Patent Description:
The gas turbine engine may include premixer injectors for providing a mixture of air and fuel for the combustors. The premixer injectors effectively mix the air and fuel. The premixer injectors may also damp out thermo-acoustic instability.

Further, in <CIT> a fuel injection nozzle is disclosed which provides providing a series of premixing chambers being in serially aligned relationship one to another. During operation of the fuel injector the premixing chambers have a liquid fluid, fuel or water, premixed with a gaseous fluid, air or fuel, respectively. The liquid fluid has a predetermined pressure and the gaseous fluid is introduced into the liquid fluid under a predetermined pressure greater than the predetermined pressure of the liquid fluid in a first mixing chamber.

Furthermore, <CIT> discloses a method of assembling a gas turbine engine. The method includes coupling a combustor in flow communication with a compressor such that the combustor receives at least some of the air discharged by the compressor. A fuel nozzle assembly is coupled to the combustor and includes at least one fuel nozzle that includes a plurality of interior surfaces, wherein a thermal barrier coating is applied across at least one of the plurality of interior surfaces to facilitate shielding the interior surfaces from combustion gases.

In one aspect, a premixer injector in a gas turbine engine includes an inlet end; an outlet end; a first wall between the inlet end and the outlet end, the first wall comprising a plurality of apertures circumferentially separated around the first wall and axially separated along the first wall, each aperture passing through the first wall; a premixer duct defined by an interior of the first wall; a second wall between the inlet end and the outlet end, the second wall at least partially surrounding the first wall; and a secondary duct defined between the first wall and the second wall.

In another aspect, a premixer injector operable to mix a fuel and an air includes a first wall enclosing a premixer duct having an inlet end for an admission of a primary air flow into the premixer duct and an outlet end for a discharge of a mixture of fuel and air, the first wall including a plurality of apertures circumferentially separated around the first wall and axially separated along the first wall, each aperture passing through the first wall; a fuel lance disposed in the premixer duct at the inlet end, the fuel lance operable to inject the fuel into the premixer duct; and a second wall positioned to at least partially surround the first wall defining a secondary duct therebetween, the second wall having an inlet end for an admission of a secondary air flow into the secondary duct, wherein the secondary air flow enters the premixer duct via the plurality of apertures and is added to the mixture of fuel and air.

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in this description or illustrated in the following drawings.

Various technologies that pertain to systems and methods will now be described with reference to the drawings, where like reference numerals represent like elements throughout. The drawings discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged apparatus. It is to be understood that functionality that is described as being carried out by certain system elements may be performed by multiple elements. Similarly, for instance, an element may be configured to perform functionality that is described as being carried out by multiple elements. The numerous innovative teachings of the present application will be described with reference to exemplary non-limiting embodiments.

Also, it should be understood that the words or phrases used herein should be construed broadly, unless expressly limited in some examples. For example, the terms "including," "having," and "comprising," as well as derivatives thereof, mean inclusion without limitation. The singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Further, the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. The term "or" is inclusive, meaning and/or, unless the context clearly indicates otherwise. The phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Furthermore, while multiple embodiments or constructions may be described herein, any features, methods, steps, components, etc. described with regard to one embodiment are equally applicable to other embodiments absent a specific statement to the contrary.

Also, although the terms "first", "second", "third" and so forth may be used herein to refer to various elements, information, functions, or acts, these elements, information, functions, or acts should not be limited by these terms. Rather these numeral adjectives are used to distinguish different elements, information, functions or acts from each other. For example, a first element, information, function, or act could be termed a second element, information, function, or act, and, similarly, a second element, information, function, or act could be termed a first element, information, function, or act, without departing from the scope of the present disclosure.

Also, in the description, the terms "axial" or "axially" refer to a direction along a longitudinal axis of a gas turbine engine. The terms "radial" or "radially" refer to a direction perpendicular to the longitudinal axis of the gas turbine engine. The terms "downstream" or "aft" refer to a direction along a flow direction. The terms "upstream" or "forward" refer to a direction against the flow direction.

