Seite - 1383 - in Book of Full Papers - Symposium Hydro Engineering

The suction effect at the point of confluence, or the point where the ejector exhaust water with higher speed mixes with the slower turbine flow, can of course only take place if the top side of the draft tube is not too severely backed up. The influence of the draft tube outlet top level is very clearly represented in the results of the model experiment and thus represents a significant boundary condition. The goal should therefore be to determine the underwater conditions very accurately, so that on the one hand during normal to full load operation no risk of air intake to the turbine occurs, but on the other hand water backup remains reduced during ejector operation. The great importance of the way in which the ejector water and the head race water are brought together is to be seen definitively in the lateral contraction effect we observed in modelling exercise. Increased efficiency from the ejector effect can only be optimized when the water flow from the ejector ramp into the tailrace enables maximum transfer of the energy from the ejector water to the head race water. The design of the mixing zone which we devised, tested and submitted for patent in this project, ensures an efficient confluence of the two flows and prevents the lateral contraction phenomenon we observed in the test series. Fig. 26 Pictures with flat bed level in the mixing area in comparison to deepened bed level, show the importance of avoiding a constricting effect, which reduces the positive ejector effect considerably [3]. 1383