Note: Descriptions are shown in the official language in which they were submitted.
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SELF SUSTAINED DETONATION APPARATUS
. =
OBJECT OF THE INVENTION
The present invention generally relates to the
field of gas detonation coating technology and, more
particularly is concerned with increasing the detonation
rate of a gas detonation coating apparatus through self
sustained detonation.
A self sustained detonation apparatus, like the
one described in the present invention, is also related to
the "Pulse Combustion Devices'". These have been developed
mainly for propulsion applications (from the early "Pulse
Jets", like the German Vi "Buzz Bomb" used in World War
II, to the more recent "Pul sed Detonation Engines", PDE's)
but have also been found to be valuable for applications
such as drying, smelting, water heating and slurry
atomization. This invention is concerned with the
development of a particular "Pulse Detonation Device" to
be used, specially but not exclusively, as a Detonation
Coating Apparatus.
BACKGROUND OF THE INVENTION
Coatings commonly protect substrates from the
effects of exposure to severe environmental conditions
such as heat, wear and corrosion. A significant factor in
the coating's protection ability relates to the manner in
which the coating is applied to the substrate. In many
industrial applications, coatings are applied via thermal
spraying techniques. Two (2) types of thermal spraying
apparatus include HVOF (High Velocity Oxygen Fuel) guns
and detonation guns.
In a HVOF gun, a continuous high temperature
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combustion creates a supersonic high energy flow stream.
A coating powder interjected into the continuous high
energy flow stream,- typically within the barrel of the
HVOF gun, forms a coating when applied to a substrate. In
contrast, the detonation gun, which operates in a pulsed
manner, uses kinetic and thermal energy from the
detonation of combustible gases to deposit powdered
coating materials onto substrates in a pulsed manner. A
combustion chamber receives a certain amount of fuel and
oxidant gas. A spark plug ignites the combustible gas
mixture to initiate combustion which transforms into
detonation. The shock wave formed by this detonation
travels at a supersonic speed from the combustion chamber
into the barrel where a suitable coating powder is
typically injected. The shock wave and further expanding
detonation products propel the coating powder out of the
barrel and deposit it onto a-substrate, thereby forming a
coating layer. This,process repeats until the substrate
obtains a sufficient coating thickness. In some detonation
spray systems, between successive ignitions, an inert gas,
such as nitrogen, is fed into the combustion chamber to
halt combustion and prevent backfire into the fuel and
oxygen supply, and to purge the combustion chamber and
barrel of combustion detonation products.
The mechanics of detonation are key to the
operation of the detonation gun. Detonation produces shock
waves that travel at supersonic velocities, as high as
4,000 meters per second (m/s), and elevated temperatures,
as high as 3,000 degrees Celsius. Detonation within the
detonation gun is controlled by the type and amount of
fuel (i.e., natural gas, propane, acetylene, butane,
etc.), the fuel and oxygen mixture ratio, the initial
pressure of the gases in the combustion chamber, and the
geometry of the combustion chamber. Cycled ignition of a
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portion of the combustible mixture creates combustion
which increases the entropy within the combustion chamber
and, in turn, propagates ignition of the combustible
mixture throughout the combustion chamber. With the
correct combination of parameters which result in
sufficient local pressure and temperature within a given
volume, accumulated combustion energy provides transition
to detonation.
At a fixed moment in time the detonation wave
front is made up of a system of individual detonation
cells. The behavior of detonation at the cell level is an
important attribute in the control and operation of a
typical detonation gun. The detonation cell is a
multidimensional structure, which is formed under
influence of both the shock wave front and transverse
shock waves. The propagation of the shock wave front,
created by detonation, is perpendicular to the inner
circumference of the combustion chamber and it is directed
from the closed end of the combustion chamber to the open
end of the combustion chamber. Transverse shock waves also
form at the inner circumference of the combustion chamber
and move toward and out the central line of the combustion
chamber. Under the current description, a detonation wave
constitutes the final case of the multidimensional
structure of the detonation front that includes a number
of traverse shock waves.
