Pulsed air combustion high capacity boiler

A pulsed combustion high capacity boiler has air inlet flapper valves at an inlet air decoupler toward a combustion chamber, with a fuel supply and an ignitor upstream of the combustion chamber along a flow path. The combustion chamber is immersed in a water jacket, and downstream of the combustion chamber a plurality of exhaust pipes are immersed in the water jacket and lead to an exhaust decoupler. The exhaust decoupler is disposed outside of the water at the periphery of the unit. An orifice assembly along the flow path defines a flow restriction producing turbulence and preferably serially combines a reduction in cross sectional area followed by a diverter having vanes to induce further turbulence. The ignitor and fuel supply are enabled by a controller that senses spark and combustion flame, and also enables a blower at least during a startup interval. The air inlet flapper admits air, opening and closing at a resonant frequency during combustion. At least the combustion chamber and the exhaust pipes are substantially enclosed in the water jacket and together define a water tank that can be part of a circulating water system. Inasmuch as the exhaust decoupler is mounted on the outer perimeter, preferably at the bell end of the boiler outside of the water jacket, the outermost wall at that end can be made of light sheet metal rather than heavy pressure-resistant plate. Due to the structure, the bell end need only withstand the pressure differential between the gaseous exhaust and ambient pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of pulsed combustion devices, and in 
particular concerns an optimized oil-fired boiler driven by 
pulse-controlled convection, having a gas/fluid heat exchanger for 
extracting heat energy from hot exhaust gases. 
According to the invention, flapper valves on an air inlet decoupler 
chamber oscillate the feed air flow. A fuel nozzle injects atomized fuel. 
Structures located downstream of the air and fuel inlets along the air 
flow path cooperate by successively diverting flow in different directions 
to induce turbulence, for example inducing toroidal and then helical flow 
and corresponding additive eddy currents for good air/fuel mixing. 
Oscillation is maintained due to the push-pull effect on pressure 
conditions of alternating expansion and exhaust of the combusting air/fuel 
mixture, which pressure conditions drive the closing and opening of the 
flapper valves and contribute to the flow of combustion gases in the draft 
direction toward the exhaust. The hot exhaust gases pass from the 
combustion chamber through a heat exchanger tube bundle that is immersed 
in a water jacket defined between the concentric walls of the combustion 
chamber and an outer housing of the unit. The tube bundle comprises an 
array of axially and radially elongated U-shaped tubes in the annular 
space between the combustion chamber and the outer walls. Each tube is 
coupled between an inner annular tube manifold plate on the upstream side 
of the flow path and an outer annular tube manifold plate on the 
downstream side, with the connections of each tube to the inner and outer 
manifold plates being angularly advanced such that the array is compactly 
compressed. The downstream or outer annular tube manifold opens into an 
exhaust decoupler chamber that is bounded by the outer bell end of the 
generally cylindrical unit such that the axial end of the housing encloses 
the gas flow path rather than the water jacket. 
2. Related Art 
Pulsed combustion devices generally comprise a combustion chamber and one 
or more exhaust pipes arranged to transfer heat into a forced air heating 
system or into the water of a boiler. Pulse combustion can involve pulsing 
the fuel and/or air that is fed to the combustion chamber. A pulse air 
system can have an inlet air valve leading to the combustion chamber, 
arranged to resonate at an oscillation frequency determined by the 
combustion chamber volume, the volume of the exhaust pipes, the length of 
the exhaust pipes, and other physical parameters (such as the speed of 
sound). 
A pulsed air combustion device can be considered to operate on an 
oscillatory pressure cycle. In general, the air/fuel charge ignited in the 
combustion chamber expands as it is heated, and flows into and through the 
exhaust pipes due to expansion with combustion and heating. Combustion 
expansion causes a local pressure increase at the area of combustion. 
There are obstructions placed upstream of the combustion area or chamber, 
namely closer to the air inlet compared to the draft or flow direction 
(including at least the inlet flapper valves). The expansion thus moves 
the gases forward in the draft direction in a higher pressure phase of the 
combustion cycle. In addition, convective flow of hot gases in the draft 
direction creates a partial vacuum at the combustion chamber in a lower 
pressure or relative vacuum phase of the pressure cycle, which assists in 
drawing a fresh air/fuel charge into the combustion chamber. With correct 
timing, including oscillation of the air inlet at the required frequency, 
pulsed combustion ensues and becomes self sustaining. The fresh air/fuel 
charge can be spontaneously ignited when exposed to still-combusting gases 
or to the latent heat of the combustion chamber. An initial cycle is 
established through ignition by a spark, pilot flame or glow plug, and the 
process is then self sustaining, with an oscillatory pressure fluctuation 
superimposed on a general flow of gases from the inlet to the outlet. 
With appropriate adjustments to the dimensions of the respective chambers 
and the valves or other means that control the feeding of air and fuel, it 
is possible to vary parameters including the resonant frequency of the 
device, the rate of fuel combustion, the ratio of air feed to fuel volume 
and the like. One objective of such adjustments is to extract as much 
thermal energy as possible from the fuel used, by complete combustion and 
efficient thermal energy transfer. There may be other objectives in 
addition to efficiency, such as maintaining a high rate of thermal energy 
transfer. Other objectives such as durability versus expense and ease of 
construction, are also involved. These objectives compete and may involve 
opposite considerations, such that they are difficult to optimize 
together. 
