Ignition processes and characteristics of charring conductive polymers with a cavity geometry in precombustion chamber for applications in micro/nano satellite hybrid rocket motors

The arc ignition system based on charring polymers has advantages of simple structure,low ignition power consumption and multiple ignitions,which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite.However,charring polymers alone need a relatively high input voltage to achieve pyrolysis and ignition,which increases the burden and cost of the power system of micro/nano satellite in practical application.Adding conductive substance into charring polymers can effectively decrease the conducting voltage which can realize low voltage and low power consumption repeated ignition of arc ignition system.In this paper,a charring conductive polymer ignition grain with a cavity geometry in precombustion chamber,which is composed of PLA and multiwall carbon nanotubes(MWCNT)was proposed.The detailed ignition processes were analyzed and two different ignition mechanisms in the cavity of charring conductive polymers were revealed.The ignition characteristics of charring conductive polymers were also investigated at different input voltages,ignition grain structures,ignition locations and injection schemes in a visual ignition combustor.The results demonstrated that the ignition delay and external energy required for ignition were inversely correlated with the voltages applied to ignition grain.Moreover,the incremental depth of cavity shortened the ignition delay and external energy required for ignition while accelerated the propagation of flame.As the depth of cavity increased from 2 to 6 mm(at 50 V),the time of flame propagating out of ignition grain changed from 235.6 to 108 ms,and values of mean ignition delay time and mean external energy required for ignition decreased from 462.8 to 320 ms and 16.2 to 10.75 J,respectively.The rear side of the cavity was the ideal ignition position which had a shorter ignition delay and a faster flame propagation speed in comparison to other ignition positions.Compared to direct injection scheme,swirling injection provided a more favorable flow field environment in the cavity,which was beneficial to ignition and initial flame propagation,but the ignition position needed to be away from the outlet of swirling injector.At last,the repeated ignition characteristic of charring conductive polymers was also investigated.The ignition delay time and external energy required for ignition decreased with repeated ignition times but the variation was decreasing gradually.

Identifying the enhancement mechanism of Al/MoO_(3) reactive multilayered films on the ignition ability of semiconductor bridge using a one-dimensional gas-solid two-phase flow model

Energetic Semiconductor bridge(ESCB)based on reactive multilayered films(RMFs)has a promising application in the miniature and intelligence of initiator and pyrotechnics device.Understanding the ignition enhancement mechanism of RMFs on semiconductor bridge(SCB)during the ignition process is crucial for the engineering and practical application of advanced initiator and pyrotechnics devices.In this study,a one-dimensional(1D)gas-solid two-phase flow ignition model was established to study the ignition process of ESCB to charge particles based on the reactivity of Al/MoO_(3) RMFs.In order to fully consider the coupled exothermic between the RMFs and the SCB plasma during the ignition process,the heat release of chemical reaction in RMFs was used as an internal heat source in this model.It is found that the exothermal reaction in RMFs improved the ignition performance of SCB.In the process of plasma rapid condensation with heat release,the product of RMFs enhanced the heat transfer process between the gas phase and the solid charge particle,which accelerated the expansion of hot plasma,and heated the solid charge particle as well as gas phase region with low temperature.In addition,it made up for pressure loss in the gas phase.During the plasma dissipation process,the exothermal chemical reaction in RMFs acted as the main heating source to heat the charge particle,making the surface temperature of the charge particle,gas pressure,and gas temperature rise continuously.This result may yield significant advantages in providing a universal ignition model for miniaturized ignition devices.

