Turbulent flame–shock interaction inducing end-gas autoignition in a confined space
Introduction
Deflagration to detonation transition (DDT) is a fundamental physical phenomenon that is widely present in mining operations, astrophysics and cosmology [1], [2], and engines, where it is a forefront topic in combustion theory. Controlled detonation initiation in some propulsion systems could revolutionize transportation, e.g., in pulse detonation engines (PDEs), whereas uncontrolled detonations can destroy facilities and result in disaster [3], [4], [5], [6], [7], e.g., gas explosions in mining operations and knock in downsized spark ignited engines. In particular, super-knock in engines can result in a very strong pressure oscillation with a high peak pressure (>300 bar), which can be destructive to engines. Di Benedetto et al. [3] attributed the oscillatory pressure-time histories and very high pressure peaks to the occurrence of the so-called “combustion-induced rapid phase transition.” Robert et al. [5] pointed out that super-knock is accompanied by the phenomenon of detonation by large eddy simulation. Qi et al. [6] verified that the autoignition and detonation phenomena exist in the end region of a combustion chamber under gasoline engine-like conditions when super-knock occurs. However, the mechanism of flame–shock interaction in a confined space was not observed adequately. Therefore, Wei et al. [8] observed the end-gas autoignition induced by the combined effect of two successive forward shock waves in a newly designed constant volume combustion chamber equipped with a perforated plate. In addition, a quasi-detonation generated by the end-gas autoignition was observed.
The aim of this brief communication is to report a new experimental result on the effect of turbulent flame–shock interaction on end-gas autoignition with a detonation wave as well as pressure oscillation in a confined space. The present work was carried out in an improved constant volume chamber, and the entire detonation occurrence process was observed. Although a stoichiometric hydrogen/air mixture was used, the fundamental mechanism of flame–shock interaction on the end-gas autoignition obtained by the present study may aid in understanding the underlying mechanism of super-knock in real engines. In addition, hydrogen has been proposed as a possible fuel for future internal combustion engines and can be produced from renewable sources. Hydrogen's wide flammability range allows higher engine efficiency than that of conventional fuels, with both reduced toxic emissions and no CO2 gases. However, knock remains the main hurdle in achieving better performance in hydrogen fueled spark ignited engines [9]. In the present work, a detonation initiation mode induced from the end gas autoignition was observed, which may benefit numerical model validation under similar schemes. And the present study may provide new insight for the science of combustion in a confined space, such as for the knocking phenomenon in downsized spark ignited engines and explosion disasters.
Section snippets
Experimental apparatus and procedure
Experiments were carried out in a constant volume combustion bomb (CVCB), which is outlined schematically in Fig. 1. This experimental apparatus is composed of a constant volume combustion vessel, a perforated plate, an intake and exhaust pipe system, an ignition system, a heating system, a time synchronizing system, an image acquisition system, and a cylinder pressure acquisition system. Note that the present experimental apparatus has been improved to observe a wider range of flame
Results and discussion
Figure 2 shows the formation and development of end-gas autoignition with detonation development using a stoichiometric hydrogen–air mixture under an initial pressure of 5 bar. Detailed discussions on the evolution of flame and shock wave propagation in the constant combustion chamber with a perforated plate can be found in previous studies [8], [10], [11]. According to the flame propagation velocity and morphology, the entire flame propagation is divided into three stages, which are the
Conclusion
In the present study, an experiment was conducted in a self-designed constant volume chamber to investigate turbulent flame–shock wave interactions and end-gas autoignition. Two main conclusions are drawn from this study.
- (1)
The shock wave oscillates in the combustion chamber and compresses the end gas several times. When the shock wave suffers a second reflection from the end wall, a detonation wave is induced with a velocity of 1987.1 m/s, which is close to the C–J velocity.
- (2)
An extremely high peak
Acknowledgments
This work was supported by the National Science Fund for Distinguished Young Scholars (Grant No. 51825603) and the National Natural Science Foundation of China (Grant Nos. 91641203 and 51606133). This paper is supported by the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology). The opening project number is KFJJ18-09 M.
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