Active clearance control with cruise mode

Flow of cooling air to a thermal clearance control system in a gas turbine engine is selectably scheduled between a normal power level versus clearance schedule and an increased efficiency cruising schedule. Selection of the cruising schedule is accompanied by a limitation on the rate of engine power increase during the period when the cruise schedule is selected.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is hereby made to copending, commonly assigned U. S. Patent 
Applications titled "Thermal Clearance Control Method for Gas Turbine 
Engines" by F. M. Schwarz and C. J. Crawley, Jr. U.S. Ser. No. 07/370,426 
and "Clearance Control Method for Gas Turbine Engine" by F. M. Schwarz, K. 
R. Lagueux, C. J. Crawley, Jr. and A. J. Rauseo U.S. Ser. No. 07/392,398, 
filed on even dated herewith and which related subject matter. 
FIELD OF THE INVENTION 
The present invention pertains to a method of operating a gas turbine 
engine in conjunction with thermal active clearance control. 
BACKGROUND 
The control of the radial clearance between the tips of rotating blades and 
the surrounding annular shroud in axial flow gas turbine engines is one 
known technique for proving engine efficiency. By reducing the blade tip 
to shroud clearance, designers can reduce the quantity of turbine working 
fluid which bypasses the blades, thereby increasing engine power output 
for a given fuel or other engine input. 
"Active clearance control" refers to those clearance control arrangements 
wherein a quantity of cooling air is employed by the clearance control 
system to regulate the temperature of certain engine structures and 
thereby control the blade tip to shroud clearance as a result of the 
thermal expansion or contraction of the cooled structure. It is a feature 
of such active clearance control systems that the cooling air flow may be 
switched or modulated responsive to various engine, aircraft, or 
environmental parameters for causing a reduction in blade tip to shroud 
clearance during those portions of the engine operating power range 
wherein such clearance control is most advantageous. 
A reduction of blade tip to shroud clearance must be achieved judiciously. 
For example, overcooling the turbine case supporting the annular shroud 
such that the shroud interferes with the rotating blade tips results in 
premature wear of the shroud or abrasion and damage to the blade tips. It 
is therefore important that the reduction in blade tip to shroud clearance 
achieved by such clearance controls systems must be designed so as to 
avoid the occurrence of blade tip and shroud interference which may 
ultimately cause deterioration of overall engine operating efficiency, or 
worse, damage to the engine internal components. 
DISCLOSURE OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
operating a gas turbine engine having an active clearance control system 
which reduces blade tip to shroud clearance during part load operation. 
According to the present invention, the method provides an alternate 
schedule of cooling air flow to the gas turbine engine for reducing blade 
tip to shroud radial clearance during periods in which the engine has 
entered a cruise mode of operation wherein its rate of increase of engine 
power is limited. The method according to the present invention further 
includes a set of criteria for determining the propriety of selecting the 
cruise mode of operation. The criteria may include environmental 
parameters, engine operating parameters, or operator input. 
Selection of the cruise mode of operation causes the flow of clearance 
control cooling air to the engine to follow an alternate flow schedule 
which results in reduced blade tip to shroud radial clearance as compared 
to the normal flow schedule. This reduced clearance increases engine 
operating efficiency at the steady state, part load, engine cruise power 
level, however, such reduced clearance is insufficient to accommodate the 
usual transient differential thermal growth between the blade tips and 
shroud following a step change in power level. 
It is therefore a feature of the present invention that the selection of 
cruise mode of operation and the corresponding alternate cooling flow 
schedule also includes a rate of change limitation on increasing engine 
power level. This limitation decreases the rate of response of the engine 
during cruise mode operation, thereby reducing the magnitude of the 
transient differential thermal growth during a change in engine power. 
Such reduced response, which may be undesirable over certain parts of the 
engine operating range, is acceptable during the cruise mode of operation 
as selected by the method according to the present invention. 
The advantage of this method is the achievement of operating efficiency 
during certain periods of engine operation without compromising engine 
response over the remainder of the operating range. Both these and other 
objects and advantages of the method according to the present invention 
will be apparent to those skilled in the art upon review of the following 
specification and the appended claims and drawing figures.

DETAILED DESCRIPTION 
FIG. 1 shows a graphic representation of the radial clearance between the 
rotating blade tips of the high pressure turbine section of a gas turbine 
engine and the surrounding annular shroud. This clearance, represented on 
the vertical axis on the .delta., is controlled by thermally heating or 
cooling the surrounding turbine case by means of a controlled flow of 
cooling air which is exhausted directly on the case exterior. Increased 
cooling air flow cools the turbine case, causing it to contract 
circumferentially thereby reducing the shroud to blade tip radial 
clearance. 
According to the control method disclosed in co-pending application 
"Clearance Control Method for Gas Turbine Engine", referenced above, blade 
tip to shroud clearance .delta. is optimally controlled responsive to 
engine level or, equivalently high rotor angular speed N.sub.2. 
