The data-acquisition (DAQ) and control system consisted of a National Instruments NI PXIe-1062Q chassis with a NI PXIe-8106 embedded controller. Included in ...
Design and operation of test-platform for high-speed combustion studies T. H. New1,, J. Li2, T. L. Chng2 and K. S. Lim2 1
School of Engineering University of Liverpool Brownlow Hill, L69 3GH United Kingdom 2
National University of Singapore 10 Kent Ridge Crescent, Singapore 119260
Abstract This paper reports on the design and operations, safety measures taken, as well as preliminary experimental results associated with a highly modular test-platform dedicated towards the study of high-speed combustion events. The test-platform was designed to be capable of controlling the overall system operations associated with and acquiring various experimental quantities simultaneously during cyclical combustion experiments. Preliminary cyclical combustion testing at 25 Hz using equivalence ratio of 1.5 non-premixed ethylene and oxygen mixtures demonstrated good repeatability and reliability of system operations. Dynamic pressure transducer measurements of a typical combustion event show that peak pressure levels ranged from 40 to 60 bars and time-of-flight velocity was as high as 2571 m/s. Compared with the estimated ChapmanJouguet pressure and velocity for the mixture, they demonstrate that successful detonation events had been produced and captured by the test-platform. In addition, results also show that significantly higher pressure levels were produced at the upstream closed-end of the test-rig and deduced to be caused by interactions between the resulting overdriven detonations and compression waves.
Present address: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798 1
Notations V
Experimentally determined time-of-flight velocity (m/s)
CJ
Chapman-Jouguet
DAQ
data-acquisition
DDT
Deflagration-to-detonation transition
EMI
Electromagnetic interference
PT
Pressure transducer
TOF
Time-of-flight
TTL
Transistor-to-transistor logic
1. Introduction Combustion of fuels is the basis of many engineering applications involving power production and combustion research has an extensive history of research and development efforts to optimize the phenomenon. However, in the face of increasing energy demand and fuel cost, there is a growing need to further enhance combustion phenomenon and explore alternative non-conventional power production phenomena. Conventional combustion process may conceivably be replaced by other processes which are significantly more energetic and efficient to begin with. Combustion typically refers to deflagrations where the flame fronts propagate at speeds lower than local sonic speeds. In contrast, more energetic forms of combustion such as detonations result when the both the shock and flame fronts are coupled together to form a detonation front and propagates at supersonic speeds.
Uniquely for detonations, almost all the critical energy transfer occurs via mass flow in strong compression shock waves with very little contributions from heat conduction (which conversely is very important in deflagrations). The structure and behaviour of a detonation front can be understood simplistically if one assumes that it comprises of a coupled shock front and flame front propagating through the reactants. The shock front is located at the leading portion of a detonation front and in its propagation, compresses and heats up the fuel-mixture such that strong chemical reactions are initiated to form a flame front behind the shock front. In turn, strong pressures resulting from the flame front due to the almost constant-volume process
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sustain the preceding shock front for an ideal detonation front. Thereafter, a balance is achieved such that both the shock and flame fronts reinforce each other’s continuing existence and propagation through the rest of the reactants. The unique mutual support and reinforcement phenomenon in detonations result in very high levels of pressures and temperatures within a very short time period, which suggest that suitablycontrolled detonations may be harnessed for power productions [1-5].
Detonations are typically highly-energetic “one-off” explosive events which consume all the reactants within very short time frames. However, detonations can be operated in “pulse” mode, whereby detonations are set off repeatedly and thus form the basis of a pulse detonation system [1-7]. For such a system, the basic operating principle is surprisingly simple which makes it even more attractive as a technology platform for power production. As shown in Fig. 1 for a typical pulse detonation operation, fuel and oxidizer are injected fully into the detonation chamber and detonated by an ignition source. The detonation front will consume and propagate through the reactants, moving at high-speeds from the ignition source location towards and out of the chamber. By virtue of the high pressure produced, the detonation imparts a very short but significant force on the system. For pulse detonation operations, the products will be purged out of the detonation chamber with fresh reactants injected and the entire sequence repeated. If the detonations are set off at a sufficiently high frequency, the force imparted on the system will be quasi-steady with a net effect resembling a steady mass-flow engine with a continuous thrust output. Due to the extremely fast propagation of the detonation front, the maximum frequency is mainly limited by how fast fresh reactants and detonation products can be injected and purged respectively. Generally, detonation frequencies up to hundreds or thousands of hertz are expected to be feasible if fast-acting valves are used in conjunction with high-pressure delivery systems.