In addition, the term "adjacent to" may mean: that an element is relatively near to but not in contact with a further element; or that the element is in contact with the further portion, unless the context clearly indicates otherwise. Further, the phrase "based on" is intended to mean "based, at least in part, on" unless explicitly stated otherwise. Terms "about" or "substantially" or like terms are intended to cover variations in a value that are within normal industry manufacturing tolerances for that dimension. If no industry standard is available, a variation of twenty percent would fall within the meaning of these terms unless otherwise stated.

<FIG> illustrates an example of a gas turbine engine <NUM> including a compressor section <NUM>, a combustion section <NUM>, and a turbine section <NUM> arranged along a central axis <NUM>. The compressor section <NUM> includes a plurality of compressor stages <NUM> with each compressor stage <NUM> including a set of stationary vanes <NUM> or adjustable guide vanes and a set of rotating blades <NUM>. A rotor <NUM> supports the rotating blades <NUM> for rotation about the central axis <NUM> during operation. In some constructions, a single one-piece rotor <NUM> extends the length of the gas turbine engine <NUM> and is supported for rotation by a bearing at either end. In other constructions, the rotor <NUM> is assembled from several separate spools that are attached to one another or may include multiple disk sections that are attached via a bolt or plurality of bolts.

The compressor section <NUM> is in fluid communication with an inlet section <NUM> to allow the gas turbine engine <NUM> to draw atmospheric air into the compressor section <NUM>. During operation of the gas turbine engine <NUM>, the compressor section <NUM> draws in atmospheric air and compresses that air for delivery to the combustion section <NUM>. The illustrated compressor section <NUM> is an example of one compressor section <NUM> with other arrangements and designs being possible.

In the illustrated construction, the combustion section <NUM> includes a plurality of separate combustors <NUM> that each operate to mix a flow of fuel with the compressed air from the compressor section <NUM> and to combust that air-fuel mixture to produce a flow of high temperature, high pressure combustion gases or exhaust gas <NUM>. Of course, many other arrangements of the combustion section <NUM> are possible.

The turbine section <NUM> includes a plurality of turbine stages <NUM> with each turbine stage <NUM> including a number of stationary turbine vanes <NUM> and a number of rotating turbine blades <NUM>. The turbine stages <NUM> are arranged to receive the exhaust gas <NUM> from the combustion section <NUM> at a turbine inlet <NUM> and expand that gas to convert thermal and pressure energy into rotating or mechanical work. The turbine section <NUM> is connected to the compressor section <NUM> to drive the compressor section <NUM>. For gas turbine engines <NUM> used for power generation or as prime movers, the turbine section <NUM> is also connected to a generator, pump, or other device to be driven. As with the compressor section <NUM>, other designs and arrangements of the turbine section <NUM> are possible.

An exhaust portion <NUM> is positioned downstream of the turbine section <NUM> and is arranged to receive the expanded flow of exhaust gas <NUM> from the final turbine stage <NUM> in the turbine section <NUM>. The exhaust portion <NUM> is arranged to efficiently direct the exhaust gas <NUM> away from the turbine section <NUM> to assure efficient operation of the turbine section <NUM>. Many variations and design differences are possible in the exhaust portion <NUM>. As such, the illustrated exhaust portion <NUM> is but one example of those variations.

A control system <NUM> is coupled to the gas turbine engine <NUM> and operates to monitor various operating parameters and to control various operations of the gas turbine engine <NUM>. In preferred constructions the control system <NUM> is typically micro-processor based and includes memory devices and data storage devices for collecting, analyzing, and storing data. In addition, the control system <NUM> provides output data to various devices including monitors, printers, indicators, and the like that allow users to interface with the control system <NUM> to provide inputs or adjustments. In the example of a power generation system, a user may input a power output set point and the control system <NUM> may adjust the various control inputs to achieve that power output in an efficient manner.

The control system <NUM> can control various operating parameters including, but not limited to variable inlet guide vane positions, fuel flow rates and pressures, engine speed, valve positions, generator load, and generator excitation. Of course, other applications may have fewer or more controllable devices. The control system <NUM> also monitors various parameters to assure that the gas turbine engine <NUM> is operating properly. Some parameters that are monitored may include inlet air temperature, compressor outlet temperature and pressure, combustor outlet temperature, fuel flow rate, generator power output, bearing temperature, and the like. Many of these measurements are displayed for the user and are logged for later review should such a review be necessary.