Preliminary discussion on the ignition mechanism of exploding foil initiators igniting boron potassium nitrate

Exploding foil initiator(EFI)is a kind of advanced device for initiating explosives,but its function is unstable when it comes to directly igniting pyrotechnics.To solve the problem,this research aims to reveal the ignition mechanism of EFIs directly igniting pyrotechnics.An oscilloscope,a photon Doppler velocimetry,and a plasma spectrum measurement system were employed to obtain information of electric characteristics,impact pressure,and plasma temperature.The results of the electric characteristics and the impact pressure were inconsistent with ignition results.The only thing that the ignition success tests had in common was that their plasma all had a relatively long period of high-temperature duration(HTD).It eventually concludes that the ignition mechanism in this research is the microconvection heat transfer rather than the shock initiation,which differs from that of exploding foil initiators detonating explosives.Furthermore,the methods for evaluating the ignition success of semiconductor bridge initiators are not entirely applicable to the tests mentioned in this paper.The HTD is the critical parameter for judging the ignition success,and it is influenced by two factors:the late time discharge and the energy of the electric explosion.The longer time of the late time discharge and the more energy of the electric explosion,the easier it is to expand the HTD,which improves the probability of the ignition success.

Investigation of high rate mechanical flow followed by ignition for high-energy propellant under dynamic extrusion loading

Investigating the ignition response of nitrate ester plasticized polyether(NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism and conditions of unwanted ignition inside charged propellant under accident stimulus.Secondly, evaluates the risk of a shell crevice in a solid rocket motor(SRM) under a falling or overturning scene. In the present study, an innovative visual crevice extrusion experiment is designed using a dropweight apparatus. The dynamic responses of NEPE propellant during extrusion loading, including compaction and compression, rapid shear flow into the crevice, stress concentration, and ignition reaction, have been firstly observed using a high-performance high-speed camera. The ignition reaction is observed in the triangular region of the NEPE propellant sample above the crevice when the drop weight velocity was 1.90 m/s. Based on the user material subroutine interface UMAT provided by finite element software LS-DYNA, a viscoelastic-plastic model and dual ignition criterion related to plastic shear dissipation are developed and applied to the local ignition response analysis under crevice extrusion conditions. The stress concentration occurs in the crevice location of the propellant sample, the shear stress is relatively large, the effective plastic work is relatively large, and the ignition reaction is easy to occur. When the sample thickness decreases from 5 mm to 2.5 mm, the shear stress increases from 22.3 MPa to 28.6 MPa, the critical value of effective plastic work required for ignition is shortened from 1280 μs to 730 μs, and the triangular area is easily triggering an ignition reaction. The propellant sample with a small thickness is more likely to stress concentration, resulting in large shear stress and effective work, triggering an ignition reaction.

Evaluating Ignition and Combustion Performance with Al-Metal- Organic Frameworks and Nano-Aluminum in HTPB Fuel

Incorporating aluminum metal-organic frameworks(Al-MOFs)as energetic additives for solid fuels presents a promising avenue for enhancing combustion performance.This study explores the potential benefits of Al-MOF(MIL-53(Al))energetic additive on the combustion performance of hydroxyl-terminated polybutadiene(HTPB)fuel.The HTPB-MOF fuel samples were manufactured using the vacuum-casting technique,followed by a comprehensive evaluation of their ignition and combustion properties using an opposed flow burner(OFB)setup utilizing gaseous oxygen as an oxidizer.To gauge the effectiveness of Al-MOFs as fuel additives,their impact is compared with that of nano-aluminum(nAl),another traditional additive in HTPB fuel.The results indicate that the addition of 15%(mass fraction)nAl into HTPB resulted in the shortest ignition delay time(136 ms),demonstrating improved ignition performance compared to pure HTPB(273 ms).The incorporation of Al-MOF in HTPB also reduced ignition delay times to 227 ms and 189 ms,respectively.Moreover,under high oxidizer mass flux conditions(79—81 kg/(m^(2)s)),HTPB fuel with 15%nAl exhibited a substantial 83.2%increase in regression rate compared to the baseline HTPB fuel,highlighting the positive influence of nAl on combustion behavior.In contrast,HTPB-MOF with a 15%Al-MOF additive showed a 32.7%increase in regression rate compared to pure HTPB.These results suggest that HTPB-nAl outperforms HTPB-MOF in terms of regression rates,indicating a more vigorous and rapid burning behavior.

