Effect Of Ignition Position Research Articles

Numerical simulation study on the effect of ignition position on premixed hydrogen deflagration characteristics in tunnels

To analyze the influence of different ignition positions on the propagation of deflagration characteristics and the disaster mechanism in the hydrogen deflagration process in the tunnel, this study compares and verifies the experimental data of tunnel model deflagration based on the CFD code GASFLOW-MPI. A combustion velocity model based on the accelerated separation effect is used, and the effect of heat transfer on the consequences of deflagration is considered. The propagation laws of pressure waves, fluid velocity and temperature in the tunnel are analyzed, and it is found that the changing trends of shock waves and fluid velocity in the tunnel are consistent, but completely opposite in the tunnel outside. The effects of different ignition positions on the hydrogen deflagration characteristics are compared and analyzed. The results show that when ignition occurs at different heights of the center of the premixed region, the impact on the deflagration characteristics is relatively small, while when ignition occurs at different horizontal distances in the premixed region, the impact on the deflagration characteristics is relatively large; the peak overpressure of side ignition is far lower than the case where ignition occurs in the center of the premixed region, but the maximum flow velocity is much higher than the center ignition. The impact on temperature is the smallest. In general, in areas close to the hydrogen/air premixed zone, side ignition results in more severe consequences, whereas in areas far from the premixed zone and outside the tunnel, central ignition leads to more severe consequences. This study provides theoretical support for understanding the disaster-causing mechanisms of hydrogen/air premixed gas deflagration in tunnels under different ignition positions.

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.

Experimental study on the effects of ignition position and inhomogeneous concentration on vented hydrogen deflagrations in a 7 m3 chamber

Vented inhomogeneous hydrogen deflagrations are conducted in a 7 m3 chamber. The effects of ignition position and inhomogeneous concentration are studied. The results show that a coupling flame structure of jet fire and flame bubble is first observed. In the case of top ignition, a M-shaped flame structure forms for average hydrogen concentration C¯H2≤ 10.14 vol%. In the cases of center ignition and bottom ignition, the flame bubble rapidly expands towards the walls and the horizontal propagation speed of upper part of flame bubble is obviously higher than that of lower part of flame bubble for C¯H2≥ 11.27 vol%. Moreover, the speed of horizontal flame front quickly decreases as the flame front moves away from the centerline of hydrogen jet. Three overpressure peaks appear in all scenarios, namely Popen, Phel and Pvib, which results from the vent failure, Helmholtz-type oscillations and thermo-acoustic oscillations respectively. The maximum overpressure dominated by Popen is nearly insensitive to C¯H2for top ignition. However, the maximum overpressure induced by center ignition or bottom ignition significantly increases when C¯H2≥ 10.14 vol%. For a given C¯H2, Pvib induced by bottom ignition is always the largest.

Combustion characteristics of nonuniform methane-air mixtures in the duct

In this study, an experimental duct was divided into two sections (ducts A and B) and filled with methane-air mixtures at various equivalence ratios. The combustion characteristics of non-uniformly distributed methane-air mixtures were studied experimentally, and the effect of ignition position on them was analyzed. The results showed that the a change in the equivalence ratio in duct A had a more visible effect on flame propagation. As the ignition position moved toward the open end, rapidly oscillating pseudo-flat structures and distorted tulip structures were observed in the right flame, and the bimodal pressure waveform was transformed into a unimodal waveform. While the propagation of the LFF (i.e., the flame front that propagated to the left open end) is the main reason for the pressure increase, the RFF (i.e., the flame front that propagated to the right closed end) also exerted a certain influence on the second peak. The stretching effect of the LFF and the Rayleigh-Taylor instability jointly cause oscillations in the RFF. We also observed that the maximum flame propagation velocity and peak overpressure varied in a consistent manner, both increasing first and then decreasing as the ignition position approaches the open end.

Effect of Ignition Position and Obstacle on Vented Methane–Air Deflagration

Effect of Ignition Position and Obstacle on Vented Methane–Air Deflagration

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.

Flame behaviors and explosion characteristics of two‐phase propylene oxide/air mixture under different ambient pressures

AbstractPropylene oxide (PO) is widely used in fuel‐air explosives and pulse detonation engines. Based on computational fluid dynamics, the effects of ignition position, initial pressure, and mass concentration on the explosion characteristics of the PO/air mixture were studied by using a 20 L spherical container. The results showed that the ignition position had a significant effect on the flame structure. When the fuel cloud was ignited at the center, the flame structure experienced spherical, elliptical, and tulip shapes. The explosion time first decreased and then increased with the increase of ignition position, while the maximum explosion pressure decreased correspondingly. The maximum explosion pressure and the temperature had an inverted “U” correlation with concentration at normal pressure. However, the overpressure was insensitive to concentration in the negative pressure condition. The maximum flame temperature showed different flame behaviors under different initial negative pressures and cloud concentrations. This study provides a theoretical basis for understanding the explosion behaviors of PO/air mixture with different ignition positions and initial vacuum pressures, which is of great significance in preventing explosion accidents.

