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

Zhiyuan Zhang, Hanyu Deng, Wenhe Liao, Bin Yu, Zai Yu

Nanjing University of Science and Technology, Nanjing 210094, China

Keywords:Micro/nano satellite hybrid propulsion Arc ignition Charring conductive polymer Ignition mechanism Ignition characteristic Repeated ignition

ABSTRACT 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.

1.Introduction

With the rapid progress in the field of space technology, the demand for micro-nano satellites in the field of navigation,communication, meteorology, military reconnaissance and scientific experiment has increased sharply [1].Propulsion system has become an indispensable part of micro-nano satellites, which can achieve orbit maneuvering and attitude control of micro-nano satellites [2].Recently developed propulsion approaches such as plasma thrusters, water-splitting thruster and solar-powered thruster are simpler and safer than chemical propulsion, which make them received extensive attention [3].However, these propulsion systems are not suitable for all applications due to their relatively small thrust.Therefore, chemical propulsion is still an indispensable choice for micro-nano satellites to achieve rapid orbital maneuvering[4].Hybrid rocket motor(HRM),which refers to a rocket propulsion system using solid fuel and liquid oxidizer as the propellants, has shown promising prospects for being used as the high-performance propulsion system of micro-nano satellites due to its multiple advantages, such as high safety in propellant storage, capability of restart, simplicity in pipe arrangement, low development cost and adjustable thrust compared with solid and liquid rocket motors[5-9].HRM can deliver impulsive maneuvers for micro-nano satellite to achieve a wide variety of applications,including rapid orbital transfer,constellation deployment and so on[6,10,11].However, practical applications of HRM for micro-nano satellite are still immature due to the lack of a lightweight, reliable, power-saving and restart-capable ignition system[12,13].

Ignition system is an essential part for high performance HRMs.Ideally, a micro-nano satellite HRM would be capable to provide dozens of burns to offer maneuvering flexibility for a wide variety of mission architectures [14].A resume of literature reveals that there are mainly four types of ignition methods applied in HRMs which can achieve multiple ignitions, including catalytic bed ignition, gas torch ignition, hypergolic ignition and arc ignition[15-24].Catalytic bed ignition can achieve multiple ignitions without complicated electronic arrangement, but it encounters various disadvantages which make it do not suit for micro/nano satellite HRMs.For instance,it has considerable ignition delay due to the preheating time needed for catalytic bed.Furthermore,it also has a risk of high-temperature degradation for the material of catalytic bed.Another issue of catalytic bed ignition system is that the payload of HRM and micro/nano satellite will decrease due to increment of mass weight of catalytic bed [25].Compared to catalytic bed ignition, gas torch ignition is an alternative approach to achieve stable and efficient ignition of micro/nano satellite HRMs[26].It can effectively achieve multiple ignitions and easily be tailored to many propellant combinations.However, it requires a secondary gaseous fuel to be carried and adds extra tubing, hardware and control valve which will increase the volume and mass of the propulsion system.Therefore, this type of igniter is not optimized for small HRMs [27,28].Hypergolic ignition based on solid fuel is also a reliable and promising method to achieve multiple ignitions of micro/nano satellite HRMs[29].It can achieve fast and efficient ignition without the need for an external ignition source,which simplify the structure of the propulsion system [30].However, the commonly used hybrid rocket solid fuel can not achieve hypergolic ignition.It is necessary to further study the selection of hypergolic additives to find a proper class of solid fuel that provides an alternative to the conventional hybrid fuel, which is hypergolic in green non-toxic oxidizer [31].

