GB2053447A

GB2053447A - Blue flame burner

Info

Publication number: GB2053447A
Application number: GB7940415A
Authority: GB
Original assignee: BLUERAY SYSTEMS Inc
Current assignee: BLUERAY SYSTEMS Inc
Priority date: 1979-07-09
Filing date: 1979-11-22
Publication date: 1981-02-04
Also published as: IT8049179A0; FR2461196A1; GB2053447B; SE8004978L; CA1141652A; IT1127517B; DE2946028A1

Abstract

A burner adapted to operate within a combustion chamber associated with a heater for burning liquid hydrocarbon fuel to produce a stable blue flame. In operation, fuel is pumped along supply line 48 to nozzle 50 which sprays fine droplets of fuel through a diffuser 70 having a central constricted region into combustion chamber defined by wall 16. Air under pressure is delivered by lower 38 through housing section 40 and apertures 43 in the form of jets for mixing with the fuel droplets. The air/fuel mixture is ignited by electrodes 60. A portion of the gaseous products of combustion is recirculated past annular baffle 78 and between diffuser 70 and nozzle 50 for entrainment with air moving to the diffuser 70. <IMAGE>

Description

SPECIFICATION Blue flame burner This invention relates to blue flame burners, and more particularly, to such burners adapted to operate with fluid heaters for burning liquid hydrocarbon fuel to produce a stable blue flame.

Those familiar with the art to which the present invention pertains, are aware of a construction wherein burner means are positioned at the combustion end of a combustion chamber, the burner means including a cylindrical tube which may be flared at the breech end, a nozzle in the tube, fuel supply means for supplying a fuel spray to the combustion chamber, means for supplying air under pressure through the tube toward the combustion chamber, and means such as a baffle to recirculate a portion of the gaseous products of combustion to the breech end of the cylindrical tube.

While such known constructions have represented a positive development in the art and have achieved a degree of commercial acceptance, experience has demonstrated that they are subject to certain disadvantages in that the burner was sensitive to both oil chemistry and nozzle spray variations resulting in two independent occurrences. First, the air metering plate shroud which surrounded the end of the burner accumulated carbon deposits which eventually resulted in shorting out of the electrodes and a safety shutdown of the burner.

Second, the flame was often quenched, especially on cold starts. This latter occurrence was manifested by burner start, flame out during transition from yellow to blue flame and reignition in a fuel rich environment that frequently resulted in sudden and rapid combustion and consequent physical damage to the unit.

The carbon deposits were principally due to the recirculation of combustion gases containing high levels of unburned hydrocarbon and the effect of low eddy currents drawing fuel back into the cylindrical tube. The recirculation of unburned hydrocarbons was in turn due in part to the incorrect trajectory of combustion air in the primary flame zone resulting in a highly substoichiometric flame envelope. In like manner, the high level of unburned hydrocarbons in the recirculating gases was caused in part by the opening between the inside of the baffle and the tube which allowed recirculation from a highly substoichiometric region of the flame. Moreover, the levels of unburned hydrocarbons greatly increased if fuel spray distribution degraded, as by nozzle degradation, air leaks and the like, or as fuel aromatics increased.

Other known constructions of interest in respect of the subject matter of the present application are found in U.S. Patent Nos.

3,399,022, 3,545,902, 3,597,134 and 3,652,194, and especially No. 3,741,166.

Accordingly, we have conceived the present invention to provide a blue flame burner which overcomes the foregoing difficulties and disadvantages. As a feature of our invention, the blue flame burner is adapted to operate with a combustion chamber associated with a fluid heater for burning liquid hydrocarbon fuel to produce a stable blue flame in which there is provided diffuser means having an upstream end defining a breech, downstream end defining a mouth and a constricted region between the breech and the mouth. The fuel nozzle of the burner forms a fuel spray in the direction of the diffuser mouth. Upstream of the breech of the diffuser means combustion air is provided under pressure and directed toward the breech.In operation the burner has the mouth of the diffuser means directed into the combustion chamber and there is also provided means defining a recirculation path for conducting a portion of the gaseous products of combustion formed in the combustion chamber back to the breech for entrainment with the combustion air moving towards the diffuser means.

More specifically, the mouth of the diffuser means is positioned within the combustion chamber at the combustion end thereof so that the region between the diffuser and the portion of the combustion chamber surrounding the diffuser comprises the recirculation path, while the nozzle itself is positioned within the diffuser substantially axially thereof and upstream a radial plane through the area of greatest constriction thereof.

Additionally, the means for providing combustion air under pressure may comprise simply a tube extending into the combustion chamber and having an end plate formed with a plurality of apertures therein opposite the diffuser breech, and means for delivering air under pressure to the tube.

In the burner, the ignition means includes a pair of electrodes positioned in a low pressure region in the diffuser adjacent the fuel nozzle and out of the spray pattern formed by the nozzle.

