Mixture-averaged Diffusion Research Articles

DC-driven subatmospheric glow discharges in the infrared-stimulated

This paper presents a conceptual framework for experimental research combined with numerical analysis on direct current (DC) glow discharges in microscale planar gas discharge-semiconductor systems (GDSS). In the experimental section, several structural and elemental analyses, including SEM, EDAX, AFM, and near-infrared absorption spectra measurements were carried out for compound semiconductor zinc selenide (ZnSe) cathode sample. Argon (Ar) was charged into the plasma reactor cell of GDSS at pressures of 100 Torr subatmospheric and 760 Torr atmospheric, respectively, by a vacuum pump- gas filling station. Glow discharge light emissions from plasma, excited under three different intensity levels (dark, weak, strong) of infrared beam illumination on ZnSe cathode electrode, were measured by using a phomultiplier tube that is sensitive to UV–Visible wavelengths. In the numerical analysis section, simulation studies were carried out on the two-dimensional gas discharge-semiconductor microplasma system (GDSµPS) cell models using the finite-element method (FEM) solver COMSOL Multiphysics DC plasma program. Calculations and predictions were based on mixture-averaged diffusion drift theory and Maxwellian electron energy distribution function. GDSµPS cell was modeled in a square chamber with planar anode/cathode electrode pair coupled at a 50 μm discharge gap. Single side of ZnSe cathode was finely micro-digitated to increase the effective surface area for enhanced electron emission to the gas discharge cell. The electrical equivalent circuit (EEC) of the proposed model was driven by 1.0 kV DC voltage source. Binary Ar/H2 gas medium in a mixture of 3:2 molar ratio was introduced to the gas discharge chamber at constant 200 Torr subatmospheric pressure. Simulations were run for normal glow discharges to exhibit the electrical fast transient glow discharge behaviours from electron field emission state to self-sustained normal glow discharge state by numerically solving the electron density (ED), electron current density (ECD) and electric potential distribution (EPD) parameters.It is figured out that binary Ar/H2 gas discharge model can undertake a major role in shaping and controlling the spatiotemporal response to transient electro-optical behavior of microplasma-based artificial electromagnetic materials configured for high-efficiency infrared-to-visible wavelength conversion applications.

Effects of molecular diffusion modeling on turbulent premixed NH[formula omitted]/H[formula omitted]/air flames

Ammonia (NH3) is a promising carbon-free energy carrier and can be used as a fuel in gas turbines for power generation. Using a blend of NH3 and hydrogen (H2) has the potential to address the poor combustion properties of NH3/air mixtures. Predictive modeling of the NH3/H2 combustion process requires a clear understanding of molecular diffusion due to the high diffusivity of H2. While the effects of molecular diffusion modeling (MDM) have been extensively investigated in laminar/turbulent hydrocarbon and pure H2 flames, comparatively less investigations have been made to explore the impact of MDM on turbulent premixed NH3/H2/air combustion characteristics as well as NO emission. To this end, four three-dimensional direct numerical simulations of temporally evolving turbulent premixed NH3/H2/air jet flames are performed in this work considering MDM methods of increased complexity: 1) unity Lewis number (UL); 2) unity Lewis number with Soret effect (UL+S); 3) mixture-averaged diffusion with Soret effect (MA+S); and 4) full multicomponent diffusion with Soret effect (MC+S), respectively. The emphasis is placed on assessing MDM effects on flame structure, NO production, and combined role of MDM and stretch rate. Comparing MA+S with UL+S under both laminar and turbulent conditions indicates that the differential diffusion effect is significantly suppressed in the turbulent NH3/H2/air flames considered here. While MDM can affect the local equivalence ratio, it is found that the choice of the MDM method has a negligible impact on the major species and NO distribution conditioned on progress variable, as well as on critical reactions. However, MDM still shows a notable impact on NO production and turbulent flame speed. Specifically, from the most detailed model (MC+S) to the most simplified model (UL), the NO production speed decreases by 30% for the fuel pathway but increases by 338% for the thermal pathway; simultaneously, the global turbulent flame speed decreases by 18%. Furthermore, it is found that both NO mass fraction and local displacement speed are very sensitive to flame curvature and strain, while MDM has a negligible impact on the responses of normalized NO mass fraction and local displacement speed to the change in flame curvature or strain. This suggests that the effects of curvature and strain should be included in flamelet-based models to accurately predict NO emission and NH3/H2/air flame propagation, even when using the assumption of unity Lewis number.

