Note: Descriptions are shown in the official language in which they were submitted.
iltUUl`JlJ U.L~' '1'11~ ll`JVl~l~'l.lUI~
Field Or the Invention:
This invention relates to the air inlet tubes for
the combustion chamber o~ a gas turbine engine and more
particularly to a film-cooled air inlet tube.
Description of the Prior Art:
Air inlet tubes for the combustion chamber o~ a
gas turbine engine are well known in the prior art with ref-
erence being specifically made to U.S. Patent No. 3,'899,882,
of common assignee as the present invention, showing such ,
structure. The inlets are normally disposed in annular
arrays through the c'ombustion chamber wall to in~ect combus-
tion air into the fuel spray pattern for mixing therewith to
promote complete combustion of the' fuel. The result .of this
projected length into the .combusti.on chamber results in the
hot combustion gases, having a temperature on the order of
2,000.+F,,flowing over the exterior surface of.the' tubes~
and inducing wakes ad~acent the downstream ~acing wa'll of
--1--
.~
6~
each tube. These wakes are generally referred to as "hot
wakes" and result in a region of high ternperature gases
intimately contacting this downstream facing wall of the
tube. Further, radiation from the combustion flame is re-
ceived directly on this downstream facing wall of the tu~e
to add to the temperature thereof. Due to these conditions,
the air inlet tube quite often becomes overheated and, gen-
erally starting ak the inlet edge of the downstream wall,
which is the most vulnerable area to the heat, becomes
burned out.
Thus, heretofore, the inlet tubes were made suffi-
ciently short so as to keep the most vulnerable portion,
i.e. the downstream edge, as far removed from the flame
front as possible and still obtain a good intermixing between
the fuel and khe air introduced through the tube. ~owever,
recent developments in a gas turbine combustor partlcularly
directed to adapting it to burn coal gas have resulted in
larger diameter combustion chambers (necessitated by an
increase in volume of the combustors to obtain similar
operating results. In such larger combustor chambers, the
short air inlet tubes do not provide sufficient penetration
of the air into the fuel such that there are pockets of
unburned fuel resulting in smoke. To prevent this smoke,
the ~nlet tubes by ~ecessity are required to extend further
into the chamber which in turn places the downstream face
and particularly the downstream edge closer to the-actual
combustion flame, resulting in overheating of the tubes so
that burnout occurs quite often.
The previously identified co-owned patent, although
directed to cooling the internal walls of the combustion
--2--
~116~ ~J~
chamber in generally the same zone o~ the chamber as the
inlet tubes, did so by permitting a portlon of the air to
enter through a gap in the mounting arrangement of the tubes
to ultimately form a film o~ cooling alr along the internal
combustor walls. It .Ls therein also stated that this
cooling air, entering ad~acent the tubes would also prevent
the combustion flame from attaching itself to the downstream
side of the inlet tube and thereby reduce the incidence of
burnout. However, the structure therein described does not
direct the air to penetrate inwardly far enough to protect
the downstream edge of the tube, which in the coal gas com--
bustor is even further from this wall cooling air and closer
to the actual combustion flame.
SUMMARY OF THE INVENTION
The present invention provldes an air inlet tube
extending generall.y radially into a gas turbine combustor
and an ad~acent radially elongated slot, one boundary of
which is the downstream wall of the air inlet tube to
provide a layer of air flowing over the downstream surface
of the tube with the radlal extent of the sl.ot providing
sufficient penetration of the air exiting therefrom to
continue to flow across the exposed downstream surface of
the tube to prevent the downstream edge of the tube from
overheating.
BRIEF V RIPTION OF THE DRAWINGS
Figure 1 is a schematic view of a gas turbine
combustion chamber having air in]et tubes projecti.ng there-
through,
Fi.gure 2 is an enlarged sectiorlal view of a.n a.ir30 inlet tube of the present invention; and
--3--
1~16A:17
~ igure 3 is a view looking down on the inlet tube
of Figure 2.
