Ignition Position Research Articles

Assessing the influence of real releases on explosions: Selected results from large-scale experiments

The research activities in the project Assessing the Influence of Real Releases on Explosions (AIRRE) included a unique series of large-scale explosion experiments with high-momentum jet releases directed into congested geometries with subsequent ignition. The primary objective for the AIRRE project was to gain improved understanding of the effect that realistic releases and turbulent flow conditions have on the consequences of accidental gas explosions in the petroleum industry. A secondary objective was to develop a methodology that can facilitate safe and optimal design of process facilities. This paper presents selected results from experiments involving ignition of a highly turbulent gas cloud, generated by a large-scale, pressurised release of natural gas. The paper gives an overview of the effect on maximum explosion overpressures of varying the ignition position relative to the release point of the jet and a congested region placed inside the flammable cloud, with either a high or a medium level of congestion. For two of the tests, involving a jet release and the medium congestion rig, the maximum overpressures significantly exceeded those obtained in a quiescent reference test. The paper presents detailed results for selected tests and discusses the effect of the initial flow field generated by realistic releases – including turbulence, net flow and concentration gradients – on relevant explosion phenomena.

Effects of methane ratio on MPDF (micro-pilot dual-fuel) combustion characteristic in a heavy-duty single cylinder engine

In this study, the characteristics of micro-pilot dual-fuel combustion with respect to the fuel mixture ratio in a single cylinder dual-fuel engine have been investigated. In order to analyze the characteristics of micro-pilot dual-fuel combustion, a metal engine and an optical single cylinder dual-fuel engine were used. The fuel mixture ratio was varied for experimental purposes; the diesel was directly injected into combustion chamber and the methane gas was supplied via intake port. The present study reports that increasing the methane mixture ratio from 0 to 97.67% changes the diesel combustion to pre-mixed combustion. As a result, the peak cylinder pressure was increased from 184 to 198 bar, and the rate of heat release was greatly advanced. In the MPDF condition, the nitrogen oxides emissions were reduced by about 90%p, and the fuel conversion efficiency increased about 5%p because of the low combustion temperature of pre-mixed combustion. However, for the same reason, the hydrocarbon emissions were increased about 95%p. The fastest combustion speed was found form the results of methane mixture ratio between 40 and 80%. In the condition of diesel combustion and micro-pilot dual-fuel combustion, the combustion periods of middle and initial were increased, respectively, resulting in the low combustion speed. The standard deviation of peak cylinder pressure, which represents the combustion variation, was correlated with initial combustion period. While the condition of methane gas mixture ratio between 40 and 80% shows the lowest combustion variation, the highest combustion variation was occurred by MPDF condition. Through the optical engine experiment, it can be found that the cycle to cycle combustion variation is ascribed to the turbulent flow and the variation of ignition position. The combustion images show that the unpredictable characteristics of the ignition position and slow flame propagation speed caused the combustion variation in micro-pilot dual-fuel combustion.

Experimental study of the effects of vent area and ignition position on internal and external pressure characteristics of venting explosion

Explosion venting can cause not only internal damage but can cause external damage to buildings. Since little attention is paid to the damage caused by both explosions simultaneously, vented explosion tests were performed in a 4.5 m3 square chamber, to investigate the internal and external pressure characteristics and their correlation. The results show that ignition location has little effect on venting pressure (P1), but significantly affects the Helmholtz oscillations and high-frequency oscillation pressure (P4). Central and front ignitions lead to the maximum P4 and longest Helmholtz oscillation, respectively; during venting, two pressure peaks (defined as Pe1 and Pe2) were recorded due to the shock wave and combustion wave, respectively. Pe1 has an inverse proportional relationship with distance. Pe2 is proportional to P1. When the vent area exceeds 0.16 m2, Pe2 under rear ignition plays a more dominant role than that of other ignition points, however, the vent area is equal to 0.16 m2, Pe2 with central ignition becomes dominant. The external pressure peaks corresponding to the external flame development were captured. The longest length of the jet flame was observed at central ignition and rear ignition forms the largest external combustion area.

