Influence of Pressure on Markstein Number Effect in Turbulent Flame Front Propagation

2013 ◽  
Author(s):  
Vlade Vukadinovic ◽  
Peter Habisreuther ◽  
Nikolaos Zarzalis ◽  
Rainer Suntz
Keyword(s):  
Flame Front ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Initial Pressure ◽  
Front Propagation ◽  
Mie Scattering ◽  
Turbulent Flames ◽  
Laser Method ◽  
Turbulent Burning Velocity ◽  
Optical Laser

In gas turbine operation a turbulent flame is employed. Thus, better understanding of the turbulent flame propagation is the key for further optimisation of turbine combustors and reduction of the environmental footprint. As turbulent flames are exposed to stretch, the effect of flame-stretch interaction must be better understood especially at higher pressures. In present study, turbulent burning velocity of two mixtures, hydrogen/air and propane/air, with negative and positive Ma, respectively are experimentally investigated in fan-stirred explosion vessel. For the investigation an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this study the influence of initial parameters as initial pressure and turbulence intensity on the flame front propagation is investigated by giving special attention on influence of Ma variation. The experiments were performed at three different pressures 1, 2, 4 bar. The RMS fluctuation velocity was varied in the range of 0–2.77 m/s. The observed results are compared and discussed in detail.

Flame stretch rate as a determinant of turbulent burning velocity

1992 ◽  
Vol 338 (1650) ◽  
pp. 359-387 ◽  
Keyword(s):  
Burning Velocity ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Stretch Rate ◽  
Dimensionless Groups ◽  
Flame Stretch ◽  
Experimental Values ◽  
Turbulent Burning Velocity

A rational basis for correlating turbulent burning velocities is shown to involve the product of the Karlovitz stretch factor and the Lewis number. A generalized expression is derived to show how flame stretch is related to the velocity field. A new dimensionless correlation of experimental values of turbulent burning velocities is presented. Dimensionless groups also are used in correlations of laminar and turbulent flame extinction stretch rates. A distribution function of stretch rates in turbulent flames, based on an earlier one of Yeung et al ., is proposed and the experimental data are well predicted by a theory based on flamelet extinction by flame stretch with this distribution. Uncertainties arise concerning the role of negative stretch rate. Laminar flamelet modelling of complex combustion appears to have a broader validity than might be expected and some explanation for this is offered.

Study of Thermo-Diffusive Effects on Iso-Octane/Air Flames at Fixed Turbulence Karlovitz Number

2011 ◽  
Author(s):  
Akihiro Hayakawa ◽  
Tomohiro Takeo ◽  
Yukito Miki ◽  
Yukihide Nagano ◽  
Toshiaki Kitagawa
Keyword(s):  
Equivalence Ratio ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Laminar Burning Velocity ◽  
Markstein Number ◽  
Flame Stretch ◽  
The Mean ◽  
Diffusive Effects ◽  
Turbulent Burning Velocity

Spherically propagating laminar and turbulent flames were studied using iso-octane / air mixtures with and without dilution. The main purpose of this study is to clarify the influence of thermo-diffusive effects on the turbulent flames. In order to examine the thermo-diffusive effects solely by separating them from the effects of flame stretch, turbulent burning velocities were compared at constant flame stretch factors. The mean flame stretch factor acting on turbulent flame front may be represented by the turbulence Karlovitz number. Thus, turbulent explosions were carried out at fixed turbulence Karlovitz numbers. The ratio of turbulent burning velocity to unstretched laminar burning velocity increased with the equivalence ratio for non-diluted mixtures at fixed turbulence Karlovitz numbers. And this ratio for CO2 diluted mixtures was larger than N2 diluted mixtures. The Markstein number that denotes the sensitivity of the flame to thermo-diffusive effects depends on the equivalence ratio and diluents of the mixture. The ratio of turbulent burning velocity to unstretched laminar one increased with decreasing Markstein number. Especially, it changed stepwise around Markstein number of zero. However, the burning velocity ratios did not increase with increasing mixture pressure although the Markstein number decreased with pressure.