Arc ignition system based on charring polymers produced by additive manufacturing technology can overcome the above mentioned shortcomings, which brings it a broad application prospect.Judson et al.[32] found that when a relatively high voltage was applied across the charring thermoplastic fuel, a conductive electrical arc would be created among the fuel surface which resulted in a small amount of pyrolyzed fuel vapor.When this event occurred simultaneous with the introduction of an oxidizing flow, the ablated hydrocarbon vapor together with residual energy from the electrical arc could rapidly seed combustion.This arc ignition technology can achieve multiple ignitions without requiring a preheated catalyst bed and an additional supply line.It also reduces the ignition power and the complexity of the ignitor,which makes it more suitable for HRM of micro/nano satellite.Mathias et al.[33] preliminary analyzed the arc formation and pyrolysis mechanism of charring polymers and found that materials containing sufficient carbon composition was the key to achieve pyrolysis and arc ignition.Although the proposed method could achieve repeated ignition with low power consumption, a relative high voltage was also needed for charring polymers to produce arc during ignition,which would increase the burden and cost of the power system of micro/nano satellite in practical application.Shota et al.[34] found that adding conductive substance to charring polymers could reduce the ignition voltage of arc ignition system.It has been proofed that a PLA-based conductive polymer mixed with carbon black at a 53 wt%in mass could achieve reliable multiple ignitions with input power less than 10 W at 40 V in N2O flow.Whitmore et al.[35,36] investigated the performance of arc ignition system with different grain structure,and found that the ignition grain with a pre combustion chamber could reduce the velocity of incoming oxidizer flow and was favor of ignition.It can be found that most of previous investigations mainly focused on overall ignition performance parameter measurements such as power consumption and ignition latency.Due to the lack of direct visualization investigation, the detailed ignition process and ignition mechanism of charring conductive polymers remain ambiguous.The ignition processes possibly vary due to the change of grain structure,which will consequently affect the performance of ignition system.Moreover, oxidizer supply modes also have an important influence on the ignition process and performance of ignition system.

In the present work, ignition processes and characteristics of charring conductive polymers with a cavity geometry in precombustion chamber had been studied.The complete ignition process in the cavity of charring conductive polymers were comprehensively revealed.The different ignition mechanisms of charring conductive polymers were also proposed.The effects of different ignition grain structure, ignition locations, voltages and oxidizer injection conditions on ignition process and performance including ignition delay time and energy required for ignition were studied in an optically accessible ignition diagnosis system.At last, the repeated ignition characteristic was also investigated.The results may provide critical information for comprehending the ignition mechanisms and characteristics of charring conductive polymers and be helpful for optimizing ignition grain design.

2.Experimental

2.1.Charring conductive polymer ignition grain preparation

A charring conductive polymer ignition grain with a cavity geometry in precombustion chamber was proposed(Fig.1(a)).It was printed by fused deposition modeling (FDM) technology.The PLA was the main component of the charring conductive polymer grain,and multiwall carbon nanotubes (MWCNT) acted as conductive additives, accounting for about 15% of the total mass.The detailed dimension of charring conductive polymer had been shown in Fig.1(b).The overall dimension of the ignition grain was kept the same with the diameter of 26 mm and the total length of 20 mm.A cavity geometry with length of 15 mm and width of 8 mm was arranged in the precombustion chamber of ignition grain.The design tolerances were set at ± 0.2 mm and the actual measurement tolerances were approximately -0.2 mm to +0.3 mm.In order to evaluate the effect of cavity configuration on ignition characteristics of charring conductive polymer ignition grain, the depths of cavity were set at 2 mm, 4 mm and 6 mm, respectively.Three different ignition positions were also set in the cavity of ignition grain.Moreover,in order to facilitate the observation of the overall ignition process, a wedge gap was left at the top of the ignition grain.The arc path was arranged in the pre-combustion chamber cavity, which could increase mixing of the oxidizer and the pyrolyzed fuel gasses.

Fig.1.(a) Schematic of charring conductive polymer ignition grain; (b) Dimension details of charring conductive polymer ignition grain; (c) Schematic of ignition grain with different depth of cavity; (d) Schematic of ignition grain with different ignition positions.

