On the deflagration-to-detonation transition in narrow tube with varying prechamber-initiator

A process of deflagration to detonation transition in propane-butane-oxygen and acetylene-oxygen mixtures in an open channel of circular cross section with a diameter of 3 mm and a length 500 mm was investigated experimentally.

Prechambers of a bigger diameter are usually used both as a method of initiation of a steady detonation for studies of the detonation propagation in the connected “narrow” channel [1] and for additional acceleration of the flame front [2]. Prechamber initiation acquires the relevance at the investigation of dynamics of flame front and detonation in the narrow channels. The critical conditions for the onset of detonation and the conditions for the propagation of the detonation wave in narrow channels have been determined [3,4]. Influence of a hydraulic resistance, boundary layer, friction and energy losses have been investigated [5]. Various flame propagating scenarios including DDT, galloping, and low speed detonation have been investigated [6]. It was written in details in [7] that duration of energy release of ignition should be taken into account for evaluation of the energy of direct initiation of detonation. Particular attention should be paid to the work [8], where the transition of a laminar flame from the prechamber into the narrow channel was studied.

The aim of this study was also to determine the pre-detonation distance in propane-butane-oxygen and acetylene-oxygen mixtures in channels. The influence of the prechamber parameters and mixture composition on the dynamics of the flame front along the channel was investigated.

The diameter of the prechamber in our investigations was 10, 16 or 20 mm. Prechamber length was varied in a range 7-34 mm. Diameter of the channel was 3 mm, length was 500 mm. The energy released in the spark gap (0.1 J) was by 2-3 orders less than the energy released during the combustion of the gas mixture in the prechamber. The measuring system consisted of 12 photodiodes FD-256 with temporal resolution less than 1 μs. To determine the boundary conditions at the entrance to the narrow channel just after the prechamber, a piezoelectric pressure transducer PCB 113A was mounted at the distance 30 mm from the channel entrance.

Depending on the size of the prechamber the dynamics of the flame front and shock waves in the channel can develop in different ways: transition from deflagration to detonation by flow of the combustion products from prechamber into narrow channel (Figure 1a), the formation of detonation due to compression waves (Figure 1b) or direct detonation initiation (Figure 1c).

Figure 1. Readings of pressure transducer (P/P₀) and photodiode (I) at the position 10 tube diameters from the channel entrance, and evolution of the velocity of the flame front along the axis of the channel (bottom figure). Prechamber diameter/length: a, 10 mm/7 mm; b, 20 mm/8 mm; c, 10 mm/20 mm; 1 – increase in pressure, 2 – flame front, 3 – first peak of pressure, 4 – pre-detonation distance, 5 – second peak of pressure, 6 – monotonous increase in pressure due to combustion, 7 – detonation wave.

Influence of equivalence ratio (ER) of mixture on the pressure profile and evolution of the flame front along the axis of the channel was investigated. For each specific configuration of the prechamber the equivalence ratio of a gas mixture can be determine, at which the dynamics of the flame front and the shock waves in the channel can occur by one of the above three scenarios.

The deflagration to detonation transition in channel in propane-butane-oxygen and acetylene-oxygen mixtures was studied for all scenarios. Figure 2 shows the dependence of DDT distance in poor propane-butane-oxygen mixture and acetylene-oxygen mixture on the ER for the scenario of flow of the combustion products from prechamber into narrow channel. For the same ER DDT distance in acetylene-oxygen mixtures is 30-40% less than in propane-butane-oxygen mixtures.

Figure 2. Pre-detonation distances for propane-butane-oxygen mixture and acetylene-oxygen mixture in dependence on ER.

Acknowledgments

The work was supported by RFBR-13-08-01227-a and by Presidium of RAS, program “Combustion and explosion”.

References

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