US2515845A

US2515845A - Flame pocket fluid fuel burner

Info

Publication number: US2515845A
Application number: US747607A
Authority: US
Inventors: Hendrik K J Van Den Bussche
Original assignee: Shell Development Co
Current assignee: Shell Development Co
Priority date: 1946-06-25
Filing date: 1947-05-13
Publication date: 1950-07-18
Anticipated expiration: 1967-07-18

Description

u y 1950 H. K. J. VAN DEN BUSSCHE 2,515,845

- FLAME POCKET mun 1mm. BURNER Filed lay 13, 1947 2 Sheets-Sheet 1 \nveni'or:

H. K. J. VAN DEN BUSSCHE W FLARE POCKET FLUID FUEL BURNER 2 Sheets-Sheet 2 Eiled lay 13. 1947 I H A r/ 4.0 15-0 L in Inches Fig 4 m w $52.: 5 O

Patented July 18, 1950 FLAME rooxnr FLUID FUEL BURNER Hendrik K. J. van den Bussche, Delft, Netherlands, assignor to Shell Development Company, San Francisco, Calii'., a corporation of Delaware Application May 13, 1947, Serial No. 747,607 In the Netherlands June 25,1946

Claims.

This invention relates to an improved burner for burning inflammable gaseous mixtures issuing in stratic form from a slit or series of slits, and is particularly concerned with attaining steady and efficient combustion at high flow velocities. The inflammable gaseous mixture may be a mixture of air (or other oxidizing gas) with a gaseous fuel, e. g., butane gas, or finely-divided liquid fuel, e. g., atomized or sprayed droplets of oil, or finely divided solid fuel, e. g., powdered coal, it may, therefore, be a gas or a dispersion of a liquid or of a solid in a gas.

The invention may be applied in any situation where such inflammable gaseous mixtures are burned, such as boilers and other types of furnaces. It is particularly useful for aircraft gas turbines, either for plain jet propulsion or for combined airscrew and. jet propulsion, wherein it is necessary that combustion occurs within a small space at high flow velocities of the inflammable fluid, and that the flame sticks to the burnerunder all circumstances.

Fuel and combustion air are customarily mixed in the combustion or flame chamber, usuall by atomization and/or vaporization, aided in some cases by turbulence. The use of fuel-air mixtures which have been carburettedbefore entering the combustion chamber offer certain advantages, but such mixtures will not always stick to the burner, i. e., the flame is subject to leaving the combustion chamber at high flow velocities, resulting in the flames being extinguished. Moreover, it has been diflicultto attain complete combustion within a limited spacefor high rates of flow.

It is an object of this invention to provide an improved burner forburning gaseous inflammable mixtures within a confined flame chamber. A further object is to provide a flame which is provided with stabilizing whirls on both sides of a stratic current of the inflammable mixture in the flame chamber, whereby the more complete and rapid combustion of the inflammable mixture can be attained even at high flow velocities.

According to the present invention the gaseous inflammable mixture is made to flow into a flame chamber in stratic form, i e., in the form of a sheet or thin wall, by issue through a slit at or near the rear of the flame. chamber, and the flame is formed so that a vortex of flame gases, i. e., hot burning or burned gases, is maintained on either side of the stream of the gaseous mixture issuing from the slit. These vortices, which may take the form of closed, annular vortices, convey the hot gases back to the point where the inflammable mixture issues into the flame chamber, thereby rapidly heating the issuing mixture and igniting it. The result of this is that 'very high velocities through the slit can be employed without blowing out the flame. With the flame chamber described herein the rotational velocity of the vortices increases with the velocity at which the inflammable gaseous mixture is supplied, so that the flame is self-stabilizing.

A flame of the type contemplated by the present invention can be insured only in flame chambers of certain shapes, as will be explained hereafter, while with other shapes the desired flames either are not attainable, or will be realized only sporadicalhr. The required shape for the flame chamber is dependent, among other things, upon the composition of the gaseous mixture, the shape of the slit, and the direction in which the mixture issues from the slit. However, once the desirability of using a flame of the type described in this specification and its characteristics are known,

the required shape can be ascertained experimentally by those skilled in the art, although it is also possible to state theoretically the requirements to which the flame chamber must conform. The shape must be such that the backward flow of air (1. e., toward the slit) between the wall of the flame chamber and the flame is prevented. Hence, the front of the flame chamber, 11. e., the end where the flame leaves the chamber, must closely conform to the desired shape of the flame.

If the slit is annular the flame chamber should have its inner walls shaped as a surface of revolution, e. g., like a circular truncated cone or circular cylinder, of sufiicient length to enclose the flame at least partially. This is true also when the slits do not form continuous annuli, aswhen they are formed of a series of apertures close together but arranged on the circumference of a circle, and when several concentric annuli of continuous slits or series of apertures are provided. But when other forms of slits are used, the flame chamber may have other cross-sectional outlines, conforming to the stratic stream of the fluid mixture, which may, for example, be boxlike, composed of four or more straight sides.

