S-Bend Transition Duct with Mixing Tube
Overview
This project looked at a specific type of diffusing transition duct that takes a swirling annular flow and expells it from a long slot exit which is offset from the axis of annulus. The testing was done in cold flow at large scale (mass flow about 2 kg/s). This duct was also studied as a primary nozzle for an air-air ejector.
The focus of the research included:
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flow distortion as a function of s-bend offset
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back pressure in annulus
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effect of swirl
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pumping with mixing tube
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comparison to CFD predictions
This project has just been completed and the results will be presented at the ASME Gas Turbine conferences in 2004 and 2005.
All flow measurements were taken with 3 and 7 hole pressure probes. The project considered two duct offsets and a single exit aspect ratio. Experiments were compared to CFD simulations using a commerical CFD package.
The results showed that the duct performed well in terms of back pressure and pumping ability. Flow simulations were generally in good agreement with experiments. The CFD was able to predict the large scale vortices generated by the offset and the pumping in teh ejecotr. The CFD tended to underpredict the back pressure in the annulus.
The following figure shows one of the s-bend ducts studied.
The following figure shows the duct mated with a simple constant area mixing tube for the ejector study. The x-y traversing rig can be seen in the bottom right of the figure.
The following figure shows a solid model of the duct mated with a mixing tube and entraining diffuser.
The figure below shows a typical CFD grid from one of the simulations of the duct driving a constant area mixing tube. The inlet and exit plenums are added so that atmospheric pressure boundary conditions can be allied far away form teh mixing tube exit and standaoff regions.
The following figures show measured exit total pressure profiles including in plane velocity vectors for the exit of the duct and mixing tube. As can be seen the offset generates a series of counter-rotating vortices in the duct.
Last updated: Oct, 2004