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Experimental Study of Deflagration-to-Detonation Enhancement Techniques in a H2/Air Pulsed-Detonation Engine
Abstract
Experiments are performed on a number of deflagration-to-detonation (DDT) enhancement techniques for use in a H2/Air pulsed-detonation engine (PDE). The mechanism, speed and location of DDT for three configurations are considered, including a Shehelkin spiral, an extended cavity/spiral and a co-annulus. High speed digital imaging is used to track flame propagation. and simultaneous time-correlated pressure traces are used to record progress of the shock structure. It is found that DDT is initiated primarily through local explosions that are highly dependent on the particular geometry. In addition to various geometries. The effect of equivalence ratio and spark timing are also investigated.
... For some fuel mixtures, the length associated with the transition (i.e., DDT runup length) can be quite large: on the order of several meters. Turbulence-enhancing DDT devices [7][8][9][10] decrease this length through flame warping and vortex generation. The Shchelkin spiral is the most commonly used DDT device in detonation research. ...
... The Shchelkin spiral is the most commonly used DDT device in detonation research. It allows the flame to propagate along the spiral rather than across the center of the spiral [8], effectively increasing the surface area and burning rate of the combustion front. However, some DDT devices generate a large pressure drop across the system due to the increased blockage. ...
... Without a turbulence-generating, DDT device, this ethylene/air PDE system would require up to ∼80 D PDE [26] to fully transition to a detonation. The Shchelkin spiral is a typical DDT device and is efficient in promoting detonation transition in a short distance with minimal blockage [8]. These two hardware configurations are used for all three initiation methods. ...
An air-breathing pulse detonation engine-crossover system is developed to characterize the feasibility of shock-initiated combustion within an air-breathing pulse detonation engine. A shock wave is transferred through a crossover tube that connects a spark-ignited, driver pulse detonation engine to the air-breathing, driven pulse detonation engine. Detonations in the driven pulse detonation engine develop from shock-initiated combustion caused by shock wave reflection. The pulse detonation engine-crossover system increases system efficiency through decreased deflagration-to-detonation transition distance while employing a single spark source to initiate a system consisting of multiple detonation tubes. The initiation effectiveness of shock-initiated combustion is compared to spark discharge initiation and detonation injection through a pre-detonator. Increasing Reynolds number enhances combustion wave acceleration. However, for all initiation methods, the system requires a device to transition the combustion wave to a detonation. Shock-initiated combustion and pre-detonator initiation produce comparable detonation transition run-up lengths. With an incident shock wave Mach number of MS = 2.36, run-up length is decreased by up to 61% compared to the spark discharge method. Similar combustion evolution is observed for an incident shock wave of strength MS = 1.87.
... The clear combustion chamber (Figure 9 ) allowed the use of a Phantom VII highspeed digital camera, which was able to record flame position along the length of the tube as a function of time from the spark [11]. This method resulted in more continuous data. ...
... Such reduced chemical mechanisms must be validated over the range of fuel types, initial pressure, and temperature conditions of interest. Many investigations of PDE thrust estimation4567891011 have focused on an ideal tube PDE which can be described as a constant cross-sectional tube, closed at one end and open at the other end, uniformly filled with quiescent fuel-air mixtures. Ideal tube configurations typically have a diameter of 0.051 or 0.076 m and a length of 1.0-1.5 m. ...
... However, more recent detailed comparisons [4,5] have shown that most of this variation could be explained on the basis of initial and boundary conditions used in the various computational studies. For an ideal PDE tube completely filled with hydrogen-air mixture (at initial conditions of 1 atm., 300 K and an equivalence ratio of 1.0), a convergence of estimations for I spf at a value of about 4160 s has been reported by the computational PDE research community91011. There is a need for detailed multi-dimensional time unsteady computations which can predict PDE performance with realistic geometry. ...
