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Abstract
This report addresses some basic issues in the structural and performance aspects of Pulse Detonation Engines (PDEs). This Device has been targeted to assist the growing demand, the Indian defense and armaments industry poised to grow exponentially. There is a requirement for a cost effective, yet highly efficient propulsion device that can drive this growth. Propulsion devices operating on the principle of Pulse Detonation Technology with the underlying principle being the Kedanacy effect. A valveless pulse detonation engine has been identified as being the suitable candidate for a low cost propulsion device. A valveless pulse detonation engine is an air breathing engine in which burning and acceleration of the air-fuel mixture causes intermittent detonations that produce shock waves. The shockwaves traveling at subsonic or supersonic speeds depending design constraints produce thrust. It is an efficient thrust producing device that can be used for military technology which include aeronautics, missile technology, aerial based systems and military automobile systems. The valueless pulse detonation engine is easy to manufacture with less complexities and exhibits superior performance efficiency and characteristics when compared with other types of jet engine-ram jet, turbojet, turboprop engines and turbofan engines. The valve-less pulse detonation engine also serves various criterion that enable its continued use as a mass produced and cheap engine; it is inexpensive to manufacture, it produces more thrust per unit of fuel consumed, specific impulse, scalability, the type of fuel used and the fuel applicability to the operation intended were considered. This report investigates theoretically the problems preventing widespread use of pulse jet engines and certain solutions are proposed to alleviate and ameliorate these problems, especially a solution to achieve deflagration from detonation. This report addresses some basic issues in the structural and performance aspects of Pulse Detonation Engines (PDEs). Performance parameters studied include thrust-specific fuel-consumption (TSFC), frequency limits, and thrust-to-weight ratio. A design surface is developed that accounts for various design limits. . Four materials for PDEs were chosen for comparison: silicon nitride, inconel, steel, and aluminum. Estimates of wall thickness and thrust-to-weight ratio are given over a range of operating conditions. Key issues and areas for further work are identified for both propulsion and performance aspects.
Chapter 1: Introduction:
Jet Engines: ¬¬¬¬¬¬these are engines of reaction type in which a fast moving fluid jet produces thrust by a scientific phenomenon: jet propulsion which is in accordance with Newton’s third law of motion. Jet engines are combusting engines that can be classified as follows: turbojets, turboprops, turbojets, ramjets and pulse jets. Of this turbojet engines are widely used commonly and are regarded as being inefficient to be operated in subsonic speeds. Other forms of jet engines except pulse jet engines and ram jet engines are still in their infancy, with development process beginning to take shape. There are several advantages that seem lucrative for the development of turbojet and turbofan engines, especially when the turbojet and turbofan engines offer more power to weight ratio compared to their predecessors the reciprocating engines used for aviation. The turbojet engines also are more compact in nature enabling their incorporation in several versatile designs. However, the main disadvantage of turbojet and turbofan engines is that, compared to a reciprocating engine of the same size, they are expensive. Because their inner mechanisms operate at high revolutions and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. Turbofan and turbojet engines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load limiting their applications to civil and military aviation rather than being used as general thrust producing engines used for common purposes. In order to incorporate the advantages of turbofan and turbojet engine, but to placate the disadvantages a pulse jet engine is selected as an ideal type of engine for general propulsion needs.
Pulse Jet Engines:
A pulse jet engine (or pulse detonation engine PDE) is a type of jet engine in which combustion occurs in pulses. These pulses are essentially rapid explosions of air fuel mixture which take place at about 150 to 200 times in the combustion chamber of the pulse jet engine. Pulse jet engines offer a low-technology varying-efficiency form of jet propulsion, unlike turbojets or rocket engines. Due to the low complexity, they are frequently made by home builders. There are two types of pulse jet engines: valve operated pulse jet engines and valveless pulse jet engines. In simple terms, a pulse jet is a pipe which is open on both ends; fuel is injected and ignited and the blast primarily goes out via the (large) exhaust pipe. However, some of the blast also goes in the other direction (via the small air intake). Due to this, the valueless pulse jet has both the air intake and exhaust pipe leading out in a same direction.
Valve Operated pulsejets
Valved pulsejet engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out of the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake.
