US3866413A - Air blast fuel atomizer - Google Patents

Abstract

An air blast fuel atomizer characterized in that the main (or secondary) fuel is atomized by centrifugal discharge of a swirling film of the fuel into a swirling annular air stream and wherein primer fuel to facilitate starting of the gas turbine engine is atomized in the throat of a venturi tube through which an air stream is flowing, the atomized primer fuel and air being discharged from the end of the venturi tube into a swirling annular stream of primary air to provide a combustible fuel-air mixture to create a pilot combustion zone to facilitate starting of the gas turbine engine, the swirling primary air stream being disposed within the swirling film of the secondary fuel. The atomizer herein is further characterized in that it has ports for introducing into the secondary air stream boost air to facilitate starting as with heavy low volatility fuels, the boost air and fuel being brought into intimate contact before the boost pressure is dissipated or alternatively, the ports aforesaid may be used for introduction of gaseous fuel into the secondary air stream for use of the atomizer in conjunction with industrial gas turbines which are generally required to operate on both liquid and gaseous fuels.

Description

United States Patent 1 Sturgess [4 1 Feb. 18, 1975 [73] Assignee: Parker-Hannifin Corp, Cleveland.

Ohio

22 Filed: Jan. 22, 1973 21 Appl. No: 325,243

[52] US. Cl 60/3974 R, 239/400, 239/404, 239/406 [51] Int. Cl. F021: 7/22, B05b 7/10 [58] Field of Search 60/3974 R, 39.49, 39.82 P, 60/3974 B, 39.71, 39.72 R, 261; 239/400, 402406, 399. 434.5

[56] References Cited UNITED STATES PATENTS 2,632,298 3/1953 Willgoos 60/3974 R 2,635,425 4/1953 Thorpe et al... 60/3974 B 2,682,302 6/1954 Cote 1 1 60/3974 R 3,002,351 10/1961 Sloan 1 60/3972 R 3,067,582 12/1962 Schirmer 60/3971 X 3,143,401 8/1964 Lambrecht 60/3972 R 3,320,744 5/1967 Blakely et al. 60/3974 R 3,612,737 10/1971 Sharan 1, 239/404 X 3,630,024 12/1971 Hopkins 1 60/3974 B 3,703,259 11/1972 Sturgess et al. 60/3974 B 3,713,588 l/1973 Sharpe 1 239/400 3,739,576 6/1973 Chamberlain 60/3974 R 3,768,250 10/1973 Kawaguchi 60/3974 R Primary Examiner-Carlton R. Croyle Assistant ExaminerRobert E. Garrett Attorney, Agent, or FirmDonnelly, Maky. Rcnner & Otto [57] ABSTRACT An air blast fuel atomizer characterized in that the main (or secondary) fuel is atomized by centrifugal discharge of a swirling film of the fuel into a swirling annular air stream and wherein primer fuel to facilitate starting of the gas turbine engine is atomized in the throat of a venturi tube through which an air stream is flowing, the atomized primer fuel and air being discharged from the end of the venturi tube into a swirling annular stream of primary air to provide a combustible fuel-air mixture to create a pilot combustion zone to facilitate starting of the gas turbine engine, the swirling primary air stream being disposed within the swirling film of the secondary fuel. The atomizer herein is further characterized in that it has ports for introducing into the secondary air stream boost air to facilitate starting as with heavy low volatility fuels, the boost air and fuel being brought into intimate contact before the boost pressure is dissipated or alternatively, the ports aforesaid may be used for introduction of gaseous fuel into the secondary air stream for use of the atomizer in conjunction with in dustrial gas turbines which are generally required to operate on both liquid and gaseous fuels.

