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O2 rapid mixed swirl torch igniter for hybrid rocket motors
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Combustion characterization of a CH4 /O2 rapid mixed swirl torch igniter for hybrid rocket motors
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School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China
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Article history: Received 26 June 2019 Received in revised form 17 November 2019 Accepted 23 December 2019 Available online xxxx
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Yi Wu ∗ , Zixiang Zhang, Fuwen Liang, Ningfei Wang
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Keywords: Oxygen/methane Torch igniter Hybrid rocket motor
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Combustion characteristics of a rapid mixed swirl torch igniter of CH4 /O2 for hybrid rocket motors has been experimentally investigated. The igniter torch consists of four tangential slits circumferentially equispaced on the internal micro-combustion chamber allows tangential injection of O2 gas, meanwhile the CH4 flows into the main channel and interacts with four tangential O2 injections flow tgo generate a swirl mixing of CH4 and O2 . The ignitability of this swirl torch igniter was verified by large number of experiments in variation of equivalence ratio and total flowrate of CH4 /O2 . The flame structure of the igniter and effect of main oxidizer injection acting as a cross flow on the flame has been investigated by using OH* and CH* chemiluminescences techniques. It is found that the torch igniter can reliably ignite instantaneously in the range of equivalence ratio 0.2∼1.4 and the effect of main oxidizer injection on the flame is tiny and can be neglected. In addition, the performance of this rapid mixed swirl flame has been investigated by using a lab scaled hybrid rocket motor and compared with the same hybrid rocket motor ignited by catalytic bed. The performance of this igniter and its comparison with catalytic bed ignition will be discussed and analyzed in detail. © 2019 Elsevier Masson SAS. All rights reserved.
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1. Introduction
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Hybrid rocket motors (HRMs) demonstrate great potential to meet the multiple requirements of high performance propulsion systems owning to its advantages compared to solid and liquid rocket motors such as high safety in propellant production, storage, transportation and testing processes, capability of restart and simplicity in pipe arrangement. Recently, numerous investigations have been performed to address the main issues relevant to high performance hybrid rocket motor such as injection optimization [1], combustion of biphasic fuels [2–4], ignition process and endburning hybrid rocket motors etc. [5–10]. Ignition system is an essential part for high performance hybrid rocket motors. A reliable ignition system must meet the requirements of successfully and precisely ignited the oxidizer/solid fuel in large range of working conditions with the optimum time delay. A resume of literature reveals that there are mainly two types of ignition systems applied in a hybrid rocket motor: the ignition system with catalytic bed for hybrid rocket motors that using selfdecomposed oxidizers such as H2 O2 etc. and the torch ignition system that using solid or liquid additional oxidizer/fuel as igni-
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Corresponding author. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.ast.2019.105666 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.
tion energy source [6,11–19]. Even though catalytic bed ignitions system has the advantages of multi-start capability and simplicities in electronic arrangement i.e. elimination of the need of electronic components for spark plug, it encounters various intrinsic disadvantages. For instance, catalytic bed ignition can only be used for oxidizers who have self-decomposed property and considerable ignition delay is necessary due to the preheating time needed for catalytic bed. Furthermore, efficiency of catalytic bed ignition is influenced significantly by the temperature of catalytic bed. Another issue of ignition by catalytic bed is that when scaling up to larger thrust classes the payload of rocket motor will decrease dramatically due to increased mass weight of catalytic bed [20]. It also demands that the material of catalytic bed must sustain high temperature conditions without degradation. Prior to catalytic bed ignition method, using an ignition torch is an alternative approach to achieve stable and efficient ignition of hybrid rocket motor. Both solid and gaseous/liquid propellant can be used in torch ignition system [12,16,19]. However, for torch ignition system using solid propellants usually they can only be used in single start of hybrid rocket motors that disables the main benefit of hybrid rocket motors i.e. multi-start capability. Torches ignition system using gas/liquid propellants has drawn attentions recently due to its multiple advantages such as multi-start capability, wide range of working operability and reusability [21]. Among various types of liquid/gas propellants, CH4 /O2 torch ignition system is a promising
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Fig. 1. Schematic of rapid mixed swirl ignition torch.
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candidate for hybrid rocket motor ignition considering its advantages of non-toxic, low cost and low vibration properties [21–24]. Different from solid or liquid rocket motor that the fuel and oxidizer are premixed or partially premixed before the moment of ignition initiated, the ignition of hybrid rocket motor is more challenging due to the non-premixed solid fuel and liquid/gaseous oxidizer. Therefore, the outlet of ignition torch in a hybrid rocket motor should be oriented to solid fuel surface so that the solid fuel can gasify/liquefy and rapidly mix with incoming oxidizer to achieve successful ignition. Therefore, characterization of fundamental combustion properties of ignition torches such as flame penetration, width and operability limit is of essential importance to achieve precise ignition and decrease the ignition delay time. In the present work, a rapid-mixed swirl ignition torch using CH4 /O2 as ignition fuels have been comprehensively investigated for its potential applications in hybrid rocket motor. A resume of literature reveals that numerous investigations have been previously performed for CH4 /O2 swirl torch flames [21–25]. For instance, Shi et al. [26–28] investigated mixing process of CH4 /O2 rapidly mixed type tubular flame in atmospheric pressure conditions and elucidated the main factors that dominated the mixing process. In the work of Shim et al. [29], the effect of gaseous methane/oxygen injection velocity ratio on the shear coaxial jet flame structure was analyzed in atmospheric pressure condition. Sanchez et al. [30] developed an swirl torch ignition system using GOx/LOx/GCH4 /LCH4 as propellants, the ignition operability limits in terms of flowrates, mixture ratio and temperature of igniter used propellants were obtained. Ivanov et al. [31] numerically studied the back and side flow regions of the plume exhausted from a typical thruster nozzles, both flows in subsonic and supersonic sections of the nozzle were studied. Biswas et al. [32,33] studied the ignition mechanisms of premixed CH4 /air and H2 /air mixtures using a turbulent hot jet generated by pre-chamber combustion. They measured the jet penetration and ignition process inside the main combustion chamber. It can be found that most of the previous investigations are limited in flame characterization or ignition process in a simplified combustion chamber. Investigations of performance of CH4 /O2 ignition torches in working conditions that similar with those encountered in practical rocket motors are still limited. Moreover, there is a lack of investigations of interaction between ignition torch flame and injection of main oxidizer in hybrid rocket motor which can potentially influence the performance of the ignition torch. In the present work, a CH4 /O2 rapid mixed swirl ignition torch system for hybrid rocket motor has been proposed and studied. A comprehensive investigation of combustion characterization of this ignition torch will be firstly addressed by using OH*/CH* chemiluminescences imaging technique including plume penetration, width and range of working operability in terms of flowrate ( Q ) and equivalence ratio (ϕ ). A simulation work was also performed by using detailed kinetic mechanism GRI-Mech 3.0 to simulate the 1D pre-mixed freely propagating CH4 /O2 flame. Distributions of radical OH and CH are calculated in different equivalence ratio conditions as an auxiliary analyze for CH4 /O2 ignition torch flame presented in the present work. Then, the influence of main
oxidizer flow injection on the ignition torch has been addressed by implementing the ignition torch to a hybrid rocket motor head i.e. incoming oxidizer injection impact acting as cross flows to torch flame. Finally, in order to evaluate the performance of the ignition torch proposed in the present work, fire experiments are performed using a lab-scale hybrid rocket motor with the ignition torch proposed in the present work. The results are compared with catalytic bed ignition method and discussed in detail.