The valved pulsejet comprises an intake with a one-way valve arrangement. The valves prevent the explosive gas of the ignited fuel mixture in the combustion chamber from exiting and disrupting the intake airflow, although with all practical valved pulsejets there is some 'blowback' while running statically and at low speed, as the valves cannot close fast enough to stop all the gas from exiting the intake. The superheated exhaust gases exit through an acoustically resonant exhaust pipe.
The intake valve is typically a reed valve. The two most common configurations are daisy valve and a rectangular valve grid. A daisy valve consists of a thin sheet of metals to act as the reed, cut into the shape of a with "petals" that widen towards their ends. Each "petal" covers a circular intake hole at its tip. The daisy valve is bolted to the manifold through its centre. Although easier to construct on a small scale, it is less effective than a valve grid.
Valveless pulsejets
Valveless pulsejet engines have no moving parts and use only their geometry to control the flow of exhaust out of the engine. Valveless pulsejets expel exhaust out of both the intakes and the exhaust, though most try to have the majority of exhaust go out of the longer tail pipe for more efficient propulsion.
The valveless pulsejet operates on the same principle as the valved pulsejet, but the 'valve' is the engine's geometry. Fuel, as a gas or atomized liquid spray, is either mixed with the air in the intake or directly injected into the combustion chamber. Starting the engine usually requires forced air and an ignition source, such as a spark plug, for the fuel-air mix. With modern manufactured engine designs, almost any design can be made to be self-starting by providing the engine with fuel and an ignition spark, starting the engine with no compressed air. Once running, the engine only requires input of fuel to maintain a self-sustaining combustion cycle.
Characteristics of valueless pulsejet engines:
A pulsejet engine is an air-breathing reaction engine employing an ongoing sequence of discrete combustion events rather than a constant level of combustion. This clearly distinguishes it from other reaction engine types such as rockets, turbojets and ramjets, which are all constant combustion devices. All other reaction engines are driven by maintaining high internal pressure; pulsejets are driven by an alternation between high and low pressure. This alternation is not maintained by any mechanical contrivance, but rather by the natural acoustic resonance of the rigid tubular engine structure. The valveless pulsejet is, mechanically speaking, the simplest form of pulsejet, and is, in fact, the simplest known air-breathing propulsion device that can operate "statically", i.e. without forward motion.
The combustion events driving a pulsejet are often informally called "explosions"; however, the preferred term is "deflagrations". They are not the violent, very high energy detonations employed in "Pulse Detonation Engines (PDEs)"; rather, deflagration within the combustion zone of a pulsejet is characterized by a sudden rise in temperature and pressure followed by a rapid subsonic expansion in gas volume. It is this expansion that performs the main work of moving air rearward through the device as well as setting up conditions in the main tube for the cycle to continue.
A pulsejet engine works by alternately accelerating a contained mass of air rearward and then breathing in a fresh mass of air to replace it. The energy to accelerate the air mass is provided by the deflagration of fuel mixed thoroughly into the newly acquired fresh air mass. This cycle is repeated many times per second. During the brief mass acceleration phase of each cycle, the engine’s physical action is like that of other reaction engines — gas mass is accelerated rearward, resulting in an application of force forward into the body of the engine. These "pulses" of force, rapidly repeated over time, comprise the measurable thrust force of the engine.
Some basic differences between valved and valveless pulsejets are:
Valveless pulsejet engines have no mechanical valve, eliminating the only internal "moving part" of the conventional pulsejet; In valveless engines, the intake section has an important role to play throughout the entire pulsejet cycle; Valveless engines produce thrust forces in two distinct but synchronized mass acceleration events per cycle, rather than just one.
Operating cycle of pulse jet engine:
The combustion cycle comprises five or six phases depending on the engine: Induction, Compression, (optional) Fuel Injection, Ignition, Combustion, and Exhaust.
Starting with ignition within the combustion chamber, a high pressure is raised by the combustion of the fuel-air mixture. The pressurized gas from combustion cannot exit forward through the one-way intake valve and so exits only to the rear through the exhaust tube.
The inertial reaction of this gas flow causes the engine to provide thrust, this force being used to propel an airframe or a rotor blade. The inertia of the traveling exhaust gas causes a low pressure in the combustion chamber. This pressure is less than the inlet pressure (upstream of the one-way valve), and so the induction phase of the cycle begins.
