1. INTRODUCTION 1.1 DESCRIPTION OF PULSE COMBUSTION Pulsating combustion is a combustion process that occurs under oscillatory conditions. That means that the state variables, such as pressure, temperature, velocity of combustion gases, etc., that describe the condition in the combustion zone, vary periodically with time. Pulse combustion is a very old technology. The phenomenon of combustion-driven oscillations was first observed in the year 1777, subsequently explained by Lord Rayleigh in the year 1878, and used in a variety of applications around the turn of the Century. Fig 1.1: General Pulse Combustion Process 1
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1. INTRODUCTION
1.1 DESCRIPTION OF PULSE COMBUSTION
Pulsating combustion is a combustion process that occurs under oscillatory
conditions. That means that the state variables, such as pressure, temperature, velocity of
combustion gases, etc., that describe the condition in the combustion zone, vary
periodically with time. Pulse combustion is a very old technology. The phenomenon of
combustion-driven oscillations was first observed in the year 1777, subsequently
explained by Lord Rayleigh in the year 1878, and used in a variety of applications around
the turn of the Century.
Fig 1.1: General Pulse Combustion Process
One of the better known examples of a pulse combustor is the German V-1
"Buzz Bomb " of World War II; Although the technology of pulse combustion has
been known for many years, devices using pulse combustion have not been implemented
widely despite their many attractive characteristics.
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Fig 1.2: Combustion chamber explosion
Compared to conventional combustion systems, their heat transfer rates are a
factor of two to five higher than normal turbulent values, their combustion intensities are
up to on order of magnitude higher, their emissions of oxides of nitrogen are a factor of
three lower, their thermal efficiencies are up to 40% higher, and they may be self-
aspirating, obviating the need for a blower. This combination of attributes can result in
favorable economic trade off with conventional combustors in many applications. Most
of the research on pulse combustors has been directed toward applied examinations of the
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engineering aspects of pulse combustors: heat transfer, efficiency, frequency of
operation, pollutant formation, etc.
There is also uncertainty over the behavior of frequency as a function of
geometry, energy input, and mass input. Zinn states that the pulse combustor can be
modeled as a Helmholtz resonator, while Dec and Keller found that the frequency of
operation is a function of the magnitude of the energy input and of the magnitude of the
mass flux. These results indicate that a Helmholtz resonator model is insufficient to
predict the frequency of operation. These fundamental questions must be answered before
the prediction of an optimum resonant condition is possible.
2. LITERATURE SURVEY
2.1 PULSE JET ENGINE
A pulse jet engine is a type of jet engine in which combustion occurs in pulses.
Pulsejet engines can be made with few or no moving parts, and are capable of running
statically. Pulse jet engines are a lightweight form of jet propulsion, but usually have a
poor compression ratio, and hence give a low specific impulse.
Pulsejet is an unsteady propulsive device with its basic components being the
inlet, combustion chamber, valve and valve head assembly and a tailpipe.
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Fig 2.1: Schematic of pulse combustion operation
2.2 TYPES OF PULSE JET ENGINES
There are two types of pulse jet engines: those with valves and those without. The
ones with valves allow air to come in through the intake valve and exit through the
exhaust valve after combustion takes place. Pulse jet engines without valves, however,
use their own design as a valve system and often allow exhaust gases to exit from both
the intake and exhaust pipes, although the engine is usually designed so that most of the
exhaust gases exit through the exhaust pipe.
A. Valved Pulsejet Engine
Valved 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 of a
intake with a one-way valve arrangement. The valves prevent the explosive gas of the
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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.
Fig 2.2: Valved Pulsejet Engine
The hot exhaust gases exit through an acoustically resonant exhaust pipe. The
valve arrangement is commonly a "daisy valve" also known as a reed valve. The daisy
valve is less effective than a rectangular valve grid, although it is easier to construct on a
small scale.
B. Valveless Pulsejet Engine
The valveless pulse jet engine operates on the same principle, but the 'valve' is the
engine's geometry. Fuel as a gas or liquid vapor 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 method 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
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compressed air. Once running, the engine only requires input of fuel to maintain a self-
sustaining combustion cycle.
