The Characteristic Modes and Structures of Bluff-Body Stabilized Flames in Supersonic Coflow Air

김지호,윤영빈,박철웅,한재원 한국항공우주학회 2012 International Journal of Aeronautical and Space Sc Vol.13 No.3

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.

The Characteristic Modes and Structures of Bluff-Body Stabilized Flames in Supersonic Coflow Air

Jiho Kim,Youngbin Yoon,Chul Woung Park,Jae Won Hahn 한국항공우주학회 2012 International Journal of Aeronautical and Space Sc Vol.13 No.3

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.

EFFECT OF FUEL STRATIFICATION ON INITIAL FLAME DEVELOPMENT: PART 2-LOW SWIRL CONDITION

엄인용,박찬준 한국자동차공학회 2008 International journal of automotive technology Vol.9 No.6

This paper is the second invstigation on the effect of fuel stratification on flame propagation. In the previous work, the characteristics under the no port-generated swirl condition, i.e., the conventional case was studied. In this work, the flame development under the low swirl condition was considered. For this purpose, the initial flame development and propagation were visualized under different axially stratified states in a modified optical single cylinder SI engine. The images were captured by an intensified CCD camera through the quartz window mounted in the piston. Stratification was controlled by the combination of the port swirl ratio and injection timing. These were averaged and processed to characterize the flame propagation. The flame stability was estimated by the weighted average of flame area and luminosity. The stability was also evaluated through the standard deviation of flame area and propagation distance and through the mean absolute deviation of the propagating direction. The results show that the flame-flow interaction determines the direction of flame propagation and that the governing roles of the two factors vary according to the stratified state and the location in the cylinder. In addition, the flame development and the initial flame stability are strongly dependent on the stratified conditions, and the initial flame stability is closely related to the engine stability and lean misfire limit. Lastly, there is no essential difference in gasoline and CNG flame propagation characteristics.

The Characteristic Modes and Structures of Bluff-Body Stabilized Flames in Supersonic Coflow Air

Kim, Ji-Ho,Yoon, Young-Bin,Park, Chul-Woung,Hahn, Jae-Won The Korean Society for Aeronautical and Space Scie 2012 International Journal of Aeronautical and Space Sc Vol.13 No.3

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.

파일럿 화염을 이용한 희석제 첨가 화염의 안정화

안태국(Taekook Ahn),이원남(Wonnam Lee) 한국연소학회 2013 KOSCOSYMPOSIUM논문집 Vol.2013 No.12

The stability of inert gas-diluted flames has been experimentally studied. Adding inert gases to a diffusion flame, which used to be very stable without dilution, makes a flame unstable. The existence of an over-ventilated pilot diffusion flame inside of a diluted diffusion flame provides an very effective tool to maintain a flame stable. The thermo-chemical effects of a pilot diffusion flame are considered the main reason for the enhancement of flame stabilization.

좁은 채널 내부의 수직 혼합 경계층에 형성된 메탄-공기 에지-화염의 안정화 기초 실험

이민정(Min Jung Lee),김남일(Nam Il Kim) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.7

Flame stabilization characteristics were experimentally investigated in a fuel-air cross flowing mixing layer. A combustor consists of a narrow channel of air steam and a cross flowing fuel. Depending on the flow rates of methane and air, flame can be stabilized in two modes. First is an attached flame which is formulated at the backward step where the methane and air streams meet. Second is a lifted-flame which is formulated within the mixing layer far down steam from backward step. The heights and flame widths of the lifted flames were measured. Flame shapes of the lifted flames were similar to an ordinary edge flame or a tribrachial flame, and their behavior could be explained with the theories of an edge flame. With the increase of the mixing time between fuel and air, the fuel concentration gradient decreases and the flame propagation velocity increases. Thus the flame is stabilized where the flow velocity is matched to the flame propagation velocity in spite of a significant disturbance in the fuel mixing and heat loss within the channel. This study provides many experimental results for a higher fuel concentration gradient, and it can also be helpful for the development and application of a smaller combustor.

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>

스파크 점화 엔진에서 초기화염 발달의 가시화

엄인용(Inyong Ohm) 한국가시화정보학회 2004 한국가시화정보학회지 Vol.2 No.2

Initial flame development and propagation were visualized under different fuel injection timings to relate the initial flame development to the engine stability in a port injection SI engine. Experiments were performed in an optical single cylinder engine modified from a production engine and images were captured through the quartz window mounted in the piston by an intensified CCD camera. Stratification state was controlled by varying injection timing. Under each injection condition, the flame images were captured at the pre-set crank angles. These were averaged and processed to characterize the flame. The flame stability was estimated by the weighted average of flame area, luminosity, and standard deviation of flame area. Results show that stratification state according to injection timing did not affect on the direction of flame propagation. The flame development and the initial flame stability are strongly dependent on the stratified conditions and the initial flame stability governs the engine stability and lean misfire limit.

부상된 수소 난류확산화염의 화염구조

오정석(Jeongseog Oh),윤영빈(Youngbin Yoon) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.9

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze coexistence of two different flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was changed from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with the increase of fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The flame stabilization was related to turbulent intensity and strain rate assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced. At the flame base, two different flame structures were found that was the partial premixed flames and premixed flame.

부상된 수소 난류확산화염에서의 이중 화염구조

오정석(Jeongseog Oh),윤영빈(Youngbin Yoon) 한국연소학회 2009 KOSCOSYMPOSIUM논문집 Vol.- No.38

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze coexistence of two different flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 ㎧. Coaxial air velocity was changed from 12 to 20 ㎧. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. Lifted flame stabilization was related to when assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced. At the flame base, two different flame structures were found that was the partial premixed flames and premixed flame.