• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1172-1177
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• 1999

Comparisons of measured turbulence properties in the unburned gas region of turbulent premixed flame stabilized by pilot flame, in cases of combusting and non-combusting flow conditions, are presented. Methane-air premixed jet at fuel equivalence ratio of 0.6 and 1.0 and Reynolds number of 7,000 was diagnosed using two-color laser velocimeter to obtain turbulence statistics. Same set of measurements was repeated at 21 locations within the unburned gas region of both combusting and non-combusting conditions. Velocity data were analyzed to evaluate the spatial distribution of turbulence properties including Reynolds stress, probability densities, joint probability densities and auto correlations. Contrary to assumptions of current theoretical models, significant influence of flame was observed in every property that was studied in the present investigation. The effective viscosity increased ten-fold when flame was on from cold flow values. The effect of mixing on joint probability as well as in turbulence intensity was suppressed by the flame. The measurements suggest that common assumptions of premixed flame model may result in sizable error in prediction of flame length and temperature distribution in near-field.

• 한국연소학회:학술대회논문집
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• 2006.10a
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• pp.60-64
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• 2006

Propagation rates ($U_{edge}$) of various premixed, twin edge-flames were measured as a function of global strain rate ($\sigma$), mixture strength, and Lewis number (Le). Using a counterflow slot-jet burner with electrical heaters at each end, both advancing (positive $U_{edge}$) and retreating (negative $U_{edge}$) edge-flames can be studied as they propagate along the long dimension of the burner. Experimental results are presented for premixed methane/air twin flames in terms of the effects of $\sigma$ on $U_{edge}$. Both low-$\sigma$ and high-$\sigma$ extinction limits were discovered for all mixtures tested. As a result, the domain of edge-flame stability was obtained in terms of heat loss factor and normalized flame thickness, and comparison with the numerical result of other researchers was also made. For low ($CH_4/O_2/CO_2$) and high ($C_3H_8$/air) Lewis number cases, propagation rates clearly show a strong dependence on Le.

• Journal of the Korean Society of Combustion
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• v.14 no.1
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• pp.25-30
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• 2009

Propagation rates ($U_{edge}$) of various premixed, twin edge-flames were measured as a function of global strain rate ($\sigma$), mixture strength, and Lewis number (Le). Using a counterflow slot-jet burner with electrical heaters at each end, both advancing (positive $U_{edge}$) and retreating (negative $U_{edge}$) edge-flames can be studied as they propagate along the long dimension of the burner. Experimental results are presented for premixed methane/air twin flames in terms of the effects of $\sigma$ on $U_{edge}$. Both low-$\sigma$ and high-$\sigma$ extinction limits were discovered for all mixtures tested. As a result, the domain of edge-flame stability was obtained in terms of heat loss factor and normalized flame thickness, and comparison with the numerical result of other researchers was also made. For low ($CH_4/O_2/CO_2$) and high ($C_{3}H_{8}$/air) Lewis number cases, propagation rates clearly show a strong dependence on Le.

• Journal of Energy Engineering
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• v.11 no.2
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• pp.166-177
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• 2002

The present study is described of the flame structure of one-dimensional, flat, premixed, laminar, coal-air flame with some addition of methane for the flame stability. A low pressure burner operating at a combustion pressure of 0.3 arm was employed in order to extend the reaction zone. Predicted results from the models considered in the present study are compared with experimental results. Comparisons are included gas temperatures, species concentrations, char analysis and measured burning velocity. Among the models, Model II $I^{*}$-d, which specified devolatilization rate constants and a char surface area factor S=4, resulted in good agreement within the present experimental ranges. The results of char analysis suggest that the extent of the reaction occurring on the panicle might be underestimated in the model so that the char surface area should be increased. A value of 4 for this factor was given by sensitivity analysis of change in char surface area. Again, model II $I^{*}$-d gave satisfactory predictions of burning velocities over most of the experimental range studied. It has been clearly shown that the particle diameter appreciably affects the rates of devolatilisation and char oxidation through the effects of thermal lag and volumetric reactive surface area, consequently laminar burning velocity.ity.

• Journal of the Korean Society of Safety
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• v.26 no.5
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• pp.1-6
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• 2011

Flame extinction and the chemistry of stoichiometric methane/air mixture were investigated numerically in the PSR(perfectly stirred reactor). For the study, PSR code was modified to be possible to unsteady calculation, and the sinusoidal fluctuation was subjected to the residence time. In the region of residence time far from the extinction limit, combustion mode was strongly dependent on the frequency. The low frequency excitation provided the quasi-steady behavior on the temperature and the concentrations of related species, but small variation of temperature was observed under high frequency. In the region of residence time near the extinction limit, the mixture subjected above 1 KHz was still reacting even though extinction had to be occurred under quasi-steady concept. The attenuation of extinction limit resulted from that chemical time was comparable to the flow time. The mean mole fractions of both NO and CO were almost same regardless of imposed frequency. However, the average mole fraction of $C_2H_2$ was decreased as increasing frequency, which implies that soot yield might be reduced at the higher frequency of flow excitation. The result provides the basic concept for flame stabilization, and it will be used to design a mild combustor.

