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Prediction of burning velocities of carbon monoxide-hydrogen-air flames

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Abstract

The predictions of the Payman and Wheeler and the Spalding mixing rules, which seek to derive the burning velocity of a mixture of fuels from knowledge of the burning velocities of its components, have been compared with the experimental results for lean hydrogen carbon monoxide air flames. It is shown that the Payman and Wheeler mixing rule fails to predict the burning velocity of the mixture, but that the Spalding mixing rule predicts it with an error of less than ten per cent, provided it is assumed that the components have the same equivalence ratios. The comparison could not be carried out for rich mixtures because of the experimental difficulty in obtaining a straight-line relation between the burning velocities and the heat loss.

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... Models (PREMIX): Li et al. [223] and Sun et al. [200] 17 Comparison of syngas laminar flames speeds for the 10/90 % H2/CO blend with available data from the literature: Dong et al. [214], Hassan et al. [198], Vagelopoulos [70], Günther and Janisch [208] and Scholte and Vaags [206] (10.5/89.5 % H2/CO). Single data points from Yumlu [207] and Scholte and Vaags [205] [198] and Scholte and Vaags [206]. Models (PREMIX) from Li [230] and Li [230] modified with the iron pentacarbonyl submechanism used in the works of Rumminger and Linteris [218] [70], Dong et al. [214], Prathap et al. [212], Sun et al. [200], Burke et al. [213], Natarajan et al. [85], Hassan et al. [198], McLean et al. [199], Yumlu [207] and Günther and Janisch [208]. ...
... Single data points from Yumlu [207] and Scholte and Vaags [205] [198] and Scholte and Vaags [206]. Models (PREMIX) from Li [230] and Li [230] modified with the iron pentacarbonyl submechanism used in the works of Rumminger and Linteris [218] [70], Dong et al. [214], Prathap et al. [212], Sun et al. [200], Burke et al. [213], Natarajan et al. [85], Hassan et al. [198], McLean et al. [199], Yumlu [207] and Günther and Janisch [208]. Models (PREMIX) are from Li et al. [230] and Sun et al. [200] 21 Comparisons of rate constants used in the Li et al. [223] and Sun et al. [200] mechanisms for: (a) H + O2 = O + OH reaction with rate constants from Hessler [231] and Hwang et al. [232]; (b) HO2 + H = OH + OH reaction with rate constants from Mueller al. [233] and adapted value of Sun et al. [200] 10 Validation of the OH* chemiluminescence methodology through comparison with experimental and numerical results for laminar flame speeds of H2/air mixtures (Burner I.D.=4 mm). ...
... Experimental results are from Lamoureux [243], Qin [244], Tse [245], Raman [246], Koroll [247], Dowdy [248], Egolfopoulos [64], Iijima [174]. Model from Li et al. [230] [207], Hassan [198], Vagelopoulos [70], Natarajan [211], McLean [199] and Sun [200]. Models are from Sun [200] and Li [230] [200], Hassan [198], Mclean [199] and numerical predictions with the kinetic mechanisms of Sun [200] and Li [230] [198] and numerical predictions with the kinetic mechanisms of Sun [200] and Li [230] [200], Hassan [198] and numerical predictions with the kinetic mechanisms of Sun [200] and Li [230] [212], Burke [177], Sun [200], Hassan [198], McLean [199] and numerical predictions with the kinetic mechanisms of Sun [200] and Li [230] [200], Burke [213], Hassan [198]. ...
Article
In the context of CO2 emission reduction, the present study is devoted to the development of a laminar flame speed measurement methodology, using the Digital Particle Image Velocimetry (DPIV) diagnostic. The latter is applied to stagnation flow flames, seen to have considerable assets for such studies. Indeed, flames stabilized in these diverging flows are planar, steady and in near-adiabatic conditions, while subtraction of strain effects on flame is intrinsically allowed. The methodology developed herein has been applied to the well-characterized methane/air mixtures for validation. An extensive comparison with the literature datasets has been provided. Both 1D (PREMIX, OPPDIF) as well as 2D (Fluent©) numerical tools have been used to confirm the reliability and accuracy of the developed approach. A particular attention has been given to the characterization of the seeding particle motion within the diverging flow, with consideration of the often-neglected thermophoretic force. Fundamental flame velocities of various syngas (H2+CO) mixtures have been investigated using multiple experimental approaches including the aforementioned counterflow methodology as well as spherical and conical flame configurations. Performed measurements from the different approaches have been confronted and flame sensitivities to stretch have been characterized for a wide range of equivalence ratios (E.R.=0.4 to 5.0) and mixture compositions (5/95 to 50/50 % H2/CO).
... In Eq.(1) αi is the mass ratio of fuel i plus the corresponding amount of oxidant and total mass of (fuel + oxidant). Yumlu (1967) simplified the Spalding mixing rule on the assumption that the heat release rate, which is proportional to Su 2 , is additive for all the components of the mixture: Harris & Lovelace (1968), as cited in Skrbić et al. (1984), proposed a predictive method considering also the effect of inerts, and adopting the volume fraction xi of any component of the gas mixture, however excluding H2, O2, N2 or CO2: ...
... For the fuels analysed in this work, the best correlation for binary and ternary fuel mixtures is that of Harris and Lovelace (1968) or alternatively the simple correlation by Yumlu (1967). The addition of hydrogen in large amount reduces the ability of any tool to reproduce the behaviour of fuel explosions. ...
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In this paper, the sensitivity of large eddy simulation (LES) to the presence of the combustion submodel was investigated for transient interactions between premixed flame fronts and toroidal vortex structures generated at the wake of a circular orifice. To this end, LES computations were run, with and without the combustion submodel, for two orifice diameters: 40 mm and 20 mm. Nonuniform unstructured grids with a cell characteristic length varying in the range of 0.5-1 mm were used. In going from the 40-mm orifice to the 20-mm orifice, both the size and velocity of the vortex increase, leading to a different regime of interaction with the flame: the vortex only wrinkles the flame front in the 40-mm case (wrinkled regime) and also disrupts the continuity of the front, giving rise to the formation of separate reaction zones (i.e., flame pockets that leave the main front), in the 20-mm case (breakthrough regime). It has been found that the impact of the combustion submodel on LES predictions is strongly dependent on the regime of interaction. Results for the 40-mm orifice are substantially the same, regardless of the presence of the combustion submodel. Conversely, at the wake of the 20-mm orifice, the intensity of the flame-vortex interaction is such that the combustion submodel is strictly needed to reproduce both the qualitative (evolution of the pockets formed and their interaction with the main front) and quantitative (flame speed) characteristics of the flame propagation correctly.