A Pulse Detonation Engine (PDE) is a propulsion device that takes advantage of the pressure rise inherent to the efficient burning of fuel-air mixtures via detonations. Detonation initiation is a critical process that occurs in the cycle of a PDE. A practical method of detonation initiation is Deflagration-to-Detonation Transition (DDT), which describes the transition of a subsonic deflagration, created using low initiation energies, to a supersonic detonation. This thesis presents the effects of obstacle spacing, blockage ratio, DDT section length, and airflow on DDT behavior in hydrogen-air and ethylene-air mixtures for a repeating PDE. These experiments were performed on a 2 diameter, 40 long, continuous-flow PDE located at the General Electric Global Research Center in Niskayuna, New York. A fundamental study of experiments performed on a modular orifice plate DDT geometry revealed that all three factors tested (obstacle blockage ratio, length of DDT section, and spacing between obstacles) have a statistically significant effect on flame acceleration. All of the interactions between the factors, except for the interaction of the blockage ratio with the spacing between obstacles, were also significant. To better capture the non-linearity of the DDT process, further studies were performed using a clear detonation chamber and a high-speed digital camera to track the flame chemiluminescence as it progressed through the PDE. Results show that the presence of excess obstacles, past what is minimally required to transition the flame to detonation, hinders the length and time to transition to detonation. Other key findings show that increasing the mass flow-rate of air through the PDE significantly reduces the run-up time of DDT, while having minimal effect on run-up distance. These experimental results provided validation runs for computational studies. In some cases as little as 20% difference was seen. The minimum DDT length for 0.15 lb/s hydrogen-air studies was 8 L/D from the spark location, while for ethylene it was 16 L/D. It was also observed that increasing the airflow rate through the tube from 0.1 to 0.3 lbs/sec decreased the time required for DDT by 26%, from 3.9 ms to 2.9 ms. M.S. Committee Chair: Timothy Lieuwen; Committee Co-Chair: Ben Zinn; Committee Member: Anthony Dean; Committee Member: Bill Wepfer
... These devices include orifice plates, centerbodies, concentric rings, and Shchelkin spirals in addition to the use of additives. The use of these devices has been shown to help promote the transition to a detonation [11,12,13,14]. Of these methods, the one most commonly used is the Shchelkin spiral [15], which is a wire coil of certain diameter and pitch. ...
... Unlike axisymmetric obstacles, the Shchelkin spiral can support additional helical modes that can propagate upstream as they grow and interact with the side walls. This increases the probability of coupling between the shock structure and turbulent flame propagation, thereby increasing the likelihood of DDT [14]. For this reason, the Shchelkin spirals were utilized in this experimental study. ...
Active flow control devices including mass injection systems and zero-net-mass flux actuators (synthetic jets) have been employed to delay flow separation. These devices are capable of interacting with low-speed, subsonic flows, but situations exist where a stronger crossflow interaction is needed. Small actuators that utilize detonation of premixed fuel and oxidizer should be capable of producing supersonic exit jet velocities. An actuator producing exit velocities of this magnitude should provide a more significant interaction with transonic and supersonic crossflows. This concept would be applicable to airfoils on high-speed aircraft as well as inlet and diffuser flow control. The present work consists of the development of a detonation actuator capable of producing a detonation in a single shot (one cycle). Multiple actuator configurations, initial fill pressures, oxidizers, equivalence ratios, ignition energies, and the addition of a turbulence generating device were considered experimentally and computationally. It was found that increased initial fill pressures and the addition of a turbulence generator aided in the detonation process. The actuators successfully produced Chapman-Jouguet detonations and wave speeds on the order of 3000 m/s.
... The tubes of smaller diameter will hold a very small amount of fuel-air mixture and if flame is able to accelerate and undergo transition in this section then we can effectively achieve the DDT with the consumption of very small amount of fuel. The study of transition of detonation from smaller tubes to large has earlier been carried out by Meyer et al. [2] and Katta et al. [3]. Earlier experiments [4] have shown large improvement of specific impulse in the case of partial filling. ...
Pulse Detonation Engine has been envisaged as a high efficieny engine as compared to the conventional
turbo engine. The propulsive force generated in the engine is due to the high pressure generated by the
detonation wave. These engines have no moving parts hence it makes it further more attractive for the
aerospace applications. If a given fuel air mixture is ignited using conventional spark ignition system it
leads to production of deflagration wave.To generate the Detonation wave two methods are used.