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2. Experimental set-up and procedures
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2.1. Rapid-mixed swirl ignition torch
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The ignition torch is made of stainless steel, which consists of a micro-combustion chamber with internal diameter of 5 mm and length of 80 mm, a spark plug and a nozzle with throat diameter of 3 mm. Four tangential slits circumferentially equispaced on the internal micro-combustion chamber allows tangential injection of O2 gas, meanwhile the CH4 flows into the main channel and interacts with four tangential O2 injections flow to generate a swirl mixing of CH4 and O2 . The mixed propellants are then ignited using a spark ignition system. The spark plug (Product No. NST-A7TI) is made of a 0.4 mm diameter iridium alloy center electrode and a special ground electrode. The ignition energy of the spark plug is around 50 mJ. A pressure transducer is added in the combustion chamber to monitor the pressure and furthermore evaluate the ignition delay. Two mass flow controller (Bronkhorst) has been used to regulate the flowrate of CH4 /O2 allowing adjusting equivalence ratio and total flowrate of ignition torch. The spark plug ignition system consists of a high voltage ignition power supply, a normal spark plug and a spark plug cap. The signal triggering device is added to facilitate the sequential control. The whole torch ignition control system and the data acquisitions are achieved by LabVIEW programming (Fig. 1).
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2.2. Optical set-up
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In order to characterize the operability range of the rapid mixed ignition torch, OH* and CH* chemiluminescences imaging technique was applied to visualize the ignition flame torch. CH* is commonly identified as a marker that would track the flame front and heat release of a reactive flow. OH* is a long lived species and may be particularly retained in reaction zone and post-reaction zone in a flame. Therefore, measurements of OH* and CH* chemiluminescences can provide useful information on the reaction zone location, combustion efficiency and approximations of heat release of the CH4 /O2 ignition torch in a non-intrusive and straightforward manner [34]. Therefore, the flame length and width can be identified by the signals. The flame is recorded by a thermoelectrically cooled, 16-bit intensified CCD camera (Pi-Max 4, Roper Scientific) with a 1024 × 1024 array. The camera is equipped with an f /2.8, f = 100 mm, achromatic UV lens (CERCO) combined with an interference bandpass filter centered at 310 nm and 430 nm having a bandwidth of 10 nm [35]. A 82 × 82 mm2 area of the flame is imaged by the ICCD camera, so that the spatial resolution is about
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Fig. 2. (a) Optical arrangement for studying the effect of main oxidizer injection on ignition torch flames. (b) Lab-scaled hybrid rocket motor used in the present work.
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Fig. 3. Schematic of experimental set-up for the lab scale hybrid rocket motor.
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80 μm per pixel. The acquisition repetition rate of the camera is kept at 20 Hz. The intensification is also kept constant so that the flames captured show a weak sensitivity to the post processing parameters. For each case the analysis is based on the averaged flame of 20 frames.
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2.3. Lab-scaled hybrid rocket motor
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In order to investigate the potential effects of main oxidizer injection on the ignition torch flame, the ignition torch system is firstly implemented to the head of a lab-scaled hybrid rocket motor to visualize the interaction between main oxidizer injection and ignition torch flame. The angle between the main oxidizer injection axial direction and the ignition torch flame is kept at 30 degree as illustrated in Fig. 2. The camera is positioned vertically face to the axial direction of the main oxidizer injection. Finally, in order to verify the performance of the ignition torch in hybrid rocket motors, fire experiments were conducted using a lab-scaled hybrid rocket motor with propellants of O2 (g) and paraffin-based solid fuel. As shown in Fig. 3, the whole test facility of the lab-scale hybrid rocket motor consists of three major parts: pipework, controls and instrumentation. In the pipework, three fluids are used, i.e., O2 serving as the oxidizer for hybrid rocket motor and as the oxidizer for ignition torch system, CH4 as the fuel for ignition torch and N2 as the combustion chamber purge, quenching the combustion after a test or in the event of a
test abort. Fig. 4 shows the experimental hybrid rocket motor configuration. With a pre-combustion chamber at the head, the length of the main combustion chamber of the hybrid rocket motor used in the current work is 240 mm; the lengths of the fore and aft ends of the combustion chamber are 20 and 40 mm respectively. The outer diameter of the solid fuel grain is 70 mm. The oxygen flow rate was controlled at 45 g/s, so that the total flow rate with the paraffin-based fuel was around 50 g/s. A conical nozzle made of red copper with a diameter of 6 mm was used in the present work. Two pressure sensors were located at the fore and aft ends of the combustion chamber, to monitor the pressure variation during the firing test. 2.4. Numerical set-up
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In order to better understand the OH* and CH* distribution in the ignition torch flames. One dimensional freely propagating flame of CH4 /O2 was simulated by using GRI-Mech 3.0 kinetic mechanism for various equivalence ratio conditions (Cosilab software package). The transport properties are deduced by using the averaged diffusion model. The Newton unsteady adaptive mesh algorithm has been used and allows an adaptive mesh refinement during the computation (‘grad’ and ‘curve’ values fixed to 1 × 10−5 ). The total final number of the grid is 300 for a physical domain of 0.1 m in order to ensure a chemical equilibrium state in the burned gases. The number of the grid was proved to be sufficient to make the simulation results grid-independent. The kinetic
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Fig. 4. O2 /Paraffin-based solid hybrid rocket motor used in the present work.
