Patent ID: 7946805

Claim:
An axial fan unit comprising: a first impeller including a first hub centered on a center axis and a plurality of first blades disposed about the center axis outside the first hub in a radial direction perpendicular or substantially perpendicular to the center axis; a first motor disposed in the first hub and operable to rotate the first impeller about the center axis to create a first airflow flowing generally axially; a second impeller disposed downstream of the first impeller in a direction of the first airflow coaxially or substantially coaxially with the first impeller, the second impeller including a second hub and a plurality of second blades disposed about the center axis outside the second hub in the radial direction; a second motor disposed in the second hub and operable to rotate the second impeller about the center axis to create a second airflow flowing in the same direction as the first airflow; and a housing arranged to radially surround the first and second impellers; wherein a radial thickness of the first hub and a radial thickness of the second hub both gradually increase in an axial direction such that radial outermost portions of the first hub and the second hub are aligned to define a straight line with a constant or substantially constant slope extending in the radial and the axial directions; and when the axial fan unit is viewed along the center axis, a cross-sectional area of a region surrounded by an outer side surface of the second hub and an inner surface of the housing at a downstream end of the second hub in the direction of the first and second airflows is smaller than a cross-sectional area of a region surrounded by an outer side surface of the first hub and the inner surface of the housing at a downstream end of the first hub and a cross-sectional area of a region surrounded by the outer side surface of the second hub and the inner surface of the housing at an upstream end of the second hub.