Modeling of flame transfer function of a diverging premixed flame with developing flame speed under velocity disturbances

조주형 대한기계학회 2023 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.37 No.10

A theoretical study was conducted to examine the flame transfer function (FTF) of a diverging premixed flame subject to uniform and/or convective velocity disturbances considering developing turbulent flame speed. The FTF under uniform velocity disturbances with developing flame speed is less oscillatory compared with constant flame speed, and the magnitude of the aforementioned FTF is decreased more rapidly with an increase in turbulent time scale or flame length. The peak overshoot of the FTF under convective velocity disturbances is observed in a certain frequency range, which is known to be a typical nature of the FTF of a diverging flame under convective velocity disturbances. Such peak overshoot is quite sensitive to how the turbulent flame speed varies along the flame front in case of developing flame speed while the amplitude of the peak overshoot rarely changes with the mean flame length in case of constant flame speed.

Effect of Low Temporal Resolution on the Uncertainty of Resolving Flame Speed of Moving Flames

Dong Hyuk Shin(신동혁) 한국연소학회 2020 한국연소학회지 Vol.25 No.4

This paper analyzes the effect of low temporal resolution on the accuracy of determining the local flame speed for experimental measurements and simulations. Flame speed measurement of moving flames requires the evaluation of a temporal derivative of flame front motion. Measurements have limited temporal resolution, which can lead to a bias error in the calculation of flame speed. The error associated with the temporal resolution was derived in the leading order effect on sampling time. In order to demonstrate the error term, we employed numerical examples. Artificial flame solutions are generated by solving the G-equation with prescribed flow fields and fine spatial/temporal resolutions. By varying the sampling time step, we demonstrated the error can lead to negative values of the measured flame speed. Furthermore, when coarse sampling time steps are used, the calculated flame speed also exhibits a spurious curvature dependency. Lastly, the associated error can be estimated by spatial gradients of the flame front for convection dominated flows.

Laminar flame speeds for n-butanol/air mixtures at elevated pressures and temperatures: An experimental and numerical study

Niladri Talukder,이기용 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.4

Laminar flame speeds of n-butanol/air premixed flames were measured experimentally and numerically at elevated pressures and temperatures for a wide range of equivalence ratios. Laminar flame speeds were obtained experimentally from the temporal evaluation of the flame front of spherically outwardly propagating flames at zero stress rate. The shadowgraph technique was employed to gain optical access to the constant volume combustion chamber. Flame propagation images were captured by a high-speed camera and MATLAB codes were used to process the images and calculate laminar flame speeds. Flame speeds have been calculated numerically using CHEMKIN-Pro based on a short reaction mechanism for n-butanol oxidation, which was derived from a previously published full reaction mechanism. Numerical predictions were in qualitative agreement with experimental data. The effects of initial pressure and temperature elevation were analyzed. Also, the effect of simultaneous elevation of initial pressures and temperatures is documented. For all experimental conditions, the maximum flame speed was found at around equivalence ratio 1.1. In general, flame speeds decreased with the elevation of initial pressure and increased with initial temperature elevation.

전기장이 튜브내 예혼합화염 전파속도에 미치는 영향에 관한 연구

류승관(S. K. Ryu),원상희(S. H. Won),차민석(M. S. Cha),정석호(S. H. Chung) 한국연소학회 2006 KOSCOSYMPOSIUM논문집 Vol.- No.-

The effect of electric fields on flame speed has been investigated experimentally by observing propagating premixed flames in a tube for methane/air mixtures. The flame speeds were measured in both the normal and micro gravity conditions to substantiate the measurements. The results show that the flame speeds were enhanced by both the AC and DC electric fields, as the flame approached to the high voltage electrode located on the one end of the tube. The enhancement of flame speed was proportional to the square root of the electric field intensity, defined as the voltage applied divided by the distance of flame from the high voltage electrode, when the electric field intensity is sufficiently large. When the electric field intensity was low, there existed critical intensities, below which the electric fields did not influence the flame speed. This critical electric field intensity correlated well with the flame speed.