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Fig. 5. Rapid-mixed swirl ignition torch flame OH* (left) and CH* (right) chemiluminescences images with different equivalence ratio (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
ϕ = 0.4–1.3 and flowrate Q = 45 nl/min.
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Table 1 Test matrix of the ignition torch flames investigated in the present work.
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mechanism of GRI-Mech 3.0 that consists of 325 elementary chemical reactions with associated 53 species involved.
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3. Results and discussion
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Experiments were firstly performed to characterize the operationability range of ignition torch in variation of equivalence ratio and flowrate of O2 /CH4 mixture by using OH* and CH* chemiluminescences technique. Then, effect of cross-flow i.e. the main oxidizer injection on the ignition torch flame was investigated. Finally, the ignition ability of the torch was verified by conducting fire experiments using a lab-scale hybrid rocket motor. The experimental conditions measured in the present work has been listed in Table 1.
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3.1. Effects of equivalence ratio and flowrates on ignition torch flame
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As illustrated in Fig. 5, the rapid-mixed swirl ignition torch has a wide range of working conditions with variations of flowrate and
equivalence ratio. It is observed that the ignition torch has a stable combustion regime for conditions of equivalence ratio ϕ > 0.2. When equivalence ratio lower than 0.2, stable ignition and combustion regime of torch flame cannot always be achieved. For OH* chemiluminescences images, it is observed that the length of torch flames increase with higher equivalence ratio for conditions of ϕ = 0.3–1.0. OH* intensity attained the maximum value at condition of equivalence ratio around ϕ = 0.9. When equivalence ratio higher than ϕ > 1, the OH* intensity begins to decrease. Meanwhile the CH* images exhibit different variation tendency that the concentration of CH* increases continually with equivalence ratio increases. Moreover, the maximum intensity area of CH* and OH* are differently located. For OH* images, the maximum intensity area is always just near the outlet of the nozzle, meanwhile for CH* image, the maximum intensity is located some distance from the nozzle outlet. The geometry of flame changes with equivalence ratio as well. It is found that for fuel rich conditions (ϕ = 1.3) the width at downstream of the flame is larger than the flames at fuel lean conditions (ϕ = 0.3). This is due to the soot radiation effect in the flames for fuel rich conditions. The radiation of the soot in flames has a wide bande which is overlaps with CH* signals. This also explains the reason why the intensity of CH* is higher than OH* for condition of ϕ = 1.3 in Fig. 5. The influence of flowrate on the flame geometry remains similar for both OH* and CH* distribution i.e. the length of the flame increases with higher flowrate as shown in Fig. 6. When flowrate higher than 55 nl/min, supersonic flow is obtained and shock diamonds are formed at the nozzle exit, meanwhile the plume is still stable. It should be noted that the OH* intensity is gener-
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Fig. 6. Rapid-mixed swirl ignition torch flame OH* (left) and CH* (right) chemiluminescences images with different flowrate Q = 35–75 nl/min and t equivalence ratio ϕ = 0.8.
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Fig. 7. OH and CH distribution of 1D laminar flame of CH4 /O2 with 300 K, P = 1 bar.
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Fig. 8. OH distribution of 1D laminar flame of CH4 /O2 with equivalence ratio 0.2–1.3, T = 300 K, P = 1 bar.
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ally higher than CH* intensity for all condition investigated in the present work i.e. ϕ = 0.8 and Q = 35–75 nl/min.
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3.2. 1 Dimensional premixed flame of CH4 /O2
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The chemiluminescences intensity of OH* and CH* observed in the previous section can be explained by 1 dimensional premixed flame simulation using detailed kinetic mechanism. As illustrated in Fig. 7, for premixed laminar flames of CH4 /O2 , the specie of CH is appeared shortly before OH and quickly disappeared at postreaction zone of the flame. Meanwhile OH appears at the beginning of the reaction and remains high concentration value at the post-reaction zone. Fig. 8 shows the OH concentration in variation of equivalence ratio. It can be seen that the mole fraction of OH firstly increases with equivalence ration increase, and then decreases with increase of equivalence ratio. The maximum value located at condition of equivalence ratio around ϕ = 0.8∼0.9. It is found that the variation of CH has similar tendency with OH. This partially explains the ignition flame torch variation tendency in Fig. 5 for conditions of equivalence ratio ϕ = 0.4∼0.7. Meanwhile for fuel rich conditions the experimental results has a different variation tendency due to the soot generation. Laminar flame speed and adiabatic flame temperature of CH4 /O2 flames in variation of equivalence ratio were calculated as well. As illustrated at Fig. 9, laminar flame speed increases with in-
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Fig. 9. CH4 /O2 laminar flame speed and adiabatic flame temperature variation versus equivalence ratio ϕ = 0.2–1.3.
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crease of equivalence ratio for ϕ = 0.2–0.9, and arrives a maximum value of 3.09 m/s at condition of equivalence ratio ϕ = 0.9. When equivalence ratio higher than 0.9, the flame speed decreases for conditions of ϕ = 0.9–1.3. Moreover, the adiabatic flame temperature of CH4 /O2 follows similar variation tendency except that the maximum value T = 3055 K located at equivalence ratio ϕ = 1.1.
