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Unsteady Flame Speed Control and DDT Enhancement Using Fluidic Obstacles
... The predetonator ignition approach can be easily operated for a typical combustion chamber. However, it still encounters some drawbacks including the requirement for an extra gas mixture supply system, which increases the complexity of the device and the possibility of failure in transition to detonation in the combustor compared to other typical ignition methods 17,18 . Generally, a detonation wave achieved by the DDT process is considered a more reliable and efficient method in combustors because of their lower requirement of ignition energy and easy operation within the chamber 2, 6 . ...
... However, when operating a particular detonation engine such as PDE, these obstacles, which are fixed within the chamber, result in pressure losses and introduce thermal reservoirs 16,17,36,37 . This leads to 25% engine thrust loss as confirmed by previous research 2, 38 . ...
... To overcome the above shortcoming of the obstacle-loaden chamber in the DDT process, the fluidic transverse jet approach is introduced, which provides a similar function as the solid obstacle and has lower pressure loss 17,39 . These fluidic crossflow jets have an advantage to be adjusted easily to form different turbulent flows and eddies by changing the jet width and stagnation pressure, which can efficiently control the DDT process 40 . ...
A combination of solid and transverse jet obstacles is proposed to trigger flame acceleration and deflagration-to-detonation transition (DDT). A numerical study of this approach is performed by solving the reactive Navier-Stokes equations deploying an adaptive mesh refinement technique. A detailed hydrogen-air reaction mechanism with 12 species and 42 steps is employed. The efficiency and mechanisms of the combined obstacles on the flame acceleration are investigated comprehensively. The effects of multiple jets, jet start time, and jet stagnation pressure on the DDT process are studied. Results show that there is a 22.26% improvement in the DDT run-up time and a 33.36% reduction in the DDT run-up distance for the combined obstacles compared to that having only solid obstacles. The jet acts as an obstruction by producing a suitable blockage ratio and introducing an intense turbulent region due to the Kelvin-Helmholtz instability. This leads to dramatic flame-turbulence interactions, increasing the flame surface area dramatically. The dual jet produces mushroom-like vortices, leading to a significantly stretched flame front and intensive Kelvin-Helmholtz instabilities, and therefore these features produce a high flame acceleration. As the jet operation time decreases, the jet obstacle almost changes its role from both physical blockage ratio as well as turbulence and vorticity generator to a physical blockage ratio. There is a moderate jet stagnation pressure that reduces the run-up time to detonation and run-up distance to detonation in the obstacle-laden chamber. While further increasing the jet stagnation pressure, it does not have a positive effect on shortening the detonation transition.
... When the main flow consists of a stoichiometric hydrogen-air mixture and the jet composition is either premixed stoichiometric hydrogen-air or pure air, the results show that the jet plays the role of a virtual obstacle but suffers from substantially lower total pressure losses than a solid obstacle with similar blockage shape. But there is no discernable difference in DDT distances between the jet composition of air and reactive mixture [11,12] . Whereas in another study, the jet made by pure air deteriorates the local kerosene-air ratio and fuel distribution, which was observed experimentally and found to be disadvantageous for flame propagation. ...
The fluidic jet turbulator has been a novel perturbation generator in the pulse-detonation engines research field for the past few years. In this paper, an experiment is performed to study the deflagration to detonation transition (DDT) process in a detonation chamber with a reactive transverse methane–oxygen mixture jet in crossflow (JICF). The jet injection arrangement is fundamentally investigated, including single jet and various double jets patterns. Corresponding two-dimensional direct numerical simulations with a multistep chemical kinetics mechanism are employed for analyzing details in the flow field, and the interaction between the vortex and flame temporal evolution is characterized. Both the experiments and simulations demonstrate that the JICF can distinctly accelerate flame propagation and shorten the DDT time and distance. The vortex stream induced by the jet distorts and wrinkles the flame front resulting in local flame acceleration. Moreover, the double jet patterns enhance flame acceleration more than the single jet injection because of the intrinsic counter-rotating vortex pairs and enhanced turbulence intensity.
... Previous studies have shown the ability of a jet in crossflow (JICF) to act as a virtual bluff-body obstacle for flame holding 9 . This led to recent exploratory research regarding the ability of a JICF to accelerate confined unsteady premixed flames 10 . Evidence of enhanced turbulence production and flame acceleration was observed. ...
