Ventilation Flow Field Characteristics of a Hydro-Generator Model- An experimental and Numerical Study

Doktorsavhandling, 2017

Hydro-generators are complex machines used to convert the mechanical energy of the water turbine into electrical energy. Electromagnetic and mechanical losses accompany this energy conversion process which will cause heat generation and temperature increase. Cooling systems are needed to remove this excess heat from hydro-generators. Cooling system should control temperature increase and its temporal and spatial uniformity. An efficient cooling and ventilation must be considered during the electro-mechanical design of a generator. Having a complete picture of the losses, the ventilation flow field characteristics and the temperatures inside a generator is essential for an optimal design of cooling system for it. \par The present work provides experimental and numerical tools essential for investigating ventilation flow attributes inside hydro-generators and also a comprehensive studies of flow based on these tools. An extensive knowledge of flow distribution inside the stator ventilation channels in different operational conditions and geometrical configurations are achieved. The obtained knowledge can be used for improvement in design of generator cooling system.  A hydro-generator model was designed and manufactured taking into consideration the needs of both the experimental and numerical methodologies. An inlet section is designed to deliver a uniform flow distribution into the machine and also to facilitate a direct and accurate measurement of the inlet flow rate. A CFD-based procedure is utilized for its design. The intake flow can either be supplied by a specifically designed radial fan connected to the rotor and co-rotating with that, or by an external centrifugal fan. Stators with three different ventilation channel geometrical configurations are used. Total pressure rake, 5-hole probe and hot-wire anemometer are used for taking measurements at stator ventilation channels outlets and generator inlet. Particle image velocimetry is carried out to reveal the flow field inside the ventilation channels.  The computational fluid dynamics simulations are performed using the FOAM-extend CFD toolbox. A block-structured mesh is generated using the ANSYS ICEM CFD mesh generator. The steady-state multiple frames of reference method is used for the numerical simulations. The frozen rotor and mixing plane approaches are used to handle the rotor-stator interaction. The flow is assumed axisymmetric, so just a section of generator model is simulated numerically. Periodic boundary conditions are imposed at the two sides of the computational section. Turbulences in the flow are modeled with Reynolds-averaged Navier-Stokes (RANS) formulation. The flow and pressure field in the generator model are analyzed in detail. The numerical and experimental results show a good agreement, which indicates the applicability of both methods. Another aspect of hydro-generator ventilation which is important for designers is the convective heat transfer coefficients. An alternative way to indirectly obtain the convective heat transfer coefficients is to conduct mass transfer experiments such as the naphthalene sublimation technique. In the present work this technique is evaluated for analysis of the local heat transfer distribution when a circular air jet impinges normal on a flat surface. The local sublimation rate from the naphthalene surface subjected to the air jet is measured and reduced to the heat transfer that would occur on the surface under analogous thermal conditions. The indirectly obtained local heat transfer distributions and its local Nusselt numbers are compared to the results of numerical simulations and other experiments. The results show that the naphthalene sublimation technique can be used to accurately estimate the local heat transfer coefficients.