Method for preventing lean flaeout at ignition of a stored energy system for driving a turbine wheel

Lean flameout upon ignition in a turbine system including a turbine wheel (10) driven by motive gases including gases of combustion from a combustor (20) may be avoided by locating an orifice (130) downstream of the servo/.DELTA.P valve (52) that controls fuel flow to the fuel inlet (22) of the combustor (20).

FIELD OF THE INVENTION 
This invention relates to so-called stored energy systems wherein stored 
fuel and oxidant are combusted to provide motive gases to drive a turbine 
wheel as in starting or operating an auxiliary power unit or an emergency 
power unit. 
BACKGROUND OF THE INVENTION 
Both commercial and miliary aircraft typically carry auxiliary power units 
(APU) and often additionally may utilize a so-called emergency power unit 
(EPU). In some instances, the functions of both are combined. 
In emergency systems, EPUs, or APUs that operate additionally as EPUs must 
be brought into full operational capacity in a relatively short period of 
time, such as two or three seconds. In the usual case, these systems 
employ a turbine wheel for driving emergency power sources such as an 
electrical generator, a hydraulic pump or both so as to provide the energy 
necessary to continue to operate the aircraft. Consequently, it is 
necessary that the turbine wheel be accelerated up to normal operating 
speed in a relatively short period of time so that if an APU is being 
utilized to provide emergency power, it can reach a self sustaining speed. 
Where an EPU is being utilized, it still must be accelerated rapidly and 
then its operation maintained for some predetermined time period. 
Because these systems are intended for operation in emergency conditions, 
it is necessary that they have an extremely high degree of reliability. 
One obviously necessary feature is the ability to start rapidly without 
fail and maintain operation for the desired time period. 
As noted above, these systems typically employ a turbine wheel, or even a 
complete turbine engine to drive the emergency power sources. Thus, in 
order that there be reliable starting of the system, ignition of fuel in a 
combustor feeding the turbine wheel must be reliably had and the continued 
combustion of the fuel maintained over the desired period of operation. 
As is typical with many turbine systems, the control of fuel flow is 
achieved by a so-called .DELTA.P valve. .DELTA.P valves conventionally 
include a variable orifice along with a valve that is operable, in 
response to command from a fuel control system, to control the pressure 
drop across the orifice. Thus, for a given pressure drop, a constant fuel 
flow will be assured. 
These valves typically respond to increases or decreases in pressure 
downstream of the valve as is well-known. As a consequence, in some cases, 
the pressure rise within a combustor associated with a turbine wheel to be 
driven and fueled by fuel via the .DELTA.P valve will result in a lesser 
pressure drop across the .DELTA.P valve which in turn means a reduction in 
fuel flow until the valve responds. Consequently, a lean flameout may 
result immediately after ignition where valve response to the pressure 
surge of ignition is slow, and intolerable situation in a piece of 
equipment adapted for emergency use. 
The present invention is directed to overcoming one or more of the above 
problems. 
SUMMARY OF THE INVENTION 
It is the principal object of the invention to provide a new and improved 
fuel system for a combustor to be associated with a turbine wheel. More 
specifically, it is an object of the invention to provide a fuel system 
for a stored energy system for operating an EPU or APU and that is not 
subject to lean flameout immediately following ignition. 
An exemplary embodiment of the invention achieves the foregoing objects in 
a system for providing hot gases to drive a turbine wheel which includes a 
combustor having a hot gas outlet adapted to be connected to a nozzle for 
a turbine. A fuel inlet is provided for the combustor and a fuel flow 
control valve which includes downstream pressure responsive flow control 
means is operable to normally provide a selected pressure drop in fuel 
flow and a corresponding constant fuel flow rate. A conduit interconnects 
the inlet and the valve and means are provided for effectively isolating 
the valve from pressure surges in the combustor. 
In a preferred embodiment, the effective isolating means comprises a 
restriction in the conduit interconnecting the inlet in the valve and more 
preferably, the restriction is in the form of an orifice. 
In a highly preferred embodiment, the fuel inlet includes an air blast 
atomizing fuel injector. 
The invention also contemplates use in a system including a turbine wheel 
adapted to rotate about an axis and having a nozzle in proximity to the 
turbine wheel for directing motive gases thereat. The combustor is 
connected to such nozzle. 
According to another facet of the invention, there is provided a method of 
preventing lean flameout immediately following ignition in a combustor for 
a turbine, which includes a combustor having an air blast atomization fuel 
injector and a .DELTA.P valve for controlling the flow of fuel to the 
injector. The method includes, according to one embodiment, the step of 
making the pressure drop due to resistance to fuel flow from the .DELTA.P 
valve to the fuel injector large in relation to the desired pressure drop 
across the .DELTA.P valve. 
