Chemical Source Term Research Articles

Application of the conditional source-term estimation model for turbulence–chemistry interactions in a premixed flame

Conditional Source-term Estimation (CSE) is a closure model for turbulence–chemistry interactions. This model uses the first-order CMC hypothesis to close the chemical reaction source terms. The conditional scalar field is estimated by solving an integral equation using inverse methods. It was originally developed and has been used extensively in non-premixed combustion. This work is the first application of this combustion model for a premixed flame. CSE is coupled with a Trajectory Generated Low-Dimensional Manifold (TGLDM) model for chemistry. The CSE-TGLDM combustion model is used in a RANS code to simulate a turbulent premixed Bunsen burner. Along with this combustion model, a similar model which relies on the flamelet assumption is also used for comparison. The results of these two approaches in the prediction of the velocity field, temperature and species mass fractions are compared together. Although the flamelet model is less computationally expensive, the CSE combustion model is more general and does not have the limiting assumption underlying the flamelet model.

New Resolution Strategy for Multiscale Reaction Waves using Time Operator Splitting, Space Adaptive Multiresolution, and Dedicated High Order Implicit/Explicit Time Integrators

We tackle the numerical simulation of reaction-diffusion equations modeling multi-scale reaction waves. This type of problem induces peculiar difficulties and potentially large stiffness which stem from the broad spectrum of temporal scales in the nonlinear chemical source term as well as from the presence of steep spatial gradients in the reaction fronts, spatially very localized. In this paper, we introduce a new resolution strategy based on time operator splitting and space adaptive multiresolution in the context of very localized and stiff reaction fronts. The paper considers a high order implicit time integration of the reaction and an explicit one for the diffusion term in order to build a time operator splitting scheme that exploits efficiently the special features of each problem. Based on recent theoretical studies of numerical analysis such a strategy leads to a splitting time step which is restricted by neither the fastest scales in the source term nor by stability constraints of the diffusive steps but only by the physics of the phenomenon. We aim thus at solving complete models including all time and space scales within a prescribed accuracy, considering large simulation domains with conventional computing resources. The efficiency is evaluated through the numerical simulation of configurations which were so far out of reach of standard methods in the field of nonlinear chemical dynamics for two-dimensional spiral waves and three-dimensional scroll waves, as an illustration. Future extensions of the proposed strategy to more complex configurations involving other physical phenomena as well as optimization capability on new computer architectures are discussed.

Climatology and trends in the forcing of the stratospheric ozone transport

Abstract. A thorough analysis of the ozone transport was carried out using the Transformed-Mean Eulerian (TEM) tracer continuity equation and the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). In this budget analysis, the chemical net production term, which is calculated as the residual of the other terms, displays the correct features of a chemical sink and source term, including location and seasonality, and shows good agreement in magnitude compared to other methods of calculating ozone loss rates. This study provides further insight into the role of the eddy ozone transport and underlines its fundamental role in the recovery of the ozone hole during spring. The trend analysis reveals that the ozone hole intensification over the 1980–2001 period is not solely related to the trend in chemical losses, but more specifically to the balance between the trends in chemical losses and ozone transport. That is because, in the Southern Hemisphere from October to December, the large increase in the chemical destruction of ozone is balanced by an equally large trend in the eddy transport, associated with a small increase in the mean transport. This study shows that the increase in the eddy transport is characterized by more poleward ozone eddy flux by transient waves in the midlatitudes and by stationary waves in the polar region. Overall, this study makes clearer the close interaction between the trends in ozone chemistry and ozone transport. It reveals that the eddy ozone transport and its long-term changes are an important natural mitigation mechanism for the ozone hole. This work also underlines the need for diagnostics of the eddy transport in chemical transport models used to investigate future ozone recovery.

Probability density function simulation of turbulent reactive gas‐solid flow in a FCC riser

AbstractA newly developed hybrid solution algorithm applicable on turbulent multiphase, gas‐solid, reactive flows is presented. A finite volume (FV) and a probability density function (PDF) method are combined. The FV technique is used to solve the total mass and momentum conservation equations together with the k‐ε turbulence equation for the gas phase and the granular temperature equation for the solid phase. The PDF method is applied to solve the species continuity equations and avoids the need to model the chemical source terms in the latter. Stochastic differential equations (SDEs) are introduced as replicas of the transported composition PDF equations. The notional‐particle‐based Monte‐Carlo technique is used to solve the PDF model equations. The hybrid FV/PDF solution algorithm is applied to simulate a fluid catalytic cracking (FCC) riser using a 12‐lump kinetic model. A good agreement between simulated results and available plant data is obtained. © 2011 American Institute of Chemical Engineers AIChE J, 2012

