Pre-vaporizing liquid fuel burner

A pre-vaporizing liquid fuel burner with an eductor mounted to the end of a burner housing having an internal chamber. The eductor has air ducts disposed in a throat of the eductor through which combustion gas and air flow. A perforated flameholder is contiguous with the gas inlet of the eductor throat. The air flowing from the ducts creates a suction at the downstream side of the eductor throat which, in turn, draws the combustion gas from the center of the flame into the throat. The streams of air and gas flow from the eductor in interleaving streams to a mixing zone and then to the chamber downstream of the eductor. The interleaving streams are non-parallel to the burner axis but flow in an axisymmetric pattern to produce turbulent secondary eddies or flow patterns for complete mixing of air and combustion gas in a short distance. Liquid fuel is sprayed into the axisymmetric flow of the air-gas mixture where it evaporates. The combined air-gas-fuel vapor mixture is then fed to the flameholder and ignited.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to burners and, more particularly, is 
directed towards pre-vaporization liquid fuel burners. 
2. Description of the Prior Art 
Oil fired burners are well known in the art. Gun type oil burners are 
characterized by a flame which is large, poorly mixed and highly 
turbulent, the flame being produced by direct spraying of fuel oil into 
air. U.S. Pat. No. 3,738,532 discloses an oil gasification burner in which 
oil is sprayed into a preheated mixing chamber and is vaporized to produce 
a combustible gas. U.S. Pat. No. 3,980,422 shows a liquified fuel burner 
in which fuel oil is gasified by being mixed with hot burned gases drawn 
from a combustion chamber, the heat of vaporization of the oil being used 
to cool the returning gases. U.S. Pat. Nos. 1,597,661; 3,174,526; 
3,277,945; 3,399,022; 3,545,902; 3,604,824; 3,620,657; 3,897,199; 
3,927,958; 3,994,665; and 4,003,691 disclose burners which utilize 
recirculated combustion gases, pre-mixing eductors, and flameholders. U.S. 
Pat. No. 3,632,284 discloses a dual fuel burner having a combustion gas 
feedback conduit to supply heat for gasification of the fuel oil. 
Generally, recirculation burners have good performance and very clean 
combustion, but they are relatively complex in design and costly to 
manufacture. In addition, such prior art burners have encountered varying 
degrees of success due to their sensitivity 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a prevaporizing liquid 
fuel burner in which a fully gaseous mixture is prepared separately and 
before combustion occurs. 
Another object of the invention is to provide a prevaporizing burner for 
liquid fuel wherein the fuel is vaporized by the heat of a precisely 
controlled mixture of combustion gases and air. The combustion gases, fuel 
vapors and air mixture are uniformly mixed, before chemically homogeneous 
combustion occurs on a perforated flameholder surface. The burner is 
characterized by a housing with an internal chamber. A perforated 
flameholder and an eductor are mounted at the front end of the chamber and 
a fuel nozzle is mounted at the back end thereof. The eductor has air 
ducts through which air flows and a throat through which combustion gas 
flows. The air flow ducts are disposed in the throat and positioned to 
direct air toward the longitudinal axis of the chamber in a vortical flow 
pattern. The air streams flowing out of the ducts create a suction at the 
downstream side of the throat which draws the combustion gas from the 
center of the flame into the eductor throat. The flame is supported on the 
perforated flameholder which is contiguous with the inlet side of the 
eductor throat. Interleaving streams of air and combustion gas flow from 
the eductor ducts and throat and are axisymmetrically mixed in a mixing 
diffuser zone in the internal chamber downstream of the eductor. Fuel is 
sprayed into the rear of the chamber through the nozzle and mixed with the 
mixture of combustion gas and air. The combined mixture of fuel vapor, 
combustion gas, and air is directed to the perforated flameholder for 
ignition and external combustion. 
Other objects of the present invention will in part be obvious and will in 
part appear hereinafter. 
The invention accordingly comprises the apparatus together with its parts, 
elements, and interrelationships, that are exemplified in the following 
disclosure, the scope of which will be indicated in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, particularly FIG. 1, there is shown a 
pre-vaporizing liquid fuel burner 10 embodying the present invention. 
