Michael Oevermann

RANS predictions of turbulent diffusion flames: comparison of a reactor and a flamelet combustion model to the well stirred approach

The flame stabilisation process in turbulent non-premixed flames is not fully understood and several models have been developed to describe the turbulence–chemistry interaction. This work compares the performance of the multiple Representative Interactive Flamelet (mRIF) model, the Volume Reactor Fraction Model (VRFM), and the Well-Stirred reactor (WS) model in describing such flames. The predicted ignition delay and flame lift-off length of n-heptane sprays are compared to experimental results published within the Engine Combustion Network (ECN). All of the models predict the trend of ignition delay reasonably well. At a low gas pressure (42 bar) the ignition delay is overpredicted compared to the experimental data, but the difference between the models is not significant. However, the predicted lift-off lengths differ. At high pressure (87 bar) the difference between the models is small. All models slightly underpredict the lift-off length compared to the experimental data. At low gas pressure (42 bar) the mRIF model gives the best results. The VRFM and WS models predict excessively short lift-off lengths, but the VRFM model gives better results than the WS model. The flame structures of the models are also compared. The WS model and the VRFM model yield a well defined flame stabilisation point whereas the mRIF model does not. The flame of the mRIF model is more diffuse and the model is not able to predict flame propagation. All models were able to predict the experimental trends in lift-off and ignition delay, but certain differences between them are demonstrated.

Numerical Studies on the Impact of Equivalence Ratio Oscillations on Lean Premixed Flame Characteristics and Emissions

Laminar and turbulent one-dimensional lean premixed methane-air flames subject to equivalence ratio oscillations are investigated numerically. For the turbulent simulations, the Linear Eddy Model is employed. Harmonic perturbations at various frequencies and with various amplitudes are considered and their influence on CO and NO emissions, heat release rate fluctuations, and burning velocity is evaluated. The results indicate a strongly nonlinear behavior of the flame response for high forcing amplitudes attributed to nonlinear effects due to the interaction of burning velocity and equivalence ratio oscillations leading to nonsinusoidal oscillations at the flame front. Furthermore, the turbulent cases reveal decreasing mean burning velocities and heat release rates with increasing amplitudes due to damping of turbulent fluctuations induced by the oscillations.

A representative linear eddy model (RILEM) for non-premixed combustion

To further improve the efficiency and emissions profiles of internal combustion engines, many new combustion concepts are currently being investigated. Examples include homogeneous charge compression ignition (HCCI), stratified charge compression ignition (SCCI), lean stratified premixed combustion, and the use of high levels of exhaust gas recirculation (EGR) in diesel engines. The typical combustion temperatures in all of these concepts are lower than those in traditional spark ignition or diesel engines. Most of the combustion models that are currently used in computational fluid dynamics (CFD) simulations were developed to describe either premixed or non-premixed combustion under the assumption of fast chemistry. The refinement of existing combustion concepts for highly efficient clean engines and the development of new ones would be greatly facilitated by the introduction of new computational tools and combustion models that are mode- and regime-independent, i.e. capable of modeling both premixed and non-premixed and also fast and non-fast chemistry. Such tools should enable more accurate simulation of combustion under non-standard conditions such as those established during low temperature combustion. This paper presents a new regime-independent combustion modeling strategy for non-premixed combustion in which the linear eddy model (LEM) is used as a representative interactive regime-independent turbulent combustion model and coupled to a 3D CFD solver. Parameters and boundary conditions that determine the evolution of the LEM are supplied by the 3D CFD calculation and updated at each time step. The LEM is then solved for the corresponding time step, providing the 3D CFD code with an updated composition state. This new representative interactive linear eddy model (RILEM) is used to simulate an n-heptane spray, demonstrating some potential to describe spray combustion processes.

Effect of the turbulence modeling in large-eddy simulations of nonpremixed flames undergoing extinction and reignition

Simulating practical combustion systems requires the approximation of the interaction between turbulence, molecular transport and chemical reactions. Turbulent combustion models are used for this purpose, but their behavior is difficult to anticipate based on their mathematical formulations, making the use of numerical experimentation necessary. Therefore, the present work explores the effect of three turbulent-combustion models, two eddy-viscosity models, and their parameters on a combustion problem which is notoriously difficult to model: flame extinction and reignition. For this purpose, two types of temporal jets are considered, and direct-numerical-simulation results are compared qualitatively with those from large-eddy simulations.

An asymptotic solution approach for elliptic equations with discontinuous coefficients

When the coefficients of an elliptic problem have jumps of several orders of magnitude across an embedded interface, many iterative solvers exhibit deteriorated convergence properties or a loss of efficiency and it is difficult to achieve high solution accuracies in the whole domain. In this paper we present an asymptotic solution approach for the elliptic problem @?@?(@b(x)@?u(x))=f(x) on a domain @W=@W^+@?@W^- with piecewise constant coefficients @b^+, @b^- with @b^+@?@b^- and prescribed jump conditions at an embedded interface @C separating the domains @W^+ and @W^-. We are in particular focusing on a problem related to fluid mechanics, namely incompressible two-phase flow with a large density ratio across the phase boundary, where an accurate solution of the velocity depends on the accurate solution of a pressure Poisson equation with equal local relative errors in the whole domain. Instead of solving the equation in a single solution step we decompose the problem into two consecutive problems based on an asymptotic analysis of the physical problem where each problem is asymptotically independent of the ratio of coefficients @e=@b^-/@b^+. The proposed methods lead to a robust and accurate solution of the elliptic problem using standard black-box iterative solvers.