Pulse combustion technology is advantageous for meeting many of the 
objectives, especially in water heating units intended for high rates of 
thermal energy transfer. Pulse boilers also have unique design concerns. 
For example, such units operate resonantly at low frequency and generate 
acoustic noise. 
Combustion boilers are advantageously compact. It is particularly important 
to make the device compact if the heat transfer fluid (water) is to be 
pressurized. If the pressure confined in a vessel is increased, it is 
obviously necessary to make the walls of that vessel thicker to withstand 
the additional pressure. Moreover, at a given pressure, if the span of a 
pressure confining wall of a vessel is increased, that is if a wall is 
widened and/or lengthened to encompass a greater area, it is also 
necessary to make that wall thicker. However, the relationship of required 
wall thickness versus wall span in a pressure vessel is a geometric 
relationship. Thus it is particularly important to keep the design 
compact. 
With thermal transfer between combustion gas and a pressurized fluid, a 
relatively small heat transfer surface provides good efficiency of heat 
transfer on the fluid side. A water jacket can substantially or only 
partly enclose a combustion chamber and its associated exhaust gas 
conduits. Heat transfer fins and surfaces can be employed to increase the 
rate of thermal transfer by increasing the surface contact area, 
particularly on the exhaust or flue gas side of the heat exchange 
surfaces. 
The extent of thermal transfer normally can be increased by more completely 
and deeply embedding the combustion and exhaust portions of such a unit in 
the water jacket. For example, the combustion chamber can be placed well 
within a large water jacket volume that encloses the combustion chamber on 
all sides. The exhaust pipes can be arranged to carry the hot exhaust 
gases a long distance through the water jacket by which heat is extracted, 
and the water or other coolant can be kept at a relatively low temperature 
such that the ultimate exhaust to the ambient atmosphere is relatively 
cool compared to the combustion temperature. However, these aspects add to 
the overall size and complexity of the unit. Lengthening the exhaust pipes 
also lowers the resonant frequency of the system. Such changes may 
increase thermal energy transfer efficiency but can have other drawbacks 
and advantageously should be optimized. 
A pulse combustion furnace for liquid or gaseous fuel is disclosed in U.S. 
Pat. No. 4,995,376--Hanson. This patent discloses and claims certain 
structural details that preferably are applied to the boiler of the 
present invention. The '376 Hanson patent is hereby incorporated by 
reference for its disclosure of structures in common with the preferred 
embodiments of the present invention. 
U.S. Pat. No. 5,242,294--Chato discloses a pulse combustion boiler which 
operates at a high resonant frequency (i.e., 440 Hz). U.S. Pat. No. 
4,951,706--Kardos discloses a flapper air check valve for oscillating the 
feed air to a combustion device. The disclosures of these references are 
also hereby incorporated. 
There is a need for a high-capacity pulse combustion boiler, having a 
compact design and an acceptable acoustic noise level, which is optimized 
for efficiency in its rate of heat transfer, and is also characterized by 
design features that contribute to efficiency while maintaining a compact 
size. Preferably, this should be accomplished without contributing unduly 
to the cost and complexity of the device and with minimum requirements for 
unduly heavy or complex construction. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a high-capacity, oil-fired 
boiler having a compact design, limited noise emissions and an optimally 
lightweight yet durable construction. 
It is another object of the invention to provide a pulse combustor having 
two or more inlet air flapper valves associated with a pressure decoupling 
inlet plenum. 
It is an object to extract added efficiency from an air-pulsed boiler by 
providing a series of sequential turbulence inducing structures along the 
gas flow path adjacent to and downstream of the fuel feed, each of the 
structures contributing a distinctly oriented form of turbulence. 
It is a further object to facilitate use of the boiler as described, with 
pressurized water or other heat transfer fluid, by placing the exhaust gas 
outlet decoupler or manifold at an external surface of a generally 
cylindrical water jacketed structure, thereby reducing the size of 
structures needed to enclose the pressurized water volume and thus 
substantially reducing the weight of material needed. 
It is another object of the invention to meet the foregoing objects in an 
oil fired pulse combustor wherein an oil pressure pump supplies a 
continuous supply of atomized oil to the system and combustion is pulsed 
by a pressure/vacuum driven air inlet valve structure, and furthermore to 
control initiation of operation by sensing combustion and coordinating 
operation of a startup blower, spark ignition device and fuel pump. 
These and other objects are accomplished by the pulse combustor disclosed 
herein. The unit can be generally tubular externally, and the flow of 
combustion air, mixed fuel and air, combusting fuel and air, and then 
exhaust, can proceed through a thermally conductive conduit disposed 
substantially along a longitudinal centerline of the combustor and leading 
into an array of thermally conductive conduits for the hot exhaust gases. 
These structures heated by combustion are immersed within a housing 
forming a water jacket. The combustor is fed with pulsed air from an inlet 
decoupler having flapper check valves arranged to oscillate the feed of 
combustion air. The combustion air is led to a point at which atomized 
fuel is inserted, namely fed by a pump. An igniting device is provided for 
initiating combustion. The combustion heats the structures along the path 
through the area of combustion. After initiation, latent heat of the 
structures at and adjacent to the combustion chamber, and the presence of 
still-combusting fuel are such that the combustion becomes self 
sustaining. Such structures preferably include, for example, a protruding 
steel bar that extends into the combustion chamber, as in the '376 Hanson 
patent mentioned above. 
