US2960823A

US2960823A - Process and apparatus for the improved combustion of liquid fuels

Info

Publication number: US2960823A
Application number: US511461A
Authority: US
Inventors: Homer M Fox
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1955-05-27
Filing date: 1955-05-27
Publication date: 1960-11-22
Anticipated expiration: 1977-11-22

Description

Nov. 22, 1960 H. M. Fox

PROCESS AND APPARATUS FOR THE IMPROVED comausnou 0F LIQUID FUELS Filed llay 27, 1955 INVENTOR. H. M. FOX

f/wmq A r TORNE vs nite States Patent Qfice Patented N 0v. 22, 1 960 rnocnss AND APPARATUS Fag THE mnovnn' iIGMBUSTIGN on LIQUID FUELS Homer M. Fox, Bartlesvilie, 01th., assignor to Phillips Petroleum Qornpany, a corporation of Delaware Filed May 27, 1955, Ser. No. 511,461

11 Claims. (Cl. 60- 3936) This invention relates to jet engines In oneaspect, this invention relates to a method for improving the combustion of fuel in a jet engine. In a more specific aspect, this invention relates to an improved jet enginewhich comprises a system for transforming fuel supplied to the jet engine into a fuel having more desirable combustion characteristics.

The most important element of a jet engine aircraft is the power plant, and this, in turn, is only as good as its combustion system. The study of the combustion chamber and the mechanics of combustion is of prime concern.

The purpose of the combustion chamber is to convert the chemical energy of hydrocarbon fuels into thermal energy. This energy is absorbed by the air flowing through the engine to provide the high-velocity exhaust jet necessary for propulsion.

The desired properties of jet engines include low frontal area for minimum drag, low weight, and operational flexibility. In some respects, frontal area and weight are inversely related to the heat release rate of the combustor employed. Thus, for a given thrust rating, a combustion process yielding a high heat release rate (expressed as B.t.u./hr./cu. ft.) will generally permit the use of an engine design having lower frontal area and weight than will a combustion process having a lower heat release rate. In other words, higher heat release rates will generally permit the design of engines ha ing higher thrust per unit of engine weight and per unit of engine frontal area. Heat release rates depend, in turn, on the stability and efiiciency of the combustion process, as well as on the heating value of the fuel It is known that fuels having high flame speeds, e.g., hydrogen, acetylene and ethylene, possess high combustion stability and efliciency. Additionally, the fuel must be in the gaseous or vaporous state before combustion can occur. Consequently, "a gaseous or vaporized fuel requires a shorter overall residence time in the combustor for complete combustion than does a fuel injected in the liquid state since the'latter must'first be vaporized. The net result is that low molecular weight fuels, such as hydrogen, acetylene and ethylene provide higher thrust per unit of engine Weight and'of frontal area than does a high molecular, liquid fuel I The ability of ajet engine to operate over a wide range of conditions is also enhanced by the use of low molecular weight, high flame speed fuels. As the temperature and pressure of the inlet air tothe combustor are decreased, e.g., as by' an increase in altitude, it becomes more difiicult to maintain stable and efiicient combustion. The use of'the more stably burning, gaseous, high flame speed fuels permits stable and eflicient combustion at lower inlet air temperature and pressure, and

thus increases the altitude operational range of the engine.

Since these low molecular weight fuels are gases under normal conditions of temperature and pressure, they have not been used as jet aircraft fuelso'wing to the 2 obvious difiiculties fostered by their gaseous state. ,Thus, a. method and means of increasing the combustion stability and efficiency ofthe usual liquid jet engine fuels are greatly desired. Such a method and means would not only increase the operational range of the engine and permit the decrease of its weight and frontal area, but would also increase the availability (and concomitantly decrease the cost) of jet fuels by permitting the inclusion of fuel components of lower volatility, lower combustion stability, and lower combustion efficiency. Also, the performance increase provided by such a method and means can be taken as an increase in thrust per unit of engine Weight and unit of engine frontal area.

