Curved Surface of Pelton Bucket Based on Differential Geometry
A 3-D free curved surface of a Pelton bucket was discretized into the 51×51 = 2,601 nodes of boundary fitted grid (BFG) for the CFD. The non-orthogonal curvilinear local frames were introduced with the natural basis vector g for the respective nodes of BFG. The natural basis vector g and its partial derivatives ∂g/∂P at each node were computed based on the differential geometry. As a result, the strong effect of the singular shape of the cutout, especially of its end-wall, on the Pelton bucket was numerically clarified and managed by the differential geometry. After the whole curved surface was approximated by the 50×50 = 2,500 local flat panels tangent to each node, the local surface area ΔS and the Gaussian curvature κ in each panel were investigated to assess the quality of the BFG.
Simulation of Unsteady Water Film Flow on Pelton Bucket
In order to simulate the complicated unsteady flow in Pelton bucket, it is necessary to apply the animated cartoon approach. In this paper, a free jet and the inner surface of a bucket is described by boundary fitted grid (BFG) with non-orthogonal curvilinear coordinates. The water flow is discretized in space and time for CFD (computational fluid dynamics). The moving grids of water film are successfully projected onto the bucket’s inner surface by a projection algorithm. The visualization result of the jet landing on bucket’s surface and the unsteady flow in the rotating buckets in 3D verifies the effectiveness of the proposed method.
EFFECT OF ENLARGED FREE JET ON ENERGY CONVERSION IN PELTON TURBINE
In Pelton turbines, when the water jet emerges in the air, its velocity decreases downstream and its diameter increases to meet the continuity. In this paper, a physical parameter, the enlarging rate of jet diameter kDj was introduced and measured experimentally. The measurement of enlarging rate was carried out in three model turbines with different distributing Y-branch and nozzle configurations under various operating conditions. The effect of jet enlarging rate on the unsteady water sheet flow in the rotating buckets has not been investigated so far. This paper also presents the comparative numerical analysis of the flow in buckets at different enlarging rates of jet. It is shown that the effect of jet enlargement is so large that the flow pattern in the bucket is drastically changed by the latter half of the frames. The jet with large enlarging rate also introduces worse energy behavior to the hydraulic performance of the Pelton turbine. Besides, the phenomenon of jet interference was also discussed by using the results of numerical simulation for one jet. It indicates that the jet with large enlarging rate takes more risk that jet interference may damage the performance of multi-nozzle Pelton turbine.