Patent Grant
7013649
1. Field of the Invention
This invention relates to fuel-air mixers for gas turbine engine combustors and, more particularly, to reducing the formation of solid carbon or coke on such fuel-air mixers.
2. Description of Related Art
Gas turbine engine combustors use fuel nozzles and fuel-air mixers for mixing and burning fuel with compressed air. The fuel is typically premixed with air in the fuel-air mixers prior to combustion in order to minimize smoke and other undesirable by-products and to maximize the efficiency of the combustion process.
Fuel-air mixers are designed to atomize the fuel and to premix it with air in order to produce efficient and complete combustion. Low pressure fuel-air mixers have been designed which incorporate primary and secondary counter-rotational air swirlers which atomize fuel by the high shear forces developed in the area or zone of interaction between counter-rotating air flows produced by the primary and secondary air swirlers. An air swirler, also referred to as a swirler cup, includes a venturi and circumferentially and downstream angled air jets formed around an axis of the venturi. The air jets swirl the air prior to intermixing with the fuel to enhance atomization as well as mixing.
A very common problem with fuel-air mixers is the formation of carbon, commonly referred to as coking on combustor parts and, in particular, venturis of the air swirlers. Solid carbon or coke is formed by impingement of liquid hydrocarbon fuel on hot metal surfaces. This results in thermal decomposition of the fuel and precipitation of solid carbon or coke on the surface. Coke is typically formed at temperatures between 400 and 900 degrees F., which is typical of the combustor inlet conditions of a modern gas turboshaft or turbofan engine. Solid carbon will oxidize or burn away at temperatures in excess of 900 degrees F.
Although these temperatures are seen during high power operation, the cooling effect of the liquid fuel impingement prevents the venturi surface from reaching temperatures high enough to allow the carbon to burn away. Tests on instrumented venturis have shown surface temperatures to be 300 to 400 degrees F. below the inlet air temperature, which results in the venturi surface being in the 400–900 degrees F. carbon formation window for most of the engine operation. The impingement of liquid fuel also prevents oxygen from reaching the surface, further contributing to carbon buildup.
The formation of carbon on the venturi surface distorts the aerodynamic shape of the surface thereby disrupting the distribution of fuel in the combustor. This results in combustor hot streaks and resulting turbine distress. The combustor temperature distortions also distort the exit temperature thermocouple readings used to monitor engine deterioration, resulting in false deterioration indications. Engine starting and altitude ignition have also been shown to be adversely affected. In severe cases, these carbon deposits have caused total blocking of the venturi passage causing fuel to be deposited outside the combustor liner, and causing casing burn-through and in flight shutdown.
Disclosed in U.S. Pat. No. 6,571,559 is a fuel nozzle positioned inside the upstream end of a radial inflow primary swirler and adjacent to the venturi, a fuel passage through the fuel nozzle from which fuel is sprayed into the venturi at a designated spray angle and, a purge airflow circumscribing the fuel passage. The purge airflow flowing substantially parallel to a longitudinal axis of the venturi to provide a boundary layer of air along the inner surface of the venturi. The boundary layer of air minimizes the amount of fuel contacting the inner surface of the venturi subsequently reducing carbon formation. Annular passages or air shrouds have been incorporated into the fuel injector tip of the fuel nozzle to admit non-swirling air for the purpose of suppressing carbon formation (see U.S. Pat. Nos. 6,571,559 and 5,123,248 as examples). The air shrouds in the fuel nozzle tips cannot always be accommodated in the fuel nozzle tips.
A gas turbine engine combustor fuel-air mixer includes a body having a substantially annular venturi positioned therein. The venturi having a longitudinal axis therethrough, an upstream end, a downstream end, and an inner surface. A primary radial jet swirler upstream of the venturi includes a plurality of radially extending primary air jets circumferentially and downstream angled with respect to the longitudinal axis. A plurality of axial jets axially extend through the primary swirler air and are circumferentially disposed around the longitudinal axis. The axial jets may have rectangular cross-sections.
An exemplary embodiment of the fuel-air mixer further includes the axial jets and the inner surface of a throat of the venturi being both radially located at about equal distances from the longitudinal axis at a radius as measured from the longitudinal axis. The axial jets are located in an insert mounted to an upstream portion of the primary radial jet swirler and axially forward of the plurality of radially extending primary air jets. The primary radial jet swirler includes a generally annular upstream portion and a conical downstream portion, the plurality of radially extending primary air jets are disposed through the conical downstream portion, and the axial jets are disposed through the upstream portion.