... Laminar burning velocities of syngas mixtures have been measured with conical flames stabilized with Mach Hebra nozzle burners [5] and with Bunsen burners [6]. Laminar flame speeds of CO/H 2 mixtures have also been measured with spherically expanding flames [7] and flat flames [8]. There are two shortcomings of these earlier flame speed studies. ...
... In fact, the deviations from the measurements are about the same levels seen in the undiluted, high preheat case; the GRI predictions are 10% (Φ=0.6) and 9% (Φ=0. 8) above the measurements, while the H 2 /CO mechanism results are 14% (Φ=0.6) and 12% (Φ=0.8) too high. ...
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Coal derived synthetic gas (syngas) fuel is a promising solution for today s increasing demand for clean and reliable power. Syngas fuels are primarily mixtures of H2 and CO, often with large amounts of diluents such as N2, CO2, and H2O. The specific composition depends upon the fuel source and gasification technique. This requires gas turbine designers to develop fuel flexible combustors capable of operating with high conversion efficiency while maintaining low emissions for a wide range of syngas fuel mixtures. Design tools often used in combustor development require data on various fundamental gas combustion properties. For example, laminar flame speed is often an input as it has a significant impact upon the size and static stability of the combustor. Moreover it serves as a good validation parameter for leading kinetic models used for detailed combustion simulations. Thus the primary objective of this thesis is measurement of laminar flame speeds of syngas fuel mixtures at conditions relevant to ground-power gas turbines. To accomplish this goal, two flame speed measurement approaches were developed: a Bunsen flame approach modified to use the reaction zone area in order to reduce the influence of flame curvature on the measured flame speed and a stagnation flame approach employing a rounded bluff body. The modified Bunsen flame approach was validated against stretch-corrected approaches over a range of fuels and test conditions; the agreement is very good (less than 10% difference). Using the two measurement approaches, extensive flame speed information were obtained for lean syngas mixtures at a range of conditions: 1) 5 to 100% H2 in the H2/CO fuel mixture; 2) 300-700 K preheat temperature; 3) 1 to 15 atm pressure, and 4) 0-70% dilution with CO2 or N2. The second objective of this thesis is to use the flame speed data to validate leading kinetic mechanisms for syngas combustion. Comparisons of the experimental flame speeds to those predicted using detailed numerical simulations of strained and unstrained laminar flames indicate that all the current kinetic mechanisms tend to over predict the increase in flame speed with preheat temperature for medium and high H2 content fuel mixtures. A sensitivity analysis that includes reported uncertainties in rate constants reveals that the errors in the rate constants of the reactions involving HO2 seem to be the most likely cause for the observed higher preheat temperature dependence of the flame speeds. To enhance the accuracy of the current models, a more detailed sensitivity analysis based on temperature dependent reaction rate parameters should be considered as the problem seems to be in the intermediate temperature range (~800-1200 K). Ph.D. Committee Chair: Jerry M. Seitzman; Committee Member: Jechiel I. Jagoda; Committee Member: Scott Martin; Committee Member: Suresh Menon; Committee Member: Timothy C. Lieuwen
... It is well known in the scientific community that several methods to measure adiabatic, unstretched, LBV have been proposed, such as the Bunsen flame [3][4][5], outwardly propagating spherical flame [6][7][8][9], stagnant flame [10][11][12][13][14], heat flux or flat flame [15][16][17] and, the annular diverging tube methods [18,19]. Each method has advantages and drawbacks, as the direct measurement of adiabatic LBV is difficult at standard thermodynamic conditions and becomes more challenging at elevated temperatures due to uncertainties that affect the performance of the kinetic model in terms of predictabilities and cause variability in LBV's data. ...
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In the context of the current energy transition, the use of biomass-derived syngas (BDS) is often recognized as a fundamental path towards decreasing fossil fuel dependency and greenhouse gas emissions. However, hydrogen-containing BDS are prone to flame instability problems. More efforts are being carried out aiming at efficiently adopting BDS in industrial combustors with CH4 co-firing or inert gas dilutions by exploring accurate knowledge of burning velocity. To do so, a deeper knowledge of the syngas combustion behaviour is strictly necessary. The objective of this study fits in this framework: in particular, a computational study has been carried out to evaluate kinetic models and present fresh insights on the effects of varying syngas mixtures such as CO/H2, CO/H2/CO2 and CO/H2/CH4 on Laminar Burning Velocity (LBV) and peak LBV location (Φ_(LBV=max)). In-detail chemical kinetic simulations of equimolar (CO: H2=1:1) forestry waste syngas were systematically carried out taking advantage of the open-source CANTERA solver. Three detailed kinetic models i.e., newly released FFCM-2, USC mech II, and modified GRI mech III were implemented to report accurate flame parameters at 1 bar and different temperature levels (from 300 K up to 450 K). On comparing the results with experiments, FFCM-2 proved to be a good kinetic model for the considered syngas mixtures CO/H2, CO/H2/CO2 and especially for CO/H2/CH4 for mixtures containing a limited share of 30 % methane at normal and moderately elevated temperature at 0.4 ≤ 𝜱 ≤ 2.1. The USC mech II performed very well for CO/H2, and CO/H2/CO2, while the modified GRI mech III model also gave agreeable predictions for CO/H2/CH4 mixture having rich methane content. Additionally, when varying syngas composition analysis was conducted at different temperatures, the progressive CO2 dilution and CH4 addition of up to 30% reduced the peak LBV and moved the peak LBV locations (Φ_(LBV=max)) towards lean ER conditions with 9% and 40% reductions, respectively; however, only the latter effect was enhanced at the elevated initial temperature. Furthermore, sensitivity analysis of respective syngas mixtures is reported at normal and elevated temperatures to explore the most sensitive intermediate reactions relative to LBV. The shift of peak LBV locations and their enhancement at elevated temperatures also open the research path to study the underlying impacts on the flame modes/regimes and structure, especially CO emissions pathways in syngas with 30% of CH4 and CO2 additions.
... Various attempts were being made earlier [1] to develop producer gas fuelled reciprocating engines, working on high octane rating, ultra clean and low energy density producer gas from gasification of wood. Earlier research studies have indicated the combustion phenomenon of CO/H2 mixtures and estimation of laminar flame speeds assuming to be either spherical flames or flat flames [2], but have not considered the effect of stretch. Most of the reported data on the combustion phenomenon of syn gas is either at stoichiometric or rich mixtures. ...