A novel flamelet manifold parametrization approach for lean CH4–H2-air flames

Hydrogen is a carbon-free energy carrier that can substantially support the decarbonization of the power generation and transportation sector in the near future. Blending H2 into natural gas represents a feasible option to continue using the current infrastructure, allowing a smooth transition to pure H2 combustion technologies. In the present study, a flamelet model is proposed to describe lean CH4–H2-air laminar Bunsen flames with inert gas as a coflow, simultaneously taking into account multiple complex physical phenomena such as differential diffusion, heat losses at the burner wall and mixing between the main flow and coflow. In most previous works based on manifold-based reduction methods, transport equations are solved for the progress variable, mixture fraction and enthalpy. In contrast, transport equations for several species and enthalpy are solved in the present study and species diffusivities are evaluated with a mixture-averaged diffusion model. The control variables of the manifold are then reconstructed with those transported variables. In order to consider the mixing between the main flow and the inert coflow, two different approaches based on a three-dimensional and a four-dimensional manifold, respectively, are formulated and assessed. Utilizing a “linear mixing” assumption, the three-dimensional manifold is parameterized by a progress variable, an approximate Bilger mixture fraction and enthalpy. The four-dimensional manifold relaxes this assumption and additionally includes the inert gas mass fraction as the fourth control variable. The accuracy of the two approaches is evaluated by comparison with detailed chemistry results and experimental measurements. Overall, results obtained with both approaches show good agreement with the reference data, both qualitatively and quantitatively, indicating that all the effects mentioned above are well captured. Slightly better agreement is achieved with the four-dimensional manifold, which shows superiority in the mixing layer between the main flow and the inert coflow.

Efficient premixed turbulent combustion simulations using flamelet manifold neural networks: A priori and a posteriori assessment

Flamelet-based reduced manifold tabulation is very useful to save computing time compared to simulations of turbulent flames with detailed kinetics. However, conventional tabulation techniques based on high-dimensional look-up tables (LUT) soon lead to tremendous memory costs. As an alternative, artificial neural networks (ANN) can be used to reduce the storage. However, it is usually difficult to obtain an ANN model that can accurately describe a high-dimensional thermochemical space. Therefore, in the present study, Flamelet Manifold Neural Networks (FMNN) are introduced to achieve sufficient accuracy by constraining the solution space to physical configurations during the sampling and training process. Different from the standard ANN, FMNN involves flame front weighting information in the sampling process and 3 different physical constraints/implications in the loss function. The FMNN models have been trained for both flames with unity Lewis number diffusion (ULD) or mixture-averaged diffusion (MAD). A priori and a posteriori assessments of the FMNN have been done, to compare with the standard ANN and conventional LUT regarding accuracy, memory storage and retrieval time. Each key component of the FMNN has been verified to enhance the model accuracy. The resulting FMNN architecture is flexible and its novel components could also be combined with other machine learning techniques (such as Residual Neural Networks or Convolutional Neural Networks) for combustion models. The final FMNN models are validated using direct numerical simulations (DNS) of turbulent flames, resulting in relative errors less than 2%, very low storage requirements, and a speed-up of the computations by a factor of 4 and more.

Modelling and experimental studies on mass transport of multiple cryoprotective agents in articular cartilage

Cartilage transplantation is an effective way to repair, reconstruct, and replace damaged articular cartilage (AC) but its use is limited by the inability to preserve AC for long periods of time. Vitrification is an ideal choice for long-term storage of AC, and multiple cryoprotective agents (CPAs) with high concentration are usually used. To obtain high cell viability, chondrocytes at all locations inside AC should be protected properly by the CPAs during cooling and rewarming. Hence, it is important to know the mass transport properties of multiple CPAs as they synergistically infiltrate AC. In this study, a mathematical model to describe the mass transport behavior of multiple CPAs in AC was developed based on the mixture-averaged diffusion model. In addition, a methodology for the simultaneous determination of dimethyl sulfoxide, glycerol, ethylene glycol, and propylene glycol by carbon-13 nuclear magnetic resonance was established. The model is applicable for predicting single- and multiple-CPA permeation into AC, and its accuracy was verified by a massive experimental dataset. Simulation results showed reverse diffusion in the multiple-CPA permeation process, which was not found in the single-CPA permeation process. This curious phenomenon shows the sharp contrast between the diffusion behavior of a binary mixture and a multicomponent mixture.

