Note: Descriptions are shown in the official language in which they were submitted.
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SCALABLE PULSE COMBUSTOR
Technical Field
In embodiments of the presently disclosed subject matter, there is provided a
scalable pulse
combustor for generation of heat energy with low NOx emissions.
Background
A pulse combustor is a device in which air and fuel are mixed in a combustion
chamber and
periodically self-ignited to create a high energy pulsating flow of combustion
products and
pressure waves, and may be used for a variety of purposes including the
production of heat
energy in applications such as high efficiency boilers, water heaters and
steam generators.
In a typical radially-configured pulse combustor, a mixture of air and fuel
enters a central
combustion chamber where it is ignited by a flame rod or spark plug. The
explosive combustion
of the air/fuel mixture generates a sudden steep rise in pressure and
temperature within the
combustion chamber, and the resultant pressure waves travel radially outward
carrying the
combustion gasses towards the perimeter of the combustor via one or more
tailpipe regions.
Simultaneously, the pressure rise prevents the flow of new volumes of air and
fuel into the
combustion chamber. Next, the rapid expansion of the combustion gasses,
together with cooling
through heat exchange at the combustor walls, generates a negative (below
ambient) pressure
within the combustion chamber. The pressure waves come to an instantaneous
rest at the
perimeter of the combustor and some combustion gasses exit the chamber, while
the rest are
taken back towards the chamber in the form of rarefaction waves. At the same
time, due to the
low pressure within the chamber, a new volume of air/fuel mixture is drawn
into the combustion
chamber. The rarefaction waves compress this new mixture volume, and with the
temperature in
the combustion chamber remaining elevated, the new air/fuel mixture is ignited
without the need
for a spark and the combustion cycle is repeated.
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PCT Publication No. WO 97/20171 describes a pulse combustor having a central
combustion
chamber surrounded by an exhaust chamber, wherein the combustion and exhaust
chambers are
partially formed between two spaced apart annular "walls" of spiral wound
coolant tubing. Each
annular wall of coolant tubing comprises an inner frustoconical region and an
outer flat-wound
region. The central combustion chamber is defined between opposing inner
frustoconical regions
of two spaced apart walls and central hubs that are welded into the apertures
at the center of each
wall, and the exhaust chamber or "tailpipe" of the combustor is defined
between the opposing
generally parallel flat-wound outer regions of the walls. A fuel nozzle
receptacle is formed
through one of the central hubs, and the opposing central hub includes an
inward facing,
generally conical surface flame spreader for dispersing the flame radially
outwardly through the
combustion chamber. The coolant tubing provides a large heat transfer area,
and water enters
each tube at the perimeter and exits at the center in order to provide a
counter flow heat exchange
process. A fuel nozzle is located in the nozzle receptacle and a spark
generator is provided in the
combustion chamber proximate the nozzle in order to ignite the fuel entering
the pulse
combustor upon startup. Practical limitations as to the maximum reasonable
radius of the
combustion chamber and tailpipe structure of a two-walled pulse combustor of
this sort have,
however, generally been found to limit the total amount of power (i.e. BTU's
of heat generation)
that may be achieved by a two-walled combustor. In particular, two-walled
pulse combustors of
the sort described in WO 97/20171 are generally not considered to be scalable
to achieve an
increased power output beyond roughly 600,000 Btu/hr.
U.S. patent number 7,473,094 describes a scalable pulse combustor comprising
two spaced apart
outer plates and a plurality of annular intermediate plates that are located
between the outer
plates to enable the generation of increased levels of power as compared to a
two-plate
combustor. Each of the outer plates has a flat outer region and a
frustoconical region inside the
flat region, and an associated central hub, and the volume between opposing
frustoconical
regions and central hubs of the outer plates defines a combustion chamber. The
intermediate
plates have a larger central opening than that of the outer plates, and are
held between the outer
plates in generally parallel spaced-apart relationship, transverse to the axis
of the combustion
chamber, to define a plurality of tailpipes therebetween, and between the
outer plates and
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adjacent ones of the intermediate plates. Akin to the two-walled (aka two-
plate) pulse combustor
described in WO 97/20171, each of the outer and intermediate plates may
comprise spiral wound
coolant tubing. Alternatively, the outer and intermediate plates may comprise
spiral coolant
passageways formed therein for conducting a cooling fluid to cool expanding
gases traveling
between the plates through the tailpipe regions. Cooling fluid (e.g. water)
enters each tube or
passageway through an inlet at the outer perimeter thereof and exits through
an outlet proximate
the center thereof in order to provide a counter flow heat exchange process.
