Note: Descriptions are shown in the official language in which they were submitted.
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COMBUSTOR CONFIGURATIONS
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority of United States provisional patent
application no.
60/695,888, filed July 5, 2005; United States provisional patent application
no. 60/706,006
filed August 8, 2005; and Canadian patent application no. 2,512,937, filed
July 28, 2005; all
of which are hereby incorporated by reference.
FIELD
The improvements relates to the field of combustors, and more particularly to
combustors for
use as burners or as engines.
BACKGROUND
Pulse combustors have been known for many years. They work on the principle
that a load of
mixed fuel and air periodically enters a combustion chamber where it ignites,
therefore giving
a pulse combustion. Some pulse combustors are specifically adapted for use as
burners. Other
pulse combustors are specifically adapted for use as engines of the pulse-jet
type. Typical
pulse-jet engines use valves which periodically allow fuel and air intake.
There is a general
need in the field of pulse combustors to enhance efficiency, durability and
thrust output of
pulse combustors.
US Patent no. 3,093,962 to Gluhareff teaches a valveless pulse-jet engine and
somewhat
discusses the use of acoustics. There is a need in the art to somewhat
elaborate on the
teachings of Gluhareff.
Furthermore, known pulse combustors are typically limited to a pulse mode of
combustion.
SUMMARY
An aim of the improvements is to alleviate some of the needs concerning
combustors.
In accordance with one aspect, the improvements provide an ejector system
comprising : a
supersonic fluid injection nozzle having an acoustic injection frequency and
amplitude; a first
resonant tube having an inlet coupled to the nozzle for receiving the injected
fluid from the
nozzle and ambient fluid entrained by the injected fluid, and an outlet for
ejecting the fluids,
the first resonant tube having a first fundamental resonance frequency
excitable by the nozzle
injection; and a second resonant tube having an inlet coupled to receive the
fluids ejected
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from the outlet of the first resonant tube outlet for receiving the ejected
fluids and additional
ambient fluid entrained by the ejected fluids, and an outlet for ejecting the
fluids received by
the inlet, the second resonant tube having a second fundamental resonance
frequency being a
sub-harmonic of the first fundamental resonance frequency..
In accordance with an other aspect, the improvements provide an ejector system
having a first
resonant tube having a first fundamental resonance frequency, an inlet and an
outlet, a second
resonant tube wider than the first resonant tube having a second fundamental
resonance
frequency, an inlet coupled to the outlet of the first resonant tube, and an
outlet, and a
supersonic fluid nozzle aerodynamically coupled to the inlet of the first
resonant tube, the
supersonic fluid nozzle having an acoustic injection frequency and amplitude
suitable to
acoustically excite the first and the second resonant tubes, the ejector
system being
CHARACTERIZED IN THAT the first resonance frequency is a harmonic of the
second
resonance frequency.
In accordance with an other aspect, the improvements provide an intake system
for a
combustor having a pulsating frequency, the intake system comprising : a
supersonic
injection fuel nozzle having an acoustic injection frequency and amplitude; a
first resonant
tube having an inlet coupled to the nozzle for receiving the injected fuel and
ambient air
entrained by the injected fuel, and an outlet for ejecting the fuel and the
air, the first resonant
tube having a first fundamental resonance frequency excitable by the fuel
nozzle; a second
resonant tube having an inlet coupled to the outlet of the first resonant tube
for receiving the
ejected fuel and air and additional ambient air entrained by the ejected fuel
and air, and an
outlet, the second resonant tube having a second fundamental resonance
frequency being a
sub-harmonic of the first fundamental resonance frequency; and a resonant
intake tube having
an inlet coupled to the outlet of the second resonant tube for receiving the
fluids ejected from
the second resonant tube outlet and additional ambient fluid entrained by
these ejected fluids,
and an outlet connected to a combustion chamber inlet of the combustor.
In some cases, the combustor further has a resonator having a fundamental
resonance
frequency which corresponds to the pulsating frequency, the resonator further
having the
combustion chamber inlet, an outlet, an oscillator for use as the combustion
chamber, having
the inlet at an acoustic center thereof and defining one end of the resonator,
and an exhaust
pipe extending from the oscillator and having the outlet at the opposite end
of the resonator,
wherein the oscillator has a resonance frequency which is an odd harmonic of
the pulsating
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frequency and at least one of the coupling between the first resonant tube and
the second
resonant tube and the coupling between the second resonant tube and the intake
tube has a
difference of area and a penetration depth suitable for the intake system to
define a high-pass
filter having a cut-off frequency between the pulsating frequency and the
oscillator
fundamental frequency.
In some cases, the odd harmonic is the third harmonic.
In accordance with an other aspect, the improvements provide a method of
ejecting fluid, the
method comprising : making high frequency noise by injecting one of an over-
expanded and
under-expanded supersonic flow of fluid into a first resonant tube at a speed
sufficient for the
fluid momentum to entrain ambient fluid through the first tube, and for the
fluid exiting the
first tube to entrain further air particles through a second tube; and driving
the first tube into
resonance using the high frequency noise, and driving the second tube into
resonance using
the resonance of the first tube.
In some cases, the ejected fluid is fuel and the ambient fluid is air.
In accordance with an other aspect, the improvements provide an acoustic
cavity for use in a
combustor having a pulsating frequency, the acoustic cavity comprising : a
resonator with a
fundamental resonance frequency which corresponds to the pulsating frequency
of the
combustor, the resonator further having an inlet, and an outlet; an oscillator
made integral to
the resonator and defining one end thereof, being for use as a combustion
chamber, having a
fundamental resonance frequency which is an odd harmonic of the pulsating
frequency and
having the inlet at an acoustic center thereof, and an exhaust pipe made
integral to the
resonator and having the outlet at an opposite end thereof, the exhaust pipe
extending from
the oscillator.