Flamelet modeling of forced ignition and flame propagation in hydrogen-air mixtures

Modeling hydrogen-air flames is challenging for several reasons. Due to its high diffusivity and reactivity, the combustion properties of hydrogen are substantially different from conventional hydrocarbon fuels. Besides differential diffusion and flame stretch, which play an important role in hydrogen combustion, the prediction of ignition events is also relevant for the safety and control of combustors operating with hydrogen. To address these effects with a manifold-based method, a novel flamelet progress variable (FPV) model is presented which consists of a tabulated manifold and a coupling strategy. In contrast to prior work, the manifold is coupled to the CFD simulation by transporting major species and enthalpy instead of transporting the flamelet control variables only. A closure approach is developed to employ a mixture-averaged diffusion model accounting for exact diffusion coefficients. Furthermore, a novel tabulation strategy is presented which is based on a recently published composition space model (CSM). The CSM is used for computing and tabulating flamelets which are consistently blended with constant-pressure homogeneous ignition solutions at elevated temperatures. The FPV model is validated for planar and curved hydrogen-air premixed flames across a range of equivalence ratios (0.7≤ϕ≤1.4). The latter are ignited by an energy deposition followed by an outward propagation as cylindrical or spherical expanding flames. It is shown that the FPV model accurately recovers the characteristics of the forced ignition process in both quiescent mixtures and a counterflow configuration, as well as the flame structures and propagation speed of the outwardly propagating hydrogen-air flames.

Computational analysis of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures

The present study aims to gain insight of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures. A specific goal is to use the validated three-dimensional (3-D) numerical model to reveal fine details of the gas and liquid phase flows as well as the resulting flame characteristics, which are challenging to obtain experimentally. To facilitate the study, 3-D formulations have been developed to explicitly solve the transport equations in both phases. A compressible solver was formulated for flame propagation in the gas phase using a one-step chemical reaction expression and mixture-averaged diffusion coefficients for the gaseous species. An incompressible solver with temperature dependent thermo-physical properties was employed to describe the convective motions and heat transfer in the liquid fuel region. Validation has been conducted for both uniform and pulsating spreads over a narrow 1-propanol tray with varying fuel depths through comparing the predicted flame front evolution with published measurements. Further qualitative comparison has also been conducted for some predicted fine features of the gas and liquid phase flows and flame spreading characteristics with published experimental observations. For both the uniform and pulsating spread, the detailed flame structure including the main diffusion flame and a small stratified premixed flame at the front have been captured. Wherever relevant, the detailed predictions were also used to shed light on some discrepancies in previously reported features in different laboratory studies and numerical simulations. Finally, the detailed 3-D predictions were used to illustrate fine features of the subsurface convective flow and its relative position to the flame front, the relative magnitudes of the subsurface flow velocity and that of the spread rate as well as the role of the thermocapillary-driven subsurface flow in the flame spread mechanism.

Assessing the impact of multicomponent diffusion in direct numerical simulations of premixed, high-Karlovitz, turbulent flames

Implementing multicomponent diffusion models in numerical combustion studies is computationally expensive; to reduce cost, numerical simulations commonly use mixture-averaged diffusion treatments or simpler models. However, the accuracy and appropriateness of mixture-averaged diffusion has not been verified for three-dimensional, turbulent, premixed flames. In this study we evaluated the role of multicomponent mass diffusion in premixed, three-dimensional high Karlovitz-number hydrogen, n-heptane, and toluene flames, representing a range of fuel Lewis numbers. We also studied a premixed, unstable two-dimensional hydrogen flame due to the importance of diffusion effects in such cases. Our comparison of diffusion flux vectors revealed differences of 10–20% on average between the mixture-averaged and multicomponent diffusion models, and greater than 40% in regions of high flame curvature. Overall, however, the mixture-averaged model produces small differences in diffusion flux compared with global turbulent flame statistics. To evaluate the impact of these differences between the two models, we compared normalized turbulent flame speeds and conditional means of species mass fraction and source term. We found differences of 5–20% in the mean normalized turbulent flame speeds, which seem to correspond to differences of 5–10% in the peak fuel source terms. Our results motivate further study into whether the mixture-averaged diffusion model is always appropriate for DNS of premixed turbulent flames.

