Investigation of the splash water velocity field in the casing of a Pelton turbine with PIV

Since the first presentation of the Pelton turbine at the end of the last century this type of hydraulic machine has undergone a significant development in operating head and unit size. In contrast to Francis- and Kaplan turbines the theoretical knowledge however is still rather poor. The flow phenomena free jet flow, unsteady flow with free surface on the buckets and two phase flow in the casing make the theoretical analysis more difficult. In particular there are almost no publications concerning the casing and the interaction between casing and turbine. The casing however is an essential part of the turbine. It has to drain off the water coming out of the buckets without reaction upon the runner. The losses due to a bad design of the casing may have a very big influence to the efficiency of the whole scheme. So we may say that the casing indirectly participates in energy transformation. Nowadays in Europe most of the activities in hydro power are concentrated on uprating and refurbishing of old units. Due to the application of modern technologies the increase of energy may be obtained with modest modifications. One of the possible modifications is the replacement of the runner. This "simple" modification of a turbine may increase the efficiency substantial. In Pelton turbines the installation of a new runner of course changes the flow leaving the buckets. In general a modification of the casing will result in modifications of the power house and is rejected due to the costs. So for uprating and refurbishing Pelton turbines the knowledge of the flow phenomena in the casing it is very important too. Up to now the investigations of the flow in the casing of Pelton turbines is limited to the visual documentation of the splash water distribution. An important step in the direction of a fundamental analysis is the measurement of the velocities of the droplet travelling in the casing and on the wall of the casing. The knowledge of the velocity field is basis for the development and verification of theoretical models for the simulation of the flow in the casing. The aim has to be the measurement of the velocity field in the casing of a Pelton turbine. Comparing and analyzing the available measurement techniques the die Particle Image Velocimetry (PIV) seems to be the most promising one. The difficulty is to open the different areas of the flow field to visual observation. Experience indicates the subdivision of the project into two parts: Investigation of the flow leaving the buckets: The velocity field around the runner shall be determined. The installation of special parts in the casing will keep clear this area from splash water disturbing the visibility. Investigation on and near the walls of the casing: The influence of the casing on the splash water will be examined. The velocity field of the droplets travelling on the wall shall open a more fundamental analysis of the influence of the casing to the splash water distribution.