Preliminary investigation into the effects of a compressor rim purge flow on OGV/pre-diffuser and combustion system aerodynamics

As aero gas turbine designs strive for ever greater efficiencies the trend is for engine overall pressure ratios to rise. Although this provides greater thermal efficiency it means that cycle temperatures also increase. Traditionally turbines have been the focus of cooling schemes to enable them to survive high temperatures. However, it is envisaged that the compressor delivery air will soon reach temperatures which mean they may require similar cooling strategies to the turbine. One such concept is akin to that of a turbine 'rim purge flow which ensures that hot, mainstream flow does not get ingested into rotor cavities. However, the main gas path in compressors is generally more aerodynamically sensitive than in turbines and introduction of a purge flow may be more penalizing. It is important to understand the impact such a flow may have on the primary gas path flow of a compressor and the downstream combustion system aerodynamics. This paper presents a preliminary investigation into the effects of a purge flow which enters the main gas path immediately upstream of the high pressure compressor outlet guide vane (OGV) row. Initial, simplified, CFD predictions clearly demonstrated the potential of the purge flow to negatively affect the OGV/pre-diffuser and alter the inlet conditions to the combustion system. Consequently, an experimental assessment was carried out using an existing fully annular, isothermal test facility which incorporated a bespoke 1.5 stage axial compressor, engine relevant outlet guide vanes, pre-diffuser and downstream combustor geometry. Using CFD to guide the process the test rig was modified to allow a metered airflow to be introduced upstream of the outlet guide vanes. Importantly the flow was directed up the face of the rotor such that it picked up a representative swirl component prior to injection into the main gas path. The experimental data confirmed the CFD results and importantly demonstrated that the degradation in the combustor inlet flow resulted in an increased combustion system loss. At the proposed purge flow rate, equal to ∼1% of the mainstream flow, these effects were small with the system loss increasing by ∼4%. However, at higher purge flow rates (up to 3%) these effects became notable and the OGV/pre-diffuser flow degraded significantly with a resultant increase in the combustion system loss of ∼13%.