In embodiments of the '094 pulse combustor that are adapted for use with a
conventional burner
(illustrated in Fig. 1A of the '094 patent), a burner nozzle operative to
ignite a fuel/air mixture
within the combustion chamber is fitted into the central hub of a first outer
plate, and the central
hub of the second, opposite outer plate comprises a generally conical flame
spreader (ref. no. 76,
Fig. 1A) for dispersing the flame. Each time a fuel/air mixture is ignited,
the flame rapidly
spreads from the burner towards the flame spreader. Flame temperature varies
along the length
of a flame, with the tip of a flame normally being hotter than its origin, and
consequently the
exhaust gasses and air surrounding the flame within a pulse combustor will
also have different
temperatures along the axial depth (i.e. from the burner nozzle to the flame
spreader) of the
combustion chamber. As a result, exhaust gas velocity will also vary along the
depth of the
combustion chamber, with the highest velocity of exhaust gases normally being
experienced at
the tailpipe that is formed between the opposite outer plate (which holds the
flame spreader) and
the immediately adjacent intermediate plate. In order to balance and maintain
exhaust gas
velocity within a desired range as between each of the several individual
tailpipes of the multi-
plate combustor, and thereby facilitate heat transfer efficiency, and the
continued pulsation and
low noise operation of the combustor, the flow of gases is accordingly
controlled in the '094
pulse combustor by adjusting the spacing between the intermediate plates to
create appropriate
resistance to flow, wherein each successive tailpipe is narrower than the
preceding tailpipe as
one approaches the flame spreader. However, as noted in the '904 patent, this
method of
adjusting the tailpipes becomes impractical if more than three intermediate
plates are used, so
conventional burner embodiments of the '094 pulse combustor are generally
limited to a
maximum total of five plates (including all outer and inner plates). In
addition, it has been found
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that during use of these conventional burner embodiments of the '904 pulse
combustor, the flame
spreader and the central hub of the opposite outer plate (i.e. the one facing
the burner) acts as a
heat sink and corrodes over time, in some cases being reduced to powder form
within only a few
days, and eventually resulting in complete failure of the hub and combustor.
The sustained high
temperatures of the flame spreader and hub (particularly after heat soak has
set in) also result in
the production of elevated levels of NOx, which renders the combustor
impractical for many
uses. Furthermore, since the cooling fluid outlets for the intermediate plates
of the '904
combustor exit through the central hub of the opposite outer plate (see Figs.
3 and 4), the outlet
tubes are located in the path of the flame and disrupt its profile, thereby
impairing proper flame
spread within the combustion chamber.
In an alternative embodiment of the '094 pulse combustor (illustrated in FIG.
1B), a specialized
burner assembly is mounted within the combustion chamber in order to minimize
the effect that
the spacing between the intermediate and outer plates has on exhaust gas
velocity. The
specialized burner assembly generally comprises an elongated hollow tube
having a plurality of
nozzle openings spaced around its cylindrical surface to help equalize gas
flow into the tailpipe
between adjacent ones of said intermediate and outer plates. The opposite
central hub in such
embodiments comprises a stainless steel plate that is referred to as a
"spreader hub" and includes
a parabolic cone structure on its inside surface for dispersing the flame
(ref. nos. 11 and 22, Fig.
1B) so that the ignited gases may escape uniformly around and along the hollow
tube. The
arrangement of the holes on each strip, the length of each strip, nozzle
profile, and the shape of
the cone govern the velocity and distribution of the flame through the
cylinder, resulting in a
flame that is generally uniformly ejected or distributed from the surface of
the cylinder, through
the nozzles, and into consecutive tailpipe gaps of the heat exchanger.
Although the use of a
specialized burner may help overcome the impracticality of adjusting tailpipe
spacing, it has
been found that the addition of the specialized burner assembly nevertheless
does not solve the
other deficiencies of the '904 pulse combustor. In particular, the spreader
hub and cone structure
of such embodiments act as a heat sink in much the same way as do the flame
spreader and hub
of the alternate embodiments described above, and are similarly susceptible to
corrosion and
failure, as well as the production of high levels of NOx.
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It is accordingly an object of the present disclosure to provide a scalable
pulse combustor for
generation of heat energy with low NOx emissions.
Summary
In embodiments of the presently disclosed subject matter, there is provided a
scalable pulse
combustor that can be deployed as the heat exchanger in high efficiency, low
NOx condensing
boilers, water heaters and steam generators. The combustor generally comprises
an annular
burner coil with a burner flange (for accommodating the nozzle of a
conventional burner/blower)
fitted into the central aperture thereof; a spaced-apart opposite annular
spreader coil with a heat
exchange hub fitted into the central aperture thereof; and a plurality of
annular intermediate
coils. Each of the burner, spreader and intermediate coils are preferably
formed of spiral wound
stainless steel tubing, with each winding directly abutting the preceding
winding so as to create
an annular "wall" or "plate", but alternate configurations such as solid
annular discs with
machined or cast internal fluid passageways may be also be used.
Each of the annular burner and spreader coils comprises a flat-wound outer
region and a
frustoconical region inside the flat region, and the volume between opposing
inner frustoconical
regions of: (1) a burner coil together with its associated burner flange, and
(2) an opposite
spreader coil together with its associated heat exchange hub, defines a
combustion chamber.
Each of the annular intermediate coils is flat-wound and has a larger central
opening than that of
the burner and spreader coils, and may preferably be dimensioned so as to
generally correspond
with or approximate the inner and outer diameters or dimensions of the flat-
wound outer regions
of the annular burner and spreader coils. The annular intermediate coils are
positioned between
the burner coil and the spreader coil in generally parallel spaced-apart
relationship, transverse to
the axis of the combustion chamber, so as to form a plurality of tailpipes
between one another,
and between an intermediate coil and the flat-wound outer region of an
adjacent burner coil or
spreader coil. In some embodiments, all of the coils are held in generally
parallel orientation in a
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vertical plane on support legs or within a frame by adjustable spacer
assemblies that affix to tabs
welded at selected points along the perimeter of each of the coils.
A cooling fluid such as water is passed under suitable selected pressure
through each of the coils,
entering through an inlet at the outer perimeter of each coil and exiting
through an outlet
proximate the center thereof so as to create a counter flow heat exchange
process between the
cooling fluid and the combustion gases within the combustor, such that a
maximum temperature
difference may be achieved at all points along the heat exchange surface
provided by the coils.
