The Siemens SGT-800 combustor has been improved using acoustic damping resulting in significantly reduced levels of thermo-acoustic pulsations for all loads. Therefore the pilot fuel ratio can be reduced leading to reduced NOx emissions, which has been verified in a number of field engines. The acoustic attenuation is achieved by introducing a soft wall located on the impingement cooled combustor wall, where a number of parameters are controlling the acoustical design. The functionality and the location of the soft wall is selected to: (1) maximize the damping effect according to acoustic calculations and measured frequencies, (2) minimize the main flame NOx emission effect due to the reduced burner air flow by the bias flow entering into the combustion primary zone, (3) minimize the effect on the turbine inlet temperature profile and (4) maximize cooling and life improvement. All this is confirmed by calculations and engine tests. The introduction of a soft wall in an existing combustor design leads to changes of the cooling layout due to the convection cooled design, since half of the local cooling flow exits through the damping holes. In addition, the local cooling is significantly improved by the reduced heat load due to the bias flow, the cooling effect of the holes and the improved efficiency of the impingement cooling, due to reduced cross flow and optimum hole positioning. Therefore an adjusted impingement plate is introduced to balance these effects in order to improve the total cooling efficiency, which is verified by CFD. Conjugate heat transfer (CHT) has been used to study the interaction between the bias jets, the combustor cross flow and the nearby geometry including the TBC layer to estimate the thermal stress, and hence the risk of high local loading for the TBC and the holes. RANS predicts significantly higher local temperature gradients compared to LES. No issues around the soft wall holes have been detected in any engine, as predicted by the LES results. Due to very long simulation times for LES CHT due to slow temperature response time of the wall, an investigation of the results using different Cp for the solid materials are presented, which can be used to significantly speed up transient CHT.

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