Numerical simulation of the unsteady flow field in a small fan using time-accurate three-dimensional Navier-Stokes solver was carried out.The numerical simulation focused on a small fan with inlet square wave total pressure distortion.The pattern and causation of flow loss caused by the pressure distortion in rotor and stator were analyzed.It is found that the influence of distortion is different at different radial positions.In the hub area

transportation-loss and mixing-loss are the main loss patterns.Distortion not only complicates them but also strengthens them.Especially in stator

inlet total pressure distortion induces large-scale vortex

which causes backflow and increases the loss.Finally

the details of flow loss both distorted and non-distorted flow fields are described and analyzed.