Computational investigation of real fluid effects in cryogenic laminar premixed CH4–O2 flames

This study investigates the effects of real fluid thermodynamic and transport properties on the structure and propagation of canonical planar, unstretched, premixed methane-oxygen flames at transcritical conditions, which is relevant to the ignition phase in a liquid rocket engine. The ideal gas law is inapplicable at such extreme conditions, and real fluid thermodynamic and transport properties are required to accurately model the combustion physics at supercritical conditions.The computational framework used in this study integrates real fluid property estimation into the steady-state, freely-propagating flame solver available in the Cantera combustion suite [Goodwin et al., 2017]. The Peng-Robinson equation of state provides closure for the thermodynamic terms in the governing equations. High-pressure thermal conductivity and mass diffusivity coefficients are obtained using the Chung and Takahashi correlations, respectively. A mixture-averaged mass diffusion model is assumed, while neglecting the Soret and Dufour effects. The reaction rate terms are modeled with the standard Arrhenius ideal gas kinetics model using the CH4–O2 mechanism from Lindstedt. The effects on laminar flame structure are presented. It is found that the choice of chemical mechanism has a much greater influence on mass burning rates than the real fluid effects. Real fluid properties produce lower mass burning rates by just ∼10% near the critical region, while the laminar flame speeds are reduced by a factor of ∼5. It is shown that using dilute gas transport properties in combination with a real fluid equation of state and differential diffusion effects may cause a qualitatively different, non-monotonic density profile in the flame.

Numerical Study of Quenching Distances for Side-Wall Quenching Using Detailed Diffusion and Chemistry

The numerical investigation of quenching distances in laminar flows is mainly concerned with two setups: head-on quenching (HOQ) and side-wall quenching (SWQ). While most of the numerical work has been conducted for HOQ with good agreement between simulation and experiment, far less analysis has been done for SWQ. Most of the SWQ simulations used simplified diffusion models or reduced chemistry and achieved reasonable agreement with experiments. However, it has been found that quenching distances for the SWQ setup differ from experimental results if detailed diffusion models and chemical reaction mechanisms are employed. Side-wall quenching is investigated numerically in this work with steady-state 2D and 3D simulations of an experimental flame setup. The simulations fully resolve the flame and employ detailed reaction mechanisms as well as molecular diffusion models. The goal is to provide data for the sensitivity of numerical quenching distances to different parameters. Quenching distances are determined based on different markers: chemiluminescent species, temperature and OH iso-surface. The quenching distances and heat fluxes at the cold wall from simulations and measurements agree well qualitatively. However, quenching distances from the simulations are lower than those from the experiments by a constant factor, which is the same for both methane and propane flames and also for a wide range of equivalence ratios and different markers. A systematic study of different influencing factors is performed: Changing the reaction mechanism in the simulation has little impact on the quenching distance, which has been tested with over 20 different reaction mechanisms. Detailed diffusion models like the mixture-averaged diffusion model and multi-component diffusion model with and without Soret effect yield the same quenching distances. By assuming a unity Lewis number, however, quenching distances increase significantly and have better agreement with measurements. This was validated by two different numerical codes (OpenFOAM and FASTEST) and also by 1D head-on quenching simulations (HOQ). Superimposing a fluctuation on the inlet velocity in the simulation also increases the quenching distance on average compared to the reference steady-state case. The inlet velocity profile, temperature boundary condition of the rod and radiation have a negligible effect. Finally, three dimensional simulations are necessary in order to obtain the correct velocity field in the SWQ computations. This however has only a negligible effect on quenching distances.

