b'Power & Energy | Engineer InnovationDespite all that has been accomplished in making gas turbine technologyComputational domaincleaner and more efficient, the industry is facing several key challenges thatPressure transducer Igniterhave arisen from the changing way in which the technology is being Fueldeployed. Originally designed forFuelproducing base-load electricity, operating continuously at full power, gas turbines are increasingly required toDLE Burnerprovide a more dynamic-load capability. With aging nuclear plantsWater cooled providing most of the steady-stateemission probebase-load and increasing wind and solarOptical capacity providing a fluctuatingaccess Combustion chamberintermittent load, gas turbines are now necessary to even out the peaks andCombustion air Cooling airtroughs between other power sources. Put simply, gas turbines are what keepFigure 2: Experimental setup, showing extent of the computational model used for LES simulationsthe lights on when the sun isnt shining, and the wind isnt blowing. combustion systems operating close to While simple gas turbines are ideal forthe lean limit. Thermo-acoustic this role, they can be started up ininstabilities are highly undesirable minutes and ramped up and downoscillations that are generated by the quickly as load requirements change,interaction between the acoustic field the operating envelope of a modernand combustion processes. low NOx gas turbine may be limited by thermo-acoustic oscillations.The most serious thermo-acoustic effects are a consequence of premixing. Thermo-acoustic oscillations Older non-premixed systems are While the overall thermodynamicincredibly stable but have very high efficiency of a gas turbine (or anyNOx emissions, explains Dr Moll. If engine) increases with the temperaturethe flame temperature becomes too of combustion, NOx formation alsolow, the heat released is not high increases rapidly with combustionenough for the combustion to be stable, temperature. Since the 1980s, gasbecause its a high-speed flow that we turbine manufacturers have beenare trying to stabilize the flame in. This experimenting with dilution as aresults in the stability issues.method of controlling combustion temperature (and therefore NOxThese instabilities can significantly formation). Although early attemptsreduce combustor performance. If not used so-called Wet Low Emissionproperly controlled, the instabilities can methods involving the injection ofcause self-sustained large amplitude water or steam to control combustionoscillations that may cause additional temperatures, most gas turbineswear and tear, or even catastrophic manufactured today employ Dry Lowdestruction if they happen to excite the Emission (DLE) combustion systems, innatural frequencies of the turbine which air is premixed with thestructure. All modern gas turbines are combustion fuel so that combustionequipped with automatic safety control occurs in a lean-premixed regime. Onesystems that prevent this from of the key success factors whenhappening but shut-down of the operating at DLE conditions is achievingturbine means loss of power enough mixing between fuel and air. generation, so needs to be avoided. Although DLE systems are extremelyThe dangerous thing about thermo-efficient in reducing NOx formation,acoustic instabilities is that they contain they do present additional engineeringa huge amount of energy, said Dr challenges, many of which are relatedMoll. For example, an SGT-800 to thermo-acoustic instabilities whichgenerates 130MW of thermal power, so are more prominent in premixedif you generate an instability, you have 7'