• Title, Summary, Keyword: Lewis number

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Experimental Study on Comparison between Buoyancy Driven and Lewis Number Induced Self-excitations in Laminar Lifted Coflow-jet Flames (층류 동축류 제트 부상화염에서 부력에 의한 자기진동과 루이스 수에 의한 자기진동 비교에 관한 실험적 연구)

  • Lee, Won June;Park, Jeong;Kwon, Oh Boong;Yun, Jin Han;Keel, Sang In
    • Journal of the Korean Society of Combustion
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    • v.19 no.2
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    • pp.21-27
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    • 2014
  • Experimental study in laminar propane coflow jet flames has been conducted to investigate self-excitations. For various propane mole fractions and jet velocities, two types of self-excitation were observed: (1) buoyancydriven self-excitation (hereafter called BDSE) and (2) Lewis-number-induced self-excitation coupled with (1) (hereafter called LCB). The mechanism of Lewis-number-induced self-excitation (hereafter called LISE) is proposed. When the system $Damk\ddot{o}hler$ number was lowered, LISE was shown to be launched. The LISE is closely related to heat loss, such that it can be launched in even helium-diluted methane coflow-jet flame (Lewis number less than unity). Particularly, The LISE becomes significant as the $Damk\ddot{o}hler$ number decreases and heat-loss is excessively large.

Experimental Study on Comparison of Buoyancy Driven and Lewis Number Induced Self-excitations in Laminar Lifted Coflow-jet Flames. (층류 동축류 제트 부상화염에서 부력에 의한 자기진동과 루이스 수에 의한 자기진동 비교에 관한 실험적 연구)

  • Ban, Gyu Ho;Lee, Won June;Park, Jeong;Keel, Sang-In;Yun, Jin-Han;Lim, In Gwon
    • 한국연소학회:학술대회논문집
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    • pp.367-369
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    • 2014
  • A study on laminar coflow jet flames diluted with helium and nitrogen has been conducted to investigate self-excitations. The stability map was provided with a function of nozzle exit velocity and fuel mole fractions of propane or methane. The results show that there exist three types of self-excitations; (1) buoyancy-driven self-excitation (BDSE), (2) Lewis number induced self-excitation coupled with buoyancy (LCB) and (3) Lewis number induced self-excitation (LISE).

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Effects of Lewis Number and Preferential Diffusion in Syngas Flame Diluted with He and Ar (He와 Ar으로 희석된 합성가스 화염에서 루이스 수와 선호확산효과)

  • Kim, Tae Hyung;Park, Jeong;Kwon, Oh Boong;Park, Jong Ho
    • Journal of the Korean Society of Combustion
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    • v.19 no.4
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    • pp.28-34
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    • 2014
  • Numerical study is conducted to grasp flame characteristics in $H_2/CO$ syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately-stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, $H_2$, and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of $H_2$ deteriorates flame temperature. Impact of preferential diffusion of H, $H_2$, and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.

A study on Self-excitations in Laminar Lifted Coflow-jet Flames (층류 동축류 제트 부상화염에서의 자기진동에 관한 실험적 연구)

  • Ban, Gyu Ho;Lee, Won June;Park, Jeong;Keel, Sang-In;Yun, Jin-Han;Lim, In Gwon
    • 한국연소학회:학술대회논문집
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    • pp.129-132
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    • 2014
  • A study on laminar coflow jet flames diluted with helium and nitrogen has been conducted to investigate self-excitations. The stability map was provided with a function of nozzle exit velocity and fuel mole fractions of propane or methane. The results show that there exist three types of self-excitations; (1) buoyancy-driven self-excitation (BDSE), (2) Lewis number induced self-excitation coupled with buoyancy (LCB) and (3) Lewis number induced self-excitation (LISE).

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Acceleration in Diffusive-thermal Instability by Heat Losses (열손실에 의한 확산-열 불안정성의 가속화)

  • Park, June-Sung;Park, Jeong;Kim, Jeong-Soo
    • Journal of the Korean Society of Combustion
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    • v.12 no.2
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    • pp.34-41
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    • 2007
  • The dynamic behaviors of counterflow non-premixed flame have been investigated experimentally to study effects of heat losses and Lewis number on edge flame oscillation, which result from the advancing and retreating edge flame motion of outer flame edge at low strain rate flame. For low strain rate flame, lateral conduction heat loss in addition to radiation heat loss could be more remarkable than the others. Oscillatory instabilities appear at fuel Lewis number greater than unity. But excessive lateral conduction heat loss causes edge flame instability even at fuel Lewis number less than unity. The excessive heat loss caused by the smaller burner diameter in which the flame length is an indicator of lateral conduction heat loss extends the region of flame oscillation and accelerates oscillatory instability in comparison to the previous study with the burner diameter of 26mm. Extinction behaviors quite different from the previous study are also addressed.