Coolant enters each coil at its perimeter where the combustion and exhaust
gases are at their
lowest temperature, and reaches its hottest point at the center where the
gases are also at their
hottest. Coolant counter flow accordingly provides a highly efficient process
for the transfer of
heat energy from the combustion and exhaust gases to the cooling fluid, and
reduces or
eliminates the possibility of a "thermal shock" occurrence. In order to avoid
potential disruption
of the flame profile within the combustion chamber, the outlet tubes of the
burner coil and all
intermediate coils are oriented so as to exit the combustor through the burner
flange. The outlet
tube of the spreader coil exits from the rear of the combustor, adjacent the
heat exchange hub for
the same reason. As outlined in further detail below, the inlets and outlets
of all of the coils may
preferably be connected to external common manifolds.
The heat exchange hub is generally disc shaped, and comprises a chamber
through which a
cooling fluid may be passed to function as a secondary heat exchanger located
at the "opposite"
end of the combustion chamber (i.e. opposite the burner/blaster), without
appreciably increasing
the external dimensions of the combustor. In some preferred embodiments, the
internal chamber
of the heat exchange hub defines a spiral passageway for the cooling fluid,
with coolant entering
the hub through an inlet adjacent the periphery thereof and exiting through an
outlet near or at
the center. In other embodiments, the internal passageway of the heat exchange
hub may be
sinusoidal or any other shape that promotes a one-way circulation of cooling
fluid through the
heat exchange hub, such that during use of the combustor, coolant may enter
the heat exchange
hub through an inlet at a first temperature and exit through an outlet at a
second, higher
temperature. By way of example, the heat exchange hub may comprise two plates
of stainless
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steel welded to one another, wherein at least one plate has had a spiral water
passageway
machined or cast into it before the other plate is welded on top. In another
example, the heat
exchange hub may comprise three components, wherein a middle element that
comprises
separator blades to form a zig-zag or sinusoidal passageway for coolant is
welded between two
outer plates that function as caps. Both the cooling fluid inlet and outlet of
the heat exchange
hub are formed along the outer rear flat surface of the hub. The lateral side
profile of the hub
comprises corrugations or "grooves" that correspond with the diameter of the
tubing from which
the spreader coil is constructed so that the heat exchange hub can be fitted
into the innermost
rows of the spreader coil without welding. Accordingly, even in cases where a
very large
temperature gradient may exist as between the heat exchange hub the adjacent
spreader coil, the
coil is free to expand/contract along the perimeter of the hub, and thermal
stresses are not
converted into shear stresses between the coil and hub. This greatly reduces
the possibility of
material failure due to destructive shear stresses generated by thermal
stresses, and thus extends
the service life of the combustor.
A separate control valve is associated with the cooling fluid inlet of the
heat exchange hub so
that the rate of coolant flow through the heat exchange hub can be separately
and individually
controlled, independently of the flow through the spreader coil, intermediate
coils, and burner
coil of the combustor. This enables the coolant within the heat exchange hub
to be maintained at
a suitable temperature that not only avoids possible overheating of the
coolant, but importantly
also prevents the heat exchange hub itself from becoming or behaving like a
heat sink. This in
turn enables control of the combustion gas temperature in the vicinity of the
heat exchange hub,
and thus also control of combustion gas velocity in the vicinity of the heat
exchange hub within
the combustion chamber.
This ability to control of the combustion gas temperature and velocity in the
vicinity of the heat
exchange hub reduces or eliminates the need to individually set the gaps
between successive
intermediate coils in the combustor (as is required in prior art multi-plate
combustors to balance
and maintain exhaust gas velocity within a desired range as between each of
the individual
tailpipes), whilst still accommodating the use of a conventional burner. Pulse
combustors having
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more than three intermediate coils may accordingly also be readily
accommodated without
requiring the use of a specialized burner assembly. Furthermore, although the
highest flame
temperatures within the combustor may be expected to occur in the vicinity of
the heat exchange
hub on account of the direction of flame travel (i.e. from the burner to the
heat exchange hub),
the temperature of the heat exchange hub can be maintained below the ¨1,500 C
that is required
for the formation of NOx, and below a temperature at which the hub becomes
susceptible to
corrosion and failure.
The burner flange is a generally annular structure with a central aperture or
bore that is
dimensioned to accommodate a conventional burner/blower that has desired
suitable flame
capacity, and with a plurality of smaller apertures or bores arranged
peripherally around the
central aperture to accommodate the cooling fluid outlet extensions and/or
tubes of all
intermediate coils (so as to permit the heated coolant from these coils to
exit the combustor at the
front and out of the main path of the burner flame). The number of peripheral
apertures in the
burner flange corresponds to the number of intermediate coils of the
combustor. The lateral side
profile of the burner flange is preferably corrugated or "grooved" akin to
that of the heat
exchange hub, with the grooves corresponding with the diameter of the tubing
from which the
burner coil is constructed so that the burner flange can be fitted into the
first rows of the burner
coil without welding.
