Pulse Jet Report
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Transcript of Pulse Jet Report
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.
3
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.
15
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
16
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
the port.
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Fig 3.3: Helmholtz Resonator
An important type of resonator with very different acoustic characteristics is the
Helmholtz resonator. Essentially a hollow sphere with a short, small-diameter neck, a
Helmholtz resonator has a single isolated resonant frequency and no other resonances
below about 10 times that frequency.
The resonant frequency (f) of a classical Helmholtz resonator, shown in Figure, is
determined by its volume (V) and by the length (L) and area (A) of its neck:
Here, f = ( S2π )√ A
LV
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Figure 3.4: A Classic Helmholtz Resonator
where S is the speed of sound in air. As with the tubes discussed above, the value of the
length of the neck should be given as the effective length, which depends on its radius.
The isolated resonance of a Helmholtz resonator made it useful for the study of
musical tones in the mid-19th century, before electronic analyzers had been invented.
When a resonator is held near the source of a sound, the air in it will begin to resonate if
the tone being analyzed has a spectral component at the frequency of the resonator. By
listening carefully to the tone of a musical instrument with such a resonator, it is possible
to identify the spectral components of a complex sound wave such as those generated by
musical instruments.
Helmholtz Resonator Analogy in Pulse Jet Engines
The simplest analytical model of the valveless pulsejet is that of a Helmholtz
resonator in a combination with a quarter wave oscillator. While their analogy is one of
the simplest forms, it allows for a wealth of understanding of the fundamental operation
of a valveless pulsejet. The model assumes that the combustion chamber and inlet can be
19
modeled as a Helmholtz resonator and the exhaust as a matched, or tuned, quarter wave
oscillator (the familiar pipe organ)
It is a classic element in the study of acoustics. The pressure of the gas within the
cavity of the resonator changes as it is alternately compressed and expanded by the influx
and efflux of the gas through the opening and thus provide the stiffness element. At the
opening, there is a radiation of sound into the surrounding medium, which leads to the
dissipation of acoustic energy and thus provides a resistance element.
Fig 3.5: Valveless Pulse Jet during operation
3.3 OPERATION OF A VALVELESS PULSE JET ENGINE
The operation of valveless pulsejet requires a fundamental knowledge about
mixing ignition, combustion and wave initiation, wave propagation and wave reflection.
Any disturbance in the fluid medium creates a wave pattern. If the propagation of the
wave is parallel to the motion of the fluid, then it is termed as longitudinal waves e.g.
sound waves. This is the mode of wave propagation that occurs in a valveless pulsejet.
20
When the deflagration begins, a zone of significantly elevated pressure travels outward
through both air masses as a "compression wave". This wave moves at the speed of sound
through both the intake and tailpipe air masses.
(Because these air masses are significantly elevated in temperature as a result of
earlier cycles, the speed of sound in them is much higher than it would be in normal
outdoor air.) When a compression wave reaches the open end of either tube, a low
pressure rarefaction wave starts back in the opposite direction, as if "reflected" by the
open end. This low pressure region returning to the combustion zone is, in fact, the
internal mechanism of the Kadenacy effect. There will be no "breathing" of fresh air into
the combustion zone until the arrival of the rarefaction wave.
Mixing of air and fuel in a Valveless Pulsejet
In the combustion chamber fuel is injected into the flow of fresh air entering the
engine. At the beginning of the charging cycle the mixture is very rich, then it gets leaned
and at the end of the cycle it gets richer again but this mixing of fuel and air in a flow
stream are affected by the parameters of molecular size, concentration, temperature, flow
velocity in the vicinity of the injector and evaporation rate, vary within wide bounds, the
mixture is very non-homogeneous. The combustion chamber consists of two distinct
layers: a highly enriched layer with fuel and combustion products from the previous cycle
and a cold layer arising at the end of the suction cycle. This mixture in-homogeneously
causes a noticeable drop in its combustible properties. The proper engine operation could
be achieved with a mixture composition of air/fuel ratio 1.1 - 1.4.