In the simplest of pulsejet engines this intake is through a venturi which causes fuel to be drawn from a fuel supply. In more complex engines the fuel may be injected directly into the combustion chamber. When the induction phase is under way, fuel in atomized form is injected into the combustion chamber to fill the vacuum formed by the departing of the previous fireball; the atomized fuel tries to fill up the entire tube including the tailpipe. This causes atomized fuel at the rear of the combustion chamber to "flash" as it comes in contact with the hot gases of the preceding column of gas—this resulting flash "slams" the reed-valves shut or in the case of valveless designs, stops the flow of fuel until a vacuum is formed and the cycle repeats.
Valve Operated Design:
There are two basic types of pulsejets. The first is known as a valved or traditional pulsejet and it has a set of one-way valves through which the incoming air passes. When the air-fuel is ignited, these valves slam shut which means that the hot gases can only leave through the engine's tailpipe, thus creating forward thrust.
The cycle frequency is primarily dependent on the length of the engine. For a small model-type engine the frequency may be around 250 pulses per second, whereas for a larger engine such as the one used on the German V-1 flying bomb, the frequency was closer to 45 pulses per second. The low-frequency sound produced resulted in the missiles being nicknamed "buzz bombs."
Valveless Design:
The second type of pulsejet is known as the valveless pulsejet.[8] Technically the term for this engine is the acoustic-type pulsejet, or aerodynamically valved pulsejet.
Valveless pulsejets come in a number of shapes and sizes, with different designs being suited for different functions. A typical valveless engine will have one or more intake tubes, a combustion chamber section, and one or more exhaust tube sections.
The intake tube takes in air and mixes it with fuel to combust, and also controls the expulsion of exhaust gas, like a valve, limiting the flow but not stopping it altogether. While the fuel-air mixture burns, most of the expanding gas is forced out of the exhaust pipe of the engine. Because the intake tube(s) also expel gas during the exhaust cycle of the engine, most valveless engines have the intakes facing backwards so that the thrust created adds to the overall thrust, rather than reducing it.
The combustion creates two pressure wave fronts, one traveling down the longer exhaust tube and one down the short intake tube. By properly 'tuning' the system (by designing the engine dimensions properly), a resonating combustion process can be achieved.
While some valveless engines are known for being extremely fuel-hungry, other designs use significantly less fuel than a valved pulsejet, and a properly designed system with advanced components and techniques can rival or exceed the fuel efficiency of small turbojet engines.
In 1909, Georges Marconnet developed the first pulsating combustor without valves. It was the grandfather of all valveless pulsejets. The valveless pulsejet was experimented with by the French propulsion research group SNECMA in the late 1940s.
The valveless pulsejet's first widespread use was the Dutch drone Aviolanda AT-21. A properly designed valveless engine will excel in flight; as it does not have valves, ram air pressure from traveling at high speed does not cause the engine to stop running like a valved engine. They can achieve higher top speeds, with some advanced designs being capable of operating at Mach .7 or possibly higher.
The advantage of the acoustic-type pulsejet is simplicity. Since there are no moving parts to wear out, they are easier to maintain and simpler to construct.
Basic Operation:
In previous article, there were seven stages of the operation of the pulse jet engine. Detailed descriptions of the pulsejet operational cycle can be found in the literature, of which an concise and abridged version is presented here. Only the essential phases of operation of the pulse jet engine have been described. The operating cycle may be divided into three phases:
1. Combustion - Reaction of an air and fuel mixture within the combustion chamber commences. The pressure begins to rise as a result of confinement of the flow. The pressure rise causes the inlet valves to close, preventing backflow. The reaction accelerates as the pressure and temperature rise; this, in turn, accelerates the pressure and temperature rise.
2. Expansion - The hot, high-pressure gases in the combustion chamber expand, forcing flow from the exhaust
3. Ingestion - The momentum of the exhaust gases causes the combustion-chamber pressure to drop below the ambient value. This allows the inlet valves to open and a fresh charge of air to enter (mixed with fuel). Eventually, the exiting exhaust flow reverses and mixes with the fresh charge. This initiates a new reaction, and the cycle begins again.
The starting of the pulsejet engines poses several challenges as with the starting operations of other types of jet engines. In the first stage of operation of the pulse jet engine, inside the combustion chamber of the pulse jet engines combustion of the air fuel mixture takes place. There are several combustion theories explaining the combustion of the air fuel mixtures.