In one embodiment of the invention, the combustion chamber has a zone of reduced diameter at the combustion end thereof and the burner is positioned with the breech of the diffuser means in that zone and in axial alignment therewith and with its mouth positioned and directed into the combustion chamber. In this case, we provide means including annular baffle means surrounding the diffuser mouth and spaced from the combustion chamber wall to define therewith the recirculation path for conducting a portion of the gaseous products of combustion back to the breech.

According to another embodiment of the invention, we position the entire diffuser means downstream of the zone of reduced diameter of the combustion chamber and in this case, we have found that we can thus eliminate the baffle means altogether, the recirculation path then comprising the region between the diffuser means and the combustion chamber wall.

The present construction produces a stable blue flame in the combustion chamber downstream of the mouth of the diffuser means.

Thus, the present construction alters the trajectory of the combustion air and effectively closes off the path of "inside" combustion gas recirculation, and increases the stoichiometric ratio along the flame envelope which in turn reduces the unburned hydrocarbon level at the source of recirculating combustion gases. Additionally, the modified electrode settings reduce the likelihood of the arc interfering with the trajectory of the oil droplets and tends to counteract the effect of eddy currents at the diffuser mouth.

It is known that the ignition speed of gases is maximized as the stoichiometric ratio approaches unity, and the known construction directed most of the combustion air into the spray core resulting in the highly substoichiometric flame envelope already mentioned. The present diffuser increases the level of air delivery to the flame envelope without delivering excessive levels that could cause autoignition and thus improves the stability range of the burner. This improved function is achieved by reason of the fact that the diffuser causes a portion of the combustion air to flow along its smooth inside surface and more uniformly distributes the air throughout the flame zone. The present diffuser also reduces the axial velocity of the gases passing through it because of its configuration and larger average crosssectional area relative to that of the known cylindrical tube.The result is an aerodynamically anchored flame front due to the reduced axial gas velocity, and increased flame speed due to improved combustion air distribution.

The diffuser also increases the air to fuel shear angle in the primary combustion zone providing more rapid fuel preparation, effectively shortening the droplet residency time in the primary zone and reducing the physical size of that zone.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention.

It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention.

Specific embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification wherein: Fig. 1 is a cross-sectional view in elevation along the longitudinal axis of a burner arrangement according to the prior art; Fig. 2 is a cross-sectional view in elevation along the longitudinal axis of a burner arrangement according to a first embodiment of the present invention; Fig. 3 is a view similar to Fig. 2, but illustrating a second embodiment of the present invention; Fig. 4 is a top plan view of a burner assembly illustrating an electrode arrangement according to the present invention; Fig. 5 is an elevational view of the structure of claim 4.

Referring now to the drawings and initially to Fig. 1 thereof, there is shown a known burner 10 supported by a support plate 12 in a combustion chamber 14 defined by a cylindrical wall 16. More specifically, the burner 10 includes an air tube 1 8 mounted in the plate 12 which merges with a cylindrical wall 19 of reduced diameter relative to wall 1 6 the end of which is joined to the end of the wall 19 by an annular end plate 20.

The burner 10 includes a fuel supply line 22 passing through the air tube 18 and projecting outwardly of an air metering plate 24 fixed across the end of the tube 20. The supply line 22 terminates in a nozzle 26 for directing a spray of fine fuel droplets towards the combustion chamber 14. A pair of electrode rods 26 extend through the tube 18 and the plate 24 and converge near the spray nozzle to ignite the fuel.

It will be noted that the fuel nozzle is positioned axially of the combustion chamber and is surrounded by a diffuser 28 which is flared as at 30 at its upstream end, and that the metering plate 24 is formed with a plurality of openings 32 centered on a circle concentric with the axis of the diffuser 28.

Finally, an annular or disc shaped baffle 34 is mounted in axial alignment with the combustion chamber 14 and with its central opening spaced from and surrounding the downstream end of the tube 28.

According to this known construction, a core of fuel spray emitted from the nozzle 26 into the combustion chamber while air jets created by the apertures 32 in the plate 24 entered the upstream end of the diffuser 28 to mix with the fuel spray to enable combustion of same in the combustion end of the chamber 14. A portion of the combustion gases were recirculated around the baffle for return to the upstream end of the diffuser 28 for passage therethrough along with the combustion air from the apertures 32.

As already stated, this described construction has acquired a degree of commercial acceptance in the field, but is subject to various problems which applicant has attacked and successfully resolved.

Referring now to Fig. 2 there is shown a construction according to a first embodiment of the present invention wherein the combustion chamber 14 is formed essentially in the same manner as that of Fig. 1 to the extent that the walls 16 and 19, support plate 12 and end plate 20 are structurally similar in each case.