... According to conventional boundary layer approaches [25][26] and PDE papers [14,17,[23][24] detonation is modelled as a Zel'dovich-von Neumann-Doering (ZND) normal wavelength, in the fuel-air mixture of a uniform tube, which is almost at rest for combustion inlet conditions, see Figure 6. Due to the difficulty of initiating air-fuel mixtures in short tubes and obtaining a constant detonation firing, methods involving deflagrative combustion processes leading to detonation reactions by the placement of optimized obstacles to create turbulent mixtures and to accelerate the gas flow (DDT), as an example, we describe the Shchelkin spiral coil, perforated plates or convergent-divergent jaws, see Figure 7, [18,19,20,21,22]. ...
PDE propulsion can work from a subsonic regime to hypersonic regimes; this type of engine can have higher thermodynamic efficiency compared to other turbojet or turbofan engines due to the removal of rotating construction elements (compressors and turbines) that can reduce the mass and total cost of propulsion system. The PDE experimental researches focused on both the geometric configuration and the thermo-gas-dynamic flow aspects to prevent uncontrolled self-ignition. This article presents a series of numerical simulations on the functioning of PDE with hydrogen at supersonic regimens.
... Chapman and Wheeler [1] were the first to place obstacles (orifice plates) in a smooth tube to promote flame acceleration and shorten X ddt . A detonation tube with a smaller diameter [2] , insertion of regular or irregular obstacles [3] , a detonation tube with divergent-convergent sections or shock-focusing end-walls [4][5] [6] have also been employed. ...
Smoked foil has been employed to visualize triple point pattern (or cell width), indicating detonation phenomena. However, the aluminum sheet also corresponds to sudden contraction in a smooth tube. It might induce early trigger on detonation initiation and result in a reduction in deflagration-to-detonation transition (DDT) run-up distance. Test results showed the thickness of aluminum sheet of less than 1.3 mm is required to eliminate the effect of smoked foil. A reduction in Xdtt is observed when the thickness of aluminum sheet increases.
... For some air-breathing, fuel mixtures, the length associated with the transition, or DDT run-up length, can be quite large, on the order of several meters [7]. Turbulence-enhancing DDT devices [8][9][10][11] decrease this length, but generate a large pressure drop across the system due to the increased blockage. ...
... It is shown that the distance between orifices of obstacles required for DTT will increase with the cell size. Meyer et al. [16] carried out experiments on a number of deflagration-to-detonation (DDT) enhancement techniques for use in a H 2 /Air pulseddetonation engine (PDE). The geometrical configurations include Shchelkin spiral, an extended cavity/spiral, and a coannulus. ...
Numerical simulation based on the Euler equation and one-step reaction model is carried out to investigate the process of deflagration to detonation transition (DDT) occurring in a straight duct. The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory (WENO) scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. In particular, effect of energy release on the DDT process is studied. The model parameters used are the heat release at q = 50,30,25,20,15,10 and 5, the specific heat ratio at 1.2, and the activation temperature at Ti = 15, respectively. For all the cases, the initial energy in the spark is about the same compared to the detonation energy at the Chapman-Jouguet (CJ) state. It is found from the simulation that the DDT occurrence strongly depends on the magnitude of the energy release. The run-up distance of DDT occurrence decreases with the increase of the energy release for q = 50 similar to 20, and increases with the increase of the energy release for q = 20 similar to 5. This phenomenon is found to be in agreement with the analysis of mathematical stability theory. It is suggested that the factors to strengthen the DDT would make the detonation more stable, and vice versa. Finally, it is concluded from the simulations that the interaction of the shock wave and the flame front is the main reason for leading to DDT.
... For some airbreathing mixtures, the length associated with the transition (i.e., the DDT runup length) can be quite large: on the order of several meters. Turbulence-enhancing DDT devices [6][7][8][9] can be used to decrease this length. Unfortunately, DDT devices increase blockage and generate an unwanted pressure drop across the system, decreasing the available pressure gain. ...