The preparation of the ignition grain was mainly a two-step process, i.e., 3D printing filament manufacturing and ignition grain 3D printing.In the present work,the raw material of charring conductive polymers was PLA containing about 15 wt% multiwall carbon nanotubes (MWCNT) provided by Dongguan Suyuan Conductive Plastic Material Co.Ltd.It was extruded and draw as printing filament by a single screw extruder with temperatures of four aeras set at 176°C,180°C,185°C and 194°C.The cooling water temperature was set at 48°C.The diameter of printing filament was 1.75 ± 0.05 mm.Subsequently, the prepared 3D printing filament was printed into ignition grain by a FDM desktop-grade 3D printer(Spriter FDM F2X).The temperatures of hot bed and nozzle were set at 90°C and 240°C respectively, during printing.The layer height was set at 0.15 mm and half an hour of printing time was required to produce an ignition grain.

2.2.Ignition test system

In order to evaluate the ignition characteristics of charring conductive polymers with a cavity geometry in precombustion chamber which was proposed in the present work, a lab-scale ignition diagnosis system had been built which could visualize the ignition process of charring conductive polymers in nitrous oxide(N2O)flow.As shown in Fig.2,the ignition diagnosis system consisted of a visual ignition combustor, a pipework system, a DC voltage source, a control system, an image and data acquisition system.In the pipework system, two fluids were used, i.e., N2O serving as the oxidizer for ignition system and N2as the combustion chamber purge,quenching the combustion after a test or in the event of test abort.The N2O tank was used to convert liquid N2O into gaseous state and the maximum pressure of the tank was 3 MPa.The system was controlled remotely by a sequential control program and a circuit control board.A typical time sequential control was listed in Table 1.

The ignition combustor was made of 30CrMnSiA steel alloys,which consisted of an injection head, an after burner, a cylindrical combustion chamber with internal diameter of 28 mm and length of 90 mm,a quartz glass window with length and width of 80 mm and 20 mm respectively being installed on the ignition combustor top wall and a nozzle with throat diameter of 4 mm at the end of ignition combustor.The conductive PLA ignition grain was fixed and separated by an insulating layer coaxially with the combustion chamber.The ignition of the grain was achieved by continuously applying the required voltage(40-60 V)from a DC voltage source,meanwhile the N2O flowed into the main channel and interacted with pyrolyzed fuel gasses to achieve ignition.The oxidizer mass flow rate moxwas governed by using the sonic orifice,which can be expressed by Eq.(1) [7,37].

Fig.2.Diagram of charring conductive polymer ignition diagnosis system.

Table 1 Ignition test system timing control.

where K is mass flow coefficient of N2O, which is approximately 0.0485 calculated according to Eq.(2),γ is specific heats and R is gas constant of N2O; poxand Toxare the pressure and temperature of the gas N2O upstream of the sonic orifice, respectively; and Deis throat diameter of the sonic orifice.The oxidizer mass flow rate can be easily adjusted by altering N2O pressure or sonic orifice throat diameter.In order to evaluate the effect of oxygen injection types on ignition performance of charring conductive polymer ignition grain,two types of injection conditions had been studied i.e.,direct injection and swirling injection.For swirling injection conditions,as illustrated in Fig.3(a), nine tangential slits circumferentially on the injector allowed tangential injection to generate a swirl flow of N2O.The swirl number of injection flow SNgbased on the geometry is 3.33[38].

where dinjis inner diameter of injector,3 mm here,dholis diameter of the injection hole, 0.5 mm here and Nholis number of injection hole, 9 here.

The voltage probe and current probe were used to measure the voltage and current applied to the ignition grain respectively.The above voltage and current were recorded and output by an oscilloscope.The ignition process was observed by a high-speed camera(PHOTRON FASTCAM Mini UX50) at 5000 frames/s.The camera is equipped with a f/2.8, f = 50 mm achromatic UV lens.A 1024×320 mm2area of the flame is imaged by the camera and the exposure time is adjusted to 1/40,000 s.For all the test cases investigated in the present work, the shooting parameters were kept consistent.

Fig.3.Schematic of (a) injector and (b) visual ignition combustor for ignition test.

The ignition characteristic parameters of charring conductive polymers mainly include ignition delay time ti,power Piand energy Q1required for ignition.ti,Qiand Piwere respectively determined with Eqs.(4)-(6),where t0,t2were the moments when the voltage was applied and the flame appeared, respectively.U was the voltage applied to the ignition grain, and I was the current in the circuit among the ignition grain surface.