The importance of achieving successful detonations repeatedly and reliably is crucial to its eventual implementation in real-world. For example, pulse detonation operations have been proposed as alternative power production techniques for aerospace applications and electricity generation applications [5]. These applications intuitively demand high operational robustness and reliability, with as little downtime as possible. In particular, studies [8, 9] have shown the feasibility of utilizing sustained detonations for electrical power generation, despite the challenges in optimizing the integration of detonation production systems and turbines. 3
However, they also verified and reiterated the need for successful and repeatable production of detonation events. The most promising detonation initiation technique appears to be using deflagration-to-detonation transition (or DDT) enhancements to enable deflagrations resulting from low-energy sources to transit to detonations [5-7, 10]. Using this technique has the distinct advantage of not requiring high-energy sources which need a significant number of heavy and bulky electrical-conditioning devices. Furthermore, the longterm reliability of high-energy sources working in the high-frequency range remains questionable. In contrast, low-energy ignitions can work under high frequencies easily due to their significantly lower energy requirements. DDT enhancements essentially involve techniques which accelerate the flame front such that it will couple with the shock front to produce a detonation front. Typically, protrusions located regularly along the deflagration path are used to promote the production of small-scale flame turbulence which accelerates the flame front. Commonly used DDT enhancement devices are typically helical Shchelkin spirals located along the detonation tube walls. The effects due to spiral configurations in terms of their blockageratio, spiral pitch, lengths have been investigated in the past and their effectiveness verified in earlier studies [10-13].
While the efficacy of various DDT enhancement devices has been studied in single-shot mode, their effectiveness and robustness under cyclical operations are less well-known. Hence, the aim of the study is to design and operate a test-platform which allows testing of various DDT enhancement devices under cyclical combustion operations. In particular, the designs of the various system components according to the practical needs of pulse detonation operations [14], operational procedures and safety measures to achieve the aims of the study will be described in the paper. Practical issues surrounding the safe delivery and injection of gaseous ethylene and oxygen as fuel and oxidizer respectively will be discussed here as well. Preliminary system test results at moderate operating frequencies will also be presented to verify the suitability of the test-platform for the intended investigations.
2. Test-platform design The test-platform consisted of custom designed fuel and oxidizer injection, ignition, DDT enhancement and pressure measurement sections (i.e. which combined together as the test-rig), off-the-shelf commercial high-
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speed data-acquisition and control subsystem, and the fuel-oxidizer delivery subsystem. A typical experiment was envisioned to proceed as follows: the fuel-oxidizer delivery subsystem provides the non pre-mixed fuel and oxidizer which is to be introduced concurrently in short bursts into the injection section; these bursts can be generated in repeated fashion for cyclical operation. This fuel-oxidizer mixture is then ignited immediately downstream within the ignition section. Figure 2 shows the sequence of one typical cycle of fuel-oxidizer injection, ignition and purging at a frequency of 25 Hz using TTL trigger signals. It should be noted that the injection of purge air is to isolate the fresh unburned fuel-oxidizer for the next cycle from the residual combustion products of the last cycle, so as to prevent any unexpected self-ignition. Following the ignition section is the DDT section which, as the term implies, serves to catalyze the deflagration to detonation process. The final section of the test platform is the pressure measurement section, where combustion events are monitored via a series of digital pressure transducers.
Figure 3 shows the design schematics of the test-rig, illustrating the ease with which changing of the various working sections can be achieved. The internal working diameter of the test-rig was designed to be 25 mm throughout and of moderate size. For strength under significant pressure and temperature conditions, stainless steel was used to construct the test-rig. Where possible, components were welded together for increased strength, leaving only removable components to be fixed by heavy-duty fasteners. For experimental safety, a reinforced concrete test-cell was specially built to house the test-rig for the study as shown in Fig. 4(a), while Fig. 4(b) shows the assembled test-rig inside the test-cell. It should be noted that combustion products were directed out of the test-cell via a fan-fitted exhaust duct located at the outlet of the test-rig. The test-rig measures approximately 1.2 m, starting from the fuel-oxidizer injection section to the pressure measurement section. It should also be mentioned that the pressure measurement section was only used to characterize the combustion events. Once the characterization stage is completed, it may be excluded for subsequent investigations. In the next few sub-sections, the individual subsystems will be described and discussed, as well as provide the design rationale driving their adoption for the intended investigations.