The Effect of Lewis Number on Instantaneous Flamelet Speed and Position Statistics in Counter-Flow Flames With Increasing Turbulence

2017 ◽  
Author(s):  
Sean D. Salusbury ◽  
Ehsan Abbasi-Atibeh ◽  
Jeffrey M. Bergthorson
Keyword(s):  
Flame Front ◽  
Turbulence Intensity ◽  
High Speed ◽  
Rayleigh Scattering ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Laminar Flames ◽  
Counter Flow

Differential diffusion effects in premixed combustion are studied in a counter-flow flame experiment for fuel-lean flames of three fuels with different Lewis numbers: methane, propane, and hydrogen. Previous studies of stretched laminar flames show that a maximum reference flame speed is observed for mixtures with Le ≳ 1 at lower flame-stretch values than at extinction, while the reference flame speed for Le ≪ 1 increases until extinction occurs when the flame is constrained by the stagnation point. In this work, counter-flow flame experiments are performed for these same mixtures, building upon the laminar results by using variable high-blockage turbulence-generating plates to generate turbulence intensities from the near-laminar u′/SLo=1 to the maximum u′/SLo achievable for each mixture, on the order of u′/SLo=10. Local, instantaneous reference flamelet speeds within the turbulent flame are extracted from high-speed PIV measurements. Instantaneous flame front positions are measured by Rayleigh scattering. The probability-density functions (PDFs) of instantaneous reference flamelet speeds for the Le ≳ 1 mixtures illustrate that the flamelet speeds are increasing with increasing turbulence intensity. However, at the highest turbulence intensities measured in these experiments, the probability seems to drop off at a velocity that matches experimentally-measured maximum reference flame speeds in previous work. In contrast, in the Le ≪ 1 turbulent flames, the most-probable instantaneous reference flamelet speed increases with increasing turbulence intensity and can, significantly, exceed the maximum reference flame speed measured in counter-flow laminar flames at extinction, with the PDF remaining near symmetric for the highest turbulence intensities. These results are reinforced by instantaneous flame position measurements. Flame-front location PDFs show the most probable flame location is linked both to the bulk flow velocity and to the instantaneous velocity PDFs. Furthermore, hydrogen flame-location PDFs are recognizably skewed upstream as u′/SLo increases, indicating a tendency for the Le ≪ 1 flame brush to propagate farther into the unburned reactants against a steepening average velocity gradient.

Role of Darrieus–Landau instability in propagation of expanding turbulent flames

2018 ◽  
Vol 850 ◽  
pp. 784-802 ◽  
Author(s):  
Sheng Yang ◽  
Abhishek Saha ◽  
Zirui Liu ◽  
Chung K. Law
Keyword(s):  
Flame Front ◽  
Flame Propagation ◽  
High Pressures ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Scaling Analysis ◽  
Turbulent Eddies ◽  
Front Instability ◽  
Cellular Flame

In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability.

Numerical Modeling of Stationary But Developing Premixed Turbulent Flames

2006 ◽  
Author(s):  
Pratap Sathiah ◽  
Andrei N. Lipatnikov
Keyword(s):  
Bluff Body ◽  
Burning Velocity ◽  
Rectangular Channel ◽  
Turbulent Flame ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Turbulent Diffusivity ◽  
Flame Thickness ◽  
Premixed Turbulent Flames ◽  
Premixed Turbulent Flame

A typical stationary premixed turbulent flame is the developing flame, as indicated by the growth of mean flame thickness with distance from flame-stabilization point. The goal of this work is to assess the importance of modeling flame development for RANS simulations of confined stationary premixed turbulent flames. For this purpose, submodels for developing turbulent diffusivity and developing turbulent burning velocity, which were early suggested by our group (FSC model) and validated for expanding spherical flames [4], have been incorporated into the so-called Zimont model of premixed turbulent combustion and have been implemented into the CFD package Fluent 6.2. The code has been run to simulate a stationary premixed turbulent flame stabilized behind a triangular bluff body in a rectangular channel using both the original and extended models. Results of these simulations show that the mean temperature and velocity fields in the flame are markedly affected by the development of turbulent diffusivity and burning velocity.

High-Speed Spectrally Resolved Imaging Studies of Spherically Expanding Natural Gas Flames Under Gas Turbine Operating Conditions

2019 ◽  
Author(s):  
Pradeep Parajuli ◽  
Tyler Paschal ◽  
Mattias A. Turner ◽  
Eric L. Petersen ◽  
Waruna D. Kulatilaka
Keyword(s):  
Natural Gas ◽  
Flame Front ◽  
Gas Turbines ◽  
High Speed ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Initial Pressure ◽  
Flame Speed ◽  
Operating Conditions ◽  
Important Species