The cross-sectional profiles of the spreader and burner coils are essentially
mirror images of one
another, with suitable changes made to the outlets and inlets as required to
accommodate
preferred orientations for cooling fluid intake and outlet extensions. In
cases where either or
both of the spreader and burner coils comprise solid discs with machined or
cast internal fluid
passageways, the corresponding heat exchange hub or burner flange may have a
lateral profile
that is slightly thicker than that of the corresponding coil, and include
removable upper and/or
lower bracket elements to enable the hub and/or burner to be fitted into its
corresponding coil
without welding in an analogous manner to the grooved embodiments described
above.
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As noted above, a cooling fluid such as water is passed under pressure through
each of the coils
when in use to generate a counter flow heat exchange process between the
cooling fluid and the
combustion gases within the combustor. In some embodiments, the cold water
inlet tubes of the
burner coil, spreader coil, all intermediate coils, and the heat exchanger hub
are all connected by
suitable tubing to a common cold water inlet manifold external to the
combustor, and similarly
the hot water outlet (i.e. exit) tubes of the burner coil, spreader coil, all
intermediate coils, and
the heat exchanger hub are all connected by suitable tubing to a common hot
water outlet
manifold. At least one of the common manifolds includes suitable valves for
controlling the
flow of coolant through the coils, and as noted above, at least one of the
inlet and outlet tubes of
the heat exchange hub (and/or at least one of the inlet and outlet manifolds
themselves) include a
separate control valve to enable the rate of coolant flow through the heat
exchange hub to be
separately and individually controlled, independently of the flow through the
combustor coils.
Embodiments in which additional valves are used to individually control of
coolant flow through
any one or more of the spreader coil, intermediate coils, and burner coil of
the combustor are
also contemplated.
In some embodiments, a flame detecting sensor may be installed inside the
combustion chamber.
As soon as flame is established, signals are sent from this sensor to a
control panel and the flame
rod or spark plug is switched off.
By employing a heat exchange hub instead of a spreader plate as in the prior
art, the oxidation
problems associated with the prior art spreader plate (which may occur due to
direct flame
impingement against a heat sink) and consequent additional maintenance/service
work that
would involve total dis-assembly of the combustor and re-welding of a new
plate are generally
avoided. Also avoided are the operational restrictions of prior art multi-
plate pulse combustors,
which are related to the need for certain optimum depth of the combustion
chamber and
readjustment of gaps between coils. In the presently described combustor, the
hub functions as a
heat exchanger and therefore it will never become a heat sink during normal
use, especially in
that it has its own flow valve and water flow rate that can be controlled
independent of water
flow rates through the coils. This virtually or entirely eliminates the
possibility of NOx
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formation because the temperature of the heat exchange hub can be maintained
below the ¨1,500
C required for NOx formation.
Flame speed and length are a function of burner head configuration, as
determined by the
manufacturer, and are essentially independent of the dimensions of the chamber
in which the
flame front propagates. In preferred embodiments of the presently described
combustor,
combustion chamber axial depth is selected to be between 50% - 75% of the
length of the flame,
and depending on the flame length, speed and ratio of flame length to
combustion chamber
depth, the taper angle 0 of the frustoconical region of the spreader coil is
set within a range of
between about 68 > 0 > 63 degrees. Embodiments where combustion chamber depth
is between
about 25% - 85% are also possible. However if the combustion chamber depth is
below about
25% of flame length, then combustion may become choked off and flame
flashbacks may be
experienced. If combustion chamber depth is above 85% of flame length, then
proper flame
distribution may not be achieved.
The term "specific heat transfer surface" defines how much heat is transferred
per unit of surface
area (e.g. per square foot or square meter), and to achieve the desired
efficiency in a scalable
pulse combustor, there should be no less than about 7500 Btu/hr for every
square foot and no
more than about 9500 Btu/hr for every square foot. For example, a 10 mBtu/hr
combustor will
require a total heat transfer surface of at least about 1,052 square feet
(i.e. 10,000,000 9500)
and at the most about 1,333 square feet (i.e. 10,000,000 7500). This gives the
"total" required
heat transfer surface. Then based on the ratio of the radial length of the
frustoconical region of
the spreader "r" to the radial length of the flat-wound region of the spreader
"R" (i.e. r/R), and
depth of the combustion chamber, one can determine how many intermediate coils
are desired
and what the ultimate radius "R" of the coils should be (or how many
intermediate coils will be
required if a certain radius "R" is mandated). Preferred values of tailpipe
gap width are typically
determined using computational fluid dynamic simulation modelling, using a
series of fluid
dynamic criteria and equations that involve the flame velocity of propagation,
the temperature
gradient along the length of the flame, the velocity of exhaust gases, and the
angle 0. In a typical
application, a tailpipe gap width of between about 4 ¨ 6 mm has been found to
be suitable.
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Brief Description of the Drawings
For a fuller understanding of the nature and advantages of the disclosed
subject matter, as well as
the preferred modes of use thereof, reference should be made to the following
detailed
description, read in conjunction with the accompanying drawings. In the
drawings, like
reference numerals designate like or similar steps or parts.
Figure 1 is an isometric view of the rear (spreader coil) side of a scalable
pulse combustor in
accordance with one embodiment of the presently described subject matter.
Figure 2 is an isometric view of the front (burner coil) side of the combustor
of Fig. 1.
Figures 3A and 3B are partially sectional side elevation views of the
combustor of Fig. 1 in
combination with a conventional burner/blower.
Figure 4 is a front elevation of the combustor of Fig. 1.
Figure 5 is a rear elevation of the spreader coil assembly of the combustor of
Fig. 1, showing the
water inlet and outlet extension tubes of the spreader coil and of the heat
exchange hub that acts
as a secondary heat exchanger.