Ignition in a Valveless Pulsejet
Initially the fuel-air ignition is done manually with the help of blower and a spark
plug. Since the pressure inside the combustion chamber is above atmospheric pressure,
the combustion products along with the air flow towards the exhaust and continue so long
as the pressure in the chamber falls below atmospheric pressure. Now the gases will
retrace its path back into the combustion chamber since the atmospheric pressure is
greater than the combustion pressure. Because of the momentum or the turbulence of the
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hot gas rushing back in, the pressure and temperature inside the combustion chamber will
increase drastically. Once the chamber temperature is above the ignition temperature of
the fuel the next ignition takes place and this cycle continues.
Combustion process in a Valveless Pulsejet
The combustion process likely exists in two phases: an initial ignition which
gradually takes over the entire combustion chamber and this increases pressure and
temperature in the chamber and thereby facilitating the evaporation of the remaining
unburned mixture, and a main combustion process occurring almost instantaneously in
the entire chamber and lasting about 25% of the entire cycle. The combustion chamber
can reach up to a maximum approximate temperature of 2000K.
Since the pressure difference between the combustion chamber and exhaust is
oscillating, there will only be intermittent flow of air to the chamber to support
combustion. A pulse jet engine is an ideal example for an unsteady combustion process.
Here the combustion process is pulsating. The potential coupling between the unsteady
components of pressure and heat release can lead to sustained, large amplitude acoustic
oscillations which being driven by heat release is referred to as a thermo-acoustic
instability. Rayleigh was the first to hypothesize the onset of the instability and define a
criterion for positive coupling.
According to Rayleigh criteria “if heat be periodically communicated to and
abstracted from a mass of air vibrating in a cylinder, the effect produced will depend on
the phase of vibration at which heat transfer takes place. If the heat be given to air at
moment of greatest compression or taken at the moment of greatest rarefaction the
vibration is encouraged. On the other hand heat is given moment of greatest rarefaction
or abstracted at the moment of greatest condensation, the vibration is discouraged.”
Expansion of gases
Due to pressure being setup only at a certain region of engine, the gases at high
pressure migrate to low pressure regions in the engine and eventually out of the engine
22
(atmosphere). This happens at a very high velocity since the potential difference in static
pressure between atmosphere and the combustion chamber is very high.
This phenomenon occurs at the cost of losing the achieved high static pressure in
combustion chamber, a very high migration velocity implies a very high volume flow rate
of the engine, hence a very quick and drastic drop in static pressure
Suction of gases
Owing to the exit of the exhaust gases at very high velocities, the static pressure
in the combustion chamber drops drastically, the drop is to such an extent that a negative
gauge pressure (partial vacuum) is setup in the combustion chamber, which forces to
cease any further exit to the combustion gases, instead the combusted products still
dwelling in the engine is sucked back into the combustion chamber along with the fresh
atmospheric air. This leads to the fresh mixing of air and fuel inside the combustion
chamber for subsequent combustions. The cyclic process is shown in the fig (3.3-1)
Fig 3.6: Working of Valveless Pulsejet Engine
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3.4 WORKING OF A VALVELESS PULSE JET ENGINE
The figure below shows a layout of a valveless pulsejet engine. It has a chamber
with two tubular ports of unequal length and diameter. The port on the right,
curved backwards, is the intake pipe. The bigger, flared one on the left is the exhaust, or
tailpipe. In some other engines, it is the exhaust pipe that is bent into the U-shape, but
the important thing is that the ends of both ports point in the same direction. When the
fuel-air mixture combusts in the chamber, the process generates a great amount of hot gas
very quickly. This happens so fast that it resembles, an explosion.
Fig 3.7: Layout of a Valveless Pulse Jet Engine
The immediate, explosive rise in internal pressure first compresses the gas inside
and then pushes it forcefully out of the chamber, two powerful spurts of hot expanding
gas are created – a big one that blows through the tailpipe and a smaller one blowing
through the intake. Leaving the engine, the two jets exert a pulse of thrust – they push
the engine in the opposite direction. As the gas expands and the combustion chamber
empties, the pressure inside the engine drops. Due to inertia of the moving gas, this drop
continues for some time even after the pressure falls back to atmospheric. The expansion
stops only when the momentum of the gas pulse is completely spent. At that point, there
is a partial vacuum inside the engine. The process now reverses itself. The outside
(atmospheric) pressure is now higher than the pressure inside the engine and fresh air
24
starts rushing into the ends of the two ports. At the intake side, it quickly passes through
the short tube, enters the chamber and mixes with the fuel. The tailpipe, however, is
rather longer, so that the incoming air does not even get as far as the chamber before the
engine is refilled and the pressure peaks.
