Method and Apparatus for Enhanced Flameholding in Augmentors
A jet engine flame holder bluffbody is provided that includes a leading edge that is rounded, a mid-body having a rectangular cross-section, and a trailing edge, where the trailing edge includes a rectangular cross-section having digitated cutouts from the rectangular cross-section, where the tailing edge projects upward according to a digitated cross-section, where material digits remain between consecutive pairs of the digitated cutouts.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/806,958 filed Aug. 24, 2010, which is incorporated herein by reference. U.S. patent application Ser. No. 12/806,958 filed Aug. 24, 2010 claims benefit of U.S. Provisional Patent application 61/236,456 filed Aug. 24, 2009, which is incorporated herein by reference.
STATEMENT OF GOVERNMENT SPONSORED SUPPORTThis invention was made with Government support under contract FA9550-08-C-0039 awarded by U.S. Air Force. The Government has certain rights in this invention.
FIELD OF THE INVENTIONThe current invention relates to jet engines. More particularly, the invention relates to jet engine flame holder bluffbodies.
BACKGROUND OF THE INVENTIONThe lean blowout limit of bluffbody flames has been studied extensively for several decades since the improvement of flame stability in the lean environment is a primary purpose for using bluffbodies in various practical combustion devices such as industrial burners and boilers, furnaces, and thrust augmentors. It is believed that the sequence of flame blowout in a fully premixed system that is stabilized by a bluffbody occurs in the following steps: (1) sporadic, local flame extinction, which produces “holes” in the flame, (2) more frequent appearance of the holes with approaching blowout, which eventually induces, (3) significant flame undulation with the appearance of a large-scale wake, and (4) the onset of blowout.
At stoichiometric equivalence ratios of the fully premixed system, the bluffbody flame exhibits little large-scale wake oscillation, while at lean blowout, the system exhibits considerable large-scale wake oscillation, so it remains unclear whether the presence of the large-scale wake oscillation has direct causality with flame blowout since both occur simultaneously as the fully premixed system is leaned out. It is believed that an alternate possibility is that at high heat release (e.g., stoichiometric operation) the flow stability is dominated by “outer modes” that imply little large-scale oscillation, and at low heat release (e.g., near blowout) the flow stability reverts to “central” oscillatory modes, so that the oscillatory motion does not necessarily dictate blowout but is merely a consequence of the lower heat release state at blowout.
These ideas are consistent with a prediction that a transition from outer modes to central modes when the ratio of burnt to unburnt gas temperatures is approximately four. The two phenomena are concomitantly observed whenever the bluffbody flame approaches blowout, and that the presence of an oscillatory wake can accelerate the flame blowout phenomenon by introducing a significant amount of low temperature fluid from the surroundings to the reaction zone such that the heat release from the chemical reactions (or rate of branching reactions) cannot keep up with the cooling effect due to the entrainment of cold gases (or rate of quenching reaction).
What is needed are geometrically modified bluffbody structures with a non-conventional geometry to generate mutually destructive incoherent vortices to assist in delaying the appearance of large-scale wake vortices.
SUMMARY OF THE INVENTIONTo address the needs in the art, a jet engine flame holder bluffbody is provided that includes a leading edge that is rounded, a mid-body having a rectangular cross-section, and a trailing edge, where the trailing edge includes a rectangular cross-section having digitated cutouts from the rectangular cross-section, where the tailing edge projects upward according to a digitated cross-section, where material digits remain between consecutive pairs of the digitated cutouts.
According to one aspect of the invention, the trailing edge is a wedge-shape along a length of the trailing edge.
In a further aspect of the invention, the trailing edge is a block-shape along a length of the trailing edge. In one aspect, the trailing edge is a rectangular recess along an upper-center portion of the solid body, where each digit is a crenellation feature. In another aspect the crenellation feature has a cross-section that is square, rectangular, semi-circular, partly-circular, elliptical, or triangular. In one aspect the crenellation feature has a top surface that is angled towards a centerline along a length of the trailing edge, angled away from a centerline along a length of the trailing edge, or rounded. In another aspect the crenellation feature has tapered sidewalls.
According to another aspect of the invention, each digitated cutout includes a cross-section shape that is square, rectangular, semi-circular, partly-circular, elliptical, or triangular.
In yet another aspect of the invention, the material digits has a cross-section shape that can be square, rectangular, circular, semi-circular, partly-circular, elliptical, or triangular.
According to a further aspect of the invention, the leading edge is a hollow structure.
In another aspect of the invention, the mid-body is a hollow structure.
According to one aspect of the invention, the trailing edge is a hollow structure.
In a further aspect of the invention, the material digit is a triangular shape, where a side of the triangular shape is curved.