Combustion can occur in two distinct modes, one is a deflagration and the other is detonation. Each mode has its own characteristic behavior which differs radically in their respective final thermodynamic states. Deflagration is typically what most people imagine when they think of combustion and explosions; it is the subsonic, constant pressure consumption of reactants into products resulting in a high temperature gas. A detonation is a violent supersonic combustion that releases an incredible amount of energy in a rather short period. Detonation is commonly referred to as knocking or pinging in traditional internal combustion engines and can lead to disastrous consequences if left unchecked. In industrial situations, detonations can occur when gasses are transported along extended lengths of pipes and can lead to accidental and sometimes fatal explosions. In the aerospace industry however, the explosive power of detonations can be harnessed for thrust or shaft power production.
The Kadenacy effect: The Kadenacy effect is an effect of pressurewaves in gases. It is named after Michel Kadenacy who obtained a French patent for an engine utilizing the effect in 1933. Operation of a pulsejet engine. There are also European and US patents. In simple terms, the momentum of the exhaust gas leaving the cylinder of an internal combustion engine creates a pressure-drop in the cylinder which assists the flow of a fresh charge of air, or fuel-air mixture, into the cylinder. The effect can be maximized by careful design of the inlet and exhaust passages.
Deflagration: Deflagration is the subsonic combustion of a fuel and oxidizer mixture usually producing a small pressure drop with significant temperature increases. Deflagration can be modeled as an isobaric process in most cases as the pressure loss that occurs during combustion is negligible. Deflagration is typical in internal combustion engines (Otto and Diesel thermodynamic cycles) and aircraft turbine engines (Brayton Cycle) and what is classically observed when a fuel and oxidizer is ignited. The flame front or reaction usually propagates through its fuel mixture at a rate of nearly 1 m/s. If the combustion is confined to a closed volume, i.e. a cylinder, thermodynamics dictates that there must be a corresponding increase in pressure from which mechanical work can be extracted.
Detonation: is the supersonic ignition of a combustible mixture where a shock wave is fueled by an exothermic (heat generating) reaction. Detonation waves propagate at supersonic speeds on the order of 2000 m/s. Detonations, which are modeled as a constant volume combustion (Humphrey and Fickett-Jacobs thermodynamic cycles) produce a higher thermal efficiency (1.3 -1.5 times) than that of a constant pressure combustion cycle at an equivalent pressure ratio and thus can result in a similar increase in fuel efficiency provided that other mechanical and related efficiencies can be maintained. The formation and propagation of a detonation wave compresses the gas ahead of it causing a dramatic increase in pressure and temperature after the combustion process. This process can be described by the one dimensional Chapman-Jouguet theory and the ZND model.
Chapter 2:Literature review:
Several Journals and Text Books were studied to gain further insight on the esoteric topic of pulse jet engines. Several Journal research publications from Science Direct were downloaded and studied. The several journals studied illustrate the current research in Pulse Detonation Technology and the progress made in Valve Less Pulse Jet engines. The papers also suggested solutions to problems faced in conventional pulse jet engines.
Journals tested the various performance characteristics of the pulse jet engines.
Paper 1: Innovative Trends in Pulse Detonation Engine, its Challenges and Suggested Solution
Authors: ApoorvGarg, Ashish Dhiman
In this paper, the status of the theoretical and experimental study of Pulse Detonation Engine was presented. Secondly, a comparison of thermal efficiency of Pulse detonation Engine and generally used propulsion system (such as Rocket Engine) was studied and it is shown that efficiency of Pulse Detonation Engine is much higher. Also, the other advantages of Pulse Detonation Engine are discussed in this paper. Further, this paper presents a theoretical investigation of the problems preventing the widespread use of Pulse detonation Engine. In the end, a review of various methods which may overcome these challenges is provided, specifically, the approach of Detonation to Deflagration (DDT) method for solving Detonation Initiation problem was discussed in detail. The paper ends on a note of promising near future when Pulse Detonation Engines will become the staple for power generation and locomotion.
