Large Eddy Simulation of Lifted Non-Premixed Jet Flames Using 2-Scalar Flamelet Model

2003 ◽  
Author(s):  
Mikikane Hirohata ◽  
Nobuyuki Taniguchi ◽  
Toshio Kobayashi
Keyword(s):  
Burning Velocity ◽  
Velocity Model ◽  
Flame Stabilization ◽  
Premixed Combustion ◽  
Premixed Flame ◽  
Jet Flames ◽  
Flamelet Model ◽  
The Difference ◽  
Partially Premixed Flame ◽  
Lift Off

In this paper we introduce the LES of lifted non-premixed jet flames based on two-scalar flamelet modeling. The flamelet G-equation for premixed combustion and the conserved scalar equation for non-premixed combustion are combined to express partially premixed flame propagation. In order to close filtered G-equation, the subgrid burning velocity model is proposed based on the concept that small triple flamelet are projected into unburnt gas from the flame-base of the lifted non-premixed flame. The calculation results are shown that wrinkling lifted flames are simulated and the difference of the lift-off height and the flame-shape with the variation of the co-flowing velocity is predicted. It is also confirmed that the conditional axial velocity near the flame base which is thought to relate the condition of the flame stabilization is on the order of two–three times of the laminar burning velocity, which agrees well with the experimental data. We hope that this method will be useful to investigate the flame stabilizing mechanism or flame controls of practical non-premixed jet flames.

The Structure of Two-Stage Counterflow n-Heptane-Air Flames

2000 ◽  
Author(s):  
S. K. Aggarwal ◽  
H. S. Xue
Keyword(s):  
Strain Rate ◽  
Reaction Zone ◽  
Equivalence Ratio ◽  
Flame Structure ◽  
Premixed Combustion ◽  
Premixed Flame ◽  
Partially Premixed Combustion ◽  
Reaction Zones ◽  
Partially Premixed ◽  
Partially Premixed Flame

Partially premixed flames are formed by mixing air (in less than stoichiometric amounts) into the fuel stream prior to the reaction zone, where additional air is available for complete combustion. Such flames can occur in both laboratory and practical combustion systems. In advanced gas turbine combustor designs, such as a lean direct injection (LDI) combustor, partially premixed combustion represents an impotent mode of burning. Spray combustion often involves partially premixed combustion due to the locally fuel vapor-rich regions. In the present study, the detailed structure of n-heptane/air partially premixed flame in a counterflow configuration is investigated. The flame is computed by employing the Oppdif code and a detailed reaction mechanism consisting of 275 elementary reactions and 41 species. The partially premixed flame structure is characterized by two-stage burning or two distinct but synergistically coupled reaction zones, a rich premixed zone on the fuel side and a ‘nonpremixed zone on the air side. The fuel is completely consumed in the premixed zone with ethylene and acetylene being the major intermediate species. The reactions involving the consumption of these species are found to be the key rate-limiting reactions that characterize interactions between the two reaction zones, and determine the overall fuel consumption rate. The flame response to the variations in equivalence ratio and strain rate is examined. Increasing equivalence ratio and/or strain rate to a critical value leads to merging of the two reaction zones. The equivalence ratio variation affects the rich premixed reaction zone, while the variation in strain rate predominantly affects the nonpremixed reaction zone. The flame structure is also characterized in terms of a modified mixture fraction (conserved scalar), and laminar flamelet profiles are provided.

Flame Stability of a Micro Can-Type Afterburner for a SOFC

2008 ◽  
Author(s):  
Yuji Yahagi
Keyword(s):  
Flame Stabilization ◽  
Premixed Flame ◽  
Baffle Plate ◽  
Air Jets ◽  
Air Jet ◽  
Nonpremixed Flame ◽  
Air Mixing ◽  
Fuel Nozzle ◽  
Lean Fuel ◽  
Partially Premixed Flame

