*Abstract*

Tandem Blades have often been under investigation, experimentally as well as numerically, but most studies have been about tandem blade stators without tip gap. This work analyzes the influence of a tip gap on the flow field of a tandem blade for core compressors.

Experiments have been conducted in a high-speed linear compressor cascade on a tandem and a reference geometry. The flow is analyzed using five-hole probe measurements in the wake of the blades and oil flow visualization to show the near surface stream lines. First, the results for design conditions are compared to measurements on corresponding blades without tip gap, establishing the change in the flow structure due to the presence of a tip gap. A first image of the flow field structure is derived based on what is already known for conventional blades under the influence of a tip gap and the here shown results. Furthermore, an incidence variation in 1-degree steps has been carried out for both reference and tandem blades, the results - similarities and differences for tandem and reference - are being discussed. The off-design case of the tandem blade for 5 degree incidence will be discussed in detail and the change in the flow topology, which suggest the occurring of a tip clearance vortex breakdown on the tandem front blade will be discussed.

 

*Introduction (excerpt)*

Tandem blades are a possible means to achieve extremely high blade loading due to very high turning, reducing the number of necessary compressor stage to achieve a set overall pressure ratio. Previous works have shown that modern conventional blade profiles may achieve these high turnings, too, at similar loss values - when operating in design conditions. However, their losses rise strongly for off-design conditions, while for tandem configurations the rise in losses is much more shallow. Another possible advantage of tandem blades lies at moderate turning levels but significantly higher relative pitch ratios. The demanded flow turning is achieved with a smaller number of blades, thus reducing the blade count within one row.

 

Tandem blades are marked by a complex secondary flow field which has been thoroughly examined for conventional blades. The main components (but not all) are the passage vortex, the horseshoe vortex and - in case of a tip clearance - a tip gap vortex. For tandem blades some of those like the horseshoe vortex are evident in double quantity for one is forming each on the front blade leading edge and the rear blade leading edge. In combination with the flow through the tandem slot (between front and rear blade, ''gap'' will be reserved here for the clearance between blade and side wall) this creates a complex secondary flow field with intricate interactions.

 

*Experimental setup (excerpt)*

The profiles for tandem and reference blades have been designed as controlled diffusion airfoils by Rolls-Royce Deutschland. The design goal was to achieve a tandem profile that has the same turning as the reference blade and then enlarging the tandem pitch ratio until the tandem working range matches that of the reference blade.

In an earlier project, the profile geometries have been studied in a two side fixed configuration. Oil flow visualizations from this project will be used later in this paper for comparison.

 

*Windtunnel and Cascade (excerpt)*

For the experiments an open high-speed wind tunnel is used. The inlet Mach number is Ma = 0.6 at a Reynolds number of 7.5 \* 10\^5. The cascade comprises of nine reference blades or respectively seven tandem blades. The blades in this setup are cantilevered and have a fixed gap of 1\\%. Measurements are carried out in the wake of the center blade. The incidence in the cascade can be varied between 0degree and 10degree.

For the oil flow visualizations, the middle-blades (three for tandem, four for reference) can be extracted from the cascade, as well as the tip clearance end-wall piece of a corresponding size.

 


An Analysis of the Secondary Flow Around a Tandem Blade Under the Presence of a Tip Gap in a High-Speed Linear Compressor Cascade

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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