The frontal surface of a detonation cell has a
convex shape. Behind the frontal surface is a reaction
zone where the chemical reactions take place. At the edge
of the cell, transverse shock waves form at substantially
right angles to the frontal surface of the detonation
cell. The transverse waves have acoustic tails that extend
from the aft edges of the transverse waves and define the
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aft edge of the detonation cell. The transverse waves move
from cell to cell and reflect off of each other and off of
any limiting structure such as the combustion chamber
wall. Once detonation has been initiated, the reaction
continues in a fairly stable fashion if subsequent
detonation cycles are initiated and maintained under
similar conditions as the previous detonations.
The shock wave moves from the closed end of the
combustion chamber toward the open end of the combustion
chamber and into the barrel. It is of _particular
importance that the combustion chamber be of-sufficient
length and sufficient diameter to complete the transition
from combustion to detonation before entering the barrel,
otherwise, the accumulated energy may dissipate within the
barrel. It is also important in the operation of a
detonation gun to produce a shock wave and direct it to
the barrel as efficiently as possible so that a large
amount of the kinetic and thermal energy of the gaseous
detonation products goes directly to carrying the powder
out of the barrel and onto the substrate. However,
reflecting transverse waves colliding with other wave
structures can collapse, thus diminishing both the speed
of the detonation wave and the transfer of detonation
energy as it travels through the combustion chamber. These
collisions reduce the amount of the energy available to be
transferred to the coating powder which decreases the
adherence characteristics between the coating and the
substrate and lowers the density of the coating itself.
The size of the detonation cell is another important attribute in the control
and operation of a
detonation gun. Cell size is a function of the molecular
nature of the fuel, the initial pressure within the
combustion chamber and the fuel/oxygen ratio. The
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particular cell size for certain conditions can be
determined experimentally._The width of a cell, Sc, is
measured along the wave front between successive
= transverse waves. The length of a cell, Lc, is the
perpendicular distance from a line tangent to the wave
front measured to the intersection point of the acoustic
tails from adjacent transverse waves. The typical ratio of
cell width, Sc, to cell length is Sc = 0.6Lc for the
detonable gases under consideration. The physical
parameters of a particular detonation gun, such as the
geometry and operating pressures, are determined by the
cell size of a particular fuel and oxygen mixture.
In a typical detonation gun the components of the
detonable mixture are fed into the combustion chamber and,
the coating powder is fed directly into the barrel by
inert gases ahead of the detonation wave. A certain gas
content system and different gases supplied from a
continuous source through a valve arrangement of the gun.
For example, the operation of the powder valve is
coordinated with the firing of the spark plug so that the
powder and carrying gases are in position along the barrel
to be properly effected by the detonation wave. Typically
the gas control valves are opened by mechanical means such
as a cam and tappets or a solenoid which pose reliability
problems in that they have rapidly moving pieces. The
powder valve is responsible for the transportation of the
powders that tend to be abrasive in nature leading to gun
life cycle and maintenance concerns. in addition, valves
pose safety concerns in that a valve that leaks, sticks
open or breaks gives an alternate and potentially harmful
path for the detonation products to escape. A further
disadvantage of these mechanisms is that they often limit
the frequency at which the gun can fire because the valve
must be opened far enough and long enough to permit the
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passage of the proper amount of gas through the valve.
The rate at which a detonation gun deposits the
coating powder on the substrate is an important economic
parameter in industrial applications. The deposition rate
is controlled, and at times limited, by a variety of
factors such as the type of fuel, the fuel supply system,
the geometries of the combustion chamber and barrel, the
powder feeder system, the purging of the system between
successive initiations and the frequency with which the
combustible gas mixture detonates. Deposition rate is
expressed as the ratio between the spray rate and the area
sprayed ("spray spot square" ). The spray rate is stated in
terms of the mass of coating powder utilized per unit
time, typically Kg/hr, and typically ranges from 1 to 6
Kg/hr. Spray rate is obviously influenced to great extent
by the rate at which the combustible gas mixture
detonates. In a typical detonation gun a spark plug is the
means to ignite the combustible gas mixture and detonates
at the maximum rate of 6 to 10 times per second. The spray
spot square is the area coated by a single detonation of
the gun and is roughly equal to the area of the barrel and
is typically expressed as mm2. A typical industrial
detonation gun has a deposition rate of about 0.001 to
0.02 Kg/mm2-hr.
In the typical detonation gun the combustible
fuels and oxygen are supplied either into a mixing chamber
or directly into the combustion chamber itself through a
series of valves. The combustible gases are supplied under
pressure of about 1 to 3 MPa from a continuous source to the valve system
before being issued into the gun. As
discussed previously, a valve system, as employed in a
typical detonation gun, raises serious concerns about
rate, reliability and safety.