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Gas turbines are low specific fuel consumption engines which are used in aviation and stationary power generation. The need for clean energy and carbon neutral technologies have emphasized for usage of alternate fuels for power production such as biomass.. Earlier research studies indicated the feasibility of utilization of low calorific value gas in Gas turbines, but reported a large quantity of Exergy destruction in combustion chamber. The present paper addresses the combustion of producer gas in a cannular combustion chamber, with a non-premixed model using Ansys-Fluent. A 25 KW gas turbine combustion chamber is considered with operating pressure of 4 bar absolute working on Producer gas with the fuel composition of 19% H2 , 20 % CO, 2 % CH4 , 10% CO2 and remaining N2. Comparative study is made with Methane and producer gas and results are indicated. Combustion chamber operating with CH4 has a lower energy density than the producer gas and flame velocities are more when producer gas is used as fuel than CH4. As Flame stabilization and the flame lengths will affect the turbine inlet temperature (TIT), the combustion chamber is designed to achieve a TIT value around 1100K (which also prevents formation of Thermal NOx). INTRODUCTION Due to increasing requirement for clean power generation and optimization of effective heat utilization by heat recovery systems using combined cycles such as IGCC has given prominence for present topic of research. Gas turbines are low specific fuel consumption thermal turbo machine which accommodates the utilization of LCV fuels for power generation. Various attempts were being made earlier [1] to develop producer gas fuelled reciprocating engines, working on high octane rating, ultra clean and low energy density producer gas from gasification of wood. Earlier research studies have indicated the combustion phenomenon of CO/H2 mixtures and estimation of laminar flame speeds assuming to be either spherical flames or flat flames [2], but have not considered the effect of stretch. Most of the reported data on the combustion phenomenon of syn gas is either at stoichiometric or rich mixtures. For devices like gas turbines for power generation and aviation applications, flames experience extinction if local mixing rates exceed combustion rates. Combustion chamber designed for aviation applications are size constrained and hence smaller in size, which leads to increasing mixing rates and creates flame extinction. Whereas gas turbines used for power generation seek lean mixtures for NOx reduction and combustion rates are lower as compared to aviation applications which leads to regions of flame extinction. In both of the situations flame instability is a major issue when utilizing LCV fuels such as producer gas having composition of 19% H2 , 20 % CO, 2 % CH4 , 10% CO2 and remaining N2. Gas turbines are generally high work output rated devices in the range of 110 MW or so and operation of gas turbine on alternate fuels demand large quantity of producer gas continuously. This requires a high rated down draft gasifier to supplement the gas continuously which is biggest limitation of operation of gas turbine. LITERATURE REVIEW As per the earlier research studies made by Francesco Fantozzi et al [3] biomass to energy conversion is particularly attractive on the micro scale where internal combustion engines such as micro turbines may be utilized coupled to an indirect gasification system. A RANS analysis has been performed in order to simulate both natural gas and syngas combustion. De biasi [4] have suggested usage of low cost and high efficiency make 30 to 50 Kw micro turbines for power generation using LCV fuels. Several other authors have indicated the importance of micro turbines and need for CFD analysis at various gas turbine operating conditions. Turbulence in gas turbine
... Laminar burning velocities of syngas mixtures have been measured with Mach Hebra nozzle burners [12] and with Bunsen burners [13]. Laminar flame speeds of CO/H2 mixtures have also been measured with spherically expanding flames and flat flames [14]. Most of these flame speed measurements are for stoichiometric and fuel-rich mixtures and with different fuels compositions. ...
Conference Paper
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Pressures on vehicle manufacturers to reduce emissions have resulted in an increased interest to improve fuel economy and enable use of fuels developed from renewable sources that can achieve a net reduction in the CO2 output per vehicle. The use of bio-gas fuels in internal combustion engines has become a real alternative to traditional liquid fuels derived from petroleum. To extract the maximum benefits from these emergent fuels through optimized engine design and calibration, a deep understanding of the behavior is necessary. The combustion process of a single cylinder research engine with optical access, four stroke PFI-SI, was experimentally investigated. High spatial resolution cycle resolved digital imaging, in the visible and UV spectral range was used to characterize the flame front propagation. A post-processing routine was developed to evaluate flame areas and various local and global morphology characteristics to have a detail understanding of the flame behavior in an engine combustion chamber. The engine was fueled with Methane as baseline fuel and compared with an equivalent syngas mixture (blend of hydrogen, methane, carbon monoxide, carbon dioxide and nitrogen). It was operated at 900 rev/min, under partial load condition. For the equivalent syngas blend the results suggest an increase in the combustion duration. The flame speed propagation was higher to methane, with a difference of 1.9 m/s. Also both fuels present a preferential flame center movement in direction of the intake valves, and the average curvature was negative. The cyclic variations in the combustion process were around 1% for syngas and 0.5% for methane, indicating a stable combustion process.
... Various attempts were being made earlier [1] to develop producer gas fuelled reciprocating engines, working on high octane rating, ultra clean and low energy density producer gas from gasification of wood. Earlier research studies have indicated the combustion phenomenon of CO/H2 mixtures and estimation of laminar flame speeds assuming to be either spherical flames or flat flames [2], but have not considered the effect of stretch. Most of the reported data on the combustion phenomenon of syn gas is either at stoichiometric or rich mixtures. ...
Conference Paper
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Gas turbines are low specific fuel consumption prime movers for power generation. The situation of increasing demand for power generation with “Carbon Neutral” technologies is the need of hour. Earlier research studies indicated the feasibility of utilization of low calorific value gas in Gas turbines, but reported a large quantity of Exergy destruction in combustion chamber. The present paper addresses the combustion of producer gas in a cannular combustion chamber, with a non-premixed model using Ansys-Fluent. A 25 KW gas turbine combustion chamber is considered with operating pressure of 4 bar absolute working on Producer gas with the fuel composition of 19% H2 , 20 % CO, 2 % CH4 , 10% CO2 and remaining N2. Combustion chamber operating with the producer gas is presented. It is observed that flame velocities are more when producer gas is used as fuel than CH4. As Flame stabilization and the flame lengths will affect the turbine inlet temperature (TIT), the combustion chamber is designed to achieve a TIT value around 1100K (which also prevents formation of Thermal NOx).