4 Claims, 5 Drawing Figures AIR BLAST FUEL ATOMIZER BACKGROUND OF THE INVENTION The starting of gas turbine engines equipped with pure air blast fuel atomizers is a rather difficult problem because of the low air energies available to distribute and break up the fuel under starting conditions. These difficulties are amplified when viscous low volatility fuels are used as in industrial gas turbines or when the inlet air temperatures and pressures are both low as during high altitude relighting of aircraft gas turbines. In such situations, it is desirable to provide a pilot zone and/or a boost system, the current practice being to use pressure atomizing primer nozzles to provide the pilot zone. However, this solution is far from ideal for the following reasons: (1) the spray characteristics of pressure atomizers are strongly dependent on fuel kinematic visocosity; (2) the primer spray if optimized for starting may result in excessive smoke at rated engine conditions due to spray cone collapse at high pressure; (3) the use of a pressure atomizer requires fuel filtration to avoid plugging of the orifice with a sacrifice in one of the advantages of the air blast system; (4) a pressure atomizing primer integrated into the air blast main fuel supply behaves as a hybrid device with the characteristics of neither a true pressure atomizer nor a pure air blast atomizer and this results in the requirement of extensive development for such hybrid device to achieve the desired performance characteristics; (5) conditions may arise where the restrictions of turn down ratio, available fuel pressure drop and engine operating line pressures with reference to valve opening and air pressure drop may preclude satisfactory matching of primer flow number and spray angle for starting.

SUMMARY OF THE INVENTION In contradistinction to the foregoing, the air blast fuel atomizer herein provides a pilot zone to ease the starting problem. to enhance the lean blow-out limit, and to give increased primary zone combustion efficiency at idle and subidle speeds with increased acceleration rates and decreased emission of unburned hydrocarbons.

The air blast fuel atomizer herein also embodies a boost system which is necessary for starting of gas turbine engines using heavy, low volatility fuels and to that end the present atomizer brings the boost air and fuel into intimate contact before the boost pressure is dissipated, and for high altitude aircraft where relighting again involves achieving fine atomization of the fuel with only low air energy available. Moreover, because relighting is not simply a question of good atomization, but also of chemical reaction rates, the present invention provides for desired chemical reaction rates with the boost system aforesaid when oxygen is employed as the boost gas.

It is another object of this invention to provide an air blast fuel atomizer which includes therewith a gaseous fuel system for operating industrial gas turbines on both liquid and gaseous fuels, the present atomizer being operative to mix the gaseous fuel with air for combustion with the mixture being of appropriate fuel/air ratio so that the flame will not be blown out, and since the atomizer herein combines the liquid and gaseous fuel atomizing construction, it compensates for the high diffusion rate of gaseous fuels which is difficult to achieve in a single flame type injector, and hence because of the requirement to also burn liquid fuel, the same atomizer herein may be used for both liquid and gaseous fuels.

It is another object of this invention to provide an air blast fuel atomizer which has wide application with the following advantages: (1) it is a low fuel pressure system; 2) fuel contamination can be handled without filtration; (3) it provides good relight characteristics; (4) it provides good lean blow-out characteristics; (5) it provides for good engine acceleration times; (6) it provides for low emission of unburned hydrocarbons at idle and sub-idle speeds; (7) it is able to handle viscous, low volatility fuels; (8) it has dual fuel capability usable with both liquid and gaseous fuels; (9) it has high altitude relight capability; (10) it has good pattern factor characteristics; l l it has potential for low emission of nitrogen oxides; and (12) it is of simple and mechanically robust construction.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-section view of an air blast fuel atomizer embodying the present invention;

FIG. 2 is a cross-section view taken substantially along the line 2-2 of FIG. 1;

FIG. 3 is a fragmentary cross-section view on enlarged scale illustrating the preferred arrangement for introducing secondary fuel to the prefilming cylinder;

FIG. 4 is a cross-section view similar to FIG. 1, except illustrating a modification in the manner of introducing primer fuel into the venturi tube throat through a pressurizing valve and an orifice between the main fuel supply line and the primer fuel supply passages rather than through a flow divider valve as employed in FIG. I; and

FIG. 5 is a cross-section view on enlarged scale along line 5-5, FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The air blast fucl atomizer 1 herein comprises a housing assembly 2 secured as shown to the wall 3 of the air pressure manifold 4 of a gas turbine engine and to the wall 5 of the combustion chamber 6, the latter connection including a spring washer 7 which compensates for tolerance variations and for differential thermal expansion and contraction in an axial direction and also a radially floating ferrule assembly 7a which compensates for tolerance variations and for differential thermal expansion and contractions in a radial direction. The housing assembly 2 is provided with a liquid fuel inlet 8 which has therein a flow divider 9 (see for example Sample, Jr. U.S. Pat. No. 3,662,959) having primary and secondary fuel delivery passages 10 and 11 of which the primary passage 10 communicates with a tube 12 extending axially into the center of the throat 14 of a venturi tube IS and of which the secondary passage ll communicates with a manifold 16 having swirl orifices 17 for flow of secondary fuel around the prefilming surface 18 of the air blast atomizer tube or prefilmer 19. The housing assembly 2 is also provided with a gas inlet 20 which communicates with a manifold 21 terminating in ports 23 around the prefilmer 19 through which high velocity jets of boost air, or oxygen, or gaseous fuel are introduced in a manner hereinafter described in detail.

