Front Ignition Research Articles

Combined effects of ignition position and hydrogen ratio on vented CH4/H2/air explosions

Experiments on the effects of ignition position and hydrogen ratio (χ) on the explosions of CH4/H2/air mixtures were experimentally investigated in a vented cylindrical vessel. Relevant experiments were conducted utilizing ignition sources situated at the rear (rear ignition), the center of the vessel (central ignition), or near the vent (front ignition) with χ varying from 0 to 1.0. Two types of cellular structures on the flame surface, owing to the diffusional-thermal instability and acoustically enhanced combustion, respectively, were observed, and the latter resulted in acoustic oscillations of the overpressure within the vessel and a unique overpressure peak p2 with amplitude sensitive to both ignition position and χ. The effects of ignition position and χ on the build-up of the internal pressure were not significant when χ ≤ 0.15. The maximum explosion overpressure in the vessel (pmax) under the explosions of rear and central ignitions (RI and CI) increased monotonically as χ was increased from 0.3 to 1.0, but a nonmonotonic trend was found in the explosions of front ignition (FI). CI could be regarded as the worst-case scenario when 0.45≤χ ≤ 1.0 except for χ = 0.7, because FI resulted in the highest pmax at χ = 0.7. A pressure peak outside the vent (pext) caused by the combustion expansion of the combustible cloud could be distinguished when χ ≥ 0.45 in the explosions of RI and CI. The amplitude of pext increased with an increase in χ. Rear ignition always led to the highest pext when χ > 0.6.

Effects of ignition location and vent area on the external explosion in vented hydrogen explosions

In the current study, effects of vent area and ignition location on vented stoichiometric hydrogen-air mixture explosion were experimentally and numerically investigated. Internal and external pressure-time curves and flame behavior were focused. For low vent coefficient Kv = 4, the maximum internal pressure results from the vent rupture for front ignition and the external explosion for center ignition and rear ignition. The maximum internal pressure for IL4 is larger than those for other ignition locations. The maximum internal pressure is induced by the internal chemical reaction and increases monotonically with Kv for different ignition locations, and the maximum internal pressure occurs at center ignition as Kv ＞ 4. Mach disk in external flame begins to appear when Kv increases to 12.75 for front ignition and 8.65 for other ignition locations. External pressure histories show two pressure peaks P1 and P2 for center and rear ignitions. Although the pressure peak P1 results from the leading pressure wave, the pressure peak P2 is induced by the venting flame and external explosion for center ignition and rear ignition, respectively. Only one pressure peak was recorded for center ignition. For front ignition and IL2, the maximum external pressure increases with Kv while it firstly increases and then decreases for center ignition, IL4 and rear ignition, the critical value is Kv = 17.36 for center ignition and IL4 while it is Kv = 8.65 for rear ignition.

Experimental Study of External Flame Evolution and Temperature Characteristics after a Methane Explosion in a Rectangular Chamber.

A series of methane-vented explosions were experimentally investigated in a 4.5 m3 rectangular chamber at P0 = 100 kPa and T0 = 298 K, and the effects of ignition positions and vent areas on the external flame and temperature characteristics were studied. The results indicate that the vent area and ignition position significantly affect external flame and temperature changes. The external flame is portioned into three stages: an external explosion, a violent flame jet with a blue flame, and a yellow flame venting. The temperature peak first rises and then reduces with increasing distance. Rear ignition produces the largest flame lengths and highest temperature, while front ignition leads to the shortest flame and smallest temperature peak. The maximum flame diameter occurs at central ignition. As vent areas increase, the coupling effect of the pressure wave and the internal flame front weakens and the diameter and peak of the high-temperature peak increase. These results can offer scientific guidance for designing disaster prevention measures and evaluating explosion accidents in buildings.

Effect of ignition position on the combustion instability of premixed methane-air in a semiopen duct

Experiments were conducted in a semiopen chamber to investigate the combustion instabilities of methane-air mixtures. The effect of ignition position (IP for short, i.e., the distance between ignition source and the closed end of a channel) on the characteristics of the pressure waveform of premixed stoichiometric methane-air mixture combustion is investigated in detail. The results reveal that the pressure waveforms demonstrate periodic oscillations when the ignition position is at the half-part of the duct on the side of the open end. The maximum pressure for the oscillating cases is 2.38 times that for the nonoscillating cases, and the maximum rate of pressure rise for the oscillating cases is one order of magnitude higher than that for the nonoscillating cases. The instant of burnout of the methane-air mixture escaping earlier from the open end coincides with the onset of the periodic oscillation. Then, the pressure oscillation is amplified with the exponentially growing amplitude through the flame-pressure interaction. The oscillation amplitude increases as the ignition position shifts toward the open end of the duct. The flame trajectory shows an intimate link with the pressure profile. The oscillating flame velocity precedes the oscillating pressure signal by a 180° phase shift. It is corroborated that the oscillation period depends on the length of the unburnt gas column located between the closed-end and the flame front, irrespective of the ignition position. The pressure oscillation amplitude is a linear function of the frequency. The growth rate of pressure oscillation is calculated through the velocity coupling mechanism. It is shown that the experimental growth rate is compatible with the calculated growth rate for premixed methane combustion. Front ignition produces the highest acoustic energy and thus a higher maximum combustion pressure.