... Several prior studies have included measurements of the flame speeds of syngas mixtures. [3][4][5] For example, stretch corrected laminar flame speed measurements with counter-flow flames 6 and spherically expanding flames [7][8][9][10] have been obtained. Various reaction mechanisms have been proposed for the H 2 /CO combustion based on these measurements. ...
Article
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Laminar flame speeds and strain sensitivities of mixtures of H 2 and air, or air highly diluted with N 2 (O 2 :N 2 1:9) have been measured for a range of equivalence ratios at high preheat conditions (~700K) using a nozzle generated, 1-D, laminar, wall stagnation flame. The measurements are compared with numerical predictions based on three detailed kinetic models (GRIMech 3.0, a H 2 /CO mechanism from Davis et al. and a H 2 mechanism from Li et al.). Sensitivity of the measurements to uncertainties in boundary conditions, e.g., wall temperature and nozzle velocity profile (plug or potential) are investigated through detailed numerical simulations and shown to be small. The flame speeds and strain sensitivities predicted by the models are in reasonable agreement with the measurements for H 2 with standard air at very lean conditions. For H 2 with N 2 diluted air, however, all three mechanisms significantly over predict the measurements. The disagreement between experimental data and the predictions for the N 2 diluted flames also increases for leaner mixtures. In contrast, the models under predict flame speeds for H 2 with both standard and N 2 diluted air for room temperature reactants, based on comparisons with measurements in the literature. Thus, we find that the temperature dependence of the hydrogen flame speed as predicted by all the models is greater than the actual temperature dependence (for both standard and diluted air). Finally, the models are found to under predict the measured strain sensitivity of the flame speed for H 2 burning in N 2 diluted air, especially away from stoichiometric conditions.
... Laminar burning velocities of syngas mixtures have been measured with conical flames, 3,4 spherically expanding flames 5 and flat flames. 6 The stretch corrected laminar flame speed measurements for H 2 /CO mixtures with counter-flow flames 7 and spherically expanding flames [8][9][10][11] have been obtained and are in fair agreement with each other. However, most of these flame speed measurements are for stoichiometric and fuel-rich mixtures; many low emissions gas-turbine approaches require lean premixed combustion. ...
Article
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Laminar flame speeds of lean H 2 /CO/CO 2 (syngas) fuel mixtures have been measured for a range of H 2 levels (20-90% of the fuel) at pressures and reactant preheat temperatures relevant to gas turbine combustors (up to 15 atm and 600 K). A conical flame stabilized on a contoured nozzle is used for the flame speed measurement, which is based on the reaction zone area calculated from chemiluminescence imaging of the flame. A O 2 :He mixture (1:9 by volume) is used as the oxidizer, rather than standard air, in order to suppress the hydrodynamic and thermo-diffusive instabilities that become prominent at elevated pressure conditions for lean H 2 /CO fuel mixtures. All the measurements are compared with numerical predictions based on two leading kinetic mechanisms: the H 2 /CO mechanism of Davis et al. and the C 1 mechanism of Li et al. The results generally agree with the findings of an earlier study at atmospheric pressure: 1) for low H 2 content (<40%) fuels, the model predictions are in good agreement with measurements at both 300 K and 600 K preheat temperature; but 2) the models tend to over predict the temperature dependence of the flame speed for medium (~40-60%) and high (>60%) H 2 content fuels, especially at very lean conditions. The elevated pressure (~15 atm) results, however, reveal that the effect is less pronounced than at atmospheric pressure. The exaggerated temperature dependence of the current models may be due to errors in the temperature dependence used for so-called "low temperature" reactions that become more important as the preheat temperature is increased. The radiation effects associated with CO 2 addition to the fuel (up to 40%) is found to be less important for medium and high H 2 content syngas fuels at elevated pressure and preheat temperature.
... 4 Laminar flame speeds of CO/H 2 mixtures have also been measured with spherically expanding flames 5 and flat flames. 6 However, most of these flame speed measurements are in stoichiometric and fuel-rich mixtures; many low emissions gas-turbine approach require lean premixed combustion. There is also substantial scatter in the reported data that could not be explained just by experimental uncertainties (see review by Andrews and Bradley 7 ). ...
Article
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Laminar flame speeds of H 2 /CO/CO 2 mixtures have been measured over a range of fuel compositions, lean equivalence ratios, and reactant preheat temperature (up to 700 K). The measurements are compared to numerical flame speed predictions based on two reaction mechanisms: GRI Mech 3.0 and a H 2 /CO mechanism. For undiluted and nonpreheated mixtures, the current results agree with previous data and the numerical calculations over most of the range tested. The measured flame speeds increase as the H 2 content of the fuel rises and for higher equivalence ratios. The most significant difference between the measurements and models is for high CO content fuel with the H 2 /CO mechanism, and the high H 2 content fuel at the leanest conditions with the GRI mechanism. For CO 2 diluted fuels, measured flame speeds decrease as predicted. However, agreement between the measurements and predictions worsens with increasing CO 2 dilution. Deviations as large as 40% are observed at lean equivalence ratios and 20% CO 2 levels. For reactant preheat temperatures below ~400K, the measured flame speeds generally match the calculated flame speeds within 10%. At higher preheat temperatures, however, the discrepancy between the measurements and the calculations increases, reaching levels of ~30% at 700 K. The measured temperature dependence is closer to the predictions from GRI Mech 3.0 than from the H 2 /CO mechanism.
... 4 Laminar flame speeds of H 2 /CO mixtures have also been measured with spherically expanding flames 5 and flat flames. 6 However, most of these flame speed measurements are in stoichiometric and fuel-rich mixtures; many low emissions gas-turbine approach require lean premixed combustion. Stretch corrected laminar flame speed measurements for H 2 /CO mixtures with counter-flow flames 7 and spherically expanding flames [8][9][10][11] have been obtained and are in fair agreement with each other. ...
