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This study reports the Mach reflection of gaseous detonation on porous wedges experimentally, in which the porous wall is consisted of equidistant inline square columns. The smoked foil technique was utilized to monitor the evolution of the triple-point trajectory and detonation cells in the Mach stem region. In addition to the wedge angle and initial pressure of gaseous mixture, this paper also focuses on the effect of porosity and pore size on the Mach reflection and its mechanism. The results show that the strength of the Mach stem is significantly weakened by the porous media compared with that on the smooth wedge, which is ascribed to the diffraction and reflection waves generated by the interaction of the Mach stem front with the pore. Furthermore, the onset of the triple-point trajectory is delayed, the angle of which is decreased. The porosity and pore size present distinct influence on the Mach stem height. With the increase in the porosity and the decrease in the pore size, the Mach stem region is attenuated more dramatically and the Mach stem is harder to be formed or even could not be observed. Furthermore, the triple-point trajectory on the porous wall exhibits local self-similarity and satisfies the frozen limit in the near field and the equilibrium limit in the far field. However, the lengths of the existence of the frozen limit and the transition to the equilibrium limit on the porous wall are found to be much shorter than the hydrodynamic thickness, and the recovery of the self-similarity depends largely on the porosity and pore size. [ABSTRACT FROM AUTHOR]

Velocity deficits, which exist universally in the process of detonation propagation, were not considered in the problem of detonation Mach reflection in previous studies. Thus, an experimental study of the Mach reflections of gaseous detonations with velocity deficits is reported in this paper. Remarkable velocity deficits were achieved by a porous wall channel. The soot foil technique was utilized to monitor the cellular pattern variation and the evolution of triple-point trajectory. Two detonable mixtures, i.e., 2H 2 +O 2 +3Ar and 2H 2 +O 2 , were used to obtain stable detonation and unstable detonation. The results show that along with velocity deficits, the detonation tends to be unstable and the cell size increases. The induction length must be taken into consideration since the specific heat ratio could be different from that of a Chapman-Jouguet (CJ) detonation. Therefore, three modes of self-similarity were determined, i.e., non-self-similarity with significant velocity deficits (mode 1), global self-similarity with moderate velocity deficits (mode 2) and localized self-similarity with slight velocity deficits (mode 3). The Mach stem height of mode 1 is higher than the non-reactive three-shock theory. The triple-point trajectory of mode 2 corresponds to that of an inert shock. In the case of mode 3 , the well-known frozen and equilibrium limits in detonation Mach reflections can be found. Considering the measured cell size (λ) as the length scale, the required transition distance for the occurrence of the equilibrium limit was found to be approximately (8–10) λ. A threshold value α i =0.95 was found above which the Mach reflection characteristics of a detonation agree well with those of a CJ detonation (α i is the overdrive degree of the incident wave). Below the threshold value, the triple-point trajectory angle decreases linearly with the increase of α i , and the Mach stem overdrive degree is a constant. [ABSTRACT FROM AUTHOR]

• Detonation diffraction through two channels was investigated experimentally. • The effect of the upper channel on the re-initiation process was studied. • The re-initiation was affected by deviation angle, upper channel height and cell size. An experimental study was performed in a rectangular duct to investigate the detonation diffraction process in stoichiometric methane-oxygen mixtures. A wedge was placed in the duct to form two channels between the wedge and the channel walls, i.e., an upper channel and a divergence channel. The upper channel heights were 27 mm, 47 mm and 67 mm, and the deviation angles (α) were 60 ° and 30 °. On the bottom wall and side wall, soot foils were placed to record the cellular structure evolution. For the larger deviation angle (α =60 °), the re-initiation mechanisms can be varied, which are affected by the upper channel height. Two overdriven detonation regions are generated behind the wedge on the bottom wall as well as the side wall for more sensitive mixtures (characterized by smaller detonation cell sizes). However, for insensitive mixtures, the re-initiation process seems to be more complicated. More regions containing finer cells can be observed on the foils. In case of α =30 ° , the detonation re-initiates at the divergence wall and propagates toward the side wall. Universally, the re-initiation is facilitated as the upper channel height increases, and equivalently, the re-initiation distance is reduced. [ABSTRACT FROM AUTHOR]

