Abstract: The inner vortical flow and aerodynamic noise in a semi open axial fan with a shroud in a central air conditioning is studied. Three dimensional unsteady flow field under the condition of uniform inflow is calculated using a large eddy numerical simulation. The results show that the tip vortex is the main feature of inner flows. The formation, development and breakdown of the tip vortex have a great effect on the aerodynamic noise. A spectral analysis is carried out by extracting the unsteady pressure fluctuation at main sound source region. There are three characteristic frequencies corresponding to the blade passing frequency, the harmonic frequency, and the vortex shedding frequency. Far field aerodynamic noise of the fan is predicted using LES/FW H acoustic analogy method. It is indicated that the aerodynamic noise radiated from the semi open low pressure axial fan is mainly the broad band turbulent noise. The predicted sound pressure level spectrum agrees well with the experimental results.