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Table 2 Ground ignition test system timing control.
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Serial number
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Data acquisition On Main oxidizer O2 On Ignition torch CH4 /O2 On Ignition spark On Ignition Torch CH4 /O2 Off Main oxidizer O2 Off N2 purge On Data acquisition Off
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Fig. 10. Swirl number variation versus equivalence ratio.
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3.3. Effects of swirl number
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The swirl number is a parameter which characterizes the swirling intensity. It can be calculated by the following equation Sw ∼
mt2 A , m2T A j
i.e. the ratio of the axial flux of tangential momen-
tum to the axial momentum at a cross section [36]. A and A t are the cross sectional area of the main channel of the igniter and the total section area of the tangential slits. mt and m T are the total mass flowrates through the tangential inlets and the test section. As illustrated in Fig. 10, the swirl number (swirl strength) decreases with increase of equivalence ratio. This variation tendency can be explained by the definition of the swirl number. When equivalence ratio decreases with constant the total flowrate of oxygen methane mixture, the tangential flowrate of oxygen increases and the axial flowrate of methane decrease leading to higher swirl number. This tendency reverses when the tangential inlet changed to methane flow i.e. the oxygen flows in to the igniter in axial direction. Meanwhile, the swirl number doesn’t change with flowrate variation with constant equivalence ratio.
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3.4. Effects of oxidizer injection
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The main injection of oxidizer acting as a cross flow can have a potential influence on the ignition torch flame and ultimately affect the performance of the ignition system. Therefore, in this section, the effect of cross-flow i.e. main oxidizer injection on the torch flame will be addressed by implanting the ignition torch at the head of a hybrid rocket motor. Experiments were first con-
ducted with different flow rate of gaseous O2 as the main oxidizer. In order to facilitate the parameters, the flowrate and equivalence ratio of ignition torch flame is kept ϕ = 1.2 and Q = 45 nl/min. Considering the higher signal to noise ratio of OH* chemiluminescences, the following experiments were only performed with OH* chemiluminescences technique. An observation of Fig. 11 reveals that with moderate flowrate O2 injection, the OH* intensity slightly increased at the case of Q = 13.2 g/s possibly due to the afterburning of fuel rich mixture of CH4 /O2 of the ignition torch. However, when the flowrate of main oxidizer injection O2 continue to increase (for the cases Q > 13.2 g/s) the downstream of torch flame has a deformed shape i.e. the height of the flame decreased due to the impact of the main oxidizer injection. Similar behavior has been observed when the cross-flow are changed to N2 indicating that the effects of cross-flow on the torch flame is dominated by transportation and the chemical kinetic effects is neglectable. It shows also that the CH4 /O2 ignition torch has a robust flow regime that the effect of main oxidizer injection on the torch flame is tiny.
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3.5. Experiments with lab-scale hybrid rocket motor
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In order to further verify the performance of the ignition torch in practical working conditions of a rocket motor, fire experiments were performed by using a lab-scaled gaseous O2 /Paraffinbased hybrid rocket motor. The sequential control of the system is achieved by controlling of series of pneumatic valves located in the feeding lines. The main injection oxidizer is started firstly, and then the ignition torch was initiated in about 3 seconds. As illustrated in Fig. 12, it can be seen that the paraffin-based solid fuel can be ignited instantaneously once the CH4 /O2 ignition torch was initiated. High pressure combustion regime is quickly established after the ignition started with a short ignition delay. The time control details are listed in Table 2. A comparison plot between our previous work that ignition of a hybrid rocket motor by catalytic bed (H2 O2 as oxidizer) and igni-
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Fig. 11. Ignition torch flame ( Q CH4 /O2
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Fig. 12. Comparison of combustion chamber pressure between hybrid rocket motor ignited by CH4 /O2 torch igniter (left) and hybrid rocket motor ignited by H2 O2 catalytic bed (right), both under 50 g/s total flowrate.
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tion of hybrid rocket motor by the CH4 /O2 ignition torch proposed in the present work was performed. Both of the two experiments are using the same type of paraffin-based fuel with same total flowrate around 50 g/s. Details of the H2 O2 /paraffin-based hybrid rocket motor can be found in Ref. [6]. It shows in Fig. 12 that for catalytic bed ignition method, there is a considerable ignition delay around 4 seconds possible due to the time needed for preheating the catalytic bed. Meanwhile, using the ignition torch presented, ignition is rapidly achieved with a short delay time.
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4. Conclusion
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Experimental investigation into an CH4 /O2 rapid-mixed swirl ignition torch has been performed. Combustion characterization of this swirl ignition torch was achieved by using OH* and CH* chemiluminescences technique in variation of flowrate and equivalence ratio. The effect of main oxidizer injection acting as a cross flow on the ignition torch was elucidated. Finally, the performance of this CH4 /O2 rapid mixed swirl ignition torch was verified by using a lab-scaled hybrid rocket motor. The interference drawn from the above studies can be summarized as follows.
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• The CH4 /O2 rapid-mixed swirl ignition torch has a wide operability range for equivalence ratio ϕ > 0.2, the flame length increases along with higher equivalence ratio value and total flowrate as well. • For fuel rich conditions, the OH* and CH* distribution is different, for CH* images the maximum value located downstream of the flame, meanwhile OH* maximum value located just behind of the nozzle outlet. • Effects of main oxidizer injection acting as a cross flow has on the ignition torch flame are limited i.e. along with the increase of flowrate of the oxidizer augmented, only the front of the ignition torch flame was slightly disturbed resulting a decrease of flame height. Moreover, the influence is dominated by flow dynamics and the influence of chemical kinetics is tiny. • The CH4 /O2 ignition torch can effectively be used in hybrid rocket motor with a short ignition delay compared with catalytic bed ignition method. Declaration of competing interest
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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.
