A Pulse Detonation Engine, Or PDE , Is a Type of Propulsion System That Has the Potential

ABSTRACT

A pulse detonation engine, or "PDE", is a type of propulsion system that has the potential to be both light and powerful and can operate from a standstill up to supersonic speeds. To date no practical PDE engine has been put into production, but several testbed engines have been built, proving the basic concept to some extent at least. In theory the design can produce an engine with an efficiency far surpassing more complex gas turbine Brayton cycle engines, but with almost no moving parts.

All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.

The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with fuel to create a flammable mixture that is then ignited. The resulting combustion greatly increases the pressure of the mixture to approximately 100 atmospheres, which then expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby pushing the aircraft forward, a series of shutters are used with careful tuning of the inlet to force the air to travel in one direction only through the engine.

CONTENTS

1) INTRODUCTION…………………………...………………………………………4 1.1 CONCEPT………………………………………………………………..4

1.2 COMBUSTION…….…………………………………………………….5

1.3 DEFLAGRATION………………………………………………….……7

(i) FLAME PHYSICS………………………………………………….8

1.4 DETONATION………………..……………………………………….10

(i) APPLICATIONS………………………………………………….12

2) BASIC PDE CYCLE……………………….………………....……………………12

3) DETONATION INITIATION IN PDE…………………………………………….14

3.1 DIRECT INITIATION………………………………………………….14

3.2 DEFLAGRATION TO DETONATION TRANSITION……….………15

(i) Factors Influencing DDT………………………………..18

(ii) Time Vs Position Graph…………….……………………..20

4) BLOCK DIAGRAM OF PDE……………………………..………………………..20

5) WORKING OF PDE………………………………………………………………..21

5.1 T-S DIAGRAM………………………………………………………..25

6) PRACTICAL PROBLEMS INVOLVED IN PDE……….…………………………26

7) CURRENTSTATE OF DEVELOPMENT…………………………………………27

8) THE FUTURE OF PDE…………………………………...………………………..27

9) REFERENCES……………………………………………..………………………28

1.INTRODUCTION

1.1 Concept

All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsoniccombustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.

The main difference between a PDE and a traditional pulsejet is that the mixture does not undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen and fuel combination process is supersonic, effectively an explosion instead of burning. The other difference is that the shutters are replaced by more sophisticated valves. In some PDE designs from General Electric, the shutters are even removed because the process can be controlled by timing on the periodic sudden pressure drops that occur after each shock wave when the "combustion" products have been ejected in one shot.

The main side effect of the change in cycle is that the PDE is considerably more efficient. In the pulsejet the combustion pushes a considerable amount of the fuel/air mix (the charge) out the rear of the engine before it has had a chance to burn (thus the trail of flame seen on the V-1 flying bomb), and even while inside the engine the mixture's volume is continually changing, an inefficient way to burn fuel. In contrast the PDE deliberately uses a high-speed combustion process that burns all of the charge while it is still inside the engine at a constant volume. The maximum energy efficiency of most types of jet engines is around 30% a PDE can attain an efficiency theoretically near 50%.

Another side effect, not yet demonstrated in practical use, is the cycle time. A traditional pulsejet tops out at about 250 pulses per second, but the aim of the PDE is thousands of pulses per second, so fast that it is basically continual from an engineering perspective. This should help smooth out the otherwise highly vibrational pulsejet engine -- many small pulses will create less volume than a smaller number of larger ones for the same net thrust. Unfortunately, detonations are many times louder than deflagrations. To know more about deflagration and detonation, we need to learn about combustion.

1.2 COMBUSTION

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.It is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. The original substance is called the fuel, and the source of oxygen is called the oxidizer. The fuel can be a solid, liquid, or gas, although for airplane propulsion the fuel is usually a liquid. The oxidizer, likewise, could be a solid, liquid, or gas, but is usually a gas (air) for airplanes.

Fig.1 Combustion [Ref no: 9]

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The temperature of the exhaust is high because of the heat that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Soot, for example, is a form of solid exhaust that occurs in some combustion processes.

During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are both present in an automobile fuel tank; but combustion does not occur because there is no source of heat. Heat is both required to start combustion and is itself a product of combustion. Also, once combustion gets started, heat source need not be provided because the heat of combustion will keep things going.

