Tianfeng Lu

Complex CSP for Chemistry Reduction and Analysis

Abstract The method of computational singular perturbation for the analysis and reduction of complicated chemical mechanisms has been extended to the complex eigensystem. The characteristic time scale for each species was defined by using the time scales of the independent modes weighted by radical pointers, and the time scale of each species normalized by a characteristic time scale of the system was used as a criterion in determining the quasi-steady-state species. Furthermore, for oscillatory modes the radical pointer and the importance index of the previous computational singular perturbation theory were redefined. Results show that the time scales of chemical species change dramatically and non-monotonically, and the oscillatory modes appear frequently in large chemical reaction mechanisms. The present method was then employed to generate a 4-step and a 10-step reduced mechanism for the high-temperature H2/air and CH4/air oxidation, respectively. The validity of these reduced mechanisms were evaluated based on the responses of the perfectly stirred reactors and the one-dimensional planar propagating premixed flames. Comparisons between the reduced and detailed chemistries over a wide range of pressures and equivalence ratios show good agreement on the flame speed, flame temperature, and flame structure. A software package based on the present algorithm was compiled to generate reduced mechanisms for complex chemical mechanisms. The validity and efficiency of the present algorithm is demonstrated.

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 Damkohler 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.

Development and validation of an n-dodecane skeletal mechanism for spray combustion applications

n-Dodecane is a promising surrogate fuel for diesel engine study because its physicochemical properties are similar to those of the practical diesel fuels. In the present study, a skeletal mechanism for n-dodecane with 105 species and 420 reactions was developed for spray combustion simulations. The reduction starts from the most recent detailed mechanism for n-alkanes consisting of 2755 species and 11,173 reactions developed by the Lawrence Livermore National Laboratory. An algorithm combining direct relation graph with expert knowledge (DRGX) and sensitivity analysis was employed for the present skeletal reduction. The skeletal mechanism was first extensively validated in 0-D and 1-D combustion systems, including auto-ignition, jet stirred reactor (JSR), laminar premixed flame and counter flow diffusion flame. Then it was coupled with well-established spray models and further validated in 3-D turbulent spray combustion simulations under engine-like conditions. These simulations were compared with the recent experiments with n-dodecane as a surrogate for diesel fuels. It can be seen that combustion characteristics such as ignition delay and flame lift-off length were well captured by the skeletal mechanism, particularly under conditions with high ambient temperatures. Simulations also captured the transient flame development phenomenon fairly well. The results further show that ignition delay may not be the only factor controlling the stabilisation of the present flames since a good match in ignition delay does not necessarily result in improved flame lift-off length prediction.

Direct numerical simulations of turbulent lean premixed combustion

In recent years, due to the advent of high-performance computers and advanced numerical algorithms, direct numerical simulation (DNS) of combustion has emerged as a valuable computational research tool, in concert with experimentation. The role of DNS in delivering new Scientific insight into turbulent combustion is illustrated using results from a recent 3D turbulent premixed flame simulation. To understand the influence of turbulence on the flame structure, a 3D fully-resolved DNS of a spatially-developing lean methane-air turbulent Bunsen flame was performed in the thin reaction zones regime. A reduced chemical model for methane-air chemistry consisting of 13 resolved species, 4 quasi-steady state species and 73 elementary reactions was developed specifically for the current simulation. The data is analyzed to study possible influences of turbulence on the flame thickness. The results show that the average flame thickness increases, in qualitative agreement with several experimental results.

Simulations of Cavity-Stabilized Flames in Supersonic Flow Using Reduced Chemical Kinetic Mechanisms (Postprint)

Abstract : The VULCAN CFD code integrated with a reduced chemical kinetic mechanism was applied to simulate cavity-stabilized ethylene-air flames and to predict flame stability limits in supersonic flows based on an experimental study. A 15-step reduced kinetic mechanism for ethylene was systematically developed through skeletal reduction with a directed relation graph and time scale reduction based on quasi-steady state assumptions. The accuracy of the reduced kinetic mechanism and its implementation in the VULCAN code were demonstrated in an auto-ignition problem with a range of parameters. 3D simulations were then carried out for cavity-stabilized flames at different fuel flow rates and turbulent Schmidt numbers. For comparison with the performance of the present reduced mechanism, a 3- and a 10-step global kinetic model were applied to simulate the same cavity combustor, and the results show that the 15-step reduced model predicts experimental results much better than the 3- and 10-step models. The importance of including accurate chemical kinetics in CFD simulations is therefore demonstrated.

Dynamic adaptive chemistry for turbulent flame simulations

The use of large chemical mechanisms in flame simulations is computationally expensive due to the large number of chemical species and the wide range of chemical time scales involved. This study investigates the use of dynamic adaptive chemistry (DAC) for efficient chemistry calculations in turbulent flame simulations. DAC is achieved through the directed relation graph (DRG) method, which is invoked for each computational fluid dynamics cell/particle to obtain a small skeletal mechanism that is valid for the local thermochemical condition. Consequently, during reaction fractional steps, one needs to solve a smaller set of ordinary differential equations governing chemical kinetics. Test calculations are performed in a partially-stirred reactor (PaSR) involving both methane/air premixed and non-premixed combustion with chemistry described by the 53-species GRI-Mech 3.0 mechanism and the 129-species USC-Mech II mechanism augmented with recently updated NO x pathways, respectively. Results show that, in the DAC approach, the DRG reduction threshold effectively controls the incurred errors in the predicted temperature and species concentrations. The computational saving achieved by DAC increases with the size of chemical kinetic mechanisms. For the PaSR simulations, DAC achieves a speedup factor of up to three for GRI-Mech 3.0 and up to six for USC-Mech II in simulation time, while at the same time maintaining good accuracy in temperature and species concentration predictions.