<FIG> is a section view of a combustor <NUM> suitable for use in the combustion section <NUM> of the gas turbine engine <NUM> of <FIG>. The combustor <NUM> includes a casing <NUM>, an inlet <NUM>, a premixer injector assembly <NUM>, a combustor liner <NUM> defining a combustor chamber <NUM> and a chamber exit <NUM>. The casing <NUM> encloses the premixer injector assembly <NUM> and the combustor liner <NUM>. The premixer injector assembly <NUM> is disposed upstream of the combustor chamber <NUM>.

The premixer injector assembly <NUM> includes a plurality of premixer injectors <NUM>. The premixer injectors <NUM> are assembled in at least one block. As illustrated in <FIG>, a number of the premixer injectors <NUM> are assembled in a primary block <NUM> and a remaining number of the premixer injectors <NUM> are assembled in a secondary block <NUM>. The primary block <NUM> is disposed upstream of the secondary block <NUM>. The premixer injectors <NUM> are parallel to each other. The premixer injectors <NUM> are orientated perpendicular to the top surface of the primary block <NUM> or perpendicular to the top surface of the secondary block <NUM>. In other constructions, it is possible that the premixer injectors <NUM> may be assembled in the primary block <NUM> and secondary block <NUM> in other configurations, such as not parallel to each other, or not perpendicular to the top surface of the primary block <NUM> or not perpendicular to the top surface of the secondary block <NUM>. It is also possible that all the premixer injectors <NUM> are assembled in single block.

<FIG> is a section view of one of the premixer injectors <NUM> suitable for use in the arrangement illustrated in <FIG>. The premixer injector <NUM> has a general cylindrical shape having an inlet end <NUM> and an outlet end <NUM>. The premixer injector <NUM> includes a first wall <NUM> and a second wall <NUM>. The second wall <NUM> at least partially surrounds the first wall <NUM>. The first wall <NUM> encloses a hollow interior defining a premixer duct <NUM>. The first wall <NUM> has a circular cross section and extends in a general straight shape. The second wall <NUM> has a circular cross section and extends in a general straight shape. The first wall <NUM> and the second wall <NUM> cooperate to define an annular chamber therebetween. The annular chamber extends from the inlet end <NUM> to the outlet end <NUM> and defines a distance between the second wall <NUM> and the first wall <NUM>. The distance is constant between the inlet end <NUM> and the outlet end <NUM> of the premixer injector <NUM>. It is possible that the distance between the second wall <NUM> and the first wall <NUM> varies between the inlet end <NUM> and the outlet end <NUM> of the premixer injector <NUM>.

The premixer injector <NUM> includes a fuel lance <NUM> disposed in the premixer duct <NUM> at the inlet end <NUM> for feeding fuel to the premixer duct <NUM>. The fuel lance <NUM> includes a liquid fuel tube <NUM> and a gas fuel tube <NUM>. The gas fuel tube <NUM> surrounds the liquid fuel tube <NUM>. The liquid fuel tube <NUM> has a liquid fuel outlet <NUM> through which the liquid fuel flows into the premixer duct <NUM>. The gas fuel tube <NUM> has a gas fuel outlet <NUM> through which the gas fuel flows into the premixer duct <NUM>. The fuel lance <NUM> includes at least one vortex generator <NUM> attached to an outer wall of the fuel lance <NUM>. Each vortex generator <NUM> has a generally triangular shape. It is possible that one or more of the vortex generators <NUM> may have any desired shapes, such as rectangular, circular, arch, etc. In the illustrated construction, the fuel lance <NUM> has a plurality of vortex generators <NUM>. The vortex generators <NUM> are attached around an outer perimeter of an outer wall of the gas fuel tube <NUM>. It is possible that the liquid fuel tube <NUM> may surround the gas fuel tube <NUM> and the vortex generators <NUM> are attached around an outer perimeter of an outer wall of the liquid fuel tube <NUM>. It is also possible that the fuel lance <NUM> may have only the liquid fuel tube <NUM> or only the gas fuel tube <NUM>.