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(a) Fuel-oxidizer delivery system and injection section Non-premixed gaseous ethylene and oxygen were used as the fuel and oxidizer here, which were channeled from remote pressurized gas cylinders regulated by pressure regulators. Note that other combinations of fuels and oxidizers can be used with the test-rig to assess other fuel-oxidizer mixtures. For example, gaseous hydrogen-oxygen, hydrogen-air mixtures may be used to study the feasibility of utilizing high-speed combustion for aerospace applications. Additionally, natural gas and liquefied petroleum gas (LPG) may also be used to investigate the potential of employing the techniques here for power generation. Flash arrestors (MESSER SIMAX-5F for ethylene and SIMAX-5OX for oxygen) and Swagelok C series check valves were located along the delivery lines to prevent flash-back incidents. Additionally, gas delivery from the gas cylinders to the test-rig was routed via 12.5 mm rigid stainless steel tubes before they passed through Alicat Scientific M series mass flow meters with 10 ms response time for metering purposes. Before entering the injection section, ethylene and oxygen were diverted equally into four identical pressure manifolds (i.e. one for ethylene and three for oxygen), where four normally-closed Alternative Fuel Systems Inc. (AFS) Gs60-05-5-C solenoid gas injectors per manifold were attached in parallel. Three additional manifolds were used to inject compressed air to purge the combustion products after each cycle, which resulted in a total of seven manifolds. Figure 4(c) show a pressure manifold where four fuel injectors were seated. All the manifolds were connected circumferentially around the injection section of the test-rig via short stainless steel tubes.
The gas injectors were controlled by AFS injector driver modules as shown in Fig. 4(d), capable of driving up to three fuel injectors per module. Digital TTL control signals were channeled into the driver modules to control their opening and closing times, so as to provide accurate ethylene and oxygen mass flows into the testrig. Furthermore, the driver modules were powered by bench-top DC power supplies as shown in Fig. 4(e), which provided power for all gas injectors as well, and simplified the set-up considerably. With the gas injectors capable of operating up to 50 Hz and mass flow per injection a relatively linear function of the source pressure and opening-time period, the mass flows required for the experiments can be determined intuitively. To control the driver module and hence gas injector opening/closing timings according to the experimental requirements, a National Instruments PXI Express data-acquisition and control system with National Instruments LabviewTM software was used. The gas injector operations were programmed such that
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each cycle of the combustion event would see the entire test-rig completely filled with ethylene and oxygen mixture before they were ignited by the ignition system. More details on the data-acquisition and control system will be elaborated later.
(b) Ignition system and section On the ignition section, eight circumferentially distributed ignition ports on the ignition section were available, in which eight Bosch automobile spark plugs were securely attached. This technique allowed higher initial flame speed, as well as decreasing DDT time delay and DDT run-up distance. Simultaneous multiple ignitions would result in more reaction zones which in turn increase the energy release and flame speed. Each spark plug was driven by a MSD Ignition Blaster Ignition control box in conjunction with a MSD Blaster 2 coil (i.e. powered by DC power supplies) as shown in Fig. 4(f). The energy levels from these spark plugs were estimated to be relatively low at approximately 120-150 mJ per spark, but sufficiently high to produce the required initial deflagrations. It should be mentioned that the spark plugs were attached such that only the prongs were protruding into the bore of the ignition section. This was to reduce tube blockage caused by the spark plugs, as well as to enhance their survivability during prolonged combustion testing. Similar to the fuel-oxidizer injection operations, the ignition system was controlled using the National Instruments data-acquisition and control system using TTL control signals.
(c) DDT enhancement section With the deflagrations initiated in the ignition section, the flame fronts would move downstream and enter the DDT enhancement section where a variety of DDT enhancement devices would be tested. Within the present study which seeks to carry out high-speed combustion studies under prolonged operations (i.e. inclusive and up to detonation events), any DDT enhancement devices used must be able to withstand the thermal and kinematic stresses in a robust manner. Therefore, modified forms of the Shchelkin spiral configuration were designed instead. In this case, rather than using a spiral of predetermined coil and wire diameter, spiral pitch and blockage-ratio, helical grooves were machined directly upon the inner walls of the DDT enhancement sections to emulate the approximate geometries of Shchelkin spirals. Two different blockage-ratios of 0% and 50% were used for the designs and shown in Fig. 5. While the initial objectives of
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the present study were to carry out preliminary testing on the reliability of the test-platform without the use of any DDT enhancement devices, some issues regarding the design and fabrication of spirals directly on the tube walls will be discussed here.