One of the prime reasons for the extra length of the tailpipe is to retain enough of
the hot exhaust gas within the engine at the moment the suction starts. This gas is greatly
rarified by the expansion, but the outside pressure will push it back and increase its
density again. Back in the chamber, the gases of the previous combustion mix vigorously
with the fresh fuel/air mixture that enters from the other side. The heat of the chamber
and the free radicals in the retained gas will cause ignition and the process repeats.
The spark plug shown on the picture is needed only at start-up. The retained hot
gas provides self-ignition and the spark plug becomes unnecessary. Indeed, if spark
ignition is left on, it can interfere with the normal functioning of the engine. In the J-
shaped and U-shaped valveless engines, gas spews out of two ports. Some valveless
pulsejet designers have developed engines that are not bent backwards, but
employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air to
come in but prevent the hot gas from getting out through the intake. A gentler, more
gradual entry would not generate the necessary swirling of gases. In addition,
turbulence increases the intensity of combustion and the rate of the heat release.
Fig 3.8: A U-shaped Pulse Jet
25
3.5 THERMODYNAMIC CYCLE FOR A PULSEJET ENGINE
The thermodynamic working principle of a pulsejet engine does not have an exact
explanation; hence a popular and commonly accepted thermodynamic model is a Lenoir
cycle.
Lenoir Cycle
The Lenoir Cycle is an idealized thermodynamic cycle, where the ideal gas undergoes
basically 3 processes to produce work. The most interesting part of this cycle is that the
output work is obtained with no energy spent on compressing the working fluid. The
cyclic process are as follows,
(1) Constant volume (isochoric) heat addition and then
(2) Adiabatic expansion and
(3) Constant pressure (isobaric) heat rejection.
As Pulsejets typically have a very small compression ratio that reaches a maximum at
around (1.7). The Lenoir three cycle process can be seen below in:
Fig 3.9: Lenoir Cycle
26
As the expansion process is isentropic and hence involves no heat interaction.
Energy is absorbed as heat during the constant volume process and rejected as heat
during the constant pressure process. Hence the (P-V) diagram from fig (3.5-1) represents
the thermodynamic process of the Lenoir cycle.
Due to the finite time of combustion and incomplete filling of the chamber with
the fresh charge, the pressure at the end of the heat supply process depends on both the
fuel-air composition and on the relative volume of the fresh mixture entering through the
inlet valve. In this case the heat supply process is not isochoric. This deviation from the
ideal process demands for implementation of modifications to the existing ideal process.
3.6 DESIGNING OF A VALVELESS PULSE JET ENGINE
Valveless Pulse jets are much simpler in design than the valved engines, but with
simplicity you have to sacrifice kgs of thrust and loose the ram air effect. The following
section breaks a valveless pulsejet engine into major components and investigates design
approaches used in other designs for each component. The most important components
are the combustion chamber, the exhaust and intake pipes, the fuel injection system, the
spark ignition system and the air assist starting system. For each of the components,
various solutions are considered to guide in designing a suitable pulsejet engine.
Combustion Chamber
The combustion chamber is arguably the most important component of a
valveless pulsejet design. For a valveless pulsejet engine, the combustion chamber
geometry is critical as any flow inconsistency can disrupt the pulsating combustion cycle,
as pressure waves may be reflected at sudden area changes. The most suitable solution
depends heavily on the selected configuration but there are several design parameters that
apply to all cases. The most significant attribute of a combustion chamber is the circular
cross section. This is because the pressure inside the combustion chamber, positive or
27
negative depending on the cycle, causes stress within the wall. This stress is more evenly
distributed by a circular cross-section design.
Fig 3.10: Comparison of conical sections
Combustion chambers also have conical sections leading into the intake and
exhaust pipes. These sections maintain smooth gas flow throughout the engine. The
above figure depicts the gas flow after combustion in both a conical section and a stepped
transition. The example on the left has a higher pressure increase because the post
ignition confinement is improved, but produces lower thrust because the gas suffers
choking due to entrance effects upon entering the exhaust, limiting the exiting velocity.