Shock wave propagation within a pulse detonation engine-crossover system is investigated, examining the properties and mechanisms of the transfer process. A shock wave is transferred through a crossover tube that connects a spark-ignited driver pulse detonation engine to a secondary, driven pulse detonation engine. Detonations in the driven pulse detonation engine develop from shock-initiated combustion, as strong shock wave reflection can cause ignition within a reactive mixture. A pulse detonation engine-crossover system can decrease deflagration-to-detonation transition length while employing a single spark source to initiate a system consisting of multiple detonation engines. Visualization of a shock wave propagating through a clear channel reveals a complex shock train behind the leading shock wave. Transverse waves connect with the leading shock wave to form a triple point that oscillates through the leading shock wave. The shock wave Mach number and decay rate remain constant for varying crossover tube geometries and operational frequencies. A temperature gradient forms within the crossover tube due to forward flow of high-temperature ionized gas into the crossover tube from the driver pulse detonation engine and backward flow of ionized gas into the crossover tube from the driven pulse detonation engine. This communication results in intermittent autoignition of the driver pulse detonation engine for higher-frequency large-crossover-tube-diameter cases. However, small-diameter crossover tubes prevent these autoignition events at higher frequencies.
... Unfortunately, one study 16 found that too much additional turbulence can reduce the effect on the DDT mechanisms signifying that there is a limit to how much turbulence can be added to the flow. The most common DDT device used is the Shchelkin spiral 17,18 . However, research has been completed on other devices including orifice plates 19 , ramps 20 , and more recently, fluid injection to create fluidic obstacles 21 . ...
... Traditional PDE technology uses a valved inlet for the fuel and air, sparked ignition, and a deflagration to detonation transition (DDT) length, often enhanced with devices that lower the time and length needed to transition to detonation, such as a spiral. 1 However, experiments at Wright-Patterson AFB have successfully initiated detonation through a split tube design, utilizing the detonation wave of a prior detonation tube. 2 Experiments at the University of Cincinnati have also investigated shock transfer related direct ignition of detonations using a crossover tube from another PDE. 3 This eliminates the DDT length and increases the percent of fuel that is burnt at the higher efficiency detonation provides. Valves tend to be among the most prone component to fail in PDE's. ...
Experiments are carried out as a follow-up to a simulation study of potential valveless, self-aspirating, static condition pulse detonation engine designs without pressurized purge. The computational study described a configuration that after detonation would self-aspirate, evacuating the hot exhaust from the pulse detonation engine (PDE) tube, reducing the internal temperature below the autoignition temperature of the fuel and bringing fresh air in. This allows for additional injection of fuel and shock initiation of a second cycle. A PDE has been built with a geometry that was determined by simulations to be most effective at self aspiration. The limits of firing frequency and fill fraction were tested. Trends in data for various configurations and off-design conditions were used to gain understanding of the operation of this type of engine. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
... Although stainless-steel detonation tubes were utilized for the majority of experiments, DDT was observed with a polycarbonate detonation tube and fast imaging system, as described by Meyer et al. [16] Figure 4 shows a typical result, with time evolving in subsequent images from top to bottom in Frames a through z. The spiral is 4 American Institute of Aeronautics and Astronautics barely perceptible in these frames, and the visibly larger vertical bands are metal supports for instrumentation ports on the polycarbonate tube. ...
Detonation initiation of hydrocarbon-air mixtures is critical to the development of the pulsed detonation engine (PDE). Conventionally, oxygen enrichment (such as a predetonator) or explosives are utilized to initiate detonations in hydrocarbon/air mixtures. While often effective, such approaches have performance and infrastructure issues associated with carrying and utilizing the reactive components. An alternative approach is to accelerate conventional deflagration-to-detonation speeds via deflagration-to-detonation transition (DDT). Analysis of hydrocarbon-air detonability indicates that mixing and stoichiometry are crucial to successful DDT. A conventional Schelkin-type spiral is used to obtain DDT in hydrocarbon-air mixtures with no excess oxidizer. The spiral is observed to increase deflagrative flame speeds (through increased turbulence and flame mixing) and produce 'hot-spots' that are thought to be compression-wave reflections. These hot spots result in micro-explosions that, in turn, then give rise to DDT. Time -of-flight analysis of high-frequency pressure-transducer traces indicate that the wavespeeds typically accelerate to over-driven detonation during DDT before stabilizing at Chapman-Jouget levels as the combustion front propagates down the detonation tube. Results obtained for a variety of fuels indicate that DDT of hydrocarbon-air mixtures is possible in a PDE. Succesful DDT in air with no oxygen enrichment was achieved with propane, 100 octane low-lead aviation gasoline, kerosene based military jet fuel JP8, and the high energy-density military jet fuel JP10.