Paper 2:Assessment of the Performance of a Pulsejet and Comparison with a Pulsed-Detonation Engine
Authors: Paul J. Litke and Frederick R. Schauer, Wright-Patterson, Daniel E. Paxson
In this paper the performance of a Solar PJ32 pulsejet engine, is evaluated under static conditions and compared with that of a pulsed-detonation engine (PDE) firing at similar inlet and operating conditions. In the series of tests suggested, thrust is calculated from combustion-chamber pressure histories and agrees with measured thrust within 5-10%. Peak combustion-chamber head pressures ranges in the conventional values, while significantly higher pressure are attained in PDEs. Airflow at the inlet of the pulsejet is measured and used to calculate specific thrust and equivalence ratio. The equivalence ratio is defined as the ratio of the actual fuel/air ratioto the stoichiometric fuel/air ratio. Stoichiometric combustion occurs when all the oxygen is consumed in the reaction, and there is no molecular oxygen(O2) in the products.Specific thrust ranges from 40-100 n/m over the range of fuel flows from lean to rich conditions. A similarly operating PDE has a specific thrust around 120 N/m, making the PDE more efficient in terms of air flow. The pulsejet equivalence ratio ranges from 0.6-1.0, with rated/peak thrust occurring at rich conditions. Typical fuel-specific impulse for the pulsejet is 1400-1500 s for rated thrust conditions, whereas PDE performance is around 1800 s. For the PDE operating in the same fill fraction range as the pulsejet, PDE Isp is estimated to be 6000-8000 s making the PDE cycle far more efficient and desirable at comparable conditions.
Paper 3:Optimization of a multiple pulse detonation engine-crossover system
Authors: Robert Driscoll, Andrew St. George, David Munday, Ephraim J. Gutmark
A novel concept of pulse jet engines, the Cross over System is studied by the authors of this article.The authors studied crossover systems in pulse detonation engines, and the effects of combustion of fuel in pulse jet engines due to shock initiated pulsing in the engines. An optimization study of the system finds auto ignition failures due to varying lengths of the pulse jet drivers, i.e. the combustion chamber geometry. Initiation performance in the driven PDE is strongly dependenton initial driven PDE skin temperature in the shock wave reflection region. The optimum initiation performance is achieved within the driven PDE by filling the driver PDE with reactants past the crossovertube entrance. Increasing operating frequency negates the detrimental effect of increased nitrogen dilution. An array of detonation tubes connected with crossover tubes is developed using optimizedparameters. Successful operation utilizing shock-initiated combustion through shock wave reflection isachieved and sustained. Results from this array show that if initially driven PDE tubes are operating successfully, all subsequently driven PDE tubes also operate successfully.
Paper 4:Analysis of the actual thermodynamic cycle of the detonation engine
Authors: Khaled Alhussan A, Mohamad Assad B, OleqPenazkov
This paper studies the thermodynamic cycles that operates the pulse detonation engine. Two variants of the limitations are analyzed: in the first case the cycles arecompared at the same degree of adiabatic compression, in the second – within the same interval of limiting temperatures. The values of the thermal efficiency and pressure intervals were compared, as well as the introduced additional criteria forassessing the thermodynamic perfection: the specific volume work, the criterion ofirreversibility, and the nondimensional value Z, characterizing the fraction of theeffective work from the mechanical work obtained per cycle.The analysis carried out has revealed that a detonation engine possesses anadvantage over some parameters, especially over the thermal efficiency, but its superiority is not absolute. This advantage by further research can be further exploited to propagate further use of pulse jet engines.
The most important criterion of the efficiency of the heat engine is the degree of the thermodynamic perfection of its operating cycle. The thermodynamic cycleofthe detonation engine is basically similar to the cycles of other heat engines interms of the structure of the elementary processes composed in its diagram. There isno doubt, however, that it has some unique features related in the first place to heatsupply to the working body, which distinguishes it from other well-knownthermodynamic cycles.
The following thermodynamic cycles are analysed under the same assumptions(this allows one to extend the results of comparison to the actualcycles typical of corresponding heat engines):
1. Brayton cycle. Gas-turbine and some other compressor engines operateaccording to this cycle, it is characterized by isobaric heat supply and removal,adiabatic compression and expansion.
2. Otto cycle. This cycle is closer to the actual cycle of the engines withforced ignition; the heat supply and removal are isochoric, the compression andexpansion are adiabatic.
3. Detonation cycle. Its heat supply obeys the Hugoniot relation, its heatremoval is isobaric, while the compression and expansion are adiabatic. This cycleis closer to the Humphrey cycle in which the Hugoniot adiabatic is replaced byisochore.
