Influence of Purge Flow Swirl at Exit to the High-Pressure Compressor on OGV/Pre-Diffuser and Combustion System Aerodynamics

A Duncan Walker,Peter A Beecroft,Bharat Koli

doi:10.1115/1.4043781

A Duncan Walker, Peter A Beecroft + Show 1 more

Open Access

https://doi.org/10.1115/1.4043781

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Abstract

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. One potential solution to managing the increasing temperatures is to employ a cooled cooling air system. In such a system, a purge flow into the main gas path downstream of the compressor will be required to prevent hot gas being ingested into the rotor drive cone cavity. However, the main gas path in compressors is aerodynamically sensitive and it is important to understand, and mitigate, the impact such a flow may have on the compressor outlet guide vanes, pre-diffuser, and the downstream combustion system aerodynamics. Initial computational fluid dynamics (CFD) predictions demonstrated the potential of the purge flow to negatively affect the outlet guide vanes and alter the inlet conditions to the combustion system. The purge flow modified the incidence onto the outlet guide vane, at the hub, such that the secondary flows increased in magnitude. An experimental assessment carried out using an existing fully annular, isothermal test facility 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 outlet guide vane and pre-diffuser flow degraded significantly with a resultant increase in the combustion system loss of ∼13%. To mitigate these effects, CFD was used to examine the effect of varying the purge flow swirl fraction in order to better align the flow at the hub of the outlet guide vane. With a swirl fraction of 0.65 (x rotor speed), the secondary flows were reduced below that of the datum case (with no purge flow). Experimental data showed good agreement with the predicted flow topology and performance trends but the measured data showed smaller absolute changes. Differences in system loss were measured with savings of around 10% at the turbine feed ports for a mass flow ratio of 1% and a swirl fraction of 0.65.

Highlights

With the projected growth of air traffic, it is essential that new technologies are introduced that will mitigate the negative impact this will have on the environment
To modify the purge flow inlet conditions the data shown in Fig. 7 were scaled to give differing mass flow ratios (MFR) and swirl fraction, SF, which is defined as the purge flow mean swirl velocity divided by the rotor blade speed (W/Ublade)
The data are presented normalized by the datum loss coefficient and in this way quickly highlight if the system performance improves or degrades with purge flow rate and swirl fraction

Summary

Introduction

With the projected growth of air traffic, it is essential that new technologies are introduced that will mitigate the negative impact this will have on the environment. To modify the purge flow inlet conditions the data shown in Fig. 7 were scaled to give differing MFR and swirl fraction, SF, which is defined as the purge flow mean swirl velocity divided by the rotor blade speed (W/Ublade). At low swirl fraction the loss core is worse than the datum (no purge flow), degrading the inner wall flow and producing a more out board biased flow at pre‐diffuser exit with an increased total pressure loss.

Results

Conclusion