Study On Internal Flow Simulation And Performance Of Pelton Turbine

Many tests of Pelton turbine illustrated that the efficiency of prototype is lower than that of model turbine, especially under off-design heads and larger discharge. This characteristic of Pelton turbine is apt to take the risk to predict the hydraulic performance of prototype from model. So far, the current IEC codes for the method of model tests specify that Pelton turbines have no scale effect between the model and its prototype, because the complicated flow mechanism of Pelton turbine has not been fully understood. Moreover, the scale effects especially the negative scale effect also accentuate the complexity of the research for Pelton turbine. This paper is dedicated to the experimental and numerical research of detailed internal flow of Pelton turbine and it performance of energy conversion to specify the characters of the scale effect and the formulas of energy conversion. In the beginning of this paper, the current status and achievements in the research field of Pelton turbine are introduced respectively by means of pipe flow, free jet and bucket flow. Based on the discussion of basic performance and its relation with various efficiencies, the paper also clarifies the cause of the negative effect, its functional mechanism and the interaction between jet interference and hydraulic deviation in Pelton turbine. The new ideas for formula of energy conversion are also put forwards accordingly. The internal flow of Pelton turbine is consisted of the one phase steady pipe flow and unsteady free surface flow such as free jet and bucket flow. The hydraulic geometry of nozzle pipe has the direct influence to the energy consistency of free jet, consequently the whole performance of the turbine. In order to reveal its fluid and energy features, this paper numerically analyzes the flow in the nozzle pipe and the effect of rod-holder on the flow distribution at the nozzle throat. Besides, the free jet flow is also simulated by CFD method. The flow characters at nozzle exit and the cause of semi-free jet are discussed as well. According to the computed result, the position/diameter of contraction jet and the enlargement rate of jet diameter are predicted under different needle strokes. In Pelton turbine, the buckets periodically penetrate into the free jet with constant rotating speed. Therefore the inlet parameters such as discharge, location, direction and velocity are also changed with the rotating of bucket. In this paper, An animated cartoon method is applied to analyze unsteady internal flow of Pelton bucket. This method shows how to discretize the real unsteady free sheet flow by a sequence of gradual time shifting frames. In every frames, the distribution of the free water sheet flow in buckets is computed Lagrangially by integrating the various acceleration on the flow particles, and the internal flow is analyzed. This paper improves the algorithm of the animated cartoon method and implements the numerical simulation of bucket flow for different bucket type to predict the flow feature and their hydraulic performance. The negative scale effect in Pelton turbine results in the energy decay of free jet and the discharge increase of spilt flow from bucket cutout. Thus this paper also discusses the energy conversion characteristic in the aspect of the decay of free jet. Firstly the parameter to evaluate the decay of free jet, enlargement rate of jet diameters is introduced. The enlargement rates for various jet under different opening are also experimentally measured through photographic analysis. This paper also presents the comparative numerical analysis of the flow in buckets under the different enlarging rate of jet to clear the effect of enragement rate on the bucket flow. Furthermore, the relation between jet interference and enragement rate is also specified numerically. This paper also involves the model test of Pelton turbine. In the model test, the energy test, characteristic of runaway test and the flow pattern observation are carried out. The model tests are also studied to specify the effect configuration of runner on the variation of efficiency of the whole turbine. The results of model test indicate that the runner has advanced hydraulic performance. According to the experimental results, the runner performance is numerically predicted, and the flow pattern inside the bucket is simulated. The comparative result with model test proves the correction of the computational method.