The Combustion Numerical Simulation of a Liquid Slag Pulverized Coal Burner in the High Temperature Secondary Air

Taking a liquid slag pulverized coal burner as the research object, the combustion characteristics of the burner in different high temperature secondary air were simulated by Fluent commercial software, and the combustion characteristics of temperature field distribution and the change of component concentration were obtained. The simulation results show that the burner can form a front ignition zone, the air vortex low temperature zone, the central DC flame and the annular flame in the hot state. It can be found that increasing the secondary air temperature can improve the combustion intensity of the burner, promote the formation of CO, and form a high temperature reduction zone, which is conducive to inhibiting pollutant emissions. The research results provide theoretical support for the application of the burner in practice.

External explosions of vented hydrogen-air deflagrations in a cubic vessel

The effects of hydrogen concentration and ignition location on the external explosion were experimentally investigated during vented hydrogen-air deflagrations in a cubic vessel. Hydrogen concentration ranging from 14 vol% to 50 vol% and three ignition locations (front, center, and back ignitions) were focused with the ambient temperature and pressure of 283 K and 100.5 kPa, respectively. The pressure histories and flame behavior inside and outside the vessel were recorded. Experimental results show that Helmholtz oscillation was always found for front ignition. Three overpressure peaks of Popen, Phel and Pvib resulting from the vent rupture, Helmholtz oscillation and flame-acoustic-oscillation were recorded respectively. For center and back ignitions, an additional overpressure peak Pext, generated by the external explosion, was monitored. Popen was the maximum one for front ignition while Pext became the maximum one with increasing hydrogen concentration for center and back ignitions. However, the maximum overpressure for back ignition was significantly larger than those for front and center ignitions. Only one external overpressure peak was recorded for front ignition. However, as hydrogen concentration was larger than 18 vol%, two external overpressure peaks were recorded for center and back ignitions, and the one induced by the external explosion for back ignition was significantly larger than that for center ignition. Moreover, external explosion affected significantly affects the flame evolution outside the vessel.

Experimental study of overpressure and temperature field behaviors of a methane-air mixture with different ignition positions, solid structure obstacles and initial turbulence levels

For gas explosion accidents occurring in buildings, the explosion characteristics may be affected by the presence of the equipment and furniture, and the temperature produced by the vented explosion threatens the safety of surrounding personal and buildings. To investigate the internal overpressure and external temperature behaviors, explosion venting tests of methane-air mixtures were conducted in a self-designed 4.5-m3 square chamber, by changing the solid obstacles, ignition positions, and initial turbulence levels. The results showed that P1 is generated due to the venting rate exceeding the volume growth rate of products after vent opening; with increasing flame propagation, flame front increases and more folds are present on the surface during venting, when the rate of combustion product exceeds the venting rate again, then P2 forms. Ignition position and obstacles have little effect on P1, but affect the Helmholtz oscillation and high-frequency pressure oscillation (P4). Front ignition leads to the longest duration of Helmholtz oscillation and central ignition results in the largest P4. In the presence of obstacles, P4 is eliminated and strengthened in the cases of rear and front ignitions, respectively. P4 decreases and then increases as a function of the number of obstacles. Front ignition leads to the lowest temperature peak, rear and central ignition results in the same temperature peak. The length of the high-temperature field is the shortest under four obstacles and front ignition, respectively. Under strong initial turbulence, the overpressure peak, length and duration of the high-temperature field are the largest.