There are different types of combustion like rapid combustion,slow combustion, complete combustion, turbulent combustion etc. But in pulse detonation engine, primary interest is on two types: deflagrationanddetonation.

1.3 DEFLAGRATION

Deflagration (Lat: de + flagrare, "to burn") is a technical term describing subsonic combustion that usually propagates through thermal conductivity (hot burning material heats the next layer of cold material and ignites it). Most "fire" found in daily life, from flames to explosions, is technically deflagration. Deflagration is different from detonation which is supersonic and propagates through shock compression

It is the set of phenomena accompanying the rapid passage of a reaction front, e.g., the front of a flame (combustion of a gas or a vapor, more rarely of a solid). In a homogeneous mixture of air and a combustible gas or vapor, a flame propagates at a constant velocity that is high but remains of the same order of magnitude of many familiar phenomena. It is of the order of 1 to 10 feet/second, hence comparable to that of a walker or a runner (as opposed to detonation that propagates several times faster than sound in air). A deflagration takes place during the rapid inflammation of the mixture of air and gas above a burner in a stove. If the amount of gas is small, this is uneventful; if the amount is important, the result may be an explosion. The flame of a gas burner is a deflagration moving at a constant velocity, in the direction opposed to that of the gas flux.

In a deflagration, the combustion products move at a subsonic velocity, in the direction opposed to that of the flame.

In engineering applications, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a gun, or a piston in an internal combustion engine) with the force of the expanding gas. Typical examples of deflagrations are combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine, a rapid burning of a gunpowder in a firearm or pyrotechnic mixtures in fireworks.

(i)Flame physics

We can better understand the underlying flame physics by constructing an idealized model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel, separated by a thin transitional region of width in which the burning occurs. The burning region is commonly referred to as the flame or flame front. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning.

There are two characteristic timescales which are important here. The first is the thermal diffusion timescale τd, which is approximately equal to where is the conductivity. The second is the burning timescale τb, which is approximately equal to where ε is the total energy released by burning per unit mass, and is the burn rate (i.e., the rate of increase of specific thermal energy).

In equilibrium, these two rates are equal: The heat generated by burning is equal to the heat carried away by heat transfer. This lets us find the characteristic width δ of the flame front:

Now, the thermal flame front propagates at a characteristic speed Sl, which is simply equal to the flame width divided by the burn time:

This simplified one-dimensional model neglects the possible influence of turbulence. As a result, this derivation gives the laminar flame speed -- hence the designation Sl.

In free-air deflagrations, there is a continuous variation in deflagration effects relative to maximum flame velocity. When flame velocities are low, the effect of a deflagration is the release of heat. Some authors use the term flash fire to describe these low-speed deflagrations. At flame velocities near the speed of sound, the energy released is in the form of pressure and the results resemble a detonation. Between these extremes both heat and pressure are released.

When a low-speed deflagration occurs within a closed vessel or structure, pressure effects can produce damage due to expansion of gases, as a secondary effect. The heat released by the deflagration causes the combustion gases and excess air to try to expand thermally as well. The net result is that the volume of the vessel or structure needs to either expand/fail to accommodate the hot combustion gases, or build internal pressure to contain them. The risks of deflagration inside waste storage drums is a growing concern among storage facilities

1.4 DETONATION

Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic speed (i.e., slower than the sound speed of the explosive material itself), and without a shock or any significant pressure change. Because detonations generate high pressures, they are usually much more destructive than deflagrations.

The simplest theory to predict the behavior of detonations in gases is known as Chapman-Jouguet (CJ) theory, developed around the turn of the 20th century. This theory, described by a relatively simple set of algebraic equations, models the detonation as a propagating shock wave accompanied by exothermic heat release. Such a theory confines the chemistry and diffusive transport processes to an infinitely thin zone.

A more complex theory was advanced during World War II independently by Zel'dovich, von Neumann, and Doering. This theory, now known as ZND (explosion) theory, admits finite-rate chemical reactions and thus describes a detonation as an infinitely thin shock wave followed by a zone of exothermic chemical reaction. In the reference frame in which the shock is stationary, the flow following the shock is subsonic. Because of this, energy release behind the shock is able to be transported acoustically to the shock for its support. For a self-propagating detonation, the shock relaxes to a speed given by the Chapman-Jouguet condition, which induces the material at the end of the reaction zone to have a locally sonic speed in the reference frame in which the shock is stationary. In effect, all of the chemical energy is harnessed to propagate the shock wave forward.