New Resolution Strategies for Multi-scale Reaction Waves: Optimal Time Operator Splitting and Space Adaptive Multiresolution


 
 
 We tackle the numerical simulation of reaction-diffusion equations modeling multi-scale reac- tion waves. This type of problems induces peculiar difficulties and potentially large stiffness which stem from the broad spectrum of temporal scales in the nonlinear chemical source term as well as from the presence of large spatial gradients in the reaction fronts, spatially very lo- calized. In this paper, we introduce a new resolution strategy based on time operator splitting and space adaptive multiresolution in the context of very localized and stiff reaction fronts. Based on recent theoretical studies of numerical analysis, such a strategy leads to a splitting time step which is not restricted neither by the fastest scales in the source term nor by restric- tive diffusive step stability limits, but only by the physics of the phenomenon. We thus aim at solving accurately complete models including all time and space scales of the phenomenon, considering large simulation domains with conventional computing resources. The efficiency is evaluated through the numerical simulation of configurations which were so far out of reach of standard methods in the field of nonlinear chemical dynamics for 2D spiral waves and 3D scroll waves as an illustration. Future extensions of the proposed strategy are finally discussed.
 
 

Calculation and Experiment of Soot Emission for a Directed Injection Engine

To calculate soot for a high-pressure common rail diesel engine a new soot model (TP model) is presented.. In soot source term phase, when zone temperature T<1500K, gas-phase kinetics is considered and the soot precursors –PAHs (Polycyclic aromatic hydrocarbons) is calculated. When zone temperature T ≥ 1500K, the gas-phase chemistry and soot source terms are calculated. The soot model integrated with the new auto-ignition models is then applied in multidimensional simulations. The TP model is then implemented in KIVA code instead of original model to carry out optimizing. The results of soot with variation of injection time, variation of rail pressure and variation of speed among TP model, KIVA standard model and experimental data are analyzed. The results indicate that the TP model can carry out optimization and CFD (computational fluid dynamics) and can be a tool to calculate for a high-pressure common rail directed injection diesel engine.

Chlorine Truck Attack Consequences and Mitigation

We develop and apply an integrated modeling system to estimate fatalities from intentional release of 17 tons of chlorine from a tank truck in a generic urban area. A public response model specifies locations and actions of the populace. A chemical source term model predicts initial characteristics of the chlorine vapor and aerosol cloud. An atmospheric dispersion model predicts cloud spreading and movement. A building air exchange model simulates movement of chlorine from outdoors into buildings at each location. A dose-response model translates chlorine exposures into predicted fatalities. Important parameters outside defender control include wind speed, atmospheric stability class, amount of chlorine released, and dose-response model parameters. Without fast and effective defense response, with 2.5 m/sec wind and stability class F, we estimate approximately 4,000 (half within ∼10 minutes) to 30,000 fatalities (half within ∼20 minutes), depending on dose-response model. Although we assume 7% of the population was outdoors, they represent 60-90% of fatalities. Changing weather conditions result in approximately 50-90% lower total fatalities. Measures such as sheltering in place, evacuation, and use of security barriers and cryogenic storage can reduce fatalities, sometimes by 50% or more, depending on response speed and other factors.

Lagrangian properties of turbulent diffusion with passive chemical reaction in the framework of the premixed combustion theory

In this paper, we analyze two effects caused by the Lagrangian nature of turbulent transfer which are usually ignored in the theory of turbulent premixed combustion. These effects are (i) the nonequilibrium behavior of the turbulent diffusion coefficient, which is important for modeling the initial stage of combustion (for example, in the spark ignition engine), and (ii) the existence of a traveling front of turbulent diffusion with finite speed, which controls the velocity of the steady state flame in strong turbulence. However, in order to derive simple and exact results, the hydrodynamical and the combustion subproblems are stated to be independent by assuming a constant density so that a passive chemical reaction is actually considered. First, we derive a parabolic diffusion equation with both diffusion and chemical source terms expressed by Lagrangian characteristics of turbulence. We show that, in general, the diffusivity of product particles is not zero in the moment of their generation by chemical...