3.Results and discussion

3.1.Analysis of ignition process and mechanism

The typical ignition processes of the charring conductive polymer ignition grain with cavity geometry were captured in Fig.4.The ignition grain was continuously applied with a voltage at 40 V.The oxidizer mass flow rate was 0.6 g/s with supply pressure set at 1 MPa.The error of the calculated mass flow rate was about 3.16%which mainly came from the change of temperature and the manufacturing error of throat of sonic orifice.

As shown in Fig.4(a), after the DC voltage source started to supply power, there was a gradual conduction between two electrodes among the surface of ignition grain under initial electric field due to the lower resistance value of charring conductive polymer.The ignition grain near the electrode tips was first pyrolyzed after being stimulated by electric energy.Fuel gasses were ejected from the ignition grain surface with the formation of residual char.The residual char near the electrode tips became brighter due to absorbed most of the electric energy and was accompanied by heat accumulation.

As the current and the accumulation of heat increased, the pyrolyzed fuel gasses and oxidizer mixture near the residual char were first ignited as soon as they reached the ignition temperature.With further increment of temperature and the chemical reaction conduction of pyrolyzed fuel gasses and oxidizer mixture near the residual char,the heat released from the pyrolyzed fuel gasses and oxidizer mixture turned larger than the heat lost, so the localized flame began to propagate and a self-sustaining combustion process started in the cavity.Meanwhile, some entrainment hot carbon particles in the flame could also be observed during propagation process, which were helpful for initial flame propagation.

A similar process of pyrolysis and energy accumulation of residual char near the electrode tips could be seen in Fig.4(b).However,different from being ignited by residual char,a hot carbon particle was separated from the residual char by the flow of oxidizer through the bottom wall of the cavity.In the process of moving to the shear layer,the carbon particle was decomposed into several high temperature small particles under the action of mainstream oxidizer flow and ignited the local mixture of oxidizer and pyrolyzed fuel gasses.The flame began to propagate by carrying a large number of carbon particles and ignited the pyrolyzed fuel gasses and oxidizer mixture in the cavity.

According to the difference of ignition processes, it could be found that there were two different ignition mechanisms of charring conductive polymer ignition grain.The first mechanism could be called heated residual char ignition and the second mechanism could be called heated carbon particle ignition.

Fig.5 presents the detailed ignition mechanisms of charring conductive polymer ignition grain.During heated residual char ignition, an increasing char structure was formed during pyrolysis near the electrode tips.The residual char absorbed external electrical energy and continuously heated local mixture of the pyrolyzed fuel gasses and oxidizer to achieve ignition and initial flame propagation in the cavity of ignition grain.Due to the need of continuously heating through the irremovable residual carbon near the electrode tips, the ignition delay was longer in this ignition mode.In contrast to heated residual char ignition, during heated carbon particle ignition, a carbon particle separated from the residual char was decomposed into several high temperature small particles, which enlarged the heat transfer area and ignited the local mixture of oxidizer and pyrolyzed fuel gasses.A strong initial flame was formed between the cavity and the shear layer by carrying a large number of carbon particles to further ignite the pyrolyzed fuel gasses and oxidizer mixture in the cavity.The reason for the two different ignition mechanisms may be due to the different on-load voltages.When the on-load voltage is high(approximately 60 V), the ignition of charring conductive polymer is inclined to heated carbon particle ignition.When the on-load voltage is low (approximately 40 V), the ignition of charring conductive polymer is inclined to heated residual char ignition.This may because that the instantaneous current increases quickly and the peak power is relatively higher under high voltage conditions.The residual char layer can absorb more energy and with a higher temperature in the same time.So, when the carbon particle is blown away from the residual char by the flow of oxidizer, it will carry higher energy and has a higher temperature which can ignite the mixture of fuel vapor and oxidant in the cavity.However,when on-load voltage is lower, the carbon particle blown away from the carbon layer has relatively low temperature,the energy it carried is not enough to ignite the mixture of fuel vapor and oxidant in the cavity.The carbon particle would quickly become dark and extinguish after leaving the char layer.Therefore, the mixture of fuel vapor and oxidant is inclined to be ignited near the irremovable residual char through continuous heat transfer before it can be ignited by the heated carbon particle.