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Laminar flame speed and strain sensitivities have been measured for mixtures of H2/CO/CO2/N2/O2 with a wall stagnation flame technique at high preheat temperature (700 K) and lean conditions. The measurements are compared with numerical predictions based on two reaction mechanisms: GRI Mech 3.0 and a H2/CO mechanism (Davis et al.). For H2:CO 50:50 fuel mixtures, both models tend to over predict the temperature dependence of the flame speed especially at very lean conditions, which confirms the trend found in an earlier study employing a Bunsen flame technique. The predicted strain sensitivities are in good agreement with the measurements. For 50:50 H2:CO fuel mixtures diluted with 40% CO2, the amount of over prediction by the models is about the same as in the undiluted case, which suggests that radiation effects associated with CO2 addition are not important for this mixture at highly preheated lean condition. For low H2 content (5 to 20%) H 2/CO fuel mixtures at 5 atm and fuel lean condition, the predicted unstrained flame speeds are in excellent agreement with the measurements, but the models fail to predicted the strain sensitivity as the amount of H 2 increases to 20%. Results are also presented for pure H2 with N2 diluted air (O2:N2 1:9) over a range of equivalence ratios. At lean conditions, the models over predict the measured flame speed by as much as 30%, and the amount of over prediction decreases as the equivalence ratio increases to stoichiometric and rich condition. The measured strain sensitivities are three times higher than the model predictions at lean conditions. More importantly, the predicted strain sensitivities do not change with equivalence ratio for both models, while the measurements reveal a clear trend (decreasing and then increasing) as the fuel-air ratio changes from lean to rich.
... cific heat at constant pressure of the mixture (m) and pure fuels (i), ΔT is the difference between the adiabatic flame temperature and the initial temperature and Su,i is the laminar flame speed of the i-unburned pure fuel/oxidant. In Eq.(1) αi is the mass ratio of fuel i plus the corresponding amount of oxidant and total mass of (fuel + oxidant). Yumlu (1967) simplified the Spalding mixing rule on the assumption that the heat release rate, which is proportional to Su 2 , is additive for all the components of the mixture: Harris & Lovelace (1968), as cited in Skrbić et al. (1984), proposed a predictive method considering also the effect of inerts, and adopting the volume fraction xi of any co ...
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The evaluation of safety parameters for binary and ternary mixtures of hydrogen and light hydrocarbons is essential for the definition of the combustion characteristics of bio-derived fuel-gas mixtures. Furthermore, with specific reference to laminar burning velocity, there is a strong need to define simple correlations, which would allow a fast prediction of global burning velocity of the mixtures starting from molar compositions and laminar burning velocity of the pure components, in analogy with Le Chatelier's rule for flammability limits. In this paper, a preliminary experimental and numerical study is performed for the assessment of safety parameters for mixtures of methane, propane and hydrogen with air at initial ambient pressure. Explosion tests have been conducted in a reinforced 5 liters steel vessel. The PREMIX module of the CHEMKIN package, coupled to the Marinov detailed reaction scheme, has been used to compute the un-stretched laminar burning velocity. For model validation, results have been compared to experimental data. Copyright © 2012, AIDIC Servizi S.r.l.
... Several studies on laminar flame speeds of syngas mixtures have been previously reported for experimental configurations including heat-stabilized [12,13], spherical [14e18], counterflow [19,20] and conical flames [21e26]. These investigations have been carefully analyzed and listed in Ref. [27] with the related experimental conditions. ...
Article
Laminar flame speeds of syngas mixtures (H2/CO/Air) have been studied using the Bunsen flame configuration with both straight and nozzle burners. The flame surface area and flame cone angle methodologies, respectively based on the OH* chemiluminescence and Schlieren imaging techniques, have been performed to extract flame speeds for a wide range of equivalence ratios (0.3 < φ < 1.2) and mixture compositions (1% < %H2 < 100%). As a result, a flame speed correlation established for lean syngas flames with 0.6 < φ < 1.0 and 10% < %H2 < 70% is proposed. A particular attention has been devoted to the development and validation of the OH* chemiluminescence methodology with the identification of important parameters governing the measurement accuracy.
... Among these parameters, Lewis number, Schmidt number, kinetic viscosity, and density can be calculated easily using CHEMKIN collection 3 with transport and thermal databanks of each species. For the laminar flame speed, similar to that used by Kalghatgi (1981), the method of Yumlu (1967Yumlu ( , 1968) was adopted in calculation. For mixtures containing two combustible fuels, the burning velocity can be defined as ...
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An extended database on blowout velocities of inert-diluted methane, pro-pane, and hydrogen jet flames in the turbulent regime was experimentally established and used to examine and verify existing theories of blowout velocity estimation. Helium, argon, nitrogen, and carbon dioxide were used as the inert diluents to generate different initial properties at the jet exit. The theories of blowout velocity estimation by Kalghatgi in the highly diluted regime were carefully examined using jet flames of different fuels diluted with inerts of different gas properties. The results showed that among the theories the blowout velocity estimation of Kalghatgi is more reliable in the extended region. On the other hand, the Accepted 11 March 2004. The financial support by National Science Council, ROC, through projects NSC90-2212-E-006-120 and NSC91-2212-E-006-039 are gratefully acknowledged.
... Laminar burning velocities of syngas mixtures have been measured with Mach Hebra nozzle burners [3] and with Bunsen burners [4]. Laminar flame speeds of CO/H 2 mixtures have also been measured with spherically expanding flames [5] and flat flames [6]. Most of these flame speed measurements are for stoichiometric and fuel-rich mixtures, while many modern low-emissions combustion approaches, especially in gas turbines, emphasize lean premixed combustion. ...
Article
Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured over a range of fuel compositions (5–95% for H2 and CO and up to 40% for CO2 by volume), reactant preheat temperatures (up to 700 K), and pressures (1–5 atm). Two measurement approaches were employed: one using flame area images of a conical Bunsen flame and the other based on velocity profile measurements in a one-dimensional stagnation flame. The Bunsen flame approach, based on imaging measurements of the reaction zone area, is shown to be quite accurate for a wide range of H2/CO compositions. These data were compared to numerical flame speed predictions based on two established chemical mechanisms: GRI Mech 3.0 and the Davis H2/CO mechanism with detailed transport properties. For room temperature reactants, the Davis mechanism predicts the measured flame speeds for the H2/CO mixtures with and without CO2 dilution more accurately than the GRI mechanism, especially for high H2 content compositions. The stagnation flame measurements for medium levels of H2 at both 1 and 5 atm, however, show lower than predicted strain sensitivities, by almost a factor of two at lean conditions (Φ=0.6–0.8). At preheat temperatures comparable to those found in gas turbine combustors, the accuracy of the flame speed predictions worsens. For example in fuels with low levels of H2, both models underpredict the measurements. In contrast, for medium H2 content fuels, both measurement techniques show that the models tend to overpredict flame speed, with the discrepancy increasing as Φ decreases and temperature increases. In general, the Davis mechanism predictions are in good agreement with the measurements for medium and high H2 fuels for preheat temperatures up to 500 K but overpredict the measurements at higher temperatures.
