Patent Application
20070029408
1. Field of the Invention
This invention resides in the field of liquid-fuel combustion engines, and particularly the fuel injectors for such engines.
2. Description of the Prior Art
Combustion engines powered by liquid fuel are used in a variety of applications. Examples are gas turbines, pre-burners, liquid-propellant rocket motors, and descent engines of space shuttles. In many of these applications, a controlled shutdown, typically involving throttling the engine in a gradual or stepwise manner, is critical to the successful operation of the engine. In thrust-producing engines, a thrust ratio of 30:1 or greater is needed upon shutdown. When throttling is performed at very high ratios or at very rapid rates, the rapid changes of pressure and the transmission of these changes throughout the system produce a combustion instability in the form of a series of low-pressure fluctuations known as “chugging.”
Attempts in the prior art to control chugging have included the foaming of the propellant by the injection of an inert gas into the propellant feed lines, as disclosed by Morrell, G., U.S. Pat. No. 3,045,424 (Jul. 24, 1962), Biehl, R. E., et al., U.S. Pat. No. 3,266,236 (Aug. 16, 1966), Jennings, J. J., U.S. Pat. No. 3,266,241 (Aug. 16, 1966), and Braue, J. W., U.S. Pat. No. 3,302,406 (Feb. 7, 1967). The purpose of the addition of the inert gas is to reduce the amount of fuel being fed without reducing the velocity. In an alternative method, the injection head contains a mechanical device that varies the injection orifice areas for the fuel and oxidizer. An injection head of this type was reported by Elverum, G., Jr., et al., “The Descent Engine for the Lunar Module,” Paper No. 67-521, AIAA 3rd Propulsion Joint Specialist Conference, Washington D.C., Jul. 17-21, 1967. The injection head in this paper supplies both fuel and oxidizer and extends into the combustion chamber. The head contains a multitude of openings for each of the two liquids plus a movable control sleeve that closes the openings, the number closed depending on the position of the sleeve. Since the injection head extends into the combustion chamber, the sleeve is exposed to the hot combustion gas.
Of further possible relevance to this invention is published literature relating to swirl-type pressure nozzles used for spray drying, and swirl-type coaxial injectors used for rocket launch engines. Swirl-type pressure nozzles are described by Marshall, W. R., Jr., in Atomization and Spray Drying, “Chapter II. Performance Characteristics of Centrifugal- or Swirl-Type Pressure Nozzles,” Chemical Engineering Progress Monograph Series, No. 2, Vol. 50, pp. 12-30, American Institute of Chemical Engineers (1954). A swirl-type coaxial injector is described by Hulka, J., et al., in “Performance and Stability of a Booster Class LOX/H2 Swirl Coaxial Element Injector,” AIAA Paper No. 91-1877, AIAA/SAE/ASME 27th Joint Propulsion Conference, Sacramento, Calif., Jun. 24-26, 1991.
The contents of all references in this section are incorporated herein in their entirety.
The present invention resides in a throttling injector that achieves thrust variation by spinning the liquid fuel around the internal wall of a swirl chamber, causing the liquid to be ejected from the chamber around the rim of an orifice in the chamber wall, with thrust variation being achieved by varying the volume of liquid entering the swirl chamber and thereby varying the depth of the layer of the peripheral liquid stream at the outlet orifice rim. The liquid stream that is ejected from the swirl chamber through the outlet orifice will thus occupy only a small fraction of the outlet orifice area at low volumetric flow rates or a majority of, and possibly the entire, outlet orifice area at higher flow rates, and various levels in between. The volumetric flow of liquid fuel that is fed through inlet orifices to the swirl chamber can be varied by continuous changes in the flow rate through an inlet line or through a series of inlet lines that feed the liquid to the chamber, or by stepwise changes produced by opening and closing different numbers of liquid fuel inlets to the swirl chamber. Variation can also be achieved by using inlets of different sizes, or by changes in the manner in which the liquid fuel is directed to individual inlets. All of these variations can be made without any substantial change in the linear velocity of liquid through the outlet orifice, and without any substantial change in the pressure drop, if any, across the orifice. Throttling of the combustion engine by decreasing the volumetric flow of fuel from the injector can thus be achieved without exposing any moving parts to hot combustion gases, and without any need for injecting gas or additives to the fuel or for maintaining a foam or dispersion of any kind in the fuel. A wide throttling range can be achieved by constructing the swirl chamber with a large number of inlets, and further flexibility can be achieved by using inlets of different sizes. Chugging can thus be avoided by reducing changes in the linear flow rate of fuel through the outlet orifice of the injector and thereby minimizing the reduction in pressure drop across the outlet orifice.
