TECHNICAL FIELD
The systems and techniques described include embodiments that relate
to techniques and systems for altering the location of deflagration-to-detonation
transition within a pulse detonation combustor. They also include embodiments that
relate to altering the ignition point for combustion within such a combustor.
^ BACKGROUND DISCUSSION
With the recent development of pulse detonation combustors (PDCs)
and engines (PDEs), various efforts have been underway to use PDCs/PDEs in
practical applications, such as combustors for aircraft engines and/or as means to
generate additional thrust/propulsion in a post-turbine stage. These efforts have been
primarily directed to the operation of the pulse detonation combustor, and not to other
aspects of the device or engine employing the pulse detonation combustor. It is noted
that the following discussion will be directed to "pulse detonation combustors" (i.e.
PDCs). However, the use of this term is intended to include pulse detonation engines,
and the like.
Typical operation of a pulse detonation combustor generates very high
speed, high pressure pulsed flow, as a result of the detonation process. These peaks
Jfe are followed by periods of significantly lower speed and lower pressure flow.
Because the operation of pulse detonation combustors and the detonation process is
known, it will not be discussed in detail herein. When a pulse detonation combustor
is used in the combustion stage of a gas turbine engine, the pulsed, highly transient
flow can produce significant pressure and heat at the location within the PDC tube at
which the combustion transitions from ordinary combustion (deflagration) into a
detonation. This may cause increased wear to the combustor at this particular
location. Because of this, such a location that experiences repeated transitions may
become a life-limiting factor for the operation of the combustor.
2
Therefore, in order to sustain long term operation of a PDC, it may be
desirable to control the location at which such a transition occurs along the length of
the combustor.
BRIEF DESCRIPTION
In one aspect of an embodiment of the methods described herein, a
technique for the operation of a pulse detonation combustor (PDC) is provided. The
PDC has a combustion tube, an inlet disposed on an upstream end of the combustion
tube, an enhanced DDT region located within the combustion tube downstream of the
H" inlet, a nozzle disposed on a downstream end of the combustion tube, a fortified
region disposed downstream of the enhanced DDT region and upstream of the nozzle,
and a combustion initiation system providing a plurality of initiation locations along
the length of the combustion tube.
In an aspect of the method, a flow of a fuel/air mixture is provided to
the combustion tube via the inlet. In a further aspect, the initiation system is activated
at a selected location along the length of the combustion tube to ignite the fuel/air
mixture within the combustion tube at the selected location. This allows selection of
an expected detonation region at which the combustion of the fuel/air mixture is
expected to transition to a detonation. In yet a further aspect, the combustion products
are exhausted from the combustion tube through the nozzle.
In an aspect of another embodiment of the methods described herein,
^ ^ the initiation system has a plurality of individual initiators, located at each of the
^ ^ initiation locations.
In an aspect of yet of another embodiment the initiation system has a
first electrode and a second electrode. The first electrode is within the combustion
tube and the second is disposed adjacent the tube, both extending from the furthest
upstream initiation location to the furthest downstream initiation location. At least
one of the two electrodes is selectively chargable or energizable along its length.
In an aspect of still another embodiment, the location of the transition
to detonation is determined, and the selected location at which the initiation system is
activated is adjusted based on the location of the transition.
3
BRIEF DESCRIPTION OF DRAWING FIGURES
The above and other aspects, features, and advantages of the present
disclosure will become more apparent in light of the subsequent detailed description
when taken in conjunction with the accompanying drawings, wherein like elements
are numbered alike in the several FIGs, and in which:
Fig. 1 is a schematic drawing showing an exemplary embodiment of a
pulse-detonation combustor (PDC) having multiple ignition sources;
^ Fig. 2 is a schematic drawing showing an embodiment of a PDC as in
Fig. 1 that has a fortified region that is physically reinforced;
Fig. 3 is a schematic drawing showing an embodiment of a PDC as in
Fig. 1 that has a fortified region that has enhanced cooling; and
Fig. 4 is a schematic drawing showing an exemplary embodiment of a
PDC that has a continuously variable ignition region.
DETAILED DESCRIPTION
In a generalized pulse detonation combustor, fuel and oxidizer (e.g.,
oxygen-containing gas such as air) are admitted to an elongated detonation chamber,
mk also referred to herein as a combustion tube, at an upstream inlet end. An ignitor is
used to initiate this combustion process, and may also be referred to as an "initiator".
Following a successful transition to detonation, a detonation wave propagates toward
the outlet at supersonic speed causing substantial combustion of the fuel/air mixture
before the mixture can be substantially driven from the outlet. The result of the
combustion is to rapidly elevate pressure within the combustor before substantial gas
can escape through the combustor exit. The effect of this inertial confinement is to
produce near constant volume combustion.