Although the use of helical grooves machined directly onto the DDT enhancement section inner wall is intuitively more robust than helical spirals, they presented their own unique set of problems as well, particularly during the fabrication stage. Machining deep internal helical grooves along a significant length of stainless steel proved to be difficult without access to specialized cutting-tools. As a compromise between cost, time and structural integrity, short stainless steel shafts were bored through accordingly to the required blockage-ratio, before spiral cutting tools were used to cut the internal helical grooves onto them. This was done with the intention to weld these shafts together to form a DDT section of desired length. It should be noted that the maximum lengths of the short stainless steel shafts were restricted by the maximum working length of the commercially available cutting tools. Due to the material used as well as the relatively deep grooves, repeated cuts along the same groove route were needed to achieve good adherence to the intended designs and finishing. Special care was taken to ensure that the geometries of the helical grooves at both ends of any one of the short stainless steel shafts matched up with the other shafts to be welded. This was accomplished by fabricating an additional spiral of similar geometry to turn it through and line up all the short shafts in the correct order. This technique also ensured that the geometry of the helical grooves was as close to the theoretical design as possible. With the short shafts lined up in the correct order, they were then welded fully around their circumferences and the spiral extracted out of the completed helical groove tube. DDT enhancement sections with straight grooves proved to be significantly less daunting in terms of fabrication and more intuitive. Thus, for the sake of brevity, their fabrication process will not be elaborated here.
As it is expected that the enhanced DDT transitions would produce significant levels of heat across a relatively short section within very short time periods, an annular cooling-chamber was fabricated and welded concentrically with the DDT section tube. Ports were available to direct water at ambient conditions (i.e. approximately 25 °C) into and out of the cooling-chamber as shown in Fig. 4(g). As it was clearly undesirable
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for the water temperature to exceed 100°C during the experiments, water temperatures were monitored and flow rates adjusted accordingly.
(d) Dynamic pressure measurement sections To assess the characteristics of the combustion events, dynamic pressure measurements were carried out along a dedicated pressure measurement section as shown in Fig. 4(h). Six equally-spaced PCB Piezotronics ICPTM 111A24 dynamic pressure transducers used with PCB Model 482 ICP sensor signal conditioners rated at approximately 68.9 bars maximum working pressure (with water-cooling jackets) located along the length of the measurement section, used in tandem with signal conditioners, were used to measure the resultant pressure profiles as the shock fronts travelled towards the test-rig exhaust end. Another similar pressure transducer was located at the upstream closed-end of the fuel-oxidizer injection section as shown in Fig. 3 earlier, though aligned in the axial direction of the test-rig. Similar to the DDT enhancement section, annular cooling was also employed here. Analog signals from these transducers were acquired by the data-acquisition system. The measured pressure profiles were used not only to determine the time-of-flight (TOF) velocities of the shock fronts as they travelled down the measurement section, but also to evaluate the sustainability of the shock front behaviour. The TOF velocities were assessed using the known distance intervals between the dynamic pressure transducers (i.e. 90 mm) and the time-intervals at which the peak pressures were registered at each of the six dynamic pressure transducers used. They could then be compared against the expected Chapman-Jouguet conditions for the fuel and oxidizer used to determine the exact state of the combustion events.
(e) Data-acquisition and platform control system The data-acquisition (DAQ) and control system consisted of a National Instruments NI PXIe-1062Q chassis with a NI PXIe-8106 embedded controller. Included in the chassis were two NI PXI-6133 and one NI PXI-6221 multifunction DAQ cards. For time-accurate data-acquisition of dynamic pressure transducers signals, NI PXI6133 DAQ cards were used as they were capable of simultaneous DAQ operations for up to eight analog channels per card at 14-bit resolution and 2.5 MS/s/channel. During the experiments however, the maximum sampling rate used was limited to 1 MS/s/channel as the response times of the dynamic pressure transducers
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were estimated to be approximately 1 µs. It was clear that vast amounts of data would be captured within very short time-spans and one concern was that the built-in buffers in the two NI PXI-6133 DAQ cards might not be able to handle and transfer the data streams to the workstation reliably. To address the concern, a dedicated NI HDD 8263 four hard-drive, one terabyte RAID enclosure was used to store the streamed data up to 200 MB/s from the DAQ cards via a NI 8262 cabled PCI-Express module plugged into the NI PXIe1062Q chassis. To further ensure measurements would be time-accurate during high-speed simultaneous sampling, NI TB-2709 and NI TB-2706 terminal boards were used together with the DAQ and analog output cards. Lastly, National Instruments LabviewTM 8.5 software was used to program and control the DAQ and control system in a graphically intuitive environment.