As already mentioned, the primary fuel atomizer comprises the centrally mounted venturi tube 15 with primary fuel being introduced into the throat 14 by way ofthe axial tube 12, passage 10, and flow divider 9. The acceleration of the primary air from chamber 4 due to well known venturi principles gives high air velocities for fuel atomization in the throat 14 where the primary fuel is thus introduced. This provides sufficiently finely atomized fuel for starting purposes under most normal operating conditions. The discharge from the venturi tube 15 is divided into a number of radial lobes 24 which serve to promote mixing of the atomized primary fuel with the swirling primary air flowing around the venturi tube 15 via the primary swirler 25, to increase the stability of the pilot zone 26, and to reduce the risk of torching from a concentrated streak of fuel in the center of the combustion chamber 6.

The swirler is disposed concentrically around the venturi tube 15 as is the secondary liquid fuel manifold 16 and orifices 17. The secondary fuel injection introduces liquid fuel from the manifold 16 with a low pressure drop through orifices 17 which are sized to pass contaminants in the fuel, and which are designed to introduce the fuel onto the prefilming surface 18 with circumferential uniformity. The preferred arrangement, as shown in FIG. 1, comprises providing a swirling discharge from the orifices 17 tangentially on the face 27 (see FIG. 3) which is contained within a slot 28 and provided with a deflector lip 29, the slot 28 assisting in circumferentially distributing the fuel prior to its flow onto the prefilmer surface 18 over the weir 30 formed by the downstream wall of the slot 28. The deflector lip 29 eliminates splashing of fuel from the slot 28 into the air stream from the swirler 25 and also prevents fuel from the slot 28 being drawn forward by any low pressure wakes from the swirler 25. In the other form, as shown in FIG. 4, the orifices 17' provide simply a swirling discharge tangentially onto the face 27.

The prefilmer 19 is here shown as a simple cylinder along which the secondary fuel from the flow divider valve 9 flows as a swirling cylindrical film and the swirler 25 serves initially to distribute the fuel into a uniform thin swirling film or sheet over the interior surface 18 of the prefilmer l9.

Secondary air is introduced through a secondary swirler 31 which is mounted with a shroud 32 concentrically about the prefilmer 19, the preferred configuration of the swirler 31 being of the radial inflow type. The annular passage 34 formed by the swirler 31, shroud 32 and the outer diameter of the prefilmer 19 is blind-ended upstream so that the radial swirling air from the swirler 31 is turned through 90 so as to flow downstream. This arrangement gives an air velocity profile in the annular passage 34 which has its maximum close to the outside diameter of the prefilmer l9 and is free from low pressure wakes. The swirling fuel sheet on surface 18, upon reaching the sharp and thin edge 35 of the prefilmer 19 moves radially outward due to its radial velocity component into the swirling high velocity secondary air stream from passage 34 where it is atomized without streaks.

Atomization of the liquid fuel is achieved in the secondary stream and combustion of this fuel takes place in the highly turbulent shear interface formed at the confluence of the swirling primary and secondary air streams. The fuel droplet size which is achieved is inversely proportional to a power of the mean velocity through the atomizer 1. However, a considerable proportion of the available air energy for atomization is taken up in acceleration of the fuel up to this velocity.

By using counter-rotating swirl in the primary and secondary streams, a high relative velocity is achieved for atomization without incurring the penalty of excessive fuel acceleration losses. Counter-rotating swirl enhances the turbulent shear region at the confluence of the flow and so maximizes the burning velocity in combustion.

Concentric with the prefilmer 19 and between the liquid fuel manifold 16 and the secondary swirler 31, is the manifold 21 which has an annular extension 36 in a downstream direction which forms part of the prefilmer 19 and this extension 36 terminates upstream of the end of the prefilmer 19 in a circumferential series of metering ports 23. The gaseous fuel, or boost air, or oxygen introduced through ports 23 is in discrete jets which mix only with the secondary air stream in passage 34, the ports 23 being placed so that excessive mixing of gas with air does not occur before combustion takes place.