Abstract Natural gas is a major fuel source for many industrial and power-generation applications. The primary constituent of natural gas is methane (CH4), while smaller quantities of higher order hydrocarbons such as ethane (C2H6) and propane (C3H8) can also be present. Detailed understanding of natural gas combustion is important to obtain the highest possible combustion efficiency with minimal environmental impact in devices such as gas turbines and industrial furnaces. For a better understanding the combustion performance of natural gas, several important parameters to study are the flame temperature, heat release zone, flame front evolution, and laminar flame speed as a function of flame equivalence ratio. Spectrally and temporally resolved, high-speed chemiluminescence imaging can provide direct measurements of some of these parameters under controlled laboratory conditions. A series of experiments were performed on premixed methane/ethane-air flames at different equivalence ratios inside a closed flame speed vessel that allows the direct observation of the spherically expanding flame front. The vessel was filled with the mixtures of CH4 and C2H6 along with respective partial pressures of O2 and N2, to obtain the desired equivalence ratios at 1 atm initial pressure. A high-speed camera coupled with an image intensifier system was used to capture the chemiluminescence emitted by the excited hydroxyl (OH*) and methylidyne (CH*) radicals, which are two of the most important species present in the natural gas flames. The calculated laminar flame speeds for an 80/20 methane/ethane blend based on high-speed chemiluminescence images agreed well with the previously conducted Z-type schlieren imaging-based measurements. A high-pressure test, conducted at 5 atm initial pressure, produced wrinkles in the flame and decreased flame propagation rate. In comparison to the spherically expanding laminar flames, subsequent turbulent flame studies showed the sporadic nature of the flame resulting from multiple flame fronts that were evolved discontinuously and independently with the time. This paper documents some of the first results of quantitative spherical flame speed experiments using high-speed chemiluminescence imaging.

A two-eddy theory of premixed turbulent flame propagation

1981 ◽  
Vol 301 (1457) ◽  
pp. 1-25 ◽  
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Experimental Measurements ◽  
Laminar Burning Velocity ◽  
Turbulent Reynolds Number ◽  
Theoretical Predictions ◽  
Premixed Turbulent Flame ◽  
Turbulent Burning Velocity

Available experimental data on the turbulent burning velocity of premixed gases are surveyed. There is discussion of the accuracy of experimental measurements and the means of ascertaining relevant turbulent parameters. Results are presented in the form of the variation of the ratio of turbulent to laminar burning velocities with the ratio of r.m.s. turbulent velocity to laminar burning velocity, for different ranges of turbulent Reynolds number. A two-eddy theory of burning is developed and the theoretical predictions of this approach, as well as those of others, are compared with experimentally measured values.

An Experimental Study on Properties of Local Burning Velocity for Hydrogen Added Hydrocarbon Premixed Turbulent Flames

2011 ◽  
Author(s):  
Masaya Nakahara ◽  
Koichi Murakami ◽  
Jun Hashimoto ◽  
Atsushi Ishihara
Keyword(s):  
Burning Velocity ◽  
Quantitative Relationship ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Laminar Flames ◽  
Mean Values ◽  
Laser Tomography ◽  
Turbulence Condition ◽  
Turbulent Burning Velocity ◽  
Local Burning

This study is performed to investigate directly the local flame properties of turbulent propagating flames at the same weak turbulence condition (u′/SL0 = 1.4), in order to clarify basically the influence of the addition of hydrogen to methane or propane mixtures on its local burning velocity. The mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δH are prepared. A two-dimensional sequential laser tomography technique is used to obtain the relationship between the flame shape and the flame displacement. The local flame displacement velocity SF is quantitatively obtained as the key parameters of the turbulent combustion. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effects of positive stretch and curvature on burning velocity. It was found that the trends of the mean values of measured SF with respect to δH, the total equivalence ratio Φ and fuel types corresponded well its turbulent burning velocity. The trend of the obtained Ma could explain the local burning velocity of turbulent flames only qualitatively. Based on the Ma, the local burning velocity at the part of turbulent flames with positive stretch and curvature, SLt, is estimated quantitatively. As a result, a quantitative relationship between the estimated SLt and the SF at positive stretch and curvature of turbulent flames could be observed for mixtures with increasing the Lewis number.

Effects of Hydrogen Addition on Swirl-Stabilized Flame Properties

2010 ◽  
Author(s):  
Shengrong Zhu ◽  
Sumanta Acharya
Keyword(s):  
Flame Front ◽  
Surface Density ◽  
Recirculation Zone ◽  
Burning Velocity ◽  
Mie Scattering ◽  
Reacting Flow ◽  
Flame Surface ◽  
Flame Surface Density ◽  
Hydrogen Addition ◽  
Methane Flame

The role of hydrogen addition to swirl-stabilized methane flames is studied experimentally. Of specific interest are flame properties including flame surface density and curvature. The measurements are based on Particle Image Velocimetry (PIV), Mie-scattering and CH-chemiluminescence imaging. Identification of the flame front and its geometric characterization provides an understanding of the flame properties. Compared to the non-reacting flow, the methane flame broadens the central recirculation zone. Hydrogen enriched flames reduce the central recirculation zone and scales down the characteristic length of the flow. With hydrogen addition, the distribution of the flame front curvature is broadened and flame surface density is increased. This indicates that hydrogen addition increases the reaction front thermo-diffusive instability, causing the flame front to be more wrinkled, and increasing the flame surface area leading to an increase in the burning velocity.

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