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Precise measurement and prediction of flame speed and laminar burning velocity are essential for premixed combustion properties characterization, turbulent combustion models validation, progress, and validation of chemical kinetic models. Besides, the problem of lack of fossil fuel, planet pollution, and production of several fuel alternatives led researchers to reexamine the process of combustion and optimize fuel consumption. So, it would be necessary to know the change of laminar burning velocity and flame speed with thermodynamic conditions to understand the impression of practical applications in all combustion systems as working pressures and temperatures are extensively higher than the atmospheric conditions. Several investigations work regarding flame speed and laminar burning velocity had been achieved. However, a detailed literature review of methods and techniques used to measure these two parameters and the effect of operating factors for different fuels focusing on biofuels is presented in this paper for ease of reviewing.
Conference Paper
Synthetic gas premixed flames have received considerable attention in the past. Recent interest in lean premixed combustion requires the fundamental combustion properties at fuel-lean conditions. This paper presents an experimental study on the laminar burning velocity of synthetic gas premixed flames at close to extinction conditions. CH4H2 and CO-H2 fuel blends at different mixture compositions simulating coal-derived synthetic gases were investigated. A water-cooled nitrogen-stabilized flat flame burner with a diameter of 6 cm was employed for this purpose. The planar laser induced fluorescence technique was employed to precisely measure the flame width. Hydroxyl radical (OH) concentration was used as the marker of reaction field. A pulsed Nd:YAG laser was used in conjunction with an optical parametric oscillator to generate a vertical laser sheet at a wavelength of 283.5 nm. An intensified CCD camera was used to capture the fluorescence signals from OH radicals. For each fuel blend mixture composition, the lean blowout condition was first determined. All measurements were then taken at this condition. For CH4-H2 fuel blends, the burning velocity near extinction varied between 6.88 cm/s and 10.13 cm/s. Burning velocities of CO-H2 fuel blends near extinction varied between 4.36 cm/s and 6.27 cm/s.
Conference Paper
Laminar flame speeds of undiluted syngas (H2/CO) mixtures have been studied at atmospheric conditions using chemiluminescence and schlieren techniques for straight cylindrical and nozzle burner apparatus. A wide range of mixture composition, from pure H2 to 1 /99 % H2/CO, has been investigated for lean premixed syngas flames. To achieve a better flame stabilization and reduce flame flashback propensity, two nozzle burners of different sizes have been designed and fabricated and were further used to compare the flame cone angle and the total surface area of the flame techniques. Results are compared to predictions using recent H2/CO mechanisms developed for syngas combustion.
Conference Paper
The paper presents the experimental measurements of the laminar burning velocity of H2 -CO mixtures. Hydrogen (H2 ) and carbon monoxide (CO) are the two primary constituents of syngas fuels. Three burner systems (nozzle, tubular, and flat flame) are used to quantify the effects of burner exit velocity profiles on the determination of laminar flame propagation velocity. The effects to N2 and CO2 diluents have been investigated as well, and it is observed that the effects of N2 and CO2 on the mixture burning velocity are significantly different. Finally, the burning velocity data of various syngas compositions (brown, bituminous, lignite and coke) are presented.
Conference Paper
Flashback is one of the major problems in lean premixed combustion of gas turbine combustor. Due to the effulgent future of co-product system and IGCC, lean premixed combustion, one of the approaches to ultra low NOx for rich hydrogen syngas fuel need farther research on anti-flashback and low pressure drop combustor. Mechanism and characteristics of methane and syngas flashback for 2 types of flame holders, i.e. ring shape and rod shape have been researched through experiment as well as numerical simulation. The partial premix model has been selected to simulate premixed combustion flashback process since it combined the advantage of PDF model and TFC model. Experiments demonstrate that, the flashback velocity of different fuel compositions or flame holder size generally can be correlated to the same dimensionless function by using Peclet number model if the structures of flame holders are the same. Peclet function curves were used to compare the anti-flashback performance of the 2 types of flame holders mentioned above with swirl holder. Boundary coaxial jet can change flashback through the wall into flashback in the core flow and significantly improve the anti-flashback performance of the ring-type flame holder on condition that the velocity of the boundary coaxial jet is in an optimal range. As the result, ring shape holder shows the best while swirl holder the worst on anti-flashback performance.
Conference Paper
Lean premixed combustion of carbon monoxide (CO), hydrogen (H2 ), and methane (CH4 ) fuel mixtures with air was investigated experimentally. Combustion at atmospheric pressure was stabilized within porous inert medium made of silicon-carbide coated carbon foam with 4 pores per centimeter. CH4 in the fuel was varied from 100% to 0% (by volume), with the remaining fuel containing equal amounts of CO and H2 . Experiments at a fixed air flow rate were conducted by varying the adiabatic flame temperature and fuel composition. Profile of CO and NOx emissions in the axial and transverse directions were taken to identify the post-combustion zone and uniformity of combustion. At a given flame temperature, fuels with CO/H2 produced lower CO and NOx emissions compared to those for CH4 . The temperature at the lean blow off limit was significantly lower (compared to CH4 ) if the fuel contained CO and H2 , each greater than 35% by volume.
Article
Laminar flame speeds and strain sensitivities of mixtures of H 2 and air or air highly diluted with N 2 (O 2 :N 2 1:9) have been measured for a range of equivalence ratios at high preheat conditions 700 K using a nozzle generated, 1D, laminar, wall stagnation flame. The measurements are compared with numerical predictions based on three de-tailed kinetic models (GRIMECH 3.0, a H 2 / CO mechanism from Davis et al. the measurements to uncertainties in boundary conditions, e.g., wall temperature and nozzle velocity profile (plug or potential), is investigated through detailed numerical simulations and shown to be small. The flame speeds and strain sensitivities predicted by the models for preheated reactants are in reasonable agreement with the measurements for mixtures of H 2 and standard air at very lean conditions. For H 2 and N 2 diluted air, however, all three mechanisms significantly overpredict the measurements, and the over-prediction increases for leaner mixtures. In contrast, the models underpredict flame speeds for room temperature mixtures of H 2 with both standard and N 2 diluted air, based on comparisons with measurements in literature. Thus, we find that the temperature dependence of the hydrogen flame speed as predicted by all the models is greater than the actual temperature dependence (for both standard and diluted air). Finally, the models are found to underpredict the measured strain sensitivity of the flame speed for H 2 burning in N 2 diluted air, especially away from stoichiometric conditions.
