Terms Of Mixture Fraction Research Articles

Soot Emission Simulations of a Single Sector Model Combustor Using Incompletely Stirred Reactor Network Modeling

AbstractThe simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so that a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of incompletely stirred reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chosen chemical mechanism and provide an estimate of the soot PSD at the combustor exit.

Nonadiabatic Flamelet Formulation for Predicting Wall Heat Transfer in Rocket Engines

A flamelet-based combustion model is proposed for the prediction of wall heat transfer in rocket engines and confined combustion systems. To account for convective heat loss due to the interaction of the flame with the wall, a permeable thermal boundary condition is introduced in the counterflow diffusion flame configuration. The solution of the resulting nonadiabatic flame structure forms a three-dimensional manifold, which is parameterized in terms of mixture fraction, progress variable, and temperature. The performance of the model is first evaluated through a direct numerical simulation analysis of an diffusion flame that is stabilized at an inert isothermal wall. The developed nonadiabatic flamelet model is shown to accurately predict the temperature, chemical composition, and wall heat transfer. Combined with a presumed probability density function closure, the model is then applied to large-eddy simulation of a single-injector rocket combustor to examine effects of heat transfer on the turbulent flame structure in rocket engines.

Characteristics of flamelets in spray flames formed in a laminar counterflow

A two-dimensional numerical simulation of a spray flame formed in a laminar counterflow is presented, and the flamelet characteristics are studied in detail. The effects of strain rate, equivalence ratio, and droplet size are examined in terms of mixture fraction and scalar dissipation rate. n-Decane (C 10H 22) is used as a liquid spray fuel, and the droplet motion is calculated by the Lagrangian method without the parcel model. A one-step global reaction is employed for the combustion reaction model. The results show that there appear large differences in the trends of gaseous temperature and mass fractions of chemical species in the mixture fraction space between the spray flame and the gaseous diffusion flame. The gas temperature in the spray flame is much higher than that in the gaseous diffusion flame. This is due to the much lower scalar dissipation rate and the coexistence of premixed and diffusion-limited combustion in the spray flame. For the spray flames, gas temperature and mass fractions of chemical species are not unique functions of the mixture fraction scalar dissipation rate. This is because the production rate of the mixture fraction, namely evaporation rate of the droplets, in the upstream region is not in proportion to its transport-diffusion rate in the downstream region. The behavior shows marked differences as the strain rate decreases, the equivalence ratio increases, or the droplet size decreases.

Extended Shvab–Zel’dovich formulation for multicomponent-fuel diffusion flames

In this paper an extension of the Shvab–Zel’dovich formulation is presented. This extended formulation, based on the Burke–Schumann kinetic mechanism, describes the combustion of multicomponent fuels in a diffusion flame in terms of mixture fraction and the excess enthalpy. Under the condition of Burke–Schumann kinetic mechanism, the multicomponent fuel is burned in a single flame. The model is applied to a diffusion flame generated by the burning of mixtures of n-heptane and hydrogen diluted in nitrogen in a counterflow configuration. Due to the very small ratio of the hydrogen molecular weight to the n-heptane molecular weight, small quantities of hydrogen (in terms of mass) in the mixture does not change significantly the properties related to the mass, like as the total heat released per unit of mass at the flame. However, properties related to the hydrogen mole fraction does change expressively with small quantities, like as the radiative energy loss from the hot region around the flame. The results show the flame properties as a function of the reciprocal scalar dissipation and hydrogen quantity in the mixture. It is observed that, by reducing the reciprocal scalar dissipation, the radiative energy loss decreases and by increasing the presence of the hydrogen, the sensitivity of the flame properties with the reciprocal scalar dissipation reduces. It is also revealed by the results, the effects of the potentiated preferential hydrogen mass diffusion in compositions in which nitrogen and n-heptane are the majority species, and the potentiated preferential n-heptane thermal diffusion in compositions in which nitrogen and hydrogen are the majority species, on the flame properties. Although, this work do not treat the extinction problem, the fluid dynamical results will be properly handled to provide information about the reciprocal scalar dissipation and the Liñán’s parameter necessary for future flame stability analyses.

Characteristics of Flamelet in Spray Flames (2nd Report, Effects of Spray Droplet Size, Equivalence Ratio and Fuel)

Characteristics of flamelet in spray flames in a laminar counterflow are investigated by a two-dimensional DNS in terms of mixture fraction and scalar dissipation rate. The motion of spray droplets is calculated by the Lagrangian method without parcel model. Decane and heptane are targeted as a fuel. The results show that for both fuels, temperature and mass fractions of chemical species cannot be clearly identified by mixture fraction and scalar dissipation rate by themselves. This behavior is caused by the fuels' evaporation in the spray flame, and becomes marked in the smaller droplet size condition due to its higher evaporation rate and in the higher equilvalence ratio condition due to its larger evaporation amount.

Fundamentals of Enclosure Fire "Zone" Models

The conservation laws are presented in control volume form and applied to the behavior of fire in enclosures. The behavior of enclosure fires is discussed and the assumptions for justifying the use of the control volume of "zone" modeling approach are presented. The governing equations are derived and special solutions are given. Flow through wall vents, room filling, and growing fires are analyzed. This paper is not a review of the litera ture. It is a theoretical presentation of compartment fire zone modeling, much of which has not been explicitly presented before or integrated into one presentation. The analysis for species in terms of mixture fraction is a distinct new contribution to zone modeling.