Burner 10 includes a housing 12 having an internal chamber 14 wherein air 
is mixed with combustion gas recirculation (CGR) to provide 
pre-vaporization of fuel, for example oil, which is sprayed into the rear 
of the chamber through a nozzle 16. CGR from the center of a flame 18 is 
drawn into a throat 20 of an eductor 22 by the suction created from the 
rearwardly flowing air streams in ducts 24 in the eductor. Flame 18 is 
supported on a perforated flameholder 38 which surrounds the inlet side 27 
of throat 20. Flameholder 38, on which flame 18 is supported and from 
which the CGR is drawn into eductor 22, is a screen in the form of a 
dome-shaped structure having a plurality of holes in the range of 0.01 of 
an inch to 0.1 of an inch. Air ducts 24, for example four air ducts, all 
have inwardly tapering sidewalls, the ducts being offset in a somewhat 
helical path from the longitudinal axis of housing 12. A blower 26 blows 
air through ducts 24 and into a forward zone 25 of chamber 14. Preferably, 
the air streams flowing out of ducts 24 in eductor 22 are directed toward 
the longitudinal axis of chamber 14 in a vortical path. The air streams 
create a suction at the downstream side or outlet 29 of throat 20 which 
draws CGR from the center of flame 18 into the mouth 27 of throat 20. The 
air flowing from ducts 24 and CGR flowing from the outlet 29 of throat 20 
exit eductor 22 in substantially interleaving streams. 
The interleaving streams of CGR and air are mixed in an intermediate mixing 
zone 28 in chamber 14, the mixing zone being directly downstream of 
eductor 22. Downstream of mixing zone 28 are flow deceleration zones 30 
and 32. The deceleration zones 30 and 32 are provided to maintain 
axisymmetric flow of the air and CGR mixture. The mixed CGR and air are 
directed to the primary fuel vaporization zone 34 which is downstream of, 
or coexists with deceleration zone 32 at the rear of chamber 14. Fuel from 
nozzle 16 is sprayed into vaporization zone 34, the nozzle being 
axisymmetric with the longitudinal axes of chamber 14 and eductor 22. 
Deflector vanes 36, which are mounted to the rear wall of housing 12 and 
project into vaporization zone 34, aid in mixing of the fuel, CGR, and 
air, and direct the mixture through passageways 37 to flameholder 38. 
The design of burner 10 is such that the fuel is completely vaporized and 
well pre-mixed with the air to produce a compact flame tight on the 
flameholder 38. The combustion gas, which is drawn from the center of the 
flame 18, provides the necessary heat for pre-vaporization of the fuel. 
The gas, air, and fuel flow path is compact and short. The various 
functions of inducting combustion gas from the center of the flame 18, 
mixing combustion gas with air in order to attain the proper temperature, 
mixing the proper amount of fuel, providing complete vaporization of fuel, 
and uniform mixing of this vapor with the air and gases are discussed 
hereinafter. 
The sequence for pre-mixing and pre-vaporizing the fuel with the combustion 
gas and air mixture begins with the eduction of combustion gas from the 
center of the flame 18 by the suction created by the air flowing through 
ducts 24 and out of the outlet 29 of throat 20 in forward zone 25. As 
previously indicated, the CGR at mouth 27 of throat 20 is surrounded by 
the flameholder 38. Hence, the flame 18 substantially covers the mouth 27 
of the burner 10. In order the prevent significant heat loss from this 
portion of the flame 18, the radius of mouth 27 is small, about equal to 
the thickness of the primary flame zone on the surrounding flameholder 38. 
Thus, the quality and quantity of the CGR is insensitive to the thermally 
buoyant flows outside of the flame 18 and the chilling effect of a cool 
furnace wall as long as it is beyond this primary flame zone. 
The design of the CGR inlet at the mouth 27 is such that the periphery of 
the inlet contiguous with the flameholder 38 is kept cool enough so that 
the flame does not flash back upstream of the flameholder. In addition, 
the metal contiguous therewith is kept cool in order to avoid either 
distortion or expensive alloys in fabrication. Cooling of the burner 
structure surrounding the CGR inlet at the mouth 27 is accomplished by 
having a conduit 39, which supplies air to eductor 22, contiguous with 
both the flameholder 38 and the CGR inlet at the mouth 27. Thus, the air 
conduit 39 that is annular to the CGR inlet at the mouth 27, and ducts 24 
provide a cooled wall for guidance of the CGR the short distance to the 
eductor throat outlet 29. Note that all flows are closely guided and that 
the CGR is cooled only by the eductor motive air. Accordingly, heat is 
conserved for fuel vaporization independent of the effectiveness of heat 
transfer through these internal burner surfaces. 
