Ratio, combustion equivalence

Combustion Equivalence Ratio. The effect of varying the equivalence ratio of combustion hydrogen to combustion oxygen was tested by operating the reactor with coal feed rate, oxygen-to-coal ratio, and carrier gas rate constant, and varying the combustion hydrogen feed rate. 

The Expression, Calculation and Importance of the Equivalence Ratio in Different Combustion Systems... 

In practice, for motors, turbines or furnaces, the conditions of combustion are frequently far from those corresponding to stoichiometry and are characterized either by an excess or by an insufficiency of fuel with respect to oxygen. The composition of the fuel-air mixture is expressed by the equivalence ratio, (p, defined by the relation / 5 r)... 

In a general manner, diesel engines, jet engines, and domestic or industrial burners operate with lean mixtures and their performance is relatively insensitive to the equivalence ratio. On the other hand, gasoline engines require a fuel-air ratio close to the stoichiometric. Indeed, a too-rich mixture leads to an excessive exhaust pollution from CO emissions and unburned hydrocarbons whereas a too-lean mixture produces unstable combustion (reduced driveability and misfiring). 

The diesel engine operates, inherently by its concept, at variable fuel-air ratio. One easily sees that it is not possible to attain the stoichiometric ratio because the fuel never diffuses in an ideal manner into the air for an average equivalence ratio of 1.00, the combustion chamber will contain zones that are too rich leading to incomplete combustion accompanied by smoke and soot formation. Finally, at full load, the overall equivalence ratio... 

Moreover, a limit to maximum density is set in order to avoid smoke formation at full load, due to an increase in average equivalence ratio in the combustion chamber. 

Where T)is flame temperature in K MC is moisture content of the waste, expressed on a total weight basis SR is defined as stoichiometric ratio or moles O2 avadable/moles O2 required for complete oxidation of the carbon, hydrogen, and sulfur in the fuel, ie, 1/SR = equivalence ratio and is temperature of the combustion air, expressed in K. In Fnglish units, this equation is as follows ... 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

Excess Air for Combustion More than the theoretical amount of air is necessary in practice to achieve complete combustion. This excess air is expressed as a percentage of the theoretical air amount. The equivalence ratio is defined as the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio. Equivalence ratio values less than... 

Cp a = specific heat of air at constant pressure AT jj = temperature rise for stoichiometric combustion D = surface average particle diameter Pa = air density Pf = fuel density

equivalence ratio B = mass transfer number... 

The model assumes that liquid evaporation is always the rate controlling step. At some point the model must fail, since as droplet size approaches zero the predicted MIE approaches zero rather than the MIE of the vapor in air. In practice, droplets having diameters less than 10-40 /rm completely evaporate ahead of the flame and burn as vapor (5-1.3). The model also predicts that the MIE continuously decreases as equivalence ratio is increased, although as discussed above, combustion around droplets is not restrained by the overall stoichiometry and naturally predominates at the stoichiometric concentration. It is recommended that the model be applied only to droplet diameters above about 20/rm and equivalence ratios less than about one. 

Experimentally deduced overall activation energies for the combustion of (a) ethylene/air and (b) n-decane/air as a function of the equivalence ratio, obtained by varying the preheat temperature. 

Vector profile of vortex ring combustion, showing induced velocities along the vortex core. (Lean propane/air mixture, equivalence ratio O = 0.8, Do = 60 mm, P= 0.6 MPa, dotted lines show the flame front taken with the ICCD camera. The right inset shows the relative position of the PIV laser sheet relative to the flame.)... 

Flames submitted to convective disturbances experience geometrical variations, which can in turn give rise to heat release unsteadiness. This process can be examined by considering different types of interactions between incident velocity or equivalence ratio modulations and combustion. The flame dynamics resulting from these interactions give rise to sound radiation and... 

T. Lieuwen and B.T. Zinn. The role of equivalence ratio fluctuations in driving combustion instabilities in low nox, gas turbines. Proc. Combust. Inst., 27 1809-1816, 1998. 

J.H. Cho and T. Lieuwen. Laminar premixed flame response to equivalence ratio oscillations. Combust. Flame, 140 116-129, 2005. 

Laser Doppler anemometry data showing the axial velocity along the centerline of a 380 mm long closed chamber during the formation of acetylene/air tulip flames of different equivalence ratios. The velocity is measured 265 mm from the ignition thus, the tulip shape is already formed before the flame reaches the measurement point. This work shows the behavior similar to the results described in Figure 5.3.9. (Adapted from Starke, R. and Roth, R, Combust. Flame, 66,249,1986.)... 

