Detonation Induction Distance Research Articles

Deflagration-to-detonation transition and the detonation induction distance estimation

Deflagration-to-detonation transition and the detonation induction distance estimation

Mechanism of deflagration-to-detonation transitions above repeated obstacles

Experiments are carried out to investigate the mechanism of the deflagration-to-detonation transition (DDT). Because, this mechanism has relevance to safety issues in industries, where combustible premixed gases are in general use. A stoichiometric gas of oxygen and hydrogen (oxy-hydrogen) is ignited in a tube, repeated obstacles are installed, and the DDT behaviours are visualized using a high-speed video camera. The pitch and height of the repeated obstacles and the initial pressure of the oxy-hydrogen premixed gas are varied in an attempt to obtain the optimum conditions that cause DDT a short distance from the ignition source. The experiments identified DDT as being essentially caused by one of the following mechanisms: (1) A deflagration wave is accelerated in terms of a vortex, which is generated behind the obstacle, and the flame acceleration induces a secondary shock wave. Eventually, the shock–flame interaction ahead of the obstacle causes DDT via a very strong local explosion. (2) Each shock wave generated by relatively weak local explosions between the obstacles is not sufficient to cause DDT directly, but DDT results from an accumulation of shock waves. The detonation induction distance is also examined, taking into account the physical and chemical parameters of the obstacles and the oxy-hydrogen premixed gas.

Detonation formation in H $_2$ -O $_2$ /He/Ar mixtures at elevated initial pressures

In this paper the formation of detonation in H\(_2\)-O\(_2\)/He/Ar mixtures at elevated initial pressures was investigated in an initiation tube for a detonation driver with an exploding wire as the ignition source. In most experiments the detonation wave was formed by a DDT process in which a reactive shock wave accelerates behind the leading shock wave and eventually leads to the onset of detonation. The onset position was found to be at the leading shock wave or behind it. Only in very sensitive mixtures at high initial pressure the direct initiation of detonation was observed. The influence of ignition energy, initial pressure and composition on the detonation induction distance was determined. The results show that the detonation induction distance increases with the decrease of ignition energy and initial pressure and with the increase of the mole fraction of helium or argon. With the same mole fraction, argon increases the induction distance more than helium. In the facility utilized the DDT upper and lower limits of hydrogen in H\(_2\)-O\(_2\) mixtures are in the ranges from 36 to 40 % and from 78 to 82 %, respectively, and the upper limits for helium and argon in stoichiometric H\(_2\)-O\(_2\) mixtures are 40 % and 36 %, respectively. High pressure peaks generated by the DDT process were measured, especially in mixtures near the DDT limits. Statistical results show that such peak pressures can be up to 6 times of the CJ-pressures.

Flame velocity for the onset of detonation

Of interest in this study was the collision of a flame with a shock wave emitted from the flame itself and propagated in the opposite direction to the flame after reflection at the end of a combustion tube. The transition to detonation occurred at a critical initial pressure for various tube lengths. It did not begin below a certain limit of the flame velocity at the collision point and it occurred with no stimulus from the reflected shock wave beyond a critical flame velocity. Relations between the detonation induction distance and the minimum flame velocity leading to the detonation and relations between the flame velocity at the collision point and the distance from the collision point and the detonation point are presented.

Investigation of the detonation of aluminum powder-oxygen mixtures.

It is shown that mixtures containing 48 to 64% (by mass) fine aluminum powder in oxygen are detonable. In 26.4-mm-diam tubes the detonation induction distances of these mix- tures are less than 1.6 m; they decrease with increasing aluminum concentration. The det- onation wave speeds for the lean mixtures are approximately 1550 m/sec and decrease slightly with increasing amounts of aluminum in the mixture. The pressures behind the detonation waves were found to be approximately 31 atm for the mixtures examined. The measured detonation speeds and pressures are approximately 9 and 14%, respectively, below the theo- retical values. This deviation is probably caused by incomplete combustion of the alumi- num particles. The detonation waves of these mixtures show the typical characteristics of spin. LTHOUGH the existence of detonation waves in solid fuel-gaseous oxidizer mixtures has been questioned by some researchers, there appears to be evidence that detonation waves can exist in heterogeneous mixtures. When high- energy shock waves were passed through mixtures .of kerosene droplets and gaseous oxidizer (Kling and Maman1), tion waves were obtained with a stoichiometric mixture only after reflection of the incident shock. The measured speeds agreed with the calculated ones. Cramer2 observed detona- tion-like waves propagating through DECH (diethylcyclo- hexane, CioH20) fuel spray-oxygen mixtures. Eraser3 was able to detonate both octane-oxygen and benzene-oxygen mixtures (also initiated with high-energy shock waves). Nicholls et al. 4 also reported detonation-like waves in heterogeneous DECH-oxygen mixtures. Although these observations support the theory that liquid-gas mixtures detonate, they do not permit us to predict the behavior of solid-fuel systems. The combustion of metal powders with gaseous oxidizers was studied by Hartmann and Greenwald.5 They reported that aluminum dust can be ignited easily in air, oxygen, carbon dioxide, and under certain conditions it can react even with nitrogen. Cassel et al. 6 measured the burning velocities of powdered aluminum in various oxygen-nitrogen atmo- spheres. For a 70% oxygen, 30% nitrogen atmosphere, they reported tube burning speeds of approximately 40 cm/sec. Bunsen-burner flame speeds of aluminum powder-oxygen mixtures have been measured previously (not reported) at this laboratory and were found to be approximately 25 cm/sec. Mixtures having low burning speeds are somewhat difficult to detonate.

Formation of detonation waves in flowing hydrogen-oxygen and methane-oxygen mixtures.

Detonation induction distances were determined in a 9-mm (i.d.) detonation tube at an initial pressure of 1 atm at ambient temperature for hydrogen-oxygen mixtures flowing at initial velocities ranging from 0-100 m/sec, and for methane-oxygen mixtures flowing at velocities from 0-30 m/sec. Some experiments with hydrogen-oxygen mixtures were conducted at 5 atm initial pressure. The induction distances for hydrogen-oxygen mixtures decrease linearly with the logarithm of the Reynolds number based on the initial linear gas velocity. Results show that the induction distance of the detonation wave propagating upstream is shorter than that of the wave propagating downstream. Induction distances in methane-oxygen mixtures were essentially constant over the range of flow velocities employed. Certain observations seem to indicate that higher turbulence levels in the initial gas flow cause an effective decrease in the quenching diameter.

On the Influence of Tube Diameter on the Development of Gaseous Detonation

AbstractStreak photographs of the transition from slow‐burning to detonation have been obtained by self‐light photography of the flame using a rotating drum camera. Experiments were performed with stoichiometric hydrogen‐oxygen mixtures contained in plastic tubes closed at one end and ignited by pilot flames. The investigation included the effect of tube diameter and distance of ignitor from closed end on the development of detonation.The plots of detonation induction distances and times as a function of tube diameter indicate the existence of asymptotes. Above a certain value of tube diameter there should be no further increase of induction distance and time with increasing tube diameter. A graph of average detonation induction velocity, that is, the ratio of average induction distance to time, shows that wall effects are noticeable only for tubes with a diameter smaller than about 1 cm. Maximum observed flame velocities, attaining the value of 3.1 km/sec, have been observed immediately following the onset of retonation before the combustion front settles down to the Chapman‐Jouguet detonation with a velocity of 2.8 km/sec. The plot of world lines on the log‐log scale revealed that the development process can be bracketed between two asymptotes, one corresponding to an acceleration which decreases, or is sometimes even negative, and the other with acceleration that increases with time. The average value of observed accelerations was of an order of 106 m/sec2.

Experimental Measurements and Theoretical Analysis of Detonation Induction Distances

Experimental measurements of the induct ion distances are given for hydrogen-oxygen, acetyleneoxygen, acetylene-air, methane-oxygen, carbon monoxide-oxygen, hydrogen-oxygen-nitrogen, hydrogen-oxygen-hel ium, hydrogen-oxygen-argon, and hydrogen-oxygen-carbon dioxide mixtures at initial pressures of 1, 5, 10 and 25 a t m . An empirical relationship involving the combust ion t e m perature, burning velocity, sonic velocity in the unburned gas and Reynolds number based on the burning velocity has been established to predict detonation induction distances of certain c o m bustible gas mixtures. The correlation is not satisfactory for mixtures containing hydrocarbons; an explanation of this discrepancy is offered.

Effects of Attenuating Materials on Detonation Induction Distances in Gases

The effectiveness of acoustic absorbing wall materials in delaying the onset of detonation in gases is explored. Experiments are reported for stoichiometric H2–O2 mixtures in two-inch diameter cylindrical tubes. The results show that the detonation induction distance may be increased by as much as a factor of two by making the walls of the tube of porous sintered bronze.