New results on the near-limit behaviors of gaseous detonations in narrow annular channels are reported in this paper. Annular channels of widths 3.2 and 5.9 mm were made using circular inserts in a 50.8 mm-diameter external tube. The length of each annular channel was 1.8 m. Detonations were initiated in a steel driver tube where a small volume of a sensitive C2H2+2.5O2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {C}_{2}\hbox {H}_{2}+ 2.5\hbox {O}_{2}$$\end{document} mixture was injected to facilitate detonation initiation. A 2 m length of circular tube with a 50.8 mm diameter preceded the annular channel so that a steady Chapman-Jouguet (CJ) detonation was established prior to entering the annular channel. Four detonable mixtures of C2H2+2.5O2+85%Ar,C2H2+2.5O2+70%Ar\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {C}_{2}\hbox {H}_{2} \,{+}\, 2.5\hbox {O}_{2}\,{+}\, 85 \ \% \hbox { Ar},\, \hbox {C}_{2}\hbox {H}_{2} \,{+}\, 2.5\hbox {O}_{2}\,{+}\, 70 \ \% \hbox {Ar}$$\end{document}, C3H8+5O2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {C}_{3}\hbox {H}_{8}\,{+}\,5\hbox {O}_{2}$$\end{document}, and CH4+2O2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {CH}_{4} \,{+}\, 2\hbox {O}_{2}$$\end{document} were used in the present study. Photodiodes spaced 10 cm throughout the length of both the annular channel and circular tube were used to measure the detonation velocity. In addition, smoked foils were inserted into the annular channel to monitor the cellular structure of the detonation wave. The results show that, well within the detonability limits, the detonation wave propagates along the channel with a small local velocity fluctuation and an average global velocity can be deduced. The average detonation velocity has a small deficit of 5–15 % far from the limits and the velocity rapidly decreases to 0.7VCJ\documentclass[12pt]{minimal}
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\begin{document}$$0.7V_{\mathrm{CJ}}$$\end{document}–0.8VCJ\documentclass[12pt]{minimal}
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\begin{document}$$0.8V_{\mathrm{CJ}}$$\end{document} when the detonation propagates near the limit. Subsequently, the fluctuation of local velocity also increases as the decreasing initial pressure approaches the limit. In the two annular channels used in this work, no galloping detonations were observed for both the stable and unstable mixtures tested. The present study also confirms that single-headed spinning detonation occurs at the limit, as in a circular tube, rather than the up and down “zig zag” mode in a two-dimensional, rectangular channel.