Acknowledgement
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This work was supported by the National Natural Science Foundation of China [Grant No. 51906016] and Starting Research Foundation from Beijing Institute of Technology.
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References
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Combustion characterization of a CH4 /O2 rapid mixed swirl torch igniter for hybrid rocket motors
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Article history: Received 26 June 2019 Received in revised form 17 November 2019 Accepted 23 December 2019 Available online xxxx
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Yi Wu ∗ , Zixiang Zhang, Fuwen Liang, Ningfei Wang
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Keywords: Oxygen/methane Torch igniter Hybrid rocket motor
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Combustion characteristics of a rapid mixed swirl torch igniter of CH4 /O2 for hybrid rocket motors has been experimentally investigated. The igniter torch consists of four tangential slits circumferentially equispaced on the internal micro-combustion chamber allows tangential injection of O2 gas, meanwhile the CH4 flows into the main channel and interacts with four tangential O2 injections flow tgo generate a swirl mixing of CH4 and O2 . The ignitability of this swirl torch igniter was verified by large number of experiments in variation of equivalence ratio and total flowrate of CH4 /O2 . The flame structure of the igniter and effect of main oxidizer injection acting as a cross flow on the flame has been investigated by using OH* and CH* chemiluminescences techniques. It is found that the torch igniter can reliably ignite instantaneously in the range of equivalence ratio 0.2∼1.4 and the effect of main oxidizer injection on the flame is tiny and can be neglected. In addition, the performance of this rapid mixed swirl flame has been investigated by using a lab scaled hybrid rocket motor and compared with the same hybrid rocket motor ignited by catalytic bed. The performance of this igniter and its comparison with catalytic bed ignition will be discussed and analyzed in detail. © 2019 Elsevier Masson SAS. All rights reserved.
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1. Introduction
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Hybrid rocket motors (HRMs) demonstrate great potential to meet the multiple requirements of high performance propulsion systems owning to its advantages compared to solid and liquid rocket motors such as high safety in propellant production, storage, transportation and testing processes, capability of restart and simplicity in pipe arrangement. Recently, numerous investigations have been performed to address the main issues relevant to high performance hybrid rocket motor such as injection optimization [1], combustion of biphasic fuels [2–4], ignition process and endburning hybrid rocket motors etc. [5–10]. Ignition system is an essential part for high performance hybrid rocket motors. A reliable ignition system must meet the requirements of successfully and precisely ignited the oxidizer/solid fuel in large range of working conditions with the optimum time delay. A resume of literature reveals that there are mainly two types of ignition systems applied in a hybrid rocket motor: the ignition system with catalytic bed for hybrid rocket motors that using selfdecomposed oxidizers such as H2 O2 etc. and the torch ignition system that using solid or liquid additional oxidizer/fuel as igni-
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Corresponding author. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.ast.2019.105666 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.
tion energy source [6,11–19]. Even though catalytic bed ignitions system has the advantages of multi-start capability and simplicities in electronic arrangement i.e. elimination of the need of electronic components for spark plug, it encounters various intrinsic disadvantages. For instance, catalytic bed ignition can only be used for oxidizers who have self-decomposed property and considerable ignition delay is necessary due to the preheating time needed for catalytic bed. Furthermore, efficiency of catalytic bed ignition is influenced significantly by the temperature of catalytic bed. Another issue of ignition by catalytic bed is that when scaling up to larger thrust classes the payload of rocket motor will decrease dramatically due to increased mass weight of catalytic bed [20]. It also demands that the material of catalytic bed must sustain high temperature conditions without degradation. Prior to catalytic bed ignition method, using an ignition torch is an alternative approach to achieve stable and efficient ignition of hybrid rocket motor. Both solid and gaseous/liquid propellant can be used in torch ignition system [12,16,19]. However, for torch ignition system using solid propellants usually they can only be used in single start of hybrid rocket motors that disables the main benefit of hybrid rocket motors i.e. multi-start capability. Torches ignition system using gas/liquid propellants has drawn attentions recently due to its multiple advantages such as multi-start capability, wide range of working operability and reusability [21]. Among various types of liquid/gas propellants, CH4 /O2 torch ignition system is a promising
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Fig. 1. Schematic of rapid mixed swirl ignition torch.
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candidate for hybrid rocket motor ignition considering its advantages of non-toxic, low cost and low vibration properties [21–24]. Different from solid or liquid rocket motor that the fuel and oxidizer are premixed or partially premixed before the moment of ignition initiated, the ignition of hybrid rocket motor is more challenging due to the non-premixed solid fuel and liquid/gaseous oxidizer. Therefore, the outlet of ignition torch in a hybrid rocket motor should be oriented to solid fuel surface so that the solid fuel can gasify/liquefy and rapidly mix with incoming oxidizer to achieve successful ignition. Therefore, characterization of fundamental combustion properties of ignition torches such as flame penetration, width and operability limit is of essential importance to achieve precise ignition and decrease the ignition delay time. In the present work, a rapid-mixed swirl ignition torch using CH4 /O2 as ignition fuels have been comprehensively investigated for its potential applications in hybrid rocket motor. A resume of literature reveals that numerous investigations have been previously performed for CH4 /O2 swirl torch flames [21–25]. For instance, Shi et al. [26–28] investigated mixing process of CH4 /O2 rapidly mixed type tubular flame in atmospheric pressure conditions and elucidated the main factors that dominated the mixing process. In the work of Shim et al. [29], the effect of gaseous methane/oxygen injection velocity ratio on the shear coaxial jet flame structure was analyzed in atmospheric pressure condition. Sanchez et al. [30] developed an swirl torch ignition system using GOx/LOx/GCH4 /LCH4 as propellants, the ignition operability limits in terms of flowrates, mixture ratio and temperature of igniter used propellants were obtained. Ivanov et al. [31] numerically studied the back and side flow regions of the plume exhausted from a typical thruster nozzles, both flows in subsonic and supersonic sections of the nozzle were studied. Biswas et al. [32,33] studied the ignition mechanisms of premixed CH4 /air and H2 /air mixtures using a turbulent hot jet generated by pre-chamber combustion. They measured the jet penetration and ignition process inside the main combustion chamber. It can be found that most of the previous investigations are limited in flame characterization or ignition process in a simplified combustion chamber. Investigations of performance of CH4 /O2 ignition torches in working conditions that similar with those encountered in practical rocket motors are still limited. Moreover, there is a lack of investigations of interaction between ignition torch flame and injection of main oxidizer in hybrid rocket motor which can potentially influence the performance of the ignition torch. In the present work, a CH4 /O2 rapid mixed swirl ignition torch system for hybrid rocket motor has been proposed and studied. A comprehensive investigation of combustion characterization of this ignition torch will be firstly addressed by using OH*/CH* chemiluminescences imaging technique including plume penetration, width and range of working operability in terms of flowrate ( Q ) and equivalence ratio (ϕ ). A simulation work was also performed by using detailed kinetic mechanism GRI-Mech 3.0 to simulate the 1D pre-mixed freely propagating CH4 /O2 flame. Distributions of radical OH and CH are calculated in different equivalence ratio conditions as an auxiliary analyze for CH4 /O2 ignition torch flame presented in the present work. Then, the influence of main
oxidizer flow injection on the ignition torch has been addressed by implementing the ignition torch to a hybrid rocket motor head i.e. incoming oxidizer injection impact acting as cross flows to torch flame. Finally, in order to evaluate the performance of the ignition torch proposed in the present work, fire experiments are performed using a lab-scale hybrid rocket motor with the ignition torch proposed in the present work. The results are compared with catalytic bed ignition method and discussed in detail.