The premixer injector <NUM> includes a secondary duct <NUM> defined by the annual chamber between the first wall <NUM> and the second wall <NUM>. The premixer injector <NUM> includes a plurality of struts <NUM> disposed in the secondary duct <NUM>, The struts <NUM> are disposed between the first wall <NUM> and the second wall <NUM> for supporting the first wall <NUM>. The plurality of struts <NUM> are disposed circumferentially around the secondary duct <NUM> at the same axial location. The struts <NUM> are axially separated from each other along the secondary duct <NUM> between the inlet end <NUM> and the outlet end <NUM>.

The first wall <NUM> is a porous wall having a plurality of apertures <NUM>. The apertures <NUM> are arranged in the first wall <NUM> between the inlet end <NUM> and the outlet end <NUM> of the premixer injector <NUM>. Each apertures <NUM> passes through the first wall <NUM>. The first aperture <NUM> is placed downstream of the outlet of the longer fuel tube. In the embodiment as illustrated in <FIG>, the first aperture <NUM> is placed downstream of the liquid fuel outlet <NUM>. The apertures <NUM> are circumferentially separated around the first wall <NUM>. A row of apertures <NUM> is formed by the apertures <NUM> at the same axial location and different circumferential locations. The apertures <NUM> are axially separated along the first wall <NUM>. A column of the apertures <NUM> is formed by the apertures <NUM> at the same circumferential location and different axially locations. The apertures <NUM> may be evenly distributed in the first wall <NUM> in one of the axial and circumferential directions, or both of the axial and circumferential directions. The apertures <NUM> may be unevenly distributed in the first wall <NUM> in one of the axial and circumferential directions, or both of the axial and circumferential directions. The number of apertures <NUM> and the distribution of apertures <NUM> in the first wall <NUM> are selected based on design requirement, such as the desired flow and acoustic behavior of the premixer injector <NUM>.

The premixer injector <NUM> includes a perforated plate <NUM> disposed between the first wall <NUM> and the second wall <NUM> at the inlet end <NUM> of the premixer injector <NUM>. The perforated plate <NUM> has a plurality of holes. The perforated plate <NUM> is placed continuously and circumferentially around the secondary duct <NUM>. It is possible that the premixer injector <NUM> includes a plurality of perforated plates <NUM> placed circumferentially around the secondary duct <NUM> and spaced apart from each other.

<FIG> is a section view of another premixer injector <NUM> suitable for use in the arrangement illustrated in <FIG>. The premixer injector <NUM> can be used in place of the premixer injector <NUM> or can be used in conjunction with the premixer injector <NUM>.

An outer wall <NUM> of the premixer injector <NUM> has a first section <NUM> and a second section <NUM> connected to each other. A diameter of the first section <NUM> is different from a diameter of the second section <NUM>. In the illustrated embodiment as shown in <FIG>, the diameter of the first section <NUM> is less than the diameter of the second section <NUM>. It is possible that the diameter of the first section <NUM> is larger than the diameter of the second section <NUM>.

The first section <NUM> starts from the inlet end <NUM> and ends upstream of the outlet of the longer fuel tube. In the embodiments as illustrated in <FIG>, the first section <NUM> ends upstream of the liquid fuel outlet <NUM>. The second section <NUM> starts from the end of the first section <NUM> and connects the end of the first section <NUM> via a planar panel <NUM> forming a step-like shaped outer wall <NUM>. The second section <NUM> extends to the outlet end <NUM>.

Thickness of the first wall <NUM> is tuned based on design requirements. For example, a thickness of the first wall <NUM> in <FIG> is thicker than a thickness of the first wall <NUM> in <FIG>. Volume of the secondary duct <NUM> is tuned based on design requirements. For example, the volume of the secondary duct <NUM> is tuned such that the secondary duct <NUM> is used as an acoustic resonator. A resonant frequency of the secondary duct <NUM> may be altered by modifying the thickness of the first wall <NUM>.

<FIG> is a section view of another premixer injector <NUM> suitable for use in the arrangement illustrated in <FIG>. The premixer injector <NUM> can be used in place of the premixer injector <NUM> or the premixer injector <NUM>, or can be used in conjunction with the premixer injector <NUM> or the premixer injector <NUM>.