• Journal of the Korean Society of Combustion
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• v.3 no.2
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• pp.41-50
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• 1998

Numerical simulations of nonpremixed $CH_4/C_2HCl_3$(Trichloroethylene, TCE)/Air flames are conducted at atmospheric pressure in order to understand the effect of hydrocabon bound chlorine on methane/air flames. A chemical kinetic mechanism is employed, the adopted scheme involving 48 gas-phase species and 445 elementray reaction steps containing 223 backward reactions. The calculated temperature, velocity, and critical strain rate are compared with the experiments for the flame (16.1% TCE by Vol.) estabilished at a strain rate of $175s^{-1}$. Whereas there is overall good agreement between predictions and the measurements, it appears that the critical strain rate is higher than measured, and some areas of further refinement in the kinetic mechanism are required.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.362-367
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• 2003

It is shown that the effect of variable parameters on flame structures and NOx emissions in the laminar partially premixed methane-air flames with a co-axial Bunsen burner. Objectives of this paper is to understand the effects of flow variables on NOx emissions and the flame structure with OH chemiluminescence, including reconstructed image by abel inversion processing at each conditions. A fuel flowrate of 200 [cc/min] was fixed and the amount of air was varied from 400 to 1200 [cc/min]. The experimental variables were equivalence ratio(${\Phi}$ fuel split percentage(${\sigma}$ and inner tube recess(x/D). Flow conditions were ranged from $1.36{\sim}4.76$(equivalence ratio), $50{\sim}100$(fuel split percentage) and $0{\sim}20$(inner tube recess). NOx analyzer and ICCD camera with a OH filter were used as a main experimental apparatus. In addition, Abel inversion, which is a kind of tomography and valuable to estimate a two-dimensional structure of co-axial flames from cubical information, was employed for combustion diagnostics. Results from this study indicate that the main effects depend on equivalence ratio and next sigma, x/D for NOx production and OH formation. Throughout Abel inversion, we could affirm the maximum position and the tendency of OH radical intensity by variants at five axial heights above the burner exit.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.6 s.237
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• pp.662-670
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• 2005

The effect of heat loss on combustion performance and burning velocity of micro combustors in various conditions were exploited experimentally. Three different gases were used, and various geometric matrixes were considered to figure out the phenomena of combustion in a micro combustor. The micro combustors used in this study were constant volume combustors and had cylindrical shape. Geometric parameter of combustor was defined as combustor height and diameter. The effect of height was exploited parametrically as 1mm, 2mm and 3 mm and the effect of diameter was parameterized to be 7.5 mm and 15 mm. Three different combustibles which were Stoichiometric mixtures of methane and air, hydrogen and air, and mixture of hydrogen and air with fuel stoichiometry of two were used. By pressure measurement and visualization of flame propagation, characteristic of flame propagation was obtained. Flame propagations which were synchronized with pressure change within combustor were analyzed. From the analysis of images obtained during the flame propagations, burning velocity at each location of flame was obtained. About $7\%$ decrease in burning velocity of $CH_4/Air$ stoichiometric mixture compared with previous a empirical result was observed, and we can conclude that it is acceptable to use empirical equations for laminar premixed flame burning velocity to micro combustions. Results presented in this paper will give fine tool for analysis and prediction of combustion process within micro combustors.

• Journal of Advanced Marine Engineering and Technology
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• v.22 no.1
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• pp.108-116
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• 1998

In this paper it is investigated the roles of key elementary reactions for NO formation in methane-air one-dimensional premixed flame and counterflow diffusion flame, which were studied numerically by using NO kinetics and $C_{2}$ -chemistry complied by Miller and Bowman. The spatial distributions of the reaction rates of 9 main elementary reactions directly related to NO formation and destruction were calculated. Integration of the rates of all reactions in the NO formation across the flame yields the quantitative reaction path diagram, which shows clearly relative importance of each reaction path in NO formation and how it changes with the type and parameters of the flame. The results show that the thermal and Fenimore mechanisms are dominant respectively for learn and rich premixed flames, and the latter is dominant for diffusion flames. In addition, it was found that the HCN recycle route is important for diffusion flame, and that the routes of mutual transformation between NO and NO$^{2}$, and between NO and HNO do not contribute to the net NO formation.

• Journal of the Korean Society of Combustion
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• v.8 no.3
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• pp.38-48
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• 2003

OH, CH and NO radical distributions have been measured and compared with the numerical analysis results in methane/air partially premixed laminar flames using 2-D LIF technique. The pick intensity of OH LIF signal is insensitive to fuel equivalence ratio: however, CH LIF intensity decreases as equivalence ratio increases and the NO concentration increases with equivalence ratio. The contribution of the prompt NO, formed near premixed reaction zone, to the total NO formation is evident from the OH, CH, and NO PLIF images in which the dilution effect of nitrogen is minimal for the highest equivalence ratio. Measured OH and NO LIF signals in counterflow flames agree with the computed concentration distributions. Both numerical and experimental results indicate that the structural change in a flame alters the NO formation characteristics of a partially premixed counterflow flame. The nitrogen dilution also changes flame structure, temperature and OH radical distributions and results in the decreased NO concentrations in a flame. The levels of decrease in NO concentrations, however, depends on the premixedness(${\alpha}$) of a flame. The larger change in the flame structure and NO concentrations have been observed in a premixed flame(${\alpha}=1.0$), which implies that the premixedness is likely to be a factor in the dilution effect on NO formation of a flame.