Parareal operator splitting techniques for multi-scale reaction waves: Numerical analysis and strategies

In this paper, we investigate the coupling between operator splitting techniques and a time parallelization scheme, the parareal algorithm, as a numerical strategy for the simulation of reaction-diffusion equations modelling multi-scale reaction waves. This type of problems induces peculiar difficulties and potentially large stiffness which stem from the broad spectrum of temporal scales in the nonlinear chemical source term as well as from the presence of large spatial gradients in the reactive fronts, spatially very localized. In a series of previous studies, the numerical analysis of the operator splitting as well as the parareal algorithm has been conducted and such approaches have shown a great potential in the framework of reaction-diffusion and convection-diffusion-reaction systems. However, complementary studies are needed for a more complete characterization of such techniques for these stiff configurations. Therefore, we conduct in this work a precise numerical analysis that considers the combination of time operator splitting and the parareal algorithm in the context of stiff reaction fronts. The impact of the stiffness featured by these fronts on the convergence of the method is thus quantified, and allows to conclude on an optimal strategy for the resolution of such problems. We finally perform some numerical simulations in the field of nonlinear chemical dynamics that validate the theoretical estimates and examine the performance of such strategies in the context of academical one-dimensional test cases as well as multi-dimensional configurations simulated on parallel architecture.

Turbulent premixed flame model based on a recent dispersion model

A model of turbulent premixed combustion is formulated based on a recent dispersion model and on earlier flame models. The dispersion model accounts for the effect of velocity correlation on turbulent dispersion without using parameters that are explicitly dependent on time or space. The earlier flame models are used as a basis for formulating an appropriate chemical source term. The resulting model is more general than the earlier models. The proposed model and one of the earlier models are validated against various experimental flames. It is shown that the proposed model is more accurate and requires less calibration. Furthermore, the proposed model is tested successfully against general criteria for premixed combustion modeling formulated in earlier works. It is therefore concluded that the dispersion model is a useful tool for premixed combustion modeling.

J056041 確率密度関数法による液相格子乱流中の反応性スカラー拡散の数値計算 : 連続競争反応の場合([J05604]乱流における運動量,熱,物質の輸送現象(4))

The aim of this study is to elucidate numerically characteristics of a diffusion field with a competitive-consecutive reaction in a liquid grid-turbulence. Numerical simulation for a nearly homogeneous diffusion field of reactive scalars has been conducted by probability density function (PDF) method. PDF method is a one of promising approach for turbulent reacting flows because the chemical source term appears in a closed form. For the molecular mixing, two models (Curl's model and the modified Curl model) are applied. At first, the results of nonreactive field and reactive field of one-step reaction are compared. It is found that the mean concentrations of reactive species become smaller than those of nonreactive species because of consumption by chemical reaction. However, the simulation results overestimate the effect of the chemical reaction as a whole.

A Simple Reaction Time Scale for Under-Resolved Fire Dynamics

A reaction time scale model is developed for use in the eddy dissipation concept (fast chemistry limit) closure of the mean chemical source term in large-eddy simulation of fires. The novel aspect of the model is to consider a scaling regime for coarse mesh resolution based on buoyant acceleration. The model computes local time scales for diffusion, turbulent advection, and buoyant acceleration and takes the minimum of these as the local mixing time. The new model is implemented in the Fire Dynamics Simulator (FDS) and tested by comparing flame height predictions to the Heskestad correlation.

On the experimental validation of combustion simulations in turbulent non-premixed jets

A Reynolds averaged Navier–Stokes (RANS) based combustion model, which incorporated the conditional source-term estimation (CSE) method for the closure of the chemical source term and the trajectory generated low-dimensional manifold (TGLDM) method for the reduction of detailed chemistry, was applied to predict the OH radical distribution in a combusting non-premixed methane jet. The results of the numerical prediction were compared with the results of a complementary experimental study in which the OH radical fields of combusting non-premixed methane jets were visualized using planar laser induced fluorescence (PLIF). It is well known within the modelling community that RANS based models are unable to capture the stochastic nature of turbulent combustion and autoignition, and are therefore unable to predict individual realizations of the flame. In this study, the agreement between the predicted OH field and a well-converged ensemble average of the experimental results was also shown to be poor. The lack of agreement between the numerical results and the ensemble averaged experimental results expose the potential significance of the known weakness in the RANS method. A statistical analysis of the experimental results was also performed. The results of the analysis showed that a minimum of 100 individual realizations was required to provide a well-converged average OH field for the combusting non-premixed jet under investigation. The significance of this result with respect to the validation of large-eddy simulations (LES) of combusting jets is discussed.