The amount of CGR educted for pre-mixing is precisely controlled to yield 
the desired final mixed temperature of CGR, air, and fuel vapor at 
thermodynamic equilibrium. Preferably, the condensation temperature or dew 
point of the mixed fuel vapor or any fraction thereof is less than the 
mixture temperature, and less than any wall temperature of a mixture 
conduit 40 to flameholder 38. Control of CGR and air flows is achieved by 
adjusting the size and shape of all conduits and ducts so that the motive 
pressure generated in eductor 22 balances the pressure losses in the rest 
of the flow path. 
On the other hand, the amount of air mixed with the CGR is controlled so 
that the equilibrium temperature is less than that which will result in 
rapid spontaneous ignition when the fuel is introduced into the air-CGR 
mixture. That is, the mixture temperature in the fuel vaporization zone 34 
provides an ignition delay time that is long compared to mixture residence 
time in the burner. These temperature limits of condensation and ignition 
delay are typically 500.degree. F. and 800.degree. F., respectively, for 
typical residential fuel use. For reliable, practical application of the 
concepts of condensation and ignition delay, precise mixing is required in 
order to avoid local regions that are either too hot or too cold, 
resulting in either catastrophic pre-ignition or poor uniformity of the 
flame and long-term build-up of carbonaceous deposits. 
As previously indicated, educted CGR and the motive air are brought into 
initial contact at the outlet 29 side of throat 20 of the eductor 22. The 
design of throat 20 is such that, in addition to achieving efficient 
pumping (i.e., high output per given input conditions), the output flow 
profile of the CGR and air mixture is spatially symmetric and 
thermodynamically uniform. Otherwise, it would be difficult or practically 
impossible to avoid the drawbacks of local hot or cold spots and to 
achieve very uniform combustion of an ideal mixture. 
Symmetric, uniform, efficient mixing is initiated in forward zone 25 by the 
use of an air nozzle shape, or air-CGR boundary wall surface, that 
effectively subdivides the air and CGR into several substantially discrete 
and interleaving flow streams. This has the effect of reducing the length 
of the mixing zone 28 needed to obtain a fairly uniform flow profile. 
Throat 20 and mixing zone 28 are precisely axisymmetric, and provide the 
structure which is required to achieve uniform subsequent process 
functions. In addition to subdividing and interleaving the air and CGR 
flow streams in zone 25, it is advantageous to direct the flows so that 
they are not parallel. Preferably, if the air injection flow is subdivided 
into streams that are not parallel to the general centerline of the mixing 
zone 28, inclined or skewed or both, then vortical, spiral, or so-called 
secondary flows are produced. The secondary flows result in complete 
mixing of air and CGR completely in a short distance of flow. This 
provides axisymmetric flow into the flow deceleration region zone 30 and 
precludes the existence of hot streaks of flow downstream where there is 
contact with the fuel-air mixture and where ignition can occur. 
In combination with the mixing zone 28, the design of the flow deceleration 
zone 30 is such that axisymmetric flow is maintained by limiting the area 
ratio across this diffuser zone 30 to less than a value of about two, if a 
diffuser is used at all, followed by an abrupt expansion of flow area in 
deceleration zone 32, all axisymmetric with the eductor inlet 27, outlet 
29, and mixing zones 28. Large ratio of diffuser areas promote asymmetric 
flow or flow separation. Abrupt or sudden expansions result in 
axisymmetric recirculation or eddy flow, which is not the case with more 
gradual transitions between large area ratios. 
The air and CGR mixture at the desired temperature flows into the primary 
fuel vaporization zone 34. Fuel is sprayed or injected into the air and 
CGR mixture and onto the walls of the chamber with a uniform distribution. 
One preferred method of fuel injection is with fuel nozzle 16 which is 
axisymmetric with the longitudinal axis of chamber 14 and eductor 22. The 
air-gas mixture and fuel are symmetrically distributed and the intensive 
mixing already existing in the air-gas stream provides satisfactory 
uniform distribution, near-equilibrium, of the fuel evaporated into the 
stream. Of course, the fuel vaporization is due to heat supplied directly 
from contact with the air-CGR stream, as well as heat supplied indirectly 
from the chamber walls that are heated by the stream. 