A fast, low-memory, and stable algorithm for implementing multicomponent transport in direct numerical simulations

Implementing multicomponent diffusion models in reacting-flow simulations is computationally expensive due to the challenges involved in calculating diffusion coefficients. Instead, mixture-averaged diffusion treatments are typically used to avoid these costs. However, to our knowledge, the accuracy and appropriateness of the mixture-averaged diffusion models has not been verified for three-dimensional turbulent premixed flames. In this study we propose a fast, efficient, low-memory algorithm and use that to evaluate the role of multicomponent mass diffusion in reacting-flow simulations. Direct numerical simulation of these flames is performed by implementing the Stefan–Maxwell equations in NGA. A semi-implicit algorithm decreases the computational expense of inverting the full multicomponent ordinary diffusion array while maintaining accuracy and fidelity. We first verify the method by performing one-dimensional simulations of premixed hydrogen flames and compare with matching cases in Cantera. We demonstrate the algorithm to be stable, and its performance scales approximately with the number of species squared. Then, as an initial study of multicomponent diffusion, we simulate premixed, three-dimensional turbulent hydrogen flames, neglecting secondary Soret and Dufour effects. Simulation conditions are carefully selected to match previously published results and ensure valid comparison. Our results show that using the mixture-averaged diffusion assumption leads to a 15% under-prediction of the normalized turbulent flame speed for a premixed hydrogen-air flame. This difference in the turbulent flame speed motivates further study into using the mixture-averaged diffusion assumption for DNS of moderate-to-high Karlovitz number flames.

Effects of Soret diffusion on turbulent non-premixed H2 jet flames

Recent studies have shown that Soret diffusion (SD), driven by temperature gradients, could play an important role in both laminar and turbulent premixed H2 flames. However, comparatively little effort has been made to investigate SD effects on turbulent non-premixed H2 flames, in spite of the relevance of these flames in safety and their potential application in clean power and propulsion systems. To this end, the impact of SD on turbulent non-premixed H2 combustion is investigated numerically in this work by comparing two three-dimensional direct numerical simulations of temporally evolving turbulent jet flames. In one simulation, a mixture-averaged diffusion (MD) model is used to approximate multicomponent transport, while in the other the MD model is supplemented with a Soret term to consider SD effects. The emphasis is placed on examining and interpreting the impact of SD on flame structure, differential diffusion, and flame-tangential diffusion. It is found that H and OH mass fractions are significantly affected by SD, while SD has a negligible impact on temperature, heat release rate and H2 mass fraction. This is due to the fact that larger SD flux of H radical is strongly coupled with the main chemical reactions. However, for H2 its larger SD flux is located in the fuel-rich zone and decoupled from the main reactions. The difference between a conserved scalar (mixture fraction Z) and a non-conserved scalar (Bilger mixture fraction ZBilger) is employed as a diagnostic parameter to characterize differential diffusion, and the results show that the effects of SD on differential diffusion are reflected in two aspects: (i) increasing the absolute value of Z−ZBilger and (ii) increasing the degree of misalignment between the gradients of Z and ZBilger. Furthermore, the analysis of the contribution of SD to flame-tangential diffusion occurring in mixture fraction isosurfaces indicates that for H radical SD can augment the relative contribution of flame-tangential diffusion, especially in the region of high scalar dissipation rate. On the other hand, for H2, SD has a negligible impact on both flame-normal and flame-tangential diffusion. The present study contributes to providing insights into how SD affects turbulent non-premixed H2 flames and into modeling SD effects with flamelet-based combustion models.

Reproducing curvature effects due to differential diffusion in tabulated chemistry for premixed flames

Tabulated chemistry reduces the cost of modeling reacting flows by transporting fewer scalars. It also has the benefit of introducing fewer terms that must be closed for Reynolds-averaged Navier-Stokes and large eddy simulation modeling. In this study of premixed lean hydrogen–air flames, an extension to an existing tabulated chemistry model was developed by including differential diffusion and thermal diffusion of multiple species, using a mixture-averaged diffusion model. The transport equations for the tabulation variables (a progress variable and a mixture fraction-like variable) have been formulated, and curvature effects are intrinsic to the transport equations derived from first principles. The mathematical derivation is general, such that other fuel/air mixtures could be examined. The new tabulated chemistry model was then validated using a range of configurations, including flames with curvature, extinction, and hot spots. The model validation indicated excellent agreement with detailed chemistry simulations for lean premixed hydrogen–air mixtures. Conditional means of the progress variable source term were shown to agree well with detailed chemistry simulations of two-dimensional freely propagating flames and three-dimensional turbulent flames. Finally, progress variable source terms three times higher than one-dimensional flat flame results were identified and validated with detailed chemistry, and super-adiabatic hot spots were also predicted in the turbulent flame configuration.