According to an inventive aspect, fuel and combustion air are fed toward 
the area of active combustion from a point upstream of an inlet orifice 
assembly along the gas flow path. The inlet orifice assembly defines an 
opening that has a smaller cross sectional area than that of the 
combustion chamber downstream of the inlet orifice assembly, and 
preferably also smaller than the dimensions of the conduit leading up to 
the inlet orifice assembly. The respective flow paths, chambers and 
openings can advantageously be circular/cylindrical. The reduced area 
inlet opening in the inlet orifice assembly provides a flow restriction 
that tends to induce an annularly rolling or toroidal form of turbulence. 
The restriction also increases the linear speed of the flow of mixed air 
and fuel in the area of the restriction as compared to points upstream and 
downstream of the restriction. The restriction can be irregular or shaped 
to provide smooth flow and minimum air pressure loss, and may involve a 
venturi effect. Downstream of the restriction is a preferably-louvered 
diverter having radially extending vanes that are canted in a 
circumferential direction and thus induce a helical component to the flow. 
By adding at least these successive distinct turbulence effects, namely an 
annular rolling turbulence downstream of the restriction and a helical 
turbulence downstream of the vanes, and optionally additional turbulence 
inducing structures encountered along the flow path, the fuel and 
combustion air are thoroughly mixed when the flow reaches the combustion 
chamber, for relatively complete and efficient combustion. 
In addition to the foregoing cross sectional restriction(s) associated with 
the inlet orifice, the combustor preferably further comprises a cross 
sectional restriction that is spaced downstream along the flow path from 
the inlet orifice, substantially comparable to the flame tube plate of the 
'376 Hanson patent. The area between the inlet orifice and the flame tube 
plate defines a mixing chamber upstream of the greater part of the zone of 
combustion, at which mixing chamber the combustion air and atomized fuel 
are turbulently mixed. The respective cross sectional restrictions, 
including the turbulence inducing structures at the inlet orifice and at 
the flame tube plate at the downstream end of the mixing chamber, are 
effective in providing a certain back pressure during the pressure phase 
of combustion to urge the expanding gases forward toward the exhaust. 
During ongoing operation (i.e., after a startup period), the structures 
along the flow path can become heated by the combusting fuel to a 
temperature above the ignition temperature of the air/fuel mixture, 
particularly structures that protrude into the flow path. These heated 
structures can provide a source of ignition after startup, while inducing 
further turbulence. Alternatively or in addition, still-combusting fuel 
that is encountered by mixed incoming air/fuel mixture can provide the 
means to ignite the incoming fuel after startup. In the preferred 
arrangement, at least one structure is disposed across the primary flow 
path of combusting gases, such as the protruding inconel steel bar of the 
'376 Hanson patent, which is heated during ongoing operation at least to 
the combustion temperature of the air/fuel mixture. 
Downstream of the combustion chamber, hot exhaust gases flow into an array 
of heat exchange tubes coupled to a circular chamber disposed axially 
adjacent to the combustion chamber. A first annular distribution plate 
directs the exhaust gases into an array of heat exchange tubes that extend 
axially back, parallel to and at a radial space from the combustion 
chamber. The heat exchange tubes curve radially outwardly and extend 
axially forward, again parallel to the combustion chamber, to a second 
annular distribution plate that is axially spaced from and radially 
outside the first annular distribution plate. Thus at least two levels of 
passing conduits, and optionally more levels, carry the heated exhaust 
gases and provide a heated surface for extraction of thermal energy. 
The heat exchange tubes and the combustion chamber are enclosed in an outer 
wall structure defining a water jacket in which the combustion chamber and 
the heat exchange tubes are immersed. The first annular distribution plate 
and the circular chamber bounded thereby are likewise immersed in the 
water jacket. The second annular distribution plate is disposed on the 
outside of the water jacket, and defines with an outer bell end of the 
device an exhaust decoupler chamber that is coupled to an exhaust conduit 
for ultimately venting the exhaust gas. 
Fuel such as fuel oil or the like is discharged into the flow upstream of 
the combustion chamber. Preferably the fuel is pumped, or otherwise is 
caused to provide a substantially continuous fuel supply during operation. 
The fuel nozzle can discharge from a point along a center line of the 
flow, axially positioned substantially flush with the inlet orifice or 
flow restriction. In a preferred embodiment, the fuel nozzle comprises a 
coaxially arranged fuel conduit along the center line, protected from 
substantial heating by a surrounding sleeve. The sleeve can be provided 
with openings that permit air flow between the fuel conduit part of the 
nozzle and the sleeve, for additional cooling effect. 
The air intake assembly supplies pulsed combustion air, for example from 
ambient air. The air intake assembly provides a means for restricting the 
rate at which combustion air is fed, and also functions as a pressure 
decoupling input plenum. The air intake assembly comprises an air 
decoupler chamber containing at least one and preferably a pair of 
bell-shaped housings, each having a check valve permitting only incoming 
flow. The check valve is in the form of an air flapper valve assembly that 
oscillates with resonant operation of the combustor. 