In the present invention, extensive mixing is accomplished by exchanges of 
flow-species or packets of flow rapidly across the entire stream. 
Extensive and intensive mixing provide substantial equilibrium before 
combustion. The preferred method for obtaining extensive mixing in and 
downstream of the vaporization zone 34 is to promote swirl flow around the 
general axis of flow of the air-gas-fuel mixture. Various methods for 
introducing swirl include skewing of the air streams about the axis of the 
eductor inlet 27; insertion of vanes 36 to swirl the flow downstream of 
the vaporization zone 34; insertion of vanes (not shown) in zone 34; and 
guiding the flow from the vaporization zone 34 to the mixture delivery 
duct 40 so that there is swirl flow in the delivery duct. In all of the 
above methods, it is desirable to either subdivide the flow 
aerodynamically into separate flow streams, or to provide sufficiently 
high swirl to cause secondary or back-mixing flows of an extensive nature. 
Delivery duct 40 provides for optimum combustion and consistency of 
operation by regulating the flow so that there is sufficient time for the 
pre-mixture to approach equilibrium and by maintaining the pre-mixture 
everywhere in the range between its dew point and pre-ignition 
temperatures. Temperature control in delivery duct 40 is provided by 
insulating the walls of the duct and preventing heat loss of the 
pre-mixture either to the surrounding structure and ambient air or to the 
input of fresh air for combustion. In the preferred embodiment shown in 
FIG. 1, external insulation 44 is provided by typical light, fibrous 
matting wrapped around the burner gun-barrel, and internal insulation 46 
is provided by impermeable sleeves or doublewall construction limiting 
direct contact of the fresh air with the hot delivery duct 40. 
Induced draft or forced draft and fuel injection can be provided to burner 
10 many of the common ways. For example, FIG. 1 shows a directly coupled 
typical fresh air blower 26 and a nozzle 16 that is supplied with fuel 
from a tank by a pump through a filter (not shown). Air and fuel supply 
systems providing variable firing rate or adjustments are compatable with 
the basic burner 10. The method of using several sequential zones in the 
chamber 14 for optimum preparation and control of the pre-mixture is 
especially suitable for use with a variable firing rate because the 
air-to-CGR ratio, hence mixture temperature, remains substantially 
constant as the air and fuel flow are varied. 
Startup consists of air and fuel flow initiation and control, possibly with 
pre-heating, and ignition. A nominally rich mixture may be ignited without 
pre-heating as a result of vaporization of light ends of fuel, in which 
case CGR causes very rapid heatup to near steady state conditions. In 
order to ensure ignition under the worst ambient conditions, as well as to 
minimize transient formation of combustion products that may form deposits 
in the long term, it is advantageous to preheat the internal structure of 
burner 10, particularly the vaporization zone 34 in chamber 14 and 
adjacent conduits. This is accomplished by the use of an electric heating 
coil 50 shown in FIG. 1, for example. With pre-heated walls, the air-gas 
stream instantly reaches the desired temperature upon ignition of the 
pre-mixture and generation of CGR. FIG. 1 shows a spark plug 52 mounted in 
the burner gun-barrel outside of the furnace 54 and a flame tube 56 from 
the spark to the main flameholder 38, internal to the delivery duct 40. 
Thus, ignition migrates or is thrown from the spark to the steady flame 
zone. In an alternative embodiment, a spark conductor is inserted in 
burner 10 and extends to a hole in the flameholder 38; or if the forward 
velocity in the delivery duct 40 is high enough to carry the flame front 
to the flameholder 38, then ignition can be anywhere in this duct. 
Shutdown of the burner is typically on demand of the heating system or 
necessary because of malfunction of the burner. The latter is conveniently 
signaled by a temperature sensor (not shown) within the body of the burner 
itself or, as is more usual but difficult, sensing the flame itself. 
As will be seen, ancillary functions and various design features do not 
specifically limit the novel basic design concept just described for a 
liquid fuel, pre-mixed, pre-vaporizing burner having superior performance. 
Since certain changes may be made in the foregoing disclosure without 
departing from the scope of the invention herein involved it is intended 
that all matter contained in the above description and depicted in the 
accompanying drawings be construed in an illustrative and not in a 
limiting sense.