Figure 6 is a partially sectional rear elevation view of the combustor of Fig.
1 with the spreader
coil removed to show the inner orientation of the water outlet extension tubes
of the first to fifth
intermediate coils exiting the combustor through the burner flange.
Figures 7A ¨ 7C are, respectively, isometric, side elevation and front
elevation views of a
representative burner coil for a scalable pulse combustor in accordance with
embodiments of the
presently described subject matter.
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Figures 8A ¨ 8C are, respectively, isometric, side elevation and front
elevation views of the
burner coil assembly of the combustor of Fig. 1, illustrating the burner coil
in combination with
the burner flange and with the burner coil inlet and outlet extension tubes.
Figures 9A ¨ 9C are, respectively, isometric, side elevation and rear
elevation views of a
representative spreader coil for a scalable pulse combustor in accordance with
embodiments of
the presently described subject matter.
Figures 10A ¨ 10C are, respectively, isometric, side elevation and rear
elevation views of the
spreader coil assembly of the combustor of Fig. 1, illustrating the spreader
coil in combination
with the heat exchange hub and with the inlet and outlet extension tubes for
both of the spreader
coil and heat exchange hub.
Figure 11 is a front elevation of a representative intermediate coil for a
scalable pulse combustor
in accordance with embodiments of the presently described subject matter.
Figures 12A ¨ 12C are, respectively, isometric, side elevation and front
elevation views of the
first intermediate coil of the combustor of Fig. 1, illustrating the first
intermediate coil in
combination with its inlet and outlet extension tubes.
Figures 13A ¨ 13C are, respectively, isometric, side elevation and front
elevation views of the
second intermediate coil of the combustor of Fig. 1, illustrating the second
intermediate coil in
combination with its inlet and outlet extension tubes.
Figures 14A ¨ 14C are, respectively, isometric, side elevation and front
elevation views of the
third intermediate coil of the combustor of Fig. 1, illustrating the third
intermediate coil in
combination with its inlet and outlet extension tubes.
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Figures 15A ¨ 15C are, respectively, isometric, side elevation and front
elevation views of the
fourth intermediate coil of the combustor of Fig. 1, illustrating the fourth
intermediate coil in
combination with its inlet and outlet extension tubes.
Figures 16A and 16B are, respectively, isometric and front elevation views of
the fifth
intermediate coil of the combustor of Fig. 1, illustrating the fifth
intermediate coil in
combination with its inlet and outlet extension tubes.
Figure 17 is a partially sectional rear elevation view of the heat exchange
hub of the combustor
of Fig. 1, showing the spiral machined or cast inner passage ways for cooling
fluid.
Figure 18 is a sectional side view of the spreader coil and heat exchange hub
of the combustor of
Fig. 1, showing the grooved lateral profile and the flat surfaces of the inner
and outer caps of the
heat exchange hub.
Figures 19A ¨ 19C are, respectively, partially sectional isometric, rear
elevation, and side
elevation views of an alternate embodiment of a heat exchange hub for a
scalable pulse
combustor in accordance with embodiments of the presently described subject
matter.
Figure 20A and 20B are, respectively, side elevation and isometric views of
the burner flange of
the combustor of Fig. 1.
Figure 21 is a schematic side elevation of the spreader coil of the combustor
of Fig. 1, showing
taper angle "0" and ratio "r/R".
Detailed Description of Specific Embodiments
The following description of specific embodiments is merely exemplary in
nature and is in no
way intended to limit the invention, its application, or uses. With reference
to Figures 1 - 4, one
embodiment of a scalable pulse combustor 10 in accordance with the presently
described subject
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matter is illustrated. Combustor 10 generally comprises an annular burner coil
12 with a burner
flange 14 fitted into the central aperture thereof; a spaced-apart opposite
annular spreader coil 16
with a heat exchange hub 18 fitted into the central aperture thereof; and five
annular intermediate
coils 20, 21, 22, 23 and 24. As illustrated in Figures 3A and 3B, the burner
head 26 of a
conventional burner/blower 28 may be accommodated within a central bore 30 of
burner flange
14, and may be secured therein by conventional fasteners such as bolts or
screws (not shown).
Each of the burner coil 12, spreader coil 16 and intermediate coils 20 ¨ 24
are preferably formed
of spiral wound stainless steel tubing, with each winding directly abutting
the preceding winding
so as to create a "wall" or "plate". However, alternate configurations such as
solid cast or
machined annular discs with internal fluid passageways are also contemplated.
Annular burner
coil 12 and annular spreader coil 16 each comprise a flat-wound outer region
(12a and 16a,
respectively) and a frustoconical inner region (12b and 16b, respectively),
whilst each of the
annular intermediate coils 20 ¨ 24 are flat-wound and have a central opening
that is larger than
that of burner coil 12 or spreader coil 16. The central opening of
intermediate coils 20 ¨ 24 is
selected with reference to the diameter of the flame produced by conventional
burner/blower 28,
such that the intermediate coils 20 ¨ 24 do not disturb the flame profile or
its boundary layer. In
some embodiments, each of the annular intermediate coils 20 ¨ 24 are further
dimensioned so as
to correspond with or approximate the inner and outer diameters of the flat-
wound outer regions
of the annular burner coil 12a and spreader coil 16a.
