End-gas autoignition and detonation in confined space

End-gas autoignition, especially with detonation development in a confined space, is a complex physical phenomenon, including premixed flame dynamics, fluid dynamics, autoignition chemistry etc., which is generally considered as the origin of knock and super-knock in internal combustion (IC) engines. Furthermore, the mechanism for detonation initiation is also related to fire safety and industrial disasters. Thus, this review focuses on the recent progress made in the fundamental understanding of the mechanisms of end-gas autoignition phenomena along with detonation combustion in confined spaces through theoretical analyses, optical diagnostics, and high-resolution numerical simulations, with emphasis on the effects of crucial physicochemical factors on the two stages of end-gas autoignition, namely autoignition occurrence and autoignition propagation. Firstly, two basic theories, namely Livengood–Wu (L–W) integral and the reactivity gradient theory, which provide theoretical foundations for understanding autoignition occurrence and autoignition propagation, respectively, are demonstrated. Specially, applications and limitations of L-W integral and the extension of Bradley's diagram to multi-dimensional conditions closer to actual circumstances are elaborated. Then, a comprehensive investigation of several pivotal physicochemical factors involved in end-gas autoignition and detonation development in confined spaces, are conducted, including flame propagation, pressure wave, inhomogeneity, turbulence, chemical reactivity and thermodynamic conditions. The results indicate that, three essential elements are included in end-gas autoignition, namely flame, pressure wave, and autoignition. The flame-pressure interaction induced end-gas autoignition and detonation can be divided into three processes: I-reactivity increase, II-critical and sensitive state, and III-coupling and detonation. The first two processes account for autoignition occurrence and the third accounts for autoignition propagation. As to autoignition occurrence, increasing turbulence flame speed can inhibit end-gas autoignition under weak pressure wave conditions, whereas it can promote end-gas autoignition under strong pressure wave conditions. As to autoignition propagation, various combustion modes can originate from a reactivity gradient induced by temperature, composition, additive, as well as a cold spot within negative temperature coefficient (NTC) region, while the existence of low-temperature chemistry (LTC) and multi-stage ignition complicates autoignition propagation. The results further indicate that an inhomogeneous field with a small characteristic length scale, and an inhomogeneous field with a large characteristic length scale but coupled with the turbulence with a small characteristic length scale and a sufficiently large turbulent velocity fluctuation, can both weaken detonation propensity. Furthermore, the fuel type, diluent gas, and thermodynamic conditions can affect both autoignition occurrence and autoignition propagation. However, the effects of fuel type and energy density on autoignition propagation may not be completely explained by Bradley's diagram, and should be considered separately. Lastly, the review discusses the stochasticity of end-gas autoignition and the similarity and difference of detonation in IC engines from deflagration to detonation transition (DDT) in ducts, and provides inspirations obtained from the present fundamental studies for the engineering applications and future prospects.