Aerodynamic Quenching and Burning Velocity of Turbulent Premixed Methane-Air Flames

2015 ◽  
Author(s):  
Girish V. Nivarti ◽  
R. Stewart Cant
Keyword(s):  
Turbulence Intensity ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Experimental Studies ◽  
Flame Speed ◽  
Flame Surface ◽  
Turbulent Flame Speed ◽  
Combustion Models ◽  
Turbulent Premixed

Industry-relevant turbulent premixed combustion models continue to rely on empirical expressions for turbulent flame speed in closure modelling for the mean turbulent reaction rate. To date, an accurate sub-model for turbulent flame speed has not been proposed for flows with high turbulence intensities. Experimental studies in the pertinent combustion regime, known as the Thin Reaction Zones (TRZ) regime, are limited by the existing techniques of turbulence generation whereas, until recently, the high computational expense involved in solving such problems has restricted theoretical studies. We investigate the behaviour of premixed flames in the TRZ regime by conducting a parametric 3D Direct Numerical Simulation (DNS) study of stoichiometric methane-air mixtures using single-step chemistry in an inflow-outflow configuration. Inflow turbulence intensity is varied while keeping the integral length scale constant across six separate simulations which span altogether a significant portion of the TRZ regime. The resulting variation of turbulent flame speed with turbulence intensity demonstrates the well-known bending phenomenon and conforms with recent experimental observations of freely-propagating premixed flames in this regime. As turbulence intensity is increased, the calculated flame surface exhibits an increasing degree of wrinkling and pocket-formation. In addition, the internal thermo-chemical structure of the flame is greatly affected when the turbulence intensity is more than an order of magnitude higher than the laminar flame speed. These qualitative observations establish the present DNS framework as a powerful tool for capturing turbulence-chemistry interactions that influence the bending phenomenon. Hence, this work forms the basis for further analysis using a detailed chemical description to investigate these interactions and, thereby, improve combustion models of industrial relevance.

An Analysis of Local Quantities of Turbulent Premixed Flames Using DNS Databases

2007 ◽  
Author(s):  
Kazuya Tsuboi ◽  
Shinnosuke Nishiki ◽  
Tatsuya Hasegawa
Keyword(s):  
Reaction Rate ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Three Dimensional ◽  
Flame Surface ◽  
Turbulent Premixed Flames ◽  
Density Ratios ◽  
Turbulent Burning Velocity ◽  
Turbulent Premixed

An analysis of local flame area was performed using DNS (Direct Numerical Simulation) databases of turbulent premixed flames with different density ratios and with different Lewis numbers. Firstly, a local flame surface at a prescribed progress variable was identified as a local three-dimensional polygon. And then the polygon was divided into some triangles and local flame area was evaluated. The turbulent burning velocity was evaluated using the ratio of the area of turbulent flame to that of planar flame and compared with the turbulent burning velocity obtained by the reaction rate.

scholarly journals Effects of pressure gradients on turbulent premixed flames

1997 ◽  
Vol 353 ◽  
pp. 83-114 ◽  
Author(s):  
DENIS VEYNANTE ◽  
THIERRY POINSOT
Keyword(s):  
Pressure Gradient ◽  
Turbulent Transport ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Flame Speed ◽  
Pressure Gradients ◽  
Turbulent Premixed Flames ◽  
Turbulent Flame Speed ◽  
Flame Brush ◽  
Turbulent Premixed

In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale lt.A simple model for the turbulent flux u″c″ is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

The structure and propagation of turbulent flames

1975 ◽  
Vol 344 (1637) ◽  
pp. 217-234 ◽  
Keyword(s):  
Turbulence Intensity ◽  
Combustion Zone ◽  
Laminar Flame ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Flame Structure ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Turbulence Scale ◽  
Turbulent Flame Speed

The influence of turbulence intensity, scale and vorticity on burning velocity and flame structure is examined by using premixed propane-air mixtures supplied at atmospheric pressure to a combustion chamber 31cm long and lOcmx 10 cm cross-section. The chamber is fitted with transparent side walls to permit flame observations and schlieren photography. Control over the turbulence level is achieved by means of grids located upstream of the combustion zone. By suitable modifications to grid geometry and flow velocity, it is possible to vary turbulence intensity and scale independently within the combustion zone in such a manner that their separate effects on burning velocity and flame structure are readily distinguished. From analysis of the results obtained three distinct regions may be identified, each having different characteristics in regard to the effect of scale on turbulent burning velocity. For each region a mechanism of turbulent flame propagation is proposed which describes the separate influences on burning velocity of turbulence intensity, turbulence scale, laminar flame speed and flame thickness. The arguments presented in support of this 3-region model are substantiated by the experimental data and by the pictorial evidence on flame structure provided by the schlieren photographs. This model also sheds light on some of the characteristics which turbulent flames have in common with laminar flames when the latter are subjected to pressure and velocity fluctuations. Finally the important role of vorticity is examined and it is found that turbulent flame speed is highest when the rate of production of vorticity is equal to about half the rate of viscous dissipation.

Hydrogen–Air–Steam Deflagration Experiment Simulated Using Different Turbulent Flame-Speed Closure Models

2018 ◽  
Vol 4 (3) ◽  
Author(s):  
Holler Tadej ◽  
Ed M. J. Komen ◽  
Kljenak Ivo
Keyword(s):  
Flame Propagation ◽  
Nuclear Power ◽  
Large Scale ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Energy Output ◽  
Combustion Model ◽  
Turbulent Flame Speed ◽  
Modeling Approach ◽  
Combustion Models

The paper presents the computational fluid dynamics (CFD) combustion modeling approach based on two combustion models. This modeling approach was applied to a hydrogen deflagration experiment conducted in a large-scale confined experimental vessel. The used combustion models were Zimont's turbulent flame-speed closure (TFC) model and Lipatnikov's flame-speed closure (FSC) model. The conducted simulations are aimed to aid identifying and evaluating the potential hydrogen risks in nuclear power plant (NPP) containment. The simulation results show good agreement with experiment for axial flame propagation using the Lipatnikov combustion model. However, substantial overprediction in radial flame propagation is observed using both combustion models, which consequently results also in overprediction of the pressure increase rate and overall combustion energy output. As assumed for a large-scale experiment without any turbulence inducing structures, the combustion took place in low-turbulence regimes, where the Lipatnikov combustion model, due to its inclusion of quasi-laminar source term, has advantage over the Zimont model.

