• Energy Research

Abstract In chemical kinetic model reduction under internal combustion engine conditions, most implementations only consider ignition related chemistry without consideration of flame speed prediction. In practice, flame propagation commonly exists in spark ignition engines, dual-fuel with pilot injection compression ignition engines, reactivity controlled compression ignition engines, and etc. Due to the inherent time-consuming nature, it is impractical to run a 1-D flame code with trial-and-error methods for model reduction, especially when starting with a large chemical kinetic model. In this paper, an improved reduction methodology is proposed for construction of a small set of species that give accurate predictions of both flame speeds and ignition delays. First, a strong correlation is found between the errors of maximum H radical and the errors in prediction of laminar flame speeds. Addition of H to the search targets in graph-based methods is conducted showing improvement in accuracy of flame speed prediction. Second, the normalized flame speed sensitivity with rate constants is analyzed for identifying a set of species that strongly influences the prediction of flame speeds. Finally, a trial-and-error based method is used for further reduction with a 0-D testbed for prediction of ignition only, while keeping the species important to flame chemistry. The newly proposed reduction methodology is used for development of accurate skeletal models predicting both ignition and flame speeds for several hydrocarbon fuels. These skeletal models include methane (27 species), propane (32 species), n-heptane (126 species), and primary reference fuel gasoline surrogates (207 species) with high fidelity to be used in engine simulations.

An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

Abstract Numerical simulations of stratified methane/air flames with various stratification configurations are conducted using an 1-D unsteady reacting flow solver. Among all the stratified flame cases investigated, rich-to-lean stratified flames show significant departures from homogeneous flames, i.e., up to 50% increase in fuel consumption speeds, primarily due to preferential diffusion of lighter species and radicals from rich burnt products. A sensitivity study of transport properties further reveals that preferential diffusion of molecular hydrogen H 2 along with radicals H, OH is the dominant factor in increasing stratified flame speeds, compared to the influence of heat diffusion and diffusion of H 2 O. Larger departure of stratified flames from homogeneous flames is observed in those cases with higher degree of stratification. In order to model transient behaviors of stratified flames with arbitrary stratification configurations, a local stratification level (LSL) model is proposed. LSL incorporates the effect of preferential diffusion by introducing a transfer function from molecular hydrogen concentration gradients to equivalence ratio gradients, and the memory effect by solving the transient model equation of LSL. The model results match well with directly simulated results quantitatively with error less than ∼10% for both lean and rich conditions.

Abstract Modes of reaction front propagation and end-gas combustion of hydrogen/air mixtures in a closed chamber are numerically investigated using an 1-D unsteady, shock-capturing, compressible and reacting flow solver. Different combinations of reaction front propagation and end-gas combustion modes are observed, i.e., 1) deflagration without end-gas combustion, 2) deflagration to end-gas autoignition, 3) deflagration to end-gas detonation, 4) developing or developed detonation, occurring in the sequence of increasing initial temperatures. Effects of ignition location and chamber size are evaluated: the asymmetric ignition is found to promote the reactivity of unburnt mixture compared to ignitions at center/wall, due to additional heating from asymmetric pressure waves. End-gas combustion occurs earlier in smaller chambers, where end-gas temperature rise due to compression heating from the deflagration is faster. According to the ξ−e regime diagram based on Zeldovich theory, modes of reaction front propagation are primarily determined by reactivity gradients introduced by initial ignition, while modes of end-gas combustion are influenced by the total amount of unburnt mixture at the time when autoignition occurs. A transient reactivity gradient method is provided and able to capture the occurrence of detonation.

Abstract The assumed β-pdf is widely used in turbulent flame simulation with the moment closure method. However, implementation of the β-pdf in turbulent flame simulation may sometimes encounter singularity difficulties in numerical calculation. The study proposes a robust β-pdf treatment to overcome these numerical difficulties. The proposed β-pdf treatment associated with the partial equilibrium chemistry model and the k -ϵ turbulence model is firstly applied to the case of H 2 /N 2 -air nonpremixed flame to demonstrate its robustness. Then, the present method is applied to the study of thermal NO formation in H 2 -air nonpremixed flames. Comparison of results shows the present model to be capable of yielding fair agreement with the measurements. It is competitive with other more sophisticated methods, such as the pdf evolution method and the conditional moment closure method.

