Chung K. Law’s research while affiliated with Tsinghua University and other places

While kinetic mechanisms play a pivotal role in simulating complex combustion problems, their extended scale often results in prohibitive computational cost, particularly when integrated with computational fluid dynamics simulations. This paper introduces the Directed Relation Graph Species Rank (DRGSR) algorithm, an efficient mechanism reduction technique designed to retain the essential species and reaction pathways while minimizing computational demands. Specifically, it incorporates a two-step approach: the first utilizes a directed relation graph to map species interactions and transform the kinetic information into a graph structure, and the second employs the PageRank algorithm and directed interaction coefficients to rank species based on their importance within the network. The DRGSR algorithm is validated through case studies involving both large- and small-scale, high-temperature and low-temperature mechanisms, specifically focusing on the ignition delay times for ethylene (C2H4) and n-heptane (C7H16). The algorithm demonstrates superior performance in reducing the number of species significantly while maintaining accuracy; for ethylene, it retains only 31 species with an error under 8%, while for n-heptane, it achieves comparable precision with fewer species compared to existing methods. The validation is extended to predicting the laminar flame speeds, and further affirms the algorithm's reliability and generalizability. A comparative analysis of the computational cost reveals that the DRGSR algorithm not only is less time-consuming, but it also simplifies the reduction process by eliminating the iterative threshold adjustments required by methods such as Directed Relation Graph (DRG), Directed Relation Graph with Error Propagation (DRGEP) and Directed Relation Graph with Error Propagation and Sensitivity Analysis (DRGEPSA). These findings indicate that the DRGSR algorithm offers a robust, efficient and reliable approach for kinetic mechanism reduction, suitable for wide ranges of engineering applications.

In response to the interest in nitrogen-containing compounds as energetic materials, an experimental and kinetic study on the low-temperature oxidation of three butyl nitrites isomers, namely n-butyl (NBN), isobutyl (IBN), and tert-butyl (TBN) was performed. By measuring their ignition delays in a rapid compression machine (RCM) under 5-15 bar at temperatures from 550 to 630 K, a two-stage ignition behavior was observed for all the three nitrites, with the first-stage delays of TBN being shorter than those of NBN and IBN. A detailed kinetic mechanism was constructed and validated against the experimental data, and the production rate was analyzed to explain the first-stage ignition behavior. Specifically, the N-O bond dissociation reaction initiated the consumption of butyl nitrites isomers in all cases studied, which produced NO and different butoxy radicals (C4H9O). In the case of TBN, the decomposition of TC4H9O produces CH3 in the first-stage ignition. The abundant CH3 radical reacts with NO2 to produce CH3O, which further yields HO2 and CH2O through the reaction with O2. The inert HO2 radical is converted to OH through the reaction HO2 + NO = OH + NO2, resulting in the first-stage ignition. Meanwhile, the decomposition of PC4H9O and IC4H9O produces n-propyl and i-propyl radicals, respectively, in the cases of NBN and IBN. The reaction sequences of n-propyl and i-propyl radicals produce less HO2 radicals compared with that in TBN, leading to longer first-stage ignition time.

The present work numerically investigates the role of wall heat loss on the propagation of stoichiometric dimethyl ether (DME)/air premixed cool flames in channels, with emphasis on the extinction limits, transition from cool to hot flames and the possibility of observing both cool and hot flame propagation in a channel of a given width and wall thermal condition. The premixed DME/air flame is simulated in a two-dimensional (2D) semi-open narrow channel employing a detailed kinetic model. The study focuses on the impact of the channel width, heat loss intensity and the initial hot kernel temperature on the formation of a steady cool or hot flame, the relevant quenching channel half-width and the transition between cool and hot flames. The results indicate that, as the wall heat loss intensity is increased, the quenching channel width for the cool flames increases as well. Transition to hot flame is observed for larger channel widths. For such a transition, the propagation of a cool flame front starts first, then a hot flame front appears behind it, and subsequently catches up with it. The critical channel width for the cool to hot flame transition (two-stage ignition) increases with heat loss intensity. As for the effect of the initial kernel temperature on the flame regime, stable cool flames were obtained for a certain range of channel widths for lower initial kernel temperatures. A non-monotonic dependence of the critical channel width for the two-stage ignition on the initial kernel temperature is shown to be related to NTC effects. Initial kernel temperatures larger than 1100 K resulted in the direct formation of a hot flame. In a preliminary study for non-stoichiometric mixtures, it was demonstrated that double cool and hot flame configuration can be observed for very lean or rich mixtures.