As noted above, key to achieving the elevated pressure of the
combustion is a successful transition from the initial combustion as a deflagration into
4
a detonation wave. This deflagration-to-detonation (DDT) process begins when a
fuel-air mixture in a chamber is ignited via a spark or other ignition source. The
subsonic flame generated from the spark accelerates as it travels along the length of
the tube due to various chemical and flow mechanics. As will be discussed below,
various design elements within the combustion tube, such as flow obstacles of various
descriptions, may be included in order to enhance the acceleration of the flame.
As the flame reaches critical speeds, "hot spots" are created that create
localized explosions, eventually transitioning the flame to a supersonic detonation
wave. The DDT process can take up to several meters of the length of the chamber,
depending on the fuel being used, the pressure and temperature of the fuel/oxidizer
'^J mix (generally referred to as "fuel/air mix", although other oxidizers may be used),
and the cross-section size of the combustion tube.
As used herein, a "pulse detonation combustor" is understood to mean
any device or system that produces pressure rise, temperature rise and velocity
increase from a series of repeating detonations or quasi-detonations within the device.
A "quasi-detonation" is a supersonic turbulent combustion process that produces
pressure rise, temperature rise and velocity increase higher than pressure rise,
temperature rise and velocity increase produced by a deflagration wave.
In addition to the combustion chamber or tube, embodiments of pulse
detonation combustors generally include systems for delivering fuel and oxidizer, an
ignition system, and an exhaust system, usually a nozzle. Each detonation or quasidetonation
may be initiated by various known techniques: such as external ignition,
mb which may include a spark discharge, plasma ignition or laser pulse, or by gas
dynamic processes, such as shock focusing, autoignition or by receiving flow from
another detonation (cross-fire ignition).
As used herein, a detonation is understood to mean either a detonation
or quasi-detonation. The geometry of the detonation combustor is such that the
pressure rise of the detonation wave expels combustion products out of the nozzle,
producing a thrust force, as well as high pressure within the exhaust flow. PDC's
may include detonation chambers of various designs, including shock tubes,
resonating detonation cavities and tubular, turbo-annular, or annular combustors. As
used herein, the term "chamber" includes pipes having circular or non-circular cross-
5
sections with constant or varying cross sectional area. Exemplary chambers include
cylindrical tubes, as well as tubes having polygonal cross-sections, for example
hexagonal tubes. In all examples described herein, combustion chambers of generally
cylindrical tubular form will be discussed; however, it is understood that these tubes
are merely exemplary, and that tubes of other cross sections that are not linear may
also be used with the techniques and systems described herein.
Within the discussion herein, the terms "upstream" and "downstream"
will be used to reference directions that are related to the flow path of the gas path
through the PDC. Specifically, "upstream" will be used to reference a direction from
which flow travels to a point, and "downstream" will be used to reference a direction
K^P from which flow travels away from a point. Therefore, for any given point within the
system, flow will proceed from the locations found upstream of that point, to that
point, and then to the locations downstream of that point. The terms may also be
generally used to identify an "upstream end" and a "downstream end" of a PDC or
other system containing fluid flow. Consistent with the use described above, an
upstream end of a system is the end into which flow is introduced into the system, and
the downstream end is the end from which flow exits the system.
Note that although local flow may include turbulence, eddies, vortices,
or other local flow phenomenon that result in unsteady or circulating flow that is
locally moving in a direction different than the overall direction that proceeds from
upstream to downstream within the system, this does not alter the overall nature of the
^ ^ upstream to downstream flow path of the system as a whole. For instance, flow
^ ^ around obstacles located within the flow path to enhance DDT may produce wake
flow that is not axial; however, the downstream direction remains defined by the axis
of the overall bulk flow, which corresponds to the axis of the combustion tube.
Within the context of a generally tubular form, such as a combustion
tube of a PDC (as will be discussed further below), the upstream and downstream
directions will generally be along the central axis of the combustion tube, with the
upstream direction pointing toward the intake end of the tube, and the downstream
direction pointing toward the exhaust end of the tube. These directions which are
6
generally parallel to the main axis of the tube may also be referred to as "axial" or
"longitudinal" as these directions extend along the lengthwise axis.
Furthermore, with reference to the axial direction of the PDC
combustion tube (or any other body having an elongated axis), a "radial" direction
will refer to a direction that extends along lines that point either directly toward the
axis (a "radially inward" direction) or directly away from the axis (a "radially
outward" direction). Purely radial directions will also be normal to the axis, while
angled radial directions may include both a radial and an upstream or downstream
component.