(e) Safety measures For experimental combustion studies, safety concerns and how they are addressed adequately are one of the most important issues to be considered. To begin with, safety procedures and measures were in-place at each significant operational stage or physical section of the experimental setup, so as to isolate and compartmentalize the risks. All experiments were carried out in a test cell, which was designed and built to withstand blast operations and located in a restricted area where human traffic was restricted. For instance, the structure design of the test-cell was designed based on the maximum anticipated pressure loading on its walls in the event of an internal blast wave produced by an accidental explosion. This pressure loading was calculated from the mass of combustible mixture present in the test cell. For safety purposes, ten times the mass of the combustible mixture available within a one-meter long detonation tube of 100 mm diameter was used for calculations. Furthermore, all experiments were carried out via full remote control from a control room which was located at a significant distance away from the test-cell. Four separate live video feeds captured within the test cell were available for monitoring purposes and in the event that any experiment became unsafe, the entire operation could be shut down immediately from the remote control room. At the same time, the system would be purged of the residual ethylene and oxygen using inert nitrogen gas. Additionally, only the test-rig with relevant equipment and measurement devices were located within the test cell, with all other devices located outside of the test cell.
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Ethylene and oxygen gas cylinders were located far away from the test cell vicinity within reinforced metal racks and they were both separated by a significant distance. All gas flow delivery made use of 12.5 mm diameter stainless steel pipes which were rigidly attached against walls as well. Within the test cell, all combustion exhausts were diverted out of the experimental area via an exhaust duct located near the end of the test-rig. Blast-proof doors, fluorescent lamps, switches and exhaust fans were used to mitigate the risk of explosions should any residual fuel and oxidizer remained within the test-cell after the experiments. Lastly, extensively documented emergency measures to be taken in the event of fuel/oxidizer leakages or unexpected fires were in place for all users.
3. Results and discussions As the primary aim of the present study was to validate the initial performance of the test-platform, a smooth DDT section without any DDT enhancement devices was used here. The secondary aim was to verify that the test-platform was able to operate at low-to-moderate frequencies in a safe and reliable manner, with test frequencies increasing from 5 to 25 Hz during this stage of testing. The equivalence ratio of ethylene and oxygen mixture used was 1.5 as determined from the mass flow meters. For the sake of brevity, only results pertaining to the highest test frequency of 25 Hz for 1 s duration will be presented here.
From the initial pressure data, abrupt pressure rises of all six pressure transducers along the pressure measurement section were found to occur every 40 ms during the 1 s operation period, corresponding to 25 Hz as intended. It was observed that peak pressure levels were approximately 40 to 60 bars and that the zero reference levels of all pressure transducers decreased with increasing operation time, which can be attributed to the thermal drift of the pressure transducers. Even though the water cooling jackets were used to cool the sensors, the sensor surfaces were apparently still influenced by the thermal build-up. Figure 6 shows the detailed pressure profiles for a captured combustion event, where evaluated TOF velocities are shown. For brevity, the pressure transducers were labeled from PT1 to PT6 with PT1 located just downstream of the DDT section exit. Using STANJAN software, the Chapman-Jouguet (CJ) detonation velocity and pressure were estimated to be approximately 2575 m/s and 39.2 bars respectively. Comparing between the measured and CJ conditions, Fig. 6 essentially shows that detonation conditions had been achieved despite the TOF velocities
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being slightly lower than the CJ velocity. Firstly, the peak pressure levels were consistently higher than the predicted CJ pressure. And secondly, there would be small uncertainties in the evaluation of the TOF velocities due to finite tolerances in the fabrication stage. Taking these considerations into account, it is reasonable to conclude that successful detonation events had been produced and captured by the test-platform.