... 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][11][12][13]. ...
This article 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 bar and time-of-flight velocity was as high as 2571 m/s. Compared with the estimated Chapman-Jouguet 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.
The detonation phenomenon occurs because the burning fuel will produce a burning wave that propagates with very high speed and the energy that propagates with it is also very high, this can be a favorable or disadvantageous thing. This study focused on developing a relationship between the detonation induction distance and the initial pressure of the hydrogen-oxygen mixture with the shchelkin spiral. The study used a horizontal 50-mm diameter detonation test tube with a total length of 2000 mm equipped with a shchelkin spiral with a pitch of 15 mm. The hydrogen-oxygen mixture was injected into the test tube with an initial pressure variation of 30 kPa to 100 kPa. From the results of the study it was found that the increase in initial pressure affects the distance of detonation induction distance where at a pressure of 100 kPa by using the shchelkin spiral the detonation induction distance was 8.5 cm and for the pressure of 30 kPa was 17 cm. As for the detonation induction distance without shchelkin spiral with initial pressure of 100 kPa and 30 kPa respectively are 70 cm and 115 cm. Abstrak. Fenomena detonasi terjadi karena bahan bakar yang terbakar akan menghasilkan gelombang pembakaran yang merambat dengan kelajuan sangat tinggi serta energi yang merambat bersamanya juga sangat tinggi, hal ini dapat menjadi hal yang menguntungkan atau merugikan. Penelitian ini difokuskan pada pengembangan hubungan antara jarak induksi detonasi dengan tekanan awal campuran hidrogen-oksigen dengan shchelkin spiral. Penelitian ini menggunakan pipa uji detonasi (PUD) horizontal berpenampang lingkaran berdiameter 50 mm dengan panjang total 2000 mm yang dilengkapi dengan shchelkin spiral yang memiliki pitch 15 mm. Campuran hidrogen-oksigen diinjeksikan ke dalam PUD dengan variasi tekanan awal 30 kPa hingga 100 kPa. Dari hasil penelitian didapatkan bahwa peningkatan tekanan awal mempengaruhi jarak induksi detonasi (detonation induction distance-DID) dimana pada tekanan 100 kPa dengan menggunakan shchelkin spiral jarak induksi detonasi adalah 8,5 cm dan untuk tekanan 30 kPa adalah 17 cm. Sedangkan untuk jarak induksi detonasi tanpa shchelkin spiral dengan tekanan awal 100 kPa dan 30 kPa masing-masing adalah 70 cm dan 115 cm. Pendahuluan Pembakaran sebagai salah satu upaya ekstrak energi berguna akan meninggalkan suatu perma-salahan yang cukup serius dalam hal keselamatan personil serta keamanan instalasi industri yang terlibat. Permasalahan keamanan dan keselamatan kerja yang terkait dengan pembakaran yaitu bahwa pada proses pembakaran dapat terjadi fenomena yang disebut detonasi. Fenomena detonasi terjadi karena bahan bakar yang terbakar akan mengha-silkan gelombang pembakaran yang merambat dengan kelajuan sangat tinggi (di atas kecepatan suara lokal) mengikuti shock wave, energi yang merambat bersamanya juga sangat tinggi. Hal tersebut dapat menjadi sumber bahaya bagi ke-selamatan personil pada industri dan keamanan instalasi pada industri tersebut karena dapat ber-ujung pada terjadinya ledakan yang berujung pada total loss bagi sektor industri yang bersangkutan. Namun pada aplikasi lain, munculnya gelombang detonasi sangat diharapkan seperti pada sistem pulse detonation engine. Pada sistem ini diharapkan detonasi yang konsisten dalam jarak yang pendek. Salah satu cara untuk memperpendek jarak induksi detonasi adalah dengan turbulizing element. Turbulizing element adalah elemen fisik yang mempengaruhi turbulensi pembakaran dan pada akhirnya dapat mempercepat laju pembakaran itu sendiri. Beberapa penelitian telah dilakukan untuk mengamati pola dan karakteristik detonasi yang berhubungan dengan tekanan awal dan jarak induksi detonasi. Sentanuhady et al. (2014) telah melakukan penelitian tentang karakteristik perambatan gelom-bang detonasi marginal (gelombang detonasi yang merambat hanya dengan satu sel detonasi saja (single cellular detonation) pada campuran hidro-gen dengan udara. Dari hasil penelitian tersebut
To reveal the mechanism of deflagration to detonation transition promoted by transverse jet, the flame acceleration and detonation initiation process of H2/Air under transverse jet was numerically investigated. According to the flame propagation characteristics and the main factors that promote flame acceleration in different periods, the process of deflagration to detonation transition was divided into four stages: initial flame development stage, turbulence-flame interaction stage, shock-flame interaction stage, detonation initiation stage. The flame acceleration process of each stage was analyzed in detail. The results indicated that the curling and stretching effect of the transverse jet and vortexes on the flame can effectively increase the flame velocity. The flame would constantly produce compression waves during acceleration. The transverse wave formed by the superposition of the compression waves would be reflected multiple times between the walls and act on the flame front, which is critical for flame acceleration. In addition, the positive feedback between the leading shock wave and the flame was also important for the continuous increase of flame velocity.