Direct estimation of edge flame speeds of lifted laminar jet flames and a modified stabilization mechanism

Jeon, Min-Kyu,Kim, Nam Il Elsevier 2017 Combustion and flame Vol.186 No.-

<P><B>Abstract</B></P> <P>The flame stabilization mechanism of a lifted flame in a laminar fuel jet has been explained based on the edge flame concept. Previous studies have employed a similarity solution between velocity and fuel concentration, and showed that a lifted flame can be stabilized when the Schmidt number, <I>Sc</I>, is within a range of either <I>Sc</I> > 1 or <I>Sc</I> < 0.5. However, two unsolved problems remained, and they were mainly answered in this study. First, the edge flame speed could not be determined from the similarity solution using the experimental results of stable lifted flames. To resolve this, the experimental relationship between the fuel flow rate and the liftoff height was measured with a higher resolution, and a new method employing an effective Schmidt number was suggested. As a result, the relationship between the edge flame speed and the fuel concentration gradient could then be directly estimated from the simple experimental values for flow rate and the liftoff height. This new method was validated for various experimental parameters including the tube diameter, air-premixing ratio, and nitrogen-dilution ratio. Second, the reason why a stable lifted flame was not obtained when <I>Sc</I> < 0.5 could not be explained theoretically. Here, the existence of a unique criterion of <I>Sc</I> > 1, for a stable lifted flame was clarified theoretically. This study will advance understanding of the characteristics and stabilization mechanism of lifted edge flames in laminar non-premixed fuel jets.</P>

동축류 제트에서 삼지화염 전파의 전기장 효과에 대한 실험적 연구

원상희(Sang Hee Won),정석호(Suk Ho Chung),차민석(Min Suk Cha) 한국연소학회 2006 KOSCOSYMPOSIUM논문집 Vol.- No.-

The effect of electric fields on the propagation speed of tribrachial flames has been investigated in a coflow jet by observing the transient flame propagation behavior after ignition. Without having electric fields, the propagation speed of tribrachial flame edges showed a typical behavior by having an inverse proportionality to the mixture fraction gradient at the flame edge. The behavior of flame propagation with the electric fields was investigated by applying high voltage to the central fuel nozzle and the enhancement of propagation speed has been observed by varying the applied voltage and frequency for AC electric fields. The propagation speed of tribrachial flame was also investigated by applying negative and positive DC voltages to the nozzle and similar improvements of the propagation speed were also observed. The propagation speeds of tribrachial flames in both the AC and DC electric fields were correlated well with the electric field intensity, defined by the electric voltage divided by the distance between the nozzle electrode and the edge of tribrachial flames.

Laminar flame speeds and Markstein lengths of methyl decanoate-air premixed flames at elevated pressures and temperatures

Talukder, Niladri,Lee, Ki Yong Elsevier 2018 Fuel Vol.234 No.-

<P><B>Abstract</B></P> <P>Laminar flame speeds and Markstein lengths of methyl decanoate (MD)-air mixtures were measured within a range of equivalence ratio at different elevated pressures and temperatures using outwardly propagating spherical flames developed inside a constant volume combustion chamber. Shadowgraph technique was employed to observe the temporal evolution of flame fronts. A numerical scheme was used to justify all the experimental data of laminar flame speeds. The numerical scheme was developed based on a short mechanism of methyl decanoate oxidation. A sound agreement was observed between the numerical predictions and experimentally obtained data of laminar flame speeds. Markstein lengths were calculated to quantify the effect of stretch on the flame front at different initial conditions. The study provided viable data of laminar flame speeds and Markstein lengths of methyl decanoate-air flames at different initial conditions which were also well conforming to the established theories of conventional fuels regarding of the effects of equivalence ratio, pressure, and temperature variation on laminar flame speed.</P>

A study on flame structure and extinction in downstream interaction between lean premixed CH₄-air and (50% H₂+50% CO) syngas-air flames