The invention will be explained in greater detail by reference to the drawings forming a part of this specification, wherein:

Figs. 1, 2 and 3 are longitudinal cross-sectional views of flame chambers showing three types of flames, of which the flame shown in Fig. 3 is the one sought in the present invention;

Fig. 4 is a graph showing the types of flames acme obtainable with one kind of flame chamber when the length and diameter are varied.

In Figs. 1 to 3, l is the wall of a flame chamber, the inner surface of which is a surface of revolution, flared slightly toward the mouth. The chamber has an opening in its rear wall for receiving a supply conduit 5, having an annular slit 6, through which an inflammable gaseous mixture issues into the flame chamber. The annular slit may be formed by mounting a circular core 1 in the conduit 5. The wall of the flame chamber 4 may be made of refractory material, such as fire brick, and may be part of the combustion chamber lining which is recessed to form the flame chamber; it may, however, also be a separate cup-shaped piece secured to the supply conduit 5, and constructed of refractory material or of metal, or metal lined with refractory material on its inner face.

It was found that, with burners of the type illustrated, three general flame forms may be formed, these forms being illustrated in Figs. 1, 2 and 3, and herein designated as forms I, II, and III, respectively. Flame form I results when no flame chamber is provided or when a flame chamber having a short length in relation to its diameter is used. In this shape of flame the burning gases advance forwardly like a slightly diverging tube. Within the tube an annular vortex 8 of burned and burning gases is formed, while external gas or air, from points outside of the flame chamber, flows in as shown by the arrows 9. Both phenomena are the result of the entraining effect of the gaseous mixture as it issues from the slit, which lowers the pressure, i. e., produces a partial vacuum. Measurements have indicated that a moderate vacuum is formed inside of the flame at the rear of the flame chamber (at the face of the core 'I) herein referred to as the core vacuum P0. The vacuum at the side wall of the refractory cone, or flame chamber cup near its rear wall, herein referred to as the cone vacuum Pk is very low, being dissipated by the inrushing air. In this form of flame the inrushing outside air cools the outside of the flame, and the mixture burns substantially only on the inside and combustion is not complete.

Flame form II, shown in Fig. 2, may be formed with a longer flame chamber. In this form the flame spreads laterally and no air can enter between the flame and the wall of the chamber. A moderately high cone vacuum Pk results, causing the formation of annular vortices l of burning and burned gases. There is also a high core vacuum Pc, but the formation of appreciable central vortices is prevented by air entering centrally as shown by the arrows ll. Rudimentary vortices l2 may occur, but these contain largely gases which are still unburned. In this form of flame the mixtures within the flame chamber burns substantially only on the outside and a considerable part of the fuel is not burned.

4 a result, it is not blown oil the slit even with very great velocities of flow through the slit. Moreover, with this shape of flame complete burning is achieved, either without or with a very slight excess of air. It follows that the shape of flame shown in Fig. 3 oflers great advantages.

The Flame form III may, also, be deflned with reference to the core and cone vacua, which, as explained above, are lower and higher, respectively than with the other flame forms. The desired flame according to this invention may further be distinguished from the other flame forms by the higher flame pressure, viz., the difference between the pressure in the conduit 6 when the gaseous mixture is not ignited and the pressure during burning. This pressure is always lower without ignition, and becomes progressively higher with Flame forms II, I, and III, respectively.

Which type of flame will prevail will depend upon several factors, as previously mentioned. Of these, the dimensions of the flame chamber are of greatest importance, but the proper dimensions depend upon the manner in which the inflammable mixture issues from the slit. The effect of these dimensions will be illustrated in connection with a burner in which a mixture of air and butane in about theoretically equivalent amounts for complete combustion was passed through an annular slit into various refractory flame chambers. The slit had an inside diameter of 26 mm. and an outside diameter of 28 mm., 1. e.

the slit was 1 mm. wide, and the butane-air mixture was given a tangential velocity by flowing it through a whirl chamber before reaching the slit to cause the ratio of the axial velocity to tangential velocity to be 4:1. The flame chambers were in all cases shaped about as shown in Figs. 1 to 3, with a flare of 7 (each wall sloped 3 from the axis). Diameters are measured at the base or narrowest part of the side walls, and the lengths from the base of the side wall to the orifice, parallel to the axis. The rear wall, behind the base,

was inclined 80 to the axis. r The graph of Fig. 4 shows the effect of usini; variously dimensioned flame chambers, the

Finally, Fig. 3 shows Flame form III, the shape of flame which is sought according to this invention. In this case the flame chamber is longer than in the preceding flgures, and the gaseous mixture, after issuing from the slit as a tubular sheet, first opens and later more or less closes, and annular vortices are formed, both inside of the tube, as shown at l3, and outside of the tube, as shown at it. With this shape of flame the core vacuum Fe is low an. the cone vacuum Pk is high. The gases burr. both on the inside and outside of the tube of gases and the flame is stabilized both on the inside and on the outside; as

lengths L being slotted as abscissae and the diameters as ordinates all in inches withflame chambers having dimensions which would be plotted in the upper, left-hand area, marked I Flame form I (having exclusively an inside vor-.

tex) is obtained; and with dimensions falling in the lower, right-hand area, marked lII, Flame form III (having both inside and outside vortices) is obtained. Flame form II can occur with dimensions corresponding to the areas marked 11 which, as can be seen, form transition areas between areas I and III. In these transition areas aerodynamic labile conditions prevail, so that, as is shown on the graph, with certain values of L and D, two and sometimes even all three flame forms can exist, which, due to some external cause or other, can suddenly change from one shape to another.