Conversely, a tapered cone that is too shallow has poor levels of post ignition
confinement, meaning thrust is also low. A good compromise is required in order to have
a practical engine.
Fig 3.11: Lockwood Hiller Combustion Chamber
28
Fig 3.12:
Logan
Combustion Chamber Section
The Logan combustion chamber section shows the implementation of the cone
sections on two different design solutions. Notably, the Lockwood-Hiller design has
steep cones while the Logan design features shallower tapers. This is because the
Lockwood- Hiller design has much larger intake and exhaust openings that allow the
flow to move relatively smoothly so post ignition confinement is the most critical
component of that design. Conversely, the Logan design has smaller openings and
requires unimpeded air flow exiting and entering the engine thus the conical section is
much shallower. From Simpson (2005), the optimum cone angle for an inline or linear
valveless configuration is approximately 30 degrees, depending on the size of the engine.
The cone section is a critical compromise between the flow of the gases and post ignition
confinement and as such, is a relatively critical consideration for our design.
Exhaust and Intake
The exhaust and intake pipes of a valveless pulsejet engine are generally straight,
circular cross-section tubes with a critical length. The length is critical as it must promote
the acoustic resonance necessary to sustain engine operation. The diameter of the pipe is
also an important consideration as it needs to allow sufficient flow to produce the
required thrust; however, some degree of pressure must be retained to aid in combustion
chamber pressure increase.
29
Fig 3.13: Standard Exhaust Runner Design
The fig shows an arbitrary exhaust pipe section. The length to diameter ratio is
not as critical as the length is the critical dimension. Generally, however, the length to
diameter ratio is 7 to 10 percent of the length (Simpson 2005) to give sufficient volume
for gas flow. This is similar for intake pipes to allow a sufficient fresh air charge into the
combustion chamber. Standard exhaust runner design also depicts the diffuser on the end
of the pipe. This is the same for both intake and exhaust and is necessary to control the
flow of gas exiting and entering the engine.
Fig 3.14: Sudden Expansion Exit Conditions
For the exit condition, this fig shows that when a sharp expansion occurs, the flow
creates turbulent eddies as it separates from these edges. This separation causes the flow
to lose energy, thus reducing the overall thrust developed by the engine. By making this
transition conical or bell-shaped these effects are negated keeping the flow smooth and
30
directing more of the energy of the flow into generating thrust from the engine.
Conversely, for the intake condition, the fig shows that the flow separates from the
surface at the sharp corner creating a vena contractor that effectively limits the cross-
sectional area through which the air can flow.
This limits the effectiveness of the intake to draw in the fresh air charge and the
exhaust to ingest the cool dense air required to confine the combustion event. Conical or
bell-shaped diffusers limit flow separation allowing smooth transition of the air into the
engine.
Fig 3.15: Entrance Flow Conditions
Fuel Injection System
Injector position and design is an important parameter in the pulsejet design. Poor
fuel delivery and injection can limit the effectiveness of combustion and in some cases,
can render the engine inoperable. Propane gas is the fuel of choice for a large majority of
pulsejets as its gaseous form does not require a nozzle for fuel atomization. Liquid fuel
systems are generally not used due to the complexity and difficulty of atomizing the fuel,
although, in the past they have been used with varying degrees of success.
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4. OBJECT AND APPROACH
Although not all waves have a speed that is independent of the shape of the wave,
and this property therefore is an evidence that sound is a wave phenomenon, sound does
nevertheless have this property .For instance, the music in a large concert hall or stadium
may take on the order of a second to reach someone seated in the nosebleed section, but
we do not notice or care, because the delay is the same for every sound. Bass, drums, and
vocals all head outward from the stage at 340 m/s, regardless of their differing wave
shapes. The speed of sound in a gas is related to the gas's physical properties. It is a series
of compressions and expansions of the air.
Fig 4.1: Propagation of Sound during the Operation of Pulse Jet
Even for a very loud sound, the increase or decrease compared to normal
atmospheric pressure is no more than a part per million, so our ears are apparently very
sensitive instruments. In a vacuum, there is no medium for the sound waves, and so they
cannot exist. The roars and whooshes of space ships in Hollywood movies are fun, but
scientifically wrong.