Simulating Flame Lift-Off Characteristics of Diesel and Biodiesel Fuels Using Detailed Chemical-Kinetic Mechanisms and Large Eddy Simulation Turbulence Model

Combustion in direct-injection diesel engines occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes. From a numerical standpoint, these spray combustion processes depend heavily on the choice of underlying spray, combustion, and turbulence models. The present numerical study investigates the influence of different chemical kinetic mechanisms for diesel and biodiesel fuels, as well as Reynoldsaveraged Navier‐Stokes (RANS) and large eddy simulation (LES) turbulence models on predicting flame lift-off lengths (LOLs) and ignition delays. Specifically, two chemical kinetic mechanisms for n-heptane (NHPT) and three for biodiesel surrogates are investigated. In addition, the renormalization group (RNG) k-e (RANS) model is compared to the Smagorinsky based LES turbulence model. Using adaptive grid resolution, minimum grid sizes of 250lm and 125lm were obtained for the RANS and LES cases, respectively. Validations of these models were performed against experimental data from Sandia National Laboratories in a constant volume combustion chamber. Ignition delay and flame lift-off validations were performed at different ambient temperature conditions. The LES model predicts lower ignition delays and qualitatively better flame structures compared to the RNG k-e model. The use of realistic chemistry and a ternary surrogate mixture, which consists of methyl decanoate, methyl nine-decenoate, and NHPT, results in better predicted LOLs and ignition delays. For diesel fuel though, only marginal improvements are observed by using larger size mechanisms. However, these improved predictions come at a significant increase in computational cost. [DOI: 10.1115/1.4007216]

Some Aspects of Chemical Kinetics in Chapman-Jouguet Detonation: Induction Length Analysis

Theine uenceofcomplex chemicalkineticson theinductionlength inChapman‐ Jouguetdetonationwasstudied, with emphases on hydrogen chemistry and applications in pulse detonation engines (PDEs). Problems studied include the role of branching‐ termination reactions on the overall reaction rate, the reduction of the detailed hydrogen oxidation mechanism to simpler ones without compromising comprehensiveness of description, the coupled ine uence of chemical reactivity and the upstream speed of sound on ignition, and the use of hydrogen as a potential ignition enhancer. Results show that the presence of the pressure-sensitive and temperature-insensitive three-body termination reactions can signie cantly prolong the ignition delay, that an operation map for PDE operation can be constructed based on the crossover temperature so that operation regimes with excessively long ignition delays can be avoided, and that while the extent of chemistry reduction for the hydrogen/air PDE system depends on the degree of parametric comprehensiveness required, a two-step reduced mechanism appears to be adequatefornear-stoichiometricdescriptions.Furthermore,itisdemonstratedthatthebenee tofthefasthydrogen chemistryismoderatedbyhydrogen’ shighspeedofsound,whichreducesthedetonationMachnumberandthereby the postshock temperature.

A reduced mechanism for biodiesel surrogates with low temperature chemistry for compression ignition engine applications

Biodiesel is a promising alternative fuel for compression ignition (CI) engines. It is a renewable energy source that can be used in these engines without significant alteration in design. The detailed chemical kinetics of biodiesel is however highly complex. In the present study, a skeletal mechanism with 123 species and 394 reactions for a tri-component biodiesel surrogate, which consists of methyl decanoate, methyl 9-decanoate and n-heptane was developed for simulations of 3-D turbulent spray combustion under engine-like conditions. The reduction was based on an improved directed relation graph (DRG) method that is particularly suitable for mechanisms with many isomers, followed by isomer lumping and DRG-aided sensitivity analysis (DRGASA). The reduction was performed for pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2 for both extinction and ignition applications. The initial temperatures for ignition were from 700 to 1800 K. The wide parameter range ensures the applicability of the skeletal mechanism under engine-like conditions. As such the skeletal mechanism is applicable for ignition at both low and high temperatures. Compared with the detailed mechanism that consists of 3299 species and 10806 reactions, the skeletal mechanism features a significant reduction in size while still retaining good accuracy and comprehensiveness. The validations of ignition delay time, flame lift-off length and important species profiles were also performed in 3-D engine simulations and compared with the experimental data from Sandia National Laboratories under CI engine conditions.

Analysis of a Turbulent Lifted Hydrogen/Air Jet Flame from Direct Numerical Simulation with Computational Singular Perturbation

The theory of computational singular perturbation (CSP) was employed to analyze the near-field structure of the stabilization region of a lifted hydrogen/air slot jet flame in a heated air coflow simulated with three-dimensional direct numerical simulation (DNS). The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11,200 with approximately 1 billion grid points. Explosive chemical processes and their characteristic time scales, as well as the species involved, were identified by the CSP analysis of the Jacobian matrix of chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the participation of species and temperature in the explosion process. Radical explosion and thermal runaway can consequently be distinguished. The CSP analysis of the simulated lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork separating the two flame fronts thereby identifies the lift-off point. A Damkohler number was defined with the time scale of the chemical explosive mode and the scalar dissipation rate to show the role of auto-ignition in affecting the lift-off point and in stabilizing the flame.