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Risultati finali
The numerical scheme for the generation of reduced chemical kinetics was significantly improved during the project. A new numerical scheme was implemented in order to increase numerical stability. This allows the generation of reduced schemes for critical regions of combustion, e.g., near flammability limits and high pressures.
A software interface for the implementation of reduced chemistry into CFD packages was developed for the use of the reduced chemical kinetics libraries for the implementation in various CFD packages.
A pre-mixed methane air test combustor has been developed. It can be operated in Equivalency ratios between 0.6 and 1.1(where 1.0 = stoichiometric) in a high-pressure vessel. The pressure range can be varied between ambient and 20bar. Air can be pre-heated before mixing with methane up to a temperature of 650K. The facility may be used to test sensors and closed loop control concepts under representative gas turbine combustion conditions.
A further result of this project was the development of a methodology for flame temperature and emissions sensor evaluation for testing solid-state gas sensors and spectroscopic temperature sensors. For the solid-state sensors a specific low-pressure device has been designed for extensive sensor calibration with synthetic as well as combustion gases. For temperature sensors an optical fibre was fitted to the combustor to allow systematic measurements.
Another development was that of concepts for gas turbine closed-loop control based on emissions and flame temperature measurements. Using the high-pressure facility and the sensors tested closed loop control concepts were studied. It was shown that oxygen and temperature sensors could be used as inputs for control. The controller was able to follow predefined step change demands in the equivalence ratio.
A database was calculated, which provides the chemical composition (no NO chemistry undertaken) and reaction rates as a function of three variables under lean gas turbine conditions for two specific pressure/ temperature inlet parameters. Better understanding of technology issues associated with temperature sensing in a staged combustion context-providing basis of future activities.
The method of measuring primary zone burn temperature by centre-axis line of sight sampling of electro-magnetic radiation has been established by this programme as being viable.
However, a range of issues have been exposed by the temperature sensing work including:
-Use of OH radiation species means that cold gas between the flame and sensor affects the results.
-Narrow band method is susceptible to temperature error due to contamination of the lens, and humidity as above. These are effects that would be common to all sensors in a redundant system.
-Broad band measurement would avoid the above problems but is inappropriate due to lack of sufficient flame opacity.
-The projected cost of the system does not give a direct cost reduction assuming a triplex redundant system. In the current world situation, loss of emissions control can equate to plant shut-down, hence a cheaper simplex system is not currently supportable.
These residual issues require evaluation to determine the most appropriate way forward prior to full development.
Auxitrol designed and manufactured an optical infrared sensor able to measure the temperature of the primary zone (pilot flame) on a real RB211 DLE gas turbine up to 1900K, 20 bars. This sensor is made of an optical head which is fitted on the top of the can, an infrared optical guide designed to be fitted on the engine, and a narrow band spectrometer which analyses the radiation coming from the combustion area. The wavelength and the spectral resolution were chosen by analysing whole infrared spectra from 1 to 4.5µm of wavelength measured by a FTIR spectrometer on a real RB211 DLE engine.
The result is a robust sensor giving a calibrated signal function only of the primary zone temperature and independent of other parameters such as secondary zone temperature, inlet air temperature and pressure, and background radiation from combustor casing and discharge nozzle.
The sensor measurements compared well with RR computed values, with an offset between them of about 50K due to the flame being cooler than average at the measurement point. The temperature was measured with an accuracy sufficient for a control loop process, as expected at the beginning of this project.
Auxitrol designed and manufactured a high temperature and high-pressure gas cell to validate radiation models of combustion species such as H2O and CO2 in conditions similar to the combustion chamber (1600 degrees Celsius, 20bars). This facility is made of a double enclosure cell with sapphire windows. This cell is used with a high temperature black body (1600 °C) to measure spectra in transmission.
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