustic and photonic measurements and flame visualization were made. Special emphasis is given to the effects of the acoustic cavity length on the stabilization characteristics. Results showed that the model combustor with the porous dump plane and the acoustic cavity exhibited dramatic attenuation of the pressure oscillation intensity by up to about 40 %. The attenuation was increased with increasing the acoustic cavity length. It was also found that the attenuation is effective not only for the fundamental resonance but also for its higher harmonics. Based on the experimental results, detailed discussions are made for the combustion instability and its attenuation characteristics in the model gas turbine combustor with porous and nonporous media.

     

On the characteristics of partially-premixed diffusion flame in a strained flow field

The characteristics and extinction of the partially-premixed diffusion flame are studied both experimentally adopting counter-flow burner system and theoretically using matched asymptotic expansion techniques. Results show that the partially-premixed diffusion flame exists at high rate of strain when the degree of partial premixing is low. The results for the practical partially premixed diffusion system indicate that the partially premixed diffusion flame plays an important role in charaterizing the turbulent flames.     

Investigation of self-excited combustion instabilities in two different combustion systems

The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300Hz are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.