"It is clear from the graph of Fig. 4 that, in order to obtain the desired Flame-form III, the flame chamber must-for a given diameter-have at least a certain maximum length, while, on the other hand, for a given length the diameter may not exceed a certain maximum value. For the particular feed composition, shape of slit, etc., of these experiments, Flame form III is usually insured when the flame chamber is at least as long as required by the inequality:

L 1.2D-.55 inch (1) atomizing cup surrounded a Flame form I is sporadically attained for lower lengths, down to about 1.2D1.50 inches.

For other conditions the locations and slopes of the lines shown on Fig. 4 will differ, but these lines will have a generally similar relation. For example, in tests with the same series of refractory flame chambers, fuel-air composition, slit and flow rate, in which the tangential velocity was reduced, form I was not so readily attained, and somewhat longer chamber became necessary; Flame form I could be insured only by making:

L 1.4D.83 inch (2) and was sporadically attained for lengths down to 1.4D1.42 inches.

When the slit was increased while maintaining the same tangential velocity as in the original tests (whereby the ratio of axial to tangential velocity was decreased) the desired flame form was stabilized and shorter lengths could be used. In this case, with a slit 2 mm. wide Flame form I could be insured when:

L 0.9lD-.51 inch (3) while Flame for I could be realized for lengths as low as 0.9lD-l.94 inches. With a slit 3 mm. wide. the requirement was mason-"0.95 inch (4) to insure Flame form I, while this form could be attained occasionally with even shorter lengths.

It was found that the rotational motion of the gaseous mixture exerts a favorable stabilizing action on the desired flame form and I prefer, for that reason, to impart to the advancing air a tangential component, although the invention may be practiced even without this feature. When the tube-like stratum issuing into the flame chamber has atangential component the vortices are deviated from closed loops by the tangential velocity; in this case the flow lines of gas particles in each eddy may be regarded as helical lines on a torus.

With flame chambers having dimensions which may be plotted in the diagonal strips of Fig. 4 marked with several Roman numerals, the flame shifts from one form to another, and no generally valid rule for predicting the form can be given. In certain experiments, however, it was found that frequently the flame starts with Flame form 1; increasing the fuel-air ratio causes a change from Flame form I into Flame form III, which then remains after the ratio was again reduced. Reducing the ratio to well below the theoretical ratio for complete combustion results in a change into Flame form II. At higher pressures of the gaseous mixture in the feed conduit Flame form I could be made to change into Flame form HI either by increasing or decreasing the fuel-air ratio.

As was suggested previously, it is possible to bum'various kinds of gaseous fuel mixtures in this burner, including those containing gaseous, liquid and solid fuels. As a specific example for burning liquid fuel, the type having a rotating by an annular air .sllt may be cited; in this case the li uid oil is thrown from the edge of the cup in e droplets into an enveloping current of air, which may have a tangential component, resultingiin an inflammable mixture of liquid oil in air. Atomiz ing burners of this general type are known U. S. Patents Nos. 2,184,471 and 2,214,568) a need not be described further herein. When such a rotating atomizing cup burner is applied, the conduit 5 shown in the drawing of this specification is the outer air duct and the core I is replaced by the rotating cup.

It is, moreover, not essential to have an annular slit for the application of the invention; for example, flat slits parallel to one another can also be used in which case, of course, care must be taken that the vortices are not hindered in their existence or continuance by the shape of the flame chamber. The vortices must either be closed in themselves or be next to a wall of the flame chamber.

I claim as my invention:

1. A burner comprising a flame chamber having a rear wall and a closed side wall and open at the front end, the length of said side wall in inches exceeding a distance equal to 0.80D0.95, wherein D is the internal diameter of the side wall near the rear wall, said length being at least 1.5 inches and the front diameter of said chamber being limited relatively to said diameter near the rear wall to provide a substantially tubiform flame chamber, and an annular slit in said rear wall having a diameter not more than about 0.73D and disposed for introducing an inflammable mixture forwardly in the form of a tubular stratum spaced from said side wall, whereby a flame formed by igniting said inflammable stratic mixture will form vortices of flame gases both within the tubular stratum and between the outside of the tubular stratum and said side wall for rapidly heating themixture issuing from the slit and thereby stabilizing it against extinction at high rates of flow.

2. The burner according to claim 1 in which the slit has a circular shape and the side wall of the flame chamber has a surface of revolution.

3. The burner according to claim 1 wherein the flame chamber has the shape of a slightly diverging irustrum of a cone and is fully open at the front end thereof.

4. The burner according to claim 1 wherein the diameter of the annular slit is not less than about 0. 8D.

5. The burner according to claim 1 wherein the length of the side wall exceeds a distance equal to 1.4D0.83D.

HENDRIKK. J. van Dxx BUSSCHE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS