Characteristics of turbulent flow in a cross-flow fan and its control using tubercles

Abstract

Flow structures in a cross-flow fan at Re = 5400 based on the blade chord length and the tip velocity at the outer radius are investigated using large eddy simulation with an immersed boundary method. An eccentric vortex near a stabilizer, flow separation on blade suction surfaces in inflow region, and a recirculation region at the corner of inflow region are obtained from simulations. To analyze the effects of these flow structures on the fan performance, the variation of the torque coefficient of each blade along the azimuthal direction is examined. Consequently, the flow separation on the blades produces oscillatory variations of the torque coefficient along the azimuthal direction, causing degraded fan performance. Moreover, the recirculation region, which is also originated from flow separation, is shown to produce a considerable loss, and the blades nearby suffer from high torque. To control the flow separations on the blades, a biomimetic control device that imitates Humpback whales tubercle structures are used. As a result, the separation on the blades in the inflow region and the oscillation of the torque coefficient are considerably attenuated. In addition, the recirculation region and the high peak of torque coefficient are also removed. The optimal shape is obtained by a parametric study and response surface method, and fan efficiency was improved up to 8.45%.