US3169367A

US3169367A - Combustion apparatus

Info

Publication number: US3169367A
Application number: US296040A
Authority: US
Inventors: Charles E Hussey
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1963-07-18
Filing date: 1963-07-18
Publication date: 1965-02-16
Anticipated expiration: 1982-02-16

Description

Feb. 16, 1965 E. HUSSEY 3,1 7

COMBUSTION APPARATUS Filed July 18, 1963 2 Sheets-Sheet 1 CENTER LINE Fig.l.

Feb. 16, 1965 c. E. HUSSEY 3,169,357

COMBUSTION APPARATUS Filed July 18, 1963 2 SheetsSheet 2 Fig.2.

WITNESSES INVENTOR WW Charles E. Hussey United States Patent 3,159,357 CUMBUSTIGN APPARATUS Charles E. Hussey, Brookhaveu, Pa, assignor to Westinghouse Electric Corporation, Pittsburgh, Pin, a corporation of Pennsylvania Filed July 18, 1963, Ser. No. 296,fl46 7 tilaims. (Cl. 6G3.37)

This invention relates to combustion apparatus, more particularly to combustion apparatus for a gas turbine power plant, and has for an object to provide improved a paratus of this type.

One of the most important, yet dificult, requirements of the combustion basket employed in gas turbine combustion apparatus is delivery of the hot products of combustion to the gas turbine with a uniform temperature profile that is free of excessive transient peak temperatures and in which the annular temperature distribution pattern niiorrn. at the turbine nozzle r ccording to the invention, the major cause of severe distortions in the temperature profile and distribution pattern is the asymmetrical flow of air surrounding the combustion basket and, in particular, the relation of this air to the primary air admission apertures in the basket. Since the primary air is employed to form the combustible mixture with the injected fuel, if the air flow field externally surrounding the basket adjacent the primary air apertures is not symmetrical, the aerodynamic recirculation process of the air and fuel mixture during combustion becomes unbalanced and the combustion flame produced is not symmetrical within the combustion basket.

In view 2 the above, it is an object of the invention to provide combustion appara us for a gas turbine in which the prh'nary air admitted to the combustion zone of a combustion basket is uniformly distributed.

A further object of the invention is to provide combustion apparatus of the above type in which the primary air is admitted to the combustion zone in a plurality of discrete jets "forming a converging radial flow pattern and having substantially uniform velocity, so that the combustion flame is stabilized in a substantially central position within the combustion basket.

A more specific object is to provide a simple yet highly edcctive arrangement for effecting uniform flow distribution of the primary air injected into the combustion basket of the above apparatus, with relatively small pressure drop loss.

Briefly, in accordance with the invention, there is provided improved fuel combustion apparatus comprising a combustion basket of the cannister type disposed within a plenum chamber and having a tubular upstream end portion defining a fuel combustion zone for generating hot gaseous products of combustion, an intermediate portion and downstream end portion for delivering the gaseous products to a gas turbine. The upstream end portion is provided with annular rows of spaced apertures for admitting prime. y air from the plenum chamber to the combustion zone to form a combustible mixture with fuel admitted thereto by suitable fuel injecting means.

The improvement comprises structure for effecting uniform flow distribution of the primary air injected into the fuel combustion zone of the above apparatus, which structure includes a tubular imperforate shield encompas ing the upstream end portion of the basket and jointly therewith defining an annular opening at one end for the primary air, the other end being closed, and a foraminuous tubular member interposed between the shield and the upstream end portion. The foraminous member the shield are in mutually convergent relation with each other to provide an annular primary air passageway communicating with the annular opening and diminishing in cross-sectional area in downstream direction "ice with respect to direction of primary air flow therethrough.

Accordingly, all of the primary air is first diverted by the shield and directed through the annular opening, then directed through the foraminous member in a great plurality of discrete jets with uniform velocity before injcction through the primary air apertures into the combustion zone.

This arrangement insures that the air approach velocity (direction and velocity) is symmetrical with respect to the combustion zone, so that uniform radial penetration of the primary air is attained in the combustion zone and the combustion flame is centered in the combustion basket.

These and other objects are effected by the invention as will be apparent from the following description and claims taken in connection with the accompanying drawings forming a part or" this application, in which:

1 is an axial sectional View of the upper half of a gas turbine provided with combustion apparatus incorporating the invention, and

FIG. 2 is a development of a frustoconical section, with some parts omitted for simplicity, taken on a radial line ll-li rotated about the central axis or" the apparatus.

