Abstract
This paper presents a detailed study of the impact of manufacturing residual riblets at the rotor hub surface of a radial inflow turbine on the flow within the rotor passages and their contribution to drag reduction. Numerical analysis has been used to study the effects of those features at design point conditions. Riblets with different height and spacing have been examined to determine the riblet geometry where the maximum drag reduction is achieved. The relative height of the riblets to rotor inlet blade height was introduced to generalise the results. At the end of this study the results were compared with the available data in literature. It was found that the introduction of riblets could reduce the wall shear stress at the hub surface, while they contribute to increasing the streamwise vorticity within the rotor passage. For the geometries tested, the minimum drag was achieved using riblets with relative height hrel = 2.5% equivalent to 19.3 wall units. The results revealed that the spacing between riblets have a minor effect on their performance, this is due to the size of the streamwise vortex above the hub surface which will be discussed in this work.
Highlights
Flow within the radial turbine rotors is highly three-dimensional and is combined with strong secondary flows, which become more complicated near the passages walls
The results reveal that the spacing between riblets has a minor effect on the wall shear stress reduction and the key parameter that affects the riblets performance is their height
It was found that residual riblets reduce the cross-stream motion of the low momentum fluids leading it to move from pressure to suction side of the rotor passage and separate the streamwise vortex from interaction with hub surface
Summary
Flow within the radial turbine rotors is highly three-dimensional and is combined with strong secondary flows, which become more complicated near the passages walls. Riblets are surface structures arranged in the streamwise direction to help in reducing wall shear stresses. They can have different shapes and arrangements as shown in figure 1. Walsh [3] studied the effect of different riblet shapes on drag reduction over flat plate surface, where 8% drag reduction was achieved using V-shaped riblets. Feng et al [6] conducted an experiment to test the effectiveness of riblets in drag reduction where they achieved 10% drag reduction by adding the riblets to the pressure surface of an axial compressor cascade. Using five grids of increasing refinement, the absolute vorticity, mass flow rate, flow velocities and angels at rotor inlet and outlet were calculated and compared. The final mesh for both rotors is shown in figure 5
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