Thus, an improved combustion chamber for a continuous combustion burner by which a portion of the fuel supplied to the combustion system is decomposed into fuel components having high flame speed characteristics is highly desirable and an object of this invention is to provide such a combustion chamber.

A further object is to provide a combustion chamber for a continuous combustion burner wherein stable combustion occurs over a wide range ofopera'ting conditions.

A further object is to provide a combustion chamber for a jet engine wherein problems of cycling and flame blowout under severe operating conditions are reduced.

In accordance with this invention, an improved combustion chamber is provided which permits a portion of the fuel supplied to the engine to be stoichiometrically burned in air and the resulting combustion products are used to decompose at least a portion of the remainder of the fuel into fuel components having'high flame speed characteristics. The decomposition process ordinarily involves predominantly cracking reactions although other reactions, such as dehydrogenation reactions, which influence the formation of hydrogen and unsaturated components, also take'place. The products from decomposing a portion of the fuel by direct heat exchange with the products of stoichiometric combustion are fuel component having high flame speeds such as hydrogen, acetylene, ethylene, propylene and the like. Broadly stated, the invention comprises a continuous combustion burner wherein a liquid'fuel is burned in air in a combustion chamber and whereinfthe hot gases thereby produced are exhausted from .the combustion chamber, means for conducting stoichiometric combustion of a portion of the fuel in air, means for quenching the stoichiometric combustion with the remainder of the fuel so as to partially decompose the remainder of the fuel in the absence of oxygen. The products from the stoichiometric combustionand the decomposition of the fuel are burned in the combustion chamber and the hot gases thereby produced are exhausted therefrom to produce thrust or to perform other work. i

In a preferred embodiment of this invention, a small proportion of the total fuel for the jet engine is utilized as the source of'heat required in a pilot chamber to furnish the energy required for the decomposition reaction of the larger, remaining proportion of the fuel. In other words, the combustion system of this invention comprises quenching combustion products in one part of the combustion chamber with fuel, instead of air, and burning the fuel so decomposed in the quenching step in another combustion Zone in which secondary air is used to quench the combustion products.

The amount of fuel supplied to the pilot chamber for stoichiometric combustion can vary over a wide range. The amount of fuel used in the pilot chamber usually amounts to at least one percent by weight of the total greater than 15 percent by weight of the total fuel;

V and spaced from the fuel decomposition chamber.

The combustion which is carried on in the pilot chamber is stoichiometric combustion so that the decomposition of the remainder of the fuel by direct heat exchange with the product of the stoichiometric combustion is carried on in an atmosphere which is substantially free of oxygen. The combustion in the pilot chamber is on a high level, and high temperatures, usually higher than 4000" F., are developed therein.

Although, the combustion chamber Which'is the subject of this invention is most advantageously used in a jet engine, the apparatus can be employed in any continuous combustion-type power plant including turboprop, turbojet and ram jet engines for aircraft and stationary gas turbines for generating power. 7

A better understanding of the combustion chamber of thisinvention will be had by referring to the accompanying drawings, in which: 7 a

Figure 1 is an over-all view of a jet engine having incorporated therein a view, partially in section, of the improved combustion chamber of this invention; and,

Fig. 2 is a partial, sectional view of a modification of the combustion chamber shown in Figure 1. 7

Referring now to Figure l, a jet engine is shown having to a combustion chamber 7 by a compressor 9 which is V driven by a turbine 11 by means not shown.

Combustion chamber 7 is a streamlined chamber having an air inlet 13 and an exhaust gas outlet 15. A flame tube 17 is axially-positioned within and spaced from combustion chamber 7. Flame tube 17 comprises a plurality of apertures or perforations 19 for the admission of primary air thereto and a plurality of such perforations 21 for the admission of secondary air thereto. The downstream end of flame tube 17 is open, attached to'and coincident with common exhaust gas outlet 15 of combustion chamber 7. V

A fuel decomposition chamber 25 is axially-positioned within and spaced from the upstream portion of flame tube 17. Decomposition chamber 25 is open at its upstream end adjacent to and spaced from the upstream end of flame tube 17 and is closed at its downstream end. The downstream end of decomposition chamber 25 is positioned generally intermediate to the upstream and downstream ends of flame tube 17. A plurality of fuel nozzles 27 are positioned in the fuel decomposition cham 'berdownstream end. Nozzles 27 are positioned so as to direct the'flow of fuel toward the upstream end of fuel decomposition chamber 25.