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An important characteristic affecting coating
quality is the supersonic velocities at which the shock
waves travel. The shock wave initiates the acceleration of
the coating powders, while the detonation products move
the coating powders to produce high density coatings with
better adhesive qualities than other spray coating
methods. The velocity of the coating powder as it exits
the barrel is influenced by, among other things, the type
of fuel used and the geometries of the combustion chamber
and barrel. Typical detonation wave velocities for
detonable gas mixtures are about 1,200 m/sec to about
4,000 m/sec. For example hydrogen-oxygen detonation wave
velocities are about 2,830 m/sec and methane-oxygen are
about 2,500 m/sec. The maximum achievable velocity in
prior art detonation gun configurations is approximately
3,000 m/sec.
Another characteristic effecting coating quality
is the temperatures surrounding the operation of a
detonation gun which effects the coating density. In order
to apply a dense coating, the powder must melt within the
barrel of the detonation gun. The higher the adiabatic
flame temperature of the combustible gas mixture, the
easier it is for the coating powder to melt. Typical
adiabatic flame temperatures for detonable gas mixtures of
concern range from about 1.,900 C to about 3,200 C, with
hydrogen-oxygen about 2,807 C and methane-oxygen about
2,757 C. The heat imparted to the powders is a function
of many parameters including the barrel geometry and the
active cooling of the barrel. These temperatures are high
enough to melt most substrate materials, however, the
discontinuous nature of the detonation within a detonation
gun and the quick heat dissipation in the atmosphere
between the gun barrel and the substrate prevents the
substrate from being adversely affected.
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The use of non-combustible gases, inert gases, in
the operation of a detonation gun also effects the quality of the coatings
produced by reducing the density of the
coating as well as adversely effecting the adhesion 5 characteristics between
the coating and the substrate.
Three common uses of non-combustible gases in detonation
gun operations include: 1. purging gases; 2. powder
carrier gases; and 3. a control on the detonation process.
Purging gases typically are inert gases and are used
primarily to purge the combustion chamber between
successive firings of the spark plug to arrest the
combustion process. This is important in the typical
detonation gun because the combustion chamber must be
filled between successive firings of the spark plug with
new amounts of combustible fuel and oxygen mixture through
a series of valves. If combustion continued in the
combustion chamber while the valves are opened it is
possible that the combustion would continue into the fuel
and oxidant gas supply and cause an explosion. One ofthe
problems with using purging gases is that they mix with
the combustible gases and lower the overall energy of the
detonation. Consequently, the heat and kinetic energy
available for transferring to the coating _powders is
reduced and coating density and adhesion are adversely
affected.
Powder carrier gases, frequently compressed air,
are typically used to transfer the coating powders from a
reservoir to the barrel of the detonation gun in front of
the detonation wave. In large quantities, these gases also
reduce the kinetic energy available for transfer to the
coating powders since they decrease the temperature and
velocity of the detonation wave front. The effect on }
coating quality is evidenced by a lower density coating
and poor adhesion to the substrate. Finally, inert gases
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are also mixed with the detonable gases as a control on the
detonation process. These gases are typically used in small
amounts to control the temperature, velocity and chemical
environment of the detonation products, and the detonation
stability.
What is needed in the art is a unique self
sustained detonation gun.
DESCRIPTION OF THE INVENTION
The present invention relates to an apparatus and
a method for producing detonation through self sustained
detonation. The self sustained detonation apparatus
comprises a combustion chamber, a means for introducing fuel
and oxidant gas to the combustion chamber to form a mixture,
a means for igniting the combustible fuel and oxidant gas
and a means for creating a secondary pressure within the
combustion chamber in order that the means for creating a
secondary pressure combines with the means for igniting to
establish an environment which provides self ignition of the
mixture and initiates subsequent detonation.
An aspect of the invention provides a self
sustained detonation apparatus, comprising: a combustion
chamber having a closed end, an open end, and a volume
sufficient to initiate detonation therein; a means for
introducing fuel to said combustion chamber; a means for
introducing oxidant gas to said combustion chamber; a means
for igniting the fuel and the oxidant gas in said combustion
chamber; and a means for creating a secondary pressure
within said combustion chamber, wherein after a detonation,
said means for igniting, in combination with said means for
creating a secondary pressure, establishes an environment
which causes fuel and oxidant gas to ignite and initiate a
subsequent detonation.
