Characteristics of unstable detonations near extinction limits

2015

Combustion and Flame

a b s t r a c tSome features of galloping detonations near the detonation limits are discussed in this paper. The experimental results previously reported in , together with additional data obtained in this study for six explosive mixtures with different reaction sensitivities, each in five different diameter tubes, are analyzed in detail. It is established that galloping detonations do not occur in highly argondiluted, stable mixtures. Only in unstable mixtures, susceptible to flow fluctuations by thermo-chemical instability, when using in small tube diameters (i.e., D 6 12.7 mm), could galloping detonations be observed. For the largest tube diameter D = 50.8 mm, the detonation wave fails completely when the initial pressure approaches the detonation limit even for all unstable mixtures. In addition, the present study shows that the initial pressure range for the occurrence of galloping detonations decreases rapidly with increasing tube diameter. A collection of existing data on galloping detonations indicates that the wavelength L of one galloping cycle is on average about 350D within experimental variations.Nonetheless, there appears to be some minor decreasing trend with increasing tube diameter (i.e., D P 12.7 mm), or with increasing detonation instability of the explosive mixture. The amplitude of the galloping cycle is also investigated and the upper velocity values show more fluctuations than the lower ones. The upper and lower values of the velocity in the galloping cycle at different conditions vary from V CJ to 1.5V CJ and 0.3V CJ to 0.65V CJ , respectively. To illustrate the role of flow instability on galloping detonations, experiments are performed with a spiral inserted into the 12.7-mm diameter tube to generate perturbations artificially. Detonation waves are observed to re-initiate quickly after passing the spiral and another cycle of galloping detonation was formed in the remaining part of the tube.

Section: Wavelength and Amplitude Of The Galloping Cyclesupporting

confidence: 93%

Section: Wavelength and Amplitude Of The Galloping Cyclesupporting

confidence: 93%

Section: Introductionsupporting

confidence: 66%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Experimental characterization of galloping detonations in unstable mixtures

2015

Combustion and Flame

“…The above mentioned characteristics are typical near-limit phenomena. Different detonable mixtures and tube geometries exhibit slightly different nearlimit behavior [7][8][9][10][11][12][13][14]. To understand the limit phenomena, an extensive study covering a spectrum of mixture properties and tube geometries is required.…”

Section: Introductionmentioning

confidence: 99%

Near-limit propagation of gaseous detonations in narrow annular channels

2016

Shock Waves

New results on the near-limit behaviors of gaseous detonations in thin annular channels are reported in this paper. The annular channels of thicknesses 3.2 mm and 5.9 mm are made by inserting smaller diameter tubes of 44.4 mm and 39.0 mm into a larger 50.8-mm diameter outer tube. The length of each annular channel is 2.0 m. Detonations are initiated in a steel driver tube where a small volume of a sensitive C2H2 + 2.5O2 mixture is used to facilitate detonation initiation. A 2-m length of circular tube with 50.8-mm diameter precedes the annular channel so that a steady Chapman-Jouguet (CJ) detonation is established prior to entering the annular channel. Four detonable mixtures of C2H2 + 2.5O2 + 85%Ar, C2H2 + 2.5O2 + 70%Ar, C3H8 + 5O2 and CH4 + 2O2 are used in the present study. Photodiodes spaced at 10 cm throughout the length of both annular channel and circular tube are used to measure the detonation velocity. In addition, smoked foils are inserted into the annular channel to monitor the cellular structure of the detonation wave. The results show that well within the limits the detonation wave propagates along the channel with a small local velocity fluctuation and an averaged global velocity can be deduced. The averaged detonation velocity has a small deficit of 5 -15% far from the limits and rapidly drops to 0.7VCJ -0.8VCJ when the onset of limit occurs. Subsequently, the fluctuation of local velocity also increases with decreasing initial pressure approaching the limit. No galloping detonations are observed for both stable and unstable mixtures in annular channels. The present study also confirms that single-headed spinning detonation occurs at the limit as in circular tube rather than the up and down "zig zag" mode in a two-dimensional rectangular channel.

Minimum tube diameters for steady propagation of gaseous detonations

2014

Shock Waves

Recent experimental results on detonation limits are reported in this paper. A parametric study was carried out to determine the minimum tube diameters for steady detonation propagation in five different hydrocarbon fuel-oxygen combustible mixtures and in five polycarbonate test tube diameters ranging from 50 mm down to a small scale of 1.5 mm. The wave propagation in the tube was monitored by optical fibers. By decreasing the initial pressure, hence the sensitivity of the mixture, the onset of limits is indicated by an abrupt drop in the steady detonation velocity after a short distance of travel. From the measured wave velocities inside the test tube, the critical pressure corresponding to the limit and the minimum tube diameters for the propagation of the detonation can be obtained. The present experimental results are in good agreement with previous studies and show that the measured minimum tube diameters can be reasonably estimated on the basis of the λ/3 rule over a wide range of conditions, where λ is the detonation cell size. These new data shall be useful for safety assessment in process industries and in developing and validating models for detonation limits.