In yet another aspect of the invention, the trailing edge has a walled structure that is thin relative to a wall thickness of a hollow the mid-body.
The current invention provides an improvement of the lean blowout limit of bluffbody stabilized flames. The flame configuration includes a hybrid of partially and fully premixed flames, which is demonstrated by injecting methane jets from a streamline-shaped bluffbody into a fully premixed methane/air crossflow. According to the invention geometric configurations of the bluffbody include a base that has two-dimensionally modified geometries and three-dimensional local cavities. According to one embodiment, the blowout limit of a hybrid configuration is extended by up to 12% (in terms of the equivalence ratio of the crossflow). Gas chromatographic sampling and particle image velocimetry (PIV) show that high fuel mole fraction regions coexist with regions of low speed flow for the modified geometries. Further, PIV analysis shows that the downstream flow fields of the modified bases generally have a larger number of incoherent vortices and lower strain rate in comparison with those of the unmodified base.
In accordance with the current invention, three flame configurations are presented. The first is a fully premixed freestream stabilized by a bluffbody (“fully premixed mode”); the second is an air freestream in which fuel is discharged from the bluffbody (“partially premixed mode”); and the third is a combination of these two, namely a fully premixed freestream with additional fuel discharged from the bluffbody “hybrid mode”. For this hybrid mode, a small amount of fuel injection is effective in enhancing stability of bluffbody stabilized flames, e.g., extending the lean blowout limit.
As presented below, the blowout limit of the flame can be extended with local cavities in the hybrid mode, according to one embodiment of the current invention. The blowout extension is not observed when the flame configuration is solely in the partially or fully premixed mode. In addition, it is shown that high local fuel mole fraction in low speed flow regions observed in the local cavity geometries is primarily attributed to the extension of flame stability in the hybrid mode.
These results are validated by gas chromatographic sampling and particle image velocimetry (PIV) measurements. Finally, the swirling strength and strain rate fields in the vicinity of the bluffbody bases are provided to compare the change in the flow field in the presence of local cavities.
As shown in
The main gas blower, which provides the required freestream flow, was originally designed as a premixed burner, but is used here for providing a room temperature, fully premixed stream in the current study, and is located centrally in a 50 cm×50 cm cross-sectional area vertical wind tunnel which can generate ˜3.5 m/s flow. The burner face has 2,184, 1/16-inchdiameter holes on the top surface whose overall diameter is 8.27 inch such that a uniform fully premixed flow can be generated over a relatively wide region. To make the fully premixed stream, air is delivered by a 1.5-hp air blower and mixed with methane ˜3 m upstream of the top surface of the gas blower. The pressure drop across the perforated top plate is measured to determine the mass flowrate; the velocity is proportional to the square root of the pressure drop. The equivalence ratio of this (fully premixed) main stream is used, which varies from 0.54 to 0.72 (confirmed by gas chromatographic measurement), as a criterion to determine the blowout limit. It should be noted that the local flow speed in the vicinity of the bluffbody base is varied, which is the second blowout criterion used in the current exemplary experimental demonstration of the invention. The variation is achieved by injecting additional air from the turbulent grid located ˜1.5 inch downstream of the perforated burner surface while the flow speed generated from the main blower is kept fixed as 0.83 m/s.
The speed of the methane jet is kept fixed at ˜1.4 m/s throughout this example. Also shown in
The turbulence grid is placed ˜1.5 inch downstream from the top surface of the perforated plate. It is composed of two, top and bottom, layers of ½ inch OD, 8-inch-long ceramic (99.9% Al2O3) hollow tube arrays (see
For the PIV studies, ˜100 mJ/pulse (532 nm), double pulsed Nd:YAG laser (New Wave, Gemini PIV) with ˜0.5 mm sheet thickness is used for illuminating 3-μm-nominal diameter Al2O3 particles seeded into the flow. The Al2O3 particles are mixed in the main premixed stream such that adequate seeding density could be achieved. The resulting Mie scattering is detected by a double exposure CCD cameras (La Vision, Flow Master).
Gas chromatographic measurements were taken for quantifying the local methane concentration in the vicinity of the bluffbody base via a Varian 3400 Gas Chromatograph equipped with Porapak Q (for nitrogen and oxygen) and Molecular sieve 5A (for methane) as columns and a thermal conductivity detector (TCD). All sampling measurements are taken in the absence of flame. For the sampling, the spatial scanning is carried out by three translation stages (along the x, y, and z coordinates) using a ˜1-mm diameter (ID), 200-mm-long stainless steel probe.
Regarding the extension of blowout limits in the presence of local (S) cavity,
This observation is consistent with previous results, where there is a minimal positive effect on blowout limit extension with a geometrical change in the bluffbody. For comparison, similar experiment was carried out but with methane injection into pure air crossflow (partially premixed mode).