Both CJ and ZND theories are one-dimensional and steady. However, in the 1960s experiments revealed that gas-phase detonations were most often characterized by unsteady, three-dimensional structures, which can only in an averaged sense be predicted by one-dimensional steady theories. Modern computations are presently making progress in predicting these complex flow fields. Many features can be qualitatively predicted, but the multi-scale nature of the problem makes detailed quantitative predictions very difficult.

Detonations can be produced by high explosives, reactive gaseous mixtures, certain dusts and aerosols.

(i)Applications

Detonations are hard to control and are used primarily for demolition and in warfare. A great deal of research is conducted on achieving or preventing detonation in various materials to improve the performance of explosives and engines. An experimental form of jet propulsion, the pulse detonation engine, uses a series of well-timed detonations to generate thrust.

Detonation in reciprocating engines is the uncontrolled supersonic explosion of the fuel-air charge, and is caused by excessively high combustion chamber temperatures. Increasing the temperature of the fuel-air charge increases the speed of combustion until the flame propagates at supersonic speeds, resulting in a pressure shockwave. This force is extremely destructive to common piston engines, and often results in holes blown through the top of pistons or cracks in cylinder heads.

2.BASIC PDE CYCLE

Fig.2 Basic PDE cycle[Ref no: 10]

1- A detonation is initiated in a detonation tube filled with reactants.

2- The detonation propagates through the detonation tube and exits at the openend.

3-The combustion products exhaust through a blowdown process.

4-At the end of the exhaust process, the tube contains expanded combustionproducts.

5-The valve opens and reactants flow into the tube, pushing the combustion products out of the tube.

6-When the tube is filled with reactants, the valve closes and the cycle repeats.

At first the air is allowed inside the combustion chamber. Fuel, controlled by a solenoid valve in the head end of the tube, is allowed to enter through a mixing element for a specified amount of time to fill a pre-defined percentage of the combustion chamber volume based on the velocity of air through the system. The fuel valve is then closed and the mixture is initiated using a spark located just downstream of the mixing element. The initial flame kernel grows as it begins to propagate down the tube. The flame speed increases as it propagates down the tube and encounters turbulence, eventually transitioning to a supersonic detonation wave. The steady-state detonation wave travels in excess of 1960 m/s (for hydrogen-air mixtures) or 1800 m/s (for ethylene-air mixtures) burning the remaining reactants and pushing the gases out of the open end of the tube, resulting in a thrust. The tube is then purged of the combustion products by the continuously flowing air and refilled with fuel to repeat the cycle

3. DETONATION INITIATION IN PDEs

Basically there are two methods in which detonation is initiated in a pulse detonation engine:

1-Direct initiation.

2-Deflagration to detonation transition.

Fig.3 PDE Engine Schematic[Ref no: 2]

3.1 DIRECT INITIATION

A detonation may form via direct initiation or deflagration-to-detonation transition (DDT). Theformer mode is dependent upon an ignition source driving a blast wave of sufficient strength such that the igniter is directly responsible for initiating the detonation. The latter case begins with a deflagration initiated by some relatively weak energy source which accelerates through interactions with its surroundings into a coupled shock wave-reaction zone structure characteristic of a detonation. Direct initiation by a concentrated source requires an extremely large energy deposition relative to deflagrative ignition. A deflagration can be ignited in a typical hydrocarbon mixture such as 1 bar stoichiometric propane-air with a 1 mJ spark, whereas direct initiation of a detonation in the same mixture requires an energy deposition of over 100 kJ. This six order of magnitude difference in ignition source energy is indicative of the general difficulty associated with employing direct initiation techniques in pulse detonation engines. On the other hand, after a small spark has created a deflagration, the transition process can take several meters orlonger and a corresponding large amount of time. The key to detonation initiation schemes applicable to pulse detonation engines is to significantly shorten the distance and time required for deflagration-to-detonation transition.