Flame simulations

In addition to the catalytic-ignition problem, this approach has been successfully implemented on opposed-flow strained-flame simulations with the inlet flow oscillating at high frequency [193]. It has also been used to model transient chemical-vapor deposition processes where the inlet flow is varies under a real-time control algorithm [324]. Although it is unlikely that a practical process-control system would be designed to induce extremely fast transients, it is important that the simulation remain stable to any potential controller command. 

Shimmering of hot gases in front of hot refractory may simulate flame Hot refractory background may cause flame simulation Electric ignition spark may simulate flame / ... 

FIGURE 20.1 (See color insert following page 530.) A comparison of temperature fields in pool fire flame simulations using RANS and LES. 

For comparison with the flame simulations, the relative CH and CN concentration profiles along the centerline of the burner were recorded followed by a calibration of the relative concentration profiles using a N2 Rayleigh calibration method. Linear LIF was used to determine the CH and CN signal intensities as a function of height above the burner. In this case the relationship between the detected LIF intensity /lip and, e.g. the CH number density Nqh is given by... 

Comparison of Eqs. (13) and (11) reveals that if one ran the CFD calculation to a computational to a convergence tolapprox = toltotai — , one could be certain that the resulting Fapprox would be a satisfactory solution of the full chemistry model, i.e. Eq. (11) would be satisfied. Methods for constructing reduced models guaranteed to satisfy Eq. (13) over a known range of conditions are discussed in detail in Section III.C below. The practical effectiveness of this overall error control approach, once one has an approximate source term fflapprox that satisfies Eq. (13) has recently been demonstrated for 1-d and 2-d steady laminar flame simulations. 

An example The temperature field computed for a partially-premixed radi-ally-symmetric methane/air flame is shown in Fig. 15. This is the same 4> — 2.464 laminar flame simulated by Bennett et al. (2000). We used the same 217 reaction full chemistry model used by Bennett et al. (2000) to compute the temperature field shown on the left-hand side of Fig. 15. On the left-hand side is shown the temperature field computed using the full chemistry model everywhere. On the right-hand side is shown the temperature field computed by the Adaptive Chemistry method using 13 different reduced models ranging in size from zero reactions to 156 reactions. As guaranteed by the error control... 

Systems are available that provide the possibility of flame simulation in a limited fraction of the scanner s lifetime. To protect a unit against such cazards the following measures can be taken ... 

Figure (12) shows the resulting flame simulation based on the LES-CMC LOI-RCCE approach as described above. Employing 4 spatial sub-domains, each with their optimally reduced model for the given conditions (fuel rich, reaction zone, p>ost-reaction zone, and fuel lean)... 

Flames simulated with complex chemistry remain flamelet-like for most investigated cases. No quenching is observed along the flame front. This result confirms predictions obtained with simple chemistry models In the absence of heat losses, localised quenching is difficult to achieve for these configurations. 

Flames simulated with complex chemistry align preferentially along extensive strain rates like simple chemistry flames. The curvature remains symmetric with near-zero mean value. 

Turanyi, T., Zalotai, L., Dobe, S., Beices, T. Effect of the uncertainty of kinetic and thermodynamic data on methane flame simulation results Phys. Chem. Chem. Phys 4,2568-2578 (2002) Turanyi, T., Nagy, T., Zsely, I.G., Cseihati, M., Varga, T., Szabo, B.T., Sedyo, I., Kiss, P.T., Zempleni, A., Curran, H.J. Determination of rate parameters based on both direct and indirect measurements, hit. J. Chem. Kinet. 44, 284—302 (2012)... 

Sikalo et al. (2014) compared several options for the application of genetic algorithms to mechanism reduction, exploring the trade-off between the size and accuracy of the resulting mechanisms. Information on the speed of solution was also taken into account, so that, for example, the least stiff system (Sect. 6.7) could be selected. An automatic method for the reduction of chemical kinetic mechanisms was suggested and tested for the performance of reduced mechanisms used within homogeneous constant pressure reactor and burner-stabilised flame simulations. The flexibility of this type of approach has clear utility when restrictions are placed on the number of variables that can be tolerated within a scheme in the computational sense. However, the development of skeletal mechanisms is rarely the end point of any reductiOTi procedure since the application of lumping or timescale-based methods can be applied subsequently. These methods will be discussed in later sections. 

The above approaches to tabulation, whilst mostly applied in the simulation of combustion problems, have a general foundation that would be relevant to many kinetic systems. However, a special class of tabulation methods has been developed for flame simulations. If a fast exothermic reaction takes place between two components (e.g. a fuel and an oxidiser) of a gaseous system, then flames are observed. In premixed flames the fuel and the oxidiser are premixed before combustion takes place, whilst in non-premixed (diffusion) flames, the fuel and the oxidiser diffuse into each other, and the flame occurs at the boundary or flame front. Premixed and non-premixed flames are two extreme cases, but in many practical flames, continuous states between these two extremes will exist. Flames can be classified as laminar or turbulent according to the characteristics of the flow. Flames are special types of reaction—diffusion systems, characterised by high spatial gradients in temperature and species concentrations, and consequently reaction rates will have a high spatial variability. 

Gicquel, O., Darabiha, N., Thevenin, D. Laminar piemixed hydrogen/air counterflow flame simulations using flame prolongation of ILDM with differential diffusion. Proc. Combust. Inst. 28, 1901-1908 (2000)... 

Yang, H., Ren, Z., Lu, T., Goldin, G.M. Dynamic adaptive chtaiustry fin turbulent flame simulations. Combust. Theory Model. 17, 167-183 (2013)... 

The possibility of determining a flame velocity corrected to the stretch in lean hydrogenous mixtures at higher than atmospheric pressure is still problematic. In particular, traditional methods of measurement result in a discrepancy between predicted and measured pressure characteristics of flame velocities at initial elevated pressure. For example, an expanding flame simulation in lean H2 + air mixtures, when real kinetics and multi-component transport processes are taken into account, results in baric index values different from those obtained experimentally. 

Flame simulations show this reaction to be a very important chain-branching step. From measurements of lean low-pressure C2H2/O2 flames (Eberius et a/., 1973) it has been concluded (Warnatz, 1981) that this reaction leads to the products CO2 + H -h H for two reasons (1) Only the formation of two H atoms can account for the fast propagation of this flame. Formation, e.g., of CO -h OH -h H, would lead to very slow propagation incompatible with the measurements (2) CO2 formation in this flame can be explained only with direct formation in the reaction of CH2 with O2. Formation of CO followed by CO -h OH CO2 -h H is too slow to reproduce the measured CO2 flame profiles. 

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