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2. Experimental set-up and procedures
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The ignition torch is made of stainless steel, which consists of a micro-combustion chamber with internal diameter of 5 mm and length of 80 mm, a spark plug and a nozzle with throat diameter of 3 mm. Four tangential slits circumferentially equispaced on the internal micro-combustion chamber allows tangential injection of O2 gas, meanwhile the CH4 flows into the main channel and interacts with four tangential O2 injections flow to generate a swirl mixing of CH4 and O2 . The mixed propellants are then ignited using a spark ignition system. The spark plug (Product No. NST-A7TI) is made of a 0.4 mm diameter iridium alloy center electrode and a special ground electrode. The ignition energy of the spark plug is around 50 mJ. A pressure transducer is added in the combustion chamber to monitor the pressure and furthermore evaluate the ignition delay. Two mass flow controller (Bronkhorst) has been used to regulate the flowrate of CH4 /O2 allowing adjusting equivalence ratio and total flowrate of ignition torch. The spark plug ignition system consists of a high voltage ignition power supply, a normal spark plug and a spark plug cap. The signal triggering device is added to facilitate the sequential control. The whole torch ignition control system and the data acquisitions are achieved by LabVIEW programming (Fig. 1).
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2.2. Optical set-up
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In order to characterize the operability range of the rapid mixed ignition torch, OH* and CH* chemiluminescences imaging technique was applied to visualize the ignition flame torch. CH* is commonly identified as a marker that would track the flame front and heat release of a reactive flow. OH* is a long lived species and may be particularly retained in reaction zone and post-reaction zone in a flame. Therefore, measurements of OH* and CH* chemiluminescences can provide useful information on the reaction zone location, combustion efficiency and approximations of heat release of the CH4 /O2 ignition torch in a non-intrusive and straightforward manner [34]. Therefore, the flame length and width can be identified by the signals. The flame is recorded by a thermoelectrically cooled, 16-bit intensified CCD camera (Pi-Max 4, Roper Scientific) with a 1024 × 1024 array. The camera is equipped with an f /2.8, f = 100 mm, achromatic UV lens (CERCO) combined with an interference bandpass filter centered at 310 nm and 430 nm having a bandwidth of 10 nm [35]. A 82 × 82 mm2 area of the flame is imaged by the ICCD camera, so that the spatial resolution is about
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Fig. 2. (a) Optical arrangement for studying the effect of main oxidizer injection on ignition torch flames. (b) Lab-scaled hybrid rocket motor used in the present work.
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Fig. 3. Schematic of experimental set-up for the lab scale hybrid rocket motor.
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80 μm per pixel. The acquisition repetition rate of the camera is kept at 20 Hz. The intensification is also kept constant so that the flames captured show a weak sensitivity to the post processing parameters. For each case the analysis is based on the averaged flame of 20 frames.
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2.3. Lab-scaled hybrid rocket motor
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In order to investigate the potential effects of main oxidizer injection on the ignition torch flame, the ignition torch system is firstly implemented to the head of a lab-scaled hybrid rocket motor to visualize the interaction between main oxidizer injection and ignition torch flame. The angle between the main oxidizer injection axial direction and the ignition torch flame is kept at 30 degree as illustrated in Fig. 2. The camera is positioned vertically face to the axial direction of the main oxidizer injection. Finally, in order to verify the performance of the ignition torch in hybrid rocket motors, fire experiments were conducted using a lab-scaled hybrid rocket motor with propellants of O2 (g) and paraffin-based solid fuel. As shown in Fig. 3, the whole test facility of the lab-scale hybrid rocket motor consists of three major parts: pipework, controls and instrumentation. In the pipework, three fluids are used, i.e., O2 serving as the oxidizer for hybrid rocket motor and as the oxidizer for ignition torch system, CH4 as the fuel for ignition torch and N2 as the combustion chamber purge, quenching the combustion after a test or in the event of a
test abort. Fig. 4 shows the experimental hybrid rocket motor configuration. With a pre-combustion chamber at the head, the length of the main combustion chamber of the hybrid rocket motor used in the current work is 240 mm; the lengths of the fore and aft ends of the combustion chamber are 20 and 40 mm respectively. The outer diameter of the solid fuel grain is 70 mm. The oxygen flow rate was controlled at 45 g/s, so that the total flow rate with the paraffin-based fuel was around 50 g/s. A conical nozzle made of red copper with a diameter of 6 mm was used in the present work. Two pressure sensors were located at the fore and aft ends of the combustion chamber, to monitor the pressure variation during the firing test. 2.4. Numerical set-up
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In order to better understand the OH* and CH* distribution in the ignition torch flames. One dimensional freely propagating flame of CH4 /O2 was simulated by using GRI-Mech 3.0 kinetic mechanism for various equivalence ratio conditions (Cosilab software package). The transport properties are deduced by using the averaged diffusion model. The Newton unsteady adaptive mesh algorithm has been used and allows an adaptive mesh refinement during the computation (‘grad’ and ‘curve’ values fixed to 1 × 10−5 ). The total final number of the grid is 300 for a physical domain of 0.1 m in order to ensure a chemical equilibrium state in the burned gases. The number of the grid was proved to be sufficient to make the simulation results grid-independent. The kinetic
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Fig. 4. O2 /Paraffin-based solid hybrid rocket motor used in the present work.