Research on the ignition strategy of a methanol/gasoline blends rotary engine using turbulent jet ignition mode

The preparation of methanol can first use renewable energy such as solar energy to produce hydrogen, and then carbon dioxide can be converted into methanol fuel by hydrogenation. Therefore, using methanol as mixed fuel in traditional gasoline rotary engines can effectively reduce the carbon emission of rotary engines. In addition, turbulent jet ignition mode is considered to effectively improve the combustion rate of rotary engines. To deeply study the ignition and combustion process of a methanol/gasoline blends rotary engine using turbulent jet ignition mode, a three-dimensional simulation model and a performance test bench of the rotary engine were established in this paper, respectively. Further, using the experimental data, the reliability of the three-dimensional calculation model was verified. Based on the calculation model, the effects of ignition position and timing on the ignition and combustion process of the methanol/gasoline rotary engine using turbulent jet ignition mode were further studied. The results showed that the ignition position and the ignition timing could significantly change the ignition and combustion process in the cylinder by changing the impact timing between the jet flame and the rotor wall, the intensity of the jet flame and the position where the jet flame enters the cylinder.

Effect of Ignition Position on Pre-chamber Jet Combustion

This study focused on the effect of ignition position in the pre-chamber on front chamber jet combustion. To investigate the effect of ignition position, we designed a pre-chamber visualized cylinder head with multiple ignition positions that can capture the pre-chamber combustion. From the visualized images, it was found that two different jet morphologies, a pre-existing jet and a pre-existing unburned mixed gas jet, are produced depending on the ignition position. In the case of the pre-existing jet, it was confirmed that the combustion pattern of the sub-chamber affects the main chamber flame spread velocity.

Effects of ignition position on the ignitability and flame behavior of stratified gasoline vapor

Stratified vapor–air mixtures are formed owing to evaporation after a leakage of gasoline in confined spaces. In this study, the effects of ignition position and ignition delay (tig), defined as the time interval between ignition and the moment of fuel spill, on the ignitability and flame behavior of the stratified vapor above a 0.8-mm-thick gasoline layer were experimentally investigated in a horizontal duct. Experimental results revealed that the ignitability of the stratified gasoline vapor depended on both ignition position and tig. The closer the ignition source was to the oil surface, the earlier the local stratified gasoline vapor became ignitable and thereafter lost flammability. A two-zone flame structure, including a blue premixed flame and a following yellow diffusion flame, was observed, and the blue flame propagated along only a certain layer of the gasoline vapor. The depth of the leading blue flame first increased and thereafter decreased with an increase in tig. The heat release owing to the yellow diffusion flame played an important role in the maximum explosion overpressure (Pmax), which closely related to both ignition position and tig. For a certain tig, Pmax was nearly independent of ignition position. The relationship between Pmax and tig varied with ignition position. The highest value of Pmax was obtained when ignition source was around the center of the duct, and the lowest value of 66 kPa appeared when ignition source was close to the duct roof.

Effect of ignition position and inert gas on hydrogen/air explosions

The effects of inert gas (i.e., He, Ar, and N2) and ignition position on flame dynamics in a half-open duct with an aspect-ratio of 10 are analyzed for hydrogen/air mixtures with constant laminar burning velocity SL. The results indicate that hydrodynamic and thermo-diffusive instabilities dominate flame propagations with ignition at the right-half part of the duct, while Rayleigh–Taylor instability dominates with ignition at the left-half part of the duct. The flame-sound interaction results in the periodic pressure oscillations. Due to decreased instability, the He-diluted flame exhibits a weaker sensitivity of explosion parameters to the ignition position. The maximum pressure Pmax is dominated by different mechanisms depending on the ignition position. Although constant SL is used, Pmax for the worst case with N2 dilution is two times that with He dilution, demonstrating the considerable effect of flame instabilities. Finally, a chemical kinetic calculation is performed to clarify the flame stabilities.