A pilot flame chamber 29 is axially-positioned within The upstream end of pilot flame chamber 29 is attached to the upstream end of flame tube 17 so that the latter also serves as the upstream end for pilot flame chamber 29. At least one perforation 31 is located in the downstream end of pilot flame chamber 29. In the embodiment shown in Figure 1, an axially-positioned perforation 31 is located in the downstream end of chamber 29. A plurality of perforations 33 are positioned in the upstream end of pilot flame chamber 29. A fuel nozzle 35 is axially-positioned in the upstream end of pilot flame chamber 29 and is positioned so as to direct the flow of fuel toward the downstream end of pilot flame chamber'29.

Liquid jet engine fuel from a fuel supply 37 is connected by a conduit 39, a conduit. 43 and a conduit41 to fuel nozzles '27, and by conduit 39, conduit 43, and a conduit 45 to fuel nozzle 35. A valve 47 is positioned in conduit 43 to permit the amount of fuel passing to nozzle 35 to be burned in pilot flame chamber 29 to be controlled so that stoichiometric combustion takes place in pilot flame chamber 29.

In the operation of the jet engine incorporating the combustion chamber of my invention, a portion of the liquid fuel, preferably a minor proportion, is passed via conduits 39, 43 and 45 to fuel nozzle 35 and burned in a portion of the total air supplied from air inlet 13 and in all the air supplied through perforations 33 to pilot flame chamber 29 under stoichiometric conditions. The remainder of the fuel is passed via conduits 39 and 41 to fuel nozzles 27. The hot combustion products from the stoichiometric combustion in pilot flame chamber 29 are exhausted therefrom through perforation 31 and quenched by liquid fuel in decomposition chamber 25. Thus, the major portion of the liquid fuel is decomposed by direct heat exchange with the products of stoichiometric combustion in pilot flame chamber 29. The resulting combustion products and decomposed fuel are exhausted from decomposition chamber 25 through its open upstream end and burned in primary air in the upstream portion of flame tube 17. The resulting products from. combustion in the upstream portion of flame tube 17 are quenched with secondary air admitted through perforations 21 in the downstream portion of flame tube 17. The resulting hot combustion gases and heated air are then exhausted to the atmosphere through outlet 15, turbine 11 and exhaust gas outlet 5.

It should be noted that the embodiment shown in Figure 1 includes a pair of perforations 33 in the upstream end of pilot flame chamber 29.- A plurality of such perforations, that is more than 2, can'be used. The number and cross-sectional area of the perforations 33 will determine the amount of air admitted to the pilot flame chamber and, since stoichiometric combustion is conducted in the pilot flame chamber, will. determine the amount of fuel which is stoichiometrically burned. As was pointed out before, the amount of fuel stoichiometrically burned varies with the characteristics of the fuel being used. In order to permit variation of the cross-sectional area of perforations 33 so that the combustion chamber can be adapted to jet engine fuels of varying chemical and physical characteristics, a modification of the apparatus of my invention is shown in Figure 2 wherein the area of perforations 33 can be varied. Thus it will be seen that the upstream end of pilot flame chamber 29 has four perforations 33 and has a pair of perforations. 31 in the downstream end of chamber 29. A plate 49 is slidably attached to the exterior of the upstream end of pilot flame chamber 29. Plate 49 has a series of perforations Sland a large, axially-positioned, perforation 53. Perforations 51 are of the same size or cross-sectional area as perforations 33. Plate 49 is slidable upon the exterior of the upstream end of flame tube 29 by a mechanical linkage 55. Thus, by adjusting the position of plate 49, it'is possible to reduce the total cross-sectional area of perforations 33 which is exposed to inlet air down to a. point at which these perforations are completely closed. Obviously, the specific arrangement for varying the area of perforations 33 is only schematically shown and other specific systems will be readily apparent to those skilled in the art. For example, such systems as butterfly valves or iris diaphragms controllable froma single source can be mounted upon the perforations 33 to permit their crosssectional area to be adjusted.

