"Cross Flow Fan" has a very complex internal structure. Despite its impeller circumferential direction is symmetrical, but it is asymmetric air flow, the relative velocity and absolute velocity is not stable, the two media flow into the impeller, the rotor wheel changing to flow along the periphery. On the inside circumference side of the impeller is able to control the entire existence of a vortex air flow, that is eccentric vortex of so-called cross-flow fan. In the center of the vortex impeller somewhere within weeks, and with the different circumstances of the throttle along the circumferential direction, it is the presence of this vortex caused by the eccentric tubular. In certain conditions, the cross-flow fan at high speed due to the eccentric vortex and greater control, cross-flow fan inside the gas can not be properly discharged or inhaled test system is abnormal situation, the so-called surging phenomenon.