Experimental study on the domino effect in explosions caused by vertically distributed methane/air vapor clouds

Chained explosions happened occasionally, leading to heavy casualties and huge property losses. However, the development regime of the chained explosions is unclear. Experiments of both single and two vertically distributed vapor clouds with different sizes, volume concentrations and ignition conditions were conducted. The experimental results show that the maximum overpressure generated by the primary methane/air mixture explosion increases with vapor diameter, but hardly changes with the ignition energy. The effect of ignition position on chained explosions is significant since the development trajectory of the primary explosion varies with the ignition position. The secondary explosion is more likely to be triggered in the case of ignition at the center of the primary vapor cloud. Because of the enhanced ignition and turbulence induced by the primary explosion, the maximum overpressure and rate of pressure rise of the secondary explosion are larger than that of the primary explosion and increase with a decrease in the separation distance between the two vapor clouds. In addition, the development of the explosion flame for the secondary explosion induced by a primary explosion is asymmetric due to the uncertain ignition point.

Effects of vessel height and ignition position upon explosion dynamics of hydrogen-air mixtures in vessels with low asymmetry ratios

The explosion characteristics of hydrogen-air mixtures in a wide range of equivalence ratios in closed vessels were investigated experimentally. The maximum explosion pressure, maximum pressure rise rate (along with deflagration index), explosion time and heat loss were evaluated. Three vessels with low asymmetry ratios (H/D) of 0.146, 0.244 and 0.341 were used to study the effect of vessel height on explosion parameters, in which H and D are the height and inner diameter of the vessel, respectively. In the H/D = 0.341 vessel, the effect of ignition position on explosion behaviors was also considered. The results show that, with the increase of the vessel height, the maximum explosion pressure increase correspondingly, and the explosion time decreases orderly. In the lowest height vessel (H/D=0.146), the ignitor-regulating column acts as a ‘turbulence-generator’. Thus, the maximum rate of pressure rise could be increased significantly while the heat loss could be decreased. Compared with the central ignition scenario, both the maximum explosion pressure and maximum pressure rise rate are decreased for the asymmetrical ignition; meanwhile, the explosion time and the heat loss during the explosion are increased.

Effect of ignition position on premixed hydrogen-air flame quenching behaviors under action of metal wire mesh

For obtaining effective inhibitory strategies for hydrogen combustion in terms of safety, the effect of ignition position on premixed hydrogen-air flame quenching behaviors under action of metal wire mesh is experimentally analyzed. Combustible hydrogen/air gases with volume ratio of 2:3 is adopted. Metal wire mesh of 60 meshes-60 layers is selected as the flame inhibitory structure. Three different ignition positions are arranged for obtaining different stages of flame entering suppression section. The results indicate that ignition position affects flame quenching results significantly. The flame quenching difficulty increases as the distance between ignition position and wire mesh location increases. Besides, the ignition position influences flame behaviors greatly. Tulip flame is actually led by the vortices in IP1 case, and the internal mechanism is attributed to the interactions of flame front and pressure waves. However, for IP3, the special tulip flame with an extremely long inversion cusp is actually led by the enhanced reverse gas flow in burnt field, and the root cause is metal wire mesh enlarges the discrepancies of combustion intensity between flame propagating toward positive and negative directions. Moreover, the ignition position changes flame tip speed and pressure greatly. As ignition position changes from IP1 to IP2 and IP3, the maximum flame tip speed toward positive direction decreases from 120.5 m/s to 105.2 m/s and 56.9 m/s, separately. Also, the maximum pressure reduces from 0.13 MPa to 0.08 MPa and 0.07 MPa, respectively, which further proves a more pronounced inhibitory effect when setting ignition position closer to wire mesh.

Effects of ignition position on explosion of premixed propane‐air in a T‐type pipe

AbstractIgnition position has an important influence on gas explosion, but few researches have been done on the ignition position in T‐type pipe. In this paper, an experimental and numerical study of horizontal and vertical ignition positions in a T‐type pipe has been presented. Results show that the ignition position has a great influence on the flame propagation velocity, flame structure and explosion pressure. In the case of vertical ignition, the maximum pressure appears near the T‐type bifurcation, and there is an obvious stress concentration on the upper side wall of the bifurcation. In the case of horizontal ignition, the maximum pressure appears at the right end of the pipe, and a weak stress concentration occurring at the splitting point. When flame passing through the bifurcation, it takes 12 ms longer than ignition on horizontal, but the time for flame propagates from ignition to the end is less. The “tulip flame” forms three times after flame propagates the bifurcation when ignition on horizontal, but there is no “tulip flame” while ignition on vertical. In the case of vertical ignition, the scale of the vortex at the bifurcation is larger than that in the horizontal. The turbulent kinetic energy is mainly in the horizontal section when ignition on vertical, while it is mainly in the vertical pipe if ignition on horizontal.