The decomposition reactions to which reference has been made ordinarily involve predominantly cracking and depolymerization reactions although other reactions which influence the formation of hydrogen and low molecular Weight unsaturated hydrocarbons also take place. 'The reaction products formed in the decomposition zone contain a substantial amount of high flame speed gaseous components, such as hydrogen, acetylene,propylene and the like, admixed with a portion which is normally liquid at atmospheric temperature and pressure. The improved combustion performance obtained by using the fuel mixtures formed by the method and means of this invention *is believed to be the result of superior piloting action of the high flame speed components in the combustion chamber at the flame holding areas as well as the result of preheating, prevaporization and the occurrence of some pre-flame reactions.

The present invention providm a method of operating jet propulsion type engines with a single fuel whereby higher combustion efficiency, higher heat release rates and higher combustion stability are obtained than was previously possible in conventional jet propulsion engines burning the same fuel. This invention is particularly effective in producing improved combustion stability and combustion efficiency in combustion systems operated under rich mixture conditions. The present invention also permits certain fuels, which heretofore have not been considered to be completely suitable fuels for jet propulsion engines because of low volatilities and excessive carbon deposition, to be effectively utilized in jet propulsion engines.

The fact that jet fuels of relatively low volatility are rendered useful by this invention is of importance because the newest high altitude, high Mach number aircraft and missiles place severe requirements on their fuels because of aerodynamic heating and decreased ambient pressures. These effects dictate the use of fuels of low volatility if excessive vapor losses from conventional vented fuel tanks are to be avoided. Also, such aircraft are limited in their performance by the volume of fuel which they can carry and, therefore, the higher volumetric heating value of heavier petroleum fractions is a desirable feature. However, this invention is applicable to combustion processes in general, such as stationary gas turbine power plants, and is not restricted to aircraft jet engines such as turbojet, turboprop, and ram jet engines.

The following examples, calculations and data are supplied to illustrate some of the advantages afforded by the improved combustion chamber of this invention. The amounts of air and fuel required in the pilot flame chamber were calculated for a combustion system using a normal decane fuel and were based upon a total air requirement of 1.0 pound per second and an over-all fuel-air ratio of .01. The energy required to decompose the normal decane fuel at the rate of 0.01 pound per second according to the following reaction was found to be 18.5 B.t.u. per second.

wherein C H is assumed to be 1,3-butadiene and C H is assumed to be l-butene. With a temperature drop of 3600 F. across the fuel decomposition chamber and using an average specific heat for air of 0.24 B.t.u. per pound per degree F., the heat available from combustion of this amount of the fuel is 865 B.t.u. per poound of air. Since 18.5 B.t.u. per second is the energy required to decompose the total amount of fuel, the amounts of fuel and air required in the pilot flame chamber is 0.0013 pound per second and 0.0200 pound per second, respectively. The stoichiometric fuel-air ratio for normal decan used was 0.066 and the heat lost from the walls of the combustion chamber and the heat conducted from the combustion in the upstream end of the flame tube in the primary air were neglected.

Another of the important advantages for the combustion chamber of my invention is that it requires less space to produce an equivalent amount of heat than is required by a conventional combustion chamber. A conventional combustion chamber operating on 20 pounds of air and 0.2 pound of fuel per second to deliver 13,680,000 B.t.u. per hour requires 6.4 cubic feet of space for the combustor. However, a combustor constructed in accordance with this invention and operating under the same conditions to yield the same amount of heat requires 6.0 cubic feet which is a saving of 0.4 cubic feet of space. The effect of temperature or flame velocity has not been included in these calculations. However, it is believed that flame speeds would be at least two times those used in these calculations at the operating temperature of the combustor. Therefore, the saving in space requirement would be even greater than that indicated by the foregoing calculations.

