Epaminondas Mastorakos

Numerical simulations of autoignition in turbulent mixing flows

Two-dimensional direct numerical simulations have been performed of the autoignition of (i) laminar and turbulent shearless mixing layers between fuel and hotter air, (ii) thin slabs of fuel exposed to air from both sides, and (iii) homogeneous stagnant adiabatic mixtures. It has been found that the time for the first appearance of an ignition site is almost independent of the turbulence time scale, varies little in individual realisations of the same flow, decreases with partial premixing, is shorter in turbulent than in laminar flows, and decreases with decreasing width of the fuel stream. The autoignition time in the turbulent flows in longer than the ignition delay time of stagnant homogeneous mixtures and this implies that the heat losses due to mixture fraction gradients associated with mixture inhomogeneities increase the autoignition time. It has also been found that ignition always occurs at a well-defined mixture fraction fMR, which is accurately predicted by previous laminar flow analyses to depend only on the fuel and oxidant temperatures and the activation energy. As a measure of the heat losses of the heat-producing regions that eventually autoignite, the time evolution of the scalar dissipation rate, conditional on the most reactive mixture fraction, is examined and used to explain successfully all the observed trends of autoignition time with turbulent time scale, flow length scale, and partial premixing. The implications of these findings for modelling and for the interpretation of experimental data are discussed.

The role of particle collisions in pneumatic transport

We analyse the dilute, steady, fully developed flow of relatively massive particles in a turbulent gas in the context of a vertical pipe. The idea is that the exchange of momentum in collisions between the grains and between the grains and the wall plays a significant role in the balance of forces in the particle phase. Consequently, the particle phase is considered to be a dilute system of colliding grains, in which the velocity fluctuations are produced by collisions rather than by the gas turbulence. The balance equations for rapid granular flow are modified to incorporate the drag force from the gas, and boundary conditions, based on collisional exchanges of momentum and energy at the wall, are employed. The turbulence of the gas is treated using a one-equation closure. A numerical solution of the resulting governing equations provides velocity and turbulent energy profiles in agreement with the measurements of Tsuji et al. (1984).

An algorithm for the construction of global reduced mechanisms with CSP data

Abstract An algorithm is presented for the construction of global reduced mechanisms, based on concepts from the Computational Singular Perturbation method. Input to the algorithm are (i) the detailed mechanism, (ii) a representative numerical solution of the problem under investigation, and (iii) the desired number of steps in the reduced mechanism. The algorithm numerically identifies the “steady-state” species and fast reactions and constructs the reduced mechanism. The stoichiometric coefficients are constant and are connected to the non “steady-state” species, while the related rates involve the slow elementary rates only. The proposed method is applied to a laminar premixed CH4/Air flame and a complex detailed chemical kinetics mechanism, consisting of 279 reactions and 49 species and accounting for both thermal and prompt NOx production. A seven-step mechanism is constructed which is shown to reproduce the species profiles and the laminar burning velocity very accurately over a wide range of values for the initial mixture composition and temperature. In addition, it is shown that the seven-step mechanism introduces much lower time scales than the detailed mechanism does. Since the proposed procedure for constructing reduced mechanisms is fully algorithmic and requires minor computations, it is very much suited for the simplification of large detailed mechanisms.

Second-order conditional moment closure for the autoignition of turbulent flows

The conditional moment closure with second-order approximation for the reaction rate and an equation for the conditional fluctuations of the temperature increments before autoignition of a turbulent nonpremixed flow has been developed for one-step chemistry. The explicit incorporation of conditional variances is necessitated due to the temperature fluctuations induced by heat losses from the reaction zone before ignition, as indicated by recent direct numerical simulations (DNS). Predicted ignition times and reaction zone structure are in very good agreement with DNS data and the differences between the first- and second-order closure are discussed.

Extinction of turbulent counterflow flames with reactants diluted by hot products

The effects of simultaneous dilution and preheat of reactants by mixing with hot combustion products are examined in terms of the stability of turbulent counterflow flames. Premixed flames were stabilized in the opposed flow of premixed natural gas/air mixtures within the flammability limits and an opposing jet composed of hot products at temperatures up to 1750 K and oxygen mole fractions down to 0.02. The gain in stability of the premixed flames was small for temperatures from 300 to 1400 K, but temperatures higher than 1550 K always ignited flames of equivalence ratio as lean as 0.2 and these could not be extinguished by straining, in agreement with expectations from laminar counterflow premixed flames. This critical temperature is close to that below which chemical reaction is not self-sustaining. Turbulent diffusion flames were stabilized in the same arrangement with the hot product stream as oxidizer and it was found that for every 0.02 of oxygen mole fraction lost to dilution, the temperature had to increase by 100 K for the same extinction strain rate and that there was no extinction at air temperatures higher than about 1700 K. Laminar counterflow flame predictions of extinction are shown to be in agreement with the measurements and also show that stability is improved in the special case of adiabatic mixing of the air with hot combustion products, so that the temperature rise and the oxygen content are related, and this explains why flames stabilized by recirculation zones, where hot products are recirculated to mix with the incoming reactants, can be stable with their high stretch rates.