A 28‐species reduced chemistry mechanism for Dimethyl Ether (DME) combustion is developed on the basis of a recent detailed mechanism by Zhao et al. (2008). The construction of reduced chemistry was carried out with automatic algorithms incorporating newly developed strategies. The performance of the reduced mechanism is assessed over a wide range of combustion conditions anticipated to occur in future advanced piston internal combustion engines, such as HCCI, SAHCCI, and PCCI. Overall, the reduced chemistry gives results in good agreement with those from the detailed mechanism for all the combustion modes tested. While the detailed mechanism by Zhao et al. (2008) shows reasonable agreement with the shock tube autoignition delay data, the detailed mechanism requires further improvement in order to better predict HCCI combustion under engine conditions.

Abstract Quantum chemistry and rate constants of reactions such as H-abstraction of diethyl ether (DEE) by H, OH, HO 2 , O and CH 3 radicals as well as DEE and DEE radicals decomposition and isomerization were carried out through high-level ab initio and RRKM master equation computations. A comparison was made between some of the calculated rate constants and literature data. A detailed kinetic mechanism for DEE ignition contains 341 species and 1867 reactions was developed mainly based on the theoretical calculation and literature data and it was then further compared with the literature measurements along with three other existing DEE models, the Yasunaga model, the Sakai model and the Tran model. Compared with the other three models, the current model can give reasonable predictions on the validated ignition data over a wider temperature range. Based on the current model, a skeletal mechanism which contains 49 species and 192 reactions was developed by using a Jacobian-aided DRGEP approach, followed with a TSA method. The skeletal mechanism can precisely represent the detailed mechanism under a wide range of compression engine related conditions for DEE ignition. Finally, reaction pathway analysis and sensitivity analysis was conducted using the current model to gain an in-depth comprehension on DEE ignition at different temperatures.

A microwave-assisted spark plug was used to extend the lean operating limit (lean limit) and reduce emissions of an engine burning methane-air. In-cylinder pressure data were collected at normalized air-fuel ratios ofλ=1.46,λ=1.51,λ=1.57,λ=1.68, andλ=1.75. For eachλ, microwave energy (power supplied to the magnetron per engine cycle) was varied from 0 mJ (spark discharge alone) to 1600 mJ. At lean conditions, the results showed adding microwave energy to a standard spark plug discharge increased the number of complete combustion cycles, improving engine stability as compared to spark-only operation. Addition of microwave energy also increased the indicated thermal efficiency by 4% atλ=1.68. Atλ=1.75, the spark discharge alone was unable to consistently ignite the air-fuel mixture, resulting in frequent misfires. Although microwave energy produced more consistent ignition than spark discharge alone atλ=1.75, 59% of the cycles only partially burned. Overall, the microwave-assisted spark plug increased engine performance under lean operating conditions(λ=1.68)but did not affect operation at conditions closer to stoichiometric.

Two new 14-step and 16-step reduced mechanisms for methane-air combustion were systematically developed by assuming the quasisteady state for 26–28 species in the starting mechanism. A series of comparison between the reduced mechanisms and the starting mechanism was carried out with the emphasis on their capabilities in predicting NO2 formation and ignition delay. The two reduced mechanisms successfully capture the complex behaviors of NO2 formation, which depends on the characteristic mixing time, pressure, and the contamination of hydrocarbon in air. The flame structure and NOx formation in diffusion flame were well predicted by the 16-step mechanism, while the 14-step showed less satisfactory performance on predicting prompt NO formation. The 16-step mechanism was shown accurate in predicting ignition delay over a wide range of equivalence ratio, temperature and pressure. The necessity of including CH2O,C2H6,C2H4, and HO2 in the reduced mechanisms was discussed.