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Kadenacy Effect
Fig 4.2: Kadenacy Effect
In the explanation of the working cycle, inertia keeps driving the expanding gas
out of the engine all the way until the pressure in the chamber falls below atmospheric.
The opposite thing happens in the next part of the cycle, when the outside air pushes its
way in to fill the vacuum. The combined momentum of the gases rushing in through the
two opposed ports causes the chamber briefly to be pressurized above atmospheric before
ignition. There is thus an oscillation of pressure in the engine caused by inertia.
The gases involved in the process (air and gaseous products of combustion) are
stretched and compressed between the inside and outside pressures. In effect, those
fluids behave like an elastic medium, like a piece of rubber. This is called the “Kadenacy
Effect”.
The elastic character of gas is used to store some of the energy created in one
combustion cycle and use it in the next.
The energy stored in the pressure differential (partial vacuum) makes the
aspiration (replacement of the burned gas with fresh fuel-air mixture) possible. Without
it, pulsejets would not work.
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4.1 PROPAGATION OF SOUND IN PULSE JETS
The phenomenon of sound is easily found to have all the characteristics we expect
from a wave phenomenon Sound waves obey superposition. Sounds do not knock other
sounds out of the way when they collide, and we can hear more than one sound at once if
they both reach our ear simultaneously. The medium does not move with the sound. Even
standing in front of a titanic speaker playing earsplitting music, we do not feel the
slightest breeze. The velocity of sound depends on the medium. Sound travels faster in
helium than in air, and faster in water than in helium. Putting more energy into the wave
makes it more intense, not faster.
Acoustic Theory
The pressure wave travels up and down the tube. When the wave front reaches an
end of the tube, part of it reflects back. Reflections from opposed ends meet and form the
so-called ‘standing wave’.
Fig 4.3: Standing Wave
34
A standing wave in a transmission line is a wave in which the distribution of
current, voltage, or field strength is formed by the superposition of two waves of the
same frequency propagating in opposite directions. The effect is a series of nodes (zero
displacement) and anti-nodes (maximum displacement) at fixed points along the
transmission line. Such a standing wave may be formed when a wave is transmitted into
one end of a transmission line and is reflected from the other end by an impedance
mismatch, i.e., discontinuity, such as an open circuit or a short. The failure of the line to
transfer power at the standing wave frequency will usually result in attenuation distortion.
In practice, losses in the transmission line and other components mean that a
perfect reflection and a pure standing wave are never achieved. The result is a partial
standing wave, which is a superposition of a standing wave and a traveling wave. The
degree to which the wave resembles either a pure standing wave or a pure traveling wave
is measured by the standing wave ratio.
Fig 4.4: Wave Formation at the Exhaust
Another example is standing waves in the open ocean formed by waves with the
same wave period moving in opposite directions. These may form near storm centers, or
35
from reflection of a swell at the shore, and are the source of microbaroms and
microseisms. Graphically, the standing wave is best represented by a double sine curve.
The same is true for the pulsejet cycle. The undulations of a single sine curve depict the
changes of gas pressure and gas speed inside a pulsejet engine very well. The doubling of
the curve – the addition of a mirror image, so to say – shows that the places where the
pressure and speed are the highest in one part of the cycle will be the places where they
are the lowest in the opposite part.
The changes of pressure and the changes of gas speed do not coincide. They follow
the same curve but are offset from each other. One trails (or leads) the other by a quarter
of the cycle. If the whole cycle is depicted as a circle – 360 degrees – the speed curve
will be offset from the pressure curve by 90 degrees. The resonance establishes a pattern
of gas pressures and speeds in the engine duct that is peculiar to the pulsejet and not
found in the other jet engines.
In some ways it resembles a 2-stroke piston engine resonant exhaust system more
than in does a conventional jet engine. Understanding this pattern is very important, for it
helps determine the way the events in the engine unfold. When considering a pulsejet
design, it is always good to remember that those machines are governed by a complex
interaction of fluid thermodynamics and acoustics.