Referring to the drawings in detail, and especially to FIG. 1, there is shown combustion apparatus generally designated "all associated with a gas turbine power plant of the axial flow type. Since the combustion apparatus ill may be employed with any suitable type of gas turbine power plant, only a portion of thepower plant sufiicient for comprehension of the invention has been shown. However, it will be understood, that the power plant includes an axial flow air compressor 12 for directing air to the combustion apparatus ill? and a gas turbine 14 connectable to the combustion apparatus ill and receiving hot products of combustion therefrom for motivating the power plant.

Only the uppenhali' of the power plant and combustion apparatus has been illustrated, since the lower half may be substantially identical and symmetrical about the center line or axis of rotation of the power plant.

The air compressor 12 includes, as well-known in the art, a multi-stage bladed rotor structure 15 cooperatively associated with a stator structure having an equal number of multi-stage stationary blades 16 for compressing the air directed therethrough to a suitable pressure value for combustion in the combustion apparatus it The outlet of the compressor 1 is directed through an annular diiusion member 17 forming an intake for a plenum chamber 13 defined by a housing structure 319. The housing 19 includes a tubular shell member 20 of frustoconical shape extending at an acute angle with the center line of the power plant, a forward annular wall member 21 connected to the external casing of the compressor 13, and a rearward annular wall member 22 connected to the outer casing of the turbine M. The turbine id, as mentioned above, is of the axial flow type and includes a plurality of expansion stages formed by a plurality of rows of nozzle blades 24 cooperativey associated with an equal plurality of rotating blades 25 mounted on'the turbine rotor 26. The turbine rotor 26 is drivingly connected to the compressor rotor 15 by a tubular connecting shaft member 27, and a tubular liner or fairing member 23 is suitabl I supported in encompassing stationary relation with the connecting shaft portion 27 to provide a smooth air fiow surface for the air entering the plenum chamber 18 from the compressor diffuser 17.

It will be noted that the housing 19 is disposed in partially overlapping relation with the compressor 12 in order to shorten the axial length of the power plant, as well-known in the art. Hence the diffuser 17 provides an air intake for the housing disposed intermediate the end walls El, 22.

Within the housing 19 there are provided a plurality of tubular combustion baskets 34) of the cellular or cannister type disposed in an annular, mutually spaced, array concentric with the center line of the power plant, as illustrated in FIG. 2. The combustion chambers 39 form combustion chambers 30a and are equally spaced from each other in the housing 19 and arranged in such a manner that their axes are substantially parallel to the outer shell member 20 and inclined at an acute angle with the center line of the power plant, Since the combustion baskets may be substantially identical, only one will be described. The combustion basket includes an upstream end portion 31 which may be formed of a plurality of (for example 5) cylindrical liner members 32 of graduated size disposed in slightly overlapping relation with each other and forming a primary combustion zone 33. Each of the liner members 32 has an annular array of apertures 34 for admitting primary air into the combustion zone 33 to support the combustion of fuel injected thereinto by a suitable fuel injector 35.

The forward wall 21 of the combustion apparatus is provided with a circular opening 38 of sufiiciently large diameter to clear the combustion basket 3i) for service purposes and is enclosed by a circular cover plate 39 attached to the wall member 21 in any suitable manner, as indicated by the bolt 40. The cover plate 39 is further provided with a central opening 41 through which the fuel injector extends. The fuel injector 35 is supplied with fuel by a suitable conduit 43 connected to any suitable fuel supply (not 'shown) and may be of the well-known 'atomizing type formed in a manner to provide a substantially conical spray of fuel within the primary combustion zone 33.

A frustoconical dome member 45 serves to close the extreme forward end of the combustion basket 36 and is provided with a central opening 45 accomodating the fuel injection nozzle 35 and cooperatively associated therewith in such a manner that substantially no air flow is bustion baskets 3i) is non-uniform and non-symmetrical so that the air would flow through some of the apertures 34 into the combustion zone 33 with a larger velocity than through some of the other apertures 34 and establish an unsymmetrical combustion pattern.