Fig.4.The representative ignition process including: (a) Heated residual char ignition; (b) Heated carbon particle ignition of charring conductive polymer ignition grain.

Fig.5.Schematic diagram of different ignition mechanisms of charring conductive polymer ignition grain.

Table 2 Ignition modes under different voltages.

The ignition conditions under different voltages were statistically analyzed in Table 2.For ignition under 40 V, the number of heated residual char ignition was 12 and carbon particle ignition was 3, the probability of carbon particle ignition was 20%.For ignition under 50 V, the number of heated residual char ignition was 7 and carbon particle ignition was 8,the probability of carbon particle ignition increased to 53.3%.When the voltage increased to 60 V,the number of heated residual char ignition was 3 and carbon particle ignition was 12, the probability of carbon particle ignition further increased to 80%.It can be seen that the ignition was indeed more inclined to heated carbon particle ignition with the increase of voltage.Besides, compared with heated residual char ignition,heated carbon particle ignition had a shorter delay and could establish a global flame in the cavity more quickly.

3.2.Effect of cavity configuration

The ignition performance of charring conductive polymer with a cavity geometry in precombustion chamber was measured at three different depths of cavity of the ignition grain.The detailed ignition grain structure could be seen in Fig.1(c).Fig.7 depicted the variation of ignition delay time and external energy required for ignition with voltages at different depths of cavity of charring conductive polymer ignition grain.The ignition position was set in the middle side of the cavity and the oxidizer was injected through a direct injector.In order to reduce the error of the results caused by accidental factors in tests,each condition had been repeated at least three times.The error bars in the text all mean standard deviation.

When the on-load voltage below 30 V under N2O flow, the ignition grain was difficult to be ignited,indicating that there was a minimum threshold to initiate ignition for charring conductive polymer.However,the ignition threshold is not analyzed here,and the relevant work requires further research in the future.So,in the following analysis in this section and subsequent sections, the ignition voltage is greater than the ignition threshold of the charring conductive polymer, which meant the ignition grain were all ignited.

Fig.7(a) demonstrated that the ignition delay time of the ignition grain decreased with increasing depth of cavity and voltages.When the depth of cavity increased from 2 to 6 mm (at 50 V), the ignition delay time dropped from 462.8 ms to 320 ms.However,the influence of depth of cavity on ignition delay time was weaker at higher voltage.On one hand,as shown in Fig.6,increasing voltage speeded up the conduction between two electrodes among the surface of ignition grain and accelerated the production of pyrolyzed gasses and residual char.The current also increased quickly and the residual char could accumulate amount of heat to ignite surrounding pyrolyzed fuel gasses and oxidizer mixture in a short time.On the other hand, increasing depth of cavity changed the structure of recirculation zone in the cavity of ignition grain.The area of recirculation zone expanded with the increasing of cavity depth.The velocity of oxidizer flowing through the bottom wall of the cavity was also slightly reduced.The energy dissipated near the residual char decreased correspondingly due to the weakening of convective heat transfer.Besides, increasing depth of cavity also decreased the thickness of ignition grain below the cavity and reduced the energy taken away by heat conduction.Therefore,the charring conductive polymers could be ignited more easily with less external energy.This can be seen in Fig.7(b), it showed a tendency for external energy required for ignition trended down with the increase of voltage and depth of cavity.The decrement of energy required for ignition resulted in shorter ignition delay at large depth of cavity.In addition, Fig.8 showed the ignition processes of charring conductive polymers at different depths of cavity at 50 V.The phenomenon shown in Fig.8 was similar to the law depicted in Fig.7(a).

Fig.6.The variation of resistance between electrodes with different voltages.

Fig.7.The variation of (a) ignition delay time and (b) energy required for ignition with voltages at different depths of cavity of ignition grain.