In accordance with an other aspect, the improvements provide a combustor
comprising : a
resonator having a fundamental resonance frequency which corresponds to a
pulsating
frequency of the combustor when operating in a pulse mode, the resonator
further having: an
oscillator for use as a combustion chamber, having a fundamental resonance
frequency which
is an odd harmonic of the pulsating frequency, and having an inlet at an
acoustic center
thereof, the oscillator defining one end of the resonator, and an exhaust pipe
extending from
the oscillator and having an outlet at the opposite end of the resonator; and
an intake system
connected to the inlet to feed the combustion chamber with fuel and air.
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In accordance with an other aspect, the improvements provide an acoustic
cavity for use in a
combustor having a pulsating frequency, the acoustic cavity having a resonator
with a
fundamental resonance frequency which corresponds to the pulsating frequency
of the
combustor, the resonator further having an inlet, an outlet, an oscillator for
use as a
combustion chamber, having the inlet at an acoustic center thereof and
defining one end of
the resonator, and an exhaust pipe extending from the oscillator and having
the outlet at the
opposite end of the resonator, the acoustic cavity being CHARACTERIZED IN THAT
the
oscillator has a fundamental resonance frequency which is an odd harmonic of
the pulsating
frequency.
In accordance with an other aspect, the improvements provide a combustor
having a
resonator with a fundamental resonance frequency, the resonator further having
an inlet and
an outlet, and a combustion chamber opposite the outlet, the combustion
chamber defining an
oscillator and having the inlet at an acoustic center thereof, the combustor
further having an
intake system connected to the inlet, the combustor being CHARACTERIZED IN
THAT the
oscillator has a fundamental resonance frequency which is a harmonic of the
resonator
fundamental frequency, the intake system is acoustically excitable by the
oscillator
fundamental frequency, and the intake system defines an acoustic high-pass
filter to reflect
the resonator fundamental frequency back into the combustion chamber.
In accordance with an other aspect, the improvements provide a method of
pulsatingly
combusting fuel in a resonator at a fundamental resonance frequency of the
resonator, the
method comprising : magnifying a harmonic frequency of the fundamental
resonance
frequency in a combustion chamber portion of the resonator; exciting an
acoustic high-pass
filter defined by an intake system connected to the combustion chamber with
the magnified
harmonic frequency; impeding the transmission of the pressure pulses from the
fundamental
resonance frequency to the intake system with the excited acoustic high-pass
filter; and
feeding fuel and air into the combustion chamber with the intake system; and
periodically
increasing the pressure in the combustion chamber by the resonance of the
resonator.
In accordance with an other aspect, the improvements provide a combustor
having a main
longitudinal axis and a pulsating frequency, the combustor comprising : a
tubular combustor
body having an outlet, a combustion chamber opposite the outlet and an exhaust
pipe
narrower than the combustion chamber between the combustion chamber and the
outlet, the
combustion chamber, exhaust pipe and outlet being in flow communication along
the main
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longitudinal axis, and the body having a resonance frequency corresponding to
the pulsating
frequency; a plurality of substantially longitudinally oriented slots
interspaced around the
combustion chamber at a longitudinal acoustic center thereof, the slots
defining an inlet to the
body; and an intake system connected to the combustion chamber inlet.
In accordance with an other aspect, the improvements provide a combustor
having an
elongated combustion chamber having an inlet proximate a longitudinal center
thereof, an
intake system connected to the combustion chamber inlet, and a tail pipe
extending from the
combustion chamber and defining an outlet thereto, wherein the combination of
the
combustion chamber and the tail pipe define an acoustic resonator having a
fundamental
resonance frequency at which fuel from the intake system is to be pulsatingly
ignited in the
combustion chamber, the combustor being CHARACTERIZED IN THAT the inlet
comprises a plurality of longitudinally oriented slots being peripherally
interspaced around
the combustion chamber.
In accordance with an other aspect, the improvements provide a combustor
comprising : a
tubular combustor body, the body having: a combustion chamber having a
plurality of
tangentially spaced apertures, and an exhaust pipe narrower than the
combustion chamber
and extending away from the combustion chamber and being in flow communication
therewith, the exhaust pipe defining an outlet of the tubular combustor body;
and an intake
system connected to the apertures to feed the combustion chamber with fuel and
air. .
In some cases, the apertures are disposed in a peripheral surface of the
combustion chamber.
In other cases, the apertures are disposed around an intake tube which
protrudes into the
combustion chamber.
In accordance with an other aspect, the improvements provide a combustor
having a main
longitudinal axis comprising : a tubular combustor body having an outlet, a
combustion
chamber portion opposite the outlet and an exhaust pipe narrower than the
combustion
chamber between the combustion chamber and the outlet, the combustion chamber
portion,
exhaust pipe and outlet being disposed along the main longitudinal axis, and
the body having
a resonance frequency corresponding to a pulsating frequency of the combustor;
an intake
system having an intake tube longitudinally penetrated into the combustion
chamber along
the main axis, opposite the outlet, the intake tube having an open end outside
the combustion
chamber, a closed end inside the combustion chamber, and plurality of
longitudinally
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oriented apertures tangentially interspaced around the intake tube at a
longitudinal acoustic
center of the combustion chamber, the slots defining an inlet to the
combustion chamber.
In accordance with an other aspect, the improvements provide a combustor
having an
elongated combustion chamber having an inlet proximate a longitudinal center
thereof, an
intake system connected to the combustion chamber inlet, and an exhaust pipe
extending
from the combustion chamber and defining an outlet thereto, wherein the
combination of the
combustion chamber and the tail pipe define an acoustic resonator having a
fundamental
resonance frequency at which fuel from the intake system is to be pulsatingly
ignited in the
combustion chamber, the combustor being characterized in that the intake
system is aligned
on an axis common to the combustion chamber and to the exhaust pipe and that
the
combustion chamber inlet is peripheral to the combustion chamber.
In accordance with an other aspect, the improvements provide a turbine system
for an in-line
combustor having an intake system, a body and an outlet aligned along a
combustor axis, the
turbine system having a power turbine adapted to extract energy from the
gasses exhausted
from the outlet into rotation, a fan positioned upstream from the intake
system, and a shaft
connecting the power turbine to the shaft, whereby energy from the exhausted
gasses is
transmitted from the power turbine to the fan by the rotation of the shaft and
the fan thereby
enhances the air intake through the intake system.