Inasmuch as the structures confining the area of combustion and the 
adjacent downstream exhaust are collectively enclosed in the water jacket, 
which forms a cylindrical water tank surrounding the heated structures, 
heat energy is collected efficiently in the water in the tank. The water 
jacket encloses the combustion gases up to the exhaust decoupler at the 
bell housing at which the combustor is coupled to the second or outer 
annular distribution plate and to the ultimate exhaust conduit. The 
exhaust decoupler at the bell housing is maintained slightly above 
atmospheric pressure, but can be at a substantially lower pressure than 
the water tank, which is optionally pressurized and is coupled into an 
external system that usefully employs the heat energy. Assuming that the 
water jacket contents are pressurized, the outer bell housing can comprise 
a relatively light sheet metal material because it confines the exhaust 
gas flow instead of a thick pressure resistant wall as would be needed to 
bound a pressurized water tank. The structure confining the water tank, 
namely the first or inner annular distribution plate, is sufficiently 
heavy to confine water tank pressure, but is correspondingly smaller in 
area than the outer bell housing. This arrangement renders the combustor 
both compact and relatively light in weight compared to the heat energy 
that it can generate. 
The turbulence inducing flow structures of the combustor and the physical 
confinement or encirclement of the combustion area and at least the 
upstream portion of the exhaust conduit(s) in the water jacket, provide 
for efficient thermal transfer to the water jacket of heat generated by 
the fuel during pulsating combustion. The water jacket preferably is 
coupled into a circulating water or steam system that makes use of the 
heat, such as a building heating system, an industrial process heat 
exchanger or another such application. This efficiency is achieved in a 
combustor that is lightweight, inexpensive and quite durable as compared 
to the heat capacity and efficiency at which it operates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A pulse combustor according to the invention as illustrated in FIGS. 1a and 
1b preferably includes, in succession along a flow path of the combustion 
and exhaust gases (generally from left to right in the figures): an 
oscillatory air inlet regulating section leading combustion air into a 
thermally conductive conduit and over and around an atomizing fuel inlet; 
certain restrictions or obstructions downstream of the fuel inlet and 
upstream of a mixing chamber, to assist in combining and mixing atomized 
fuel and inlet combustion air; a further restriction or flame tube plate 
disposed at the downstream side of the mixing chamber; an ignitor for 
initiating operation; and a heat exchanger in thermal contact with the 
conduit, in particular the portions of the conduit defining the combustion 
chamber and the exhaust pipes. 
In a preferred embodiment, the pulse combustor can be used as a water 
heater or boiler. The conduit(s) at least at the combustion chamber and 
along adjacent portions of the exhaust path are substantially enclosed in, 
or otherwise placed in thermal communication with water in a tank, thus 
forming a water jacket around the heated portions of the combustor. Heat 
generated through combustion is transferred insofar as possible to the 
water in the tank. The ultimate exhaust gas discharged along an exhaust 
path exiting the combustor is relatively cool compared to the combustion 
temperature, however combustion is relatively complete. The water tank of 
the pulse combustor can be coupled into various systems in which heat 
energy is usefully employed, such as a circulating water system in which 
the water heated at the combustor is circulated through one or more heat 
exchangers (not shown) at which heat energy is extracted and the water is 
routed back to the combustor. The further heat exchangers that transfer 
heat energy to an application can be associated with a building heating 
system, an industrial process or the like. The coolant can be water 
circulated wholly or partly in liquid form or as steam. Other coolant 
liquids and gases are also applicable. 
FIGS. 1a and 1b show a preferred embodiment of the pulse-combustion boiler 
according to the invention. The flow of fuel and air in the boiler 
proceeds from left to right in FIGS. 1a-1c, into and through a combustion 
chamber 12 at which the air/fuel mixture is burned. The combustion chamber 
12 in the embodiment shown is defined by a generally cylindrical flame 
tube 14, which extends substantially along the axial center of the 
cylindrical part or the boiler housing shown. 
The inlet portion of the flame tube and the path downstream of the flame 
tube where the exhaust is directed have restrictions that add turbulence 
and flow resistances to generate some back pressure at the combustion 
chamber. A partially obstructed opening 72 is provided at an inlet orifice 
assembly 70, upstream of and leading into the combustion chamber along the 
gas flow path. The inlet orifice assembly defines an upstream end of a 
mixing chamber 20 and a flame tube plate 16 forms another restricted 
opening 17 at the downstream end of the mixing chamber 20. Mixing is not 
limited to the mixing chamber 20, nor is combustion necessarily limited to 
the portion of flame tube 14 that is downstream of mixing chamber 20, 
preferably slightly downstream of the fuel nozzle 32. However the 
respective obstructions contribute turbulence and help to force expanding 
combustion exhaust gases downstream during at least part of the resonant 
combustion cycle. The increases and decreases in pressure applied 
backwards along the flow path also open and close the air check valves or 
flapper valves at the air inlet, as discussed in more detail below. 
Fuel is injected into the combustion air path by nozzle 32, adjacent to the 
upstream end of combustion chamber 12. Nozzle 32 is preferably protected 
and kept relatively cool by a coaxial sleeve 36 surrounding nozzle 32. 