Fig.8 also showed initial flame propagation processes of charring conductive polymers at different depths of cavity of the ignition grain.Flame luminosity images were acquired from the top view of the combustor at a flame rate of 5 kHz and the acquisition rate was sufficient to capture the transient features of ignition and initial flame propagation process.It could be seen that after ignition in the 2 mm depth cavity of the ignition grain as shown in Fig.8(a),the initial flame first propagated towards the cavity leading edge,and then anchored there.After a short period of time, the initial flame grew stronger and propagated back towards the cavity rearwall.Finally,a steady flame was formed in the cavity and started to propagate out of the cavity and the ignition grain.It took more than 235.6 ms for the flame to propagate completely out of the ignition grain.By comparison, as the depth of cavity increased, the propagation velocity of the initial flame became faster obviously.About 144 ms and 108 ms were needed for the flame to propagate completely out of the ignition grain after ignition in the 4 mm and 6 mm depths of cavity,respectively.The increasing depth of cavity enlarged the size of low-speed recirculation zone in the cavity.The pyrolyzed fuel gasses and oxidizer mixture could be kept in the cavity longer and became fully mixed.Therefore, within the same amount of time after ignition, the chemical reaction in the cavity turned stronger and the flame propagation speed became faster.

Fig.8.Ignition process of charring conductive polymer ignition grain with different depths of cavity at 50 V.

It can be seen that cavity configuration has great influence on ignition and initial flame propagation.As the depth of the cavity decreased,a more deteriorated flow field environment was formed inside the cavity, which was not beneficial to ignition, and would affect the subsequent initial flame propagation process.

3.3.Effect of different ignition positions

The ignition processes and characteristics of charring conductive polymers with a cavity geometry in precombustion chamber were measured at three different initial ignition positions (fore,middle and rear side of the cavity, as shown in Fig.1(d)) of the ignition grain.Fig.9 showed the variation of ignition delay with voltages at different ignition positions.The depth of cavity was set at 4 mm and the oxidizer was injected through a direct injector.In order to reduce the error of the results caused by accidental factors in tests,each condition had been repeated at least three times.

Experimental results showed that the ignition delay time of charring conductive polymer ignition grain became longer first,and then shorter as the ignition position continued to move towards the rear wall of the cavity.Specially,the ignition delay time were 307.4 ms and 245.4 ms respectively when ignited in the fore and rear sides of the cavity,which were 18%and 34.5%shorter than that of 374.6 ms when ignited in the middle side of the cavity (at 50 V).The reason may be that the velocities were relatively low in the fore and rear sides of the cavity near the bottom wall.The energy lost near the residual char from convection heat transfer was correspondingly lower [39].The charring conductive polymers could be ignited more easily by applying less external energy, as seen in Fig.10.As a result, the ignition delay time shortened accordingly when ignited in the fore and rear sides of the cavity.Furthermore, the ignition processes of charring conductive polymers at different initial ignition positions of the ignition grain at 50 V were displayed in Fig.11.The phenomenon presented in Fig.11 was similar to the discovery illustrated in Fig.9.

Fig.9.The variation of ignition delay time with voltages at different initial positions.

Fig.10.The variation of energy required for ignition with voltages at different initial positions.

Fig.11 also depicted the initial flame propagation process of charring conductive polymer ignition grain at different ignition positions at 50 V.As shown in Fig.11,after ignition in the fore side of the cavity, the initial flame first appeared at the cavity leading edge.Then the initial flame grew stronger and propagated towards the cavity rear-wall under the action of the main flow.A steady flame was formed in the cavity with less than 36.8 ms and started to propagate out of the cavity and the ignition grain.It took more than 176.6 ms for the flame to propagate completely out of the ignition grain.By comparison, after ignition in the middle side of the cavity,the initial flame first propagated back towards the cavity leading edge among the bottom wall of the cavity, and then anchored there.After a short time, the initial flame grew stronger and propagated towards the cavity rear-wall.A steady flame was formed in the cavity using less than 26.6 ms and started to propagate out of the cavity and the ignition grain.It took a shorter time(about 144 ms) for the flame to propagate completely out of the ignition grain.Similar to ignition in the middle side of the cavity,the initial flame which was ignited in the rear side of the cavity still first propagated back towards and near the cavity leading edge,and then propagated towards the cavity rear-wall.A steady flame was formed in the cavity with less than 8.2 ms before starting to propagate out of the cavity and the ignition grain.Among all examined ignition positions, this took the shortest time (about 91.8 ms) for the flame to propagate completely out of the ignition grain.