Large Eddy Simulation of Stratified Combustion in Spray-guided Direct Injection Spark-ignition Engine

Stratified combustion in gasoline engines constitutes a promising means of achieving higher thermal efficiency for low to medium engine loads than that achieved with combustion under standard homogeneous conditions. However, creating a charge that leads to a stable efficient low-emission stratified combustion process remains challenging. Combustion through a stratified charge depends strongly on the dynamics of the turbulent fuel-air mixing process and the flame propagation. Predictive simulation tools are required to elucidate this complex mixing and combustion process under stratified conditions. For the simulation of mixing processes, combustion models based on large-eddy turbulence modeling have typically outperformed the standard Reynolds averaged Navier-Stokes methods. Therefore, we investigated spray-guided stratified combustion in a single cylinder engine using large-eddy turbulence modeling with a variant of the flame speed closure (FSC) model for premixed turbulent combustion. This model reveals the influence of the mixture composition on the flame speed. The effect of fluctuations in the composition were accounted for by using a presumed probability density function (PDF) approach for the mixture fraction. The fuel injection process was modeled with a standard Lagrangian spray model. More importantly, the measured in-cylinder pressure traces for three different loading cases with varying injection and ignition timings (leading to different levels of stratification) were accurately reproduced by the simulation. High-speed video images were used to evaluate the ability of the model to accurately simulate flame propagation under stratified conditions. The influence of mixture fluctuations on flame propagation was also investigated.

LES Investigation of ECN Spray G2 with an Eulerian Stochastic Field Cavitation Model

Due to an ongoing trend of high injection pressures in the realm of internal combustion engines, the role of cavitation that typically happens inside the injector nozzle has become increasingly important. In this work, a large Eddy Simulation (LES) with cavitation modeled on the basis of an Eulerian Stochastic Field (ESF) method and a homogeneous mixture model is performed to investigate the role of cavitation on the Engine Combustion Network (ECN) spray G2. The Eulerian stochastic field cavitation model is coupled to a pressure based solver for the flow, which lowers the computational cost, thereby making the methodology highly applicable to realistic injector geometries. Moreover, the nature of the Eulerian stochastic field method makes it more convenient to achieve a high scalability when applied to parallel cases, which gives the method the edge over cavitation models that are based on Lagrangian tracking. The result of the Eulerian stochastic field simulation is compared against that from a typical single volume fraction solver for validation. Vortex structures and its correspondence to cavitation are shown, and the behavior of the size Probability Density Function (PDF) at different probe locations at different times are acquired to demonstrate the capability of the Eulerian stochastic field model to capture cavitation and potentially providing more statistical information in each cell as compared to the typical single volume fraction solver. Major cavitation zones that are observed in the result indicates the important role of cavitation in ECN spray G2, therefore inferring a need to take cavitation into consideration in future spray studies.

Building Blocks for a Strictly Conservative Generalized Finite Volume Projection Method for Zero Mach Number Two-Phase Flows

Building blocks for a generalized fully conservative finite volume projection method for numerical simulation of immiscible zero Mach number two-phase flows on Cartesian grids are presented, focusing on the crucial issues of interface propagation, fluid phase conservation and discretization of the singular contribution due to surface tension, each in a discretely conservative fashion. Additionally, a solution approach for solving Poisson-type equations for two-phase flows at arbitrary ratio of coefficients is sketched. Further, (intermediate) results applying these building blocks are presented and open issues and future developments are proposed.

Numerical studies of turbulent particle-laden jets using spatial approach of one-dimensional turbulence

To challenge one of the major problems for multiphase flow simulations, namely computational costs, a dimension reduced model is used with the goal to predict these types of flow more efficiently. One-dimensional turbulence (ODT) is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps that represent the effect of individual turbulent eddies on property fields. As the particle volume fraction is in an intermediate range above 10(-5) for dilute flows and under 10(-2) for dense ones, turbulence modulation is important and can be sufficiently resolved with a two-way coupling approach, which means the particle phase influences the fluid phase and vice versa. For the coupling mechanism the ODT multiphase model is extended to consider momentum transfer and energy in the deterministic evolution and momentum transfer during the particle-eddy interaction. The changes of the streamwise velocity profiles caused by different solid particle loadings are compared with experimental data as a function of radial position. Additionally, streamwise developments of axial RMS and mean gas velocities along the centerline are evaluated as functions of axial position. To achieve comparable results, the spatial approach of ODT in cylindrical coordinates is used here. The investigated jet configuration features a nozzle diameter of 14.22 cm and a Reynolds number of 8400, which leads to a centerline inlet velocity of 11.7 m/s. The particles used are glass beads with a density of 2500 kg/m(3). Two different particle diameters (25 and 70 mu m) were tested for an evaluation of the models capability to capture the impact of a varying Stokes number and also two different particle solid loadings (0.5 and 1.0) were evaluated. It is shown that the model is capable of capturing turbulence modulation of particles in a round jet.

Investigation of Equivalence Ratio Fluctuations on the Dynamics of Turbulent Lean Premixed Methane/Air Flames with a Linear-Eddy Model

Heat release fluctuations generated by equivalence ratio fluctuations may interact with the acoustics of a gas turbine combustion chamber leading to unwanted combustion instabilities, which remains a critical issue in the development of low emission, lean premixed gas turbine combustors. The present article addresses this topic by numerical investigations of one-dimensional lean premixed methane/air flames subject to prescribed sinusoidal equivalence ratio fluctuations. Compared to previous investigations, we focus on turbulent conditions and emission predictions using the one-dimensional linear-eddy model (LEM) and detailed chemistry. Within the limitations of the one-dimensional LEM the approach allows to investigate the fully non-linear regime of flame response to equivalence ratio fluctuations under turbulent conditions. Results for different forcing amplitudes and turbulence levels indicate a strongly non-linear behavior for high forcing amplitudes.