This paper is fundamental studies on an afterburner for a 20kW class home cogeneration solid oxide fuel cell (SOFC) hybrid system. The proposed burner is a micro size can-type with a baffle plate having multi air holes set annularly and an opposite arranged single fuel and air nozzle in the center. This geometry is suitable to enhance the fuel and air mixing and to stabilize the flame in the ultra lean fuel of the effluent from SOFC stack in the MGT. The blow off limits and flame shape are discussed with the flow structure behind the baffle plate which measured by using a particle image velocimetry (PIV). The formed flames can be classified into four groups which are a premixed flame, a partially premixed flame, a partially nonpremixed flame, and a nonpremixed flame depend on the pilot air jet velocity, the baffle plate holes air jets velocity, and the clearance of the fuel nozzle exit and baffle plate, even when the flow rate of the fuel is same. When the premixed flame formed side by the fuel nozzle, the fuel is preheated approximately 750K. The counter-rotating vortices are formed behind the baffle plate and the vortices play a key role for the fuel and air mixing as well as the flame stabilization. The pilot jet not only controlled the flame position but also enhanced the fuel and air mixing. Especially, the pilot jet is important to form the premixed flame near the blow off conditions, and the desirable velocity is close to the air jets velocity of the baffle plate holes. However, there are some ineffective conditions for the pilot air jet.

Multi-Dimensional Flamelet Manifolds for Laminar Partially Premixed Flame

2015 ◽  
Vol 1092-1093 ◽  
pp. 520-524
Author(s):  
Chang Min Cao ◽  
Tao Hong Ye ◽  
Yu Xin Wu ◽  
Wei Wei
Keyword(s):  
Flame Structure ◽  
Direct Calculation ◽  
Premixed Flame ◽  
Dimensional Manifold ◽  
Flamelet Model ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
Low Dimensional ◽  
Low Dimensional Manifold ◽  
Chemistry Tabulation

The aim of this paper is to evaluate the performance of thermo-chemistry tabulation approaches for numerical simulation of laminar partially premixed flame by comparing with the results of detail chemistry. Two thermo-chemistry tabulation approaches are considered: the multidimensional flamelet manifolds (MFM) method and Flame Prolongation of Intrinsic low-dimensional manifold (FPI) method. The fuel streams with different equivalence ratios (ΦF=1.8,ΦF=2.464) are analyzed. In both of the equivalence ratios the results obtained from MFM method are in closer agreement with the direct calculation than FPI method. It is concluded that multi-dimensional flamelet manifolds can capture more precisely partially premixed characteristic of the multi-regime flame structure, comparing to single-dimensional flamelet model FPI.

Aero-Thermodynamic Consideration of Single-Crystal-Silicon Premixed-Fuel Microscale Can Combustor

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Moriaki Namura ◽  
Toshiyuki Toriyama
Keyword(s):  
Single Crystal ◽  
Physical Interpretation ◽  
Burning Velocity ◽  
Single Crystal Silicon ◽  
Velocity Model ◽  
Flame Stabilization ◽  
Crystal Silicon ◽  
Thermodynamic Consideration ◽  
Stable Combustion ◽  
Silicon Bulk

This paper describes the aero-thermodynamic design, microfabrication and combustion test results for a single-crystal-silicon premixed-fuel microscale can combustor. The combustion chamber volume is 277 mm3, and the microscale can combustor was fabricated by silicon bulk micromachining technology. Hydrogen fuel-air premixing was performed in the combustion test. The operation space in which stable combustion occurred was experimentally determined from the combustion temperature and efficiency for various mass flow rates and equivalence ratios. The expression for the combustion efficiency under conditions where the overall rate of heat release is limited by the chemical kinetics was consistent with the burning velocity model. The flame stabilization, the range of equivalence ratios and the maximum air velocity that the combustor can tolerate before flame extinction occurs were in agreement with the well - stirred reactor (WSR) and combustion loading parameter (CLP) models. A proposed aero-thermodynamic design approach based on these three models provides a physical interpretation of the experimental results in the operation space of stable combustion. Furthermore, this approach provides a unified physical interpretation of the stable combustion operation spaces of microscale combustors with various dimensions and configurations. Therefore, it is demonstrated that the proposed aero-thermodynamic approach has an important role in predicting the preliminary aerodynamic design performances of new microscale combustors.