Variations of the maximum Karlovitz number and laminar burning velocities with the equivalence ratio, showing the accessible domain when the maximum/= 170Hz is operated, (a) CH4/air mixtures (b) CH4 diluted with 20-60% N2 (c) CH4 diluted with 20-60% CO2 and (d) combined plots of these maximum-Ka = 170 Hz) lines from (a-c) for comparison. (From Yang, S.I. and Shy, S.S., Proc. Combust. Inst, 29,1841, 2002. 

Yang, S.l. and Shy, S.S., Global quenching of premixed CH4/air flames Effects of turbulent straining, equivalence ratio, and radiative heat loss, Proc. Combust. Inst., 29,1841,2002. 

Marzouk,Y.M., Ghoniem, A.F., and Najm, H.N., Dynamic response of strained premixed flames to equivalence ratio gradients, Proc. Combust. Inst., 28,1859, 2000. 

Turbulent mass burning rate versus the turbulent root-mean-square velocity by Karpov and Severin [18]. Here, nis the air excess coefficient that is the inverse of the equivalence ratio. (Reprinted from Abdel-Gayed, R., Bradley, D., and Lung, F.K.-K., Combustion regimes and the straining of turbulent premixed flames. Combust. Flame, 76, 213, 1989. With permission. Figure 2, p. 215, copyright Elsevier editions.)... 

Vapor-phase fuel-distribution image converted to an equivalence-ratio field downstream of the maximum liquid-phase fuel penetration. Quantitative planar images are obtained in the optical engine using PLRS. (From Espey, C., Dec, J.E., Litzinger, T.A., and Santavicca, D.A., Combust. Flame, 109,65,1997.)... 

Chapter 6.2, contributed by S.S. Shy, is devoted to the problem of flame quenching by turbulence, which is important from the point of view of combustion fundamentals as well as for practical reasons. Effecfs of turbulence straining, equivalence ratio, and heat loss on global quenching of premixed furbulenf flames are discussed. 

The modeling of steady-state problems in combustion and heat and mass transfer can often be reduced to the solution of a system of ordinary or partial differential equations. In many of these systems the governing equations are highly nonlinear and one must employ numerical methods to obtain approximate solutions. The solutions of these problems can also depend upon one or more physical/chemical parameters. For example, the parameters may include the strain rate or the equivalence ratio in a counterflow premixed laminar flame (1-2). In some cases the combustion scientist is interested in knowing how the system mil behave if one or more of these parameters is varied. This information can be obtained by applying a first-order sensitivity analysis to the physical system (3). In other cases, the researcher may want to know how the system actually behaves as the parameters are adjusted. As an example, in the counterflow premixed laminar flame problem, a solution could be obtained for a specified value of the strain... 

To obtain hygienic combustion, it is essential to adjust the equivalence ratio 0 to an ideal value. This value characterises the ratio of the fuel quantity needed for a stoichiometric combustion to the fuel quantity supplied. In most of the common gas appliances, the air supply slightly exceeds the amount of air needed for complete stoichiometric combustion. The exact value for the surplus of air - often referred to as lambda (X) - depends on the configuration of the burner system in question. 

Generally, the volume fraction of single flue gas compounds for a known combustion system, an adjusted equivalence ratio and a given composition of the fuel gas supplied can be regarded as fixed. In order to adjust the air and gas supply, several different strategies have been developed. The most important strategies to predict correlations between the concentration of flue gas compounds and an ideally adjusted combustion are explained below. 

The air and gas supply of domestic appliances is usually adapted using the fraction of C02 in the flue gas. The changes in the minimum required amount of air and in the total flue gas flow rate cannot be taken into account, but are negligibly small. This also applies to the maximum C02 fraction for the combustion of common natural gas, which differs by less than 1%. Therefore the variations in equivalence ratio 0 remain within tolerable limits. 

Furthermore factors such as stoichiometric value, heat load and design of the burner as well as the combustion chamber have a significant impact on the emission of pollutant gases. Depending on the reaction of a combustion system to a changing equivalence ratio decisions can be made how to minimize the pollutant emissions by adapting the flow rate of air or gas. A combustion control system based on monitoring the CO fraction in the flue gas could thus be considered. 

These examples show that there are in fact correlations between the concentration of certain substances in the flue gas and the equivalence ratio of the combustion. Integrating sensors into gas appliances would be a good way of improving their safety, reliability and efficiency. 

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