The same gas manifold 21 can also serve for a boost gas to facilitate starting. For low pressure loss, large scale gas turbines operating on viscous fuels with cold day inlet conditions, an air pressure drop to suitably atomize the liquid fuel may not be available on cranking the engine, even with a venturi air blast primary fuel supply. In such case, a boost start may be necessary and may comprise starting on gaseous fuel with a switch over to liquid fuel when sufficient engine speed is achieved. If gaseous fuel is not available, the manifold 21 can be used with high pressure air from an auxiliary compressor and, in this case, the metering orifices 23 produce high velocity discrete jets of air which locally break up the fuel sheet flowing over the lip 35 of the prefilmer 19. This procedure results in an atomized fuel spray with a wide distribution of droplet sizes and fuel rich regions which are ideal for ignition purposes.

For aircraft high altitude application, the boost gas in manifold 21 may be pure oxygen which is supplied to the inlet 20 through a suitable selector and pressure reducing valve which is keyed into the relight sequence when ignition is selected by the pilot. When the selector and pressure reducing valve is opened, oxygen flows into the manifold 21, and the high velocity jets of oxygen issuing from the metering ports 23 finely atomizes the liquid fuel from the prefilmer 19. The close proximity of fuel and oxygen satisfies the criterion of intimate mixing and the combination of finely atomized fuel and oxygen in proximity to a sparking ignitor plug will result in ignition of the fuel and temperature rise in the combustion chamber 6. Jet pipe thermocouples or the like sense the temperature rise and shut off the oxygen supply at a predetermined value for the engine to be self-sustaining. Inasmuch as oxygen is not required for every engine start, additional feedback control to the oxygen regulator valve of at least a reference pressure would be a necessary incorporation. The oxygen-boost system could also be used for making fast emergency starts in a combat situation at sea level on a 60F day with 12 centistoke JP5 fuel and a manual override would be provided for this. However, normal operation on the oxygen-boost system at sea level is not recom- Mach numbers, and high flight altitudes, and serve to improve the chemical reaction rates where ignition is residence time limited, i.e., at high flight Mach num bers and high flight altitudes. In both cases, however, the oxygen also lowers the effective altitude at which combustion is taking place, thereby increasing combustion efficiency and temperature rise with consequent reduction in acceleration times.

In an industrial gas turbine installation, the combustor will comprise the liner 5 having a domed upstream closure member 37 with an opening therein to receive the air blast fuel atomizer l. The liner 5 is enclosed in a casing 3 and the combustor may be of the tubular, tubo-annular or annular type having a plurality of circumferentially spaced apart openings. The flow through the combustor can be either straight through or reverse in nature without departing from the present invention. Means 38 are provided for cooling the domed member 37 and provision is made as aforesaid to accommodate axial and radial movements of the liner 5 relative to the atomizer 1 whether such movements be due to assembly tolerances or thermal growth of the liner 5 during operation of the turbine. The passages 4 formed by the liner 5 and the casing 3 are adapted to deliver a flow of pressurized air from a suitable source such as a compressor.

As aforesaid, the construction shown in FIGS. 1 to 3 is of a type which employs a flow divider valve 9 operative to supply primer fuel to the throat 14 of the venturi tube 15 until a specified fuel pressure is reached and at that time the flow divider valve 9 is opened in known manner by the fuel pressure to permit fuel flow to the secondary liquid fuel manifold 16.

The means 38 for cooling the dome 37 herein is shown as comprising a baffle plate and air is admitted through the dome 37 by the ports 39 and flows between the baffle plate 38 and the dome 37 thereby cooling the latter. In annular combustors, the baffle 38 fulfills another purpose and that is because the liner 5 is closedended by the dome 37, the issuing fuel-air flow satisfies its entrainment appetite by drawing in additional air from the surroundings, some of this additional air being part of the recirculation zone and is derived from the air ports 40. Some of it, however, is the dome cooling air from annular passage 41 where the issuing fuel-air flow expands outward from the atomizer 1 centerline until it encounters the cooling air from passage 41 which it draws into itself to grow and keep moving downstream. This entrainment phenomenon can be used to advantage. In annular combustors, the aim is to produce a circumferentially uniform discharge temperature to give a long life to the turbine entry guide vanes and to achieve this it is necessary to submerge the point sources produced by individual fuel injectors by aerodynamic mixing of the combustion products with relatively cool air injected through the liner 5 at some downstream station. The difficulties associated with doing this can be greatly eased by using the entrainment phenomenon with specially shaped baffle plates 38 to produce an apparent near-line-source. This distortion of the issuing fuel-air flow has become to be termed as a forced recirculation zone.