Catalytic combustion has been proposed to overcome the obstacles when burning ammonia as a carbon-free fuel, such as difficult ignition, narrow flammability limits and combustion instability. To this direction, ignition of gas-phase combustion of ammonia in catalytic reactors, surface-gas chemistry coupling and, especially, the resulting impact on product selectivity, turn to be the key questions. As such, this work numerically investigated catalytic and coupled catalytic-gaseous combustion of premixed ammonia/oxygen/nitrogen mixtures in catalytic micro-channels coated with platinum (Pt), which is one of the most widely used catalysts in combustion and emission control systems. NH3 conversion and N2 selectivity were examined for a wide range of operating conditions, including pressure (1 to 5 bar), equivalence ratio (0.1 to 0.5), channel height (0.5 to 3.0 mm) and catalyst temperature (800 to 1300 K). The roles of gas-phase ignition and interplays between the catalytic and the gaseous reaction pathways were further clarified. Results showed that gas-phase combustion could be ignited in the very narrow catalytic microchannels and was promoted by higher wall temperature, equivalence ratio and pressure. Under all investigated conditions, significant amounts of NO were produced from the catalytic pathway, which however could be substantially consumed by the gaseous pathway via the reaction of NO with NH2. The practical implications are that NO emissions from NH3 catalytic burners can be readily suppressed via proper reactor design and the selection of operating conditions. Hence, this work provides key information for the design and optimization of NH3 catalytic combustion systems.

Effects of bio-fuel diethyl ether (DEE) addition on the near-field spray tip characteristics of iso-octane have been investigated at different ambient temperatures experimentally by using high-speed imaging with a long-distance microscope. The near-field spray screening is focused on the early stage of ∼0.2 ms after starting the injection and the measuring length scale is less than 3.0 mm below the nozzle exit. Results show that, first, the DEE blending ratio and ambient temperature significantly affect the state of the previous injection residual fuel, which leads to four typical near-field spray tip patterns: mushroom, umbrella, helmet, and vapor hemisphere, as the liquid phase residual fuel is vaporized. Second, quantitative description of the near-field spray tip has been conducted. High ambient temperature and DEE blending ratio favor faster tip penetration because of the enhanced residual fuel vaporization and reduced axial obstruction. Finally, different near-field spray tip patterns resulting from the residual fuel state are shown as function of the ambient temperature and DEE blending ratio, followed by a conceptual scheme regarding the state of the residual fuel and its evolution after the start of injection, at different ambient temperatures and DEE blending ratios.

This study presents a novel method for optimizing combustion kinetic mechanisms using variational inference, addressing the longstanding challenge of balancing accuracy and computational efficiency. The primary focus is on the uncertainty quantification and minimization of pre-exponential factors in combustion reactions, a critical aspect of kinetic mechanism optimization. The initial phase involves identifying the most sensitive parameters through local sensitivity analysis, followed by the application of artificial neural networks (ANN) as surrogate models for each experimental condition. This approach significantly accelerates the optimization process while maintaining gradient propagation essential for variational inference. A key advancement in this research is the implementation of variational inference in place of traditional Markov chain Monte Carlo (MCMC) methods. This shift not only results in a marked improvement in the computational speed, but it also maintains a high degree of accuracy in parameter optimization. This is illustrated through a detailed comparison of posterior distributions and uncertainty constraints obtained via both MCMC and variational inference methods, employing a methanol mechanism as a test case. Moreover, the inclusion of covariance in the variational inference effectively addresses the over-constraint of parameter uncertainties observed in earlier methods, offering a more realistic depiction of parameter interdependencies. The results show complete consistency in covariance matrices inferred from both MCMC and variational inference, validating the accuracy of the new probabilistic model. In terms of computational efficiency, the variational inference method demonstrates an enhancement of more than three orders of magnitude compared to the ANN-MCMC method. This significant reduction in time cost paves the way for applying the method to larger-scale mechanisms and data sets.

The separate and coupled chemical and transport effects on the freely-propagating H2-O2-Br2 flames in the doubly-infinite domain are studied for fixed equivalence ratio and different bromine fractions. Contrary to previous studies, it is shown that the laminar flame speed monotonically decreases with the increasing bromine fraction, and slight flame enhancement is observed in the mass burning velocity. On the other hand, stronger chemical inhibition and transport enhancement have been respectively identified on the lean and rich sides, accounting for the rich-shift of the peak flame speed. Substantial preferential diffusion effects due to the co-existence of heavy bromine species and light hydrogen species can enhance and inhibit flame propagation. Sensitivity analysis indicates that, compared to reactions in the bromine sub-mechanism, key reactions in the hydrogen-oxygen mechanism H + O2 = O + OH (R1) may exert stronger influences on the laminar flame speed. Furthermore, the chemical and thermal structures exhibit multiplicity depending on the equivalence ratio and bromine concentration. Bromine reactions introduce additional reaction layers situated in the preheat zone close to the upstream unburnt mixtures. Reaction Br2 + H = Br + HBr (R41) with the largest reaction rate releases considerable heat, imposing considerable ambiguity on the definition of flame thickness based on the maximum temperature gradient. HBr + H = H2 + Br (R33) reverses in the leading layer in cases with relatively lower reactivity, while in the middle reaction zone R33 generates H2 and hence increases the local equivalence ratio.