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The method includes the steps of: (a) supplying
fuel and oxidant gas to the combustion chamber; (b) igniting
the fuel and oxidant gas to produce a detonation wave; and
(c) creating a secondary pressure within the combustion
chamber wherein the secondary pressure coupled with the
means for igniting combines to provide the appropriate
conditions for self ignition of the fuel and oxidant gas to
initiate the next detonation.
Another aspect of the invention provides a self
sustained detonation apparatus, comprising: a barrel having
an inlet and an exit; a means for introducing a powder to
the apparatus such that the powder departs the apparatus
through said exit; a combustion chamber having a closed end,
an open end, an ignition section, a converging section, and
a volume sufficient to initiate detonation prior to said
converging section, said ignition section extending from
said closed end to said converging section, said converging
section having an upstream opening and a downstream opening
which forms said open end, said upstream opening converges
to said downstream opening at a sufficient angle R to cause
detonation waves to reflect off said converging section back
toward said ignition section to create a secondary pressure
within said combustion chamber; a means for introducing fuel
to said combustion chamber; a means for introducing oxidant
gas to said combustion chamber; a means for preventing
combustion from extending from said combustion chamber into
said means for introducing oxidant gas; and a means for
preventing combustion from extending from said combustion
chamber into said means for introducing fuel; and an
initiating element capable of attaining a temperature
sufficient to ignite the fuel and oxidant gas in combination
with the secondary pressure, said initiating element located
in said combustion chamber.
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A further aspect of the invention provides a
method for sustained detonation in a detonation coating
apparatus, comprising the steps of: supplying fuel and
oxidant gas to a combustion chamber; igniting said fuel and
oxidant gas to create a detonation wave; creating a
secondary pressure within said combustion chamber; and
heating an initiating element to a sufficient temperature to
ignite additional fuel and oxidant gas within the combustion
chamber at said secondary pressure, which will cause a
subsequent detonation.
The present invention will now be described and
explained in greater detail with reference to the
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embodiments shown in the drawings. The features shown and
described in the specification and the drawings are merely
exemplary and may be used in other embodiments of the
invention either individually or in any desired
combination there of.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view, partially in section, of
one embodiment of the detonation gun of the present
invention.
FIG. 1A is an enlarged view of one embodiment of
the initiating element of the present invention
illustrated in FIG. 1.
FIG. 1B is an -enlarged view of an alternative
embodiment of an initiating element of the present
invention illustrated in FIG. 1. _
FIG. 2A is a plan view of a section of a
detonation gun of the present invention. --
FIG. 2B is a pressure versus time graph of a
detonation gun of the present invention.
FIG. 3A is an alternate plan view of a section of
a detonation gun of the present invention.
FIG. 3B is an alternate pressure versus time graph
of a detonation gun the present invention. FIG. 4 is a reflection secondary
pressure versus
angle f3 graph of a -detonation gun of the present
invention.
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PREFERRED EMBODIMENT OF THE INVENTION
Ignition of a combustible gas mixture, which is a
combination of fuel and oxidant gas, is dependent upon the
temperature and pressure within a combustion chamber and
the composition of the combustible gas mixture, while
detonation is dependent upon the temperature and pressure
within the combustion chamber and the volume thereof. For
the following description assume that the composition of
the combustible gas mixture remains constant. Therefore,
as pressure within the combustion chamber increases, the
required ignition temperature decreases and vice versa,
within certain limits. Upon ignition of the combustible
gas mixture, the temperature and pressure increase, to a
level where the amount of energy accumulated attains its
detonation point, detonation is initiated and a detonation
wave begins to propagate through the combustion chamber.
FIG. 1 illustrates a detonation coating apparatus,
such as a detonation gun generally designated 10. The
detonation gun 10 includes a fuel supply 12, an oxidant
gas supply 14, a mixing chamber 16, a combustion chamber
26, a barrel 36, a powder supply 38, a spark plug 40 and
an initiating element 44.