A further advantage for the combustion chamber of this invention is that turbulent flow of air to the primary combustion zone is not required as is true in a conventional combustion chamber. In the latter, a considerable portion of the primary air must be turbulent in order to aid the vaporization of the fuel and this results in a pressure drop across the combustor. For example when there is a 4 percent pressure drop and a 8:1 pressure ratio in a conventional combustion chamber, each percent pressure drop results in a 1 percent drop in the efficiency of the power plant. However, in the combustion chamber of this invention, the fuel is vaporized completely before it leaves the fuel combustion chamber and therefore, the pressure drop for aiding the vaporization of the fuel is not required.

It will be obvious to those skilled in the art that many substitutions, changes and modifications can be made in the light of the foregoing disclosure which will be Within the spirit and scope of my invention.

I claim:

1. A continuous combustion burner for burning a liquid fuel in air, which comprises, in combination, a combustion chamber having an air inlet at its upstream end and a product gas outlet at its downstream end, a flame tube axially-positioned within and spaced from said combustion chamber, said flame tube having a plurality of perforations about the upstream portion thereof adjacent to said combustion chamber upstream end for admission of primary combustion air thereto and having a plurality of perforations about the downstream portion thereof adjacent to said combustion chamber downstream end for admission of secondary combustion air thereto and having a product gas outlet at its downstream end attached to and coincident with said combustion chamber product gas outlet, an axially-positioned fuel decomposition chamber within, and spaced from said upstream portion of said flame tube, said fuel decomposition chamber being open at its upstream end adjacent to and spaced from said flame tube upstream end and being closed at its downstream end, said decomposition chamber downstream end being positioned generally intermediate to said upstream and downstream ends of said flame tube, a plurality of first fuel nozzles positioned in said fuel decomposition chamber downstream end, said first fuel nozzles being positioned so as to direct fuel toward said fuel decomposition upstream end, an axially-positioned pilot chamber positioned within and spaced from said fuel decomposition chamber, the upstream end of said pilot chamber being attached to the upstream end of said flame tube so that said flame tube upstream end also serves as the upstream end for said pilot chamber, at least one perforation in said pilot chamber downstream end, a second fuel nozzle axially-positioned in said flame tube upstream end and positioned so as to direct fuel into said pilot chamber, a plurality of perforations in said pilot chamber upstream end, a first conduit means for supplying fuel to said first fuel nozzles, a second conduit connected to said first conduit means for supplying fuel to said second fuel nozzle and a valve means positioned in said second conduit for controlling the amount of fuel supplied to said second conduit so that stoichiometric combustion of said fuel in air occurs in said pilot chamber.

2. A burner in accordance with claim 1 further comprising a means for varying the cross-sectional area of said plurality of perforations in said pilot chamber upstream end.

3. In a continuous combustion burner wherein a liquid fuel is burned in air in a combustion chamber comprising means for quenching the combustion with air and means for exhausting the hot gases thereby produced from said combustion chamber, a pilot flame chamber, a fuel decomposition chamber, said pilot flame chamber being positioned Within the fuel decomposition chamber and the fuel decomposition chamber being positioned Within the combustion chamber, means for stoichiometrically burning a portion of said fuel in a portion of said air in one end of the pilot flame chamber, means for exhausting the products of combustion from the opposite end of the pilot flame chamber into the fuel decomposition chamber, means for introducing the remainder of said fuel into the fuel decomposition chamber at one end thereof and adjacent to the last mentioned exhaust means, and means at the opposite end of the fuel decomposition chamber for exhausting a mixture of combustion gases and decomposed fuel gases into the combunstion chamber whereby said gases pass in indirect heat exchange and countercurrent to the combustion gases flowing through the pilot flame chamber.