In Fig. 2 we have illustrated a housing assembly 34 which is removably attached by bolts to the support plate 12 so that the burner assembly can readily be installed.

The housing 34 includes a lower section 36 formed with suitable air inlet openings and contairiing a blower 38 driven by an electric motor (not shown) in known manner and which may also drive a fuel pump for a purpose to be described.

The housing 34 also includes an upper, generally tubular section 40 formed with a hinged partition 42 carrying an ignition transformer 44 and a mounting flange 46 for securement to the heat exchanger flange 12. The section 40 extends forwardly through the flange 12 in similar fashion as tube 1 8 of Fig. 1 and terminates in a metering plate 41 formed with openings 43 centered on a circle concentric with the axis section 40.

The fuel supply is controlled by conventional equipment as disclosed in U.S. Patent No.

3,741,166 and delivers fuel to the supply line 48 in the air tube section 40, which line extends forward to the nozzle 50.

The electrodes 60 extend forward through ceramic insulating sleeves 62 fixed in a pair of suitably located openings in the plate 41 and the electrode buss bars contact respective high voltage terminals 68 of transformer 44 when the hinged partition 42 is closed.

A diffuser 70 having an upstream end defining a breech 72, a downstream end defining a mouth 74 and a constricted region 76 between the breech and the mouth, defines opposed arcuate configurations in cross section, as shown, having a radius of curvature of the order of 4.5 inches, while the diameter of the breech and mouth is of the order of 3 inches. The diffuser is mounted by means not shown, such as thin spaced tabs extending between the breech end of the diffuser and the tubular section 40.

An annular baffle 78 is supported by suitable means, not shown, to surround the diffuser mouth and is spaced from the combustion chamber wall 1 6 to define therebetween a recirculation path for conducting a portion of the gaseous products of combustion formed in the combustion chamber back to the diffuser breech for entrainment with the combustion air from the apertures 43 in plate 41 and movement through the diffuser.

In operation, fuel is pumped to the nozzle 50 to spray a cone shaped core of fine fuel droplets into the combustion chamber while air under pressure is delivered by the blower 38 through the housing section 40 and the apertures 43 in the form of jets to the breech of the diffuser for mixing with the fuel droplets. Following ignition by the electrode rods 60, a portion of the combustion gases are recirculated, as described, to the diffuser breech.

The advantage derived by such recirculation is known to those skilled in the art and is set forth in U.S. Patent No. 3,741 ,1 66. However, in known constructions, as in Fig. 1, the tube 28 was spaced from the end of the tube 18 to provide a breech width of about 3/8 inches, whereas we have found that by employing the diffuser as here shown and described and by widening the breech to the order of 5/8 inches, we obtain higher recirculation rates while showing the axial velocity of the products passing through the diffuser and raising the primary flame zone temperature, both occurrences serving to broaden the stability range of the burner.The diffuser also serves to increase the airto-fuel shear angle the primary combustion region resulting in more rapid fuel preparation and effectively shortening the residency time required of the fuel droplets in the primary zone and reducing the physical size of the primary zone.

Turning now to Fig. 3, it will be seen that the combustion chamber 14, the air, fuel and ignition means are similar to that of Fig. 2 so that no detailed discussion thereof is deemed necessary at this time. It will be noted, however, that the tubular housing section 40 extends further into the combustion chamber than in the previous embodiment so that the metering plate is almost flush with the annular end plate 20. Moreover, the contour of the diffuser is formed on a radius of the order of 1.5 inches, the breech and mouth diameters are of the order of 4 inches and the baffle 78 is omitted.

It should be noted that this metering plate 41 is designed with reduced swirl angles, i.e., angles based upon the angle of the air metering plate jet from a plane passing through the nozzle orifice and the centerline of the jet with a minimum jet L/D ratio of .60. Proper swirl angles serve to enhance burner stability by improving mass transport rates in the combustion process.

Optimized swirl angles are necessary since increased swirl tends to reduce recirculation rates which will have detrimental effects on the process.

We have been able to reduce the swirl angle from about 200 to about 50 due to improved distribution and mixing of combustion gases in the combustion chamber. Moreover, because of venturi geometry, there is no need for a convergence angle in the air metering plate jets.

Convergence of course refers to the angle that the air metering plate jet makes with a plane normal to a plane passing through the nozzle orifice and the centerline of the jet.

In the present embodiment, we are able to realize the advantages described in connection with that of Fig. 2 while eliminating the baffle because the larger diffuser alters the combustion gas patterns to permit recirculation of gases that are cool enough to permit advantageous recirculation rates and to prevent autoignition of the fuel droplets.

In both embodiments we have also repositioned the electrodes 60, as shown in Figs.

4 and 5, so that while the gap between the rod tips shown in Fig. 4 maintain the conventional 1/8 inch spacing, these tips are actually located 1/8 inch from the nozzle end, thus drawing them back from the fuel spray pattern and into a low pressure region in the diffuser.