Effects of ignition location, obstacles, and vent location on the vented hydrogen-air deflagrations with low vent burst pressure in a 20-foot container

A series of experiments on vented lean hydrogen-air deflagrations with low vent burst pressure was performed in a 20-foot container. The effects of hydrogen concentration, ignition position, vent location, and obstacles were studied, and all tests were performed at about 290 K and 101 kPa. For the cases with end vent, three overpressure peaks of P1, P2, and P3 were recorded, resulting from the vent rupture, flame propagation to the vent, and the Helmholtz oscillation, respectively. Moreover, at the middle and later stages of the vented explosion, the acoustic oscillations with high-frequency were observed due to the thermo-acoustic-vibration coupling caused by the instability of lean hydrogen combustion. The peak overpressure for the case with the obstacle is lower than that without the obstacle. Vent rupture occurs quickly for front ignition and low hydrogen concentrations. For the tests with high hydrogen concentration such as 24 vol% and 28 vol%, since the polyethylene film can reach its static activation pressure and rupture, the peak overpressure is much larger. For the cases with the ceiling vents, only one overpressure peak P1, corresponding to the rupture of the vent cover, was recorded and its value increases with the hydrogen concentration. The obstacle causes the propagation direction of the flame to change, and resultantly the flame spreads upward and burns the polyethylene film, which decreases the internal overpressure rise time. This phenomenon is particularly evident for the tests with high hydrogen concentrations. Except for 16 vol%, the obstacle reduces the peak overpressure.

Experimental study on the effects of ignition location and vent burst pressure on vented hydrogen-air deflagrations in a cubic vessel

A series of experiments was conducted in a cubic vessel of 0.5×0.5×0.5m to investigate the effects of ignition location and vent burst pressurePvon vented stoichiometric hydrogen-air deflagrations inside and outside the vessel. The results show that two overpressure peaks resulting from the vent rupture and the flame-acoustic interaction respectively, were always found for front ignition. Another overpressure peak generated by the external explosion was observed for center ignition andPv<69.8kPa. For the back ignition, three overpressure peaks successively produced by the vent rupture, the flame venting and the external explosion were recorded forPv<57.3kPa. The maximum peak overpressure inside the vessel always increases withPv, and the maximum peak overpressure for back ignition is significantly larger than those for front and center ignitions, and the latter two cases almost have the same values. The external overpressure-time curves present a single peak for front ignition and two peaks for back ignition. The external overpressure increases withPvfor front and back ignitions. However, the external overpressure increases forPv<57.3kPa, but it is insensitive toPvasPv>57.3kPa for center ignition. The external flame speed increases withPvfor front ignition while it is independent of the latter for center and back ignitions. The maximum length of the external flame for back ignition is much larger than that for front ignition and slightly larger than that for center ignition.

Effect of ignition position on vented hydrogen–air deflagration in a 1 m3 vessel

This study investigates the effect of the ignition position on vented hydrogen-air deflagration in a 1 m3 vessel and evaluates the performance of the commercial computational fluid dynamics (CFD) code FLACS in simulating the vented explosion of hydrogen-air mixtures. First, the differences in the measured pressure-time histories for various ignition locations are presented, and the mechanisms responsible for the generation of different pressure peaks are explained, along with the flame behavior. Secondly, the CFD software FLACS is assessed against the experimental data. The characteristic phenomena of vented explosion are observed for hydrogen-air mixtures ignited at different ignition positions, such as Helmholtz oscillation for front ignition, the interaction between external explosion and combustion inside the vessel for central ignition, and the wall effect for back-wall ignition. Flame-acoustic interaction are observed in all cases, particularly in those of front ignition and very lean hydrogen-air mixtures. The predicted flame behavior agree well with the experimental data in general while the simulated maximum overpressures are larger than the experimental values by a factor of 1.5–2, which is conservative then would lead to a safe design of explosion panels for instance. Not only the flame development during the deflagration was well-simulated for the different ignition locations, but also the correspondence between the pressure transients and flame behavior was also accurately calculated. The comparison of the predicted results with the experimental data shows the performance of FLACS to model vented mixtures of hydrogen with air ignited in a lab scale vessel. However, the experimental scale is often smaller than that used in practical scenarios, such as hydrogen refueling installations. Thus, future large-scale experiments are necessary to assess the performance of FLACS in practical use.

The effect of ignition location on explosion venting of hydrogen–air mixtures

The effect of ignition location and vent burst pressure on the internal pressure-time history and external flame propagation was investigated for vented explosions of hydrogen–air mixtures in a small cylindrical vessel. A high-speed camera was used to record videos of the external flame while pressure transducers were used to record pressure-time histories. It was found that central ignition always leads to the maximum internal peak overpressure, and front ignition resulted in the lowest value of internal peak overpressure. The internal peak overpressures are increased corresponding to the increase in the vent burst pressure in the cases of central and rear ignition. Because of the effect of acoustic oscillations, the phenomenon of oscillations is observed in the internal pressure profile for the case of front ignition. The pressure oscillations for the cases of rear and central ignition are triggered by external explosions. The behavior of flames outside the chamber is significantly associated with the internal pressure of the chamber so that the velocity of the jet flame is closely related to the internal overpressure peak.