Abstract In this study, we report an experimental investigation of detonation propagation in a tube filled with bluff bodies. Experiments were carried out in a 6 m long rectangular cross-section (112 mm × 107 mm) tube. Firstly, the effect of an array of cube bodies (60 mm × 60 mm × 60 mm) on the detonation propagation characteristics were studied. Hydrogen, ethylene and acetylene mixed with air and hydrogen-oxygen diluted with argon were used as the test mixtures. Evenly spaced photodiodes were mounted on the top wall to recorded the optical signals, from which the detonation velocity could be determined. Soot foils were adopted in hydrogen-oxygen-argon mixture to record the evolution of the cellular structure. The results show that the flame accelerates rapidly in the obstructed tube. The critical conditions for deflagration to detonation transition (DDT) are found to be consistent with L / λ > 7, where L is the modified characteristic geometrical size for the tube with repeated cubes and λ is the detonation cell size. The long soot foils indicate that the detonation modes are influenced significantly by the bluff bodies. Near the limits, only some traces of transverse waves and shock waves can be observed, making the distinction between choked flame and quasi-detonation blurred. Secondly, the effect of a single bluff body on the detonation diffraction was investigated. Stoichiometric hydrogen-oxygen and those diluted with argon at sub-atmospheric pressures were used as the test mixtures. In the vicinity of the bluff body, a soot foil was used to record the detonation cellular structure evolution, from which the effect of the geometry of the bluff body on the diffraction and re-initiation processes was studied. Three transition regimes are observed: (1) a detonation propagates continuously over the bluff body; (2) the cellular structure initially fails and then recover due to the two symmetrical diffraction waves collision or interaction between shock waves and the bottom wall; (3) a detonation quenches and then re-initiate by flame acceleration. The re-initiation distance was found to be dependent on mixture sensitivity (chemical length scale) and the geometry of the bluff bodies (physical length scale). [ABSTRACT FROM AUTHOR]

Highlights • The effects of different hollow plates on 2H 2 + O 2 detonation behaviors were studied. • The effective diameter is an appropriate indicator for the characterization of the limits. • Long soot foils were adopted to study the cellular pattern evolution. • The re-initiation processes of 2H 2 + O 2 detonation were analyzed. Abstract In this study, experiments were carried out in a circular tube (5300 mm long, 48 mm inner-diameter) filled with orifice plates to investigate the detonation behaviors of stoichiometric hydrogen-oxygen mixture. Two kinds of orifice plates, i.e., round orifice plates and square orifice plates were used to generate obstacle configurations with various spacings. The blockage ratios of these orifice plates are 0.56. Evenly spaced photodiodes were mounted on the tube wall to record the detonation velocity. Long soot foils were adopted to study the evolution of the cellular structure. The detonation propagation limits (also the quasi-detonation limits) were found to be in agreement with d eff /λ > 1, where d eff and λ are the effective diameter of the orifice and the detonation cell size. The measured velocity as well as the propagation limits are independent of the opening geometry as long as the effective diameters are identical. Soot foils obtained with various sensitivity (or initial pressure) exhibit that the re-initiation of detonation occurs via a variety of modes. Well within the limits, the detonation could recover via the generation of hot spots. In some cases, a detonation can propagate without the formation of hot spots. Near the limits, only a few transverse wave traces or streaks can be observed, indicating that the propagation mechanisms are changed. [ABSTRACT FROM AUTHOR]

The present study reports the detonation behaviors of syngas-oxygen mixtures in three 2 m long tubes, including two circular tubes ( D = 32 mm and 18.5 mm, D is the inner diameter) and a square tube ( H = 32 mm, H is the inner side). Three stoichiometric syngas-oxygen mixtures, i.e., CO+H 2 +O 2 , 2CO+H 2 +1.5O 2 , 3CO+H 2 +2O 2 were used. Evenly spaced photodiodes were used to determine the detonation velocity while the soot foil technique was adopted to record the cellular structure. The results indicate that well with the limits, the detonation propagates at a steady velocity close to the Chapman-Jouguet value. The velocity deficits are more prominent at decreased initial pressure (equivalently, larger cell size and less sensitivity). At the limiting pressure, the velocity deficits of three mixtures in various tubes are approximate 14%–17%. The experimental velocity deficits were compared with a modified model based on Fay's theory, in which the cell size as well as the corresponding cell length are measured. The experimental velocity deficit is in excellent agreement with the theoretical prediction. The effective diameter, D e , rather than the hydraulic diameter, is found to be a more appropriate parameter for the characterization of the detonation velocity in both round and square tubes. Further, a linear correlation between the normalized detonation velocity and ( D e · p 0 ) − 1 is observed. The cellular structure shows that the single-headed spin occurs in all tubes when the detonation limits are approached. [ABSTRACT FROM AUTHOR]