Valveless pulsejets, have no moving parts and use only their geometry to control
the flow of exhaust out of the engine. Valveless engines expel exhaust gases out of both
the intake and the exhaust, most try to have the majority of exhaust go out the longer tail
pipe, for more efficient propulsion.
Fig 2.3: Valveless pulsejet engine
Fuel is drawn into the combustion chamber through the intake valve in either as
an air-gas mixture or in liquid form. The intake valve then closes and a spark plug is used
to ignite the fuel in the combustion chamber. The fuel then expands rapidly and tries to
fill the entire chamber in order to escape. The closed intake valve forces the fuel to the
rear of the combustion chamber and allows the exhaust gases to exit through the exhaust
valve.
2.3 HISTORY OF VALVELESS PULSE JET ENGINES
The idea of pulsed combustion was conceived even before the use of steady state
combustion employed in gas turbine engines. Over the past hundred years various
number of valveless pulsejet designs have been invented and tested. These are classified
into three main systems
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Inline systems
U-shaped systems
Linear systems
2.3.1: Inline systems
The systems, which have an intake pipe, combustion chamber and exhaust pipe,
all on the same axis with intake and exhaust held in opposite directions are called inline
systems. The advantage of this system is that when the engine has positive forward air
velocity the intake has air rushing into it creating a ram-air effect, similar to ram jet
engines.
Moreover the fabrication and fitting of inline systems is much easier than any
other systems. The disadvantage is that these engines have lower thrust than other
systems because the hot air exiting the intake after combustion does not to contribute to
net thrust and actually creates negative trust that has to be overcome.
To overcome this many complicated and mostly infeasible aerodynamic valves
have been created to allow the ram air effect to work without allowing the air to move
back through so as to increase thrust. However none have been proven effective.
Marconnet Design
In 1909 Georges Marconnet developed the first pulsating combustor without
valves. It was the father of all valveless pulse jets. Marconnet found that a blast inside a
chamber would prefer to go through a bigger exhaust opening rather than squeezing
through a relatively narrow intake. In addition a long diffuser between the intake and the
combustion chamber would direct the charge strongly towards exhaust, the way a trumpet
directs sound.
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Fig 2.4 Marconnet’s Valveless Pulsejet Engine
Schubert design
The principle of the valveless pulsating combustor was discovered by Lt.William
Schubert of the US NAVY in the early 1940s. Schubert’s design was called a “Resojet”
on the account on its dependence on resonance.
The taper less attachments of the inlet tube to the combustion chamber in
Schubert’s design creates strong turbulence for better mixing of fuel and air so that high
intensity combustion takes place. Schubert carefully calculated the geometry of the intake
so that the exhaust gas could not exit by the time the pressure inside fell below
atmospheric.
The resistance of a tube to the passage of gas depends steeply on the gas
temperature. Thus, the same tube will offer a much greater resistance to outgoing hot gas
than to the incoming cold air. The impedance is inversely proportional to the square root
of the gas temperature. This degree of irreversibility seems to offer the possibility for the
cool air necessary for combustion to get in during the intake part of the cycle, but for the
hot gas to encounter too much resistance to get out of the intake during the expansion
part.
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Fig 2.5: Schubert’s valveless pulsejet engine
2.3.2: U-shaped systems
The U-shape design overcomes the shortfall of the inline design by bending the
exhaust pipe by 180 degrees, so that the exhaust and intake are aligned in the same
direction. The advantage of this design is that the thrust generated by the inlet contributes
to the net thrust of the engine as it flows in the same direction as the exhaust. The
disadvantage is that the ram-air affect is lost. Moreover fabrication is quite complex.
Lockwood-Hiller
The U-shaped Lockwood-Hiller engine was invented by Raymond Lockwood. It
is said that the Lockwood was the most effective pulse jet engine ever developed.
The air fuel mixture is generated by mixing fuel which is injected through a jet
built into the side of the combustion chamber or on a strut projecting into the chamber or
on two crossed struts spanning the front part of the chamber. The chamber is the drum
like broad part of the engine. The short straight tube attached to the combustion chamber
is the inlet. And the long U tube attached to the combustion chamber is the tail pipe. The
tailpipe is fitted with a flare at the end.
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Fig 2.6: U-shaped Lockwood Hiller engine
The Lockwood-Hiller design is the most successful example of U-shaped designs
in both performance and efficiency. Conversely it is difficult to construct because of
numerous cone sections are to be fabricated for it.