In accordance with the invention, the above effects are avoided by provision of structure for effecting uniform flow distribution of the primary air entering the upstream end portion 31 into the primary combustion zone 33. This flow distribution structure comprises a tubular imperforate shield structure. 57 of substantially circular cross-section attached at one end to the wall member 21, as indicated at 58, and having its opposite end 59 extendingsubstantially to the juncture between the upstream end portion 31 and the intermediate portion 43. In other words, the shield member 57 is substantially coextensive with the upstream end portion 31. The shield member 57 is substantially larger in diameter than that of the upstream end portion 31 so that it is disposed in encompassing spaced relation therewith and jointly therewith at one end defines an annular primary air inlet 6% adjacent the jointure between the upstream end portion 31 and the intermediate portion 48, while the opposite end is closedp Accordingly, all of the air from the chamber 13 admitted emitted therethrough into the combustion zone 33. Also, i

a suitable electrical ignitor 47 is provided for igniting the fuel and air mixture in the combustion zone 33.

The combustion basket 30 further includes an elongated intermediate cylindrical portion 48 provided with l a plurality of annular rows of apertures 49 for admitting secondary air into the combustion chamber during operation, and a downstream end portion or transition member 50 having a forward portion 51 of cylindrical shape disposed in encompassing and slightly overlapping relation with the intermediate portion 48 and a rearward tubular portion 52 that progressively changes in contour frorn'circular cross-section at the jointure with the cylindrical portion 51 to arcuate cross-section at its outlet end porion 53. The arcuate extent of the outlet 53 is such that, jointly with the outlets of the other combustion baskets, a complete annulus is provided for admitting the hot products of combustion from the combustion baskets 39 to the

blades

24 and 25 of the turbine 14, thereby to provide full peripheral admission of motivating gases to the turbine 14, as well-known in the art and hence not shown.

As thus far described, the structure is substantially conventional, and in operation, pressurized air from the com pressor 12 is directed into the plenum chamber 18 through the diffuser member 17 to fill the chamber with air at a substantial pressure and then flow into the combustion baskets 3010 provide hot motive gases for motivating the turbine 14. During such flow the air, as indicated by arrows A, moves radially outwardly into the plenum chamber 18 and then reverses in direction with most of the air being directed towards the forward wall 21 of the combustion apparatus with various directions of flow as it goes around the combustion baskets 3d and the spaces be tween adjacent combustion baskets. Accordingly, the

air apertures 34 in the upstream end portion of the comto the primary air apertures 34 must flow through the primary air inlet 69;

' A foraminous tubular member 62 of substantially circular cross-section is interposed between the shield member 57 and the upstream end portion 31 of the combustion basket and is provided with a large number of perforations 63jthrough which the primary air flows before it'is directed to the primary air apertures 34. The foraminous member 63 may be made in any suitable manner, and, as illustrated, is preferably formed of perforated sheet metal. Hence, for simplicity, it will hereafter he referred to as a screen. The screen 62, as illustrated, is of frustoconical shape converging in downstream direction with respect to flow of gases through the combustion basket 30 and is attached to the combustion basket 36 at 64 in a substantially fluid tight manner. The forward'end portion 65 of the screen is disposed in spaced relation'with the upstream end portion 31 and is clamped to the wall member 21 by the cover plate 39. Since the shield member 57 is also attached to the wall member 21, a seal is provided to prevent flow of air entering the upstream end portion 31 through this end of the structure. 7

The shield 57 and the screen 62 are so proportioned and shaped'with respect to each other thatrthey converge in upstream direction or, conversely, they diverge in downstream direction to provide a primary air passageway 66 of gradually varying annular cross-sectional area. The variation in cross-sectional area of the primary air passageway 66 is such that it is of largest cross-sectional area adjacent the primary air inlet 66 and of smallest crosssectional area at the extreme forward end of the structure 57, 62; This feature is importannsince' as the primary air is directed through the primary air inlet 6%, it is progressively diverted through the apertures 63 in the screen in discretejets directed to the primary air apertures 34 in radially convergent directions. Hence, the primary air passageway reduces in cross-sectional area as the demand for air flow therethrough is reduced because of the diversion of the air flow through the apertures in the screen.