Numerical Simulation of Premixed Propane/Air Flame Propagation Using a Dynamically Thickened Flame Approach

2013 ◽  
Vol 444-445 ◽  
pp. 1574-1578 ◽  
Author(s):  
Hua Hua Xiao ◽  
Zhan Li Mao ◽  
Wei Guang An ◽  
Qing Song Wang ◽  
Jin Hua Sun
Keyword(s):  
Numerical Simulation ◽  
Flame Propagation ◽  
Numerical Study ◽  
Vortex Motion ◽  
Combustion Process ◽  
Single Step ◽  
Premixed Combustion ◽  
Premixed Flame ◽  
Flame Surface ◽  
Arrhenius Kinetics

A numerical study of premixed propane/air flame propagation in a closed duct is presented. A dynamically thickened flame (TF) method is applied to model the premixed combustion. The reaction of propane in air is taken into account using a single-step global Arrhenius kinetics. It is shown that the premixed flame undergoes four stages of dynamics in the propagation. The formation of tulip flame phenomenon is observed. The pressure during the combustion process grows exponentially at the finger-shape flame stage and then slows down until the formation of tulip shape. After tulip formation the pressure increases quickly again with the increase of the flame surface area. The vortex motion behind the flame front advects the flame into tulip shape. The study indicates that the TF model is quite reliable for the investigation of premixed propane/air flame propagation.

Large-Eddy Simulation of Combustion Oscillation in Premixed Combustor

1999 ◽  
Author(s):  
Tomoya Murota ◽  
Masaya Ohtsuka
Keyword(s):  
Large Eddy Simulation ◽  
Present Method ◽  
Premixed Combustion ◽  
Compressible Turbulence ◽  
Flamelet Model ◽  
Turbulent Premixed Flame ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Combustion Oscillation ◽  
Combustion Flow

To analyze combustion oscillation in the premixed combustor, a large-eddy simulation program for premixed combustion flow was developed. The subgrid scale (SGS) model of eddy viscosity type for compressible turbulence (Speziale et al., 1988) was adopted to treat the SGS fluxes. The fractal flamelet model, which utilizes the fractal properties of the turbulent premixed flame to obtain the reaction rate, was developed. Premixed combustion without oscillation was analyzed to verify the present method. The computational results showed good accordance with experimental data (Rydén et al., 1993). The combustion oscillation of an “established buzz” type in the premixed combustor (Langhorne, 1988) was also analyzed. The present method succeeded in capturing the oscillation accurately. The detailed mechanism was investigated. The appearance of the non-heat release region, which is generated because the supply of the unburnt gas into the combustion zone stagnates, and its disappearance play an important role.

Effect of Acoustic Feedback on Lagrangian Coherent Structures in a Backward Facing Step Combustor With a Partially Premixed Flame

2017 ◽  
Author(s):  
Ramgopal Sampath ◽  
S. R. Chakravarthy
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Pressure Oscillation ◽  
Velocity Fields ◽  
Premixed Flame ◽  
Time Resolved ◽  
Acoustic Feedback ◽  
Acoustic Characterization ◽  
Partially Premixed ◽  
Partially Premixed Flame

The thermoacoustic oscillations of a partially premixed flame stabilized in a backward facing step combustor are studied at a constant equivalence ratio in long and short combustor configurations corresponding to with and without acoustic feedback respectively. We perform simultaneous time-resolved particle image velocimetry (TR-PIV) and chemiluminescence for selected flow conditions based on the acoustic characterization in the long combustor. The acoustic characterization shows a transition in the dominant pressure amplitudes from low to high magnitudes with an increase in the inlet flow Reynolds number. This is accompanied by a shift in the dominant frequencies. For the intermittent pressure oscillations in the long combustor, the wavelet analysis indicates a switch between the acoustic and vortex modes with silent zones of relatively low-pressure amplitudes. The short combustor configuration indicates the presence of the vortex shedding frequency and an additional band comprising the Kelvin Helmholtz mode. Next, we apply the method of finite-time Lyapunov exponent (FTLE) to the time-resolved velocity fields to extract features of the Lagrangian coherent structures (LCS) of the flow. In the long combustor post transition with the time instants with dominant acoustic mode, a large-scale modulation of the FTLE boundaries over one cycle of pressure oscillation is evident. Further, the FTLEs and the flame boundaries align each other for all phases of the pressure oscillation. In the short combustor, the FTLEs indicate the presence of small wavelength waviness that overrides the large-scale vortex structure, which corresponds to the vortex shedding mode. This behaviour contrasts with the premixed flame in the short combustor reported earlier in which such large scales were found to be seldom present. The presence of the large-scale structures even in the absence of acoustic feedback in a partially premixed flame signifies its inherent unstable nature leading to large pressure amplitudes during acoustic feedback. Lastly, the FTLE boundaries provide the frequency information of the identified coherent structure and also acts as the surrogate flame boundaries that are estimated from just the velocity fields.

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