Main Annulus Gas Path Interactions - Turbine Stator Well Heat Transfer

This paper summarises the work of a 5-year research programme into the heat transfer within cavities adjacent to the main annulus of a gas turbine. The work has been a collaboration between several gas turbine manufacturers, also involving a number of universities working together. The principal objective of the study has been to develop and validate computer modelling methods of the cooling flow distribution and heat transfer management, in the environs of multi-stage turbine disc rims and blade fixings, with a view to maintaining component and sub-system integrity, whilst achieving optimum engine performance and minimising emissions. A fully coupled analysis capability has been developed using combinations of commercially available and in-house computational fluid dynamics (CFD) and finite element (FE) thermo-mechanical modelling codes. The main objective of the methodology is to help decide on optimum cooling configurations for disc temperature, stress and life considerations. The new capability also gives us an effective means of validating the method by direct use of disc temperature measurements, where otherwise, additional and difficult to obtain parameters, such as reliable heat flux measurements, would be considered necessary for validation of the use of CFD for convective heat transfer. A two-stage turbine test rig has been developed and improved to provide good quality thermal boundary condition data with which to validate the analysis methods. A cooling flow optimisation study has also been performed to support a re-design of the turbine stator well cavity, to maximise the effectiveness of cooling air supplied to the disc rim region. The benefits of this design change have also been demonstrated on the rig. A brief description of the test rig facility will be provided together with some insights into the successful completion of the test programme. Comparisons will be provided of disc rim cooling performance, for a range of cooling flows and geometry configurations. The new elements of this work are the presentation of additional test data and validation of the automatically coupled analysis method applied to a partially cooled stator well cavity, (i.e. including some local gas ingestion); also the extension of the cavity cooling design optimisation study to other new geometries. INTRODUCTION The requirement for ever more efficient gas turbine engines is leading to increased gas path temperatures, creating increasingly hostile environmental conditions for the adjacent turbomachinery and support structures. Cooling air systems are designed to protect vulnerable components from the hot gas that would otherwise be entrained into the cavities communicating with the main annulus, through the inevitable gaps between rotating and static parts. These cooling flows are bled from the compressor stages and reduce the engine efficiency as they can represent around 20% of the total main gas path flow. These performance penalties manifest themselves in two ways, i.e. having a direct impact on thermodynamic cycle performance, resulting from imperfect work extraction in the turbines, and in the spoiling effect of the efflux at the point where it re-enters the turbine main annulus flow, causing a reduction in stage efficiency. It is desirable therefore to minimise these cooling flows, to levels consistent with maintaining the optimum component lives and the mechanical integrity of the engine. The various cooling air and gas flows involved are illustrated for a typical multi-stage turbine, in Figure 1. Figure 1 Typical turbine stator well Cooling air Hot gas ingestion Cooling air Cooling air Hot gas ingestion