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Due to the low chemical activity and large detonation cell size, methane-air continuous rotating detonation (CRD) is hard to be realized in a traditional small annular chamber without extra additives. In order to investigate the characteristics of methane-air CRD wave in a small chamber, three hollow chambers with Laval nozzles are adopted in this research. The methane-air CRD can be realized in a quite small chamber (diameter of 80 mm) under very particular condition. The operating range enlarges with the increase of chamber diameter. The CRD wave propagation frequencies vary apparently with chamber diameter while the propagation velocities stay in a quite narrow range of 1744–1812 m/s, and the relative deviation is within 4%. The CRD wave propagation process is very stable, and the propagation velocity accounts for 99% of the theoretical C-J velocity in the chosen test. Optical observation results illustrate that the reduction in chamber diameter or increase in nozzle contraction ratio will lead to CRD deterioration. By comparing the frequencies of high-frequency pressure and optical observations, the coupling of inducing shock wave and combustion flame is proved quantitatively. • Realizing methane-air continuous rotating detonation (CRD) in a small hollow chamber with diameter of 80 mm. • Analyzing the influences of chamber diameter on CRD realization and propagation characteristics. • Analyzing the flame structure in different chambers though optical observation. • Verifying the couple of inducing shock wave and rotating flame quantitatively. [ABSTRACT FROM AUTHOR]

Methane-air Continuous Rotating Detonation (CRD) has been firstly achieved in this paper in the hollow chamber with a Laval nozzle, and the diameter of the chamber is just 100 mm. The contraction ratio of the Laval nozzle is a key factor for the CRD realization. CRD can only be obtained when the contraction ratio is no less than 4, but its ER operating range decreases in the increase of contraction ratio from 4 to 10. For all the success cases, the average propagation frequency and velocity are in the ranges of 5.32–5.65 kHz and 1670.48–1774.10 m/s, respectively, and the velocity deficits are less than 10%. Based on the high-speed photography images, the approach of chemiluminescence intensity integral is proposed in this paper, and the propagation characteristics of the flame are analyzed quantitatively. The propagation velocities of the flame and shock wave are agreed well with each other, indicating that the typical feature of detonation wave, i.e., the coupling of the flame and the shock wave, is verified quantitatively. • Methane-air CRD is firstly achieved without additional oxygen and hydrogen. • Realization is in the small hollow chamber with the diameter of 100 mm. • The average velocity of success cases is in the range of 1670.48–1774.10 m/s. • Chemiluminescence intensity integral is proposed as a new image processing method. • The velocity coupling of flame and shock wave is verified quantitatively. [ABSTRACT FROM AUTHOR]