The fuel and the oxidant gas can be combined in
the mixing chamber 16 to produce a substantially
homogenous combustible gas mixture prior to entering the
combustion chamber 26. The fuel supply 12 provides fuel
(i.e., natural gas, propane, etc.) to the mixing chamber
16 while the oxidant gas supply 14 provides oxidant gas
( i. e., oxygen or air) . It is generally preferred that the
combustible gas mixture enter the ignition section 28 of
the combustion chamber 26 in order that combustion occur
as close as possible to the closed end 46, thereby
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allowing energy resulting from combustion to accumulate
and detonate prior to reaching the barrel 36. Both the
fuel supply 12 and the oxidant gas supply 14, furnish fuel
and oxidant gas, respectively, at a positive pressure and
flow rate sufficient to supply the combustion chamber 26
with fuel and oxidant gas during the time interval between
the reflection pressure peak and the subsequent detonation
pressure peak.
In order to prevent combustion or detonation from
extending into the mixing chamber 16, oxidant gas supply
14, or fuel supply 12, the combustible gas mixture
preferably passes through a labyrinth 25 prior to entering
the ignition section 28. The labyrinth 25 is created, for
example, when a first hole 22 of a first bushing 18 and a
second hole 24 of a second bushing 20 overlap to form a
passageway. The passageway has a size large enough to
allow the combustible gas mixture to flow easily from the
mixing chamber 16 to the ignition section 28 but small
enough to prevent a detonation cell from passing through
the passageway from the ignition section 28 to the mixing
chamber 16, oxidant gas supply 14, or fuel supply 12.
Preventing a detonation cell from passing through the
passageway minimizes the possibility of combustion
extending from the combustion chamber 26 into the mixing
chamber 16, the fuel supply 12 or the oxidant gas supply
14.
When the combustible gas mixture enters the
ignition section 28, a spark creating device, such as a
spark plug 40 or an initiating element 44, ignites the
combustible gas mixture causing combustion. Combustion
increases the temperature and pressure within the
combustion chamber 26, thereby increasing the energy
level, due to the volume thereof, and transitioning to
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detonation. Detonation produces a supersonic detonation
wave of numerous detonation cells. The detonation wave
preferably passes from the ignition section 28 through the
diverging section 30, intermediate section 32 and
converging section 34 before entering the barrel 36. (See
FIG. 2A).
Detonation preferably occurs prior to the barrel
36 in order to efficiently propagate powder onto a
substrate 42. It is especially preferred that detonation
occur prior to the converging section 34 so that
detonation waves may reflect off the converging section 34
and create a reflection pressure within the combustion
chamber 26. The barrel 36 is an elongated chamber which
the detonation wave passes through prior to exiting the
detonation gun 10. A powder supply 38 typically introduces
coating powder to the detonation wave as it passes through
the barrel 36. The barrel 36 preferably has a sufficient
overall length such the temperature of the powder
introduced to the detonation wave has sufficient time to
increase beyond its melting point, thereby increasing the
density of the final coating. Although the powder supply
38 can be oriented to supply powder to the combustion
chamber 26, it is preferably located a sufficient
distance, measured from the open end 47 of the combustion
chamber 26, along the barrel 36 to prevent the powder from
entering and adhering to the interior of the combustion
chamber 26.
while each detonation wave is traveling through
the combustion chamber 26 and out the barrel 36, a
subsequent ignition, combustion, detonation cycle is
progressing in the ignition section 28 of the detonation
gun. Since detonation is an exothermic reaction which
releases significant energy, mostly in the form of heat of
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expanding detonation products, it causes the temperature
of the initiating element 44 to increase. When the
initiating element 44 attains a sufficient temperature for
a given reflection pressure (discussed below), the two
parameters create an environment which causes the
combustible gas mixture to ignite and detonate. The
initiating element 44 is generally located in the ignition
section 28 of the combustion chamber 26 and preferably on
the closed end 46 such that combustion may occur as close
as possible to the closed end 46 allowing maximum time for
combustion energy to be accumulated and to initiate
detonation.
Referring to FIG. lA, the initiating element 44
consists of a capacitor portion 48a and an insulating
portion 50a. The capacitor portion 48a is fabricated from
a material having a heat capacity capable of absorbing
sufficient energy from detonation to establish combustion
when a reflection pressure occurs. Preferably, the
capacitor portion 48a is constructed of a material having
a heat capacity such that it absorbs energy at a rate
which allows the capacitor portion 48a to rise to a
minimum temperature, sufficient to ignite the combustible
gas mixture, in less than about ten -detonations and
preferably between about 2 and about 10 detonations. Once
the capacitor portion 48a attains the minimum temperature,
the spark plug 40 can be disconnected or switched off.
