Fuel oxidation calculation

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

In order to calculate the thermal NO formation rate from the preceding expression, it is necessary to know the concentrations of 02, N2, O, and OH. But the characteristic time for the forward reaction (8.49) always exceeds the characteristic times for the reaction systems that make up the processes in fuel-oxidizer flame systems thus, it would appear possible to decouple the thermal NO process from the flame process. Using such an assumption, the NO formation can be calculated from Eq. (8.52) using local equilibrium values of temperature and concentrations of 02, N2, O, and OH. 

The mean rate of energy release S and the rate of consumption/production of species j, Vj, are calculated on the basis of the detailed or reduced reaction mechanism of fuel oxidation in air and a single-point bimodal normalized PDF of temperature P T, T) in the turbulent flame brush ... 

Methane fuel has alternative and more complex equilibrium electrochemical oxidation routes and two of these are examined because of their resemblance to practical routes. The first is via a reformer and the second is via direct oxidation, now achievable in the laboratory. Both analyses involve approximate equilibrium constants, but the second direct oxidation calculation route is seen to be more in error, and the numerical answer less accurate, than that of the reformer route. 

Methane and methanol provide more simple examples of fuel oxidation. Methane, CH4, is natural gas. Methanol, CHiOH, is a liquid sometimes used as a cooking fuel- The steps in the calculation of the RQ f(jr methane combustion follow ... 

For gaseous flames, the LES/FMDF can be implemented via two combustion models (1) a finite-rate, reduced-chemistry model for nonequilibrium flames and (2) a near-equilibrium model employing detailed kinetics. In (1), a system of nonlinear ordinary differential equations (ODEs) is solved together with the FMDF equation for all the scalars (mass fractions and enthalpy). Finite-rate chemistry effects are explicitly and exactly" included in this procedure since the chemistry is closed in the formulation. In (2). the LES/FMDF is employed in conjunction with the equilibrium fuel-oxidation model. This model is enacted via fiamelet simulations, which consider a laminar counterflow (opposed jet) flame configuration. At low strain rates, the flame is usually close to equilibrium. Thus, the thermochemical variables are determined completely by the mixture fraction variable. A fiamelet library is coupled with the LES/FMDF solver in which transport of the mixture fraction is considered. It is useful to emphasize here that the PDF of the mixture fraction is not assumed a priori (as done in almost all other flamelet-based models), but is calculated explicitly via the FMDF. The LES/FMDF/flamelet solver is computationally less expensive than that described in (1) thus, it can be used for more complex flow configurations. 

Primary air is usually defined by a percentage of the stoichiometric air calculated for the total amount of fuel. In the case of PF, it provides a transport air stream that can or can not be considered in the total amount of primary air. Since the very hot secondary air has to be entrained into the fuel-primary air jet, it can have an important impact on the fuel-oxidant macro-mixing. The flow patterns of the secondary stream are mainly determined by the design of cooler uptake and the kiln hood itself. The relationship between primary air jet momentum and secondary air velocity has a significant impact on the flame geometry as well as on the heat transfer to the material and refractory lining. 

The temperatures of several common analytically useful flames are given in Table 1. These are the so-called theoretical temperatures, calculated for stoichiometric fuel-oxidant gas mixtures by Snelleman. They are roughly one hundred degrees higher than most measured temperatures. Moreover, the stoichiometric mixtures do not give the highest attainable temperatures these are reached at somewhat higher fuel-to-oxidant ratios, especially for the air-acetylene flame, due at least in part to air entrainment. Fuel richness also alters rates and extents of chemical reactions in flames. In any case, the tabulated values show the relative temperatures of useful flames. 

Critical experiments were used to benchmark both the uranium oxide and mixed-oxide calculations. Several sets of criticals were looked at before choosing e qieri-ments that most closely matched the assembly s pin pitch and fuel types. The uranium oxide fuel system was benchmarked using uiqniblished data, received by personal communication, from five critical experiments done at the Oak Ridge National Laboratory and one B W critical e3q >eriment The variables iiwestigated in modeling the uranium oxide e]q>eriments included ... 

The flame temperatures are influenced by a large number of factors. The flame temperature can be increased with the addition of excess oxidant, such as nitrate ammonium, or the increase of the fuel-oxidant. The adiabatic flame temperature can be calculated using the heat capacity of the products, the ignition temperature and the heat of combustion, assuming that no heat is lost in the system. The flame temperature measurements are almost always much smaller than the calculated adiabatic values. Irradiated losses, incomplete combustion and air heating contribute to a deaease in the actual flame temperature. 

Recent research on SCS has investigated the role of fuel in the control of particle size and microstructure of the products under different fuel-oxidant ratios (Table 2.3). The fuel-oxidant ratio, however, is not always calculated using a thermodynamic modeling and/or theory of the propellants. 

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