approach velocity'of the air withrespect to the primary With the above arrangement, all of the primary air is constrained to flow in axial directions'into the primary air inlet 60, as indicated by arrows B. That is, they are directed in substantially cylindrical flow patterns with respect to the axis of the combustion basket 30 and directed in upstream direction before passing through the openings 63 in the screen 62. As the air flows through the openings 63 in the screen, the direction of flow is rendered normal to the wall of the screen'and a great plurality of discrete jets, asindicated by the arrows C, are obtained; Each of these jets C has substantially the same approach velocity with respect to the primary apertures 34 and are directed theretbrough with equal velocity and in substantially radially inward direction. Accordingly, the depth of penetration of the jets B into the primary combustion zone 33 is substantially uniform so that as the atomized fuel is injected by the injector 35 into the combustion zone 33, the resulting fuel and air mixture is stabilized. The air jets D through the first row of primary apertures (nearest the fuel injector 35) provide the initial fuel/air mixture and initiate combustion. This row of air jets forms a substantially symmetrical recirculation pattern E. The recirculation flow pattern E thus attained is substantially toroidal, and is centered or symmetrical with respect to the axis of the upstream end portion 31. The air jets D through the remaining rows of apertures 34 are deflected uniformly in downstr am direction as indicated by the arrows F and serve to complete combustion. Hence, the combustion fiame is substantially centered in the upstream end portion 31, and the gases formed by combustion in the primary combustion zone 33 flow downstream therefrom at a uniform rate and temperature into the intermediate member 43. The gases then flow through the transition member 59 into the arcuate outlet 53 at a uniform rate and with a substantially uniform temperature distribution, so that the blading of the turbine 14 is subjected to motivatin gases at a substantially uniform value during operation.

Furthermore, with this arrangement, the variation in the temperature level of the gases is substantially minimized so that the temperature of the gases flowing through the outlet 53 is maintained within more reasonable and uniform limits.

As ell-known in the art, as the combustion gases generated in the primary combustion zone 33, in the manner described above, ilow through the intermediate member 38, secondary air is admitted through the apertures 4? to substantially dilute and cool the combustion gases to an acceptable level that may be safely employed in the turbine. Although some imbalance may exist in the air fiow through the secondary air apertures 49, this efiect is of a substantially minor nature and is relatively insignificant with respect to its effect on the uniformity of the gas flow through the combustion basket Ell, since this air is not employed in the combustion process.

it will be understood that the terms rimary air and secondary air as employed in this specification and the claims appended hereto, respectively denote combustion air or air that is employed for fuel combustion, and diluent air or air that is employed to dilute and cool the hot products of combustion formed in the primary combustion zone 33. These terms are well-known to those versed in this art and have the above specific meanings.

The screen 62 is formed with a sufficient number of apertures whose total cross-sectional area is at least larger than the total cross-sectional area of the primary air apertures 3d, so that the major pressure drop that is occasioned by the primary air is attained in the primary air apertures 34. The openings lid in the screen 62 are smaller, and more closely spaced, than the primary air inlets 34, and are efiective to straighten the air flow of the primary air and to increase the velocity during flow therethrough to'a moderate degree. However, the pressure drop across the apertures 53 in the screen 62 is of a relatively smaller order, that is, about 8 to 15% of the total pressure drop.

With the converging primary air passageway es, as the primary air flows therethrough the velocity of the primary air is progressively accelerated, thereby further enhancing the flow straightening characteristics of the screen 62.

Although only one embodiment of the invention has been shown, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof.

-I claim as my invention:

1. Combustion apparatus for a gas turbine power plant comprising,

casing structure having an intake for receiving air under pressure,

a tubular combustion basket disposed in said casing structure and defining a combustion chamber,

said combustion basket havin a tubular upstream end portion provided with a plurality of circumferentially spaced apertures for admitting primary air to said chamber and a tubular downstream end portion having a discharge outlet for connecting to a gas turbine,

means disposed adjacent said upstream end portion for injecting fuel into said combustion chamber to provide a combustible mixture with the primary air, and

means for effecting uniform flow distribution of said primary air into said upstream end portion,

said last mentioned means including a tubular imperforate shield disposed in encompassing spaced relation with said upstream portion,

said shield and said basket jointly defining an annular primary air inlet communicating with said apertures, and

a foraminous tubular member interposed between said shield and said upstream end portion,

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet.