In accordance with an other aspect, the improvements provide a method of
tuning a
combustor having a body defining a resonator and a combustion chamber in the
body, the
method comprising : selecting a combustion chamber shaped to define an
oscillator which
has a fundamental resonance frequency which is the third harmonic of the
resonator
fundamental frequency at operation temperature.
In accordance with an other aspect, the improvements provide a method of
tuning an ejector
having a high frequency fluid nozzle, a first resonant tube and a second
resonant tube, the
method comprising : selecting a first stage resonant tube which has a
fundamental resonance
frequency which is a harmonic of the fundamental resonance frequency of the
second
resonant tube.
In the present specification, when reference is made to a resonant frequency,
it is to be
understood that what is meant is the resonant frequency during operation,
which may depart
from resonant frequency at rest due to temperature variations.
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DESCRIPTION OF THE FIGURES
Further features and advantages of the present improvements will become
apparent from the
following detailed description, taken in combination with the appended
figures, in which:
Fig. 1 is a perspective view of an L-shape pulse combustor in accordance with
the
improvements;
Fig. 2 is a cross-sectional view of the pulse combustor of Fig. 1 showing the
structural
elements thereof;
Fig. 3 is a cross-sectional view of the pulse combustor of Fig. 1 showing the
acoustic
elements thereof;
Fig. 4 is a schematic view illustrating variations in pressure over time in
the pulse combustor
of Fig. 1;
Fig. 5A and Fig. 5B are a side and a top cross-sectional views, respectively,
showing the
ejector system of the pulse combustor of Fig. 1;
Fig. 6 is a perspective view of an in-line combustor in accordance with the
improvements;
Fig. 7 is a cross-sectional view of the combustor of Fig. 6 showing the
structural elements
thereof;
Fig. 8 is a cross-sectional view of the combustor of Fig. 6 showing the
acoustic elements
thereof;
Fig. 9A is a side views, partly sectioned and enlarged, showing the slotted
inlet of the
combustor of Fig. 6;
Fig. 9B is an enlarged view of the slotted inlet of Fig. 9A;
Fig. 9C is a front cross-sectional view of the slotted inlet of Fig. 9A;
Fig. 9D is a front cross-sectional view of an alternative to the slotted inlet
of Fig. 9A;
Fig. 9E is a side cross-sectional view of an alternative to the slotted inlet
of Fig. 9A;
Fig. 10 is a side view, partly sectioned, showing the intake system of the
combustor of Fig.
6;
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Figs. 11 A, is a side view showing an other configuration of an in-line
combustor in
accordance with the improvements;
Figs. 11 B and 11 C are cross-section views showing alternate configurations
of an in-line
combustor in accordance with the improvements;
Fig. 12 is a perspective view showing a turbine engine using a plurality of in-
line combustors;
and
Fig. 13 is a perspective view of a Hiller-Lockwood engine having the ejector
system of Fig.
5A.
DETAILED DESCRIPTION
Referring to the drawings, and more particularly to Fig. 1, an example of a
pulse combustor
10 having a configuration referred to herein as "L-shaped configuration" is
shown. The pulse
combustor 10 can be used as a burner to generate a hot air flow and can
alternately be used as
a pulse-jet engine to generate thrust. Typically, when the combustor 10 is
used as a burner, it
will be mounted to a fixed frame. When it is used as an engine, it will be
mounted to a
displaceable vehicle. The pulse combustor 10 generally includes a body 12 and
an intake
system 14. The body 12 has a generally tubular shape. In the example, the body
12 has an
irregular surface of revolution tubular shape aligned along a main axis 16.
The intake sysfem
14 will be recognized by those skilled in the art to include an ejector system
18 and is aligned
along an intake axis 20. The main axis 16 and the intake axis 20 are
transverse, thus giving
the L-shape configuration. In the illustrated example, the angle between the
main axis 16 and
intake axis 20 is of 90 degrees, with the axes being in a common plane. Other
angles can be
used as well.
Referring now to Fig. 2, it is seen that the body 12 includes two portions : a
combustion
chamber 22 and an exhaust pipe 24. The body 12 has an inlet located in the
combustion
chamber 22 and an outlet 28 at the end of the exhaust pipe 24. The combustion
chamber 22 is
wider than the exhaust pipe 24. The combustion chamber 22 includes a nose cone
30 opposite
to the outlet 28 and having the pointed end outwardly oriented, a converging
section 32
bridging the combustion chamber 22 to the exhaust pipe 24 and a generally
cylindrical
section 34 between the nose cone 30 and the converging section 32. The inlet
26 is located in
the cylindrical section 34.
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The intake system 14 includes an intake tube 36 having an inlet 36a and an
outlet 36b, and
the outlet 36b is connected to the combustion chamber inlet 26. In this
example, the intake
tube 36 is flared and defines a diverging section between the intake tube
inlet 36a and the
intake tube outlet 36b. This contributes to slowing the gasses entering the
combustion
chamber 22 from the intake system 14. The ejector system 18 is coupled to the
intake tube
inlet 36a. The ejector system 18 includes a supersonic fuel nozzle 38 coupled
to the inlet 40a
of a first tube 40, and a second tube 42 having an inlet 42a coupled to the
outlet 40b of the
first tube 40. The outlet 42b of the second tube 42 is coupled to the inlet
36a of the intake
tube 36. The intake system 14 includes a first tube coupling 44. The first
tube coupling 44
includes an adjustment of the relative position between the supersonic fuel
nozzle 38 and the
first tube 40, and adjustment of the cross-sectional area of the first tube
40. The intake system
14 also includes a second tube coupling 46 and an intake tube coupling 48.
The tubes 40, 42, 36 of the intake system 14 can be maintained in position
relative to each
other in many possible ways. In one example where the combustor is used as a
burner, each
component of the intake system can be mounted to a common frame (not shown).
In another
example where the combustor is used as an engine, the tubes can be connected
to one another
with suitable brackets. The exact choice thereof is left to those skilled in
the art.