The “outlet orifice” as used in the preceding paragraph is to be distinguished from the inlet orifices supplying the liquid fuel to the swirl chamber. A system parameter that is variable in the practice of the present invention is the discharge coefficient Cd, also known as the flow coefficient, at the outlet orifice. The discharge coefficient Cd is defined as the product of an area ratio Ca and a velocity coefficient Cv. The area ratio Ca is the portion of the outlet orifice opening are that is occupied by liquid during ejection of the liquid from the swirl chamber divided by the total area of the outlet orifice. The velocity coefficient Cv reflects the angle of the ejected liquid relative to the axis of the outlet orifice. For outlet flow that is fully axial, Cv=1, while for outlet flow that is fully tangential, CV is zero. Thus, in any realistic situation, the value of CV is between zero and 1. For a 45° angle, for example, Cv is approximately 0.7.
Thus, when the liquid flows in only a thin layer over the rim of the outlet orifice around a core of air or gas along the orifice axis, Cd will be only a small fraction, and as the thickness of the flowing layer increases, Cd will rise toward 1, reaching a maximum when the outlet orifice is flooded. When the linear velocity of the fuel through the outlet orifice is constant, the volumetric flow rate of fuel injected into the combustion chamber through the outlet orifice is directly proportional to Cd. Accordingly, the thrust in a rocket engine whose fuel is supplied through the outlet orifice is likewise directly proportional to Cd with a constant linear velocity of fuel. A constant linear velocity can be maintained by fixing the linear velocity of fuel passing through each inlet and varying the number of inlets in simultaneous use by allowing each inlet only a fully open or a fully closed condition. This can be achieved in conventional ways, notably by continuously monitoring the pressure drop across the inlet and adjusting the flow rate accordingly, or by supplying the inlets from a pressurized source and maintaining the source at a constant pressure.
For a liquid bipropellant rocket engine, the two propellants can be injected through separate injectors, each with its own swirl chamber, although with certain types of propellants, the injector can be configured to inject both propellants simultaneously. In most cases, however, multiple injectors positioned in a distinct spatial arrangement will be used for a single combustion chamber.
These and other features of the invention, as well as various preferred embodiments, are described in greater detail below.
The swirl chamber is a flow-through receptacle, preferably a body of revolution around a longitudinal axis, that receives an incoming liquid steam and causes the liquid to circulate within the chamber in a swirling motion around the axis to form a vortex of the liquid before the liquid leaves the chamber. The outlet orifice is thus preferably circular, and preferably has a width or diameter that is smaller than other internal portions of the chamber so that diameter of the swirling path of the liquid decreases as the liquid approaches the outlet orifice. The outlet orifice can have the same diameter as the swirl chamber itself, although in preferred embodiments of the invention, the diameter ratio of the widest section of the chamber to the outlet orifice is about 2:1 or greater, or more preferably about 3:1 to about 6:1. This decrease in diameter increases the dispersion angle of the fuel ejected through the outlet orifice into the combustion chamber or engine, forming a stream in the form of a hollow cone whose angle depends on the geometry of the swirl chamber.
