Characteristics of the Convective Heat Transfer Coefficient at the End Winding of a Hydro Generator

Large hydro generators convert the mechanical energy of water turbines into electrical energy. This process is not free of heat losses which have to be dissipated from the machine. For this, different ventilation methods can be applied for an air cooled hydro generator. These methods have not changed very much in the last decades, but there has been a strong enhancement of the calculation and design approaches. State of the art computation methods are thermal and flow networks in combination with 3-dimensional computational fluid dynamics (CFD). The focus of this paper is a CFD analysis of the end winding region as basis for development of correlations between the convective wall heat transfer coefficient (WHTC) and speed and flow rate parameters. These correlations are used as boundary conditions for thermal networks. Furthermore there is also a focus on the influence and sensitivity of the numerical settings on the correlations. This work deals with a reduced numerical model which is designed to calculate a hydro generator fast and accurately by using a steady-state simulation with the mixing plane method. The mixing plane approach is predestined for this kind of design. Considering only one slot section of the end winding bars and stator ducts, the advantages of the stage model can take its full effect. The main advantages are the use of large pitch ratios, periodicities in circumferential direction and the averaging of the fluxes on bands.