4. Carnot cycle. This is a standard (equivalent) cycle with which a comparisonis made. Here the heat supply and removal are isothermal, the compression andexpansion are adiabatic.
Paper 5: Detonation propagation with velocity deficitsin narrow channels
Authors: K. Ishii, M. Monwar
Propagation limits of detonations in narrow channels have been studied with afocus on velocity deficitsand variation in cell widths. A channel was formed by a air of metal plates of 1500 mm length which were inserted in a detonation tube of 0.5 mm inner diameter. Test gases were hydrogen–oxygen mixturesdiluted with argon or nitrogen, which were selected as representatives of regular and irregular mixture systems. The velocity deficits predicted using the concept of negative boundary layer displacement thicknesswere compared to those obtained experimentally. From good agreement between the predicted and theexperimental velocity deficits, the cell width enlarged in the channel was calculated using the induction zonelength behind the decelerated leading shock front. Although this calculation underestimates the cell widths,the calculated cell widths were found to be well predicted when they were multiplied by an appropriate proportionality factor. It is found that for given mixtures, a combination of the calculated velocity deficit andthe number of cells in a channel contributes to the prediction of propagation limits of detonations.In conclusion, the propagationbehavior of detonations in narrow channels wascategorized into five modes: stable, quasi-stable,galloping, single head, and failure mode. Amongthese modes the velocity deficit was experimentally evaluated only for the stable and quasi-stablemodes, which can be treated as steady propagation.
Paper 6:Experimental investigations on pulse detonation rocket engine withvarious injectors and nozzles
Authors: Yu Yan, Wei Fan, Ke Wang, Xu-dong Zhu, Yang Mu
This research article further avers Pulse detonation engines (PDEs) potential to become a revolutionary approach to propulsion.The engine of simple construction can be easily manufactured. The pulse detonationrocket engine (PDE) used in this study are 30 mm in inner diameter and 860 mm in length.Liquid kerosene, gaseous oxygen and nitrogen were used as fuel, oxidizer and purge gas,respectively for the purposes of this experiment. Two-phase detonation generating is harder than gaseous detonation dueto liquid fuel atomization and mixing of two-phase reactants. It is a difficult task forliquid fuel and gaseous oxidizer to mix and form uniformly distributed mixture in theentire long engine during filling process in a short time. Therefore the velocities of fueland oxidizer must be well designed to achieve not only the requirement of filling theentire engine but also the requirement of liquid fuel atomization and reactants mixing.Four injectors were tested to improve the atomization of liquid fuel and mixing processof reactants for performance enhancement of PDE. Injector with small fuel exit areaand large gas exit area was found to be effective for liquid fuel atomization andreactants mixing process. From this study we understood that the nozzles are critical components in improving the performance of PDE.Four kinds of bell-shaped converging–diverging nozzles were also tested here in orderto enhance the performance of PDE. It was found from the results and conclusion of this study that a nozzle with high contraction ratio and high expansion ratio generated the highest thrust augmentation of up to 25 to 30%.
Chapter 2: Design of valveless pulse jet engine:
Oxidizer and Fuel Selection
Selections of a fuel and oxidizer affect net thrust or work produced by a PDE cycle due to the large variation in detonation velocities, compression ratios, and temperatures produced by various types of fuels. It is typically best to use gaseous form reactants because of their lower detonation energy requirements although liquid fuels can be used if atomized prior to ignition. Even after atomization though, liquid fuels would require more power from a direct ignition system or a longer deflagration-to-detonation transition section. There is a strong dependence on stoichiometric ratio for cell size, initiation charge, and critical tube diameter. It is thus important to ensure stoichiometric or near stoichiometric fuel balances entering the combustion chamber. The stochiometric ratios of pulse jets have not been studied exclusively, albeit pulsejets require a very lean air-fuel mixture at max-thrust stage.
Detonation Initiation
Detonation initiation is currently one of the most critical problems in contemporary PDE development. Initiation of a detonation requires significantly more input energy than that of deflagration. For detonations there exists a critical initiation energy for which it is the smallest amount of energy deposition that will cause a direct initiation of a detonation. A detonation will be initiated if the energy release couples with the generated shock waves. If energy release occurs too far behind the shock wave or if the shock waves are weak, a detonation will not be initiated and result in a deflagration with modest pressure increases. There are generally two types of initiation modes, direct initiation and detonation transition. Direct initiation is usually caused by blast waves created by rapid energy addition either from the discharge of solid or gaseous explosives, exploding wires or high energy spark discharges. Detonation transition is usually carried out by means of flame acceleration via obstacle-wave interaction.