Large-Scale Homogeneous Hydrogen-Air-Steam Deflagration Experiment Simulated Using Two Turbulent Flame Speed Closure Models

2016 ◽  
Author(s):  
Tadej Holler ◽  
Varun Jain ◽  
Ed M. J. Komen ◽  
Ivo Kljenak
Keyword(s):  
Flame Propagation ◽  
Nuclear Power ◽  
Large Scale ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Energy Output ◽  
Combustion Model ◽  
Turbulent Flame Speed ◽  
Combustion Models

The CFD combustion modeling approach based on two combustion models was applied to a hydrogen deflagration experiment conducted in a large-scale confined experimental vessel. The used combustion models were Zimont’s Turbulent Flames Speed Closure (TFC) model and Lipatnikov’s Flame Speed Closure (FSC) model. The conducted simulations are aimed to aid identifying and evaluating the potential hydrogen risks in Nuclear Power Plant (NPP) containment. The simulation results show good agreement with experiment for axial flame propagation using the Lipatnikov combustion model. However substantial overprediction in radial flame propagation is observed using both combustion models, which consequently results also in overprediction of the pressure increase rate and overall combustion energy output. As assumed for a large-scale experiment without any turbulence inducing structures, the combustion took place in low-turbulence regimes, where the Lipatnikov combustion model, due to its inclusion of quasi-laminar source term, has advantage over the Zimont model.

Fuel Variation Effects in Propagation and Stabilization of Turbulent Counter-Flow Premixed Flames

2018 ◽  
Author(s):  
Ehsan Abbasi-Atibeh ◽  
Sandeep Jella ◽  
Jeffrey M. Bergthorson
Keyword(s):  
High Speed ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Mass Diffusion ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Flame Surface ◽  
Differential Diffusion ◽  
Turbulent Flame Speed

Sensitivity to stretch and differential diffusion of chemical species are known to influence premixed flame propagation, even in the turbulent environment where mass diffusion can be greatly enhanced. In this context, it is convenient to characterize flames by their Lewis number (Le), a ratio of thermal-to-mass diffusion. The work reported in this paper describes a study of flame stabilization characteristics when the Le is varied. The test data is comprised of Le ≪ 1 (Hydrogen), Le ≈ 1 (Methane), and Le > 1 (Propane) flames stabilized at various turbulence levels. The experiments were carried out in a Hot exhaust Opposed-flow Turbulent Flame Rig (HOTFR), which consists of two axially-opposed, symmetric turbulent round jets. The stagnation plane between the two jets allows the aerodynamic stabilization of a flame, and clearly identifies fuel influences on turbulent flames. Furthermore, high-speed Particle Image Velocimetry (PIV), using oil droplet seeding, allowed simultaneous recordings of velocity (mean and rms) and flame surface position. These experiments, along with data processing tools developed through this study, illustrated that in the mixtures with Le ≪ 1, turbulent flame speed increases considerably compared to the laminar flame speed due to differential diffusion effects, where higher burning rates compensate for the steepening average velocity gradient, and keeps these flames almost stationary as bulk flow velocity increases. These experiments are suitable for validating the ability of turbulent combustion models to predict lifted, aerodynamically-stabilized flames. In the final part of this paper, we model the three fuels at two turbulence intensities using the FGM model in a RANS context. Computations reveal that the qualitative flame stabilization trends reproduce the effects of turbulence intensity, however, more accurate predictions are required to capture the influences of fuel variations and differential diffusion.

Boundary Layer Flashback Prediction for Turbulent Premixed Jet Flames: Comparison of Two Models

2019 ◽  
Author(s):  
Alireza Kalantari ◽  
Nicolas Auwaijan ◽  
Vincent McDonell
Keyword(s):  
Boundary Layer ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Operating Conditions ◽  
Pollutant Emissions ◽  
Jet Flames ◽  
Turbulent Flame Speed ◽  
Turbulent Premixed

Abstract Lean-premixed combustion is commonly used in gas turbines to achieve low pollutant emissions, in particular nitrogen oxides. But use of hydrogen-rich fuels in premixed systems can potentially lead to flashback. Adding significant amounts of hydrogen to fuel mixtures substantially impacts the operating range of the combustor. Hence, to incorporate high hydrogen content fuels into gas turbine power generation systems, flashback limits need to be determined at relevant conditions. The present work compares two boundary layer flashback prediction methods developed for turbulent premixed jet flames. The Damköhler model was developed at University of California Irvine (UCI) and evaluated against flashback data from literature including actual engines. The second model was developed at Paul Scherrer Institut (PSI) using data obtained at gas turbine premixer conditions and is based on turbulent flame speed. Despite different overall approaches used, both models characterize flashback in terms of similar parameters. The Damköhler model takes into account the effect of thermal coupling and predicts flashback limits within a reasonable range. But the turbulent flame speed model provides a good agreement for a cooled burner, but shows less agreement for uncooled burner conditions. The impact of hydrogen addition (0 to 100% by volume) to methane or carbon monoxide is also investigated at different operating conditions and flashback prediction trends are consistent with the existing data at atmospheric pressure.

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