Development of a monochromatic crystal backlight imager for the recent double-cone ignition experiments

We developed a monochromatic crystal backlight imaging system for the double-cone ignition(DCI) scheme, employing a spherically bent quartz crystal. This system was used to measure the spatial distribution and temporal evolution of the head-on colliding plasma from the two compressing cones in the DCI experiments. The influence of laser parameters on the x-ray backlighter intensity and spatial resolution of the imaging system was investigated. The imaging system had a spatial resolution of 10 μm when employing a CCD detector. Experiments demonstrated that the system can obtain time-resolved radiographic images with high quality, enabling the precise measurement of the shape, size, and density distribution of the plasma.

The Effect of Ignition Parameters on the Combustion Characteristics of an Aviation Piston Engine

A cylinder combustion simulation model was established for a two-stroke aviation piston engine used in a small unmanned aerial vehicle. The influence of different ignition system parameters on the combustion process of aviation kerosene was studied using this model. The research results showed that under the working conditions of 5500 r/min and 50% throttle opening, as the ignition energy increased, the peak values of average cylinder pressure and average temperature increased, and the combustion duration shortened, The advance of the combustion center of gravity increases the tendency of the engine to knock. Under the same operating conditions, as the ignition timing advances, the peak values of average pressure and average temperature in the cylinder increase, gradually approaching the top dead center, and the tendency of engine detonation increases more significantly.

Numerical study of inhibition mechanism of high-pressure hydrogen leakage self-ignition with the addition of ammonia

Hydrogen and ammonia have attracted increasing attention as carbon-free fuels.Ammonia is considered to be an effective energy storage and hydrogen storage medium.However,a small amount of unremoved NH3 is still present in the product during the decomposition of ammonia to produce hydrogen.Therefore,it is very essential to investigate the self-ignition of hydrogen-ammonia mixtures in order to accommodate the various scenarios of hydrogen energy applications.In this paper,the effect of NH3 addition on the self-ignition of high-pressure hydrogen release is numerically investigated.The RNG k-εturbulence model,EDC combustion model,and 213-step detailed NH_(3)/H_(2) combustion mechanism are used.CHEMKIN-Pro programs for zero-dimensional homogeneous and constant volume adiabatic reactor models are used for sensitivity analysis and ignition delay time of the chemical reaction mechanism.The results showed that the minimum burst pressure required for self-ignition increased significantly after the addition of ammonia.The maximum temperature and shock wave intensity inside the tube decreases with increasing ammonia concentration.The ignition delay time and H,HO2,and OH radicals reduce with increasing ammonia concentration.H and HO2 radicals are suggested as indicators for tracking the second and third flame branches,respectively.

Thermal and ignition properties of hexanitrostilbene(HNS) microspheres prepared by droplet microfluidics

HNS-IV(Hexanitrostilbene-IV) is the main charge of the exploding foil initiators(EFI), and the microstructure of the HNS will directly affect its density, flowability, sensitivity, and stability. HNS microspheres were prepared using droplet microfluidics, and the particle size, morphology, specific surface area, thermal performance, and ignition threshold of the HNS microspheres were characterized and tested. The results shown that the prepared HNS microspheres have high sphericity, with an average particle size of 20.52 μm(coefficient of variation less than 0.2), and a specific surface area of 21.62 m^(2)/g(6.87 m^(2)/g higher than the raw material). Without changing the crystal structure and thermal stability of HNS-IV, this method significantly enhances the sensitivity of HNS-IV to short pulses and reduces the ignition threshold of the slapper detonator to below 1000 V. This will contribute to the miniaturization and low cost of EFI.