Elements of Resonance
In acoustic terms, the combustion chamber is the place of the greatest impedance,
meaning that the movement of gas is the most restricted. However, the pressure swings
are the greatest. The chamber is thus a speed node but a pressure antinode. The outer
ends of the intake and exhaust ports are the places of the lowest impedance. They are the
places where the gas movement is at the maximum and the speed changes are the greatest
– in other words, they are speed antinodes. The pressure swings are minimal, so that the
port ends are pressure nodes. The pressure outside the engine is constant (atmospheric).
The pressure in the combustion chamber seesaws regularly above and below
atmospheric. The pressure changes make the gases accelerate through the ports in one
36
direction or another, depending on whether the pressure in the chamber is above or below
atmospheric. The distance between a node and an antinode is a quarter of the wavelength.
This is the smallest section of a standing wave that a resonating vessel can accommodate.
In a valveless pulsejet, this is the distance between the combustion chamber (pressure
antinode) and the end of the tailpipe (pressure node). This length will determine the
fundamental wavelength of the standing wave that will govern the engine operation. The
distance between the chamber and the end of the intake is rather shorter. It will
accommodate a quarter of a wave of a shorter wavelength. This secondary wavelength
must be an odd harmonic of the fundamental.
5. PROCEDURE ADOPTED
5.1 MATERIAL EMPLOYED
The material used in the manufacturing process is Mild Steel. Mild steel is a type of
steel that only contains a small amount of carbon and other elements. It is softer and more
easily shaped than higher carbon steels. It also bends a long way instead of breaking
because it is ductile. It is used in nails and some types of wire, it can be used to make
bottle openers, chairs, staplers, staples, railings and most common metal products. Its
name comes from the fact it only has less carbon than steel.
Some mild steel properties and uses:
Mild steel has a maximum limit of 0.2% carbon. The proportions of manganese
(1.65%), copper (0.6%) and silicon (0.6%) are approximately fixed, while the
proportions of cobalt, chromium, niobium, molybdenum, titanium, nickel,
tungsten, vanadium and zirconium are not.
37
A higher amount of carbon makes steels different from low carbon mild-type
steels. A greater amount of carbon makes steel stronger, harder and very slightly
stiffer than a low carbon steel. However, the strength and hardness comes at the
price of a decrease in the ductility of this alloy. Carbon atoms get trapped in the
interstitial sites of the iron lattice and make it stronger.
What is known as mildest grade of carbon steel or 'mild steel' is typically low
carbon steel with a comparatively low amount of carbon (0.16% to 0.2%). It has
ferromagnetic properties, which make it ideal for manufacture of many products.
The calculated average industry grade mild steel density is 7.85 gm/cm3. Its
Young's modulus, which is a measure of its stiffness is around 210,000 MPa.
Mild steel is the cheapest and most versatile form of steel and serves every
application which requires a bulk amount of steel.
The thickness of 2mm for both the pulsejets is maintained.
5.2 COMPONENTS OF THE PULSE JET
A Valve less Pulse Jet Engine has three major components,
1. Inlet
2. Combustion chamber
3. Exhaust tail pipe
Inlet
Inlet is the smallest component in a pulsejet engine designed for air intake. The
inlet is a 100mm long open cylinder with 38mm inner diameter and 42mm outer
diameter, maintaining 2mm thickness.
38
Photo 5.1: Air Inlet
Combustion Chamber
The combustion chamber is a combination of cylindrical section and tapered section.
Fig 5.1: Combustion chamber
Combustion chamber is equipped with 4 sockets for fuel injectors and spark plug
to inject the fuel and to initially ignite it to start the combustion process. The air drawn
inside is combusted to high pressures and temperatures.
39
Photo 5.2: Combustion Chamber undergoing Turning Operation
Exhaust Tail Pipe
The exhaust tail pipe is the largest part of a pulsejet engine. The length of the tail
pipe determines the resonant characteristics of Pulse Jet. The higher the tail length, the
greater the hot gases retained in it and the temperature increase is high in the combustion
chamber.
As the pulse combustion is the consequence of a combustion instability that is
driven into resonance via this resonance pipe. Hence the exhaust pipe is also called as
“Resonance Pipe”.