2. Combustion apparatus for a gas turbine power plant comprising casing structure having an intake for receiving air under pressure,

an axially elongated tubular combustion basket dis:

posed in said casing structure and defining a combustion chamber,

said combustion basket having a tubular upstream end portion defining a combustion zone and provided with a plurality of circumferentially and axially spaced apertures for admitting primary air to said combustion zone, and a tubular downstream end portion having a discharge outlet for connecting to a gas turbine,

means disposed adjacent said upstream end portion for injecting fuel into the combustion zone of said combusti-on chamber to provide a combustible mixture with the primary air,

said intake being disposed intermediate said upstream and downstream end portions, and

means for effecting uniform flow distribution of said primary air into said combustion zone through the apertures in said upstream end portion, including a tubular imperforate shield disposed in encompassing spaced relation and axially co-ex-tensive with said upstream portion,

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet and jointly forming an annular primary inlet passage of diminishing cross-sectional area.

3. Combustion apparatus for a gas turbine power plant compri ing tubular casing structure having an intake for receiving air under pressure,

a plurality of tubular combustion baskets disposed in a mutually spaced annular array in said casing structure and defining a plurality of combustion chambers,

at least one of said combustion baskets having a tubular upstream end portion provided with a first plurality of circumferentially spaced apertures for admitting primary air to said chamber, an intermediate portion provided with a second plurality of circumferentially spaced apertures for admitting secondary air to said chamber, and a tubular downstream end portion having a hot gas discharge outlet for connection to a gas turbine,

said last mentioned means including a tubular imperforate shield disposed in encompassing paced relation with said upstream portion, a

a fo-raminous tubular member of substantially frustoconical shape interposed between said shield and said upstream end portion,

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet, and

said forarninous member being formed in such a manner that it imposes a smaller pressure drop to the primary :air flow than the primary apertures in said upstream end portion.

4. Combustion apparatus for a gas turbine power plant,

comprising V casing structure defining a plenum chamber, means for admitting pressurized air to said plenum chamber, a tubular combustion basket mounted in said casing structure and defining a combustion chamber,

said combustion basket being axially elongated and having a tubular upstream wall portion providedwith :a plurality of circumferentially and axially spaced apertures for admitting primary air to said chamber, and a tubular downstream wall portion having a discharge outlet for connection to a gas turbine, a a

means disposed concentrically with and adjacent said upstream wall portion for injecting fuel into said combustion chamber to provide a combustion mixture with said primary air, and a means for effecting uniform primary air flow distribution through said apertures, comprising, 7

a tubular imperforate shield disposed in encompassing radially spaced relation and axially coextensive with said upstream wall portion,

a foraminous tubular member interposed between and concentric with said shield and said upstream wall portion, 7

said shield and said foraminous member jointly defining an annular primary air inlet at said one end communicating with said apertures, and

said foraminous member having one end portion in abutment with said basket and its opposite end portion circumferentially spaced from said basket,

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet and jointly forming an annular primary inlet'passage of diminishing cross-sectional area, and

means for restricting air flow into said basket past said opposite end of said foraminous member. 7

5; Combustion apparatus for a gas turbine power plant,

: comprising tubular casing structure defining a plenum chamber, means for admitting pressurized air to said plenum chamber, 7 a plurality of tubular combustion baskets mounted in mutually spaced annular array inlsaid casing structure,

0 L3 j each'of said combustion baskets being axially elongated and defining a combustion chamber, and further comprising a tubular upstream end portion provided with a plurality of circumterentially and axially spaced apertures for admitting primary air to said chamber, an intermediate portion for admitting secondary air to said chamber, and a tubular down- ,stream end portion having a discharge outlet for conveying hot gaseous products of combustion to a gas turbine,

means disposed concentrically with and adjacent said,

a foraminou tubular member interposed between and V concentric with said shield and said upstream end portion, said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet and jointly forming an annular primary inlet passage of diminishing cross-sectional area, the total cross-sectional area of the openings in said foraminous member being greater than the total cross-sectional area of said primary apertures. 6. Qombustion apparatus for a gas turbine power plant, comprlslng casting structure defining a plenum chamber, -means for admitting pressurized air to said plenum chamber, a tubular combustion basket mounted in said casing structure and defining a combustion chamber, saidcombustion basket being axially elongated and having a tubularupstream end portion provided with a plurality of circumferentially and axially spaced apertures foradmitting primary air to said chamber, and a tubular downstream wall portion having a dis- 7 charge outlet'for connection to a gas turbine, means disposed concentrically with and adjacent said I upstream end portion for injecting fuel into said combustion chamber to provide a combustible mixture with said primary air, and

means for effecting uniform primary air flow distribution through said apertures, comprising a tubular imperforate shield disposed in encompassing relation and axially coextensive with said upstream end portion,

said shield and said basket jointly defining at one end an annular primary air inlet communicating with said apertures, and

a foraminous tubular member interposed between and concentric with said shield and said upstream wall portion, V i

