US Patent 7946805 Fan unit including tapered airflow passage

An axial fan unit includes two or more impellers which rotate about a center axis in opposite directions relative to each other to create an axial airflow. Each impeller includes a hub and a plurality of blades disposed around the hub. The impellers are surrounded by a housing in a radial direction that is perpendicular or substantially perpendicular to the center axis. On a cross-sectional plane of the axial fan unit including the center axis, a distance between the hubs of the impellers and the housing decreases from an upstream side to a downstream side in a direction of the axial airflow.