As illustrated in Figure 3B, the volume delineated between frustoconical inner
region 12b of
burner coil 12 together with its associated burner flange 14, and opposing
frustoconical inner
region 16b of spreader coil 16 together with its associated heat exchange hub
18, defines a
combustion chamber 32 having a central axis that extends between the burner
flange 14 and the
heat exchange hub 18. Support legs 34 with adjustable brackets 36, preferably
comprising
stainless steel bolt and nut spacer assemblies that affix to tabs 38 welded at
selected points along
the perimeter of each of the coils, are provided to hold the annular burner
coil 12, spreader coil
16 and intermediate coils 20 ¨ 24 in generally parallel spaced-apart
relationship at a selected
distance from one another and oriented transverse to the axis of the
combustion chamber 32,
14
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thereby forming a plurality of tailpipes 40 between adjacent intermediate
coils, and between an
intermediate coil and the flat-wound outer region of an adjacent burner coil
or spreader coil. In
other words, in the specific embodiment illustrated, the tailpipes 40 comprise
the gaps formed on
either side of each of intermediate coils 21 ¨ 23, the gap formed between
intermediate coil 20
and outer region 12a of burner coil 12, and the gap formed between
intermediate coil 24 and
outer region 16a of spreader coil 16.
As outlined in the summary above, a cooling fluid such as water is passed
under suitable selected
pressure through each of the annular burner coil 12, spreader coil 16 and
intermediate coils 20 ¨
24 when the combustor 10 is in use in order to generate a counter flow heat
exchange process
between the cooling fluid and the combustion gases within the combustor 10.
Heat exchange
hub 18 functions as a secondary heat exchanger with its own independently
controllable coolant
flow. The cooling fluid enters each of the coils through an inlet at the outer
perimeter of each
coil and exits through an outlet proximate the center thereof so as to create
the counter flow heat
exchange process between the cooling fluid and the combustion gases within the
combustor,
such that a maximum temperature difference may be achieved at all points along
the heat
exchange surface provided by the coils. Coolant enters each coil at its
perimeter where the
combustion and exhaust gases are at their lowest temperature, and reaches its
hottest point at the
center where the gases are also at their hottest. Coolant counter flow
accordingly provides a
highly efficient process for the transfer of heat energy from the combustion
and exhaust gases to
the cooling fluid, and reduces or eliminates the possibility of a "thermal
shock" occurrence.
In the illustrated embodiment, the coolant inlets of coils 12, 16 and 20 ¨ 24,
and heat exchange
hub 18, are connected by stainless steel tubing to a common cold water inlet
manifold 44, and
the coolant outlets of coils 12, 16 and 20 ¨ 24, and heat exchange hub 18, are
connected by
stainless steel tubing to a common hot water outlet manifold 58. Figure 1
illustrates the
connection by between common cold water inlet manifold 44 and burner coil 12,
spreader coil
16, intermediate coils 20 ¨ 24 and heat exchange hub 18, respectively, via
inlet tubes 46, 48, 50-
54 and 56, respectively. Figure 2 illustrates the connection between common
hot water outlet
manifold 58 and the burner coil 12, spreader coil 16, intermediate coils 20 ¨
24 and heat
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exchange hub 18 via outlet tubes 60, 62, 64-68 and 70, respectively. Control
valves 60a, 62a,
64a-68a and 70a of outlet tubes 60, 62, 64-68 and 70, respectively, are
provided adjacent the hot
water outlet manifold 58 to enable individual control of the rate of coolant
flow through the heat
exchange hub 18, as well as. through each of burner coil 12, spreader coil 16
and intermediate
coils 20 ¨ 24.
A separate control valve 70a is associated with the cooling fluid outlet 70 of
the heat exchange
hub 18 so that the rate of coolant flow through the heat exchange hub 18 can
be separately and
individually controlled, independently of the flow of coolant through the
spreader coil 16,
intermediate coils 20 ¨ 24, and burner coil 12 of the combustor 10. This
enables the coolant
within the heat exchange hub 18 to be maintained at a suitable temperature
that not only avoids
possible overheating of the coolant, but importantly also prevents the heat
exchange hub 18 itself
from becoming or behaving like a heat sink. This in turn enables control of
the combustion gas
temperature in the vicinity of the heat exchange hub 18, and thus also control
of combustion gas
velocity in the vicinity of the heat exchange hub 18 within the combustion
chamber 10. This
ability to control of the combustion gas temperature and velocity in the
vicinity of the heat
exchange hub 18 reduces or eliminates the need to individually set the gaps
between successive
intermediate coils 20 ¨ 24' in the combustor 10 whilst still accommodating the
use of a
conventional burner 28. Pulse combustors having more than three intermediate
coils may
accordingly also be readily accommodated without requiring the use of a
specialized burner
assembly. Furthermore, although the highest flame temperatures within the
combustor 10 may
be expected to occur in the vicinity of the heat exchange hub 18 on account of
the direction of
flame travel (i.e. from the burner 28 to the heat exchange hub 18), the
temperature of the heat
exchange hub 18 can be maintained below the ¨1,500 C that is required for the
formation of
NOx, and below a temperature at which the hub 18 becomes susceptible to
corrosion and failure.
As best seen in Figure 6, hot water outlet (exit) tubes 64 ¨ 68 of
intermediate coils 20 ¨ 24,
respectively, all exit the combustion chamber 32 through the burner flange 14.