Spark Initiation
Many experimental direct initiation tests are conducted through the use of solid explosives and are based on the equivalent mass of explosive tetryl (C7H5N5O8) with a blast energy value of 4.2 MJ/kg. Varying the amount of explosive material can then be used to equate the energy required for direct ignition to other methods of initiation. For "sensitive mixtures" like ethylene the required energy can be in the tens of kilojoules and less sensitive mixtures can scale up to the hundreds or even thousands of kilojoules. Direct initiation of detonation then can require very large power input for high cycle frequencies. Confinement by tubes or channels will decrease the critical energy required since blast waves decay more slowly when compared to unconfined cases. Increasing initial pressure or temperature will also slow the decay and reduce critical energy requirements. In detonation transition, a detonation wave can be created either by deflagration-to-detonation transition (DDT) or shock-to-detonation transition (SDT). DDT employs the use of obstacles in the path of combustion wave to accelerate it to CJ velocity. SDT uses directed or focused shockwaves along with obstacles to initiate a detonation wave. Detonation transition generally requires a large pre-detonation section or transition section for a self-sustaining wave to form and can be impractical for many applications.
In general, detonation initiation (direct or through transition) is sensitive to the followingconditions:
-Detonation Cell Size (A function of the fuel and oxidizer combination)
-Initial Temperature
-Initial Pressure
-Geometrical cross-sectional area
-Wall porosity
Deflagration to Detonation Transition (DDT)
In some situations the energy required for direct initiation of detonation may be prohibitively high. This can be due to large combustion chamber sizes, particularly insensitive fuel choices, very low temperature conditions, or low pressures. Deflagration-to-detonation transition (DDT) and shockto-detonation transition (SDT) are two methods commonly employed to achieve the detonation with significantly reduced energy requirements. In some cases an overdriven detonation wave, one that propagates at a speed greater than the speed of a CJ detonation wave, can also be used to reduce the critical diameter requirement needed for successful transition of a detonation wave from a tube of small diameter to a tube of larger diameter. Critical conditions for DDT require that the cell width be smaller than a specified fraction of the tube or obstacle dimensions, the expansion ratio (ratio of burned to unburned gas volume) must be larger than a minimum value, and that the deflagration speed exceed a minimum threshold. For simple situations, transition to detonation is possible only if the detonation cell width is smaller than the tube diameter (unobstructed tube) or smaller than the obstacles' aperture (obstructed tube). For a successful transfer of a detonation wave from one section to a larger or essentially unconfined volume, there exists a critical tube diameter which is generally accepted to be on the order of thirteen times the detonation cell width. In DDT a subsonic combustion wave (deflagration or flame) is accelerated to a supersonic combustion wave (detonation). The DDT process can be divided into four phases as described:
• Deflagration initiation - A relatively weak energy source such as an electric spark is used to create a flame.
• Flame acceleration - Increasing energy release rate and the formation of strong shock waves are caused by flame acceleration.
• Formation and amplification of explosion centers - One or more localized explosion centers form as pockets of reactants reach critical ignition. The explosion centers create small blast waves which rapidly amplify in the surrounding mixture.
• Formation of a detonation wave: The amplified blast waves and existing shock-reaction zone complex merge into a supersonic detonation front which is self-sustaining.
Methods of Flame Acceleration
The exact physics of flame acceleration are unknown yet recent work into studying detonation transitions has yielded a new explanation of the role that obstacles play in flame acceleration. Simulations from existing research documents showed that the deflagration propagates along the unobstructed center of the orifice plates leaving the mixture between orifice plates untouched near the wall. Gas expansion due to delayed burning in the pockets produces a jet flow in the unobstructed part of the tube. This jet flow allows the flame tip to propagate faster which then produces new pockets and creates a chain reaction leading to flame acceleration. The simulation also showed a strong reduction in the acceleration rate with higher initial flow Mach numbers and mitigation of flame acceleration was observed as soon as the flame speed became comparable to the gas speed of sound.