Effects of I-EGR and Pre-Injection on Performance of Gasoline Compression Ignition(GCI)at Low-Load Condition

Gasoline compression ignition(GCI)has been considered as a promising combustion concept to yield ultralow NOX and soot emissions while maintaining high thermal efficiency.However,how to improve the low-load performance becomes an urgent issue to be solved.In this paper,a GCI engine model was built to investigate the effects of internal EGR(i-EGR)and pre-injection on in-cylinder temperature,spatial concentration of mixture and OH radical,combustion and emission characteristics,and the control strategy for improving the combustion performance was further explored.The results showed an obvious expansion of the zone with an equivalence ratio between 0.8∼1.2 is realized by higher pre-injection ratios,and the s decreases with the increase of pre-injection ratio,but increases with the increase of i-EGR ratio.The high overlap among the equivalentmixture zone,the hightemperature zone,and the OH radical-rich zone can be achieved by higher i-EGR ratio coupled with higher preinjection ratio.By increasing the pre-injection ratio,the combustion efficiency increases first and then decreases,also achieves the peak value with a pre-injection ratio of 60%and is unaffected by i-EGR.The emissions of CO,HC,NOX,and soot can also be reduced to low levels by the combination of higher i-EGR ratios and a pre-injection ratio of 60%.

Interactions Between Surface Reactions and Gas-phase Reactions in Catalytic Combustion and Their Influence on Ignition of HCCI Engine

The catalytic combustion of methane in a mierochannel whose surface was coated with platinum（Pt） catalyst was studied by numerical-simulation. The effects of gas-phase reactions on the whole catalytic combustion process were analyzed at a high inlet pressure. A sensitivity analysis of the detailed mechanisms of the surface reaction of methane on Pt revealed that the most sensitive reactions affecting the heterogeneous ignition are oxygen adsorption/desorption and methane adsorption, and the most sensitive reactions affecting the homogeneous ignition are OH and H2O adsorption/desorption. The combustion process of the homogeneous charge compression ignition（HCCI） engine whose piston face was coated with Pt catalyst was simulated. The effects of catalysis and the most sensitive reactions on the ignition timing and the concentration of the main intermediate species during the HCCI engine combustion are discussed. The results show that the ignition timing of the HCCI engine can be increased by catalysis, and the most sensitive reactions affecting the ignition timing of the HCCI engine are OH and H2O adsorption/desorption.

Application study on plasma ignition in aeroengine strut–cavity–injector integrated afterburner

To increase the thrust-weight ratio in next-generation military aeroengines,a new integrated afterburner was designed in this study.The integrated structure of a combined strut–cavity–injector was applied to the afterburner.To improve ignition characteristics in the afterburner,a new method using a plasma jet igniter was developed and optimized for application in the integrated afterburner.The effects of traditional spark igniters and plasma jet igniters on ignition processes and ignition characteristics of afterburners were studied and compared with the proposed design.The experimental results show that the strut–cavity–injector combination can achieve stable combustion,and plasma ignition can improve ignition characteristics.Compared with conventional spark ignition,plasma ignition reduced the ignition delay time by 67 ms.Additionally,the ignition delay time was reduced by increasing the inlet velocity and reducing the excess air coefficient.This investigation provides an effective and feasible method to apply plasma ignition in aeroengine afterburners and has potential engineering applications.

A review on ignition mechanisms and characteristics of magnesium alloys

Magnesium alloys have become more attractive because of their low density and electromagnetic shielding effectiveness in the aerospace industry.However,some unpredictable situation may lead to the ignition of magnesium alloys.In this review,the thermodynamic conditions and transfer processes of magnesium alloys ignition are analyzed from the point of mechanisms.The criteria of ignition are emphasized.In addition,ignitability and flammability test systems are compared.And a more suitable method to assess the potential ignition and flammability risks of magnesium alloys in extreme environments is recommended.Furthermore,the ignition characteristics of magnesium alloys are discussed in detail.It was found that the ignition of magnesium alloys is a complex process determined by internal properties such as thermo-physical properties,oxide film properties,chemical compositions and geometrical parameters,as well as the external environment such as gas species,oxygen concentration and oxygen pressure.Ignition temperature is not physical constants of materials.It is not simply assumed that ignition may occur when the temperature of Mg alloys reaches a certain ignition point.Finally,the unsolved issues in the ignition of magnesium alloys are pointed out and the future investigation are suggested for improving the safety and reliability of magnesium alloys in the aerospace applications.