A fluidic obstacle has been proposed as an alternative to conventional deflagration-to-detonation transition (DDT) enhancement devices for use in a Pulsed Detonation Engine (PDE). Experimental results have been obtained utilizing unsteady reacting and steady non-reacting flow to gain insight on the relative performance of a fluidic obstacle. Using stoichiometric premixed hydrogen-air, transition to detonation has been achieved using solely a fluidic obstacle with comparable DDT distances to that of a physical orifice plate. Flame acceleration is achieved due to the intense turbulent mixing characteristics inherent of a high-velocity jet and the blockage created by the virtual obstacle. Turbulence intensity (T.I.) measurements, taken downstream of both obstacles with hot-film anemometry during non-reacting steady flow, show a conservative trend that a fluidic obstacle produces approximately a 240% increase in turbulence intensity compared to that of a physical obstacle. Ignition times were reduced approximately 45%, attributable to the increase in upstream T.I. levels relative to the fluidic obstacle during the fill portion of the PDE's cycle. Transition to detonation was obtained for injection compositions of both premixed stoichiometric hydrogen-air and pure air.
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.
Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.
Jet-in-crossflow (JICF) is proved to be advantageous to accelerate flame propagation and stimulate the deflagration-to-detonation transition (DDT) process in the recent decade. Studies focused on the performance of the jet of combustible mixture or reactive gas on facilitating the DDT process are carried out, and the results show that the combustible JICF can promote the flame transiting to detonation efficiently. Although most investigations attribute the ability of the JICF on promoting DDT to the turbulent effect induced by the jet, the combustion-enhancement effect of the jet medium hasn't been considered. In our previous work, it is proposed that the non-reactive gas jet in an optimal condition can accelerate flame propagation and promote the precursor shock wave formation. Hence, the performance of the multi-jet of non-reactive gas on facilitating the DDT process is worth investigating. In this paper, three different double jet placement methods are designed: parallel, staggered, and opposite positioned, and their performances on accelerating flame propagation and promoting the DDT process are investigated systematically. The results indicate that the staggered jet can accelerate the flame to detonation within a short run-up distance whereas the parallel jet and the opposite jet can only make the flame accelerate slightly. The schlieren images of flame evolution affected by double jet indicate that the complicated reactivity gradients generate on the flame surface while the flame is disturbed by the double jet. The interaction between the staggered jet and the flame induces local explosions which results in a prominent increase of the flame area and a large volumetric burning rate. For the opposite jet, its negative effect of combustion-inhibition of the aggregated CO2 overwhelms the positive effect of turbulence-enhancement on the flame acceleration.
We studied the mechanisms of flame acceleration (FA) and deflagration to detonation transition (DDT) triggered by a combination of solid and jet obstacles. The Navier–Stokes equations with a detailed hydrogen–air kinetics model were utilized. Vast Kelvin–Helmholtz instabilities generate intensive turbulence–flame interactions, leading to an increase in surface area and high propagation velocity. The jet position has a significant effect on the FA and DDT. A choking flame and detonation flame are obtained by the transverse jet with different positions and mixing times even though in a lower blockage ratio.
Two-dimensional numerical simulation is performed with the open-source program AMROC to study the effects of transverse jets (act as fluidic obstacles within a detonation tube) on the flame acceleration and deflagration to detonation transition (DDT). The slot transverse jets have been studied and compared with conventional solid obstacles in tubes. The jet initial parameters, such as mixture composition, stagnation temperature, pressure, and mass flow rate, are investigated. The results demonstrate that a hydrogen-oxygen-argon reactive fluidic obstacle leads to the shortest DDT distance and time compared with solid obstacles and fluidic obstacles composed of pure oxygen or argon. The fluidic obstacles can induce more vorticities to accelerate flame propagation. The DDT distance and time decrease with the jet initial temperature, pressure, and mass flow rate rise, while a high jet initial stagnation temperature is counterproductive to shorting DDT distance and time. The local static pressure rise plays an important role in flame acceleration when increasing the initial pressure of the fluidic obstacle. Higher jet pressure and a wider jet induce more compression waves, which can make the initial flame front more unstable and accelerate the flame as well.
This paper reviews the state of knowledge on flame acceleration and deflagration-to-detonation transition (DDT) in smooth ducts and ducts equipped with turbulence-producing obstacles. The objective is to bring to light the basic understanding of the phenomenon and its application to explosion safety. The scope of the review is restricted to homogeneous gas-phase combustion with emphasis placed on experimental investigation.