The invention contemplates that the step of making the resistance large be 
accomplished by placing an orifice in the path of fuel flow from the 
.DELTA.P valve to the fuel injector. 
Other objects and advantages will become apparent from the following 
specification taken in conjunction with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An exemplary embodiment of a stored energy system made according to the 
invention is illustrated in the drawing in the environment of an EPU. 
However, it should be understood that the invention is applicable to APUs 
and other turbine systems as well. 
With reference to the drawing, a gas turbine wheel 10 is seen to be mounted 
on a shaft 12 to be rotatable about the axis defined thereby. The shaft 12 
in turn is connected to a power unit 14 which may include an electrical 
generator, one or more hydraulic pumps, etc., which provide electrical or 
hydraulic energy for loads (not shown). 
A nozzle is shown schematically at 16 in proximity to the turbine wheel 10 
for directing motive gases against the same. The nozzle 16 is connected to 
the outlet 18 of a combustor, generally designated 20, to receive motive 
gases, including gases of combustion, therefrom. 
The combustor 20 includes a first fuel inlet 22 remote from the outlet 18 
and a second fuel inlet 24 in proximity to the outlet 18. Also included is 
an oxidant inlet 26. 
As somewhat schematically illustrated in the FIGURE, the oxidant inlet 26 
surrounds the fuel inlet 22. Advantageously, the high velocity of the 
oxidant as it enters the combustor 20 may be utilized to achieve a high 
degree of atomization of fuel entering by the fuel inlet 22 by use of any 
conventional air blast atomization fuel injector. Not untypically, such an 
injector surrounds the point of fuel release with a discharge opening for 
high velocity air under pressure to break up and atomize the fuel stream. 
In the present invention, the pressurized gas will be the oxidant which, 
as mentioned hereinafter, need not necessarily be air. Thus, the term "air 
blast atomization fuel injection" is not intended to be restricted to air, 
but only to denote a particular type of structure. 
Fuel is provided to the inlets 22 and 24 from a fuel tank 28 as will be 
seen. The fuel tank 28 includes an internal bladder and a pressurizing 
inlet 32 connected to a source 34 of air under pressure via a control 
valve 36. When the valve 36 is opened, the bladder 32 will be pressurized 
to expel fuel from the tank 28 via an outlet 38. The fuel system also 
includes a relief valve 40 and a fill port 42 on the inlet side of the 
fuel tank 28. 
The outlet 38 is connected to a vent cap 44 as well as a fill port 46 and a 
filter indicator 48. A fuel flow line 50 extends from the filter indicator 
48 to a primary fuel flow control servo/.DELTA.P valve 52 of a 
conventional construction and to a secondary fuel flow control 
servo/.DELTA.P valve 54, also of conventional construction. The valve 52 
is connected via a shut off valve 56 to the first inlet 22 while the valve 
54 is connected via a shut off valve 60 to the second fuel inlet 24. As 
can be seen, the valves 58 and 60, when not establishing fluid 
communication from the valves 52 and 54 to the combustor 24 have 
connections to the source 34 which operate to purge the respective lines 
to prevent residual fuel from gumming up the fuel lines over a period of 
time. 
The fuel injected into the combustor 20 at the first inlet 22 is atomized 
and combusted with oxidant received at the inlet 26 to provide hot gases 
of combustion. Fuel injected into the combustor outlet 18 by the second 
inlet 24 does not appreciably participate in the combustion process, if at 
all. Rather, the same is vaporized and/or thermally cracked by the hot 
gases of combustion resulting from fuel introduced at the inlet 22 to 
increase the volume and mass flow of the motive gases being applied to the 
turbine wheel 10 by the nozzle 16. 
The source 34 may also be connected to an air atomization valve 62 which in 
turn is connected to the first inlet 22 to provide for air atomization of 
fuel thereat and to a purge line 64 for directing a purging flow to the 
combustor when fuel is not being flowed thereto. Also included is an 
ignitor 66. 
The oxidant system includes a pressure vessel 70 which may be charged with 
an oxidant such as air, oxygen enriched air or even molecular oxygen in 
some instances. Charging is accomplished through a fill port 72 and fill 
valve 74. A pressure transducer 76 for monitoring the pressure of the 
charge of oxidant within the vessel 70 is also provided as is a pressure 
relief valve 78. 
A line 80 extends from the outlet 82 of the vessel 70 to a shut-off valve 
84. Downstream of the shut-off valve is a pressure regulator 86 which 
provides, when the shut-off valve 84 is opened, oxidant to a conduit 92 at 
constant pressure. 
The conduit 92 in turn is connected to the oxidant inlet 26. 