The insulating portion 50a of the initiating
element 44 is preferably fabricated from a material such
as ceramic which prevents energy stored within the
capacitor portion 48a from transferring to the closed end
46 of the combustion chamber 26 in order that the
combustible gas mixture may ignite consistently from the
same location.
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FIG. 1B illustrates an alternate initiating
element 44 that does not require a spark creating device,
such as a spark plug 40, to first ignite the combustible
gas mixture. Rather, the capacitor portion 48b of the
initiating element 44 is heated from an external source,
such as electrically, to a temperature sufficient to
ignite the combustible gas mixture. Once the detonation
gun 10 is operational and the initiating element has
achieved the desired temperature, the external power
supply for the capacitor portion 48b may be switched off.
The energy resulting from the detonation will maintain the
temperature of the capacitor portion 48b above the minimum
ignition temperature necessary to ignite the combustible
gas mixture at the given reflection pressure.
The operating pressure within the combustion
chamber is influenced by the behavior of the detonation
cells. Prior to ignition, the pressure within the
combustion chamber is controlled by the,fuel and oxygen
supply pressures and the geometry of the combustion
chamber. After ignition of the combustible gas mixture the
local pressure within the combustion chamber increases and
reaches a maximum when detonation occurs. This initial
maximum pressure is referred to as the detonation pressure
peak (P1), see FIG. 2B. When the detonation wave travels
down the barrel a rarefaction pressure peak (P3) is
measured within the combustion chamber. The rarefaction
pressure peak (P3) is the minimum pressure within a
detonation cycle. Under certain conditions, a positive
pressure peak is then subsequently measured within the
combustion chamber due to the presence of reflected waves
from the detonation wave front. This subsequent pressure
peak is referred to as a secondary pressure peak or
reflection pressure peak (P2) which is the second highest
pressure peak
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within a detonation cycle.
Referring to FIG. 2B, which illustrates the
pressure profile for a part of a detonation cycle, time
(T), between the detonation pressure peak (P1) and the
reflection pressure peak (P2) . (Pi) is the peak of the
initial pressure due to detonation, while (P2) is the peak
of the secondary pressure. As stated above, the secondary
pressure is formed by the reflection of the initial
detonation wave off the walls of the converging section 34
back toward the closed end 46 of the combustion chamber 26
creating a"reflection pressure" within the combustion
chamber 26.
The reflection pressure increases the local
pressure within the combustion chamber 26 to a level,
which combined with the temperature of the initiating
element 44, is sufficient to ignite the combustible gas
mixture in the combustion chamber 26. The detonation cycle
time is decreased by reducing length (L) of the combustion
chamber 26. (See FIG. 2A). Decreasing length (L) decreases
the time (T) between the detonation pressure peak (P1) and
the reflection pressure peak (P2) (see FIG. 2B versus FIG.
3B). Operating the detonation gun 10 at its minimum
detonation cycle time (T) increases the detonation rate.
As mentioned above, once detonation occurs, the
detonation wave travels from its point of initiation
toward the barrel 36. The maximum distance a detonation
wave must travel to reach the barrel 36 is a distance (L) .
(See FIG. 2A). The distance (L) is measured from the
closed end 46 of the combustion chamber 26 to the open end
47 of the combustion _chamber 26 which is also the
downstream end of the converging section 34.
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The time (T) required to complete a detonation
cycle is dependent upon the time it takes a detonation
wave to be supported from its point of initiation to the
converging section 34 and back toward the ignition section
28. The maximum distance a detonation wave may possibly
travel is, therefore, 2L if the point of initiation is
exactly at the initiating element 44. As the length (L)
decreases, the time (T) of the detonation cycle also
decreases. As seen in FIG. 3A, the length (L') decreased
upon removing the intermediate section 32 of the
combustion chamber 26, thereby, as is shown in FIG. 3B,
decreasing time (T') as compared to time (T).
