Simulation of airflow and heat transfer in small alternators using CFD

An overview is given of the first stage of work aimed at modelling the cooling of alternators used in electrical generating sets. The general approach taken was to use computational fluid dynamics (CFD) to model a test rig that was reduced in complexity compared with a 27.5 kVA alternator, and to experimentally validate the models using the test rig. The test rig design was reached by removing the internal fan and exciter assemblies from the commercial alternator, and by replacing the rotor and stator with mild steel cylinders. Cartridge heaters were embedded in both the rotor and stator of the test rig to provide well-defined variable heat sources, and local temperatures were measured. CFD models were created to represent this test rig using STAR-CD and included both the solid and fluid regions. The models enabled the simultaneous calculation of the airflow field and resulting heat transfer processes when the rotor and shaft assembly was both rotating and stationary. In general, predicted steady-state temperatures correlated well with experimental measurements. This work demonstrated the potential offered by CFD as a means of analysing the cooling of alternators in the longer term, but also highlighted the complexity of the required models.