Those familiar with the principles of ejectors will understand that the
coupling 44 between
the nozzle 38 and first tube 40 is such that the fuel exiting the supersonic
fuel nozzle 38 at
high velocity transfers some of its momentum to adjacent particles of air
which entrains a
flow of air through the first tube 40 with the flow of fuel. The coupling 46
between the first
tube 40 and second tube 42 allows fuel and air exiting the first tube 40 to
transfer some of its
angular momentum and entrain more air through the second tube 42. The air
mixes with the
fuel as it travels through the first and second tubes. Similarly, the coupling
48 between the
second tube 42 and the intake tube 36 allows fuel and air exiting the second
tube 42 to
transfer some of its angular momentum and entrain more air through the intake
tube 36 and
into the combustion chamber 22.
The mixture of air and fuel entering the combustion chamber 22 is pulsatingly
ignited at a
pulsating frequency of the pulse combustor 10, and the combustion products are
exhausted
out the exhaust pipe outlet 28. This combustion mode will be referred to
herein as the pulse
mode. The basic cycle of the pulse mode will be outlined further below with
relation to
combustor acoustics.
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In this example, the fuel used is gas and can be propane. The supersonic fuel
nozzle 38 is
adapted to inject such a gaseous fuel. When a gaseous fuel is used, it can be
advantageously
pre-heated inside the combustion chamber 22. One way to achieve this is to use
a coil 50 in
the combustion chamber 22 through which the gaseous fuel circulates prior to
reaching the
fuel nozzle 38. A fuel source (not shown) is connected to a fuel inlet 52. The
fuel inlet 52 is
connected to the coil 50 through the combustion chamber wall. The coil 50 has
a fuel outlet
54 which exits through the combustion chamber wall and is connected to the
fuel nozzle 38.
During operation of the pulse combustor 10, heat may thus be transferred from
the
combustion chamber 22 to the fuel through the coil 50.
It will be seen from the description below that the acoustics of the pulse
combustor 10 is an
important consideration in maximizing its output power. It was demonstrated by
experiment
that the coil 50 has a tendency to absorb acoustic vibrations by vibrating,
which has been
shown to lower the combustor's efficiency or power output. For this reason, it
can be
advantageous to secure the coil 50 to the combustion chamber in a manner to
minimize its
tendency to vibrate. However, one will understand that it is practically
impossible to entirely
eliminate coil vibration. Resulting coil vibration has a tendency to propagate
through the
material of the conduit connecting the coil outlet 54 to the fuel nozzle 38,
especially if this
material is rigid. Such vibrations have been known to negatively influence the
injecting
action of the fuel nozzle 38. For this reason, it has been found advantageous
to use a flexible
hose 56 at some point between the coil outlet 54 and the fuel nozzle 38. The
flexible hose 56
dissipates energy from the coil vibration and minimizes the amount of
vibration which affects
the fuel nozzle 38.
It is to be understood that instead of a gaseous fuel, a liquid fuel can be
used with a
supersonic fuel nozzle which is adapted to inject a liquid fuel. In this case,
there is typically
no need to preheat the fuel in the combustion chamber and it is to be
understood therefore
that the coil and flexible hose can be entirely omitted. Such an embodiment is
depicted for
example in Fig. 3. When selecting a fuel, one must consider the flame
propagation speed
relatively to the pulsating frequency and the size of the combustion chamber
of the pulse
combustor.
At combustor startup, an igniter such as an igniter electrode 58 is used to
start the pulse
combustor 10 into operation. In the example, the igniter electrode 58
positioned in the wall of
the exhaust pipe 24, near the converging section 32 of the combustion chamber
22, is used to
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start the combustor 10. However as it will be seen below, once the combustor
10 has warmed
up, it can be autonomous and maintain its operating cycle by automatically
lighting the fuel at
the pulsating frequency. At that point, no igniter is necessary to maintain
the combustor into
operation. For this reason, it is believed that using an igniter which is
permanently part of the
combustor is not essential in certain embodiments.
Referring to Fig. 3, a pulse combustor skeleton being quite similar to that
illustrated in Fig. 2
is depicted. One difference between Fig. 2 and Fig. 3 lies in the fact that a
liquid fuel is used
in Fig. 3 instead of a gaseous fuel, and the coil has therefore been entirely
omitted. Apart
from that, Fig. 3 is used to illustrate the acoustic components of the
combustor rather than to
focus on the structural, aerodynamic or thermodynamic requirements, since an
understanding
of the acoustic operating principles of the combustor is useful to understand
the
improvements. For clarity, reference numerals in the one hundred series will
be used to
identify corresponding elements in Fig. 3. It will be understood by those
skilled in the art of
acoustics that the requirements to guide pressure waves of sound are much
different from the
requirements to guide fluid mechanics, and that some structural changes which
may greatly
affect the visual geometry of components may affect their acoustic behaviour
only negligibly.
Referring to Fig. 3, the body 12 and the intake system 14 of the pulse
combustor 10 (Fig. 2)
can be seen to define an acoustic cavity 110. The body 12 (Fig. 2) acts as an
acoustic
resonator 112 which may in some ways be compared to a closed cylinder air
column, and
thus bearing resemblance to a wind instrument such as the clarinet. The
resonator 112 can be
said to have a fundamental resonance frequency similar to that of a closed
cylinder air
column. The fundamental resonance frequency of the resonator 112 is thus a
function of the
length of the resonator 112 and a function of the speed of sound. Since the
speed of sound is a
function of temperature, and the temperature changes both as a function of
position and time,
the speed of sound varies depending of the position in the resonator 112. The
combustion
chamber (22) is typically warmer than the exhaust pipe (18). The temperature
varies over
time with the operating cycle of the combustor, therefore in the instant
description, when
reference is made to a resonance frequency, it is understood that what is
meant is average
resonance frequency. This resonance frequency may be quite different than the
resonance
frequency at ambient temperature, especially when referring to the combustion
chamber 22,
exhaust pipe 24 and intake tube 36.
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Typically, the pulsating frequency of the combustor 10 coincides with the
resonator
fundamental frequency, although it is envisaged that the combustor 10 could
alternately
pulsate at a harmonic of the resonator fundamental frequency, such as the
third harmonic for
example. It is the important pressure variations at the pulsating frequency in
the combustion
chamber that drive the main combustor operating cycle. For indicative
purposes, the pulsating
frequency of one example of a pulse combustor was of about 145 Hz.