Kim, J.S.,Park, J.,Kwon, O.B.,Bae, D.S.,Yun, J.H.,Keel, S.I. Pergamon Press ; Elsevier Science Ltd 2011 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.36 No.9

Downstream interactions between lean premixed flames with mutually different fuels of (50% H<SUB>2</SUB>+50% CO) and CH<SUB>4</SUB> are numerically investigated particularly on and near lean extinction limits in order to provide fundamental database for the design of cofiring burners with hydrocarbon and syngas under a retrofit concept. In the current study the anomalous combination of lean premixed flames is provided such that even a weaker CH<SUB>4</SUB>-air flame temperature is higher than a stronger syngas-air flame temperature, and, based on a deficient reactant concept, the effective Lewis numbers Le<SUB>eff</SUB>∼1 for lean premixed (50% H<SUB>2</SUB>+50% CO)-air mixture and Le<SUB>D</SUB><1 for CH<SUB>4</SUB>-air mixture. It is found that the interaction characteristics between lean premixed (50% H<SUB>2</SUB>+50% CO)-air and CH<SUB>4</SUB>-air flames are quite different from those between the same hydrocarbon flames. The lean extinction boundaries are of slanted shape, thereby indicating strong interactions. The upper extinction boundaries have negative flame speeds while the lower extinction boundaries have both negative and positive flame speeds. The results also show that the flame interaction characteristics do not follow the general tendency of Lewis number, which has been well described in interactions between the same hydrocarbon flames, but have the strong dependency of direct interaction factors such as flame temperature, the distance between two flames, and radical-sharing. Importance of chain carrier radicals such as H is also addressed in the downstream interactions between lean premixed (50% H<SUB>2</SUB>+50% CO)-air and CH<SUB>4</SUB>-air flames.

0.57의 L/D 비를 가지는 폭발챔버에서 전파하는 화염과 다중 장애물의 상호작용에 관한 실험적 연구

박달재 ( Dal Jae Park ) 한국안전학회(구 한국산업안전학회) 2012 한국안전학회지 Vol.27 No.6

Experimental investigations were performed to examine the characteristics of propagating flame fronts around multiple bars within a rectangular chamber. The explosion chamber is 400 mm in height, 700×700 mm(2) in cross section and has a large top venting area, A(v) of 700×210 mm(2). This results in a value of 0.44 for A(v)/V(2/3) and a L/D value of 0.57. The multiple obstacles of length 700 mm with a blockage ratio of 30% were placed within the chamber. Temporally resolved flame front images were recorded by a high speed video camera to investigate the inte-raction between the propagating flame and the obstacles. Results showed that the flame propagation speeds before the flame impinges onto the obstacle almost equal to the laminar burning velocity. As the propagating flame impinged on the obstacle, the central region of flame began to become concave, this resulted in the flame deceleration in the region. As the flame interacted with the modified flow filed generated behind the central obstacle. the probability density functions(PDFs) of the local flame displacement speed were extensively distributed toward higher speeds.

직접수치해법을 이용한 난류 예혼합 화염전파속도 연구

한인석(Insuk Han),허강열(Kang-Yul Huh) 한국연소학회 2006 KOSCOSYMPOSIUM논문집 Vol.- No.-

Flame surface area is a critical parameter determining turbulent flame speed. Three-dimensionaldirect numerical simulations(DNS) were conducted to figure out the evolution process of flame surface area. Fully compressible Navier-Stokes equations are solved to reproduce premixed flame embedded in isotropic decaying turbulent flow. The tangential straining and curvature of propagating surface affect development of flame area. In this study, four different turbulent intensity flows and three different Le number flames are investigated to force changes in straining and curvature effects. Consistent results are obtained for the probability density functions (PDF) of strain and curvature with previous researches. It is revealed that displacement speed, which is a speed of flame surface relative to unbumt flow, controls the balance between sink and source of flame surface area.