^ ^ A "circumferential" direction will be used to describe any direction
that is perpendicular to a purely radial direction at a given point, and also has no axial
component. Thus, the circumferential direction at a point is a direction that has no
components parallel to either the axis or the radial direction through that point.
One embodiment of a PDC is shown in Figure 1. The PDC 100
includes a valve 110 or other inlet on the upstream end of a combustion tube 120, also
referred to as a combustion chamber, through which air or other oxidizer is introduced
to the PDC during the fill phase of operation. Fuel is injected through an injector 130
near the upstream end of the combustion tube as well. Note that in alternate
embodiments, fuel and oxidizer may be mixed upstream of the tube and both
introduced together through the valve 110. The choice of whether to pre-mix or inject
^ ^ does not alter the nature of the discussion made herein, but may be varied based on
the nature of the fuel to be used, its pressure, the form of the fuel (e.g.: atomized
liquid, gas, vaporized liquid, etc.), and other factors.
The combustion tube 120 extends axially and ends in a nozzle 140,
through which combustion products will exit the tube during operation. An initiation
system 150, as discussed further below, is also included to begin the combustion
within the fuel/air mixture. The tube is desirably long enough to allow sufficient
space for the flame front of the combustion of the fuel/air mixture to accelerate and
achieve DDT.
7
Although the length required to achieve transition to detonation may
vary with various operating conditions (as will be discussed further below), it is
generally desirable to add features to the design and operation of the tube that increase
the rate at which the flame front accelerates. This helps to ensure that DDT is
achieved within the tube during operating conditions. An enhanced DDT region 160
is shown in the combustion tube 120, generally located downstream of the
introduction of fuel (whether by fuel injector 130 or by premixed flow through the
valve 110) and at least part of the initiation system 150, but upstream of the nozzle
140.
^ ^ The enhanced DDT region 160 in the embodiment illustrated in Figure
^ ^ 1 includes a plurality of obstacles 170 that are disposed at various axial stations along
the length of the tube 120 in the enhanced region. Such obstacles may take various
forms as are known in the art, which may include but are not limited to: plates
extending inwardly from the inner surface of the tube; bolts or other obstructions
which extend radially inward from the surface of the tube; perforated plates or flow
restrictions; surface texturing features, such as dimples, ridges or flanges; or spiral
tubes that extend along the length of the enhanced region.
The enhanced DDT region 160 accelerates the flame front at a faster
rate than the flame would accelerate in the absence of any obstacles, and thereby
helps the combustion run-up to the speed necessary to achieve transition to detonation
in less space (and time) than would be required in the absence of the enhanced region.
Such mechanisms provide the benefit of accelerating the flame front,
^m but also generally have larger surface areas and less structural strength than the
primary structure of the combustion tube. Because the durability of enhancements
such as obstacles 170 is generally less than that of the tube 120 itself, the obstacles
will become the life-limiting parts if not protected from the conditions associated with
the transition to detonation, as will be discussed further below).
In addition to varying based on the size and configuration of the tube
120 and the specific fuel/oxidizer mix used, the amount of run-up necessary to
produce DDT also varies based on factors such as the pressure and temperature of the
fuel/air mix within the combustion tube. As the pressure is increased, the length of
8
the run-up to DDT will decrease. Similarly, an increase in the temperature of the
fuel/air mixture will decrease the run-up distance required.
During operation of a PDC 100 which is part of a larger system, such
as a hybrid PDC-turbine powerplant for an aircraft, the PDC will be operated at a
variety of speed and throttle settings. These will vary the pressure of the mixture
being fed to the PDC, based on changes due to the ambient pressure varying from sealevel
to flight altitude, as well as pressure changes due to the effectiveness of the
compressor which feeds air to the PDC.
In a hybrid PDC-turbine engine, the compressor may be driven by
^ turbines placed downstream of the combustor exhaust. Therefore, the amount of
^ ^ compression achieved is also affected by the power output of the turbine, reflected by
the throttle settings for the engine. As a result, significant changes in the pressure and
temperature of the mixture fed to the PDC 100 can be experienced as the engine is
operated at conditions varying from ground idle (low power, high ambient pressure,
low compression) to take-off power (high power, high ambient pressure, high
compression) to high altitude cruise (moderate power, low ambient pressure,
moderate compression), to idle descent (low power, low but increasing ambient
pressure, low compression). Temperatures may also vary with altitude, as well as
with the heat soak of the engine's components, and ram-air effects can alter the
pressure of the mixture as well.