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Experiments have been performed to determine the blowout of jet diffusion flames with pure fuels, oxygenated fuels, mixed fuels, and diluted fuels. Stability tests were conducted with pure hydrocarbons at the C2 level to determine the effects of structural differences in the fuels. Diffusion flame blowout models were also used to correlate and interpret the data. Ethylene is more stable than ethane because of the additional heat release from the double-carbon bond and ethane is more stable than dimethyl ether. The blowout pressures of mixtures of ethylene and ethane are not linear contributions of component blowout pressures. Stability tests were conducted with ethylene diluted with air and nitrogen. Since both diluents have similar properties and also have a similar density to ethylene, factors in the blowout process, such as the laminar flame speed and air–fuel mass ratio, were isolated and measured. Stability tests with hydrogen diluted with helium, nitrogen, carbon dioxide, and sulfur hexafluoride were also conducted. The diluted hydrogen diffusion flames become less stable as the complexity of the diluent increases.
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Effects of positive flame stretch on the laminar burning velocities of CO/H-2/air mixtures were studied both experimentally and computationally for outwardly propagating spherical laminar premixed flames having concentrations of hydrogen in the fuel mixture of 3-50% by volume, fuel-equivalence ratios of 0.6-5.0, and pressures of 0.5-4.0 atm, Both measured and predicted ratios of unstretched to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding constant Markstein numbers for each reactant mixture and pressure, Effects of stretch on laminar burning velocities were modest at low hydrogen concentrations, but approached earlier results for hydrogen/air flames as hydrogen concentrations increased. Predicted and measured name properties were in reasonably good agreement using several contemporary chemical reaction mechanisms.
Article
Laminar flame speeds and strain sensitivities of mixtures of H2 and air or air highly diluted with N2 (O2:N2 1:9) have been measured for a range of equivalence ratios at high preheat conditions (∼700 K) using a nozzle generated, 1D, laminar, wall stagnation flame. The measurements are compared with numerical predictions based on three detailed kinetic models (GRIMECH 3.0, a H2/CO mechanism from Davis et al. (2004, "An Optimized Kinetic Model of H2/CO Combustion," Proc. Combust. Inst., 30, pp. 1283-1292) and a H2 mechanism from Li et al. (2004, "An Updated Comprehensive Kinetic Model of Hydrogen Combustion," Int. J. Chem. Kinet., 36, pp. 566-575)). Sensitivity of the measurements to uncertainties in boundary conditions, e.g., wall temperature and nozzle velocity profile (plug or potential), is investigated through detailed numerical simulations and shown to be small. The flame speeds and strain sensitivities predicted by the models for preheated reactants are in reasonable agreement with the measurements for mixtures of H2 and standard air at very lean conditions. For H2 and N2 diluted air, however, all three mechanisms significantly overpredict the measurements, and the over-prediction increases for leaner mixtures. In contrast, the models underpredict flame speeds for room temperature mixtures of H2 with both standard and N2 diluted air, based on comparisons with measurements in literature. Thus, we find that the temperature dependence of the hydrogen flame speed as predicted by all the models is greater than the actual temperature dependence (for both standard and diluted air). Finally, the models are found to underpredict the measured strain sensitivity of the flame speed for H2 burning in N2 diluted air, especially away from stoichiometric conditions.
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The flame extinction limits of syngas (H(2)-CO) flames were measured using a twin-flame counterflow burner Plots of extinction limits (%f: volumetric percent of fuel in air) versus global stretch rates were generated at different fuel blend compositions and were extrapolated to determine the flame extinction limit corresponding to an experimentally unattainable zero-stretch condition. The zero-stretch extinction limit of H(2)-CO mixtures decreases with the increase in H(2) concentration in the mixture. The average difference between the measured flame extinction limit and the Le Chatelier's calculation is around 7% of the mean value. The measured OH chemiluminescence data indicates that regardless of blend composition the OH radical concentration reduces to a critical value prior to the flame extinction. The measured laminar flame velocity close to the extinction indicates that regardless of fuel composition, the premixed flame of hydrogen fuel blends extinguishes when the mixture laminar flame velocity falls below a critical value.
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The burning velocities of carbon monoxide-air mixtures containing various amounts of water have been determined using a flat-flame burner with heat extraction. The addition of water is then treated as being equivalent to that of molecular hydrogen and oxygen in order to obtain a binary fuel mixture of hydrogen and carbon monoxide to which existing mixing rules are applied. This treatment shows that the burning velocities of carbon monoxide-air flames containing water can be predicted with less than eight percent of error when the Spalding mixing rule is used. Calculations have shown that the heat release rates, corresponding to a homogeneous reaction, are identical if the same amount of hydrogen or water is added to the mixture, thus confirming the equivalence of these two compounds.
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Decades of research have underlined the undeniable importance of the Lewis number (Le) in the premixed combustion field. From early experimental observations on laminar flame propagation to the most recent DNS studies of turbulent flames, the unbalanced influence of thermal to mass diffusion (i.e. Le ≠ 1), known as nonequidiffusion, has shed the light on a wide range of combustion phenomena, especially those involving stretched flames. As a result the determination of the Lewis number has become a routine task for the combustion community. Recently, the growing interest in hydrogen/hydrocarbon (HC) fuel blends has produced extensive studies that have not only improved our understanding of H2/HC flame dynamics, but also, in its wake, raised a fundamental question: which effective Lewis number formulation should we use to characterize the combustion of hydrogen/hydrocarbon/air blends? While the Lewis number is unambiguously defined for combustible mixtures with a single fuel reactant, the literature is unclear regarding the appropriate equivalent formulation for bi-component fuels. The present paper intends to clarify this aspect. To do so, effective Lewis number formulations for lean (φ = 0.6 and 0.8) premixed hydrogen/hydrocarbon/air mixtures have been investigated in the framework of an existing outwardly propagating flame theory. Laminar burning velocities and burned Markstein lengths of H2/CH4, H2/C3H8, H2/C8H18 and H2/CO fuel blends in air were experimentally and numerically determined for a wide range of fuel compositions (0/100% → 100/0% H2/HC). By confronting the two sets of results, the most appropriate effective Lewis number formulation was identified for conventional H2/HC/air blends. Observed deviations from the validated formulation are discussed for the syngas (H2/CO) flame cases.
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The thesis examines the current testing standards for diesel fuels and establishes the relevance of such testing standards to bio-derived diesel fuels. There is a need for more detailed kinetics information for biodiesel fuels to allow exploration of issues in engine and fuel design. Flame studies can provide overall chemical kinetic information that is currently lacking in the literature for bio-diesel fuels. An experimental apparatus to measure laminar flame speeds was designed and implemented to convey overall chemical reaction rate information. This work addressed three major aspects of such design: combustion chamber, auxiliary systems (gases and fuel supply, ignition, and control) and measurement systems. Test rig characterization was attempted; however, critical ignition and fueling issues were uncovered during experimentation. Suggestions for future work provide solutions and improvement pathways to the current design.