Sleeve 36 has one or more holes leading incoming combustion air into the 
annular space between nozzle 32 and sleeve 36 for cooling purposes. The 
air and atomized fuel are combined in proper proportion to optimize fuel 
combustion. The air and fuel preferably are mixed thoroughly by turbulence 
at mixing chamber 20, by obstructions at inlet assembly 70 and flame tube 
plate 16, and also by obstructions such as steel bar 18 protruding into 
the flow path downstream of flame tube plate 16. 
In a preferred embodiment the combustion chamber 12 is substantially 
integrally formed as a machined one-piece casting, for example of iron. In 
a preferred mode of operation, air is supplied in excess of that required 
to achieve a stoichiometric combustion reaction. This ensures that a high 
proportion of the fuel is combusted, for good efficiency and so as to 
generate minimal by products that may accumulate and necessitate 
maintenance. The pulse combustor can include a controller (not shown) for 
varying the proportions of air and fuel (for example by control of the 
fuel pump), selectively or as a function of sensed operational parameters. 
A means for at least starting ignition is disposed in the combustion area 
12. The ignition means provides an initial spark or similar ignition 
source, for at least initiating combustion of the air/fuel mixture passing 
into and through in the combustion chamber. Alternative embodiments for an 
ignition means can include, for example, one or more electric arc 
generating devices (spark plugs), electrically heated elements (glow 
plugs,) pilot flames or the like. 
In a preferred embodiment the combustible mixture is ignited by an electric 
arc or spark generated by producing back EMF in a voltage step-up 
transformer to strike an electric arc between electrodes 39, shown in 
detail in FIG. 7. The electrodes can define a diverging or "Jacobs Ladder" 
type arrangement 37, as shown. A controller stores electromagnetic energy 
in the transformer by coupling the primary winding of the transformer to a 
dc voltage component. An inductively spiked back-EMF pulse is generated on 
the secondary by abruptly decoupling the dc voltage, which can be done 
repetitively during ignition to generate successive arcs between diverging 
conductors extending into the path of the air/fuel mixture. The arc is 
initially struck between the conductors at the point where they are 
closely spaced, and the arc moves outwardly along the divergence. 
In a startup phase, the controller operates the ignitor repetitively while 
supplying the inlet with forced air by switching on power to a blower that 
forces air along the flow path. After ignition commences, resonance drives 
the flow, including expansion of combusting gases followed by a drop in 
pressure as the expanded exhaust is drafted out, thereby drawing 
additional air and entrained fuel into the combustion chamber. When the 
ignition spark is detected at the ignitor, for example by a UV cell 
coupled to the controller, a fuel pump is enabled by the controller to 
initiate fuel flow. Ignition commences. When a flame is detected in the 
combustion chamber by a temperature sensor, UV scanner or other similar 
sensor, and preferably is maintained for a minimum time interval 
sufficient to ensure that the combustion chamber and the combusting 
air/fuel mixture therein has been heated to a temperature that will 
sustain combustion, the controller may disable the inlet blower and 
discontinue generating the ignition spark. 
As air flows and fuel is pumped into the combustion chamber, pulsed 
combustion ensues and reaches a steady state. The flow is driven 
resonantly in a cycle of alternate expansion of the burning combustion 
gases, and convective draw of hot gases through the exhaust conduits, the 
latter also drawing combustion air through the flapper valve air inlet to 
continue the process. 
The fuel nozzle assembly 30, comprising nozzle 32 and sleeve 36, supplies 
fuel adjacent to the upstream end of the combustion chamber 12, and 
preferably at the inlet to a mixing chamber 20. The illustrated embodiment 
operates using atomized fuel oil, which can be discharged using known 
nozzle types. Other types of fuel such as natural gas are also possible. 
The nozzle assembly is disposed substantially in an air plenum chamber 31 
upstream of mixing chamber 20, and discharges in the direction of flow, 
preferably in a diverging pattern of atomized fuel droplets. 
During operation, fuel is continuously supplied to fuel nozzle assembly 30 
at a constant rate via a fuel line coupled to a pump (not shown). Such 
operation can be continuous or controlled by a demand thermostat 
arrangement in a known manner. In a preferred embodiment, the fuel pump is 
operable at least at two distinct flow rates, i.e., a higher flow rate and 
a lower flow rate, selected by the controller and/or by user switch 
inputs. The flow rate also may be sensor controlled, for example on 
startup the controller can operate the pump at a lower input rate for 
initial spark ignition, and then can switch over automatically to the 
higher input rate after a main flame is detected and conditions indicate 
that combustion has become self-sustaining. Additionally the user may be 
provided with means to alter the fuel flow rate to adjust or switch the 
level at which the combustor generates heat. 
With reference to FIGS. 1b, 1c and 3, an inlet orifice assembly 70 leading 
into mixing chamber 20, divides the plenum chamber 31 from the combustion 
area 12. The inlet orifice assembly includes an inlet opening 72 which is 
smaller in flow cross section than the internal dimension of the 
combustion chamber downstream of the orifice assembly 70. This provides a 
flow restriction. Restrictions along the flow path have at least two 
distinct functions due to the back-and-forth motion of the gas that is 
caused by the oscillating pulsed flow. Restriction(s) along the flow path 
provide back pressure during the combustion or positive pressure phase of 
each cycle of pulsed combustion, namely a pressure component directed 
opposite the flow path due to expansion of the combusting air/fuel 
mixture. The restrictions along the flow path thus form a backstop 
structure against which the force of expanding combustion gases can act to 
force the exhaust gases forward along the flow path. In the reduced 
pressure phase of the resonant cycle, convective draw due to expanded 
exhaust gas disposed in the conduits downstream of the combustion chamber, 
draws inlet air forward along the flow path. 