Numerical study of gas-phase interactions of phosphorus-containing compounds with co-flow diffusion flames

Computational studies have been performed for methane-air cup-burner (i.e., co-flow diffusion) flames with addition of phosphorous containing compounds (PCCs). The PCCs are: phosphoric acid, trimethyl phosphate (TMP), and dimethyl methylphosphonate (DMMP), and these are added to the oxidizer or fuel flow. Calculations (time-dependent) employing a 2D axisymmetric domain, detailed gas-phase chemistry and mixture-averaged diffusion, are employed to unravel the chemical inhibition and flame structure. A CH4O2 chemical mechanism is combined with one for phosphorous in flames, and has 77 species and 886 reactions. Radiation heat losses are modeled in the energy equation via an assumption of optically-thin behavior, with plank-mean averaged emission from CO2, H2O, CO, CH4, and soot. The effect of the PCC on the CO2 volume fraction required for extinction (the minimum extinguishment concentration [MEC]) is used as the metric for inhibitor effectiveness, when agent is added to either the air or fuel stream. For flames inhibited by only CO2, the MEC from calculations and experiment are in good agreement; however, with DMMP added to the air (< 1%), the calculated MEC is significantly greater. Moreover, addition of the PCC to the air stream as opposed to the fuel stream is about ten times more effective. As flame inhibitors, DMMP, TMP, and phosphoric acid behave similarly, lowering the volume fraction of radicals and the rate of heat release at the reaction kernel (i.e., the flame-stabilizing peak reactivity spot) in the flame base, promoting flame blow-off. The three compounds behave differently, however, with regard to the trailing flame. While all three increase the peak temperature in the trailing flame, DMMP and TMP, which contain three methyl groups, result in higher maximum flame temperature and combustion enhancement there, with a two-zone flame structure, whereas phosphoric acid does not.

Validation of a mixture-averaged thermal diffusion model for premixed lean hydrogen flames

The mixture-averaged thermal diffusion model originally proposed by Chapman and Cowling is validated using multiple flame configurations. Simulations using detailed hydrogen chemistry are done on one-, two-, and three-dimensional flames. The analysis spans flat and stretched, steady and unsteady, and laminar and turbulent flames. Quantitative and qualitative results using the thermal diffusion model compare very well with the more complex multicomponent diffusion model. Comparisons are made using flame speeds, surface areas, species profiles, and chemical source terms. Once validated, this model is applied to three-dimensional laminar and turbulent flames. For these cases, thermal diffusion causes an increase in the propagation speed of the flames as well as increased product chemical source terms in regions of high positive curvature. The results illustrate the necessity for including thermal diffusion, and the accuracy and computational efficiency of the mixture-averaged thermal diffusion model.

A multi-timescale and correlated dynamic adaptive chemistry and transport (CO-DACT) method for computationally efficient modeling of jet fuel combustion with detailed chemistry and transport

A correlated dynamic adaptive chemistry and transport (CO-DACT) method is developed to accelerate numerical simulations with detailed chemistry and transport properties in a reactive flow. Different sets of phase parameters, which govern the transport properties and chemical reaction pathways, respectively, are proposed to identify the correlated groups for transport properties and reaction pathways in both temporal and spatial coordinates. The correlated transport properties and reduced chemical mechanisms in phase space are dynamically updated by different user-specified threshold values. For the calculation of detailed transport properties, the mixture-averaged diffusion model is employed. For the on-the-fly generation of reduced chemistry, the multi-generation path flux analysis (PFA) method is used. In the present method, the chemical reduction and transport properties calculation are only conducted once for all the computation cells in the same correlated group within the pre-specified thresholds. Therefore, without sacrificing accuracy within the range of uncertainty of mechanisms and transport properties, the CO-DACT method can eliminate all redundant chemistry reductions and transport properties calculations in temporal and spatial coordinates when the transport properties and chemical reaction pathways are correlated due to the similarities in phase space. The CO-DACT method is further integrated with the hybrid multi-timescale (HMTS) method to achieve efficient integration of chemistry. Simulations of outward propagating spherical premixed flames and one dimensional (1D) diffusion ignitions of a jet fuel surrogate mixture, as well as an unsteady spherical propagating diffusion flame of a DME/air mixture are conducted to validate the present algorithm. The impact of the selection of threshold values as well as the dependence of numerical errors on pressure and equivalent ratio are also examined. The results demonstrate that the CO-DACT method can increase the computation efficiency for transport properties by at least two-order of magnitudes. Moreover, it is robust, accurate, and improves the overall computation efficiency involving a large kinetic mechanism.