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A flame extinction model is incorporated in a computational procedure containing the “k-ϵ-g” model of turbulence and applied to the prediction of burner stability features of some furnace trials performed at the International Flame Research Foundation. The fuels are low calorific value gases. As a prelude to the application, the lifting of and blow off of turbulent jet diffusion flames are examined as well as the stability behaviour of baffle stabilized premixed flames. The extinction model presumes that reaction will cease when the diffusional rate between the fine turbulence scales exceeds the reaction rate, both of which quantities are simply characterized. Its performance across the range of flows studied is remarkably good. Diffusion flame blow off is well represented by straightforward considerations of jet similarity and fuel stoichi-ometry.
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The problem of ignition and extinction for the flow of a compressible fluid with competitive/chain reactions has been considered in the present paper, A typical system of hydrogen-carbon monoxide fuel mixture forming one end of the opposed jet and air the other end has been considered for the numerical study. An efficient numerical procedure has been developed for the solution of two coupled and non-linear second order differential equations describing the chemical kinetics. The present study reveals that the mass flow rates at extinction (Apparent Flame Strength) for the mixture can be obtained by using a simple mixing rule similar to those used in theories of flame propagation. The results of maximum volumetric heal release rates have been obtained and compared with the experimental results of Bittker and Brokaw.
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The effects of CO addition on the characteristics of premixed CH4/air opposed-jet flames are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position, temperature, and velocity are performed in stoichiometric CH4/CO/air opposed-jet flames with various CO contents in the fuel. Thermocouple is used for the determination of flame temperature, velocity measurement is made using particle image velocimetry (PIV), and the flame front position is measured by direct photograph as well as with laser-induced predissociative fluorescence (LIPF) of OH imaging techniques. The laminar burning velocity is calculated using the PREMIX code of Chemkin collection 3.5. The flame structures of the premixed stoichiometric CH4/CO/air opposed-jet flames are simulated using the OPPDIF package with GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position, temperature, and velocity of the stoichiometric CH4/CO/air flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that as the CO content in the fuel is increased from 0% to 80%, CO oxidation (R99) increases significantly and contributes to a significant level of heat-release rate. It is also shown that the laminar burning velocity reaches a maximum value (57.5 cm/s) at the condition of 80% of CO in the fuel. Based on the results of sensitivity analysis, the chemistry of CO consumption shifts to the dry oxidation kinetics when CO content is further increased over 80%. Comparison between the results of computed laminar burning velocity, flame temperature, CO consumption rate, and sensitivity analysis reveals that the effect of CO addition on the laminar burning velocity of the stoichiometric CH4/CO/air flames is due mostly to the transition of the dominant chemical kinetic steps.
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The effects of additives, inert or otherwise, on the burning velocity of a propane-air mixture have been studied experimentally, by a schlieren technique, with flames stabilized on a tube burner. The Spalding mixing rule predicted the effects with a fair degree of accuracy, except for the effect of hydrogen sulphide. With hydrogen sulphide, the experimental burning velocities were far smaller than the calculated values implying that this gas acts as an inhibitor in the flame reaction.
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New measurements of burning velocity are reported for selected CO/H2 mixtures in air, to provide datafor examining the rate of the CO+OH reaction. Burning velocities were obtained from measurements on constant-pressure expanding spherical flames. Flame speeds over the first 35 mm of travel were measured by high-speed schlieren cine photography and extrapolated to infinite radius, using a simple phenomenological model, to yield one-dimensional (1D) values. Extensive computational testing shows that the burning velocities obtained are accurate to within 3%. Experimental burning velocities were then compared with 1D computed values, obtained using full kinetics.Fuel mixtures of 95% CO+5% H2 and 50% CO+50% H2 with air were studied across the stoichiometricrange, along with stoichiometric mixtures with varying H2/CO ratio. The 95+5% mixture was the prime object of study, since computation revealed that burning velocities for this mixture had the greatest sensitivity to the CO+OH reaction. For the 95+5% mixture, the maximum burning velocity was 0.65 ±0.02 m s−1 at 51.7% fuel; the stoichiometric value was 0.34 m s−1. For the 50+50% mixture, the corresponding velocities were 1.82±0.06 (at 47.2% fuel) and 1.20 m s−1.Computed 1D burning velocities agreed well with experiment when the 1976 recommendation of Baulchet al. for the rate of the CO+OH reaction was used. A more recent recommendation gave values more than 20% too high. Further computation revealed that the reaction exerts its influence on the burning velocity of these flames at low temperatures, in a relatively narrow region around 1165 K. Use of different (artificially derived) rate expressions, but having a common value at 1165 K, produced essentially the same burning velocities. Implications of this for flame chemistry are explored.
Article
Several properties are studied of fuel-rich (CO:N2O = 1.5:1) and stoichiometrie (CO:N2O = 1:1) carbon monoxide/nitrous oxide flames with varying water content up to 10%. Flame temperatures, ranging from 2680 to 2860°K. are measured with the line-reversal method, and compared with calculated adiabatie values. Excess concentrations of H and OH radicals are determined by the LiOH/Li method; the ratios [H]/[H2O] appear to exceed their (calculated) equilibrium values by a factor 1.5 to 3.0. even after 10 msec rise time of the flame gases. Burning velocities of the various mixtures, estimated from the height of the combustion cones, range from 30 to 75 cm/see and are markedly dependent on the water content of the mixture. The absolute spectral intensity of the (quasi) continuous emission of the burnt pases was measured and was found to have the same dependence on CO and O concentrations as in CO/air flames. Electron concentrations of 109 to 1010 cm−3, measured with a HF resonance method, are tentatrvely attributed to (thermal) ionization of NO.
Article
The temperature dependence of burning velocity has been investigated by means of cooled porous metal burners and fine quartz-coated Pt-Pt 10 per cent Rh thermocouples. It has been shown that, for nearly all of the mixtures studied, the logarithm of the mass burning velocity is a linear function of the reciprocal of the measured flame temperature in the range from 1300° to 1900°K. A comparison with adiabatic burning velocities suggests that this correlation may be valid at higher temperatures. It is suggested that the results imply that the high-temperature flame reactions are rate-limiting in the flames studied.