Numerical simulation has been performed to investigate the DDT (Deflagration-to-Detonation Transition) mechanism by solving fully compressible reactive flow for hydrogen/air mixtures in tube with sudden cross-section expansion. The results reveal the acceleration action of abrupt cross-section on DDT, which is validated by comparing the run up distance and time with corresponding long annular tube and single tube. Detailed discussion of flow field variations finds that the DDT process in cross-section abrupt tube can be divided into three stages (flame acceleration, transition to detonation, and detonation propagation stages respectively) according to different flame modes. Particularly, it is found that formation of vortex could accelerate DDT by promoting turbulent mixing of hot products and cold reactants. Further comparative analysis on DDT characteristics of cross-section abrupt tube with different annular gap lengths shows that different mechanisms dominate in the single tube zone. The conclusions in present study support the cross-section abrupt tube as a means to enhance DDT and provide an alternative potential in practical pre-detonation initiator and pulse detonation engine applications.
Formation of detonation waves in a tube is a complex phenomenon and depends upon many factors like ignition energy, presence of a deflagration to detonation transition (DDT) enhancement device and spatial distribution of fuel, etc. In the present study, gaseous octane-air mixtures have been examined by varying the equivalence ratio linearly along the axial direction of the detonation tube though the overall stoichiometry was maintained in the tube. Three different conditions have been modelled and studied, which includes small, moderate and large, fuel density gradient in axial direction with equivalence ratio ranging from 1 to 2 near the ignition zone. A series of simulation study have been conducted and the analysis of simulation results reveal that the DDT onset is significantly affected by the initial fuel distribution at the ignition zone as well as on fuel density gradient in a detonation tube. It has been observed that a moderate gradient in the fuel density distribution is favourable for onset of detonations. From the study of pressure plots for above mentioned conditions it has been found that the presence of large gradients in fuel density has adverse effect on the stability of detonation wave.
An experimental study is conducted on a Pulse Detonation Engine-Crossover System to investigate the feasibility of repeated, shock-initiated combustion and characterize the initiation performance. A PDE-crossover system can decrease deflagration-to-detonation transition length while employing a single spark source to initiate a multi-PDE system. Visualization of a transferred shock wave propagating through a clear channel reveals a complex shock train behind the leading shock. Shock wave Mach number and decay rate remains constant for varying crossover tube geometries and operational frequencies. A temperature gradient forms within the crossover tube due to forward flow of high temperature ionized gas into the crossover tube from the driver PDE and backward flow of ionized gas into the crossover tube from the driven PDE, which can cause intermittent auto-ignition of the driver PDE. Initiation performance in the driven PDE is strongly dependent on initial driven PDE skin temperature in the shock wave reflection region. An array of detonation tubes connected with crossover tubes is developed using optimized parameters and successful operation utilizing shock-initiated combustion through shock wave reflection is achieved and sustained. Finally, an air-breathing, PDE-Crossover System is developed to characterize the feasibility of shock-initiated combustion within an air-breathing pulse detonation engine. The initiation effectiveness of shock-initiated combustion is compared to spark discharge and detonation injection through a pre-detonator. In all cases, shock-initiated combustion produces improved initiation performance over spark discharge and comparable detonation transition run-up lengths relative to pre-detonator initiation. A computational study characterizes the mixing processes and injection flow field within a rotating detonation engine. Injection parameters including reactant flow rate, reactant injection area, placement of the fuel injection, and fuel injection distribution are varied to assess the impact on mixing. Decreasing reactant injection areas improves fuel penetration into the cross-flowing air stream, enhances turbulent diffusion of the fuel within the annulus, and increases local equivalence ratio and fluid mixedness. Staggering fuel injection holes produces a decrease in mixing when compared to collinear fuel injection. Finally, emulating nozzle integration by increasing annulus back-pressure increases local equivalence ratio in the injection region due to increased convection residence time.
