Tip clearance control method

Abstract

A method of controlling tip clearance of rotor blades from a surrounding casing in a gas turbine engine. The method comprising the steps of performing a first run of the gas turbine engine, shutting down the gas turbine engine and starting the gas turbine engine for a second run. The method further comprises measuring at least one performance parameter during or after the first engine run but before a time selected at which air within the gas turbine engine, soaked to the approximate temperature of one or more surrounding engine components, is substantially displaced as a consequence of the second run. The thermal condition of at least part of the engine in accordance with the measured performance parameters is then recorded. Finally the recorded thermal condition is retrieved and the temperature of the casing is controlled during at least part of the second run in a manner dependent on it.

1 . A method of controlling tip clearance of rotor blades from a surrounding casing in a gas turbine engine, the method comprising the steps:
 a) performing a first run of the gas turbine engine;   b) shutting down the gas turbine engine;   c) starting the gas turbine engine for a second run;   d) measuring at least one performance parameter during or after the first engine run but before a time selected at which air within the gas turbine engine, soaked to the approximate temperature of one or more surrounding engine components, is substantially displaced as a consequence of the second run;   e) recording the thermal condition of at least part of the engine in accordance with the measured performance parameters;   f) retrieving the recorded thermal condition and controlling the temperature of the casing during at least part of the second run in a manner dependent on it   
     
     
         2 . A method according to  claim 1  where the recorded thermal condition is used in control of the casing temperature to account for residual heat present in relevant parts of the gas turbine engine as a consequence of the first run. 
     
     
         3 . A method according to  claim 1  where the recorded thermal condition is accounted for in the generation of a thermal model following starting of the engine for the second run, which is then used in control of the casing temperature. 
     
     
         4 . A method according to  claim 1  where the thermal condition recorded is in accordance with performance parameters measured at starting of the gas turbine engine for the second run. 
     
     
         5 . A method according to  claim 1  where the performance parameters measured include the temperature of gas in a cavity around a disc from which extend the rotor blades. 
     
     
         6 . A method according to  claim 3  where the thermal condition recorded is in accordance with performance parameters measured at a particular time during the first engine run. 
     
     
         7 . A method according to  claim 6  further comprising recording a time stamp corresponding to the time of taking the performance parameters on which the recorded thermal condition is dependent and using the time stamp in addition to the recorded thermal condition in controlling the temperature of the casing during at least part of the second run. 
     
     
         8 . A method according to  claim 7  where at least one cooling curve, each indicative of approximate cooling over time for one/or more relevant parts of the engine following shut-down, are used in generation of the thermal model to account for the estimated temperature of the relevant parts of the engine at start-up. 
     
     
         9 . A method according to  claim 8  where the ambient temperature is recorded throughout the period between engine shut-down and start-up for the second run and used to adjust the at least one cooling curve to account for ambient temperature. 
     
     
         10 . A method according to  claim 9  where the time stamp is set at the time of taking of measurements used to define the recorded thermal condition. 
     
     
         11 . A tip clearance control system fore controlling tip clearance of rotor blades from a surrounding casing in a gas turbine engine, the system comprising at least one sensor each arranged to measure a performance parameter of the engine, a memory arranged to record a thermal condition of at least part of the engine dependent on the measured performance parameters and a processing system arranged to retrieve the recorded thermal condition and control the temperature of the casing in a manner dependent upon it during at least part of a second run, the tip clearance control system being further arranged to measure the performance parameters during or after a first engine run but before a time selected at which air within the gas turbine engine, soaked to the approximate temperature of one or more surrounding engine components. is substantially displaced as a consequence of a second run. 
     
     
         12 . A tip clearance control system according to  claim 11  where the memory is arranged to record the thermal condition of at least part of the engine in accordance with performance parameters measured by the sensors at starting of the gas turbine engine for the second run. 
     
     
         13 . A tip clearance control system according to  claim 11  where the non-volatile  memory is arranged to record the thermal condition of at least part of the engine in accordance with performance parameters measured by the sensors at a particular time prior to first run engine shut-down, and to further record a time stamp, the processing system being arranged to retrieve at engine second run start-up the recorded thermal condition and the time stamp from the non-volatile memory and control the temperature of the casing in a manner dependent on them. 
     
     
         14 . A tip clearance control system according to  claim 11  where a disc temperature sensor is arranged to supply the disc temperature to the processing system as one of the measured performance parameters. 
     
     
         15 . A gas turbine engine comprising the tip clearance control system of  claim 11 .