The present study is motivated by the desire to provide a flame stabilization environment without the use of a sudden expansion or bluff body that incurs a thrust penalty. A transverse slot jet issuing into a confined cross flow is used to produce a low-speed recirculation zone similar to that formed downstream of a rearward-facing step, a configuration often employed for flame holding. The study of the nonreacting flowfield characteristics of the induced recirculation is conducted to identify parametric aspects of the recirculation zone and study the effects of placing a diffuser upstream of the test section. For combustion conditions, the flame stabilization limits are documented. Altering the equivalence ratio of the transverse jet relative to that of the main flow allows for secondary control of the stabilization limits. Flowfield characteristics for a representative combustion case are presented. Combustion tends to enhance shear and create higher turbulence levels. Intense heat release rates occur downstream of the recirculation zone. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
: An in-house computational and experimental program to investigate and develop an air breathing pulse detonation engine (PDE) that uses a practical fuel (kerosene based, fleet-wide use, JP type) is currently underway at the Combustion Sciences Branch of the Turbine Engine Division of the Air Force Research Laboratory (AFRL/PRTS). PDE's have the potential of high thrust, low weight, low cost, high scalability, and wide operating range, but several technological hurdles must be overcome before a practical engine can be designed. This research effort involves investigating such critical issues as: detonation initiation and propagation; valving, timing and control; instrumentation and diagnostics; purging, heat transfer, and repetition rate; noise and multi-tube effects; detonation and deflagration to detonation transition modeling; and performance prediction and analysis. An innovative, four-detonation-tube engine design is currently in test and evaluation. Preliminary data are obtained with premixed hydrogen/air as the fuel/oxidizer to demonstrate proof of concept and verify models. Techniques for initiating detonations in hydrogen/air mixtures are developed without the use of oxygen enriched air. An overview of the AFRL/PRTS PDE development research program and hydrogen/air results are presented.
The effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top-and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three-dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas. (author)
Pulse detonation engines (PDEs) are currently attracting considerable research and development attention because they promise performance improvements over existing airbreathing propulsion devices. Because of their inherently unsteady behavior, it has been difficult to conveniently classify and evaluate them relative to their steady-state counterparts. Consequently, most PDE studies employ unsteady gasdynamic calculations to determine the instantaneous pressures and forces acting on the surfaces of the device and integrate them over a cycle to determine thrust performance. A classical, closed thermodynamic cycle analysis of the PDE that is independent of time is presented. The most important result is the thermal efficiency of the PDE cycle, or the fraction of the heating value of the fuel that is converted to work that can be used to produce thrust. The cycle thermal efficiency is then used to find all of the traditional propulsion performance measures. The benefits of this approach are 1) that the fundamental processes incorporated in PDEs are clarified; 2) that direct, quantitative comparisons with other cycles (e.g., Brayton or Humphrey) are easily made; 3) that the influence of the entire ranges of the main parameters that influence PDE performance are easily explored; 4) that the ideal or upper limit of PDE performance capability is quantitatively established; and 5) that this analysis provides a basic building block for more complex PDE cycles. A comparison of cycle performance is made for ideal and real PDE, Brayton, and Humphrey cycles, utilizing realistic component loss models. The results show that the real PDE cycle has better performance than the real Brayton cycle only for flight Mach numbers less than about 3, or cycle static temperature ratios less than about 3. For flight Mach numbers greater than 3, the real Brayton cycle has better performance, and the real Humphrey cycle is an overoptimistic (and unnecessary) surrogate for the real PDE cycle.
For flame propagation in tubes or channels filled with obstacles, it was observed that deflagration to detonation transition (DDT) occurred mostly in the vicinity of an obstacle. Furthermore, onset of detonation originated near the upstream side of the obstacle. This observation suggests that the interaction between the leading shock front of an accelerated flame with the obstacles along its path plays an important role in the transition process. To understand this interaction, initiation of detonation resulting from a collision of a shock wave with obstacles (repeated baffles and wedge-baffle combination) was studied in a 9 × 9 cm channel containing hydrogen-oxygen mixtures. Stroboscopic schlieren photographs showed that shock-focusing, from shock diffraction and reflection, can create local hot spots capable of causing a strong ignition develops into a detonation wave. For collision of a shock wave with a wedge-baffle combination in a stoichiometric H2O2 mixture at 7.9 kPa (60 torr) initial pressure, the critical incident shock Mach number was measured to be about 2.25. This critical value depends on the wedge angle as well as the size of the baffle obstacle. Even though the effects of turbulence in DDT have been well documented, present results suggest that for highly accelerated flames (with flame speed approaching that of a CJ deflagration), local heating by shock-focusing, resulting from a collision of the leading shock with obstacles, is sufficient to cause transition to detonation.
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.