As shown in the dotted bar, no significant extension of blowout limit is observed among the various geometries. It is believed that this observation with the partially premixed mode (no appreciable cavity effect) is primarily due to the limited amount of the pilot fuel injected via the fuel jets on the bluffbody that is too small to sustain largescale flames. However, the cavity entrained jet fuel is capable of enhancing flame stability in the presence of the fuel containing freestream in the hybrid mode.
For the case of the fully premixed mode, an Ozawa curve that correlates blowout limits of premixed subsonic flames to Damkohler number is used for validation of the blowout limit observed in the current example (
Da=exp[8.75(φ−1)2]
where Da and φ are the Damkohler number and the equivalence ratio, respectively. Also, a simple representation of the Damkohler number is defined as follows:
where H, Uo, α, SL, p and T are the bluffbody thickness (1 inch), freestream velocity (1.7 m/s, blowout velocity of R base in fully premixed mode), thermal diffusivity, maximum laminar burning velocity, pressure, and temperature of the freestream, respectively. A value of thermal diffusivity presented in a literature was used here, which is 0.213 cm2/s for a lean CH4/air mixture at room temperature and 1 atm, and the laminar burning velocity of a methane/air mixture of equivalence ratio 0.5 at 1 atm is approximately 10 cm/s.
The calculated Damkohler number for the R base at the blowout limit (q Dabo) in the fully premixed mode is approximately 7 (1/Dabo=0.14). The data point (0.5 equivalence ratio at 1/Dabo=0.14) is indicated (black dot) in the graph (
Alternatively, another measure of blowout (dimensionless DeZubay parameter) is employed to confirm the above observation.
where Dz, U, p, T, and L are the modified DeZubay parameter, flow velocity (ft/s), pressure (psi), temperature (° R), and the half bluffbody thickness (0.5 inch), respectively. The modified DeZubay parameters corresponding to the flow velocities of the current exemplary demonstration of the invention (1.5 and 2 m/s) are 2.1 and 2.9 (equivalence ratio ranges between 0.55 and 0.4), respectively.
In
The resulting nominal flow speeds of the stream are 1.5 m/s (fully premixed mode) and 2.8 m/s (hybrid mode). In the hybrid mode (black solid bar), it is shown that the presence of the local cavity in the R+S, D+S, and T+S geometries extends the blowout limit from 0.62±0.02 (R and D) and 0.61±0.02 (T) to 0.55±0.02 (R+S), 0.56±0.02 (D+S), and 0.58±0.02 (T+S).
This trend is consistent with what was observed in
The different blowout dependence on the geometric modification observed in the hybrid and the fully premixed modes show that the way of interaction between the small pure methane stream (namely, partially premixed stream) and the local cavities are a key aspect of the observed blowout limit extension. In this context, the variation in the mole fraction and the flow fields in the vicinity of the six base geometries is of interest. Thus, in the following sections, the measurements of temporally averaged mole fraction/flow fields using gas chromatographic sampling and particle image velocimetry (PIV) will be presented.
Regarding the fuel mole fraction field in the vicinity of the base
Along the y coordinate (
This high mole fraction can be also expected from PIV results for the geometry (see later
Comparing simultaneously the mole fraction and velocity fields is of interest. This is because when considering that the total amount of fuel injection is the same for every geometry investigated, one can find that the result shown in
As shown in
Another interesting observation is in the gradients of fuel mole fraction. For the R and D geometries, the mole fractions increase by over 100% (from ˜2.5% to ˜6%) within ˜4 mm distance, but for the local cavity geometries, the variations in mole fraction is much smaller, less than 25% in the highest case. It is believed that the way the fuel distributed and the lesser mole fraction gradient in the presence of the local cavities effectively assist in improving blowout limits in the hybrid mode flow.
Further, regarding
In the following section, the difference in flow fields in the absence/presence of a local cavity will be briefly discussed.
Turning now to flow fields in the vicinity of the base,
As shown in the representative instantaneous images (especially R, D, R+S, and D+S), recirculating flow structures, which are believed to play a key role in bluffbody flame stabilization, can be found for all four geometries.
For example, one can clearly find that the strong recirculating pairs whose centers are at y˜10 mm and y˜6 mm in the R geometry and y˜10 mm and y˜20 mm in the D geometry in these specific images. For the R+S and D+S geometries, the structure resides at the height of y˜5 mm (R+S) and y˜-2 mm (D+S).