In Fig. 4, the pressure variations at pulsating frequency 200 are
schematically depicted as a
sinusoidal curve, although it is understood that the actual pressure
variations depart from this
curve. Combustion mainly takes place during the rising pressure portion 210 of
the cycle and
a negative pressure portion 220 of the cycle follows. During the negative
pressure portion
220, air and fuel enter the combustion chamber 22 through the inlet 26 (Fig.
2). Although the
combustion mainly occurs during the positive portion 210 of the cycle, some
combustion
lingers on during the negative pressure portion when the combustor is warm,
especially near
the wall of the combustion chamber 22, perhaps due to boundary layer effect.
Once the
pressure rises, the fresh air/fuel mixture present in the combustion chamber
22 is ignited by
the boundary layer and combustion or more precisely, an explosion occurs. The
energy
deployed by the cyclic explosions adds to the resonance frequency of the
resonator and helps
sustain this resonance in a manner similar to how the edge-tone principle
generates energy
which contributes to sustain the resonance in some musical wind instruments.
To maintain a satisfactory flow of air and fuel into the combustion chamber
22, it is desired
that the velocity impulse from the positive pressure pulses 210 of the
explosion be guided to
exhaust through the exhaust pipe 24 rather than through the intake system 14.
One way to
contribute to this goal is to augment the ejecting action of the ejector and
to use the
momentum of fuel and air in the first tube 40 and second tube 142 to counter
the pressure
pulses exiting the intake tube 36. In the following discussion, this way, and
other ways to
contribute to this goal will be discussed.
To reduce the travel of the pressure pulses or waves through the inlet 26 and
intake system 14
at the pulsating frequency, it is desired to increase the intake system
impedance at that
frequency. One skilled in the art of acoustics is aware of basic acoustic
filter theory and will
recall that an acoustic high-pass filter can be constructed using a T junction
or a side-branch
opening in a duct or pipe. If both the radius and the length of the side
branch are smaller than
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a wavelength of the plane waves in the duct then the acoustic impedance of the
side-branch
opening becomes a function of the side-branch opening area and of the side-
branch length.
Turning back to Fig. 3, it is seen that in the intake system 14 (Fig.2), the
intake tube
coupling 148 includes an adjustment of penetration depth and area difference.
Although the
visual appearance of the coupling 148 between these tubes is much different
from the
appearance of a side branch in a straight pipe, it was found that the acoustic
behaviour is
actually quite similar, with the difference of area between the second tube
and the intake tube
being equivalent to the opening area of the side branch, and the penetration
depth of the
second tube into the intake tube being equivalent to the length of the side
branch. The
coupling 148 between the second tube and the intake tube can therefore act as
a precisely
selected high-pass filter 114 if the penetration depth and the difference of
area are precisely
chosen. A similar discussion can be made of the coupling 146 between the first
tube 140 and
the second tube 142. In Figs. 5A and 513, the penetration depth 160 of the
first tube 140 into
the second tube 142 and the difference of area 162 between the first tube 140
and second tube
142 are identified for further clarity.
Referring to Figs. 5A and 513, it will be discussed how to enhance the
ejecting action of the
ejector system 118. A first point which may be made is that as it is known to
those skilled in
the art of fluid mechanics, a well rounded inlet creates less energy loss and
eases the
penetration of fluid relatively to a sharp edged inlet. This phenomenon is
used
advantageously in the case of the first and second tubes (140, 142) which have
rounded inlets
(140a, 142a) and straight edge outlets (140b, 142b). Hence the impedance to a
flow of air is
greater when traveling into the outlet than when traveling into the inlet, and
thus contributes
to enhance the ejecting action.
One parameter to consider when adjusting the ejecting action of the ejector
system 118 is the
adjustment of the position of the nozzle 138 relative to the first tube inlet
140a. Typically, a
small gap will be present between the nozzle 138 and the first tube inlet
140a. One can
contemplate the effect of the position of the nozzle 138 by monitoring the
amount of fluids
ejected from the second tube outlet 142b.
Furthermore, it will be understood that the first tube 140 and the second tube
142 act as open
ended cylinders acoustically and that as such, they have a fundamental
resonance frequency
which is a function of the length of the tube in addition to being a function
of the speed of
sound. However, the diameter of the tube does not have much influence on the
acoustics. For
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aerodynamic, fluid mixing and momentum considerations, the second tube will
generally be
chosen to be longer than the first tube and will therefore have a lower
fundamental frequency.
It was found that most supersonic fluid nozzles (which are fuel nozzles when
the ejector
system 18 is used with the pulse combustor 10) generate pressure vibrations
while they inject
fluid. Of three possible fluid nozzle types, both the over-expanded and under-
expanded types
produce pressure vibrations. It was found that when using a typical over-
expanded nozzle,
high amplitude and high frequency pressure waves (or noise) resulted which
excited the first
tube 140 into resonance. In turn, the resonance of the first tube 140 enhanced
the ejecting
action of the first tube. The fluid nozzle 138 therefore acted as a high-
frequency noise
generator having a frequency and amplitude suitable to acoustically excite the
first tube 140.
It was found that by selecting a first tube 140 having a fundamental resonance
frequency
which was a harmonic of the second tube fundamental resonance frequency, the
resonance of
the first tube 140 was transmitted to the second tube 142 and excited the
second tube 142 into
resonance. In the tests, the first tube fundamental resonance frequency were
selected to be the
third harmonic of the second tube fundamental resonance frequency. The
resonance of the
second tube 142 further enhanced the ejecting action of the injector.
Similarly, if a third tube is coupled to the second tube, such as an intake
tube for example, the
resonance frequency of the third tube can be selected to be in tune with the
resonance
frequency of the second tube.
In this specification, the term sub-harmonic is used to designate the inverse
of a harmonic.