Research on the influence of dual spark ignition strategy at combustion process for dual cylinder free piston generator under direct injection

The direct injection technology is easy to achieve stratified combustion and greatly improves the fuel economy and power of free piston generator (FPG), but it also exacerbates the cyclic combustion fluctuation caused by the structure. Therefore, based on the dual spark ignition strategy, the influence of the ignition position and ignition time on the combustion process for dual cylinder FPG under direct injection was studied. The research results show that compared with the changes in the ignition angle position, the combustion process of direct injection FPG is more sensitive to changes in moment of ignition. Furthermore, with the ignition timing advances from 6°CA BTDC to 18°CA BTDC, the indicated thermal efficiency rises constantly at a rate of 25.1% and reached a high value of 33.9% in the same ignition position, although the uniformity of the mixture in the cylinder and the concentration of the mixture around the spark plug decrease gradually. While, in the same time of ignition, the increase rate of the indicated thermal efficiency reaches 6.6% at the maximum along with the increase of the spark plug angle and the increasing rate drops continuously along with the advancement of the ignition timing. Furthermore, compared with single spark ignition, the dual spark ignition can increase the cylinder pressure by 30% and the indicated thermal efficiency by 2% respectively.

Propane-air mixture deflagration hazard with different ignition positions and restraints in large-scale venting chamber

In order to better assess the hazards of explosion accidents, propane-air mixture deflagrations were conducted in a large-scale straight rectangular chamber (with a cross-section of 1.5 m × 1.5 m, length of 10 m, and total volume of 22.5 m3). The effect of initial volume, ignition position, and initial restraints on the explosion characteristics of the propane-air mixtures was investigated. The explosion overpressure, flame propagation, and flame speed were obtained and the computational fluid dynamics (CFD) software was used to simulate the flame-propagation process and field flow for auxiliary analysis. The hazards of large-scale propagation explosion under weak and strong constraints were evaluated and the different phases of flame propagation under weak and strong constraints were discriminated. Results indicate that the hazards caused by propane deflagration under weak constraint are mainly caused by flame spread. And the maximum overpressure under strong constraint appeared at the front part of the chamber under the large-scale condition, which is consistent with the previous small-scale test. Moreover, the simulations of flame structures under weak and strong constraint are in good agreement with experimental results, which furthers the understanding of large-scale propane deflagration under different initial conditions in large-scale spaces and provides basic data for three-dimensional CFD model improvement.

Experimental investigation of the ignition and combustion characteristics of electrically controlled solid propellant

The ignition and combustion characteristics of electrically controlled solid propellant (ECSP) are investigated via high-speed photography, thermogravimetry, and differential scanning calorimetry with the application of an electrical voltage across electrodes imbedded in the ECSP. The effects of electrode polarity, electrode material, electrode surface roughness and electrode area on the ignition characteristics of ECSP were studied, and the response of ECSP to heat and electricity is studied separately. Experiments are performed to film the ignition process and the combustion flame and to measure the temperature distribution of the combustion wave. ECSP ignition and burning were localized to the electrode with small electrode surface area and to the interface between the electrode and the insulator when the area ratio was not 1:1. Furthermore, the electrode area ratio, electrode surface roughness, electrode material, and electrode polarity affect the ignition position. Under atmospheric conditions, the ECSP began and stopped with the application and removal of power and the preference of the ignition occurring the interface between the ECSP and the electrode coated insulation. Moreover, the current density distribution result matched well with the ignition and combustion experimental results. A clear “double-platform” phenomenon was observed in the temperature distribution of the combustion wave. The combustion flame of ECSP was determined to be the diffusion flame.

