Gas Turbine Transition Duct Gap Assessment for Unsymmetrical Thermal Boundary Conditions

Abstract

The transition duct (TD) in a gas turbine (GT) twin shaft variant provides an aerodynamic coupling between its high-pressure gas generator module and low-pressure power module. Since the TD defines the flow passage, it interfaces with the high-pressure rotor and shroud at the forward end and the low-pressure rotor and stator at the aft end. Normally, the GT twin shaft variant is equipped with part-load capability. To fulfill this need and to comply with the emission norms, only desired number of burners, adjacent to each other, are used to burn fuel. Using some of the burners during the GT operation is referred to as staging. The GT operation under staging conditions result in non-uniform temperature distribution in the angular locations at any axial position and thus non-uniform thermal growth in the radial and axial directions. This non-uniform thermal growth in radial and axial direction leads to the interface definition very challenging. During the staging operation, the rotor parts experiences uniform radial and axial growth at all the angular locations. Whereas the interfacing stator parts experience temperature distribution like that of the TD and results in non-uniform thermal growth in the radial and axial directions. Appropriate interface definition is vital for efficient operation of the GT. Any interference condition of the TD with rotating parts result in rubbing and with stationary parts result in thermal binding, impacting the GT normal operation. Any generous gap adversely impacts the GT performance due to consumption of more cooling medium. Thus, an assembly gap which results in no interference and consumption of less cooling medium throughout the staging operation is considered as optimum assembly gap. Thorough gapping assessment is performed considering all the transient time points to ensure that the gap values are set optimally. This paper is intended to describe the steps followed in assessing the anticipated interference and gap situations at various interfaces.