The flow restriction(s) increase the linear speed of the fuel/air mixture 
in the area of the cross sectional restriction, compared to flow at the 
same rate through a larger cross section, and also induce turbulence in 
the form of eddies on the downstream side of the restriction. Both the 
speed increase and the turbulence contribute to fuel/air mixing and thus 
to combustion efficiency. 
In the preferred embodiment shown, the inlet opening 72 provides a 
reduction in diameter or restriction of the cross sectional area of the 
flow path. Downstream of the restriction the gases are turbulent, 
including a generally toroidal eddy current motion. In addition, 
downstream of the inlet opening 72 is a diverter 73 having a series of 
circumferential vanes 74 which induce a second turbulence effect to 
facilitate mixing of the fuel and combustion air. In the preferred 
embodiment, the diverter comprises a frustoconical member 76 connected to 
a supporting ring 77 by a series of radially elongated vanes 74. Each vane 
comprises a sheet metal blade that is tilted relative to a plane normal to 
the axis, in a circumferential direction, to divert the flow in a 
circumferential direction as it moves axially forward, i.e., to impart a 
helical flow. The toroidal eddy currents and the helical diversion add 
together two or more distinct forms of turbulence to provide a complex 
meandering flowpath for droplets of fuel and surrounding combustion air, 
and a good mixing effect. The diverter opening 75 likewise is preferably 
smaller than inlet opening 72 to augment the turbulence-inducing effect of 
the orifice assembly 70 by forming a restriction in cross sectional area, 
further improving mixing of fuel and combustion air. 
Each flapper valve assembly 50 is enclosed in an inlet air decoupler 
assembly 40 which communicates with the plenum chamber 31 of the fuel 
nozzle assembly 30. The inlet decoupler serves as a receiver and 
distributor for the combustion air, as well as an expansion space that 
muffles sound waves emanating from the resonant combustion process taking 
place further on, particularly in combustion chamber 12. 
As shown in FIGS. 2a and 2b, each flapper valve assembly 50 comprises two 
circular plates, namely a backer plate 51 and an orifice plate 52. The 
backer plates and orifice plates of the respective valves are each aligned 
along a common central axis and fixed in a predetermined spaced 
relationship by a spacer 53. A plurality of guide pins 54 are fixed 
between plates 51, 52. Preferably, the orifice plate 52 has a number of 
circular apertures and the backer plate 51 has a number of circular and 
slot apertures. A flapper wafer 55 having alignment holes corresponding to 
guide pins 54, is movably disposed between orifice plate 52 and backer 
plate 51. The flapper wafer 55 oscillates between the plates 51, 52, along 
the path of guide pins 54 through each combustion cycle. The flapper wafer 
55 includes a plurality of slots that are aligned with the slot apertures 
of backer plate 51 and misaligned with the circular apertures of orifice 
plate 52. Thus, in one position the flapper wafer 55 blocks backward flow 
and in the other position forward flow is permitted, the valves being 
arranged to pass air from the inlet decoupler 40 and to block air from 
returning, in the manner of check valves that oscillate open and closed at 
the resonant pulsed combustion frequency. The flapper wafer is formed from 
a flexible, heat and wear resistant material. Preferably, the flapper 
wafer comprises a Teflon coated fiberglass wafer. Each flapper valve 
assembly 50 is mounted in an air conduit, which communicates with the air 
plenum 31. 
Referring again to FIGS. 1b and 1c, downstream of the combustion chamber 
12, hot combustion gases flow through an array of exhaust tubes 62, 
coupled to a circular chamber 63 disposed axially adjacent to the 
combustion chamber 12. In the embodiment shown, a first annular 
distribution plate 65 has a relatively smaller diameter than a second 
annular distribution plate 67. These distribution plates form axially 
spaced annular rings at different radii, with the space between them 
traversed by tubes 62, which loop axially back and radially outwardly to 
occupy a substantial part of the volume of the water jacket. The first 
distribution plate 65 directs the exhaust gases from the combustion 
chamber 12 into the exhaust tubes 62. The exhaust tubes in the embodiment 
shown extend longitudinally back, parallel to, and at a radial space from 
the combustion chamber 12. The exhaust tubes then curve radially outwardly 
and then forward, again parallel to the combustion chamber, to the second 
annular distribution plate 67. Second distribution plate 67 is axially 
spaced from and radially outside the first annular distribution plate 65. 
The exhaust tubes can be oriented such that respective back and forward 
extending portions of each tube are radially aligned. However preferably 
the exhaust tubes are radially skewed, as best shown in FIGS. 5a and 5b. 