2.3.3: Linear systems
There are many designs of valveless pulsejet engines that cannot be categorized
by either U-shape or inline designs. These engines are generally variations of inline
designs with the intake moved to the side of the combustion chamber. The typical feature
of the linear engine is that the intake emanates from the side of the combustion chamber.
The advantage of this type of engine is that the physical size is smaller than an equivalent
U-shaped engine making integration into airframe more practical.
These engines are also simpler to manufacture than U-shape design. The
disadvantage of this design is the tuning difficulty for optimized performance as the
intake length is directly proportional to exhaust length. Net thrust outputs are
considerably greater than inline while performance is less than the equivalent U-shape
design as the efficiency is limited by intake position.
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Argus design
The capped tube design was first invented by the Argus Company (manufacturer
of German V-1 bombs). It consisted of combustion chamber (plenum chamber), which
formed a bottle shape design capped over with a hemispherical top. Fuel was injected
through a nozzle located on the tip of the cap and protected from the chamber with metal
grid. The grid functioned as a heat sink and prevented gas from burning at the nozzle.
Pressurized air was forced into the plenum chamber continuously using a
compressor, the combustion took place and the hot gases expanded. The continuous
supply of the compressed air into the plenum chamber prevented hot gas from getting out
of the plenum chamber and almost all of it were thrust into the exhaust. The engine did
not self-sustain or resonate due to the reasons of smaller plenum chamber and exhaust
length.
Fig 2.7: Capped tube-Argus
2.4 WHY VALVELESS PULSEJETS
A valveless pulse jet engine is a simple and ordinary engine. It is just a piece of
metal tube cut to the required dimensions. In a valveless pulsejet engine there are no
mechanical valves but they do have aerodynamic valves which for the most part resist the
flow in a single direction. They have no mechanically moving parts and so they are more
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reliable. All valveless engines have low thrust output, high fuel consumption and overall
poor performance.
Fig 2.8: A 4-Pound Valveless Pulse Jet
Pulsejets can be used on a large scale as industrial drying systems, and there has
been a new surge to study and apply these engines to applications such as high output
heating, biomass conversion, and alternative energy systems, as pulsejets can run on
almost anything that burns including particulate fuels such as sawdust or coal powder.
2.5 APPLICATION OF PULSEJET ENGINE
Pulse jet engines have been used in many functional jets; they can also be used for
a variety of other applications such as:
Ground Applications: 1) Water heating,
2) Biomass fuel conversion,
3) Heat generators and
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4) Orchard fields
Flight Applications: 1) In radio controlled aircraft and
2) Target drone aircraft and control line.
Merits
Pulse jet engines are easy to build on a small scale and can be constructed using
few or no moving parts. This means that the total cost of each pulse jet engine is much
cheaper than traditional turbine engines. Pulse jet engines do not produce torque like
turbine engines do, and have a higher thrust-to-weight ratio.
De-Merits
While pulse jet engines can be beneficial to many industries, they do have several
disadvantages. For example, pulse jet engines are very loud which only makes them
practical for military and industrial purposes. Also, pulse jet engines do not have very
good thrust specific fuel consumption levels. Likewise, pulse jet engines use acoustic
resonance rather than external compression devices to compress fuels before combustion.
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3. PRINCIPLE OF OPERATION OF PULSE JET ENGINES
3.1 RIJKE TUBE
Rijke's tube turns heat into sound, by creating a self-amplifying standing wave. It
is an entertaining phenomenon in acoustics and is an excellent example of resonance.
Fig 3.1: Rijke Tube
The Rijke tube is simply a cylindrical tube with both ends open and a heat source
placed inside it. The heat source may be a flame or an electrical heating element. It has a
wire gauze inside about one quarter the way from the bottom. Traditionally, the tube is
positioned vertically on a stand or even held in a hand and the heat source is introduced
from below into the tube. For certain ranges of position of the heat source within the tube,
the Rijke tube emits a loud sound. This phenomenon was discovered by Rijke around
1850, and is therefore called the Rijke phenomenon. Sound production in the Rijke tube
is a classic example of a thermo-acoustic phenomenon.