Experimental investigation on the dynamic responses of vented hydrogen explosion in a 40-foot container

To investigate the structural dynamics of a container subjected to a vented hydrogen explosion, 48 field tests were conducted in a 40-foot container with roof vents and an end vent. The effects of the hydrogen concentration, ignition position, and obstacles on the evolution of the dynamic responses were investigated. Three stages were generally observed for displacements: (1) At the stage of the vent rupture, the displacement could be approximated as a quasi-static response, and there was a linear relationship between the peaks of positive overpressure and displacement. (2) Structural deformation appeared as reciprocating vibration at the stage of Helmholtz oscillation. (3) The structure exhibited relatively weak irregular fluctuation when high-frequency acoustic oscillation occurred. Two types of the structural acceleration with low and high amplitudes resulting from Helmholtz oscillation and acoustic oscillation, respectively, were clearly observed. For the end-vented explosion, multiple peaks were observed for the displacement at the quasi-static stage due to the rupture, discharge, and external explosion. Moreover, the displacement was sensitive to hydrogen concentration, whereas the number of obstacles and the ignition position had significant influences on the peak acceleration for roof venting. This work conducted the fundamental explanation for the evolution law of structural responses induced by vented hydrogen explosions from the perspective of structural dynamics and enriched the experimental accumulation in a large-scale container with congestion in this field.

Quantitative 1-D LIBS measurements of fuel concentration in natural gas jets at high ambient pressure

Fuel concentration information is critical to the development of high-efficiency combustion system of direct-injection natural gas engines. In this study, a one-dimensional (1-D) measurement approach based on laser induced breakdown spectroscopy (LIBS) is developed to quantitatively measure 1-D fuel concentration distributions in gas jets at high ambient pressures. Except for the spectral dimension, another dimension of the intensified charge coupled device (ICCD) is employed for the spatial resolution of measurements. The effects of laser energy and ambient pressure on plasma dynamics are investigated to explore the feasibility of 1-D LIBS in concentration measurements at high ambient pressure. Since the peak intensity ratio (PIR) of H656/N746 shows significant spatial variations in the plasma volume, a 1-D calibration strategy is developed by establishing respective calibration curves at different spatial positions to quantify the fuel concentration distributions. The measurement uncertainties are evaluated, including both the systematic and random errors. The temporal variations of the 1-D fuel concentration distributions in methane jets are presented, and the shot-to-shot fluctuations are analyzed in detail. The results show that the coefficient of variation of equivalence ratio is lowest at the end of injection (EOI), suggesting that ignition at the EOI could enable a relatively stable flame kernel formation. Along the radial direction of the jet, a high ignitable mixture percentage appears at approximately 3.0 mm from the jet axis at the EOI, suggesting an optimal ignition position. The results reveal that the 1-D LIBS could be a powerful tool for calibration of numerical models and optimal designs of combustion engines taking advantage of high-pressure gas jets.

Assessing the thermal safety of solid propellant charges based on slow cook-off tests and numerical simulations

A method is proposed to assess the thermal safety of solid propellant charges by measuring the temperature and pressure of propellant specimens during cook-off tests and determining the reaction kinetics. In this work, two propellants—ammonium perchlorate/hydroxy-terminated polybutadiene/aluminum (AP/HTPB/Al), and ammonium perchlorate/hydroxy-terminated polyether/aluminum (AP/HTPE/Al)—are taken as examples to demonstrate this method in detail. Specifically, by implementing two kinds of cook-off tests—viz., multipoint temperature monitoring, and combustion pressure measurement—the thermal reaction temperature before ignition and the combustion pressure after ignition of the two propellants were respectively determined. Then, the kinetic parameters of the thermal decomposition reaction model and the parameters of the combustion reaction model were numerically simulated and calibrated to achieve mathematical descriptions of the entire process of cook off of two solid propellant charges. On this basis, the technology of grid-node separation calculation was employed to simulate and predict the rupture of a solid rocket motor case, thereby quantitatively describing the severity of the cook-off reaction. The results demonstrate that, at a heating rate of 1 K min−1, the ignition positions of the two propellant charges in the solid rocket motor are located in the annular area where the side wall and the front of the casing are joined. Compared with those of the AP/HTPE/Al charge, the ignition time of the AP/HTPB/Al charge is longer (12,714 s for AP/HTPB/Al vs. 9701 s for AP/HTPE/Al), the temperature of the casing before ignition is higher (501.5 K for AP/HTPB/Al vs. 466.2 K for AP/HTPE/Al), the reaction after ignition is more intense, and the deformation of the casing is more serious.