Deflagration to detonation (DDT) experiments with stoichiometric hydrogen-methane-air mixtures at ambient pressure (1 atm) and temperature (293 K) were carried out in a 112 × 112 mm by 6 m long tube with various obstacle configurations. The hydrogen fraction in fuel mixtures ranged from 0 to 1. Two types square orifice plates with opening sides of 86.8 mm and 70.8 mm (blockage ratios of 0.4 and 0.6), spaced at 1, 2, 3 and 4 times the tube height were employed. Flame signals were detected by photodiodes spaced evenly, from which the average velocity could determined. The detonation cell size was obtained using soot foil technique. CJ detonations can still propagate in less sensitive mixtures with larger detonation cell sizes. At the DDT limits, the ratio of the height of the orifice plate (d) and the detonation cell size (λ) increases with the increase of the blockage ratio and the decrease of obstacle spacing. This indicate that in a square tube with square orifice plates, the detonation initiation process as well as the detonation propagation mechanism govern the DDT limits. [ABSTRACT FROM AUTHOR]

Explosions can occur in different oxidizer atmospheres in confined spaces, which should be given attention in regard to the fire safety in process industries. In this study, experiments were carried out in a closed cylindrical vessel (7.3 L) to compare the explosion behaviors of hydrogen (H 2) and ethylene (C 2 H 4) with air, nitrous oxide (N 2 O), and oxygen (O 2) at various equivalence ratios. The initial pressures of the hydrogen and ethylene mixtures were 50 kPa and 30 kPa, respectively. Explosion indices such as the maximum explosion pressure (P m a x ), maximum rate of pressure rise ( (d p / d t) m a x ) and deflagration index (K G) were determined experimentally. The results showed that the pressure evolutions obtained with N 2 O and O 2 exhibited strong oscillations due to flame instabilities and cellular structures. The maximum explosion pressure increased in the order of air, O 2 and N 2 O while the maximum rate of pressure rise increased in the order of air, N 2 O and O 2. Unlike those of air and O 2 , the maximum values of (d p / d t) m a x obtained with N 2 O were found to be at the fuel-lean side. This indicated that the instability feature of N 2 O supported combustions at the fuel-lean side. Compared with air, the maximum explosion pressure obtained in N 2 O and O 2 atmospheres could be very close to the adiabatic pressure when considering no heat loss during an explosion. Therefore, there was a flame acceleration and deflagration to detonation transition (DDT) mechanism in the explosions with N 2 O and O 2. Additionally, two dimensionless pressure rise coefficients were derived and discussed. The flames in very lean H 2 -N 2 O were very unstable, resulting in scattered coefficients as a function of equivalence ratio. At the fuel-rich side, the dimensionless coefficients were very close. In practice, the potential DDT mechanism in the explosions, especially in N 2 O and O 2 atmospheres, needed to be considered. [ABSTRACT FROM AUTHOR]

• Effect of a perforated plate on the explosion indices of syngas-air was discussed. • The heat loss during an explosion to the perforated plate was derived. • The dual effects of the perforated plate on some explosion indices were analyzed. • The turbulence enhancement induced by an obstacle was evaluated. Explosions may occur in the presence of obstacles, which was not given enough considerations. In this study, an experimental study was conducted to investigate the influence of a perforated plate on the explosion characteristics of syngas-air mixtures in a constant volume vessel. The syngas, containing 50% H 2 and 50% CO, was selected as the test fuel. The maximum explosion pressure, heat loss to the vessel wall as well as the perforated plate, explosion time and deflagration index were examined. The turbulence enhancement factor, which was defined as the amplification of the burning velocity when a perforated plate was introduced, was also evaluated. The maximum explosion pressure was always decreased with a perforated plate due to the heat transferred to the obstacle. In addition, the maximum explosion pressure decreased as the distance between the obstacle and ignitor decreased. The heat loss to unit area of the obstacle was larger than that of the vessel walls. When a perforated plate existed, the explosion time was decreased. The maximum pressure rise rate could be affected by the obstacle in both positive and negative ways, depending on the blockage ratio and location of the perforated plate. Lastly, the turbulence enhancement factor was found to be up to 3 with a 0.99 blockage ratio. [ABSTRACT FROM AUTHOR]