4. The apparatus of claim 3 in which means is provided for introducing the remainder of the fuel entering the fuel decomposition chamber in countercurrent contact with the products of combustion from the pilot flame chamber.

5. A continuous combustion burner for a fuel in air which comprises in combination a combustion chamber having an air inlet and a product gas outlet, aflame tube positioned within the combustion chamber, a decomposition chamber within the flame tube, a pilot flame chamber within the decomposition chamber, means for stoichiometrically burning a portion of said fuel in a portion of said air in the pilot flame chamber, means for exhausting the products of combustion from the pilot flame chamber into the fuel decomposition chamber, means for introducing the remainder of said fuel into the fuel decomposition chamber at one end thereof and adjacent to the above mentioned exhaust means, means at the opposite end of the fuel decomposition chamber for exhausting a mixture of combustion gases and decomposition gases into the flame tube whereby said gases are passed in indirect heat exchange and countercurrent to the combustion gases flowing through the pilot flame chamber, means for passing primary air from the air inlet into the flame tube about the decomposition chamber and means for passing secondary air from the air inlet into the flame tube down stream of the decomposition chamber to quench the combustion reaction therein.

6. A method of burning liquid fuel to power a jet engine which comprises burning a first portion of said fuel in a pilot zone with a stoichiometric amount of air to form hot combustion gases, contacting said hot gases in a decomposition zone with a second portion of said fuel whereby the fuel thus contacted is decomposed in the absence of air to lower molecular weight fuels, passing the resulting mixture of combustion gases and said lower molecular weight fuels to a combustion zone, and burning said mixture with air to form combustion products for powering said engine.

7. The process of claim 6 in which the amount of fuel stoichiometrically decomposed comprises at least one percent of and not greater than 15 .percent by weight of the total liquid fuel.

8. A method of burning liquid fuel to power a jet engine which comprises burning a first portion of said fuel in a pilot zone with a stoichiometric amount of air to form hot combustion gases, contacting said hot gases in a decomposition zone with a second portion of said fuel whereby the fuel thus contacted is decomposed in the absence of air to lower molecular weight fuels, passing the resulting mixture of combustion gases and said lower molecular weight fuels to a combustion zone, said mixture en route passing in indirect heat exchange relation with said pilot zone and said combustion zone, and burning said mixture with air to form combustion products for powering said engine.

9. A method of burning liquid fuel to power a jet engine which comprises burning a first portion of said fuel in a pilot zone with a stoichiometric amount of air to form hot combustion gases, contacting said hot gases in a decomposition zone with a second portion of said fuel whereby the fuel thus contacted is decomposed in the absence of air to lower molecular weight fuels, passing the resulting mixture of combustion gases and said lower molecular weight fuels to a combustion zone, said mixture en route passing in indirect heat exchange relation with said pilot zone and said combustion zone, burning said mixture with primary air to form combustion products for powering said engine, and quenching said combustion products with secondary air.

10. The apparatus of claim 5 in which a variable air inlet means is provided for the pilot flame chamber.

11. The process of claim 9 in which the amount of feed stoichiometrically decomposed comprises at least one percent and not greater than 15 percent by weight of the total liquid fuel.

References Cited in the file of this patent UNITED STATES PATENTS 1,757,855 Chilowsky May 6, 1930 2,059,523 Hepburn et a1. Nov. 3, 1936 2,201,965 Cook May 21, 1940 2,206,553 Nagel July 2, 1940 2,523,096 Clements Sept. 19, 1950 2,628,475 Heath Feb. 17, 1953 2,635,426 Meschino Apr. 21, 1953 2,655,786 Carr Oct. 20, 1953 2,697,910 Brzozowski Dec. 28, 1954 2,767,233 Mullen et al. Oct. 16, 1956 FOREIGN PATENTS 712,843 Great Britain Aug. 4, 1954