More particularly, the point at which each U-shaped exhaust tube connects 
to the first or inner distribution plate 65 is angularly displaced from 
the point at which that tube connects to the second or outer distribution 
plate 67. In the embodiment shown the skew is equal to the angular 
distance between one tube and the next. This skew is such that the 
respective back and forward axially extending tube portions are angularly 
offset from a radial extension line, and the curving ends of the tubes are 
compactly stacked or angularly overlapped as shown in FIG. 5b. This 
arrangement increases the length and surface area of the tubes as compared 
to an arrangement in which the tubes are radial rather than skewed, to 
provide for an efficient and compact design. 
The exhaust tubes 62 and combustion chamber 12 are enclosed in an outer 
wall structure defining a water jacket 64 in which the combustion chamber 
and exhaust tubes 62 are completely immersed. The first annular 
distribution plate 65, the first partition plate 68 (shown in FIG. 1c), 
and the circular chamber 63 bounded thereby, are likewise fully immersed 
in the water jacket 64. Immersion of these relatively-hotter upstream 
elements provides good transfer of thermal energy from the combustion 
gases to the circulating water. 
According to another inventive aspect, the second annular distribution 
plate 67 and the second partition plate 69 are not fully immersed. Instead 
these relatively cooler downstream elements are disposed at the outside 
boundary of the water jacket. That is, bell housing 61 bounds a portion of 
the exhaust flow path together with second annular plate 67 and second 
partition plate 69, such that the exhaust decoupler 60 is disposed at the 
outside boundary of the unit rather than being immersed within and bounded 
on all sides by the water jacket (compare inner structures 63, 65). The 
volume inside bell housing 61 contains exhaust air and thus is pressurized 
at most only slightly above the ambient atmospheric pressure that is 
incident on the opposite side of bell housing 61. In this manner, bell 
housing 61, which is the boundary element having the greatest span, need 
not withstand a substantial pressure difference. 
The water enclosed in the water jacket preferably is pressurized in the 
typical application of the invention to a boiler. Thus the inner 
distribution plate 65 and inner partition plate 68 must withstand a 
pressure differential between the water or vessel pressure and the 
pressure within the exhaust decoupler, only slightly above atmospheric 
pressure. Thus there is a substantial pressure difference across plates 
65, 68. These plates are accordingly made of a thick and durable material 
and are securely bolted or otherwise mounted, as needed to confine the 
pressure vessel. In contrast, the pressure difference across the bell 
housing is minimal, so the bell housing can be constructed of relatively 
light material such as thin sheet metal and need not be as securely 
mounted. In short, according to the invention the exhaust decoupler is not 
located at a position immersed on all sides within the water jacket, as 
might be considered appropriate for maximum thermal transfer between the 
exhaust gases and the water in the water jacket. Instead, the exhaust 
decoupler is in contact with the water jacket but is located outside of 
the water jacket, bounding the outer periphery of the combustor unit. As a 
result, the bell end or bell housing is made a relatively lighter or 
thinner material than the inner elements that confine the water jacket and 
need to be made of a heavy pressure-resistant material. However, these 
inner elements span a shorter distance than the bell housing because they 
are internal of the bell housing. At a given pressure in a pressure 
vessel, an increase in linear dimensions or span of a vessel defining wall 
may require a geometrically corresponding increase in thickness. However 
according to the invention the pressure vessel defining elements are 
internal and therefore smaller than the outer elements. The invention 
allows the overall combustor unit to be constructed from lighter 
materials, at considerably reduced expense, without compromising the 
structural integrity of the boiler, and with minimal effect on thermal 
transfer efficiency. 
The exhaust decoupler 60 is positioned on the bell end of boiler, 
downstream of the combustion chamber 12, and communicates with the 
combustion chamber via the array of exhaust pipes 62. The exhaust 
decoupler 60 serves as a receiving chamber for exhaust gases and as an 
expansion space to muffle sound waves emanating from the pulsed combustion 
in combustion chamber 12. In the preferred embodiment as shown, 
twenty-four 1.25" outer-diameter exhaust pipes couple the combustion 
chamber 12 to the exhaust decoupler 60. 
The water jacket 64 has inlet and outlet ports (not shown) for circulating 
water through the tank under power of a pump or similar element in an 
external circulating water system adapted to make use of the heat energy. 
Heat generated by combustion is transferred to the circulating water by 
contact between the hot gases, the thermally conductive structures 
separating the combustion chamber from the water jacket, and the water or 
other coolant contained in the water jacket. The rate of thermal transfer 
can be enhanced by increasing the surface area of contact, for example by 
providing heat transfer fins (not shown) to increase the surface area of 
contact. Normally, such fins are only employed on the gas side of the heat 
exchange path because contact between the denser water and the surface 
more effectively conducts thermal energy than contact on the gas side. 
The sequence of operation of the apparatus can be appreciated with further 
reference to FIG. 6, which shows a schematic relay ladder diagram with 
associated relays, actuators, sensor and switches. When the power on/off 
switch is closed, and there is sufficient water to close a level 
responsive switch, and the temperature is between an operating temperature 
and a high limit temperature, a call for heat is enabled. The pulse 
combustion process begins by activating a starter blower to initially 
pressurize the system, and preferably by first operating at a low speed 
blower level for a pre-purge period, for example, five seconds, after 
power-on and heat-enabled, as provided in the operating control. 