Because all of these operational factors can change the pressure and
temperature of the fuel/air mixture being fed into the PDC, the amount of run-up
Gk required to reach detonation will vary during the operation of the PDC. As a result,
the particular point at which detonation will be achieved will not always be at the
same distance downstream from the point of which the mixture is ignited. The axial
location downstream from ignition at which DDT occurs has been observed to vary
by up to 1 foot when the pressure is increased from one atmosphere to twenty
atmospheres, using the same tube and enhanced DDT regions.
At the point of transition to detonation, the pressure and heat produced
in the combustion process are maximized. This results in this region of the tube
experiencing higher mechanical loads than the remainder of the tube, including the
9
region downstream of the transition point, even though the combustion wave may
remain a detonation downstream from the point of DDT.
Instrumentation placed on the combustion tube have been used to
observe strain in the combustion tube at the point of DDT that may be as high as five
times higher than the strain associated with the theoretical pressure of a fully formed
detonation. Although testing has indicated that the pressure falls away from this peak
downstream of the transition point, downstream pressure may still be higher than the
pressure expected for an ideal Chapman-Jouguet detonation. In addition to the higher
pressure loading experienced at the point of DDT, experiments have shown that
^_ increased heating occurs at this point as well.
^ ^ Because of the increased energy release at the transition point, the PDC
is subjected to higher mechanical loading in the transition region. In order to
compensate for the higher energy release in this region, techniques can be adopted to
allow the PDC to better withstand these exceptionally high pressure and heat loads.
In general, the techniques will be related to either physically strengthening the PDC
tube in the region where the highest pressure loads will be experienced (as will be
discussed below with regard to Figure 2) or by increasing the ability of the PDC to
dissipate excess heat where the highest heat loads will be experienced (as will be
discussed below with regard to Figure 3).
However, such fortifications of the PDC 100 generally require adding
structure or cooling capability, which can increase the cost, complexity, and weight of
the PDC. Therefore, it is generally desirable to provide such fortification in as small a
Wm region of the PDC as possible. In addition, the upstream advance of the transition
point as pressure increases during operation can lead to DDT occurring within the
enhanced DDT region 160 in tubes that do not provide for sufficient separation
between the enhanced DDT region and the nozzle 140. Adding additional length to
the PDC tube 120 is undesirable because of the associated weight such additional
structure adds, as well as producing additional volume to fill during the fill phase, and
additional tube through which pressure drops may occur. However, allowing the
transition to occur within the enhanced DDT region is likely to damage the obstacles,
surface features, or other enhancements within this region, resulting in poor
10
performance, or an inability to achieve detonation at lower pressure operating
conditions.
Because the run-up distance is constrained by the factors indicated
above, the only way to adjust the detonation position within the PDC tube for a given
set of input conditions is to change where the combustion run-up begins, i.e. to select
a point of ignition for the combustion that results in a run-up to DDT that locates the
transition within a desired region, generally the fortified region. Such techniques can
also be used to ensure that detonation transition does not occur within the enhanced
DDT region, as well being useful to produce quasi-detonations, if desired. In one
^ ^ embodiment, this is accomplished with an initiation system 150 having a plurality of
^ ^ initiators located at different axial stations within the combustion tube of the PDC.
Combustion initiation may be performed by a variety of techniques, as
mentioned above. The initiation system illustrated in Figure 1 has a plurality of
individual initiators disposed at different points along the length of the tube 120. In
the illustrated embodiment, the initiation devices, which are also referred to as
ignitors, are spark ignitors, similar to those used as spark plugs in automotive engines.
While such spark ignition is simple to control and drive, the techniques discussed with
regard to this embodiment apply generally to any ignitor or initiation system that be
placed at separate discrete locations within the tube.
As can be seen in the Figure, a first ignitor 182 is located at a point
fairly far upstream along the tube 120, at an axial station downstream of the fuel
injector 130, but well upstream of the enhanced DDT region 160. A second ignitor
i ^ 184 is located just upstream of the enhanced DDT region, while a third ignitor 186 is
located within the enhanced DDT region itself. It will be understood that such
positioning can be varied, and additional ignitors might be located at additional
stations along the tube without deviating from the principles described herein.
In operation, the PDC system 100 of Figure 1 can use one or more of
the ignitors 182, 184, 186 to start the combustion of the fuel/air mixture once the tube
is sufficiently filled. For instance, in low pressure operation (for example, at initial
power up from idle), run-up may take a longer distance, and therefore the use of the
first initiator 182 located the farthest upstream within the tube 120 may be used to
start the combustion. When higher pressure operation is called for (for example,
11
operating at high power settings with maximum compression being provided by the
compressor), the shorter run-up required allows the use of an initiator further
downstream to still achieve complete transition to detonation at the desired location
within the PDC combustion tube.