Experiments are carried out as a follow-up to the proof of concept study of potential valvelessly self-aspirating static condition pulse detonation engine designs without pressurized purge. The computational study described a configuration that after detonation would self-aspirate, evacuating the hot exhaust from the pulse detonation engine (PDE) tube, reducing the internal temperature below the autoignition temperature of the fuel and bringing fresh air in. This allows for additional injection of fuel and shock initiation of a second cycle. A PDE has been built with a geometry that was determined by simulations to be most effective at self aspiration. The aerovalve headwall has an adjustable exhaust gap. The ejector can be removed and replaced with longer and wider ejectors at various locations. The limits of firing frequency were determined with regard to the ability to avoid autoignition, and the ability to achieve detonation at chapman jouguet predicted speed. Trends in data for a range of headwall aerovalve spacing, ejector size and placement, and a new ignition system were tested to fill out the performance capabilities of this PDE. In addition, the control cases of nearly closed and completely open end in place of the aerovalve were tested to verify the effect of the aerovalve on the system. Fill fraction and oxygen injection were altered to see trends on their influence of detonability and frequency limitation due to autoignition. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
An effect of a spiral turbulent ring, so-called Shchelkin spiral, on a detonation performance was studied experimentally for acetylene and oxygen mixture. A couple of dynamic pressure transducers were used to calculate a detonation wave velocity by a time difference between two pressure peaks. In addition, impulse was measured by a load cell and the impulse was used to analyze the spiral effect on the detonation performance. A CFD analysis was adopted to calculate mass flow rates of the propellants and the minimum filling time. The maximum velocity and pressure were measured at the equivalence ratio of 2.4, and the measured values showed similar trend to C-J conditions calculated from CEA. For the shorter chamber with the short spiral, the maximum detonation velocity was appeared. In contrast, the longer chamber without the spiral showed the maximum thrust performance.
Pulsed detonation engines have the potential to provide thrust over a wide operating range for a fraction of the cost of conventional turbine engines. These promised returns have given rise to several vigorous research programs in academia, industry, and government labs. To capture the uhra-fast events composing a detonation wave. researchers are forced to use megahenz range data acquisition systems for relatively long testing intervals. producing gigabytes of purely numeric test data. Since human inspection of such enormous data sets is virtually impossible, a computational framework of highly automated analysis tools is necessary to facilitate the interpretation and analysis of pulsed detonation engine experiments.
Current research by the US Air Force and Navy is concentrating on obtaining detonations in a pulse detonation engine (PDE) with low vapor pressure, kerosene based jet fuels. These fuels have a low vapor pressure and the performance of a liquid hydrocarbon fueled PDE is significantly hindered by the presence of fuel droplets. A high pressure, fuel flash vaporization system (FVS) has been designed and built to reduce and eliminate the time required to evaporate the fuel droplets. Four fuels are tested: n-heptane, isooctane, aviation gasoline, and JP-8. The fuels vary in volatility and octane number and present a clear picture on the benefits of flash vaporization. Results show the FVS quickly provided a detonable mixture for all of the fuels tested without coking or clogging the fuel lines. Combustion results validated the model used to predict the fuel and air temperatures required to achieve gaseous mixtures with each fuel. The most significant achievement of the research was the detonation of flash vaporized JP-8 and air. The results show that the flash vaporized JP-8 used 20 percent less fuel to ignite the fuel air mixture twice as fast (8 ms from 16 ms) when compared to the unheated JP-8 combustion data.