Initially, combustion air is fed under blower power; the ignitor is 
operated periodically for a starting interval through an ignition 
transformer; and if the UV scanner detects a spark or flame, fuel is 
provided. At 15 seconds from the start of this function, the blower is 
switched to high speed by time delay relay TDR-1. Combustion proceeds and 
after a time may become self-sustaining with the convection and expansion 
of hot exhaust gases being sufficient to draw combustion air and to expel 
the exhaust. After a timed interval, the starter blower may be 
deactivated. 
Sinusoidal pressure waves are generated by pulsed combustion fed by 
oscillatory air from the flapper valves in a resonant manner. The pulsed 
pressure waves provide the necessary force to draw combustion air and to 
drive exhaust flow. Air flow input through the decoupler forces the 
flapper wafer 55 of each flapper assembly 50 against its backer plate 51, 
allowing air to pass through each flapper assembly and into the air plenum 
31. Pressure during the combustion part of the cycle closes the flapper 
assemblies in the manner of check valves. The pressure against the closed 
flappers contributes to moving the exhaust gases forward toward the 
exhaust. The flow is also driven in part by convective drawing of the 
exhaust, and in part by back pressure from expansion of combusting gases 
against obstructions along the flow path during a part of the resonant 
combustion cycle. 
Atomized fuel is introduced through the fuel nozzle assembly 30, through 
the inlet orifice assembly 70, and into the combustion chamber 20. The 
fuel combines with sufficient air at least to form a combustible air/fuel 
mixture, and preferably to form a combustible mixture with excess oxygen 
for clean burning. Mixing is facilitated by turbulence due to 
obstructions, preferably successive forms of turbulence involving eddy 
currents oriented in different directions. The air/fuel mixture is ignited 
initially by the spark generated from the transformer, the spark being 
struck between diverging electrodes in a "Jacob's Ladder" type electrode 
arrangement. After a startup interval combusting fuel and residual heat in 
the combustion chamber ignite the incoming air/fuel mixture without the 
need for an additional source of ignition. Sensors and controls are 
provided to enable fuel flow upon detection of a spark, to change the rate 
of fuel flow and may disable the combustion air blower upon detection of a 
main flame, and generally to enable or disable operation of the entire 
unit depending on temperature, flow and coolant levels remaining at safe 
levels. Combustion continues with the main flame and the heated surfaces 
in the chamber sustaining combustion so long as fuel and air continue to 
be supplied. 
The ratio of combustion air to fuel is determined in part by the structure 
of the flapper valve. By increasing the axial space occupied by spacer 53, 
a higher ratio of air to fuel is obtained. Preferably the combustor is 
operated with a combustion air feed rate greater than a stoichiometric 
rate needed precisely to react with all the available oxygen with all the 
available hydrocarbons (i.e., air is oversupplied to the combustion 
reaction), whereby the combustor is clean burning. 
Oxidation of the fuel oil and propagation of the flame continue along the 
flow path and into the combustion chamber 12. One or more restrictions or 
flow directing vanes or the like proved turbulence. Combustion gases 
exiting the combustion chamber are distributed through the twenty-four 
exhaust pipes, which are immersed in the volume of water in tank 64. 
Upon exiting the exhaust pipes, the combustion gases are further expanded 
and decoupled in the manifold referred to as the exhaust decoupler 60. The 
combustion chamber 12 and the exhaust pipes 62 are each substantially 
enclosed within the cylindrical water tank 64, thus facilitating heat 
transfer from the hot combustion gases to the water at the point where the 
temperature difference between the combustion gases and the water is high 
and the air/fuel mixture flow is relatively turbulent and high in speed, 
for efficient thermal transfer. From the exhaust decoupler 60, which is 
disposed in part at the relatively cooler outer perimeter of the unit, 
outside the volume enclosed by the water jacket, the flue gases are 
vented. Preferably the flue gases pass to the outdoors through a chimney 
(not shown). 
During the low pressure or drawing phase of the combustion cycle, oxygen 
bearing gas passes through the apertures of the flappers, the orifice 
plate 52 and backer plate 51. During the high pressure phase of the 
combustion cycle, expanding gases force flapper wafer 55 away from the 
backer plate 51 and against the orifice plate 52. This blocks the 
apertures in the orifice plate 52, creating back pressure which forces the 
combustion exhaust to advance through exhaust pipes 62. As the cycle 
continues, back pressure drops off with convective flow of the exhaust, 
and a partial vacuum arises. Wafer 55 then is lifted from orifice plate 52 
and bears against backer plate 51, again permitting inlet combustion air 
to be drawn in. These phases are resonantly cycled at a relatively low 
frequency, for example about 65-75 Hz, the precise frequency being 
determined by the respective dimensions of the combustor and by various 
physical parameters. 
The flapper valve assemblies 50 regulate the feed of combustion air, and 
the travel of the flapper wafer, determined in part by the size of the 
spacer, is a factor in the flow rate. The flow rate can be adjusted by 
providing for a choice of spacers of different sizes, such that the boiler 
can be operated at higher or lower levels of excess air, as required. 
The invention having been disclosed in connection with the foregoing 
variations and examples, additional variations will now be apparent to 
persons skilled in the art. The invention is not intended to be limited to 
the variations specifically mentioned, and accordingly reference should be 
made to the appended claims rather than the foregoing discussion of 
preferred examples, to assess the scope of the invention in which 
exclusive rights are claimed.