The availability of multiple initiators in such an embodiment also
allows for the possibility of continued operation if one initiator is to fail, or if the
particular operating point of the engine is best served by triggering multiple initiators
simultaneously. These operating techniques may result in less efficient operation of
the PDC than if no failure had occurred, but can allow operation to continue, rather
^ than requiring the PDC to be shut down due to a single initiator failure.
^ * As discussed above, use of different initiators under different operating
conditions can be used to control the location of the transition to detonation within the
PDC tube. In most situations, it will be most desirable to control this location such
that it is made to occur within the region of the tube constructed to best handle the
repeated increased stresses associated with the transition. This region, referred to as
the "fortified region" herein, is shown in the embodiment illustrated in Figure 2, as
well as that shown in Figure 3.
Figure 2 schematically shows an embodiment of a PDC 200 that
includes the features shown in Figure 1 and also identifies an area of local
fortification positioned downstream of the enhanced DDT region 160 and upstream of
the exhaust nozzle 140. This fortified region 210 may be set up in various ways to
better resist the destructive effects that might be caused by the increased pressure and
^fc heat loads associated with the detonation transition.
As shown in the Figure, the fortified region 210 may contain an
additional sleeve 220 of material that surrounds the combustion tube 120 in the
fortified region and provides reinforcement against physical stresses. The additional
thickness of material may also provide for increased capacity to absorb heat.
It will be recognized that alternative forms of structural reinforcement
to the sleeve may also be used. These may include: discrete bands wrapped around
the tube in place of the sleeve; a variation in cross-sectional thickness of the wall of
the tube in the region being reinforced; longitudinal flanges extending along the
outside of the reinforced region; variations in material composition that provide
12
different strength, flexibility, or heat resistance in the reinforced region; and such
other techniques as are known in the art.
Strain gauges 230 are also included and are disposed upon the
combustion tube 120 at various locations along its length. These may be placed in
regions near where transition to detonation is expected to occur. The strain gauges
can be used to identify where the strain in the material of the tube is largest, and
therefore to determine approximately where DDT is occurring. This information can
be used to select the appropriate ignitor 182, 184, 186 to activate during operation in
order to move the point of transition to the desired location and to maintain DDT
4 } within the fortified region 210 of the tube. In a particular embodiment, the strain
gauges are generally disposed upon the outer surface of the combustion tube so as to
protect them from the effects of the combustion and detonation waves within the tube.
Figure 3 shows a schematic view of a PDC 300 system that includes
the features of Figure 1 and a fortified region 210 that includes improved heat
resistance. In the illustrated embodiment, the combustion tube 120 of the PDC 300 is
disposed within a cooling fluid path 310. In operation, a cooling fluid with a lower
temperature than the temperature of the wall of the combustion tube is passed through
the fluid path in order to absorb heat from the tube and transfer the heat into the
cooling fluid. In the illustrated embodiment, the cooling fluid path is a reverse-flow
fluid path, i.e. the flow through the cooling fluid path is along the outside of the
combustion tube in a direction which is upstream with respect to the combustion tube.
^ P Those of skill in the art will recognize that other cooling fluid path geometries may be
used, and that a reverse-flow path is not required for effective operation of every
possible embodiment.
In addition, the illustrated embodiment shows a reduced cross sectional
area 320 of the cooling fluid path 310 in the fortified region 210. This reduced crosssectional
area increases the flow speed through this region, which increases the heat
transfer from the combustion tube 120 to the cooling fluid in this area, and provides
for a greater resistance to high heat for this portion of the tube. The reduced crosssectional
area also results in an increased pressure drop within the cooling fluid in this
13
region, so it is desirable to minimize the portion of the cooling fluid path that has this
reduced cross sectional area.
It will be appreciated that in the illustrated embodiment, the cooling
fluid is air that is passed through the valve 110 into the PDC combustion tube 120
later on to be mixed with fuel and burned. Such a flow arrangement allows for
extraction of heat from the combustion tube while also pre-heating the charge of air
being input into the tube. This arrangement is not required in order to provide for a
fortified region 210 with enhanced cooling, and other arrangements may be used as
are known in the art.
For instance, in alternative embodiments, the cooling fluid may flow in
a direction that is downstream with respect to the combustion tube along the outside
of the combustion tube. In another alternative embodiment, the cooling fluid may be
bypass air from elsewhere within the engine system, or air taken from the ambient
flow around the engine. In further alternative embodiments, the cooling system could
make use of liquid as a cooling fluid, or other cooling techniques could be applied as
are known in the art.
