The effect of channel size and equivalence ratio of ethylene/oxygen mixtures on detonation velocity deficits were experimentally investigated in miniature square channels in this paper. The premix mixtures were spark ignited under ambient pressure and temperature, and detonation waves were formed through deflagration-to-detonation transition (DDT) process in the smooth and straight channel in which no cross-sectional area change existed. The reaction wave in the small channels went through flame acceleration, detonation transition, and overdriven detonation before stably propagated along the chanel at a value lower than the Chapman-Jouguet velocity. As the channel size decreased from 1.2 mm to 0.6 mm, both DDT time and DDT distance reduced while the detonation velocity deficit increased. In the 0.8 mm channel, the propagation speed of detonation wave dropped after a certain distance after detonation transition, and then fluctuated around a new value. Low-speed detonation mode appeared when the channel size was 0.6 mm. Moreover, the detonation velocity deficit gradually increased with the decrease of equivalence ratio within the test range. Non-dimensional detonation velocities V/V_(CJ) becomes invariant as non-dimensional number D_H/λ is larger than 1.4. But the deficit is not only a function of D_H/λ for near-limit conditions when the channel dimension approaches or becomes smaller than the cell size.
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