Figure 7 shows the comparison of pressure levels registered by two pressure transducers: one located along the pressure measurement section and just downstream of the DDT enhancement section (i.e. PT1 shown in Fig. 6 above), while another located at the upstream closed-end of the fuel-oxidizer injection section but aligned in the axial direction (see Figs. 3(b) and 3(c)). The measured mass flow rates of ethylene and oxygen are also included in the figure. It should be mentioned that the axially-aligned pressure transducer was centrally mounted at the closed-end of the injection section to examine the resultant force in the axial direction. The results show that multiple detonations were achieved at the intended frequency except for a missing instance at approximately 0.35 s. Nonetheless, the overall repeatability of the test-rig remained satisfactory. Interestingly, very high pressure levels (i.e. larger than 137 bars from the figure) were registered by the pressure transducer located at the closed-end. There was some initial concern over whether these high pressure levels were resultant of successful combustion/detonation events or electromagnetic interferences (EMI) associated with the ignition circuits. To investigate further, Fig. 8 shows the details of the pressure profile associated with the first “shot” shown earlier in Fig. 7. From the figure, it can be deduced that the pressure peak registered by the pressure transducer at the closed-end occurred later than the signal peaks observed in the mass flow meter measurements, the latter of which were caused by EMI. The time difference between the pressure and signal peaks was approximately a hundred microseconds, which indicates that the pressure peaks observed were not caused by the ignition circuits. Instead, this suggests that the pressure peaks registered by the pressure transducer at the closed-end might have resulted from overdriven detonations.
Overdriven detonations have been shown to typically appear during the DDT process [15] or from direct initiation of detonations [3]. And for a successful CJ detonation here, calculations show that it would require 58 s to propagate from the ignition location to the closed-end location. In the event where a directlyinitiated overdriven detonation was produced (i.e. with higher associated propagation velocity), the 12
propagation time would have to be shorter than 58 s. However, since the propagation period was almost a hundred microseconds as mentioned earlier, it is unlikely that the pressure peak was caused by overdriven detonation from direct detonation initiation. Instead, it is more likely that an overdriven detonation from the DDT process propagated toward the closed-end with a series of compression waves ahead of it. These compression waves then subsequently reflected off the closed-end to further compress and enhance the strength of the overdriven detonation, which in turn led to very high pressure levels at the closed-end. On the other hand, the rarefaction waves behind the overdriven detonation would weaken it to the level typical of CJ detonations [15]. This will explain why, relative to the ignition location, typical CJ detonation pressure levels were observed along the downstream pressure measurement section with significantly higher pressure levels at the upstream closed-end. Further investigations will be conducted in the future to elucidate the transition location so as to understand the wave propagation phenomenon within the test-rig.
4. Conclusions This paper reports on the design and preliminary test operations of a test-platform dedicated towards providing robust and consistent test-platform for studying detonations produced from helical and parallel spiral DDT enhancement devices using a variety of fuel-oxidizer combinations. Detailed descriptions of the test-platform sub-systems, including the detonation test-rig, dynamic pressure measurement system, highspeed data-acquisition system, fuel-oxidizer delivery system and test cell construction have been provided here for a comprehensive understanding. In particular, practical issues surrounding the machining of helical spirals directly on the inner DDT enhancement section walls have been discussed. Preliminary experimental testing using equivalence-ratio of 1.5 ethylene-oxygen mixtures at 25 Hz test frequency revealed that successful detonation events have been regularly produced by the test-rig, even though DDT enhancement devices were not used.
Interestingly, closer inspection of the dynamic pressure measurements revealed that overdriven detonations were likely to have been produced, which in turn led to very high pressure levels at the immediate upstream region of the ignition section but much lower CJ pressure levels downstream due to their interactions with compression/rarefaction waves. These results essentially verified the design and intended operation of the
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test-platform. Immediate future plans would be to begin testing on DDT enhancement sections with machined helical and straight grooves and to investigate their associated DDT behavior, with the smooth DDT section acting as the benchmark case. On the other hand, longer-term plans may see the study looking into reducing the length of the DDT enhancement sections, while retaining system robustness in producing reliable detonation events.
Acknowledgments The authors gratefully acknowledge the financial support of the research project by the National University of Singapore.
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List of captions Fig. 1 Basic operation cycle of a pulse detonation-based device Fig. 2 Sequence of a typical cycle of fuel-oxidizer injection, ignition and purging at 25 Hz operating frequency Fig. 3 Design schematics of the test-rig in the (a) 3D-view, (b) side-view and (c) cross-section view Fig. 4 Test cell and various experimental components used for the present study Fig. 5 Design schematics of the 0% and 50% blockage-ratio helical-grooved DDT enhancement devices Fig. 6 Dynamic pressure profile measured by pressure transducers and TOF velocities evaluated Fig. 7Dynamic pressure profiles measured by pressure transducers downstream of the DDT enhancement section (PT1) and upstream of the fuel-oxidizer injection stream, as well as the mass flow rate profiles taken by the mass flow meters Fig. 8 Detailed pressure and mass flow rate profiles associated with the first “shot” shown in Fig. 7 earlier
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