Effect of ignition position on vented hydrogen–air explosions

Among the factors affecting explosion venting, ignition position deserves more detailed investigation because it significantly affects the propagation of flame and the relief of internal pressure during a vented explosion. Explosion venting of hydrogen–air mixtures with equivalence ratio ranging from 0.6 to 5.0 was experimentally investigated in the cases of back, central and front ignition. The results show that front ignition always leads to the minimum internal and external overpressure. Central ignition results in the maximum internal overpressure for all equivalence ratios and back ignition produces the maximum external overpressure for hydrogen-rich mixtures. Both the maximum internal and external overpressures first increase and then decrease with the increase of hydrogen equivalence ratio. The evolution of the external flame includes the formation of a fireball and a jet flame. The maximum lengths of the external flame are nearly identical when hydrogen–air mixtures are back and centrally ignited, which are larger than those in the front ignition case. For equivalence ratio of 3.0 and 5.0, the interaction of internal flame with the backward propagating pressure wave resulted from external explosion leads to the appearance of fine cellular structure and the oscillation of the internal pressure.

Experimental Study on the Ignition and Combustion Mechanisms of a Methane–Air Mixture in a Divided Constant-Volume Combustion Chamber

An experimental study on the ignition and combustion mechanisms of a methane–air mixture in a divided constant-volume combustion chamber was conducted in this paper. The effects of the initial pressure, equivalence ratio, and orifice diameter on the mechanism pattern were studied using the pressure curve and rate of pressure rise, and the jet speed and the flame propagation speed in the lower chamber were measured. The ignition and combustion mechanisms of the methane–air mixture in a divided constant-volume combustion chamber can be classified into three patterns: (i) chemical chain ignition and burning, (ii) composite ignition and burning, and (iii) flame front ignition and burning. Reasonable explanations for each pattern were then presented. Characterization parameters were defined so that the mechanism pattern can be easily confirmed under any condition. The appropriate increase in the initial pressure, equivalence ratio, and orifice diameter were helpful in obtaining a higher burning speed. Ultimately, analyses of the jet and flame propagation speeds in the lower chamber proved that pattern (ii) was the most advantageous for increasing the burning speed.

Self-ignition of polymerization fronts with convection: the ‘rainstorm effect’

AbstractThis work is devoted to an experimental and numerical study on the influence of natural convection on the ignition of polymerization reaction fronts. Numerical simulations confirm experimental findings. Indeed, it has been observed that under the same initial and boundary conditions the front ignition can occur either in the centre of the reactor or at the top. The difference between these two cases is determined by the viscosity of the medium and by other parameters that can influence the convective motion of a liquid monomer. If the viscosity is low, weak convective motion in the beginning of the development of the reaction will lead to the accumulation of the hotter monomer at the top of the reactor. In this case, front ignition occurs at the top and propagates downwards (rainstorm effect). If the viscosity is sufficiently high, the convective motion does not essentially change the position of the emerging hot spot; the front appears at the centre of the reactor and propagates almost spherically outwards.

Numerical modeling and experimental study of the frontal polymerization of the diglycidyl ether of bisphenol A/diethylenetriamine epoxy system

AbstractFrontal polymerization is a process in which a spatially localized reaction zone propagates through a monomer by converting it into a polymer. In particular, the heat produced during the curing process is exploited to promote the reaction of the monomer lying next to the propagating front, making this latter able to self‐sustain. This approach represents an alternative solution to traditional polymerization methods and can be successfully applied to the preparation of many polymeric materials. In this study, frontal polymerization was numerically modeled to better understand it and to provide the basis for processing simulation. A finite‐difference method was used to solve the thermal problem coupled with the equation describing the cure evolution for a reactor with a cylindrical geometry. The implicit backward time–centered space method was used. First, a one‐dimensional model, able to describe the process in an adiabatic tube, was developed. The front ignition was simulated as if it were a hot surface warming one end of the reactor to trigger reactant polymerization. The model was able to predict the formation of a reactive front advancing in the unreacted zone with a constant speed. The influence of the chemical and physical properties of the resin on process evolution was also investigated. By applying the alternate direction implicit method, a more detailed two‐dimensional model able to describe a three‐dimensional problem for a cylindrical reactor was also developed. With this model, it was possible to study the influence of boundary conditions on process evolution, considering a convective heat exchange with the environment through the reactor walls. Diglycidyl ether of bisphenol A, cured with diethylenetriamine (DETA), was used as the model system. Differential scanning calorimetry was used to produce a phenomenological model able to describe the cure process and to determine the physical properties of the resin. The validity of the approach was confirmed experimentally using a small cylindrical reactor. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1756–1766, 2005