Flame stabilization is the act of maintaining combustion in the presence
of a high-speed premixed flow, and continues to be an important process
that influences the performance and limitations for propulsion
applications. A common approach for current generation flame holders
involves the employment of a low-speed recirculation zone where hot
combustion products are maintained and act as a continuous ignition
source. The recirculation zone is often induced using a wake-generating
bluff body that is submerged in the flow, or through the use of a
rearward facing step. A fluidic-based flame holder using a transverse
slot jet issuing into a cross flow offers potential thrust and
efficiency benefits for propulsion. The transverse slot jet flame holder
has been shown to develop a low-speed recirculation zone capable of
stabilizing a stationary flame, analogous to a rearward-facing step
(i.e. a wall-bounded bluff body). Turbulent flame structures were
investigated for various flame holders. The role of baroclinic torque on
turbulent flame structures evolution and the flowfield will be
described. Comparisons will be made to a rearward-facing step flame
holder. The details of the turbulent flow with and without combustion
will be described, showing the potential advantages achieved using
fluidics. The fluidic flame holder provides competitive flame holding
performance to the mechanical counterpart, while having enhanced
combustion rates that result in higher combustor efficiencies and/or
shorter burners.
This paper summarizes a 10-year theoretical and numerical effort to understand the deflagration-to-detonation transition (DDT). To simulate DDT from first principles, it is necessary to resolve the relevant scales ranging from the size of the system to the flame thickness, a range that can cover up to 12 orders of magnitude in real systems. This computational challenge resulted in the development of numerical algorithms for solving coupled partial and ordinary differential equations and a new method for adaptive mesh refinement to deal with multiscale phenomena. Insight into how, when, and where DDT occurs was obtained by analyzing a series of multidimensional numerical simulations of laboratory experiments designed to create a turbulent flame through a series of shock–flame interactions. The simulations showed that these interactions are important for creating the conditions in which DDT can occur. Flames enhance the strength of shocks passing through a turbulent flame brush and generate new shocks. In turn, shock interactions with flames create and drive the turbulence in flames. The turbulent flame itself does not undergo a transition, but it creates conditions in nearby unreacted material that lead to ignition centers, or “hot spots,” which can then produce a detonation through the Zeldovich gradient mechanism involving gradients of reactivity. Obstacles and boundary layers, through their interactions with shocks and flames, help to create environments in which hot spots can develop. Other scenarios producing reactivity gradients that can lead to detonations include flame–flame interactions, turbulent mixing of hot products with reactant gases, and direct shock ignition. Major unresolved questions concern the properties of nonequilibrium, shock-driven turbulence, stochastic properties of ignition events, and the possibility of unconfined DDT.
The process of flame acceleration inside the tubes and channels depends on several parameters such as nature of the fuel involved, composition of the mixture and configuration of the enclosure itself. The wall roughness and the presence of obstacles in the flame path act as a turbulence generator causing continuous flame acceleration. In some situations the flame can reach a sufficiently high speed to allow the transition of the deflagration into a detonation.A considerably large amount of experimental data on flame speed and DDT run-up distance for several mixtures have been accumulated. Nevertheless simple relationships, based on the most relevant parameters governing the phenomenon, could be useful for design purpose and safety assessment.The present paper suggests some simplified formulas for the evaluation of flame speed and DDT run-up distance of flammable mixtures for both smooth and obstacles filled tubes.
Two-dimensional reactive Navier-Stokes equations for an acetylene–air mixture are solved numerically to simulate the interaction of a shock wave and an expanding flame front, the formation of a flame brush, and deflagration-to-detonation transition (DDT). The effects of viscosity, thermal conduction, molecular diffusion, and chemical reactions are included. A new method for adaptive mesh refinement was used to ensure that the structure of the flame front was resolved. The shock–flame interactions, through the Richtmyer-Meshkov instability, create and maintain a highly turbulent flame brush. The turbulence is not Kolmogorov turbulence, but it is driven at all scales by repeated shock–flame interactions. Pressure fluctuations generated in the region of the turbulent flame brush create, in turn, hot spots in unreacted material. These hot spots may then transition to DDT through the gradient mechanism. Repeated shock–flame interactions and merging shocks in unreacted material lead to the development of a high-speed shock that moves out in front of the turbulent flame. The region between this shock and the flame is subject to intense fluctuations generated in the flame. The simulations show that the interactions of shocks and flames create the conditions under which deflagration-to-detonation transition may occur.
Written for the Dept. of Mechanical Engineering. Typewritten MS. Thesis (Ph.D.) -- McGill University. Bibliography: leaves 93-98.
―Some Experiments on Shock–Flame Interaction
- T Scarinci
- G O Thomas
Scarinci, T., Thomas, G. O., ―Some Experiments on Shock–Flame Interaction,‖ UCW/det905, Department of Physics, University of Wales, Aberystwyth, Wales, UK. 1990.