In addition to the length of the combustion
chamber 26, the detonation rate is also a function of the
reflection pressure peak (P2) intensity. As the intensity
of the pressure peak (P2) increases, the slope of the
curve from the rarefraction pressure peak (P3), which is
the minimum pressure the combustion chamber 26 experiences
during a detonation cycle, to the reflection pressure peak
(P2) increases. If the intensity of the reflection
pressure peak (P2) increases, the combustible gas mixture
will ignite sooner since the pressure within the
combustion chamber 26 will attain the pressure necessary
to ignite the gas in a shorter period of time. In order to
attain the maximum detonation rate, the intensity of the
pressure within the combustion chamber 26 must be at its
maximum value, which is the maximum reflection pressure
(Pmax). (See FIG. 4).
The reflection pressure is a function of angle i3,
which is the angle at which the converging section 34
retracts toward the barrel 36. (See FIG.s 3A and 4). As
angle 9 increases, the pressure within the combustion
chamber 26 increases until the pressure attains its
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maximum reflection pressure (Pmax). If angle 9 continues
to increase after the pressure reaches the maximum
reflection pressure (Pmax), the pressure within the
combustion chamber 26 begins to decrease. In order to
continually initiate detonation, the pressure within the
combustion chamber 26 must exceed a critical pressure
(Pc), which is the minimum pressure required to initiate
detonation at a given initiating element 44 temperature.
Consequently, angle 9 must remain below a maximum critical
angle (f3max) and above a minimum critical angle (i3min).
For example, the maximum critical angle (l3max) for an
oxygen/natural gas detonable mixture at a ratio of about
2 to about 7 is generally about 50 and preferably about
35 , while the minimum critical angle (f3min) is about 8
and preferablv about 15 .
Unlike the converging section 34 which creates a
reflection pressure, the diverging section 30 is designed
to maintain the stability of the detonation process. The
diverging section 30 expands from the ignition section 28
at an angle O. (See FIG. 3A) . After ignition the detonable
gas mixture passes through the diverging section 30, its
combustion front expands and its speed decreases which, in
turn, allows the pressure within the diverging section 30
to increase facilitating transition to the detonation.
Angle 6 is generally greater than about 15 and preferred
to be about 30 to about 75 in order to decrease the
speed of the combustion front and to increase the local
pressure after the ignition section 28, inside of the
diverging section 30.
For example, a detonation gun 10 having a
diverging section 30 with an angle 6 of 30 and a
converging section 34 with an angle 9 of 15 was employed
to coat a substrate with an Amperit 526.062 coating powder
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being introduced to the detonation gun 10 at a spray rate
of about 4 kg/hr. Natural gas was supplied to the ignition
section 28 at a flow rate of 10 liters/minute while oxygen
and air, combined to form the oxidant gas, were supplied
at respective flow rates of 47 liters/minute and 12
liters/minute. The deposition efficiency was 80%, i.e.80
% of the powder introduced to the detonation gun 10,
adhered to the substrate 42. Furthermore, a detonation
rate of 55 detonations per second was achieved.
The detonation gun of the present invention not
only surpassed the detonation rate of prior art detonation
guns by a factor of more than S while using similar gas
flow rates and maintaining equivalent quality
characteristics such as coating density and porosity, but
this detonation gun is capable of attaining a de,tonation
rate up to or exceeding about 100 - 300 detonations per
second. In addition, this detonation gun is self
sustained, such that the detonation rate of the detonation
gun is not limited by the constrains of any firing device
such as a spark plug. Rather, this detonation gun ignites
a combustible ressure originates.
It will be understood that various modifications
may be made to the embodiments disclosed herein. For
example, the initiating element 44 may not be necessary if
the temperature of the fuel and oxidant gas is sufficient
to combine with the pressure within the combustion chamber
26 to ignite the combustible gas mixture. In addition, the
initiating element 44 may be heated by a means other than
electricity, or the capacitor portion 48a, 48b of the
initiating element 44 may-be constructed of a material that
has a higher heat capacity *an specified within. Furthermore,
compressing the combustion Chamber 26 could cause a second
pressure peak within the combustion chamber 26 sufficient
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WO 98/29191 PCT/EP97/07300
-20-
to ignite the combustible mixture. Therefore, the above_
description should not be construed as limiting, but
merely as exemplifications of the preferred embodiments.
Those skilled in the art will envision other modifications
within the scope and spirit of the claims appended hereto.