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Fig. 5. Rapid-mixed swirl ignition torch flame OH* (left) and CH* (right) chemiluminescences images with different equivalence ratio (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
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Table 1 Test matrix of the ignition torch flames investigated in the present work.
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mechanism of GRI-Mech 3.0 that consists of 325 elementary chemical reactions with associated 53 species involved.
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3. Results and discussion
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Experiments were firstly performed to characterize the operationability range of ignition torch in variation of equivalence ratio and flowrate of O2 /CH4 mixture by using OH* and CH* chemiluminescences technique. Then, effect of cross-flow i.e. the main oxidizer injection on the ignition torch flame was investigated. Finally, the ignition ability of the torch was verified by conducting fire experiments using a lab-scale hybrid rocket motor. The experimental conditions measured in the present work has been listed in Table 1.
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As illustrated in Fig. 5, the rapid-mixed swirl ignition torch has a wide range of working conditions with variations of flowrate and
equivalence ratio. It is observed that the ignition torch has a stable combustion regime for conditions of equivalence ratio ϕ > 0.2. When equivalence ratio lower than 0.2, stable ignition and combustion regime of torch flame cannot always be achieved. For OH* chemiluminescences images, it is observed that the length of torch flames increase with higher equivalence ratio for conditions of ϕ = 0.3–1.0. OH* intensity attained the maximum value at condition of equivalence ratio around ϕ = 0.9. When equivalence ratio higher than ϕ > 1, the OH* intensity begins to decrease. Meanwhile the CH* images exhibit different variation tendency that the concentration of CH* increases continually with equivalence ratio increases. Moreover, the maximum intensity area of CH* and OH* are differently located. For OH* images, the maximum intensity area is always just near the outlet of the nozzle, meanwhile for CH* image, the maximum intensity is located some distance from the nozzle outlet. The geometry of flame changes with equivalence ratio as well. It is found that for fuel rich conditions (ϕ = 1.3) the width at downstream of the flame is larger than the flames at fuel lean conditions (ϕ = 0.3). This is due to the soot radiation effect in the flames for fuel rich conditions. The radiation of the soot in flames has a wide bande which is overlaps with CH* signals. This also explains the reason why the intensity of CH* is higher than OH* for condition of ϕ = 1.3 in Fig. 5. The influence of flowrate on the flame geometry remains similar for both OH* and CH* distribution i.e. the length of the flame increases with higher flowrate as shown in Fig. 6. When flowrate higher than 55 nl/min, supersonic flow is obtained and shock diamonds are formed at the nozzle exit, meanwhile the plume is still stable. It should be noted that the OH* intensity is gener-
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Fig. 6. Rapid-mixed swirl ignition torch flame OH* (left) and CH* (right) chemiluminescences images with different flowrate Q = 35–75 nl/min and t equivalence ratio ϕ = 0.8.
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Fig. 7. OH and CH distribution of 1D laminar flame of CH4 /O2 with 300 K, P = 1 bar.
ϕ = 1.0, T =
Fig. 8. OH distribution of 1D laminar flame of CH4 /O2 with equivalence ratio 0.2–1.3, T = 300 K, P = 1 bar.
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ally higher than CH* intensity for all condition investigated in the present work i.e. ϕ = 0.8 and Q = 35–75 nl/min.
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3.2. 1 Dimensional premixed flame of CH4 /O2
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The chemiluminescences intensity of OH* and CH* observed in the previous section can be explained by 1 dimensional premixed flame simulation using detailed kinetic mechanism. As illustrated in Fig. 7, for premixed laminar flames of CH4 /O2 , the specie of CH is appeared shortly before OH and quickly disappeared at postreaction zone of the flame. Meanwhile OH appears at the beginning of the reaction and remains high concentration value at the post-reaction zone. Fig. 8 shows the OH concentration in variation of equivalence ratio. It can be seen that the mole fraction of OH firstly increases with equivalence ration increase, and then decreases with increase of equivalence ratio. The maximum value located at condition of equivalence ratio around ϕ = 0.8∼0.9. It is found that the variation of CH has similar tendency with OH. This partially explains the ignition flame torch variation tendency in Fig. 5 for conditions of equivalence ratio ϕ = 0.4∼0.7. Meanwhile for fuel rich conditions the experimental results has a different variation tendency due to the soot generation. Laminar flame speed and adiabatic flame temperature of CH4 /O2 flames in variation of equivalence ratio were calculated as well. As illustrated at Fig. 9, laminar flame speed increases with in-
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Fig. 9. CH4 /O2 laminar flame speed and adiabatic flame temperature variation versus equivalence ratio ϕ = 0.2–1.3.
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crease of equivalence ratio for ϕ = 0.2–0.9, and arrives a maximum value of 3.09 m/s at condition of equivalence ratio ϕ = 0.9. When equivalence ratio higher than 0.9, the flame speed decreases for conditions of ϕ = 0.9–1.3. Moreover, the adiabatic flame temperature of CH4 /O2 follows similar variation tendency except that the maximum value T = 3055 K located at equivalence ratio ϕ = 1.1.
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Table 2 Ground ignition test system timing control.