For example, if the first tube has a fundamental resonance frequency which is
the third
harmonic of the second tube fundamental resonance frequency, the second tube
fundamental
resonance frequency can be said to be at the third sub-harmonic of the first
tube fundamental
resonance frequency. The expression in tune indicates that the resonance
frequency of a first
element is either the same, a harmonic of, or a sub-harmonic of the resonance
frequency of a
second element.
Comparing Figs 2 and 3, another acoustic component which may advantageously be
used
with the combustor 10 is to use a combustion chamber 22 shaped to act as an
oscillator 122.
The nose-cone 30 of the combustion chamber 22 acts as a same-phase reflector
130 to the
acoustic pressure wave caused by the explosion. Furthermore, using the nose
cone 30 shape
instead of a flat end is advantageous because it flattens out the incoming
pressure wave,
diminishing the amplitude and increasing the period with the reflection. The
converging
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section 32 of the combustion chamber 22 acts as a partial inverse-phase
reflector 132 which
reflects a portion of the outgoing pressure wave back into the oscillator 122
with the opposite
phase. The combined action of the reflector 130 and the partial reflector 132
is to trap and
magnify a frequency which is higher than the pulsating frequency in the
oscillator 122. The
oscillator 122 can thus be said to also have its own fundamental resonance
frequency. By
adjusting the length of the oscillator 122 taking into account the speed of
sound at operating
temperatures, the fundamental resonance frequency of the oscillator 122 can be
selected to be
a harmonic of the pulsating frequency of the resonator 112. Due to the closed-
end cylinder
acoustic characteristics of the resonator 112, the odd harmonics are favoured.
Typically the
third harmonic is selected, although it will be understood that another
harmonic such as the
fifth harmonic can also be selected.
In Fig. 4, an exemplary sinusoidal curve 230 representing the oscillator 122
resonating at the
third harmonic of the pulsating frequency 200 is shown, although it will be
understood that
the actual pressure curve may depart somewhat dramatically from this
sinusoidal curve
illustration. In the combustion chamber 22, the pulsating frequency 200 and
the oscillator
frequency 230 are superposed, which gives rise to a resulting curve which has
a more
complex form. Other harmonic frequencies and noise are also present in an
actual pulse
combustor 10 during operation.
The resonator 112 therefore acts as a low-pass filter 112a allowing the
pressure waves at the
pulsating frequency out of the combustion chamber 22 and through the exhaust
pipe 24, but
at least partially maintains the pressure waves at the oscillator resonance
frequency in the
oscillator 122. The cut-off frequency of this low-pass filter l 12a can be
said to be between
the pulsating frequency and the oscillator frequency.
The oscillator 122 can also be said to have an acoustic center 123 where the
oscillator
pressure variations are optimized. This acoustic center 123 may somewhat
depart from the
longitudinal center of the combustion chamber. For instance, if a gas fuel is
used and a coil
50 is present in the aft portion (32) of the combustion chamber 22, the coil
50 will decrease
the mean temperature in that portion of the combustion chamber 22, therefore
lowering the
speed of sound in that portion relatively to the fore portion (30) of the
combustion chamber
22. As a result, the acoustic center 123 will be shifted toward the fore
portion of the
combustion chamber 22 relatively to the longitudinal center.
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It is understood that the intake tube 36 can be seen from one acoustic
perspective as an open
cylinder resonator. The diameter and the length of the intake tube 36 are
guided by
aerodynamic, thermodynamic and momentum considerations. However, it has been
found
that by using an intake tube having a length giving a fundamental resonance
frequency which
is equal to the oscillator resonance frequency or a harmonic thereof results
in the oscillator
driving the resonance of the intake tube. Furthermore, the length of the
equivalent acoustic
cylinder formed between the nose cone tip and the intake tube outlet, and the
length of the
equivalent acoustic cylinder formed between the small end of the converging
section and the
intake tube outlet are chosen to have acoustic resonance frequencies which are
harmonics of
the pulsating frequency.
The resonance of the intake tube 36 contributes to maximize the ejecting
action of the ejector
18. To maximize the resonance-driving effect of the oscillator 112, the
penetration depth and
the area difference defining the acoustic coupling 148 between the second tube
142 and the
intake tube 136 and the acoustic coupling 146 between the first tube 140 and
the second tube
142, can be selected to create a high-pass filter 114 in which the cut-off
frequency is between
the pulsating frequency of the resonator 112 and the resonance frequency of
the oscillator
122. Another factor to influence the resonance-driving effect of the
oscillator 122 is to place
the combustion chamber inlet 26 (thereby placing the intake tube outlet 36b)
at the
longitudinal acoustic center 123 of the oscillator/combustion chamber.
Another factor in increasing the acoustic impedance of the intake tube 36 to
the pressure
pulses from the explosions in the combustion chamber is to give a funnel shape
to the intake
tube 36 by making it a converging section with the larger end acting as the
outlet 36b and
being connected to the combustion chamber inlet 26.
The result is that the arrangement of acoustic components as shown in Fig. 3
defines an
acoustic cavity 110 which can advantageously be used in a pulse combustor 10
because the
resonance frequencies can be chosen in a manner in which they interact during
combustor
operation to obtain increased combustor efficiency, power or thrust.
Typically, when the combustor 10 is started, the temperature of the combustion
chamber 22,
intake tube 36 and exhaust pipe 24 are below the normal operating
temperatures, and the
resonance frequencies will be out of tune. The combustor will start with a
lower power output
until the components are heated up and a steady-state regime is reached and
sustained.
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Turning now to Fig. 6, an example of another configuration of a combustor 310
is shown,
which will be referred to herein as the "in-line configuration". For clarity,
this in-line
combustor 310 will be described using reference numerals in the three hundred
series.
Similarly to the L-shape pulse combustor 10 shown in Fig. 1, the in-line
combustor 310 also
includes a body 312 generally disposed on a main axis 316 and an intake system
314
disposed on an intake axis 320. However, in the case of this in-line combustor
310, the
intake axis 320 and the main axis 316 coincide and the intake system 314 is
oriented away
from the body 312 in a direction opposite from the direction of the exhaust
pipe 324. As it
will be understood, this in-line configuration procures some significant
advantages relative to
the L-shape configuration, especially when the combustor is used as an engine
for propulsion,
rather than as a burner for hot air generation.