An additional advantage can accrue from the forced recirculation zone. The dimensions of the recirculation zone generated by a swirling flow can be related to the swirl strength. The swirl strength of the issuing flow in the present invention is determined independently of the recirculation zone and can be quite high. This, under natural conditions, would result in a long recirculation zone with a high residence time. This tends to result in increased emission of oxides of nitrogen from the combustor. With a forced recirculation, dimensions of the zone can be distorted by the baffles 38 to those appropriate to low or zero swirl strengths, even when there is appreciable swirl in the issuing flow. This results in shorter combustors and reduced nitrogen oxide emissions. These are achieved without sacrificing the hot and vigorous recirculation necessary at low engine speeds to avoid emission of unburned hydrocarbons.

Referring now to FIGS. 4 and 5, there is shown a fuel inlet control where all of the liquid fuel is passed through a pressurizing valve 45 to flow simultaneously to the throat 14 of the venturi tube 15 and to the liquid fuel manifold. The flow split between the two in this case is determined by an orifice 46 in the passage 47 leading to the venturi throat 14 via the streamlined web 48 having divergent discharge passages 49.

By way of summary, it can be seen that the liquid fuel is atomized from the lip 35 of the pretilmer 19 by the swirling secondary air flow issuing from the annular passage 34 and is intimately mixed with such air. The gaseous fuel injected through the metering ports 23 mixes with some of the secondary air flow in the annular passage 34. For both fuels, combustion takes place in the turbulent high shear region generated at the interface between the swirling primary and secondary air flows. The swirling annular jet discharging from the prefilmer 19 generates a region of low pressure in its center along the atomizer l centerline and induces a reverse flow of gases flowing toward the atomizer l with a swirl component opposite to that of the issuing flow. The reverse flow of gases is made up of hot products of combustion originating from the flame together with a quantity of fresh air admitted through the liner 5 by the ports 40, some of which is drawn into the low pressure region generated by the issuing flow. The hot gas is carried forward by the flow of gases mixed with the issuing flow and serve to ignite the fuel contained therein. By this means combustion is sustained.

If the primary (or intermediate) and secondary air flows are swirling in opposite directions, the net swirl or the issuing flow will be reduced as one of these overcomes the other. This maximizes the tubulent shear and combustion rates at the confluence of the two flows but reduces the recirculated flow. This causes a reduction in the lean blowout characteristics of the combustor. If the primary (or intermediate) and secondary air flows are swirling in the same direction, the net swirl would be increased, combustion rates reduced, and lean blowout characteristics enhanced. Whichever approach is used, the principles of the present invention are not departed from.

I therefore, particularly point out and distinctly claim as my invention:

1. A fuel nozzle for a gas turbine engine comprising a housing having passages therein adapted for communication with fuel and air pressure sources; primary and secondary atomizing means in said housing to supply combustible fuel-air mixtures to said engine; said primary atomizing means comprising a venturi tube having a throat in communication with said air and fuel passages whereby fuel introduced into said throat is atomized by the air flowing through said venturi tube with the fuel-air mixture being conducted to the engine from the downstream end of said venturi tube; said secondary atomizing means comprising a prefilming tube concentrically surrounding said venturi tube defining therewith and with said housing annular air passages which at their upstream ends are communicated with said air pressure source, said prefilming tube at its upstream end communicating with said fuel passage for flow of fuel in film form around the interior of said prefilming tube to the downstream end thereof from which it is discharged with a radially outward velocity component into the annular air stream around said prefilming tube; said annular air passages having swirl means associated therewith to impart swirling motion to the annular air streams flowing therethrough; said housing having an additional passage with metering orifices up stream of the downstream end of said prefilming tube and downstream of the associated swirl means to introduce high velocity discrete jets of boost air, or oxygen, or gaseous fuel into and through the swirling annular air stream which surrounds said prefilming tube to impinge on the film of fuel as the latter emerges with a radial outward velocity component from the downstream end of said prefilming tube.