Pressure relief of underground ammunition storage under missile accidental ignition

Safety of underground ammunition storage is an important issue, especially during the accidental ignition of missiles. This work investigates the pressure and temperature distribution of the multi-layer underground ammunition storage with a pressure relief duct during the accidental ignition process of the missile. A large-scale experiment was carried out using a multi-layered restricted space with a pressure relief duct to simulate the underground ammunition store and a solid rocket motor to simulate the accidental ignition of the missile. The results show that when the motor gas mass flow increased by 5.6 times, the maximum pressure of the ammunition storage increased by 5.87 times. At a certain motor flow rate, when the pressure relief exhaust area at the end of the relief duct was reduced by 1/2, the maximum pressure on the first layer did not change. But the rate of pressure relief was reduced and the time delayed for the pressure of ammunition store to drop to zero. In this experiment, when the motor ignition position was located in to the third layer ammunition chamber, the maximum pressure was reduced by 32.9% and also reduced the rate of change of pressure. In addition, for the experimental conditions, the theoretical analysis of the pressure relief of the ammunition storage is given by a simplified model. Based on the findings, some suggestions to the safety protection design of ammunition store are proposed.

Experimental study of lean hydrogen-air mixture combustion in a 12m3 tank

Hydrogen can be released to the nuclear power plant containment atmosphere during a reactor core melt accident, and the potential hydrogen explosion need to be mitigated by all efforts. One of the current hydrogen management methods in pressurized water reactors (PWR) is to prevent detonation with slow combustion using ignitor as designed in AP1000. A HYdrogen MIddle scale Test facility (HYMIT) was setup to study the hydrogen behavior and mitigation efficiency of ignitor. The main component of HYMIT is a steel cylinder tank whose height is 4 m and diameter is 2 m, and the tank volume is about 12 m3. The mixed gas content, gas temperature and pressure, and arrival of flame can be measured with gas analyzer, thermocouple array, piezoelectric pressure sensors, and photodiode array. The current paper studied combustion behavior of hydrogen-air homogeneous mixture in various hydrogen volume ratio up to 10% at different ignition positions, the results showed that for tests with less than 10% hydrogen concentration the hydrogen consumption ratio increased when ignition location descending from top to bottom, and the maximum pressure and temperature increased accordingly, and for 10% hydrogen concentration tests the consumption ratio, the maximum pressure and temperature were not affected by the ignition location,. The apparent flame velocities were estimated with both photodiode sensors and thermal couples and they were in good agreement, the maximum upward apparent flame velocity measured was 2.27 m/s. A minimum actual radius was recommended to estimate the initial quasi-laminar flame velocity using pressure measurement.

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.

Experiment Study of Ignition Characteristics in An Axial-flow-injector Burner for Stirling Engine

To investigate the ignition characteristics of an axial-flow injection burner for a Stirling engine, a combustion chamber was designed. Diesel was used as fuel and oxygen as oxidant. The experiments of ignition characteristics were carried out with an electric plug igniter. The ignition characteristics under different combustion chamber pressure, pre-oxygen supply time, oxygen supply flow and ignition position were studied. The experimental results show that, with the increase of the pressure, the ignition time of the burner increases gradually, and the ignition success rate decreases gradually. The oxygen flow rate is related to ignition time in a certain range, while the pre-oxygen supply time has little effect. With the ignition position moving downward, the ignition time decreases gradually.