In addition to the cooling fluid path having a reduced cross-section in
the fortified region, other alternative embodiments may make use of surface features
within the cooling fluid path in order to improve the heat transfer through the tube in
this region. For instance, in an alternative embodiment, turbulators may be disposed
^ ^ on the outer surface of the combustion tube within the fortified region to increase the
local flow vorticity in this region in order to increase the heat transfer from the
surface into the cooling fluid. Other alternative embodiments may use a flow path
that has increased mass flow in the fortified region, or a separate cooling system with
a greater heat transfer capacity for this region of the combustion tube.
In other alternative embodiments, ribbing on the outer surface of the
tube, or fins disposed along the outer surface of the tube may be used to increase the
surface area available for heat transfer into the cooling fluid. Still other alternative
14
embodiments may make use of impingement cooling in this region, or additional
cooling techniques as are known in the art.
In operation, the systems described herein operate on the basic PDC
cycle: the tube 120 is filled with a mixture of fuel and air, air being introduced
through a valve 110 or inlet and fuel through a fuel injector 130; the fuel/air mixture
is ignited using the initiation system 150; the combustion propagates and accelerates
through the mixture, transitioning into a detonation as it accelerates down the length
of the combustion tube; the exhaust products are blown out of the exhaust end of the
tube through the nozzle 140; and then a new charge of air is introduced into the tube
^ } to clear out any exhaust products and begin the fill process for the next detonation
cycle.
In particular, in order to take advantage of the plurality of combustion
initiation locations along the length of the tube, additional steps may be performed. In
one embodiment, the strain gauges 230 (or other instrumentation) are used to
determine the location along the length of the combustion tube 120 at which transition
to detonation occurs for each cycle. Once that location is determined, it is possible to
know whether detonation is occurring within the desired region of the combustion
tube or not. This will generally be desired to occur within the fortified region,
although in particular alternate embodiments, detonation could be desirable in other
portions of the tube for particular operating conditions, for example, in the throttling
embodiment described below.
If detonation transition is not occurring within the desired region, a
different initiation location may be selected that adjusts the starting point of the runup
to detonation in order to relocate the detonation of a subsequent cycle within the
desired region. For instance, if detonation is being detected moving further upstream
and outside of the fortified region 210, initiation of a subsequent cycle may be made
using an ignitor that is located at a further downstream location within the tube in
order to shift the detonation back into the fortified region.
15
In other embodiments, the system may use a control map that identifies
the appropriate initiation location to be used for a variety of operating conditions and
parameters. These can include the pressure and temperature of the fuel/air mixture,
the power or throttle setting requested for the PDC (or the engine as a whole), the
operational status of various portions of the system, such as the ignitors and the
obstacles in the enhanced DDT region, and the ambient temperature and strain history
of specific regions within the combustion tube.
In practice, these techniques may be combined to provide both a
control map for default settings, as well as a closed-loop system that responds to
^ p particular conditions within the engine. For instance, while an ignition location might
be chosen based on the control map in order to locate transition within the fortified
region based on the operating conditions, the choice of ignition location may be
varied slightly around this base position in order to spread out the peak stress and
thermal load in subsequent cycles. In this way, wear along the length of the fortified
region may be evened out so as to prevent a pre-mature failure in one portion of the
system due to extended periods of time being spent in a particular operating mode that
places the DDT at a single location.
Techniques such as those described above can be used to improve the
operational lifetime of the PDC and its components. By keeping the detonation
transition within those portions of the tube best able to survive the additional stresses
and heat imposed by the DDT, the overall life of the PDC is improved. Furthermore,
^ P even within fortified regions, the periodic relocation of the transition point can reduce
the repeated stresses felt by any one point within the region, prolonging the life of the
fortified region as well. In addition, by detecting when detonation is not occurring
properly, or is happening in portions of the tube that can be damaged by detonations,
for instance the enhanced DDT region, those regions are protected from wear that
would otherwise reduce the operational life of those components as well.
In addition to improving the operational life of various components,
the techniques described herein can be used to produce a throttling effect across
multiple tubes. For instance, there may be operating conditions in which it is or
16
desirable to achieve only a quasi-detonation (an accelerated flame front at a higher
speed and pressure than a deflagration, but less than the Chapman-Jouguet detonation
pressure achieved by a fully shock-driven combustion front) rather than a full
detonation. In these conditions, using an ignition location that is further downstream
than that which would result in transition within the combustion tube will result in no
actual DDT, and therefore will eliminate the increased energy release (and its heat and
pressure peaks) at that point associated with the transition. This helps to preserve the
life of the mechanical systems, while still providing an increase in efficiency over a
pure deflagration system.
^ P In such operating modes, the systems and techniques described herein
can be used to make sure that an ignition station far enough downstream is selected
that no detonation peak is achieved before the flow is blown out through the nozzle.