The reason for the above differences may be caused by different flow states at different positions in the cavity of the ignition grain.When ignited in the fore side of the cavity, the initial flame and pyrolyzed fuel oxidizer mixture were directly transmitted out of the cavity by convection and had a short residence time in the cavity.The chemical reaction in the cavity was weaker and the flame propagation speed was slower.In contrast, as the ignition position continued to move towards the rear wall of the cavity,the pyrolyzed fuel gasses and oxidizer mixture could stay in the cavity for longer time to become fully mixed.Therefore,in the same time after ignition, the chemical reaction in the cavity would become stronger and the flame propagation speed was faster.

Fig.11.Ignition process of charring conductive polymer ignition grain at different ignition positions at 50 V.

3.4.Effect of different injection scheme

In order to evaluate the influence of oxidant injection schemes on the ignition performance of charring conductive polymer ignition grain, swirling injection condition had been also studied.Fig.12 depicted the variation of ignition delay time and external energy required for ignition under different injection conditions.The grain was applied to voltage of 50 V and was tested at different ignition positions (fore, middle and rear side of the cavity).The depth of cavity was set at 4 mm.In order to reduce the error of the results caused by accidental factors in tests, each condition had been repeated at least three times.

According to the results displayed in Fig.12(a),the ignition delay time of the charring conductive polymer ignition grain seemed to become shorter under swirling injection condition when ignited in the middle and rear side of the cavity.Specially, the ignition delay time were 244.4 ms and 197.6 ms respectively when ignited in the middle and rear side of the cavity under swirling injection condition, which were 35%and 19.5% shorter than that of 374.6 ms and 245.4 ms respectively when ignited under direct injection condition.Possible explanation could be that as the oxidant flowed towards the rear wall of the cavity, its tangential velocity decreased significantly near the bottom wall of the cavity, which correspondingly decreased the energy dissipated near the electrode tips due to convection heat transfer [40].Therefore, the charring conductive polymers could be ignited more easily with less external energy, as shown in Fig.12(b), which correspondingly reduced the ignition delay time.It worth noting that the ignition delay under the swirling condition was much longer when ignited in the fore side of the cavity, because this ignition position was located below the outlet of the swirl injector.The oxidizer flow had high tangential velocities and fast flowed through the ignition grain surface which increased the heat convection among the surface of ignition grain near the electrode tips.Therefore,it was necessary to provide a large external energy to achieve the ignition critical state,which led to a longer ignition delay time value.

Fig.12.The variation of (a) ignition delay time and (b) energy required for ignition at different injection conditions and ignition positions.

Fig.13.Ignition process of charring conductive polymer ignition grain under swirling injection condition at 50 V.

In addition,it could be seen in Fig.13 that a significant difference in initial flame propagation process of charring conductive polymer with a cavity geometry in precombustion chamber had been observed.In swirling injection condition, the oxidant entered into the cavity directly and propagated spirally towards the rear wall of the cavity instead of being sucked into the cavity by the shear layer in condition of direct injection.The initial flame mainly propagated towards the cavity rear wall after ignition in the middle side of the cavity.As the strength of the swirling decreased along the axial direction of the ignition grain, it existed a pressure gradient along the axial direction and a depression zone at the inlet of the ignition grain[41].Part of the flames and carbon particles would propagate back towards cavity leading edge along both sides of the cavity.