It is interesting, however, that the recirculating pattern is more clearly shown and the strength of the recirculating flow in this region of interest is more intense in the non-cavity geometries than in the cavity geometry cases. It is likely because either intense recirculation zones of the local cavity geometries reside in a region where the laser light cannot illuminate, e.g., inside of the cavity, and/or the geometries tends to generate small, multiple vortices rather than one large vortex pair (see
Here, it is noteworthy it was observed that the vortices generated from the local cavity geometries are more incoherent, i.e., the shedding frequency, size, and locations are more variable, as described in our previous study. Evidence of the incoherent vortices can also be found in the present study as shown in the averaged PIV images of
Unlike the above four geometries, the T and T+S geometries have more direct (i.e., non-recirculating) flow pattern along the slanted side surfaces in the region of interest for which laser illumination was present. It seems that the flow pattern is inconsistent with our previous observations, which showed two large, nearly symmetric recirculations along the side surfaces in ˜5.7 m/s vitiated flow in the absence of additional air injection. It is believed that a slight misalignment between the direction of the fully premixed flow and the air injection in the current configuration induces a small skew of the local flow (to the right) in the region of interest. The skewed flow, as clearly seen in the averaged field, seems to eliminate the recirculation zone on the left face in the region of interest and create bent flow along their vertices at y˜0 (clockwise direction). Although it is not shown here, one can see elements of a recirculation zone in the other side (right face) of the T and T+S bases (e.g., near x=y=−5) from the relatively upright averaged flow field at x˜-5 to −10, y=0 in
According to the current invention, the improvement of the blowout limit in the cavity geometries is due to its ability to distribute the fuel in a less diluted manner, where geometries according to R and R+S and the D, D+S, T and T+S are more robust to the flow asymmetry.
Turning to
The current invention provides complex back-steps in the geometry to generate multiple and incoherent vortices. In
The strain rates of the averaged velocity field in the six example test geometry cases are illustrated in
In another aspect of the invention, the leading edge 1202, the mid-body 1204, and the trailing edge 1206 are hollow structures.
The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.
Claims
1. A jet engine flame holder bluffbody, comprising:
- a. a leading edge, wherein said leading edge is rounded;
- b. a mid-body, wherein said mid-body comprises a rectangular cross-section; and
- c. a trailing edge, wherein said trailing edge comprises said rectangular cross-section having digitated cutouts from said rectangular cross-section, wherein said tailing edge projects upward according to a digitated cross-section, wherein material digits remain between consecutive pairs of said digitated cutouts.
2. The jet engine flame holder bluffbody of claim 1, wherein said trailing edge comprises a wedge-shape along a length of said trailing edge.
3. The jet engine flame holder bluffbody of claim 1, wherein said trailing edge comprises a block-shape along a length of said trailing edge.
4. The jet engine flame holder bluffbody of claim 3, wherein said trailing edge comprises a rectangular recess along an upper-center portion of said solid body, wherein each said digit comprises a crenellation feature.
5. The jet engine flame holder bluffbody of claim 4, wherein said crenellation feature comprises a cross-section shape selected from the group consisting of square, rectangular, semi-circular, partly-circular, elliptical, triangular.
6. The jet engine flame holder bluffbody of claim 4, wherein said crenellation feature comprises a top surface selected from the group consisting of angled towards a centerline along a length of said trailing edge, angled away from a centerline along a length of said trailing edge, and rounded.
7. The jet engine flame holder bluffbody of claim 4, wherein said crenellation feature comprises a tapered sidewall.
8. The jet engine flame holder bluffbody of claim 1, wherein each digitated cutout comprises a cross-section shape selected from the group consisting of square, rectangular, semi-circular, partly-circular, elliptical, and triangular.
9. The jet engine flame holder bluffbody of claim 1, wherein said material digits comprise a cross-section shape selected from the group consisting of square, rectangular, circular, semi-circular, partly-circular, elliptical, and triangular.
10. The jet engine flame holder bluffbody of claim 1, wherein said leading edge comprises a hollow structure.
11. The jet engine flame holder bluffbody of claim 1, wherein said mid-body comprises a hollow structure.
12. The jet engine flame holder bluffbody of claim 1, wherein said trailing edge comprises a hollow structure.
13. The jet engine flame holder bluffbody of claim 1, wherein said material digit comprises a triangular shape, wherein a side of said triangular shape is curved.
14. The jet engine flame holder bluffbody of claim 1, wherein said trailing edge comprises crenellation structures and a rectangular recess having a wall thickness up to a thickness of a wall thickness of a hollow said mid-body.
Type: Application
Filed: Jun 5, 2013
Publication Date: Nov 20, 2014
Inventors: Wookyung Kim (Glastonbury, CT), Mark Godfrey Mungal (Cupertino, CA), Heinz Guenter Pitsch (Aachen)
Application Number: 13/910,432
International Classification: F23R 3/18 (20060101);