Large eddy simulation of bluff-body stabilized flames using a multi-environment filtered density function model

A multi-environment turbulent combustion model, originally developed for Reynolds averaged Navier–Stokes equations (RANS), was extended to large eddy simulation (LES) and was used to model a bluff-body stabilized flame series (HM1 and HM3) that ranges from near equilibrium to near global extinction. The multi-environment filtered density function model (MEFDF) depicts the filtered density function (FDF) as a weighted summation of a small number of multi-dimensional Dirac delta functions in composition space. It is derived from the transport FDF equation using the direct quadrature method of moments (DQMOM). The MEFDF method with multiple reactive scalars retains the unique property of the joint FDF model of treating the chemical source term exactly. It was coupled with a 19-species reduced mechanism based on quasi-steady-state assumption and an In Situ adaptive tabulation (ISAT) algorithm for chemical source term integration. The model successfully captures the level of local extinction of the two flames exhibited in experiment. The predicted radial profiles of mixture fraction, temperature and CO mass fraction are in good agreement with measurements. The current work highlights the potential of LES/MEFDF approach as a tool for modeling of turbulent flame stability and combustion dynamics as well as pollutant emissions.

Phase optimized skeletal mechanisms for engine simulations

Adaptive chemistry is based on the principle that instead of having one comprehensive model describing the entire range of chemical source term space (typically parameters related to temperature, pressure and species concentrations), a set of computationally simpler models are used, each describing a local region (in multidimensional space) or phases (in zero-dimensional space). In this work, an adaptive chemistry method based on phase optimized skeletal mechanisms (POSM) is applied to a 96 species n-heptane–isooctane mechanism within a two-zone zero-dimensional stochastic reactor model (SRM) for an spark-ignition (SI) Engine. Two models differing only in the extent of reduction in the phase mechanism, gave speed-up factors of 2.7 and 10. The novelty and emphasis of this study is the use of machine learning techniques to decide where the phases are and to produce a usable phase recognition. The combustion process is automatically divided up into an ‘optimal’ set of phases through machine learning clustering based on fuzzy logic predicates involving a necessity parameter (a measure giving an indication whether a species should be included in the mechanism or not). The mechanism of each phase is reduced from the full mechanism based on this necessity parameter with respect to the conditions of that phase. The algorithm to decide which phase the process is in is automatically determined by another machine learning method that produces decision trees. The decision tree is made up of asking whether the mass fraction values were above or below given values. Two POSM studies were done, a conservative POSM where the species in each phase are eliminated based on a necessity parameter threshold (speed-up 2.7) and a further reduced POSM where each phase was further reduced by hand (speed-up 10). The automated techniques of determining the phases and for creating the decision tree are very general and are not limited to the parameter choices of this paper. There is also no fundamental limit as to the size of the original detailed mechanism. The interfacing to include POSM in an application does not differ significantly from using the original detailed mechanism.

Micromixing effects on series–parallel and autocatalytic reactions in a turbulent single-phase gas flow

A hybrid solution algorithm is implemented to simulate turbulent reactive single-phase gas flow in an isothermal tubular reactor. This algorithm is a combination of a Finite Volume (FV) and a Probability Density Function (PDF) method. The FV method is used to solve the mass and momentum conservation equations combined with a standardized k– ε model for single-phase gas flow. The PDF method is applied to solve the species continuity equations. The advantage of using the PDF method is the fact that there is no need of any closures for chemical reaction source terms in a turbulent flow . The mesomixing and the micromixing contributions in the PDF equation are closed using the gradient-diffusion model and the Interaction-by-Exchange-with-the-Mean (IEM) model, respectively. This hybrid solution algorithm is applied to simulate a series–parallel and an autocatalytic reactive single-phase gas flow. The mechanical-to-scalar time-scale ratio, i.e. the IEM model parameter, is found to have an influence on the simulation results that cannot be neglected. As expected, in series–parallel reactions, the desired product selectivity increases when the reaction rate coefficient, corresponding to its formation, increases. Moreover multiple solutions are observed in the autocatalytic reaction for a given feed ratio, Damköhler (Da I ) and Péclet (Pe) numbers. To validate the hybrid FV–PDF solution algorithm, the calculated results are compared with the results obtained when using the Reynolds-averaged species continuity equation model. A good agreement is observed when infinite-rate mixing is applied.

Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: a chemical explosive mode analysis

A chemical explosive mode analysis (CEMA) was developed as a new diagnostic to identify flame and ignition structure in complex flows. CEMA was then used to analyse the near-field structure of the stabilization region of a turbulent lifted hydrogen–air slot jet flame in a heated air coflow computed with three-dimensional direct numerical simulation. The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11000 with over 900 million grid points. Explosive chemical modes and their characteristic time scales, as well as the species involved, were identified from the Jacobian matrix of the chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the contribution of species and temperature in the explosion process. Radical and thermal runaway can consequently be distinguished. CEMA of the lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork preceding the two flame fronts thereby identifies the stabilization point. A Damköhler number was defined based on the time scale of the chemical explosive mode and the local instantaneous scalar dissipation rate to highlight the role of auto-ignition in affecting the stabilization points in the lifted jet flame.

Large-eddy simulation of kerosene spray combustion in a model scramjet chamber

Large-eddy simulation (LES) of kerosene spray combustion in a model supersonic combustor with cavity flame holder is carried out. Kerosene is injected through the ceiling of the cavity. The subgrid-scale (SGS) turbulence stress tensor is closed via the Smagorinsky's eddy-viscosity model, chemical source terms are modelled by a finite rate chemistry (FRC) model, and a four-step reduced kerosene combustion kinetic mechanism is adopted. The chamber wall pressure predicted from the LES is validated by experimental data reported in literature. The test case has a cavity length of 77 mm and a depth of 8 mm. After liquid kerosene is injected through the orifice, most of the droplets are loaded with recirculation fluid momentum inside the cavity. Due to lower velocity of the carrier fluid inside the cavity, sufficient atomization and evaporation take place during the process of droplet transportation, resulting in a rich fuel mixture of kerosene vapour accumulating inside the cavity. These rich fuel mixtures are mixed with fresh air by the approaching mixing layer at the front of the cavity and are thus involved in burning accompanied with the approaching boundary layer separation extending towards upstream. The combustion flame in the downstream impinges onto the rear wall of the cavity and is then reflected back to the front of the cavity. During the recirculation of hot flow, heat is compensated for evaporation of droplets. The circulation processes mentioned above provide an efficient flame-holding mechanism to stabilize the flame. Comparisons with results from a shorter length of cavity (cavity length of 45 mm) show that, due to insufficient atomization and evaporation of the droplets in the short distance inside the cavity, parts of the droplets are carried out of the cavity through the boundary layer fluctuation and evaporated in the hot flame layer, thus resulting in incomplete air fuel mixing and worse combustion performance. The flow structures inside the cavity play an important role in the spray distribution, thus determining the combustion performance.

Free Turbulent Reacting Jet Simulation Based on Combination of Transport Equations and PDF

In this paper, a method based on the combination of transport equations and PDF (Probability Density Function) calculations is tested through the study of a free axisymmetric turbulent jet and non-premixed flames. This method solves the difficulties related to the closure of the chemical source term. PDF calculations provide the statistical properties of the flow by which the mean values of physical quantities are calculated. In addition, Favre averaged quantities, i.e. temperature, species concentration and velocity fields, can be obtained by using PDF. A comparison has been made with previous experimental and numerical studies in the same field. The model is then applied to the investigation of a diffusive turbulent flame for which experimental data exist. The results obtained show good accuracy for velocities, main species and temperature and are very encouraging for use in combustion simulations.

Simulation of reactive nanolaminates using reduced models: III. Ingredients for a general multidimensional formulation

A transient multidimensional reduced model is constructed for the simulation of reaction fronts in Ni/Al multilayers. The formulation is based on the generalization of earlier methodologies developed for quasi-1D axial and normal propagation, specifically by adapting the reduced formalism for atomic mixing and heat release. This approach enables us to focus on resolving the thermal front structure, whose evolution is governed by thermal diffusion and heat release. A mixed integration scheme is used for this purpose, combining an extended-stability, Runge–Kutta–Chebychev (RKC) integration of the diffusion term with exact treatment of the chemical source term. Thus, a detailed description of atomic mixing within individual layers is avoided, which enables transient modeling of the reduced equations of motion in multiple dimensions. Two-dimensional simulations are first conducted of front propagation in composites combining two bilayer periods. Results are compared with the experimental measurements of Knepper et al. [ 22], which reveal that the reaction velocity can depend significantly on layering frequency. The comparison indicates that, using a concentration-dependent conductivity model, the transient 2D computations can reasonably reproduce the experimental behavior. Additional tests are performed based on 3D computations of surface initiated reactions. Comparison of computed predictions with laser ignition measurements indicates that the computations provide reasonable estimates of ignition thresholds. A detailed discussion is finally provided of potential generalizations and associated hurdles.