• Effect of an orifice plate on the explosion behaviors of H 2 /CH 4 /air was studied. • The maximum explosion pressure was decreased by an orifice plate. • The obstacle played a dual role in the maximum pressure rise rate. We reported an experimental study on the explosion characteristics of hydrogen-methane-air mixtures in a closed vessel at normal pressure and temperature. The explosion parameters, such as the maximum explosion pressure, the explosion time, the maximum pressure rise rate and the deflagration index were derived based on the pressure-time evolution. The effect of the blockage ratio as well as the position of the orifice plate on these explosion parameters were analyzed. The results show that the maximum explosion pressure was decreased in the presence of an orifice plate due to the heat transfer to the baffle. At the early stage of the flame propagation, the flame was suppressed. Subsequently, the flame speed was increased at the later stage in some cases. Combined with the various reactivities of the mixtures, the explosion times obtained in all cases were very close. For a given mixture and an orifice plate position, the maximum pressure rise rate and the deflagration index were increased with the increase of blockage ratio. Besides, the maximum pressure rise rate as well as the deflagration index were decreased when the orifice plate was close to the ignitor and oppositely, increased when the orifice plate was away from the ignitor. [ABSTRACT FROM AUTHOR]

• The detonation behaviors in annular tubes with circular obstacles was investigated experimentally. • Smoked foil technique was adopted to record the cellular pattern evolution. • The diffraction-retonation of detonation wave was analyzed in detail. • Special vertical bands was observed, and the formation mechanism of which was analyzed. Experiments were performed to investigate the detonation behaviors of stoichiometric hydrogen-oxygen mixtures (at sub-atmospherical pressures) in annular tubes with circular obstacles. The annular tube was 1000 mm long and 48 mm in diameter (D). The circular obstacles were with the diameters of 37.20 mm and 30.36 mm, corresponding to the BR = 0.57, 0.36 (BR : blockage ratio), respectively. Two types of obstacle spacing S was set in the test, which are S = D and S = 2 D. The detonation velocities were obtained based on the time of arrival of the wave front monitored by pressure transducers. Smoked foils were used to register the detonation cellular evolution though the test section. The results showed that the propagation limit in this test was 2 ω / λ ≈ 1 (ω indicates the channel width, λ represents the cell width), which was consistent with the previous results in round tubes with orifice plates. The obstacle spacing presented no noticeable effect on detonation propagation limit. However, it played a significant role on detonation velocity. What is more, larger flow blockage for BR = 0.57 caused the more violent velocity fluctuation and higher propagation limit than in smaller blocking ratio. The process of diffraction-retonation for interaction between wave front and obstacle was analyzed in detail. In further, special vertical bands with periodic variation characteristic were observed between two adjacent obstacles. The vertical bands were spaced as same as the obstacle spacing and attached very fine cells which were caused by the head-on collision of the detonation wave propagating in the compressed region with high temperature and pressure. [ABSTRACT FROM AUTHOR]

• The detonation propagation in convergent annular channels is investigated experimentally. • Smoked foil technique was adopted to study the cellular pattern evolution. • Detonation velocity presenting different variation in three convergent channels were observed. • The influencing mechanisms of viscous boundary layer and Mach reflection were explored. Methane (CH 4) is an environmental-friendly fuel with a wide range of applications. The explosion and detonation hazards with respect to methane in tubes with different shapes should be paid in special attention. In this study, experiments were performed to investigate the propagation characteristics of detonation waves in annular channels with convergent cross-sections in stoichiometric methane-oxygen mixture at sub-atmospherical pressures (10 kPa-40 kPa). The annular channels with convergent cross-sections were assembled by inserting 0.5-m long cones into a 48-mm circular tube. Three conicities (d end / L) were employed, i.e., d end / L = 0.048, 0.072, 0.096, in which d end and L represent the bottom diameter and the length of the cone, respectively. The detonation velocity was obtained based on the time of arrival of the wave front. Smoked foils were used to register the detonation cellular evolution in the annular channels. The results show that the convergence of the channel has a great impact on the detonation velocity and cell sizes. Depending on the conicity, the detonation velocity and cellular structure exhibit different behaviors. For the smallest conicity, the continuous velocity deficit can be observed after detonation propagating into the annular tube. However, the characteristic of detonation velocity evolution shows distinct with the increase of the conicity, which is governed by the chemical scale (the detonation cell size) as well as the physical geometry (the channel width). The interaction between Mach reflection, boundary layer effect and the wall can be served as the dominating reason for the velocity behavior. [ABSTRACT FROM AUTHOR]