A secondary air swirler is located downstream of the primary radial jet swirler and circumferentially disposed about and radially spaced apart from the venturi. The secondary air swirler includes a plurality of secondary swirler vanes disposed between the venturi and a spaced apart bellmouth-shaped fairing. At least some of the axial jets have jet centerlines that intersect primary swirler centerlines of corresponding ones of the primary air jets downstream of outlets of the primary air jets.
A fuel injector assembly incorporating the gas turbine engine combustor fuel-air mixer includes a fuel nozzle disposed in the annular upstream end of the primary radial jet swirler in alignment with the longitudinal axis.
The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where:
Illustrated in
The combustor 11 is enclosed by a casing 30 which together with the outer liner 18 defines an annular outer passage 32. The dome-shaped end 22 includes a plurality of apertures 36 for supplying compressor discharge air 14 to the fuel-air mixers 28. Each fuel-air mixer 28 includes a body 38 having a substantially annular venturi 40 positioned therein. The venturi 40 has a longitudinal axis 42 therethrough and includes an upstream end 44, a downstream end 46, and an inner surface 48. The upstream end 44 of the venturi abuts a primary radial jet swirler 50. The primary radial jet swirler 50 is illustrated as a tubular ferrule defined by a generally annular upstream portion 52 and a conical downstream portion 54 terminating at a radial flange 55. A plurality of radially extending primary air jets 56 are disposed through the conical downstream portion 54. The primary air jets 56 are circumferentially and downstream angled with respect to the longitudinal axis 42 so that compressor discharge air 14 entering the primary air jets 56 is swirled to produce primary swirler jet airflow 58. The venturi 40 is positioned with respect to the primary radial jet swirler 50 to enable the primary swirler jet airflow 58 to enter the venturi 40 in a swirling manner.
A fuel nozzle 60 is disposed in the annular upstream end 44 of the primary radial jet swirler 50 in alignment with the longitudinal axis 42 of the venturi 40 to provide a fuel injector assembly 61. The fuel nozzle 60 includes a fuel passage 62 for spraying fuel 64 into the venturi 40 where it is atomized and mixed with the primary swirler air. A secondary air swirler 66 downstream of the primary radial jet swirler 50 is circumferentially disposed about and radially spaced apart from the venturi 40. Compressor discharge air 14 flows into the secondary air swirler 66 and is directed by a plurality of secondary swirler vanes 72 disposed between the venturi 40 and a spaced apart bellmouth-shaped fairing 68 and substantially equidistant from one and the other angled to induce a swirl on the flowing compressor discharge air. The secondary swirler vanes may be angled in the same or different tangential direction as the primary swirler air jets 56. The fairing 68 extends aft of the venturi's downstream end 46. The fairing 68 is positioned and spaced so as to, in combination with the venturi 40, form a fluid passageway 70 through which secondary swirler air flows. This secondary swirler air intermixes with the primary swirler jet airflow 58 and fuel 64 mixture aft of the downstream end 46 of the venturi 40, thereby, further atomizing and mixing the fuel and air for combustion. The fuel-air mixer arrangement described typically operates at temperatures approaching 1000 degrees F. The centrifugal effect of the fuel intermixing with the primary swirler jet airflow in the venturi 40 results in fuel wetting the inner surface 48 of the venturi 40 which lowers the surface temperature and, under certain conditions, initiates predominately carbon formation and, in some cases, coke formation, commonly referred to as carboning.
Referring to
In
At least some of the axial jets 71 extend axially through the primary radial jet swirler 50 into some of the primary air jets 56 are radially located inwardly of a circumference C of the primary radial jet swirler 50 defined by radially inwardmost points 77 of the primary air jets 56. These axial jets 71 have jet centerlines 82 that intersect primary swirler centerlines 88 of corresponding ones of the primary air jets 56 at an intersection point 92 downstream of discharges or outlets 90 of the primary air jets 56 as illustrated in
Prior art fuel-air mixer designs delivered purge airflow to the fuel-air mixer using a shroud defined by an annular air passage in the fuel nozzle as disclosed and illustrated in U.S. Pat. No. 6,571,559. This is not practical on an smaller gas turbine engines, smaller than a GE CF6 for example, when using a dual passage fuel injector. Also fuel injector heat shielding is seriously compromised by placing the air shroud on the injector. Putting the purge on the swirler allows much more flexibility in fuel injector design as is done in the present invention.
The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims:
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| Number | Date | Country | |
|---|---|---|---|
| 20050262843 A1 | Dec 2005 | US |