Effect of Ignition Position on Methane Explosion in Spherical Vessel with a Pipe

Abstract An experimental system consisting of a spherical vessel with a horizontal pipe was established. By changing the pipe length, the effect that the ignition position has on methane explosion in an airtight spherical vessel with a pipe was studied, and the regularities of Pmax, (dP/dt)max and flame transmission were analyzed in detail. The results show that different ignition position leading to different propagation of flame and pressure wave. As the ignition position moved from the wall of vessel to the end of pipe, the Pmax in the apparatus first increased, then decreased when ignition occurs behind the center of vessel, and finally increased when ignition occurs behind the middle of pipe. The same trend was observed for (dP/dt)max. When ignited near the end of pipe, the Pmax and (dP/dt)max were large, and the explosion was the most dangerous. In addition, gas oscillation was observed during the explosion process in the pipe, and it was more serious when igniting at the wall of vessel, the center of vessel and the end of pipe. Furthermore, the influence that structure effect had on the gas explosion was determined. With ignition in spherical vessel, the longer the pipe is, the weaker the explosion intensity in vessel is, but the greater the explosion intensity in pipe is. With ignition in the pipe, the longer the pipe is, the greater the explosion intensities are in both the vessel and pipe. The influence that the ignition position has on a gas explosion was revealed, and the reason was analyzed. This research has great theoretical value and practical significance for preventing and controlling potentially disastrous accidents in complex units. The research results provide a theoretical foundation for antiknock and explosion venting in technical vessels and pipes.

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.

Steady Heat Transfer Analysis of Leaf Spring Rotary Engine

This paper presents the heat transfer simulation of a new kind of rotary engine called Leaf Spring Rotary Engine. The structure and the principle of the prototype engine were introduced. The thermodynamic models including heat transfer model were presented. The contrast of the performance parameters between the heat transfer condition and the ideal condition was presented and the effect of the heat transfer to the performance of the engine was analyzed. It showed that the heat transfer loss would account for 24% of the input energy at the rated speed of 3000r/min. At the same time, the effect of ignition position to the performance of the engine was analyzed. The work would be used in the combustion system design and the performance optimization.

Effect of ignition position on the run-up distance to DDT for hydrogen–air explosions

The method described in this paper enabled reliable and accurate positioning of an overdriven detonation by calculation of shock wave velocities (detonation and retonation) for hydrogen explosions in a closed 18 m long horizontal DN150 pipe. This enabled an empirical correlation between the ignition position and the run-up distance to DDT to be determined. It was shown that the initial ability of the flame to expand unobstructed and the piston-like effect of burnt gas expanding against the closed end of the tube contributed to initial flame acceleration and hence were able to affect the run-up distance to overdriven detonation. Flame speeds and rates of initial pressure rise were also used to explain how these two competing effects were able to produce a minimum in the run-up distance to DDT. The shortest run-up distance to DDT, relative to the ignition position, for this pipe and gas configuration was found when the ignition position was placed 5.6 pipe diameters (or 0.9 m) from the closed pipe end. The shortest run-up distance to DDT relative to the end of the pipe was recorded when the ignition source was placed 4.4 pipe diameters or 0.7 m from the pipe end.

Numerical analysis of release, dispersion and combustion of liquid hydrogen in a mock-up hydrogen refuelling station

Computational Fluid Dynamics (CFD) simulations of hydrogen dispersion and combustion in accident scenarios in a mock-up liquid hydrogen (LH 2) refuelling station have been performed with the codes ADREA-HF and REACFLOW. The assumed accident scenarios are caused by a hose break during LH 2 car refuelling. Two main configurations have been taken into account: a mitigated case and a non-mitigated case. For each configuration, 5 different ambient conditions are assumed: a case with no wind at all and four cases with a 5 m/s wind speed, coming from the south, north, east and west direction. The numerical simulations show the effect of the wind on the dispersion and explosion of the hydrogen flammable cloud. The effect of ignition position has also been investigated. The expectation that the wind plays a positive role in increasing the dispersion of the flammable cloud and therefore in reducing the overpressures in case of explosions does not hold true for all wind directions. Depending on the wind direction and on the station layout, the wind can have the effect of trapping the flammable cloud with more reactive concentrations and higher level of turbulence within the station, generating a similar level of overpressure as in the stagnant case although the amount of flammable mixture is much smaller than in the stagnant case.