In this case, since
combustor 10 comprises five intermediate coils, burner flange 14 has five
corresponding
peripheral apertures to accommodate the five outlet (exit) tubes 64 ¨ 68. The
number of
16
=
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peripheral apertures in the burner flange thus corresponds with the number of
intermediate coils
of the combustor, such that for a combustor having, say, ten intermediate
coils, the burner flange
would comprise ten corresponding peripheral apertures. As best seen in Figures
5 and 10,
coolant inlet tube 48 of spreader coil 16 and coolant inlet tube 56 of heat
exchange hub 18, as
well as coolant outlet tube 62 of spreader coil 16 and coolant outlet tube 70
of heat exchange hub
18, are routed along the rear side of combustor 10 so as not to pass through
the combustion
chamber 32.
Referring to Figures 17 and 18, heat exchange hub 18 of combustor 10 is
generally disc shaped,
and comprises an internal chamber 72 through which a cooling fluid may be
passed to function
as a secondary heat exchanger located at the "opposite" end of the combustion
chamber 10 (i.e.
opposite the burner/blaster 28), without appreciably increasing the external
dimensions of the
combustor 10. Internal chamber 72 of heat exchange hub 18 defines a spiral
passageway for the
cooling fluid, with coolant entering the hub through inlet 56 adjacent the
periphery of the hub 18
and exiting through outlet 70 at the center of hub 18. Both the inlet 56 and
the outlet 70 are
formed along the outer rear flat surface 74 of hub 18 so that the inlet and
outlet tubes do not pass
through combustion chamber 32 as discussed above. The lateral side profile 76
of hub 18
comprises corrugations or "grooves" that correspond with the diameter of the
tubing from which
spreader coil 16 is constructed so that hub 18 can be fitted into the
innermost rows of spreader
coil 16 without welding. Accordingly, even in cases where a very large
temperature gradient
may exist as between the heat exchange hub 18 the adjacent spreader coil 16,
coil 16 is free to
expand/contract along the perimeter of hub 18, and thermal stresses are not
converted into shear
stresses between the coil 16 and hub 18. This greatly reduces the possibility
of material failure
due to destructive shear stresses generated by thermal stresses, and thus
extends the service life
of the combustor 10.
Figures 19A ¨ 19C illustrate an alternative embodiment of a heat exchange hub
18x comprising a
plurality of internal water spreaders 78 in place of the spiral internal
chamber 72 of the
embodiment shown in Figures 17 and 18. As with the hub 18 of Figures 17 and
18, water
spreaders 78 of this alternate embodiment promote a one-way circulation of
cooling fluid
17
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through the heat exchange hub, such that during use of the combustor 10,
coolant may enter the
heat exchange hub through an inlet at a first temperature and exit through an
outlet at a second,
higher temperature. In order for the flow of coolant to complete a full
"circuit" through the
entirety of the alternative heat exchange hub 18x of Figure 19, coolant inlet
56x is positioned at
one end along rear flat surface 74x of the hub 18x (as is the case with hub 18
of Figures 17 and
18) corresponding to a first end of the internal maze created by water
spreaders 78, and coolant
outlet 70x is positioned at the opposite end of alternative hub 18x (unlike
hub 18, in which outlet
is positioned at the center) corresponding to the opposite end of the internal
maze created by
water spreaders 78. As with heat exchange hub 18 of combustor 10, the lateral
side profile 76x
of hub 18x also comprises corrugations or "grooves" that correspond with the
diameter of the
tubing from which spreader coil 16 is constructed so that hub 18x can be
fitted into the innermost
rows of spreader coil 16 without welding. This is best illustrated in Figure
19C.
Referring to Figures 20A and 20B, burner flange 14 of combustor 10 is a
generally annular
structure with a central aperture or bore 30 that is dimensioned to
accommodate a conventional
burner/blower with desired flame capacity. A plurality of smaller apertures or
bores 80 are
arranged peripherally around the central aperture to accommodate the cooling
fluid outlet
extensions and/or tubes of all intermediate coils 20 ¨ 24 (so as to permit the
heated coolant from
these coils to exit the combustor at the front and out of the main path of the
burner flame). In the
illustrated embodiment, there are five bores 80, one to accommodate the outlet
tube of each of
intermediate coils 20 ¨ 24. As noted above, the number of peripheral apertures
in a given burner
flange corresponds with the number of intermediate coils of the combustor in
which it is
employed. As with heat exchange hub 18 discussed above, the lateral side
profile of burner
flange 14 comprises corrugations or "grooves" 82 that correspond with the
diameter of the tubing
from which burner coil 12 is constructed so that burner flange 18 can be
fitted into the innermost
rows of burner coil 12 without welding.
Every conventional burner/blower assembly is equipped with an ignition system
(e.g. an ignition
rod or spark plug). In the illustrated embodiment, a conventional
burner/blower 28 is secured in
combustor 10, with burner head 26 installed through the central aperture 30 of
burner flange 14
18
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of burner coil 12. A volume of air and gas mixture is ignited by the ignition
rod or spark plug
(not shown) of conventional burner/blower 28 as it leaves the burner head 26
and enters the
combustion chamber 32. The combustion results in instantaneous rise of
pressure inside the
combustion chamber 32. This generates pressure waves that carry the exhaust
products radially
outwards through the tailpipe 40 gaps between the coils 12, 16 and 20 ¨ 24
towards the perimeter
of the coils. As well, this rapid rise in pressure stops the flow of fresh air
and gas mixture into
the combustion chamber 32. 'At the same time, cold water flows through each
coil 12, 16, and 20
¨ 24 from the perimeter towards the center of each coil, resulting in a
counter flow heat exchange
between the water and hot exhaust gases. Rapid expansion of the exhaust gases
(carried by said
pressure waves) together with cooling of said gases through said counter flow
heat exchange
results in a negative pressure being created inside the combustion chamber 32.