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Serial number
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Name of action
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0.0 4.5 7.5 8.0 8.5 17 17.5 25.0
Data acquisition On Main oxidizer O2 On Ignition torch CH4 /O2 On Ignition spark On Ignition Torch CH4 /O2 Off Main oxidizer O2 Off N2 purge On Data acquisition Off
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Fig. 10. Swirl number variation versus equivalence ratio.
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3.3. Effects of swirl number
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The swirl number is a parameter which characterizes the swirling intensity. It can be calculated by the following equation Sw ∼
mt2 A , m2T A j
i.e. the ratio of the axial flux of tangential momen-
tum to the axial momentum at a cross section [36]. A and A t are the cross sectional area of the main channel of the igniter and the total section area of the tangential slits. mt and m T are the total mass flowrates through the tangential inlets and the test section. As illustrated in Fig. 10, the swirl number (swirl strength) decreases with increase of equivalence ratio. This variation tendency can be explained by the definition of the swirl number. When equivalence ratio decreases with constant the total flowrate of oxygen methane mixture, the tangential flowrate of oxygen increases and the axial flowrate of methane decrease leading to higher swirl number. This tendency reverses when the tangential inlet changed to methane flow i.e. the oxygen flows in to the igniter in axial direction. Meanwhile, the swirl number doesn’t change with flowrate variation with constant equivalence ratio.
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3.4. Effects of oxidizer injection
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The main injection of oxidizer acting as a cross flow can have a potential influence on the ignition torch flame and ultimately affect the performance of the ignition system. Therefore, in this section, the effect of cross-flow i.e. main oxidizer injection on the torch flame will be addressed by implanting the ignition torch at the head of a hybrid rocket motor. Experiments were first con-
ducted with different flow rate of gaseous O2 as the main oxidizer. In order to facilitate the parameters, the flowrate and equivalence ratio of ignition torch flame is kept ϕ = 1.2 and Q = 45 nl/min. Considering the higher signal to noise ratio of OH* chemiluminescences, the following experiments were only performed with OH* chemiluminescences technique. An observation of Fig. 11 reveals that with moderate flowrate O2 injection, the OH* intensity slightly increased at the case of Q = 13.2 g/s possibly due to the afterburning of fuel rich mixture of CH4 /O2 of the ignition torch. However, when the flowrate of main oxidizer injection O2 continue to increase (for the cases Q > 13.2 g/s) the downstream of torch flame has a deformed shape i.e. the height of the flame decreased due to the impact of the main oxidizer injection. Similar behavior has been observed when the cross-flow are changed to N2 indicating that the effects of cross-flow on the torch flame is dominated by transportation and the chemical kinetic effects is neglectable. It shows also that the CH4 /O2 ignition torch has a robust flow regime that the effect of main oxidizer injection on the torch flame is tiny.
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3.5. Experiments with lab-scale hybrid rocket motor
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In order to further verify the performance of the ignition torch in practical working conditions of a rocket motor, fire experiments were performed by using a lab-scaled gaseous O2 /Paraffinbased hybrid rocket motor. The sequential control of the system is achieved by controlling of series of pneumatic valves located in the feeding lines. The main injection oxidizer is started firstly, and then the ignition torch was initiated in about 3 seconds. As illustrated in Fig. 12, it can be seen that the paraffin-based solid fuel can be ignited instantaneously once the CH4 /O2 ignition torch was initiated. High pressure combustion regime is quickly established after the ignition started with a short ignition delay. The time control details are listed in Table 2. A comparison plot between our previous work that ignition of a hybrid rocket motor by catalytic bed (H2 O2 as oxidizer) and igni-
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Fig. 11. Ignition torch flame ( Q CH4 /O2
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Fig. 12. Comparison of combustion chamber pressure between hybrid rocket motor ignited by CH4 /O2 torch igniter (left) and hybrid rocket motor ignited by H2 O2 catalytic bed (right), both under 50 g/s total flowrate.
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tion of hybrid rocket motor by the CH4 /O2 ignition torch proposed in the present work was performed. Both of the two experiments are using the same type of paraffin-based fuel with same total flowrate around 50 g/s. Details of the H2 O2 /paraffin-based hybrid rocket motor can be found in Ref. [6]. It shows in Fig. 12 that for catalytic bed ignition method, there is a considerable ignition delay around 4 seconds possible due to the time needed for preheating the catalytic bed. Meanwhile, using the ignition torch presented, ignition is rapidly achieved with a short delay time.
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4. Conclusion
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Experimental investigation into an CH4 /O2 rapid-mixed swirl ignition torch has been performed. Combustion characterization of this swirl ignition torch was achieved by using OH* and CH* chemiluminescences technique in variation of flowrate and equivalence ratio. The effect of main oxidizer injection acting as a cross flow on the ignition torch was elucidated. Finally, the performance of this CH4 /O2 rapid mixed swirl ignition torch was verified by using a lab-scaled hybrid rocket motor. The interference drawn from the above studies can be summarized as follows.
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• The CH4 /O2 rapid-mixed swirl ignition torch has a wide operability range for equivalence ratio ϕ > 0.2, the flame length increases along with higher equivalence ratio value and total flowrate as well. • For fuel rich conditions, the OH* and CH* distribution is different, for CH* images the maximum value located downstream of the flame, meanwhile OH* maximum value located just behind of the nozzle outlet. • Effects of main oxidizer injection acting as a cross flow has on the ignition torch flame are limited i.e. along with the increase of flowrate of the oxidizer augmented, only the front of the ignition torch flame was slightly disturbed resulting a decrease of flame height. Moreover, the influence is dominated by flow dynamics and the influence of chemical kinetics is tiny. • The CH4 /O2 ignition torch can effectively be used in hybrid rocket motor with a short ignition delay compared with catalytic bed ignition method. Declaration of competing interest
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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.
Acknowledgement
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This work was supported by the National Natural Science Foundation of China [Grant No. 51906016] and Starting Research Foundation from Beijing Institute of Technology.
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