Moisture content thresholds for ignition and rate of fire spread for various dead fuels in northeast forest ecosystems of China

Fuel moisture content is one of the important factors that determine ignition probability and fire behaviour in forest ecosystems.In this study,ignition and fire spread moisture content thresholds of 40 dead fuel were performed in laboratory experiments,with a focus on the source of ignition and wind speed.Variability in fuel moisture content at time of ignition and during fire spread was observed for different fuels.Matches were more efficient to result in ignition and spread fire with high values of fuel moisture content compared to the use of cigarette butts.Some fuels did not ignite at 15%moisture content,whereas others ignited at 40%moisture content and fire spread at 38%moisture content in the case of matches,or ignited at 27%moisture content and spread fire at 25%moisture content using cigarette butts.A two-way ANOVA showed that both the source of ignition and the wind speed affected ignition and fire spread threshold significantly,but there was no interaction between these factors.The relationship between ignition and fire spread was strong,with R2=98%for cigarette butts,and 92%for matches.Further information is needed,especially on the density of fuels,fuel proportion(case of mixed fuels),fuel age,and fuel combustibility.

Study on the ignition process of a segmented plasma torch

Direct current plasma torches have been applied to generate unique sources of thermal energy in many industrial applications. Nevertheless, the successful ignition of a plasma torch is the key process to generate the unique source （plasma jet）. However, there has been tittle study on the underlying mechanism of this key process. A thorough understanding of the ignition process of a plasma torch will be helpful for optimizing the design of the plasma torch structure and selection of the ignition parameters to prolong the service life of the ignition module. Thus, in this paper, the ignition process of a segmented plasma torch （SPT） is theoretically and experimentally modeled and analyzed. Corresponding electrical models of different stages of the ignition process axe set up and used to derive the electrical parameters, e.g. the variations of the arc voltage and arc current between the cathode and anode. In addition, the experiments with different ignition parameters on a home-made SPT have been conducted. At the same time, the variations of the arc voltage and arc current have been measured, and used to verify the ones derived in theory and to determine the optimal ignition parameters for a particular SPT.

MECHANISM ON DISTRIBUTION OF PILOT FUEL SPRAY AND COMPRESSING IGNITION IN PREMIXED NATURAL GAS ENGINE IGNITED BY PILOT DIESEL

Numerical simulations of pilot fuel spray and compressing ignition forpre-mixed natural gas ignited by pilot diesel are described. By means of these modeling, the dualfuel and diesel fuel ignition mechanism of some phenomena investigated on an optional engine bytechnology of high-speed CCD is analyzed. It is demonstrated that the longer delay of ignition indual fuel engine is not mainly caused by change of the mixture thermodynamics parameters. Theanalysis results illustrate that the ignition of pre-mixed nataral gas ignited by pilot dieseltaking place in dual fuel engine is a process of homogenous charge compression ignition.

Ignition of nanothermites by a laser diode pulse

Experimental investigation has been carried out for laser ignition and combustion of nanothermites based on aluminum and oxides of copper,bismuth and molybdenum.Ultrasonic mixing of nanosized powders was used to produce compositions.For thermite ignition,initiating laser pulse with a maximum intensity of 770 W/cm2 was generated by a laser diode with a wavelength of 808 nm.The ignition delay times,the minimum initiation energy density,and the average burning rate at various thermite densities and mass fractions of components were determined by recording the emission of radiation of the reaction products using a multichannel pyrometer jointly with a high-speed video camera.The effect of adding carbon black on the threshold parameters of a laser pulse was also studied.Based on the obtained results,certain assumptions were put forward with regard to the mechanism of nanothermites’ignition by laser radiation and their burning.In particular,the assumptions were made on the two-stage process of the reaction initiation and jet burning mechanism of porous nanothermites.

