THERMOCOUPLES IN GAS TURBINES

A complex engine like a gas turbine needs to be thoroughly instrumented in order to be safely and correctly operated: the most important parameter to be monitored is definitely temperature.

The operation of a two shaft jet engine, schematically shown in Figure8, can be simply described with the following steps:

• air is aspirated from the atmosphere (station 0) to the low pressure compressor inlet (station 2) and compressed till the low pressure compressor exit (station 2.5); an increase in air temperature and pressure results;
• air is compressed from the high presssure compressor inlet (station 2.5) to the high pressure compressor exit (station 3); an increase in air temperature and pressure results;
• in the combustor compressed air is mixed with fuel and combustion takes place: the gas mixture exits the combustor (station 4) at higher temperature than at the combustor inlet (station 3) and with almost the same pressure;
• combustion gases are expanded in the high pressure turbine from station 4 to station 4.5 with a reduction in pressure and temperature: the high pressure turbine drives the high pressure compressor as they share the same shaft;
• combustion gases are expanded in the low pressure turbine from station 4.5 to station 5 with a reduction in pressure and temperature: the low pressure turbine drives the low pressure compressor as they share the same shaft; part of the power produced in the turbine is used to drive the compressor and part is converted in useful work, that is thrust in the jet engine;
• gases are then released to the atmosphere through a diffuser (from station 5 to station 8).

Figure8: Schematic drawing of a two shafts gas turbine

The following stations are commonly instrumented with thermocouples:

• station 2
• station 2.5
• station 3
• station 4
• station 4.5
• station 5

and usually each station has at least 8 thermocouples at different angular locations. All of these temperature measurements aregas temperature measurement.

The main reasons to monitor continuously the gas turbine temperature are:

• performance evaluation: the knowledge of inlet and exit temperatures allows performance engineers to calculate the efficiency of compressors and turbines;
• engine control: the maximum power from the jet engines is not always required during the flight of an airplane. Maximum power is required at take off, but lower power level are required for instance during cruise. The control of the jet engine through all the different operational conditions is a complex engineering problem where temperature monitoring at the different stations plays a major role;
• health monitoring of high temperature components: a temperature increase at the turbine inlet (T₄) would increase the efficiency of the engine and, as a result, reduce the fuel consumption at the same power or increase the available power with the same fuel consumption. However, increasing the temperature T₄ is not easy: components already face very demanding temperatures at the high pressure compressor exit and the high pressure turbine inlet and serious components damage and failure can occur for the turbine blades and last stages of the high pressure compressor blades if the temperature is above the capability of the used materials. Temperature limits are implemented in the control of the engine to avoid temperatures above the melting points of alloys and to have limited creep deformation for rotating blades. Furthermore the temperature history of the components can be used to estimate their residual life.

The acquisition of the temperature measurement from a thermocouple immersed in a flowing gas through the walls of the gas duct is not trivial. In fact the temperature sensed at the junction of the thermocouple, T_J, is different from the total temperature of the gas, T_T, which is the useful thermodynamic parameter for the engine. T_J is a result of the thermal equilibrium between the thermocouple and the environment around the thermocouple. In particular heat transfer occurs:

• through conduction along the wires and the sheath of the thermocouple,
• through radiation to/from the walls and the blades/vanes surfaces,
• through convection at the boundary layer around the thermocouple.

Conduction and radiation give rise to two measurement errors called conduction error and radiation error respectively.

Furthermore, obtaining the total temperature of a flowing gas is technically difficult. This would require to stop the gas adiabatically in order to convert the kinetic energy of the flowing gas in a temperature increase: so the static temperature of the flowing gas, T_S, would be converted in the total temperature T_T.

The gas flowing around a thermocouple immersed in the gas is slowed down, but not stopped and not adiabatically: for this reason a measurement error arises, called velocity error.

The recovery factor α , defined as

Equation3

takes into account conduction error, radiation error and velocity error.

A good thermocouple design is achieved if the recovery factor is very close to 1.

A shield around the thermocouple, as shown in Figure9, is a common measure to increase the recovery factor: in fact it reduces the radiation and velocity errors.

Figure9: Schematic diagram showing a shield around a thermocouple.

Furthermore, it should be mentioned that the thermocouple always lags behind the real gas temperature during transient: the thermocouple has its own time response, which can be improved through careful design.