In Fig. 6, the in-line combustor 310 uses gaseous fuel as it can be understood
from the fuel
line 356 which extends between the fuel nozzle 338 and a coil (not shown) in
the combustion
chamber 322. This feature is similar to the features illustrated in Fig. 2.
However, it will be
understood that a liquid fuel can also be used and that the coil can be
omitted. For simplicity,
this latter case is illustrated in Figs. 7 and 8.
Referring to Figs. 6, 7 and 8, it can be seen that at least some of the
structural and acoustic
components of the in-line combustor 310 are equivalent to the structural and
acoustic
components of the L-shaped pulse combustor 10 illustrated in Figs. 1, 2 and 3,
without
limiting the advantages of this in-line configuration in terms of performance
and additional
capabilities. For the sake of clarity and simplicity, the in-line combustor
example will
therefore be described on the basis of comparisons made relative to the L-
shape pulse
combustor 10 example.
Like the L-shape pulse combustor 10, the in-line combustor 310 also includes a
body 312
which has a combustion chamber 322 connected to an exhaust pipe 324, with an
328 outlet in
the exhaust pipe 324 and an inlet 326 in the combustion chamber 322. However,
in this
example, the inlet 326 is peripheral to the combustion chamber 322 instead of
being an
aperture 26 on one side of the combustion chamber 22 as was the case for the L-
shape
combustor 10.
Furthermore, like in the case of the L-shape combustor 10 example, the in-line
intake system
314 also basically functions on the principle of an ejector and includes an
ejector system 318
with a supersonic fuel nozzle 338, a first tube 340 and a second tube 342.
Instead of being
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simply cylindrical, the second tube 342 has a cylindrical portion 341 and a
flared end portion
343 which covers a portion of the nose cone 330. The cross-sectional area
defined between
the nose cone 330 and the flared portion 343 is substantially equal to the
cross-sectional area
of the cylindrical portion 341. The intake system 314 also includes an intake
tube 336, but the
intake tube 336 in this case has the shape of a cowl that covers the portion
of the combustion
chamber 322 extending from the peripheral inlet 326 and the outlet 342b of the
second tube
342. The cross-sectional area between the combustion chamber 322 and the
intake tube 336
can be chosen to be equivalent to the cross-sectional area of the cylindrical
intake tube 36
which was used in the L-shape combustor example (Fig. 1), for example. In this
case, the
intake tube 336 overlaps a portion of the second tube 342, and it will be
understood that this
overlap distance equates the penetration distance 160 (Figs. 5A and 5B) and
can be selected
together with the area difference 162 to achieve desired filtering
characteristics, as can be
seen from the discussion above.
One skilled in the art will understand that the choice of the irregular shape
of the second tube
342 and intake tube 336 has only a minimal influence on the acoustic,
aerodynamic and
momentum characteristics of these components. These irregular shapes are a
good example
of how the structural shape of certain components of the combustors 10, 110,
310 can be
greatly varied while not substantially affecting their function in combustor
operation. This
should serve as an illustration of how the reader of the instant specification
must bear in mind
the essential acoustic, aerodynamic and/or momentum characteristics of the
combustor
components rather than to only look at the simple structural characteristics
or visual
appearance of the combustor components.
One special feature of the in-line configuration is that the intake system 314
can benefit from
the effect of ram air when the engine is displaced in the surrounding air.
During engine
displacement, surrounding air is "caught" in the area between the fuel nozzle
338 and the first
tube 340, in the area between the first tube 340 and the second tube 342, and
the area between
the second tube 342 and the intake tube 336. This ram air increases the amount
of air flow
through the intake system 314, which opposes the direction of the pressure
pulses in the
intake tube 336. This ram air can thus be used to increase combustor
efficiency.
When ram air is used with the intake system 314, the combustor 310 can enter a
mode which
will be referred to herein as the ram mode. During the transition to the ram
mode, the
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pulsating frequency can vary. In the ram mode, the acoustic considerations can
become of
lesser importance relatively to other aerodynamic and thermodynamic
considerations.
Referring now to Figs. 9A to 9E, discussion will be made of the peripheral
inlet to the
combustion chamber. One will understand that when designing the inlet (26 or
326 for
example) of a combustor, one attempts to satisfy various requirements. First,
the inlet should
have a sufficient overall size (area) so as to allow a satisfying amount of
air and fuel to flow
into the combustion chamber. To enhance the acoustic interaction between the
intake tube
and the oscillator as was discussed above, the inlet should have a sufficient
longitudinal
length to allow the longitudinal waves reflected from the reflector and the
partial reflector to
form appropriate waves in the intake tube. Furthermore, one does not wish that
the overall
area defined by the combustion chamber inlet be too important because this has
the effect of
lowering the impedance of the intake tube to the pressure pulses from the
pulsating frequency
and consequently increases the amount of discharge through the intake tube at
the expense of
the amount of thrust or hot air discharge through the exhaust pipe.
To satisfy the requirement of inlet overall size and inlet longitudinal length
along the
combustion chamber discussed above, it has been found advantageous to provide
the inlet
326 as a plurality of apertures. In this example, the apertures are slots 327
which are
tangentially interspaced around the combustion chamber wall relative to the
main axis 316
(Fig. 6). The slots 327 can be longitudinally oriented to optimize their
actual longitudinal
length. Alternately, they can be somewhat angled. Further, to maximize the
acoustic
interaction between the intake tube 336 and the oscillator 122 formed by the
combustion
chamber 322, the slots 327 can be longitudinally positioned at the acoustic
center 323 of the
oscillator. The main acoustic length of the intake tube 336 can then be
calculated from the
inlet 336a to the nearest end 327a of the longitudinal slots 327. In other
examples, the
apertures can be provided as tangentially spaced sets of longitudinally
aligned apertures, or in
other aperture configurations which are not necessarily longitudinally aligned
but which
satisfy similar acoustic and aerodynamic criteria.
One way to further enhance intake flow through the slots 327 and to reduce
backflow from
the combustion chamber 322 is to provide the slots with a rounded edge 327b
which
penetrates into the combustion chamber 322. This is more clearly shown in
Figs. 9B and 9C.