The conduit 92 includes an oxidant flow control servo valve 98 in series 
with a venturi 100. The venturi 100 acts as a choked orifice and minimizes 
flow losses in the conduit 92. A pressure transducer 102 and a temperature 
transducer 104 may also be connected to the branch 92 between the servo 
valve 98 and the venturi 100. The servo valve 98 is operable to vary the 
flow through the conduit 92 to achieve desired combustion conditions 
within the combustor 20 in response to signals received from a known servo 
control system 106. The signals are received on a line 108 and position 
feedback information is provided on a line 110. 
The lines 112 and 114 respectively connect the temperature transducer 104 
and pressure transducer 102 to the servo control 106 and a line 116 
connects a speed sensor 118 associated with the shaft 12 to the servo 
control 106. The valves 52 and 54 are respectively connected to the servo 
control 106 by means of lines 120 and 122. Thus, loading on the turbine 
wheel 10 may be determined by determining shaft speed sensed by the sensor 
116 and information to that effect provided to the servo control to vary 
fuel flow through the valves 52 and 54 as well as oxidant flow through the 
valve 98 as appropriate. Pressure variations as well as the effect on 
varying temperature on mass flow rate may be determined through use of the 
transducers 102 and 104 to provide suitable control information. 
As mentioned previously, the valve 52 is a servo/.DELTA.P valve of 
conventional construction. That is to say, its inner workings include a 
variable orifice along with a valve that is operable to control the 
pressure drop across the variable orifice in accordance with some sort of 
command. Any given commanded pressure drop will result in a constant flow 
rate associated with that particular pressure drop. And because pressure 
drop across the internal variable orifice is determined by the 
relationship of the pressure upstream of the valve 52 to the pressure 
downstream of the valve 52, it can be appreciated that any increase in the 
downstream pressure will decrease the pressure drop, thereby reducing the 
low rate of fuel through the valve 52. 
In most cases, when an increase in pressure downstream of the valve 52 is 
felt, the servo/.DELTA.P valve 52 responds rapidly to restore the desired 
pressure drop and thus maintain the desired fuel flow rate. However, where 
large changes in pressure occur suddenly, the valve 52 may not respond as 
rapidly as desired. In such a case, the flow of fuel through the valve 52 
is reduced because of the reduced pressure drop. Thus, combustion within 
the combustor 20 goes "lean" and if the flow of fuel is not restored to an 
appropriate level promptly, a flameout due to lack of sufficient fuel, or 
so-called "lean flameout" occurs. 
In starting up a system such as shown in the FIGURE, before ignition occurs 
within the combustor 20, the internal pressure thereat may be on the order 
of 100 psi. Immediately upon ignition of fuel in the combustor 20, the 
pressure jumps to approximately 300 psi. When such occurs, the pressure 
immediately downstream of the valve 52 rapidly escalates, changing the 
pressure drop across the valve 52; and such can result in lean flameout. 
The problem may be accentuated where the fuel injector is an air blast 
atomization type of injector as is preferred. Frequently, fuel injectors 
of this type act almost like an eductor with the pressurized gas creating 
a very low pressure on the fuel 22, almost drawing the fuel out of the 
fuel line 22. This, of course, creates a very low pressure downstream of 
the valve 52 to begin with and maximizes the pressure drop associated with 
any given fuel flow rate. 
To avoid the problem, the invention contemplates the provision of means for 
effectively isolating the servo/.DELTA.P valve 52 from large pressure 
surges occurring in the combustor 20 as when ignition of fuel is achieved 
during start up of the system. The invention provides for isolation of the 
valve 52 by intentionally incorporating a resistance in the fuel flow line 
between the valve 52 and the fuel inlet 22 to the combustor 20, preferably 
in the form of an orifice 130. The orifice 130 effectively creates a back 
pressure acting on the servo/.DELTA.P valve 52 at all times so that when 
the pressure surge occurs downstream of the orifice 130, it is in a large 
part isolated from the servo/.DELTA.P valve 52 by the presence of the 
orifice 130 and has a lesser effect on fuel flow. Consequently, while fuel 
flow may momentarily diminish somewhat, the diminishment is insufficient 
to result in a lean flameout. 
Stated another way, placing the orifice 130 between the combustor 20 and 
the servo/.DELTA.P valve 52 increases the resistance to fuel flow between 
the two to the point where it becomes large in relation to the commanded 
pressure drop controlling flow through the servo/.DELTA.P valve. The surge 
in pressure that occurs in the combustor 20 upon ignition becomes 
relatively small in comparison to the pressure drop between the combustor 
20 and the servo/.DELTA.P valve 52 and has a commensurately diminished 
effect on the operation of the servo/.DELTA.P valve 52. 
Thus, through the relatively simple and inexpensive expedient of 
introducing the orifice 130 into the fuel flow path downstream of the 
servo/.DELTA.P valve, lean flameout upon ignition is avoided.