The inlets are configured to direct the incoming liquid along directions that do not intersect the axis of the swirl chamber and to thereby cause the liquid to encircle the axis in a circular or swirling motion. The inlet direction can be described as “tangential” although it is not strictly necessary that the angle of entry be so close to the wall of the chamber that the incoming liquid immediately contacts the wall. The outlet orifice is coaxial with the longitudinal axis of the swirl chamber so that the ejected liquid is evenly and uniformly distributed around the orifice rim. The inlets are configured such that all liquid entering the swirl chamber follows a unidirectional circular or spiral flow path, i.e., that all of the entering liquid flows in the clockwise direction or all of it flows in the counter-clockwise direction. Flow of the liquid toward the outlet orifice of the swirl chamber can be achieved by the force of the incoming liquid, or by shaping the inlet orifices to direct the flow in a plane that is at an angle. Thus, the direction of flow emerging from any single inlet can be perpendicular to, although not intersecting, the axis of the chamber, or the incoming flow can be in a plane that is at an acute angle, generally less than 30°, to the perpendicular to immediately produce a spiral flow pattern of the liquid. A spiral flow pattern can also be promoted by placing spiral grooves along the internal wall of the swirl chamber.
In preferred embodiments of the invention, a single source of fuel or propellant is used to supply each of the inlets, or when two or more propellants are fed through separate inlets, each of the inlets for the same propellant. Flow of propellant through the inlet in these embodiments is achieved by pressurization of the source. When inlets of different cross sectional areas are present, a uniform flow rate through all inlets from the same pressurized source can be achieved by engineering the inlet interiors to control the pressure drop. The high flow rates that would otherwise result from excessive pressure drops, for example, can be reduced by the inclusion of internal orifices or other types of flow restrictors. Narrow inlets with cross sections at the low end of the range may for example require such restrictors.
The number of inlets to the swirl chamber may vary widely and is not critical to the invention, provided that there be a plurality of inlets and the means to open individual inlets or groups of inlets of the plurality to flow and to select the inlets that are thus actuated. In preferred embodiments of the invention, the number of inlets is three or more, up to as many as 100 or more, with the capability of choosing between actuating only one of the inlets, two or more but less than all of the inlets, or all of the inlets, either by automation or at the direction of an operator of the injector. The position of the inlets on the inner wall of the swirl chamber is not critical. The inlets can be axially or circumferentially separated from each other, or both. Reactive forces from the jets of incoming liquids can be balanced by arranging the inlets symmetrically around the longitudinal axis of the swirl chamber for actuation in opposing pairs or groups of three or more. In certain embodiments, however, it may be beneficial to position either all inlets or groups of inlets in a common plane perpendicular to the longitudinal axis to cause all liquid flowing through the swirl chamber to follow a the same spiral path.
Variations in the volumetric flow rate can be achieved either by selectively actuating, i.e., supplying liquid fuel through, individual inlets or groups of inlets. In preferred embodiments, the range of variation can be such that the ratio of the highest cross sectional area to the lowest cross sectional area of actuated inlets is at least about 2, preferably at least about 4, and most preferably at least about 6. These different cross sectional areas can be achieved by individually actuating inlets of different cross sectional areas or by combining different numbers of inlets of the same cross sectional area, or both.
As noted above, when the linear velocity of the liquid ejected through the outlet orifice remains constant, or substantially constant, the volumetric flow rate of liquid ejected through the orifice, and hence the thrust in the engine into which the fuel is ejected, are directly proportional to the discharge coefficient Cd. The value of Cd is empirically correlated with the dimensions of the swirl chamber and the inlet and outlet orifices by the relation shown in
In the above formula, Ro is the radius of the outlet orifice; ri is the collective inlet orifice radius for all actuated inlets, i.e., the square root of the ratio of the total of the cross sectional areas of all actuated inlets to π; and Rs is the radius of the widest portion of the swirl chamber.
The definitions above assume that the swirl chamber has a circular cross section, and that the inlet and outlet orifices are likewise circular. Throughout this specification and claims, any reference to a radius, area, or cross section of an inlet refers to the projection of the inlet onto a plane that is perpendicular to the axis of the inlet, recognizing that the actual inlet opening through which the inlet empties into the swirl chamber will in many cases be at an angle to this plane due to the curvature of the swirl chamber wall and therefore often elliptical in shape or otherwise asymmetrical about the inlet axis.