A steady flame could be formed in the cavity in less than 6.2 ms and started to propagate out of the cavity and the ignition grain.It took a shorter time about 90.2 ms for the flame to propagate completely out of the ignition grain compared with that under the direct injection condition.Besides,the initial flame intensity under swirl condition was higher than that under direct flow condition.Possible causes could be that swirl injection tremendously prolonged the existing period of oxidizer in the cavity and consequently promoted the mixture of oxidizer and fuel pyrolyzed from ignition grain.Therefore, within the same amount of time after ignition, the chemical reaction in the cavity became stronger and the flame propagation speed turned faster.

3.5.Repeated ignition characteristic

To further study the repeated ignition characteristic of charring conductive polymer,multiple ignition operations were performed.The depth of cavity of the ignition grain was 4 mm and the ignition position was set at middle side of the cavity.The oxidizer was directly injected into the ignition combustor.The value of the voltage applied to the ignition grain was 50 V.The repeated ignition process of charring conductive polymer was actualized sequentially,as shown in Fig.14.In order to reduce the error of the results caused by accidental factors in tests,the multiple ignition tests had been repeated three times.

According to the results displayed in Fig.15, the ignition delay time and external energy required for ignition decreased with the increase of repeated ignition times.This could be attributed to the fact that the ignition grain had formed conductive char layer among the surface between two electrodes after the first ignition, which shortened the time for charring conductive polymer to form conductive circuit and pyrolysis in the subsequent ignition.Besides,the current was enhanced quickly in a short time which increased the energy and accumulated heat in the same time.Due to the significant reduction in ignition delay time,the energy loss through convection was reduced accordingly.The external energy required for ignition was also reduced.However, the variation of ignition delay time and external energy required for ignition decreased with increasing of ignition times.The trend of conduction and current on the surface of ignition grain became similar after repeated ignitions.

Fig.14.Repeated ignition process of charring conductive polymer ignition grain.

Fig.15.The variation of ignition delay time and energy required for ignition under multiple operations.

4.Conclusions

The ignition processes and characteristics of charring conductive polymer ignition grain with a cavity geometry in precombustion chamber for hybrid propulsion applications of micro/nano satellite were investigated in this paper.The main conclusions from the above studies can be summarized as follows:

(1) Cavity configuration has great influence on ignition and initial flame propagation.As the depth of the cavity decreased,values of ignition delay time and external energy required for ignition increases from 320 to 462.8 ms and 10.75 J to 16.2 J, respectively (at 50 V).A more deteriorated flow field environment is formed inside the cavity, which is not beneficial to ignition, and will affect the subsequent initial flame propagation process.

(2) The rear side of the cavity is the ideal ignition position which has a shorter ignition delay time and a faster flame propagation speed compared to other ignition positions.The ignition delay time is 245.4 ms when ignites in the rear side of the cavity, which is 34.5% shorter than that of 374.6 ms when ignites in the middle side of the cavity (at 50 V).The external energy required for ignition is also decreased from 15.1 to 8.7 J.

(3) Compared to direct injection scheme, swirling injection can provide a more favorable flow field environment in the cavity, which is favorable for ignition and initial flame propagation,but the ignition position needs to be away from the outlet of swirling injector.

(4) The increase in repeated ignition times leads to the decrease of ignition delay time and external energy required for ignition.However, the variation of ignition delay time and external energy required for ignition decreases with the increasing of ignition times.The trend of conduction and current on the surface of ignition grain become similar after repeated ignition.

(5) There are two different ignition mechanisms of charring conductive polymer ignition grain proposed in this paper.For heated residual char ignition, the residual char produced by pyrolysis achieves ignition by absorbing external electrical energy and continuously heats local mixture of the pyrolyzed fuel gasses and oxidizer near the electrode tips.For heated carbon particle ignition, carbon particle separated from the residual char decomposes into several high temperature small particles under the action of the shear layer and ignites the local mixture of oxidizer and pyrolyzed fuel gasses to achieve ignition.Increasing the on-load voltage can promote charring conductive polymer to be inclined to heated carbon particle ignition,which has a shorter delay and can establish a global flame in the cavity more quickly.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded in part by the Fundamental Research Funds for the Central Universities (Grant No.30920041102) and National Natural Science Foundation of China(Grant No.11802134).