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Initiation of Gaseous Premixed Flame (예혼합기체 연료의 화염생성에 관한 연구)

  • 백승욱
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.13 no.1
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    • pp.135-139
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    • 1989
  • The flame initiation of an infinite fuel system exposed to a planar ignition kernel has been investigated numerically. The aim has been to promote an understanding of the flame initiation by using the simplest mathematical formulation which retains the essential physical features. It has been found that there exists a minimum ignition energy below which a combustion wave cannot be initiated. For a fixed value of Lewis number, the same flame progation velocity has been obtained irregardless of the amount ignition energy supplied. Furthermore, for a fixed energy input there is a maximum Lewis number over which the flame cannot be initiated.

Diffusion-flame instability in the premixed-flame regime (예혼합화염 영역에서 확산화염의 불안정성에 관한 연구)

  • Lee, Su-Ryong;Kim, Jong-Su
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.21 no.9
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    • pp.1218-1229
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    • 1997
  • The diffusional-thermal instability of diffusion flames in the premixed-flame regime is studied in a constant-density two-dimensional counterflow diffusion-flame configuration, to investigate the instability mechanism by which periodic wrinkling, travelling or pulsating of the reaction sheet can occur. Attention is focused on flames with small departures of the Lewis number from unity and with small values of the stoichiometric mixture fraction, so that the premixed-flame regime can be employed for activation-energy asymptotics. Cellular patterns will occur near quasisteady extinction when the Lewis number of the more completely consumed reactant is less than a critical value( ~ =0.7). Parametric studies for the instability onset conditions show that flames with smaller values of the Lewis number and stoichiometric mixture fraction and with larger values of the Zel'dovich number tend to be more unstable. For Lewis number greater than unity, near-extinction flame are found to exhibit either travelling instability or pulsating instability.

A Study on Self-excitation in Laminar Lifted Coflow-jet Flames (층류 동축류 제트 부상화염에서의 자기진동에 관한 연구)

  • Van, Kyu Ho;Lee, Won June;Park, Jeong;Kim, Tae Hyung;Park, Jong Ho
    • Journal of the Korean Society of Combustion
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    • v.20 no.1
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    • pp.43-51
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    • 2015
  • A study on laminar jet flames in coflow air diluted with helium has been conducted to investigate self-excitations for various propane mole fractions and nozzle exit velocities. The stability map was represented as a function of nozzle exit velocity and fuel mole fraction for propane. The results show that two types of self-excitation were observed : (1) buoyancy-driven self-excitation (hereafter called BDSE) and (2) Lewis-number induced-self-excitation coupled with (1) (hereafter called LCB) near extinction limit for 9.4 mm nozzle diameter. It was shown that with 0.95 mm nozzle diameter, Lewis-number-induced self-excitation (hereafter LISE) and BDSE could be separated. The differences between the two self-excitations were shown and discussed.

On the Origin of Oscillatory Instabilities in Diffusion Flames (확산화염의 진동불안성의 기원에 대해서)

  • Kim, Jong-Soo
    • Journal of the Korean Society of Combustion
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    • v.10 no.3
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    • pp.25-33
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    • 2005
  • Fast-time instability is investigated for diffusion flames with Lewis numbers greater than unity by employing the numerical technique called the Evans function method. Since the time and length scales are those of the inner reactive-diffusive layer, the problem is equivalent to the instability problem for the $Li\tilde{n}\acute{a}n#s$ diffusion flame regime. The instability is primarily oscillatory, as seen from complex solution branches and can emerge prior to reaching the upper turning point of the S-curve, known as the $Li\tilde{n}\acute{a}n#s$ extinction condition. Depending on the Lewis number, the instability characteristics is found to be somewhat different. Below the critical Lewis number, $L_C$, the instability possesses primarily a pulsating nature in that the two real solution branches, existing for small wave numbers, merges at a finite wave number, at which a pair of complex conjugate solution branches bifurcate. For Lewis numbers greater than $L_C$, the solution branch for small reactant leakage is found to be purely complex with the maximum growth rate found at a finite wave number, thereby exhibiting a traveling nature. As the reactant leakage parameter is further increased, the instability characteristics turns into a pulsating type, similar to that for L < $L_C$.

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Acceleration in Diffusive-thermal Instability by Heat Losses (열손실에 의한 확산-열 불안정성의 가속화)

  • Park, June-Sung;Park, Jeong;Lee, Kee-Man;Kim, Jeong-Soo;Kim, Sung-Cho
    • 한국연소학회:학술대회논문집
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    • pp.145-152
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    • 2007
  • The dynamic behaviors of counterflow non-premixed flame have been investigated experimentally to study effects of heat losses on edge flame oscillation, which result from the advancing and retreating edge flame motion of outer flame edge at low strain rate flame. For low strain rate flame, lateral conduction heat loss in addition to radiation heat loss could be more remarkable than the others. Oscillatory instabilities appear at fuel Lewis number greater than unity. But excessive lateral conduction heat loss causes edge flame instability even at fuel Lewis number less than unity. The dramatic change of burner diameters in which flame length is an indicator of lateral conduction heat loss was applied to examine the onset condition of edge flame oscillation and flame oscillation modes. Especially, extinction behaviors quite different from the previous study were observed.

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