The premixer injector <NUM> includes a third wall <NUM>. The third wall <NUM> at least partially surrounds the second wall <NUM>. A third duct <NUM> is defined between the second wall <NUM> and the third wall <NUM>. At least one bar <NUM> is disposed between the third wall <NUM> and the second wall <NUM>. The bar <NUM> is circumferentially separated around the third duct <NUM>. The bar <NUM> may be evenly or unevenly distributed around the third duct <NUM> in the circumferential direction. The bar <NUM> is an acoustically stiff boundary. The bar <NUM> is placed closer to the outlet end <NUM> than to the inlet end <NUM>. It is possible that the bar <NUM> may be placed at any desired location between the inlet end <NUM> and the outlet end <NUM>. The third wall <NUM> has a plurality of openings <NUM>. The openings <NUM> are disposed downstream of the bar <NUM>. The openings <NUM> are circumferentially separated around the third wall <NUM>, evenly or unevenly. The openings <NUM> are axially separated along the opening <NUM>, evenly or unevenly. A row of the openings <NUM> is formed by the openings <NUM> at the same axial location and different circumferential locations. A column pf the openings <NUM> is formed by the openings <NUM> at the same circumferential location and the different axial locations.

The second wall <NUM> is a porous wall including a plurality of orifices <NUM>. The orifices <NUM> are distributed along the second wall <NUM> between the inlet end <NUM> and the outlet end <NUM> and spaced apart from each other. The orifices <NUM> are circumferentially separated around the second wall <NUM>. The orifices <NUM> are axially separated along the second wall <NUM>. The orifices <NUM> are axially unevenly distributed along the second wall <NUM>. It is possible that the orifices <NUM> are axially evenly distributed along the first second wall <NUM>. The orifices <NUM> may be circumferentially evenly around the second wall <NUM>. It is also possible that the orifices <NUM> may be circumferentially unevenly around the second wall <NUM>. A row of the orifices <NUM> is formed by the orifices <NUM> at the same axial location and different circumferential locations. A column of the orifices <NUM> is formed by the orifices <NUM> at the same circumferential location and the different axial locations. The orifices <NUM> are axially staggered with the apertures <NUM> in the first wall <NUM> along the secondary duct <NUM>. It is possible that the orifices <NUM> may be distributed at the same axial location as the apertures <NUM> in the first wall <NUM> along the secondary duct <NUM>. The orifices <NUM> may be circumferentially staggered with the apertures <NUM> in the first wall <NUM> around the secondary duct <NUM>. It is possible that the orifices <NUM> may be distributed at the same circumferential location as the apertures <NUM> in the first wall <NUM> around the secondary duct <NUM>.

In operation of the gas turbine engine <NUM> of <FIG>, with reference to <FIG>, air from the compressor section <NUM> flows into the combustor <NUM> through the inlet <NUM> and is injected to the premixer injectors <NUM>. Fuel from a fuel source (not shown in <FIG>) enters the premixer injectors <NUM>. Air and fuel are mixed in the premixer injectors <NUM>. The mixture of air and fuel enters the combustor chamber <NUM>, as indicated by the arrow line, and is ignited in the combustor chamber <NUM>. The ignited mixture of air and fuel exits the combustor chamber <NUM> through the chamber exit <NUM> and enters the turbine section <NUM>.

In operation of the gas turbine engine <NUM> of <FIG>, with reference to <FIG>, <FIG>, and <FIG>, air from the compressor section <NUM> is split to the primary air flow <NUM> and the secondary air flow <NUM> at the inlet end <NUM> of the premixer injector <NUM>, the premixer injector <NUM>, or the premixer injector <NUM>. The primary air flow <NUM> includes a majority portion of the air and flows into the premixer duct <NUM>. The secondary air flow <NUM> includes the rest of the air and flows into the secondary duct <NUM>. The fuel lance <NUM> injects the fuels into the premixer duct <NUM> to mix with the primary air flow <NUM>. Vortices may be generated on the primary air flow <NUM> by the vortex generator <NUM> to improve the mixture. The secondary air flow <NUM> enters the premixer duct <NUM> from the secondary duct <NUM> through the plurality of apertures <NUM> along the first wall <NUM>. The secondary air flow <NUM> are mixed together with the mixture of the fuels and the primary air flow <NUM> in the premixer duct <NUM>. The mixture of the fuels and the primary air flow <NUM> and the secondary air flow <NUM> are discharged out of the premixer injector <NUM> or the premixer injector <NUM> or the premixer injector <NUM> at the outlet end <NUM>. The discharge is ignited to form a flame <NUM>.

