b'Engineer Innovation | Power & Energyan almost infinite amount of power tosuch as the thermo-acoustic behavior of drive it, operating at thousands ofa pre-mixed gas turbine combustor.cycles per second. Uncontrolled they have the potential to cause lots ofInstead Dr Moll and his colleagues damage adopted a more fundamental approach to simulating turbulence, using Although thermo-acoustic instabilitiesSimcenter STAR-CCM+. Large-eddy are well understood, and well managedsimulation (LES) is a transient technique in industrial gas turbines operating atin which the large scales of the close to full load, they can becometurbulence are solved explicitly, and the more problematic under partial loads,small-scale motions are modeled. or as we shall discuss later, for differentWhether a turbulent eddy is resolved or mixes of fuels. modeled depends on the local grid resolution, a consequence of which is Large Eddy simulation that LES calculations require fine Previously the only way of investigatingcomputational meshes, and small dynamic thermo-acoustics effects wastime-steps. In comparison to the on an actual engine on a test-bed, saidstationary RANS methods (which model Dr Moll. You have to run the engine,rather than resolve turbulence), LES is measure the dynamics, stop the enginecomputationally very expensive, and and then work out how you are goinguntil recently prohibitive at an industrial to damp out the oscillations and runscale:again to see if the problem is fixed.The barrier until recently has been Not only is this trial and errordown to computational cost - these are approach time-consuming andexpensive calculations, explained Dr expensive, but it also doesnt give muchMoll 10 years ago, LES simulations insight into the combustion instabilitieswere completely out of the question, its that ultimately lead to the thermo- only in the last 2-5 years that these acoustic vibration. Dr Moll and histypes of calculations have become colleagues turned to computationalfeasible on an industrial scale.fluid dynamics (CFD) in order to improve their understanding of howIn a recent ASME paper entitled: Large these thermo-acoustic oscillationsEddy Simulation and Experimental originate, aiming to damp them out inAnalysis of Combustion Dynamics in a future generations of gas turbines.Gas Turbine Burner Dr Moll and his colleagues validated an LES simulation Although CFD is used extensively in theof a gas turbine burner fitted to an design of gas turbine combustionatmospheric combustion rig. This systems, the majority of simulations useburner, from the Siemens SGT-800 gas Reynolds-averaged (RANS) turbulenceturbine, is a low NOx, partially premixed models. These model the overallburner, where preheat air temperature, influence of turbulence, instead offlame temperature, and pressure drop predicting the motion of individualacross the burner are identical to engine turbulent eddies. While this approachfull load conditions. The simulation works well for understanding most fluidcaptured all the geometric detail dynamics, it cannot reasonably predictpresent in the real combustion an inherently transient phenomenachamber, with a computational mesh ofthe simulations revealed the mechanisms behind the pressure fluctuations we observed on the rig,Dr Daniel Moll Combustion engineer with Siemens Energy8'