This reduces the energy of the exhaust gas flow, and can therefore be used as a
throttling mechanism that would not be possible with a single ignition location along
the length of the tube. Such operating modes may also be beneficial for use when
temperature limits in the fortified region of the tube are exceeded, and a temporary
reduction in heat release into the tube is required. This technique does not require
altering the fill-fraction of the combustion tube.
In operation, a single engine may have multiple tubes, all firing into a
single turbine located downstream of the PDC nozzles. The techniques described
herein, and the systems described with respect to a single PDC, may be applied to
^ P each PDC within a multiple tube system. This may provide advantages not just in
positioning the detonation of each of the tubes in the same manner as the other tubes
for a given operating point, but for having different tubes operate in such a way to
achieve their detonations at slightly different points. This can be significant for
controlling vibration or resonance effects, as well as distributing the heat and thermal
loads of the transition point across a broader length of the engine.
For instance, in a system having a plurality of tubes, not all tubes need
be operated to produce DDT at the same location. This may be used to allow shared
fortifications (such as cooling) to be more effectively distributed among the various
17
tubes. The throttling techniques described above may also be used on some tubes and
not others within the cycle in order to allow an over-stressed tube to be cooled while
still operating.
It will be recognized that although the systems are described above
with respect to the particular embodiments illustrated in the Figures, that various
alternatives to the specific configurations illustrated can be used. For instance,
although the spark initiators in Figure 1 have been illustrated as all having the same
circumferential station in the combustion tube (i.e., they are all shown as descending
from the top of the tube), the initiators can be distributed at various circumferential
flfe positions around the tube, as may be desirable for packaging reasons.
In addition, it may be desirable to place multiple initiators at the same
axial station along the tube in order to provide redundancy or improved ignition
performance. In some embodiments the ignitors at the same axial station may be
triggered simultaneously in order to distribute the ignition kernels within the tube. In
other embodiments, the ignitors at a single station may be used separately. In other
embodiments, it may be desirable to use more than one ignition location within a
single detonation cycle to address damage to some of the ignitors or to help speed up
the acceleration of the flame front.
The placement of initiators within the enhanced DDT region,
especially for high pressure operation, may also be desirable in certain embodiments
^ ^ and operating techniques. Variations in placement within the enhanced DDT region
are also possible. For instance, in some embodiments, placement of an initiator in the
wake of a flow obstacle within the enhanced DDT region may be useful to achieve
ignition with lower ignitor power, as well as providing protection from the direct
impact of the propagating flame front on the initiator, which may improve the
operating lifetime of those initiators located within the combustion region.
An alternate embodiment for an initiation system that can provide a
continuously variable location for ignition along the length of the initiator is
illustrated in schematic form in Figure 4. The PDC systems described with respect to
18
Figures 1-3 showed an initiation system 150 that made use of individual initiators,
specifically spark ignitors, each of which were disposed at discrete locations along the
axial length of the tube. However, other ignition systems may be configured to
provide for a variable initiation location that is not limited to discrete locations, but
can be varied continuously within a region. I
In the embodiment of a PDC 400 illustrated in Figure 4, a plasma
initiation system 410 is illustrated that would provide such a feature. Although the
other features of Figure 1 are present in essentially the same manner, the individual
ignitors are replaced with a pair of plasma electrodes: an inner electrode 420 disposed
^ P generally centrally within the combustion tube 120; and an outer electrode 430 that is
disposed along the wall of the combustion tube. Both electrodes extend over an axial
length of the PDC. At least one of the electrodes is able to be partially energized so
that only a portion of its length is charged. Although not required for the operation
described herein, this is easier to achieve with the outer electrode by forming it from a
plurality of coils that spiral along the tube, and which can be electrically connected to
the control system at a variety of positions. By energizing more coils along the outer
electrode, a control system can effectively energize as much or as little of the outer
electrode as is desired.
Because the plasma initiator 410 works by creating a highly ionized
region where a plasma can form, the initiator will only create the desired plasma
between the energized portions of the two electrodes. Energizing only a portion of
^ P the outer electrode will allow the control system to position the plasma between the
energized portion of the outer electrode 440, and the nearest portion of the inner
electrode 420. In this way, the control system can locate the plasma, and therefore the
combustion ignition of the fuel/air mixture, anywhere along the energizable length of
the electrodes.
This embodiment can provide a more precise degree of control over
the selected initiation location, and can be especially effective when a small variation
in ignition point is desired, for instance to fine tune operation around a base operating
19
point, or to produce small variations in the detonation point to limit continuous overstress
to a single point within the fortified region.
