Direct numerical simulation of nonpremixed ignition under gasoline compression-ignition engine conditions

IF 6.2 2区工程技术 Q2 ENERGY & FUELS

Combustion and Flame Pub Date : 2025-08-18 DOI:10.1016/j.combustflame.2025.114393

Zisen Li , Evatt R. Hawkes , Armin Wehrfritz , Bruno Savard

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引用次数: 0

Abstract

We present an analysis of the ignition process in thermochemical conditions relevant to gasoline compression-ignition (GCI) engines using direct numerical simulation (DNS). Two-dimensional DNS modelling the interaction of turbulence with an igniting double mixing layer are carried out. Three different primary reference fuel (PRF) blends, PRF0, PRF70, and PRF90, to span a range of different possible compression ignition scenarios are investigated. The fuel chemistry is shown to significantly affect the ignition process and the transition to a fully burning high-temperature flame. All three cases exhibit a diffusively supported cool flame which propagates towards richer mixtures faster than expected from homogeneous ignition delays. High-temperature combustion (HTC) initiates in rich mixtures in the PRF0 case, in both rich and lean mixtures in the PRF70 case, and in lean mixtures in the PRF90 case, which is consistent with expectations from homogeneous ignition delays. Budget analysis shows that HTC flames are diffusively supported in all cases, and as a result progress more rapidly from the ignition location to surrounding mixtures than homogeneous ignitions suggest. A quantitative model is proposed for the premixed flame propagation speed in the stratified and autoignitive mixtures. By considering the effects of normalised residence time of reactant at the flame surface, the conditional mean turbulent flame speed, conditioned upon mixture fraction, can be related to 1D referenced laminar flame speeds. The mechanism of consumption of the stoichiometric surface is examined by considering both displacement speed statistics and by tracking each single edge flame front. In the PRF0 case the results show the stoichiometric surface is consumed mostly by propagating HTC fronts that are almost parallel to it, which is referred to parallel consumption mode, while results in the PRF70 and PRF90 cases show signatures of edge-flame propagation as a secondary mechanism. For edge-flame mode the contribution of tangential-to

Z

diffusion to the displacement speed prevails over that of normal-to-

Z

diffusion. Overall the results demonstrate significant fuel-chemistry effects on the evolution of the ignitions, which will probably translate into significant differences in flame structure in a practical GCI engine.

Novelty and significance statement

This work presents the first DNS of turbulent, nonpremixed autoignition targeting fuel chemistry effects in gasoline compression ignition (GCI) engines. The novelty further arises from two aspects. First, it is the first study to quantitatively model flame displacement speed in autoignitive, stratified mixing layers using the residence time concept. Second, the evolution of edge flame fronts is tracked in complex turbulent flows to enable temporal characterisation of edge flame dynamics and reveal how tangential-to-mixture-fraction diffusion varies across different propagation modes. The significance lies in the implications for practical GCI engine design, as fuel chemistry significantly affects the flame structure, akin to how unravelling the diesel flame structure advanced engine design. These findings also highlight the need to improve practical CFD models, such as incorporating residence time into level-set-based approaches for accurate flame speeds, or characterising conditional fluctuations arising from mixed edge flame modes in flamelet or CMC models.

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汽油压燃发动机非预混点火直接数值模拟

本文采用直接数值模拟（DNS）对汽油压缩点火（GCI）发动机在热化学条件下的点火过程进行了分析。对湍流与点火双混合层的相互作用进行了二维DNS建模。研究了三种不同的主要参考燃料（PRF）混合物，PRF0， PRF70和PRF90，以跨越一系列不同可能的压缩点火场景。研究表明，燃料化学性质对点火过程和向充分燃烧的高温火焰的过渡有显著影响。所有三种情况都表现出扩散支持的冷火焰，其向更丰富的混合物传播的速度比均匀点火延迟的预期快。高温燃烧（HTC）在PRF0的富混合气中开始，在PRF70的富贫混合气中开始，在PRF90的贫贫混合气中开始，这与均匀点火延迟的预期一致。预算分析表明，在所有情况下，HTC火焰都是扩散支持的，因此从点火位置到周围混合物的进展比均匀点火所表明的要快。提出了层状自燃混合物中预混火焰传播速度的定量模型。考虑反应物在火焰表面的归一化停留时间的影响，以混合物分数为条件的条件平均湍流火焰速度可以与一维参考层流火焰速度相关。通过考虑位移速度统计和跟踪每个单边火焰前缘，研究了化学计量表面消耗的机理。在PRF0的情况下，结果表明，化学计量表面主要是通过传播几乎平行于它的HTC前沿被消耗，这被称为平行消耗模式，而在PRF70和PRF90的情况下，结果显示边缘火焰传播作为次要机制的特征。对于边缘火焰模式，切向Z扩散对位移速度的贡献大于法向Z扩散。总体而言，研究结果表明燃料化学对点火演化有显著影响，这可能会在实际GCI发动机中转化为火焰结构的显著差异。本文首次对汽油压缩点火（GCI）发动机中紊流、非预混自燃的燃料化学效应进行了研究。这种新颖性进一步产生于两个方面。首先，本文首次采用停留时间概念定量模拟自燃分层混合层中火焰位移速度。其次，在复杂湍流中跟踪边缘火焰锋面的演变，以实现边缘火焰动力学的时间特征，并揭示切向-混合分数扩散在不同传播模式下的变化。其意义在于对实际GCI发动机设计的启示，因为燃料化学会显著影响火焰结构，类似于揭示柴油火焰结构对发动机设计的影响。这些发现还强调了改进实用CFD模型的必要性，例如将停留时间纳入基于水平集的方法以获得准确的火焰速度，或者在小火焰或CMC模型中描述混合边缘火焰模式产生的条件波动。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Combustion and Flame 工程技术-工程：化工

CiteScore

9.50

自引率

20.50%

发文量

631

审稿时长

3.8 months

期刊介绍： The mission of the journal is to publish high quality work from experimental, theoretical, and computational investigations on the fundamentals of combustion phenomena and closely allied matters. While submissions in all pertinent areas are welcomed, past and recent focus of the journal has been on: Development and validation of reaction kinetics, reduction of reaction mechanisms and modeling of combustion systems, including: Conventional, alternative and surrogate fuels; Pollutants; Particulate and aerosol formation and abatement; Heterogeneous processes. Experimental, theoretical, and computational studies of laminar and turbulent combustion phenomena, including: Premixed and non-premixed flames; Ignition and extinction phenomena; Flame propagation; Flame structure; Instabilities and swirl; Flame spread; Multi-phase reactants. Advances in diagnostic and computational methods in combustion, including: Measurement and simulation of scalar and vector properties; Novel techniques; State-of-the art applications. Fundamental investigations of combustion technologies and systems, including: Internal combustion engines; Gas turbines; Small- and large-scale stationary combustion and power generation; Catalytic combustion; Combustion synthesis; Combustion under extreme conditions; New concepts.