CFD of Air Flow in Hydro Power Generators for Convective Cooling, using OpenFOAM

Paper i proceeding, 2010

Hydroelectric power generation plays an important role in the total electric power generation in Sweden. Almost half of the electric power in Sweden is generated by hydro electric power plants and any modifications and improvements of the system would lead to a significant contribution to the total electric energy production. Two large sources of losses in the electric generators are the thermal and ventilation losses. The electric resistance in the generator system causes heat generation in windings and coils, which decreases the total efficiency of the stator in delivering power and causes material through thermal stresses in components. The generators are thus cooled by air flowing through the rotor and stator. This paper will focus on ventilation of axially air-cooled generators through the stator cooling channels in the stator wall. The name axial suggests that the air movement in the gap between rotor and stator is along the rotors axis of rotation. It is important to have a good understanding of the complicated flow field in the air passages in the generator in order to be able to design the cooling of the system. The air flow is driven by the rotational movement of the rotor and its appended poles, which will act as a fan, into the radially extended stator channels. The air thus cools the stator body and the stator coils. The flow in the generator here is in the present work modeled with a multiple-reference-frame method, which includes source terms for rotation. This ensures that Coriolis effects are taken into account in the simulations. The flow is simulated employing a low-Reynolds number turbulence model where the fluid flow is solved throughout the boundary layer. The computational domain is generated without inlets and outlets so that the volume rate of flow through the generator is determined by the solution, rather than by an imposed inlet volume flow rate. Some parts of the surrounding environment are thus included in the simulation to allow for recirculation of air in the domain. The establishment of the flow depends on the pressure distribution in the domain. The development of the pressure distribution has been validated for the laminar flow between two concentric circular cylinders where the inner cylinder rotates and the outer is at rest. The numerical results for both pressure and velocity distributions have been in agree with analytical results, which will also be presented in the paper.