In the case of the Rijke tube air can move in and out of both ends. A heated metal
mesh placed a quarter of the way up from the bottom heats the air flowing past it. This
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flow of air is a combination of the convection current caused by the transfer of heat from
the metal mesh and the sound wave that is set up for the condition of two open ends.
For half of the oscillation cycle of the sound wave air moves in from both ends as
it flows towards the center generating a pressure antinode (displacement node) there.
Even though some of the air moving past the hot metal mesh has already been heated
during the cycle prior to this, some additional cool air flows in, passing through it and
acquiring thermal energy and further increasing the pressure, thus reinforcing the
oscillation. For the remaining half cycle air passing by the metal mesh while flowing
outward from the center of the tube is already heated and therefore energy transfer is
minimal.
The sound comes from a standing wave whose wavelength is about twice the
length of the tube, giving the fundamental frequency. Lord Rayleigh, in his book, gave
the correct explanation of how the sound is stimulated. The flow of air past the gauze is a
combination of two motions. There is a uniform upwards motion of the air due to
a convection current resulting from the gauze heating up the air. Superimposed on this is
the motion due to the sound wave. For half the vibration cycle, the air flows into the tube
from both ends until the pressure reaches a maximum. During the other half cycle, the
flow of air is outwards until the minimum pressure is reached. All air flowing past the
gauze is heated to the temperature of the gauze and any transfer of heat to the air will
increase its pressure according to the gas law.
As the air flows upwards past the gauze most of it will already be hot because it
has just come downwards past the gauze during the previous half cycle. However, just
before the pressure maximum, a small quantity of cool air comes into contact with the
gauze and its pressure is suddenly increased. This increases the pressure maximum, so
reinforcing the vibration. During the other half cycle, when the pressure is decreasing, the
air above the gauze is forced downwards past the gauze again. Since it is already hot, no
pressure change due to the gauze takes place, since there is no transfer of heat. The sound
wave is therefore reinforced once every vibration cycle and it quickly builds up to very
large amplitude. This explains why there is no sound when the flame is heating the gauze.
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All air flowing through the tube is heated by the flame, so when it reaches the gauze, it is
already hot and no pressure increase takes place.
Fig 3.2: Working of a Rijke Tube
When the gauze is in the upper half of the tube, there is no sound. In this case, the
cool air brought in from the bottom by the convection current reaches the gauze towards
the end of the outward vibration movement. This is immediately before the pressure
minimum, so a sudden increase in pressure due to the heat transfer tends to cancel out the
sound wave instead of reinforcing it.
The position of the gauze in the tube is not critical as long as it is in the lower
half. To work out its best position, there are two things to consider. Most heat will be
transferred to the air where the displacement of the wave is a maximum, i.e. at the end of
the tube. However, the effect of increasing the pressure is greatest where there is the
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greatest pressure variation, i.e. in the middle of the tube. Placing the gauze midway
between these two positions (one quarter of the way in from the bottom end) is a simple
way to come close to the optimal placement.
The Rijke tube is considered to be a standing wave form of thermo
acoustic devices known as "heat engines" or "prime movers".
3.2 THE HELMHOLTZ RESONATOR
Helmholtz resonance is the phenomenon of air resonance in a cavity, such as
when one blows across the top of an empty bottle. The name comes from a device created
in the 1850s by Hermann von Helmholtz. The "Helmholtz resonator", which he, the
author of the classic study of acoustic science, is used to identify the
various frequencies or musical pitches present in music and other complex sounds. The
Helmholtz resonator can best be demonstrated by taking a normal soft drink bottle and
blowing over the mouth of the bottle. When air is forced into a cavity, the pressure inside
it increases. When the external force pushing the air into the cavity is removed, the
higher-pressure air inside will flow out. The cavity will be left at a pressure slightly lower
than the outside, causing air to be drawn back in. This process repeats with the magnitude
of the pressure changes decreasing each time.
The air in the port (the neck of the chamber) has mass. Since it is in motion, it
possesses some momentum. A longer port would make for a larger mass, and vice-versa.
The diameter of the port is related to the mass of air and the volume of the chamber. A
port that is too small in area for the chamber volume will "choke" the flow while one that
is too large in area for the chamber volume tends to reduce the momentum of the air in