We believe that the construction and operation of our novel blue flame burner will now be understood and that the advantages thereof will be fully appreciated by those persons skilled in the art.

Claims

1. A blue flame burner adapted to operate with a combustion chamber associated with a fluid heater for burning liquid hydrocarbon fuel to produce a stable blue flame, comprising: diffuser means having an upstream end defining a breech, a downstream end defining a mouth and a constricted region between said breech and said mouth; a fuel nozzle and a fuel supply line for delivering fuel to said nozzle, said nozzle being directed such that, in use, a fuel spray is formed in said diffuser means in the direction of said mouth; means upstream of said breech for providing combustion air under pressure directed toward said breech; ignition means for igniting said fuel spray; said burner being adapted to operate with said mouth of said diffuser means directed into the combustion chamber; means defining a recirculation path along which a portion of the gaseous products of combustion formed in said combustion chamber, when said burner is in use, is conducted to said breech for entrainment with said combustion air and movement through said diffuser means; whereby, in use, a stable blue flame is produced in said combustion chamber downstream of said mouth of said diffuser means.

2. A blue flame burner as claimed in claim 1, wherein said mouth of said diffuser means is positioned within said combustion chamber at the combustion end thereof, whereby the region between said diffuser and the portion of the combustion chamber surrounding said diffuser comprises said recirculation path.

3. A blue flame burner as claimed in claim 1 or claim 2, wherein said fuel nozzle is positioned within said diffuser means.

4. A blue flame burner adapted to operate with a combustion chamber associated with a fluid heater for burning liquid hydrocarbon fuel to produce a stable blue flame, comprising: diffuser means having an upstream end defining a breech, a downstream end defining a mouth and a constricted region between said breech and said mouth; fuel nozzle means positioned within said diffuser means and a fuel supply line for feeding liquid fuel to said nozzle means, said nozzle means being directed such that, in use, a fuel spray is formed in the direction of said mouth; means upstream of said breech for providing combustion air under pressure directed toward said breech; ignition means for igniting said fuel spray; said combustion chamber having a combustion end of larger cross sectional area than said diffuser means; said burner being adapated to operate with said diffuser means entirely within said combustion end of said combustion chamber; means defining a recirculation path along which a portion of the gaseous products of combustion formed in said combustion chamber, when said burner is in use, is conducted to said breech for entrainment with said combustion air and movement through said diffuser means; whereby, in use, a stable blue flame is produced in said combustion chamber downstream from said mouth of said diffuser means.

5. A blue flame burner adapted to operate a combustion chamber associated with a fluid heater for burning liquid hydrocarbon fuel to produce a stable blue flame, comprising: diffuser means having an upstream end defining a breech, a downstream end defining a mouth and a constricted region between said breech and said mouth; fuel nozzle means positioned within said diffuser means and a fuel supply line for feeding liquid fuel to said nozzle means, said nozzle means being directed such that, in use, a fuel spray is formed in the direction of said mouth; means upstream of said breech for providing combustion air under pressure directed toward said breech; ignition means for igniting said fuel spray; said combustion chamber having a wall of larger cross sectional area than said diffuser means and having a zone of reduced diameter at the combustion end thereof; said burner being adapted to operate with said breech of said diffuser means in said zone of reduced diameter of said combustion chamber and in axial alignment therewith and with said mouth positioned and directed into said combustion chamber; means including annular baffle means surrounding said diffuser mouth and spaced from said combustion chamber wall defining therewith a recirculation path along which a portion of the gaseous products of combustion formed in said combustion chamber, when said burner is in use, is conducted to said breech for entrainment with said combustion air and movement through said diffuser means; whereby, in use, a stable blue flame is produced in said combustion chamber downstream from said mouth of said diffuser means.

6. A blue flame burner as claimed in any one of claims 3 to 5, wherein said diffuser constricted region defines a throat, and said fuel nozzle is disposed substantially axially of said diffuser means and upstream of a radial plane through the area of greatest constriction of said throat.

7. A blue flame burner as claimed in any one of the preceding claims, wherein said ignition means include a pair of electrodes positioned in a region in said diffuser which is under low pressure when said burner is in use, which is adjacent said nozzle, and which is out of the spray pattern formed by said nozzle when said burner is in use.

8. A blue flame burner as claimed in any one of the preceding claims, wherein said means for providing combustion air under pressure directed toward said breech comprises a tube extending into said combustion chamber and having an end plate thereon formed with a plurality of apertures therein opposite said breech and means for delivering air under pressure to said tube.

9. A blue flame burner as claimed in claim 1 substantially as hereinbefore described with reference to, and as illustrated in, Fig. 2 or Fig. 3 of the accompanying drawings.