CFD predictions for cement kilns including flame modelling, heat transfer and clinker chemistry

Abstract Clinker formation in coal-fired rotary cement kilns under realistic operation conditions has been modelled with a commercial axisymmetric CFD code for the gaseous phase including a Monte Carlo method for radiation, a finite-volume code for the energy equation in the kiln walls, and a novel code for the species and energy conservation equations, including chemical reactions, for the clinker. An iterative procedure between the predictions for the temperature field of the gaseous phase, the radiative heat flux to the walls, and the kiln and clinker temperature is used to predict the distribution of the inner wall temperature explicitly, including the calculation of heat flow to the clinker. It was found that the dominant mode of heat transfer between the gas and the kiln walls is by radiation and that the heat lost through the refractories to the environment is about 10% of the heat input and a further 40% is used for charge heating and clinker formation. The predictions are consistent with trends based on experience and limited measurements in a full-scale cement kiln.

Extinction and temperature characteristics of turbulent counterflow diffusion flames with partial premixing

Abstract A turbulent counterflow diffusion flame of natural gas stabilized between two opposed jets discharging from straight tubes of 25.4 mm diameter fitted with turbulence-promoting perforated plates has been examined in terms of its appearance, extinction limits, and mean and fluctuating temperatures as measured by numerically compensated fine-wire thermocouples. It was observed that the flame was flat, blue, and located around the symmetry plane, that its appearance did not change with initial premixing of the fuel stream with air, and that a premixed flame could also be stabilized provided the equivalence ratio was smaller than that of the rich flammability limit. The bulk velocity for extinction of the nonpremixed flame increased with tube separation and with initial premixing, but decreased with an increase in turbulent intensity. The extinction data collapsed to a single curve to within 20% if the total strain rate acting on the flame (bulk plus small-scale turbulent) was plotted as a function of the air volume fraction in the fuel stream, implying a critical total strain rate for extinction that depended only on the degree of partial premixing. Partial premixing increased the resistance of the flame to straining, from about 350 s −1 for pure fuel to about 600 s −1 for an air volume fraction of 0.8, consistent with experiments and predictions for laminar counterflow flames and with experimental data from piloted turbulent jet flames. The present results for the total strain rate at extinction provide a quantitative description of the effect of partial premixing on flame stability and can be used to predict the extinction of nonpremixed flames in other geometries. The maximum mean temperature of the flame did not change as extinction was approached and was about 1300 ± 50 K for all flow conditions measured, while the rms fluctuations were about 450 K; this insensitivity is attributed to a low-frequency (as indicated by high-pass filtering the temperature time series) flame motion, which also resulted in broad temperature probability density functions.

Modeling evaporation effects in conditional moment closure for spray autoignition

Simulations of an n-heptane spray autoigniting under conditions relevant to a diesel engine are performed using two-dimensional, first-order conditional moment closure (CMC) with full treatment of spray terms in the mixture fraction variance and CMC equations. The conditional evaporation term in the CMC equations is closed assuming interphase exchange to occur at the droplet saturation mixture fraction values only. Modeling of the unclosed terms in the mixture fraction variance equation is done accordingly. Comparison with experimental data for a range of ambient oxygen concentrations shows that the ignition delay is overpredicted. The trend of increasing ignition delay with decreasing oxygen concentration, however, is correctly captured. Good agreement is found between the computed and measured flame lift-off height for all conditions investigated. Analysis of source terms in the CMC temperature equation reveals that a convective–reactive balance sets in at the flame base, with spatial diffusion terms being important, but not as important as in lifted jet flames in cold air. Inclusion of droplet terms in the governing equations is found to affect the mixture fraction variance field in the region where evaporation is the strongest, and to slightly increase the ignition delay time due to the cooling associated with the evaporation. Both flame propagation and stabilization mechanisms, however, remain unaffected.

Global reduced mechanisms for methane and hydrogen combustion with nitric oxide formation constructed with CSP data

Reduced mechanisms for methane-air and hydrogen-air combustion including NO formation have been constructed with the computational singular perturbation (CSP) method using the fully automated algorithm described by Massias et al. The analysis was performed on solutions of unstrained adiabatic premixed flames with detailed chemical kinetics described by GRI 2.11 for methane and a 71-reaction mechanism for hydrogen including NO x formation. A 10-step reduced mechanism for methane has been constructed which reproduces accurately laminar burning velocities, flame temperatures and mass fraction distributions of major species for the whole flammability range. Many steady-state species are also predicted satisfactorily. This mechanism is an improvement over the seven-step set of Massias et al, especially for rich flames, because the use of HCNO, HCN and C2H2 as major species results in a better calculation of prompt NO. The present 10-step mechanism may thus also be applicable to diffusion flames. A five-step me...

Diesel engine simulations with multi-dimensional conditional moment closure

First-order elliptic Conditional Moment Closure (CMC), coupled with a computational fluid dynamics (CFD) solver, has been employed to simulate combustion in a direct-injection heavy-duty diesel engine. The three-dimensional structured finite difference CMC grid has been interfaced to an unstructured finite volume CFD mesh typical of engine modelling. The implementation of a moving CMC grid to reflect the changes in the domain due to the compression and expansion phases has been achieved using an algorithm for the cell addition/removal and modelling the additional convection term due to the CMC cell movement. Special care has been taken for the boundary conditions and the wall heat transfer. An operator splitting formulation has been used to integrate the CMC equations efficiently. A CMC domain reduction of the three-dimensional problem to two- and zero-dimensions through appropriate volume integration of the CMC equation has been explored in terms of accuracy and computational time. Additional considerations have been reported concerning the initialization of the CMC domain in conserved scalar space during transient calculations where the probability density function of the mixture fraction changes drastically with time as during fuel injection. Predictions compare favourably with the experimental pressure traces for tests at full and half load. A balance of terms in the CMC equation allows conjectures on the structure of the flame and its expansion across the spray after autoignition.