Combustion and Flame最新文献

Numerical simulations of two typical flame stabilization modes in a cavity-assisted supersonic combustor were performed using improved delay detached eddy simulation and three hydrogen oxidation mechanisms with different levels of fidelity. The simulation results with Burke's detailed mechanism agree well with the experimental measurements in terms of flame morphology and wall pressure, in both jet-wake and cavity flame modes. The comparative study shows that, lacking necessary intermediate species, Eklund's reduced mechanism and Marinov's global mechanism incorrectly yield jet wake stabilization mode under low inflow stagnation temperature $T_0$. Through computational singular perturbation analysis, a sequential radical triggering mechanism was identified for flame stabilization, wherein the reaction R1: $H+O_2=O+OH$ dominates in fuel jet wake forming OH and O radicals, the reaction R2: $H_2+O=H+OH$ controls the reaction between H₂ and O forming the OH radical pool, and then the heat release completes via R3: $H_2+OH=H+H_{2}O$. However, their activation differs in the two stabilization modes. The role of transport is key in the cavity flame mode, where the colder stream inhibits auto-ignition in the jet wake, activating low-temperature chemistry, and delaying R2 in the cavity region. Thus, the presence of H₂O₂ and HO₂ species was found to be essential for accurately reproducing the flame stabilization in the cavity flame stabilization mode, whereas their effect is marginal in jet wake mode. In fact, the jet-wake flame stabilization is characterized by auto-ignition under high inflow stagnation temperatures, with the chain-branching reaction R2 activating in the fuel jet-wake, causing an explosive dynamic therein. These findings suggest the H₂O₂ and HO₂ species and associated low-temperature reactions are necessary for the accurate prediction of the flame stabilization mode under low $T_0$, whereas their absence does not affect the prediction of the flame mode under high $T_0$, in which case all three chemical mechanisms give reasonably good agreements in flame characteristics and engine overall performances.

In the present study, flame acceleration driven by an array of multiple ignition kernels, equally spaced in the axial direction of a channel closed at one end, is studied numerically. In order to demonstrate the effects of the proposed ignition configuration on the flow dynamics, free-slip wall boundary condition is adopted. It is demonstrated that the burning of multiple flame kernels generates a powerful upstream flow, propelling the flame kernels, with the leading kernel’s tip undergoing a strong exponential acceleration. In a channel with smooth walls, such a powerful acceleration process is limited in time. The present study is mainly focused on the dynamics of the flame during the exponential stage of acceleration. The dependence of the acceleration rate, total acceleration time, and the maximum flame velocity on the initial distance between the kernels, the number of ignition kernels, and the thermal gas expansion coefficient, is quantified. Remarkably, the initial distance between the kernels has a weak influence on the above mentioned characteristics of flame dynamics as long as it is sufficiently large. It is observed that the acceleration rate increases with the kernel number. Notably, a significantly large maximum flame tip velocity can be achieved using the multi-kernel configuration as compared to the single-point ignition scenario. The acceleration rate demonstrates a nearly linear dependence on the thermal expansion coefficient, thus, the increase in thermal expansion results in a considerably stronger flame acceleration process.

Novelty and Significance Statement

For the first time, the flame acceleration process upon simultaneous ignition of a multi-point array of hot kernels equally spaced at the centerline of a channel was systematically studied. The work emphasizes the potential for achieving an extremely high flame speed within a very short time as compared to the single-ignition method. The acceleration rate, total acceleration time, and maximum achievable flame velocity were quantified. In a multi-point ignition scenario, we demonstrate that the burning of the fresh mixture between the kernels increases an overall cumulative gas expansion, propelling the leading tip. This result suggests a method for creating a powerful flame acceleration, which is an essential step in deflagration-to-detonation transition. Notably, the powerful acceleration is achieved in channels with smooth walls, which is important for DDT applications, in which using obstructed channels may be associated with a range of operational problems. The simplicity of the setup is also emphasized.

Hydrogen atom abstraction reactions by ṄH₂ radicals play a crucial role in determining the reactivity of ammonia/fuel binary blends. Esters are a typical component of environmentally friendly and economically promising biofuels. The feasibility of the ammonia/biofuel dual-fuel approach has been proven in practical engines. [Energy and Fuels 22 (2008) 2963] and [Int. J. Energy Res. 2023 (2023) 9920670]. ṄH₂ radicals play a critical role in the combustion and pyrolysis chemistry of ammonia and N-containing-rich fuels. In ammonia/biofuels hybrid combustion, ṄH₂ radicals can react with biofuel molecules in a reaction class that is particularly important especially when sufficient ammonia is blended in order to eliminate NO_x emissions. To help unravel the chemistry of ammonia/biofuel blends, a systematic theoretical kinetic study of H-atom abstraction from eleven alky esters of C_nH_2n+1COOCH₃ (n = 1–4), CH₃COOC_mH_2m+1 (m = 1–4), and C₂H₅COOC₂H₅, by ṄH₂ radicals is performed in this work. The geometry optimization, frequency, and zero-point energy calculations for all related species, as well as the hindrance potential energy surface for low frequency torsional modes in the reactants and transition states, were performed at the M06–2X/6–311++G(d,p) level of theory. Intrinsic reaction coordinate calculations were performed to validate the connections between the transition states and expected minima energy species. The energies of all of the species involved were calculated at the QCISD(T)/cc-pVXZ (X = D, T, Q) and MP2/cc-pVYZ (Y = T, Q) levels of theory and then extrapolated to the complete basis set. Rate constants of 39 reactions were calculated using the Master Equation System Solver (MESS) program in the temperature range of 500 – 2000 K. These rate constants for different H-atom abstraction sites are provided and can be extrapolated to larger esters. The kinetic effects from the functional group are also illustrated by performing detailed comparisons with the previous studies of ṄH₂ radical reactions with alkanes, alcohols and ethers.

The Quasi-Steady State Approximation (QSSA) can be an effective tool for reducing the size and stiffness of chemical mechanisms for implementation in computational reacting flow solvers. However, for many applications, the resulting model still requires implicit methods for efficient time integration. In this paper, we outline an approach to formulating the QSSA reduction that is coupled with a strategy to generate C++ source code to evaluate the net species production rates, and the chemical Jacobian. The code-generation component employs a symbolic approach enabling a simple and effective strategy to analytically compute the chemical Jacobian. For computational tractability, the symbolic approach needs to be paired with common subexpression elimination which can negatively affect memory usage. Several solutions are outlined and successfully tested on a 3D multipulse ignition problem, thus allowing portable application across chemical model sizes and GPU capabilities. The implementation of the proposed method is available at https://github.com/AMReX-Combustion/PelePhysics under an open-source license.

Novelty and Significance

A symbolic method is proposed to write analytical chemical Jacobians. The benefit of the symbolic method is that it is easy to implement and flexible to any elementary reaction type. Its benefit is shown in the context of QSS-reduced chemistries: there, constructing an analytical chemical Jacobian is complex since one must include the effect of traditional elementary reactions and algebraic closure for the QSS species. To the authors’ knowledge, there is no open-source package available to construct analytical Jacobians of QSS-reduced chemistries. We expect this work to facilitate the use of analytical Jacobians in arbitrarily complex chemical mechanisms. The proposed method was integrated into an open-source suite of reacting flow solvers https://github.com/AMReX-Combustion/PelePhysics to facilitate its dissemination.