Photo 5.3: Exhaust Pipe before Machining
40
The diameter of the pipe should be somewhat larger 8% than that of the inlet pipe
and the tube length needed to be able to be adjusted for frequency manipulation so that
the flow of combusted air moves with a certain velocity. These are the main important
factors to design the exhaust pipe.
The cause of designing the exhaust pipe with a larger diameter than that of the
inlet pipe gives the pulse jet the ability to pump out most of the combustion products
while re-inhaling the fresh charge of air from the inlet portion
Fig 5.2: Exhaust
Fuel Injectors
We have produced three fuel injectors of varying injection holes, one of 0.8mm, the
other with 1mm and finally with 1.2mm holes drilled at equidistant positions from the injector
head which has been brazed to lock the fuel and create the injection effect.
41
Photo 5.4: Fuel Injectors
Fuel
Propane is a three-carbon alkane with the molecular formula C3H8, normally a
gas, but compressible to a transportable liquid. A by-product of natural gas processing
and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches,
barbecues, portable stoves, and residential central heating. Propane is one of a group of
liquefied petroleum gases.
Propane undergoes combustion reactions in a similar fashion to other alkanes. In
the presence of excess oxygen, propane burns to form water and carbon dioxide.
C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat
Propane + Oxygen → Carbon dioxide + Water
When not enough oxygen is present for complete combustion, incomplete
combustion occurs when propane burns and forms water, carbon monoxide, and carbon
dioxide.
C3H8 + 4.5 O2 → 2 CO2 + CO + 4 H2O + Heat
Propane + Oxygen → Carbon dioxide + Carbon monoxide + Water
42
5.3 DIMENSIONS
Name of the section Length
(in mm)
Inner diameter
(in mm)
Cross Sectional
Areas (mm2)
Flow Volume
(mm3)
Inlet 100 38 4534.16 453416
Combustion chamber 100 80 20096 2009600
Tapered section 72 80 Variable
Exhaust tail pipe 400 42 5538.96 2215584
Table 5.1: Dimensions of a single pulsejet engine
Fig 5.3: Pulse Jet Exploded Diagram
43
Photo 5.5: Finished Pulse Jet undergoing TIG Welding
44
6. RESULTS
In this section, the Experimental setup and procedure along with the results are
given. A schematic diagram for each experimental setup is given. The procedure for
starting and operating the engine is presented at the beginning and the procedures for
measuring variables like mass flow, noise levels etc. in different operations are given
after the schematic diagram of setup for their measurement. The results are given in a
tabular form for each operation.
6.1 PROCEDURE FOR STARTING AND OPERATING THE ENGINE
Gaseous Propane from the cylinder as fuel is supplied to the combustion chamber
(directly or using injectors) using fuel pipes through fuel regulating valve(s). The
pressure of the gas from the cylinder is maintained using pressure regulator. Fuel is
ignited either manually or using spark plug and pressurized air is provided i.e. air is
blown in through inlet initially till the engine attains self-sustaining combustion.
The minimum fuel flow for self-sustaining combustion is maintained using fuel
regulating valve. Once the engine is self-sustained and resonating, the required variables
can be measured using the specific instruments. At first experiments were done on single
pulsejet engine to tune the engine and freeze the dimensions and also to determine the
minimum fuel flow required for self-sustained resonating combustion and the range of
flow for which the engine operates with ease and smooth.
With the fixed dimensions, the noise levels and temperatures in the combustion
chamber were measured for single engine in a single operation. Simultaneously fuel mass
flow rate is also measured from micro motion mass flow meter.
45
6.2 EXPERIMENTAL SETUP - 1
In this setup the engine is tested and the length of the inlet is fixed to 160mm and
different fuel flow ranges for engine operation are determined. The fuel is injected
directly into the combustion chamber perpendicular to the air flow through an injector of
5x1mm holes. The schematic diagram of the test setup is shown in the following figure.
The mass flow readings were noted from micro motion mass flow meter.
Fig 6.1: Experimental Setup - 1
Table 6.1: Fuel Flow Rate Range for Engine Direct fuel Injector 1mm
46
Trial No.