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet and jointly forming an annular primary inlet passage of diminishing cross-sectional area, and

wall structure extending across the Opposite end of said shield and said basket for restricting flow of primary air into said upstream end portion, whereby all of the primary air flow into said upstream end portion passes through said primary inlet and said foraminous member.

7. Combustion apparatus for a gas turbine power plant,

comprising tubular casing structure having a central axis and defining a plenum chamber,

an annular intake concentric with said axis for admitting pressurized air to said plenum chamber,

a plurality of tubular combustion baskets mounted in mutually spaced annular array in said casing structure about said intake and inclined with said axis,

each of said combustion baskets being axially elongated and defining a combustion chamber, and further comprising a tubular upstream end portion provided with a plurality of circumferentially and axially spaced apertures for admitting primary air to said chamber, an intermediate portion for admitting secondary air to said chamber, and a tubular downstream end portion having a discharge outlet for conveying hot gaseous products of combustion to a gas turbine,

said intake being disposed adjacent said downstream end portion of the combustion baskets,

means disposed concentrically with and adjacent said upstream portion for injecting fuel into said combustion chamber to provide a combustible mixture with said primary air, and

means associated with at least one of said combustion baskets for eifecting uniform primary air flow distribution through said apertures comprising a tubular imperforate shield disposed in encompassing relation and axially coextensive with said upstream end portion,

a foraminous tubular member interposed between and concentric with said shield and said upstream end portion,

said shield and said foraminous member converging towards each other in downstream direction with relation to said air inlet and jointly forming an annular primary inlet passage of diminishing cross-sectional area,

the total cross-sectional area of the openings in said foraminous member being greater than the total cross-sectional area of said primary apertures.

References Cited in the file of this patent UNITED STATES PATENTS 2,611,599 MacCracken Sept. 23, 1952 2,630,679 Sedille Mar. 10, 1953 3,044,263 Hennig July 17, 1962

Claims

1. COMBUSTION APPARATUS FOR A GAS TURBINE POWER PLANT COMPRISING, CASING STRUCTURE HAVING AN INTAKE FOR RECEIVING AIR UNDER PRESSURE, A TUBULAR COMBUSTION BASKET DISPOSED IN SAID CASING STRUCTURE AND DEFINING A COMBUSTION CHAMBER, SAID COMBUSTION BASKET HAVING A TUBULAR UPSTREAMS END PORTION PROVIDED WITH A PLURALITY OF CIRCUMFERENTIALLY SPACED APERTURES FOR ADMITTING PRIMARY AIR TO SAID CHAMBER AND A TUBULAR DOWNSTREAM END PORTION HAVING A DISCHARGE OUTLET FOR CONNECTING TO A GAS TURBINE MEANS DISPOSED ADJACENT SAID UPSTREAM END PORTION FOR INJECTING FUEL INTO SAID COMBUSTION CHAMBER TO PROVIDE A COMBUSTIBLE MIXTURE WITH THE PRIMARY AIR, AND MEANS FOR EFFECTING UNIFORM FLOW DISTRIBUTION OF SAID PRIMARY AIR INTO SAID UPSTREAN END PORTION, SAID LAST MENTIONED MEANS INCLUDING A TUBULAR IMPERFORATE SHIELD DISPOSED IN ENCOMPASSING SPACED RELATION WITH SAID UPSTREAM PORTION, SAID SHIELD AND SAID BASKET JOINTLY DEFINING AN ANNULAR PRIMARY AIR INLET COMMUNICATING WITH SAID APERTURES, AND A FORAMINOUS TUBULAR MEMBER INTERPOSED BETWEEN SAID SHIELD AND SAID UPSTREAM END PORTION, SAID SHIELD AND SAID FORAMINOUS MEMBER CONVERGING TOWARDS EACH OTHER IN DOWNSTREAM DIRECTION WITH RELATION TO SAID AIR INLET.