Consequently,
with the pressure inside the combustion chamber 32 being below that of the
surrounding ambient
atmospheric pressure, the exhaust gases reaching the perimeter of the coils of
the scalable
combustor come to an instantaneous rest; some exit the combustor and the
remaining exhaust
gases return towards the combustion chamber through rarefaction waves (i.e.
waves moving in
opposite direction to the pressure waves at lower velocities). The negative
pressure created
inside the combustion chamber 32 draws a new volume of air/gas mixture into
the combustion
chamber from the burner 28. The rarefraction waves entering the combustion
chamber compress
this new mixture volume, and with the temperature of the combustion chamber
still being high,
this new volume of air and gas mixture is ignited, another combustion occurs,
and the cycle is
repeated.
The flame tip impinges on the heat exchange hub 18, which is functioning as a
secondary heat
exchanger. Furthermore, all water outlet tubes associated with internal coils
20 ¨ 24 exit the
combustion chamber through the perimeter of burner flange 14, which is above
the burner head
26. As well, the diameters of the hollow central sections of the intermediate
coils 20 ¨ 24 are
always larger than flame diameter. As such, the flame profile and its velocity
remain un-
disturbed along the depth of the combustion chamber 32 throughout the flame
length. The flame
impinges on the flat surface of the heat exchange hub 18 and spreads over the
inner frustoconical
region 16b of the spreader coil 16. The length "r" (see Figure 21) of the
frustoconical section of
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the spreader coil 16 and the taper angle 0 (Figure 21) are so calculated that
the flame is spread
uniformly along the said lengths and over the intermediate coils and the
burner coil, thus
maintaining the combustion gases at a uniform temperature along the depth of
the combustion
chamber. Therefore, all tailpipe 40 gaps between adjacent coils can be the
same and there is no
need to adjust each gap separately.
As noted above, by employing a heat exchange hub 18 instead of a spreader
plate as in the prior
art, the oxidation problems asociated with the prior art spreader plate (which
may occur due to
direct flame impingement) and consequent additional maintenance/service work
that would
involve total dis-assembly of the combustor and re-welding of a new plate are
avoided. Also
avoided are the operational restrictions of prior art multi-plate pulse
combustors, which are
related to the need for certain optimum depth of combustion chamber and
readjustment of gaps
between coils. In the presently described combustor, the heat exchange hub 18
will never
become a heat sink during normal use, especially in that it has its own flow
valve and water flow
rate that can be controlled independent of water flow rates through the coils.
This virtually or
entirely eliminates the possibility of NOx formation because the temperature
of the heat
exchange hub can be maintained below the ¨1,500 C required for NOx formation.
Flame speed and length are a function of burner head configuration, as
determined by the
manufacturer, and are essentially independent of the dimensions of the chamber
in which the
flame front propagates. In preferred embodiments of the presently described
combustor,
combustion chamber depth is selected to be between 50% - 75% of the length of
the flame, and
depending on the flame length, speed and ratio of flame length to combustion
chamber, the taper
angle 0 of the frustoconical region of the spreader coil is set within a range
of between about 68
> O? 63 degrees. Embodiments where combustion chamber depth is between about
25% - 85%
are also possible. However if the combustion chamber depth is below about 25%
of flame
length, then combustion may be choked off and flame flashbacks may be
experienced. If
combustion chamber depth is above 85% of flame length, then proper flame
distribution may not
be achieved.
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Specific heat transfer surface defines how much heat is transferred per unit
of surface area (e.g.
per square foot or square meter), and to achieve the desired efficiency in a
scalable pulse
combustor, there should be no less than about 7500 Btu/hr for every square
foot and no more
than about 9500 Btu/hr for every square foot. For example, a 10 mBtu/hr
combustor will require
a total heat transfer surface of at least about 1,052 square feet (i.e.
10,000,000 9500) and at the
most about 1,333 square feet (i.e. 10,000,000 7500). This gives the "total"
required heat
transfer surface. Then based on the ratio of the radial length of the
frustoconical region of the
spreader "r" to the radial length of the flat-wound region of the spreader "R"
(i.e. r/R ¨ see
Figure 21), and depth of the combustion chamber, one can determine how many
intermediate
coils are desired and what the ultimate radius "R" of the coils should be (or
how many
intermediate coils will be required if a certain radius "R" is mandated).
Preferred values of
tailpipe gap width are typically determined using computational fluid dynamic
simulation
modelling, using a series of fluid dynamic criteria and equations that involve
the flame velocity
of propagation, the temperature gradient along the length of the flame, the
velocity of exhaust
gases, and the angle 0. In a typical application, a tailpipe gap width of
between about 4 ¨ 6 mm
has been found to be suitable.
The present description is of the best presently contemplated mode of carrying
out the subject
matter disclosed herein. The description is made for the purpose of
illustrating the general
principles of the subject matter and not to be taken in a limiting sense; the
described subject
matter can find utility in a variety of implementations without departing from
the scope of the
invention made, as will be apparent to those of skill in the art from an
understanding of the
principles that underlie the invention.
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