Two main sources of high losses in small axial turbines are the tip leakage loss and the Reynolds number related loss. The extents of these losses are directly related to blade geometry. Due to limitation in manufacturing capabilities and the prohibitive cost of precision engineering often the manufactured blade is very different from the design or the ideal blade shape and as a result the component efficiency degrades. Reynolds number effect plays a very important role in the low efficiency of small axial turbines. The effect of low Reynolds number is essentially separation of the flow resulting in high losses. Unlike in large turbines, where the Reynolds number is above 10e5, depending on size the Reynolds number over which a small turbine operates can be as low as 10e4. At this range the flow is laminar over a large extent of the blade and is very susceptible to laminar separation resulting in high losses. Since due to the aerodynamics this is inevitable, the only resort for an engineer is to design a blade that will delay separation to a point which will either result in turbulent separation or a transitional-laminar separation resulting in a smaller separation bubble. The design of such blades requires in depth knowledge on the field of flow transitional methods. An existing flow transition model has been implemented with simple modifications for separated flow in an in-house CFD code. Once validated against available experimental data, a parametric study has been conducted, where the effects of Reynolds number, velocity ratio, the turbulence levels and the location of maximum loading has been tested on a simulated turbine flow. Two different velocity distributions were tested. The combined results provided an understanding of the aerodynamic behaviour of small low Reynolds number axial turbine blades and provided a basis of better blade design.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:490816 |
| Date | January 2007 |
| Creators | Kazi, Sheila K. |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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