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

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 the stroke-to-bore ratio on the performance of a dual-piston free piston engine generator

The free piston engine generator (FPEG) is considered as one of the next generation efficient energy conversion device because of its compact structure, high geometric power ratio and low pollution. This paper investigated the effect of stroke-to-bore (S/B) ratio on the system operation characteristics and engine performance, constructed a detailed numerical model in MATLAB/Simulink and verified the experimental data whose difference value could be controlled within 5%. The effect of five S/B ratios (0.84, 0.91, 0.99, 1.07 and 1.14) and three compression ratios (8, 9 and 10) was analysed at a constant bore diameter. The simulation results indicated that the operation frequency increased from 28.2 Hz to 48.3 Hz when the S/B ratio decreased from 1.14 to 0.84. The highest indicated power is 4.1 kW when the S/B ratio is 0.84 and the compression ratio (CR) is 10. While for high thermal efficiency and fuel economy design, larger S/B ratio and higher operating compression ratio should be selected while keeping the periodic energy input unchanged. The heat transfer loss decreased from 29.0% to 20.4% when the S/B ratio increased from 0.84 to 1.14. And in the long stroke, ignition position needs to lean back (from 6.8 mm to 24.8 when S/B increased from 0.84 to 1.14) so as to keep the compression ratio unchanged under different S/B ratios.

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.

Experimental Study on the Explosion Characteristics of CH4/O2/N2 Mixtures with Different Oxygen Enrichment Coefficients and Ignition Positions.

In this paper, the flame propagation characteristics and overpressure oscillation characteristics of CH4 explosion were studied under different ignition positions (IPs) and oxygen enrichment conditions in a half-open tube. The distances between the IP and the closed end of the tube are 0, 250, 500, and 750 mm. The oxygen enrichment coefficient (φ) values used in the experiment are 0.21, 0.3, and 0.4. The experimental results show that the IP and oxygen enrichment coefficient have an important influence on the flame structure and overpressure oscillation. Only when the oxygen enrichment coefficient φ = 0.21, a tulip flame will be formed. The IP close to the outlet can make the air participate in the combustion more quickly. With the increase of the oxygen enrichment coefficient, the combustion-induced rapid phase transition phenomenon is more likely to occur, and the maximum overpressure value and the overpressure rise rate of flame will increase. It is worth noting that after increasing the oxygen enrichment coefficient, the IP has less influence than the oxygen enrichment coefficient on the overpressure rise rate.

On explosion characteristics of premixed syngas/air mixtures with different hydrogen volume fractions and ignition positions

This study analyzes the effect of both ignition position and hydrogen volume fraction on the flame propagation structure and overpressure wave. The interpretations are based on the propagation dynamics of the premixed syngas/air explosions at different ignition positions with different hydrogen volume fractions investigated at a constant equivalence ratio of 1 in a semi-open rectangular combustion chamber experimentally. The initial temperature T0 and pressure p0 were 298 K and 0.1 MPa, respectively. The experimental results are consistent with those obtained using the proposed analytical theory, indicating that the proposed theory predicts the early flame acceleration stage and the flame front motion well under some limited conditions. Flame–wall interaction and combustion instability decrease the theoretical flame front position than the experimental value and increase the wall contact time than the experimental value in some cases. At the same ignition location, both the overpressure growth rate and the maximum overpressure increase with increasing hydrogen volume fraction. When ignited at 0 mm off the closed end, the increased volume fraction of hydrogen would cause the initial location of the tulip flame to approach the opening end of the duct, but at an earlier time. The closer the ignition position to the opening end is, the more intense the overpressure and flame leading edge oscillation are, and the period of overpressure is similar to that of the flame oscillation. Finally, the ignition position and hydrogen volume fraction have obvious effects on overpressure, the structure and development of the flame front.