In operation of the gas turbine engine <NUM> of <FIG>, with reference to <FIG>, the secondary air flow <NUM> also flows into the third duct <NUM> through the plurality of orifices <NUM> along the second wall <NUM> so that the third duct <NUM> becomes a wave resonator. The wave resonator may be a ¼ wave resonator or any desired wave resonator. The third duct <NUM> may also become a high frequency dynamics damping resonator. The secondary air flow <NUM> also flows to an exterior of the premixer injector <NUM> from the third duct <NUM> through the plurality of openings <NUM> along the third wall <NUM>. The secondary air flow <NUM> flows to the exterior of the premixer injector <NUM> is used as purge air to purge the third duct <NUM> functioned as the wave resonator and/or the high frequency dynamics damping resonator.

The arrangement of the premixer injector <NUM>, the premixer injector <NUM>, or the premixer injector <NUM> distributes the injection of a portion of the air, the secondary air flow <NUM>, into the premixer duct <NUM> along the premixer duct <NUM> to mix with the fuels, rather than injecting all air into the premixer duct <NUM> from the inlet end <NUM>. Such an arrangement improves air-fuel-ratio damping capability. The arrangement also improves the acoustic attenuation in the combustion section <NUM>. The arrangement mitigates boundary layer flashback by leaning the air-fuel mixture near the first wall <NUM>. The arrangement mitigates coking, auto ignition, and flashback while operating on liquid fuel by creating an air buffer at the first wall <NUM> to inhibit wall-wetting. The premixer injector <NUM>, the premixer injector <NUM>, and the premixer injector <NUM> are each designed with a sufficiently high pressure drop across the first wall <NUM> such that the fuels are not ingested into the secondary duct <NUM>.

Although an exemplary embodiment of the present disclosure has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, and improvements disclosed herein may be made without departing from the scope of the disclosure in its broadest form.

Claim 1:
A premixer injector (<NUM>; <NUM>; <NUM>) operable to mix a fuel and an air, the premixer injector (<NUM>; <NUM>; <NUM>) comprising:
an inlet end (<NUM>) receiving air which is split to a primary air flow (<NUM>) and a secondary air flow (<NUM>) at the inlet end (<NUM>);
an outlet end (<NUM>);
a first wall (<NUM>) enclosing a premixer duct (<NUM>) having the inlet end (<NUM>) for an admission of the primary air flow into the premixer duct (<NUM>) and the outlet end (<NUM>) for a discharge of a mixture of fuel and air, the first wall (<NUM>) including a plurality of apertures (<NUM>) circumferentially separated around the first wall (<NUM>) and axially separated along the first wall (<NUM>), each aperture (<NUM>) passing through the first wall (<NUM>);
a fuel lance (<NUM>) disposed in the premixer duct (<NUM>) at the inlet end (<NUM>), the fuel lance (<NUM>) operable to inject the fuel into the premixer duct (<NUM>); and
a second wall (<NUM>) positioned to at least partially surround the first wall (<NUM>) defining a secondary duct (<NUM>) therebetween, the second wall (<NUM>) having an inlet end (<NUM>) for an admission of the secondary air flow (<NUM>) into the secondary duct (<NUM>), wherein the secondary air flow enters the premixer duct (<NUM>) via the plurality of apertures (<NUM>) and is added to the mixture of fuel and air, wherein the secondary air flow (<NUM>) are mixed together with the mixture of the fuels and the primary air flow (<NUM>) in the premixer duct (<NUM>) and the mixture of the fuels and the primary air flow (<NUM>) and the secondary air flow (<NUM>) are discharged out of the premixer injector (<NUM>; <NUM>; <NUM>) at the outlet end (<NUM>) to be ignited to form a flame (<NUM>).