FIG. 1 shows blade tip to shroud clearance .delta. on the vertical axis 
with high rotor speed N.sub.2 on the horizontal axis 12. The sloping curve 
14 represents the steady state blade tip to shroud clearance over a range 
16 of normal power operation at maximum normal power level 18, it can be 
seen that the blade tip to shroud clearance .delta. is equivalent to the 
minimum required clearance, .delta..sub.min 20 and increases as engine 
power is reduced within the operating range 16. As disclosed in the 
above-mentioned application, the reason for the increased excess clearance 
at part power operation is represented by dashed curves 22, 24 showing the 
transient departure of blade tip to shroud clearance from the steady state 
curve 14 in response to a step increase in engine power from part load to 
the maximum normal power 18. 
It has been observed that, while a gas turbine engine must be free to 
operate within the entire range 16 at all times, extended periods of 
engine operation within the normal power range 16 yet well below the 
maximum normal power 18, are known to occur regularly as the aircraft 
reaches cruising altitude and maintains such altitude, air speed, and 
engine power level setting for extended periods in route to a known 
destination. 
The present invention improves upon the schedule shown in FIG. 1 by 
reducing the excess clearance between the rotating blade tip and 
surrounding shroud during periods of engine operation at extended, steady 
state cruising conditions. This is best illustrated by the alternate 
clearance curve 26 shown beneath the normal curve 14 repeated from FIG. 1. 
Curve 26 is achieved by increased cooling air flow at part load operation 
as compared to the normal clearance curve 14 which, without other 
modification to engine operation, could result in a serious under 
clearance or interference between the blade tips and shroud following a 
step in engine power. 
This is illustrated by the hypothetical transient curve 28, representing 
the envelope of radial clearance decrease, which drops not only below the 
required minimum .delta..sub.min 20, but is also shown as falling below 
zero clearance, thereby indicating radial interference between the 
rotating blade tips and surrounding annular shroud. As noted in the 
above-referenced application, the excess clearance provided at part power 
operation by the clearance curve 14 and corresponding cooling air flow 
schedule is sized to accommodate this transient deviation as represented 
by curve 28. 
As will be appreciated by those skilled in the art, the magnitude of the 
transient deviation from the steady state curves 14, 26 is a function of 
the rate of change of engine power in response to a step change in demand. 
Hence, by reducing the response rate of the engine to a demanded increase 
in engine power, the magnitude of the departure from the steady state 
clearance curve 14, 26 may be reduced at the expense of engine response 
time. The present invention is based on the recognition that while 
unacceptable for the totality of the expected engine operating range, the 
limitation on the rate of power increase in response to a step change in 
demand may be acceptable within certain defined periods of aircraft and 
engine operation. 
As an example of such periods wherein the cruise mode of operation 
according to the present invention may be used, the engine operating range 
of a passenger aircraft will be considered. During takeoff and climb to 
altitude, the cooling air flow through the active clearance control 
portion of the engine or engines on the aircraft may be regulated to 
achieve the normal operating curve 14 as shown in FIG. 1. This curve 
permits timely response of the engine power to changes in demand as may be 
required to execute climb out, turning, etc. Once the aircraft has reached 
the desired cruising altitude, the method according to the present 
invention provides for the selecting of the "cruise mode" wherein the 
alternate cooling air flow schedule is implemented, resulting in the 
clearance response 26 as shown in FIG. 2. Upon entering the cruise mode of 
operation, a limitation is place on the rate of change of engine power in 
response to the pilot demand, thus resulting in the reduced transient 
deviation as represented by curve 32 in FIG. 2. 
By reducing the need for excess clearance as a result of the response 
limitation on engine power changes, the method according to the present 
invention permits the reduction in excess clearance between the blade tips 
and shroud thereby improving overall engine operating efficiency by 
reducing the amount of working fluid bypassing the blade rotor stages 
within the engine. As will be appreciated by those skilled in the art, the 
slower engine response time during such periods of operation is acceptable 
to operators and pilots as the very nature of cruising operation implies 
steady state, relatively unchanging engine power output. Actions by the 
aircraft pilot to change altitude, accommodate reduced fuel rate, or 
counteract headwinds, etc., and which require changes in engine power 
level can readily be accommodated in the cruise mode although response 
time has been somewhat increased. 
The cruise mode of operation is deselected according to the method of the 
present invention as the aircraft nears its final destination wherein it 
descends and begins landing maneuvers. Cooling air flow is again 
controlled responsive to the normal flow schedule resulting in the larger 
excess clearance at part load power shown by the curve 14. 
Selection of the reduced clearance, increased response time cruise mode of 
operation according to the present invention may be achieved by a variety 
of selective processes, including by not limited to pilot control, 
altitude sensing, interaction with aircraft course and position control 
system, etc. The overall criteria for selecting cruise mode is that the 
aircraft and engines should be reasonably expected to be entering a future 
period of extended, steady state operation wherein no immediate, quick 
response increase in engine power should be expected. Likewise, the 
deselection of cruise mode may follow a step change in engine power level 
demand outside of a preselected range or percentage thus indicating that 
the engine or aircraft has reached the end of the extended period of 
steady state operation.