Experimental investigation on electrical characteristics and ignition performance of multichannel plasma igniter

Relighting of jet engines at high altitudes is very difficult because of the high velocity, low pressure, and low tempera- ture of the inlet airflow. Successful ignition needs sufficient ignition energy to generate a spark kernel to induce a so-called critical flame initiation radius. However, at high altitudes with high-speed inlet airflow, the critical flame initiation radius becomes larger; therefore, traditional ignition technologies such as a semiconductor igniter （SI） become infeasible for use in high-altitude relighting of jet engines. In this study, to generate a large spark kernel to achieve successful ignition with high-speed inlet airflow, a new type of multichannel plasma igniter （MCPI） is proposed. Experiments on the electrical char- acteristics of the MCPI and SI were conducted under normal and sub-atmospheric pressures （P = 10-100 kPa）. Ignition experiments for the MCPI and SI with a kerosene/air mixture in a triple-swirler combustor under different velocities of inlet airflow （60-110 m/s）, with a temperature of 473 K at standard atmospheric pressure, were investigated. Results show that the MCPI generates much more arc discharge energy than the SI under a constant pressure; for example, the MCPI generated 6.93% and 16.05 % more arc discharge energy than that of the SI at 30 kPa and 50 kPa, respectively. Compared to the SI, the MCPI generates a larger area and height of plasma heating zone, and induces a much larger initial spark kernel. Furthermore, the lean ignition limit of the MCPI and SI decreases with an increase in the velocity of the inlet airflow, and the maximum velocity of inlet airflow where the SI and MCPI can achieve successful and reliable ignition is 88.7 m/s and 102.2 m/s, respectively. Therefore, the MCPI has the advantage of achieving successful ignition with high-speed inlet airflow and extends the average ignition speed boundary of the kerosene/air mixture by 15.2%.

Analysis of flowout gas field simulations and ignition methods for sulphuric gas wells

To avoid potential human casualties and environmental pollution,flowout gas from sulphuric gas wells showing high concentrations of volatile gas must be neutralized by controlled ignition.Simulation model is built by using CFD software for flowout gas,and ignition methods are analyzed.The simulation results indicate that the optimal ignition zone is located between 150mm and 570mm above the gas flowout device.Two ignition methods,electronical and chemical,are developed.12 and 6 experimental tests are performed respectively for these two methods.Results from the above tests verify that both approaches are successful in igniting the gas promptly and safely.In addition,our experience proves that the former way is more suitable for the fixed position ignition case,while the latter is more suitable for the long-distance or emergent ignition case.These two approaches can potentially be applied to a wide range of situations other than the fixed position ignition case and long distance ignition case.

Influence of Ethanol Addition on the Spray Auto-ignition Properties of Gasoline and Its Relationship with Octane Number

In this study,the spray auto-ignition properties of binary primary reference fuels(PRFs)of 2,2,4-trimethylpentane and n-heptane with different research octane numbers(RONs)were measured according to the industry standard NB/SH/T 6035 to determine their ignition delay times at various initial temperatures.Furthermore,the auto-ignition properties were investigated after blending the PRFs with various amounts of ethanol.The results revealed a very good correlation between the derived cetane number and the RON for the PRFs in both the presence and absence of ethanol.In addition,a concept of ignition delay sensitivity was developed for ethanol-containing fuels that exhibited a close relationship with the octane sensitivity,which is defined as the RON minus the motor octane number(MON).Finally,the developed method was applied to conveniently estimate the RON and MON values of several ethanol-containing fuels by simply measuring their auto-ignition properties.