2. The nozzle of claim 1 wherein said metering orifices are disposed to introduce such high velocity jets axially along the exterior of said prefilming tube to impinge upon the film of fuel immediately upon emergence from the downstream end of said prefilming tube.

3. A fuel nozzle for a gas turbine engine comprising a housing having passages therein adapted for communication with fuel and air pressure sources; primary and secondary atomizing means in said housing to supply combustible fuel-air mixtures to said engine; said secondary atomizing means comprising a prefilming tube around said primary atomizing means and defining therewith and with said housing annular air passages which at their upstream ends are communicated with said air pressure source, said prefilming tube at its upstream end communicating with said fuel passage for flow of fuel in film form around the interior of said prefilming tube to the downstream end thereof from which it is discharged with a radially outward velocity component into the annular air stream around said prefilming tube; said housing having an additional passage with metering orifices upstream of the downstream end of said prefilming tube to introduce high velocity discrete jets of boost air, or oxygen, or gaseous fuel into and through the annular air stream which surrounds said prefilming tube to impinge on the film of fuel as the latter emerges with a radial outward velocity component from the downstream end of said prefilming tube.

4. The nozzle of claim 3 wherein said metering orifices discharge such jets axially along the exterior of said prefilming tube to impinge upon the film of fuel immediately upon emergence from the downstream end of said prefilming tube.

Claims (4)

1. A fuel nozzle for a gas turbine engine comprising a housing having passages therein adapted for communication with fuel and air pressure sources; primary and secondary atomizing means in said housing to supply combustible fuel-air mixtures to said engine; said primary atomizing means comprising a venturi tube having a throat in communication with said air and fuel passages whereby fuel introduced into said throat is atomized by the air flowing through said venturi tube with the fuel-air mixture being conducted to the engine from the downstream end of said venturi tube; said secondary atomizing means comprising a prefilming tube concentrically surrounding said venturi tube defining therewith and with said housing annular air passages which at their upstream ends are communicated with said air pressure source, said prefilming tube at its upstream end communicating with said fuel passage for flow of fuel in film form around the interior of said prefilming tube to the downstream end thereof from which it is discharged with a radially outward velocity component into the annular air stream around said prefilming tube; said annular air passages having swirl means associated therewith to impart swirling motion to the annular Air streams flowing therethrough; said housing having an additional passage with metering orifices upstream of the downstream end of said prefilming tube and downstream of the associated swirl means to introduce high velocity discrete jets of boost air, or oxygen, or gaseous fuel into and through the swirling annular air stream which surrounds said prefilming tube to impinge on the film of fuel as the latter emerges with a radial outward velocity component from the downstream end of said prefilming tube.

2. The nozzle of claim 1 wherein said metering orifices are disposed to introduce such high velocity jets axially along the exterior of said prefilming tube to impinge upon the film of fuel immediately upon emergence from the downstream end of said prefilming tube.

3. A fuel nozzle for a gas turbine engine comprising a housing having passages therein adapted for communication with fuel and air pressure sources; primary and secondary atomizing means in said housing to supply combustible fuel-air mixtures to said engine; said secondary atomizing means comprising a prefilming tube around said primary atomizing means and defining therewith and with said housing annular air passages which at their upstream ends are communicated with said air pressure source, said prefilming tube at its upstream end communicating with said fuel passage for flow of fuel in film form around the interior of said prefilming tube to the downstream end thereof from which it is discharged with a radially outward velocity component into the annular air stream around said prefilming tube; said housing having an additional passage with metering orifices upstream of the downstream end of said prefilming tube to introduce high velocity discrete jets of boost air, or oxygen, or gaseous fuel into and through the annular air stream which surrounds said prefilming tube to impinge on the film of fuel as the latter emerges with a radial outward velocity component from the downstream end of said prefilming tube.

4. The nozzle of claim 3 wherein said metering orifices discharge such jets axially along the exterior of said prefilming tube to impinge upon the film of fuel immediately upon emergence from the downstream end of said prefilming tube.

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