To illustrate the use of
The actuation of the inlets, i.e., the opening of inlets to allow passage of liquid fuel through the inlets to the swirl chamber, is done in a selective manner, which term is used herein to denote that the injector is provided with the capability of actuating less than all of the inlets or all of the inlets at the choice of an operator or of control components in the injector. Throttling is thereby achieved by decreasing Cd in stages by stepwise reduction of the number of actuated inlets. The means of achieving this selectivity or stepwise reduction is not critical to the invention and can vary widely. In a relatively primitive form, for example, the selectivity can be achieved by individual on-off valves, operated either manually, by pressure actuation, or by solenoid. Alternatively, selectivity can be achieved by a stepping valve or a series of stepping valves. A further alternative is the use of a movable closure such as a sleeve or piston enclosed in a flow distribution chamber that is common to all inlets or to series of inlets, the sleeve or piston selectively closing either individual inlets or groups or inlets according to the position of the sleeve or piston in the chamber, or successively closing a row of inlets, the number that are closed depending on the position of the sleeve or piston. As noted above, the liquid fuel to all inlets can be supplied from a common reservoir through individual supply lines or groups of lines. The flow distribution chamber will be positioned between the reservoir and these supply lines.
The supply lines can be molded, drilled or cast into the wall of the swirl chamber. When a large number of supply lines are desired, particularly those that are of small dimensions, the swirl chamber wall can be formed by platelet technology. Platelet technology is well known in the art, and a representative description can be found in U.S. Pat. No. 5,387,398 (Mueggenburg et al., issued Feb. 7, 1995) and U.S. Pat. No. 5,804,066 (Mueggenburg et al., issued Sep. 8, 1998), the contents of each of which are incorporated herein by reference in their entirety. As described in these patents, individual platelets (thin metallic sheets) are chemically etched through masks, then laminated by either diffusion bonding, roll bonding, or brazing. Diffusion bonding is achieved by hot-pressing the platelets together at pressures of 1000 to 3000 psi (6.9 to 20.7 MPa) and temperatures of 450° C. to 550° C. The thickness of each platelet will range from about 0.001 inch (0.00254 cm) to about 0.025 inch (0.064 cm). The total number of platelets in the laminate will be determined by the desired dimensions of the swirl chamber, the number and arrangement of the supply lines, the anticipated stress load, and other general matters of construction, as well as the ability to withstand the conditions expected to be encountered during use. In most cases, the number of platelets will range from 10 to 2,500, and preferably from 20 to 500. Copper, steel, and aluminum are suitable platelet materials, although other metals can be used as well.
As the descriptions above indicate, this invention is capable of implementation in a variety of ways. The invention and its scope can be readily understood however by a detailed examination of specific embodiments. One such embodiment is shown in the drawings and described below.
The three inlets shown in
As noted above, in certain embodiments of the invention, smaller inlets may include a flow restrictor to reduce the flow that might otherwise result from an excessive pressure drop across the inlet when larger inlets are closed.
Injectors in accordance with this invention are useful for any kind of liquid fuel or propellant. For bipropellants that combust upon contact, the propellants must be kept out of contact until the reach the combustion chamber. This can be done by using separate injectors for the two propellants, or by using specially constructed injectors that contain a length of tubing extending along the axis of the swirl chamber such that one propellant can be fed through the tubing in a non-swirling configuration and the other around the tubing in a swirling configuration. When the bipropellant combination is fed through multiple injectors, however, it is preferable to use individual injectors for the two propellants, arranged in close proximity in an alternating one-dimensional or two-dimensional array.
The foregoing description focuses on particular embodiments of the invention for purposes of explanation and illustration. Further embodiments and modifications of the above will be apparent to those skilled in the art upon reviewing this description, such embodiments and modifications falling within the scope of the invention.