In the recording of refractive index fields, the exceedingly high monochromatic brightness of laser light sources opens up new possibilities where applications to very fast or highly self-luminous phenomena are concerned. The properties of laser light, however, pose special problems, as well as presenting special opportunities. These are examined in relation to schlieren recording, deflexion mapping, shadowgraphy and interferometry. The ultimate aim is the development of a 'versatile' optical system which is capable of fulfilling all these functions with only minor readjustments in its optical components. This is achieved, for laser light, without either the expensive apparatus, or the considerable intensity losses with which such systems are otherwise associated.
The experimental study of transition to detonation has been enhanced recently by two novel techniques. One exploits simply the fact that a self-sustained detonation front, unlike any other wave associated with the transition process, is capable of leaving imprints on the wall along which it travels. The other is based on the adaptation of an amplitude modulated, giant pulse, laser system as a light source for stroboscopic schlieren photography. The insight gained by the utilization of these techniques into the wave processes accompanying the onset of detonation is unparalleled in the long history of the study of these phenomena. The results demonstrate that the transition can take place in various modes depending on the wave interaction processes which occur ahead of the accelerating flame.
An introduction is given to the problem and principal research themes of the deflagration-to-detonation transition phenomenon. The key ideas of flame acceleration and detonation initiation are briefly discussed. Recent research is described with an emphasis on photographic studies of the propagation mechanisms of quasi-detonations. Theoretical notions about the spontaneous development of detonation are reviewed. Relationships between hotspots, reaction waves, and shock wave amplification are emphasized.
Transition from high speed flame to detonation in tubes was studied in an extensive series of experiments with the aim being to establish quantitative limiting criteria for the onset of transition. The experiments were carried out in three long tubes each with a different diameter. The tubes were 18 meters each and had internal diameters of 5, 15 and 30 cm, respectively. A matrix of fuel-air mixtures at atmospheric initial pressure and room temperature was studied over a broad range of equivalence ratios. The fuels were hydrogen, acetylene, ethylene, propane and methane. High speed flame propagation and transition to detonation were achieved in a controlled manner within each tube using the well-known flame acceleration technique of obstructing obstacles pioneered long ago in the experiments of Wheeler. In the present experiments the entire tube length was filled with orifice ring obstacles, equispaced one tube diameter apart, to ensure that the maximum terminal flame speed is achieved in all cases within the available length of each tube. The results show that transition to detonation in tubes invariably occurs from a minimum level of flame speed corresponding roughly to the speed of sound of the combustion products. Since the flame speed in a tube is directly coupled to the flow field that it generates ahead of itself, this minimum flame velocity requirement implies that an adequate intensity of turbulent shear mixing is required to form the required explosive pocket of gas inherent in the genesis of detonation. There is also a necessary condition for transition to detonation in that the minimum transverse tube dimension, corresponding to the orifice opening diameter d in this study, must be sufficiently large to accomodate at least one transverse cell width characteristic of the mixture in the tube. That is, the quantitative criterion for transition is that λ/d≤1. Once established, the detonation wave in the tube within the obstacle field is observed to propagate at a steady velocity with a substantial velocity deficit which can be as high as 40% below the theoretical C-J value. In the limit when d/λ→13, the detonation propagation asymptotically approaches the C-J level as expected.
Deflagration to detonation transitions and strong deflagrations for a large range of stoichiometric alkane (CH4, C2H6, C3H8, C4H10) and air mixtures and alkene (C2H4, C3H6, C4H8) and air mixtures have been studied systematically by varying surface roughness via the introduction of Shchelkin spirals. The use of such spirals, rather than repeated obstacles such as orifice plates, is preferred, as improved acceleration and higher flame speeds are obtained due to reduced momentum losses. The results obtained indicate that deflagration to detonation transitions (DDT) and quasi-detonations can be obtained for all fuels tested. Using short obstacles it is demonstrated that quasi-stable strong deflagrations of considerable duration, supported only by a smooth-walled tube, can easily be established and typically serve as a basis for further acceleration and transition to detonation. The effects of different obstacle exit velocities on the duration of the strong deflagration phase and on DDT parameters have been investigated. By further increasing the obstacle length, DDT via quasi-detonations has been investigated and it is demonstrated that the relative detonability of the fuels under strongly turbulent conditions differs systematically from that obtained in smooth tube experiments.