Lean blow-out prediction in an industrial gas turbine combustor through a LES-based CFD analysis

摘要

The use of lean premixed flames to control combustion temperature and limit nitric oxides (NO_x) formationis a common practice in all the applications of land-based gas turbines irrespective of their sizeand power production capacity. Despite the capability of lean premixed combustion to nominally reduceNO_x emissions well below the most strict international regulations, such technology still requires improvementsto overcome the impact on whole engine operability due to possible onset of combustioninstabilities and flame lean blow-out (LBO). Both phenomena involve highly unsteady processes resultingin challenging numerical modeling. Particularly critical is the prediction of lean blow-out mechanismswhere the role of finite rate chemistry and turbulence cannot be easily simplified. The current studyaims at proving the capability of a CFD methodology based on Large Eddy Simulation (LES) and on anextended version of the Turbulent Flame Closure (TFC), to describe and properly catching the dynamicsof flame extinction at the leanest operating conditions in typical turbulent lean premixed flames adoptedin gas turbine combustors. The methodology is first validated on a laboratory test case where detailed experimentalresults are available. The extended TFC model well describes the LBO process observed in anatmospheric swirl stabilized burner investigated at the University of Cambridge. The proposed approachis then applied to the investigation of a real scale combustor operated with a swirl lean burner, representingthe standard device adopted by Baker-Hughes on the heavy-duty 16 MW class engine Nova LT16.An accelerated numerical procedure is used to trigger the LBO, considering both constant firing temperatureand constant pilot split conditions. An excellent agreement is observed with respect to the dataobtained on a full-annular test rig, confirming the validity of the developed LBO modeling strategy whichcan be used in design phase to improve the overall engine operability.