Transmission of near-limit detonation wave through a planar sudden expansion in a narrow channel

Transmission of single-cell and spinning detonation waves in C ₂H ₄+3(O ₂+βN ₂) mixtures through a 2-D sudden expansion experimentally studied using high-speed cinematography and soot film visualization. Nitrogen dilution ratio, β, is utilized to control cell size and detonation mode. Detonation wave of ethylene/oxygen/nitrogen mixture was initiated via DDT in the 1mm×1mm cross-section and 250mm long initiator channel before propagating into the 3mm×1mm receptor channel. Visualizations show that detonation waves were extinct and accompanied with abrupt decrease in visible reaction front propagating velocities right after passing through the sudden expansion. However, re-acceleration of the reaction front and re-initiation of the detonation wave were observed downstream in the expanded receptor section. Two re-initiation modes with large disparity in the re-initiation distance were experimentally characterized. For mixtures with nitrogen dilution ratio equals 0.3 or less, the cellular detonation front propagated with single cell in the initiator section before entering the sudden expansion. The re-initiation distance was less than 50mm and was likely achieved via shock reflection. Velocity characterization shows that steady propagating speed of the detonation wave is ∼100m/s higher in receptor section than in the initiator section. Since the cell size became larger than 1mm for mixtures with β≥0.3, the detonation wave propagated in spinning detonation mode before transmitting into the expanded section. The reaction front would have to go through another DDT process to reach detonation state in the receptor section, and the re-initiation distance was increased to more than 150mm. Moreover, step height of the sudden expansion was proposed as the characteristic length scale to obtain a unified non-dimensional correlation between re-initiation distance and detonation cell size.