The various embodiments described herein may be used to provide
improvements in operating life and efficiency for PDCs. They may also be used to
provide for a more flexible control environment for operation of a PDC. Any given
embodiment may provide one or more of the advantages recited, but need not provide
all objects or advantages recited for any other embodiment. Those skilled in the art
will recognize that the systems and techniques described herein may be embodied or
carried out in a manner that achieves or optimizes one advantage or group of
Gm advantages as taught herein without necessarily achieving other objects or advantages
as may be taught or suggested herein.
This written description may enable those of ordinary skill in the art to
make and use embodiments having alternative elements that likewise correspond to
the elements of the invention recited in the claims. The scope of the invention thus
includes structures, systems and methods that do not differ from the literal language
of the claims, and further includes other structures, systems and methods with
insubstantial differences from the literal language of the claims. While only certain
features and embodiments have been illustrated and described herein, many
modifications and changes may occur to one of ordinary skill in the relevant art.
Thus, it is intended that the scope of the invention disclosed should not be limited by
the particular disclosed embodiments described above, but should be determined only
^ p by a fair reading of the claims that follow.
20
Parts List
Number Item Notes
100 PDC
110 Valve
120 Combustion tube
130 Fuel injector
140 Nozzle
150 Initiation system
160 Enhanced DDT region
170 Obstacles
•
182 First Individual initiators/ignitors
184 Second ignitor
186 Third ignitor
"200 PDC
210 Fortified region
220 Sleeve
230 Strain gauges
~300 PDC
310 Cooling fluid path
320 Reduced cross-sectional area
lOQ PDC
410 Plasma initiation system
420 Inner electrode
430 Outer electrode
440 Active portion of electrode

WE CLAIM:
1. A method of operating a pulse detonation combustor (PDC) having a combustion
tube, an inlet disposed on an upstream end of the combustion tube, an enhanced
DDT region located within the combustion tube downstream of the inlet, a nozzle
disposed on a downstream end of the combustion tube, a fortified region disposed
downstream of the enhanced DDT region and upstream of the nozzle, and a
combustion initiation system providing a pliu-ality of initiation locations along the
length of the combustion tube, the method comprising:
providing a flow of a fuel/air mixture to the combustion tube via the inlet;
activating the initiation system at a selected location along the length of the
combustion tube to ignite the fuel/air mixture within the combustion
tube at the selected location, in order to select an expected detonation
region at which the combustion of the fuel/air mixture is expected to
transition to a detonation;
exhausting the combustion products from the combustion tube through the
nozzle.
2. The method of Claim 1, wherein the initiation system comprises a plurality of
individual initiators, at least one of which is disposed at each of the plurality of
^ ^ initiation locations, and the activating step comprises using the individual initiator
located at the selected location.
3. The method of Claim 2, wherein at least one of the plurality of individual
initiators is disposed upstream of the enhanced DDT region and at least one of the
plurality of initiators is disposed within the enhanced DDT region.
4. The method of Claim 1, wherein the initiation system comprises a first electrode
disposed within the tube and extending at least from the furthest upstream
initiation location to the furthest downstream initiation location, and a second
electrode disposed adjacent to the tube, wherein the activating step comprises
22
charging the electrodes to opposite electrical polarities and selectively charging at
least one of the electrodes only in the selected location.
5. The method of Claim 1, wherein the expected detonation region is the fortified
region of the combustion tube.
6. The method of Claim 1, further comprising;
detecting the location of the transition to detonation of a previous combustion; and
adjusting the selected location at which to activate the initiation system based on
^ ^ the location of the transition of the previous detonation.
7. The method of Claim 6, wherein the detection of the location of transition is
performed by measuring the strain in the combustion tube at multiple locations
along the length of the combustion tube.
8. The method of Claim 6, wherein no transition to detonation is found to occur
within the combustion tube.
9. The method of Claim 1, further comprising varying the selected initiation location
based on at least one of the pressure of the fuel/air mixture and the temperature of
the fuel/air mixture.
10. The method of Claim 1, further comprising varying the selected initiation location
^ ^ based on a commanded throttle setting for the PDC.
11. The method of Claim 1, wherein the selected detonation region is not within the
length of the combustion tube, such that no detonation of the fuel/air mixture
occurs within the combustion tube.
12. The method of Claim 1, further comprising:
detecting a mechanical failure within the enhanced DDT region; and
adjusting the selected location at which to activate the initiation system in order to
compensate for the mechanical failure.
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13. The method of Claim 12, wherein the mechanical failure results in reduced
effectiveness of the enhanced DDT region in accelerating the combustion of the
fuel/air mixture.
14. The method of Claim 1, further comprising:
detecting a mechanical failure within the enhanced DDT region; and
adjusting the selected location at which to activate the initiation system in order to
compensate for the mechanical failure.