Inlet Length
(mm)
Tail Pipe length
(mm)
Cylinder Pressure
(psi)
Mass Flow
(kg/min)
1. 100 400 90 0.056-0.063
2. 100 400 90 0.042-0.049
3. 100 400 90 0.061-0.0673
4. 100 400 90 0.0605-0.067
It is very clear from the above table that the engine has a wide range of fuel flow
rate ranging between 0.042 kg/min and 0.0673 kg/min at a cylinder pressure of 90psi.
6.3 EXPERIMENTAL SETUP - 2
In this setup again single engine was operated with the same dimensions and a
fuel injector with 5x2 holes of 0.8mm diameter was used to inject the gaseous hydrogen
into combustion chamber perpendicular to the air flow. The engine was operated twice
and the range of fuel flow rate was determined at a cylinder pressure of 90psi. The below
figure is the schematic diagram of the test setup with the fuel injector.
Fig 6.2: Experimental Setup - 2
47Trial No.
Inlet Length
(mm)
Tail Pipe length
(mm)
Cylinder Pressure
(psi)
Mass Flow
(kg/min)
1. 100 400 90 0.034-0.044
2. 100 400 90 0.0338-0.0446
Table 6.2: Fuel Flow Rate range for Engine with Fuel Injector 0.8mm
It was observed that the usage of fuel injectors has reduced the minimum fuel
flow rate required for engine operation. It was also observed that the engine starts at the
flow rate of 0.034 kg/min.
Trail no. Pressure
(psi)
Noise levels For
engine run (dB)
1 100 89.7
2 100 82.8
3 100 80.3
4 100 86.8
5 100 88.9
Table 6.3: Noise levels for Pulsejet
Table 6.4: Temperatures in the Combustion Chamber during Engine Operation
The temperatures are manually determined using multi point temperature meter.
48
Trail no. Pressure
(psi)
Fuel mass
Rate(kg/min)
Temperature in
Combustion Chamber
(oC)
Noise level for
Pulsejet
(dB)
1 90 0.0525 – 0.0545 1096 84.6
2 90 0.0525 – 0.0545 1112 85.1
3 90 0.0525 – 0.0545 1136 84.9
4 90 0.0525 – 0.0545 1152 84.2
5 90 0.0525 – 0.0545 1184 83.8
7. CONCLUSIONS AND FUTURE STUDY
As we knew that valveless pulsejet engines are easy to build and operate but they
produce a lot of noise during its operation, we identified the need to reduce the noise
levels.
In order to further improve on the pulsejet research, several things can be done
during the testing phase that would enhance the results. A pressure transducer that
measures a pressure differential of 30 psi which would be compared with theoretical
results to see how close to optimum the pulsejet is performing.
Additionally, further research in the effects of optimum fuel to air ratios and
maximizing flow into the inlets could potentially show improvements in thrust and
efficiency within the engine. Potentially something such as gasoline or Jet A fuel could
be an alternate fuel source to the pulse jet and with acceptable fuel injection, these fuels
could allow for greater thrust from the engine and overall lighter setup that could be used
on a unmanned air vehicle or RC plane.
FUTURE STUDY
In Future they might be used in any of the ground applications efficiently so they
are non-pollutant and eco-friendly engines. The fuels which are used for the operation of
these engines are also cheap and can be helpful in all the needs.
The design of the plenum chambers can also be changed into elliptical shape
design in where we can expect to yield good results in reducing the sound
49
8. REFERENCES
[1] Anderson, R., LukacsT., ‘Design and Build of a Pulsejet’
[2] Bruce Simpson, ‘Enthusiasts Guide To Design, Construction and Operation of
valveless Pulse jets.’
[3] Cottrill, L., “First Principles: The Kadenacy Effect,”
[4] Franco Marcenaro, Advanced Pulsejet Theory.
[5] Ogorelec, B., “Valveless Pulsejet Engines 1.5,” http://www.pulse-jets.com/valveless/
[6] Sulprizio, C., “Lockwood Valveless Pulsejet,” Senior Project, Cal Poly Univ.,
WEBSITES
[a] pulsecombustion.com
[b] aardvark.com
[c] xjets.com
[d] wikipedia.org
[e] aiaa.org
[f] pulsejetengines.com
[g] pulse-jets.com
[h] jetzilla.com
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APPENDIX
.
51