DESCRIPTION OF THE PRE~E~RED_E~BODIME~T
Referring to Figure 1, a gas turbine combustion
chamber 10 is schematically shown which comprises a stepped
cylindrical configuration, as is well known in the art, with
each successive cylindrical portion 12, 14, being larger and
connected to the ad~acent upstream cylindrical portion in
spaced relation as through a wiggle strip 16 to permit
cooling air entry along the walls of the chamber. Fuel is
introduced at the upstream end 18 of the chamber through a
fuel nozzle 20 providing an atomized fuel spray pattern.
~ombustion air is in~ected into the fuel spray through an
annular array of air scoops or air inlet tubes 22 pro~ecting
into the combustion chamber transverse to the flow of fuel
and the combustion gases through the chamber to provide
sufficient penetration of the air into the fuel spray for
complete combustion of the fuel in the primary or flame zone
of the chamber.
The resulting combustion, once ignited by a spark
igniter (not shown) is continuous, resulting ln a flow of
hot gases across the surface of the air inlet tubes 22.
This flow results in "hot wakes" or relatively slowly
moving hot gases ad~acent the downstream facing portion of
the tubes 22. These hot gases come in intimate heat transfer
contact with the downstream surfaces of the tubes to cause
this portion to become hotter than the upstream surface of
the tube. ~urther~ this downstream facing wall, in addition
to being the portion of the tube closest to the actual
3 combustion flame and thus contacted by the hot combustion
--4--
1~6~
gases, also directly recelves radiation from the flame, with
the resulting heat absorbed by this portion o~ the tube
causing the tube to overheat and burn out. ~his burnout
starts at the vulnerable downstream innermost edge of the
tube and generally proceeds until the tube requires replace-
ment.
As previously stated, the overheating is parti-
cularly critical as the tube length increases to obtain
sufficient air penetration in enlarged diameter combustion
chambers particularly suited for low BTU fuels such as coal
gas~ To prevent the burnout of the air inlet tubes, a film
cooling slot is provided adjacent the downstream face of the
tube to direct a layer of air to flow over the downstream
surface and prevent the hot gases in the combustion chamber
from intimately contacting such surface. With such lntlmate
heat transfer contact prevented, the tube is able to normally
withstand the temperature increase caused by the heat absorbed
from the other sources without any burnout.
Thus, referring to Figures 2 and 3, the inlet tube
22 according to the present invention comprises an annular
flange 26 for attachment, as by welding, to the wall 28 of
the combustion chamber, with the main air flow tube 30 pro-
~ecting radially inwardly through an aperture 32 in the
wall.
The inlet opening through the flange 26 is larger
than the outside diameter of the main tube 30 to provide a
gap or slot 36 between the downstream facing wall 38 of
the main air tube 30 and the shortened wall 4C~ A radially
inwardly extending shortened wall 40, having an arcuate
0 configuration conforming to the outer periphery of
-5-
~:116~17
the opening 34 ~ith the opposed terminal ends thereofattached to the side of the main air flow tube 30, de~ines
the inwardly extending cooling air inlet 36 that directs a
layer or film of cooling alr to flow over the downstream
face 38 of the tube 30 with sufficient radial penetration to
flow along the complete length of the tube 30 to the inner-
most end 42 thereof and thereby minimize heat transfer from
the hot gases within the chamber to the downstream wall of
the air inlet tube. It is seen from Fig. 3 that the opening
34 and tube 30 are generally elliptical with the ma~or axis
in alignment with the direction of hot gas flow through the
chamber to minimize the downstream surface 38; however, any
configuration would also be enhanced by the cooling air slot
36.
It is apparent that the gap opening 36 and the
radial length of the shortened wall 40 are dimensions whlch
may vary according to the various parameters such as differ-
ential in pressures between inner and outer wall-, the length
of the main air flow tube 30, the temperature within the
combustion chamber, and the desired maximum temperature of
the downstream face 38 of the tube 30, along with other
considerations which may vary for each particular engine or
application. However, for the most part, the desired effect
is a layer of cooling air directed over the downstream
facing wall of the inwardly pro~ecting air tube to prevent
burnout which generally starts at the downstream terminal
edge of the tube. It is also apparent that a cooled air
inlet tube as above described could be disposed at any of
various positions within the combustion chamber when air
penetration is required (i.e. as downstream dilution air)
--6--
and where burnout of the tube ends may be a potential problem
due to increased temperatures wlthin the chamber.