The backflow is thus faced to a re-entrant edge having a high loss
coefficient, whereas the
intake flow faces a rounded edge having a low loss coefficient. Flow into the
combustion
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chamber 322 is thus favoured relative to backflow. This configuration can be
realized in sheet
metal by punching the slots 327 in prior to rolling the cylindrical section
334 of the
combustion chamber 322, for example.
In the example illustrated in Figs 9A to 9C, the rounded edges of the slots
are oriented
generally perpendicularly to the combustion chamber wall or to the combustor
central axis.
However, it will be understood that to favour a vortex flow, the rounded edges
of the slots
can be tangentially slanted relatively to the combustion chamber wall or the
central axis, in a
common tangential direction, such as the slots 427 shown in Fig. 9D for
example. This
changes the configuration of the penetrating flow of fuel and air, and can
increase the
resistance to backflow. In Fig. 9E, it is shown that the slots 427 can also
have edges which
are longitudinally slanted.
Referring to Fig. 10, it can be seen that in this example of an in-line
combustor 310, the
intake system 314 is held into position by a plurality of fins 370, 372, 374.
In the example,
four fins are used between each two components. The fins 370, 372, 374 are
longitudinally
oriented and are made thin in order to minimize their resistance to inflowing
air. For the first
tube coupling 344, first tube fins 370 connect the first tube inlet 340a to
the nozzle 338. For
the second tube coupling 346, second tube fins 372 connect the second tube
inlet 342a to the
first tube 140. For the intake tube coupling 348, intake tube fins 374 connect
the intake tube
inlet 336a to the flared end portion 343 of the second tube 342. The end 337
of the intake
tube 336 can be welded directly to the combustion chamber 322, slightly past
the far end
327b of the slots 327. The penetration depth 360a of the first tube into the
second tube and
the penetration depth 360b of the second tube into the intake tube are also
identified. If the
intake system 314 is made of inetal tubes and nozzle, the fins can be affixed
by welding. It
will be understood that this latter structural configuration is an example
only and that many
other ways to hold the components of the intake system into operating position
can
alternately be used.
Turning now to Figs. 11 A to 11 C, an alternate configuration of a combustor
510 is shown. In
this example, the combustion chamber 522 also has a plurality of apertures
interspaced near
an acoustic center thereof, and in this case, the apertures are also
longitudinally oriented slots
527. The slots 527 also define an inlet 526 to the combustion chamber 522 and
the intake
system 514 is connected to this inlet 526. However, in this configuration, the
slots 527 are
disposed around the intake tube 536, and the intake tube 536 penetrates into
the combustion
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chamber 522. In the illustrated configurations, the intake tube 536 has a
closed end. In Fig.
I 1 B, the closed end is a flat end 529, whereas in Fig. I 1 C, the closed end
is a rounded end
531. In these examples, the intake system 514 is aligned with the axis of the
body 512 and is
oriented opposite the tail pipe 524. The combustion chamber 522 has a flat
front end 530
instead of the nose cone shown in the preceding examples. The flat front end
530 bridges and
closes the opening between the end of the straight section 534 and the intake
tube 536.
Turning now to Fig. 12, a schematic view of a turbine system 600 to recuperate
energy from
the hot gasses exhausted by in-line combustors 610 is shown. The combustors
610 are
aligned with their axes being circumferentially interspaced around a central
axis 605. A
power turbine 680 is positioned at the outlet 628 of the in-line combustor
exhaust pipes 624.
A fan 682 is positioned ahead of the in-line combustor intake systems 614 and
is connected
to the power turbine 680 by a shaft 684 along the central axis 605. In
operation, the power
turbine 680 rotates by extracting power from the combustion gasses and
transmits the rotation
to the fan 682 via the shaft 684. The fan 682 thus generates a stream of air
which adds to the
ejecting action of the ejector system 618 and increases the flow of air into
the combustion
chamber 622. Other turbine-based systems to recuperate energy from the
combustion gases
may alternately be used as well.
It shall be noted here that although combustors having generally cylindrical
bodies were
described above, other shapes can be used instead. For example, the combustion
chamber and
body can be made with a generally ellipsoidal cross-section in order to make
them somewhat
flatter. Further, although the use of the slotted intake was found satisfying
in the in-line
combustor described above, other types of apertures can be used instead. In
one alternate
embodiment, an annular slot making the entire periphery of the combustion
chamber can be
used instead of the plurality of slots. In a case where the combustion chamber
has an
ellipsoidal cross-section, a separate aperture on each respective flat side of
the combustion
chamber can be used instead of the plurality of slots, for example.
Further, although an intake system using three stages defined by the first
tube, second tube
and intake tube was used in the examples above, it is to be understood that a
different number
of stages can also be used.
The ejector system described above can be used in other applications than in
an intake
system. It can also be used in other types of intake systems. For example, in
Fig. 13, an
ejector system 718 is shown on an engine known as the Hiller-Lockwood engine
710.
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In the tests, combustors were made of steel. However, other materials can be
used as well.
One consideration is that the materials used have suitable acoustic
properties. One other
consideration is that the materials used have sufficient resistance to heat,
especially for the
combustion chamber and exhaust pipe.
In the example given above, the ejector system was used to eject a mix of fuel
and air and
was made part of an intake system of a combustor. However, it is believed that
many other
applications can also be made to the acoustically enhanced ejector system. It
can be used with
fluid nozzles which inject other types of fluids which may or may not be fuel,
and with
different ambient fluids. In the examples given above, the injected fluid was
fuel and the
ambient fluid was air.
It will be noted that other types of intake systems can be used to feed fuel
and air to a
combustor having an oscillator at a resonance frequency which is an odd
harmonic of the
pulsating frequency.
It will be understood that the overall size and shape of the combustors
described and
illustrated above can be varied to generate a combustor which is adapted to a
specific
application.
As can be seen therefore, the examples described above and illustrated are
intended to be
exemplary only. Hence, the scope of the improvements is intended to be limited
solely by the
scope of the appended claims.
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