Etude aérodynamique sur maquette d'un étage terminal de turbine à vapeur, dans les régimes partiels de fonctionnement

Aéro-dynamique study of a terminal stage of steam turbine using a scale-model, in partial load operating conditions

¹ Alsthom-Atlantique Belfort
² C.E.T.I.M. Senlis

Abstract

It is known that in turbo-generator sets rotating at constant speed, if the initial stage is of the "partial injection" type, it operates under different conditions from the terminal stage, the mismatching increasing as one moves further away from optimum load conditions. At a first approximation, the diagrams portraying the speed of all other stages are maintained. The only thermodynamic variable is the specific mass of the fluid, which decreases in proportion to pressure, i.e. virtually in proportion to the load of the turbine. The question of partial injection is presently under study at the CETIM. Our purpose here is to make an aerodynamic study of a terminal stage over the whole range of variation of volume flow QV at the exhaust, or rather : (QV)* =QV/(QV)opt a variable which depicts the rate of kinetic mismatching well. The increasing use of cooling towers associated with the growing scarcity of traditional cold sources induces a wide range of condensation pressure Pc over the year for a terminal turbine stage. The maximum to minimum Pc ratio can thus reach 5 so that for quasi-constant power of the set, the volume flow at the exhaust can change over a very great range (e.g. from 0.2 to 1). In general, a terminal stage is required to operate "without matching" when the power of the set and/or condensation pressure differ from optimum operating conditions. Thus, for an identical volume flow (Q V) : "at the exhaust, the power consumed or transferred by the stage varies between two limit values, characterised either by the volume flow QV or by the change in condensation pressure. Several experiments have, of course, a1ready been performed on turbines in operation, in particular in France, by EDF and manufacturers, but it appeared to be of interest to use a scale-model on which measurements an visual examination are particularly easy in order to understand clearly the changes in flow conditions as a function of volume flow. A series of tests has thus been performed for this purpose using a 1/8 th scale-model of a terminal stage used in industry (useful vane legnth : 850 mm) at 2400 Rpm. The sections of the blade profiles can evolve considerably with the hub ratio being of the order of Z and the change in the degree of reaction at nominal point increasing from a few % at the base to approximately 70 % at the top of the blade. The compressibility effects of the real turbine are of course not reproduced here. By contrast, the trend of three-dimensional flow as a function of volume flow is certainly quite similar to the real case. An analytical approach has been developed to study the operation of a stage over the whole range of volume flow at turbine exhaust.