Parallel on-the-fly adaptive kinetics in direct numerical simulation of turbulent premixed flame

A new numerical framework for direct numerical simulation (DNS) of turbulent combustion is developed employing on-the-fly adaptive kinetics (OAK), correlated transport (CoTran), and a point-implicit ODE solver (ODEPIM). The new framework is tested on a canonical turbulent premixed flame employing a real conventional jet fuel mechanism. The results show that the new framework provides a significant speed-up of kinetics and transport computation, which allows DNS with large kinetic mechanisms, and at the same time maintains high accuracy and good parallel scalability. Detailed diagnostics show that calculation of the chemical source term with ODEPIM is 17 times faster than with a pure implicit solver in this test. OAK utilizes a path flux analysis (PFA) method to reduce the large kinetic mechanism to a smaller size for each location and time step, and it can further speed up the chemical source calculation by 2.7 times in this test. CoTran uses a similar correlation method to make the calculation of mixture-averaged diffusion (MAD) coefficients 72 times faster in this test. Compared to conventional DNS, the total CPU time of the final framework is 20 times faster, kinetics is 46 times faster, and transport is 72 times faster in this test.

Effects of non-unity Lewis number of gas-phase species in turbulent nonpremixed sooting flames

Turbulence statistics from two three-dimensional direct numerical simulations of planar n-heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20–30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors’ concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers.

Optimized Reduced Chemistry and Molecular Transport for Large Eddy Simulation of Partially Premixed Combustion in a Gas Turbine

A methodology is discussed to automatically determine the parameters of closed budget equations for chemical species mass fractions and energy, in order to simulate spatially filtered flames as required in large eddy simulation (LES). The method accounts for the effects of LES filtering on chemistry and transport by simultaneously optimizing, for a reduced number of species, the Arrhenius reaction rates and a correction to mixture-averaged molecular diffusion coefficients. The objective is to match, for a given filter size, spatially filtered canonical one-dimensional flames simulated with detailed chemistry solutions. This approach is designed for quite well-resolved LES, in which most of the unresolved fluctuations result from flame thickening due to spatial filtering, thus featuring weak levels of sub-grid scale flame wrinkling. Methane-air partially premixed combustion is addressed. A four-step reduced reaction mechanism involving seven species is developed along with mass and heat molecular transport properties. The optimization is performed at atmospheric pressure and at 3 bar, for ranges of fresh gas temperatures [300–650 K] and equivalence ratios [0.4–1.2]. Comparisons with the filtered detailed chemistry solution of a planar propagating front show that the laminar flame speed, the adiabatic flame temperature, the species profiles in the reaction zone, and the flow chemical composition and temperature at equilibrium are adequately predicted. The new sub-grid scale modeling approach is then applied to three-dimensional LES of an industrial gas turbine burner. Good agreement is found between the quantities predicted with LES and experimental data, in terms of flow and flame dynamics, axial velocities, averaged temperatures, and some major species concentrations. Results are also improved compared to previous simulations of the same burner.

A reduced multicomponent diffusion model

The diffusion models for multicomponent mixtures are investigated in planar premixed flames, counterflow diffusion flames, and ignition of droplet flames. Discernable discrepancies were observed in the simulated flames with the mixture-averaged and multicomponent diffusion models, respectively, while the computational cost of the multicomponent model is significantly higher than that of the mixture-averaged model. A systematic strategy is proposed to reduce the cost of the multicomponent diffusion model by accurately accounting for the species whose diffusivity is important to the global responses of the combustion systems, and approximating those of less importance. The important species in the reduced model are identified with sensitivity analysis, and are found to be typically among those in high concentrations with exception of a few radicals, e.g. H and OH, that are known to participate in critical reactions. The reduced model is validated in simulating the propagation of planar premixed flames, extinction of counterflow non-premixed flames and ignition of droplet flames. The reduced model was shown to feature similar accuracy to that of the multicomponent model while the computational cost was reduced by a factor of approximately 5 for an n-heptane mechanism with 88 species.