Analysis Of Aeroacoustic Noise And Internal Flow Characteristics Of The Cross Flow Fan

Cross-flow fans are featured of a compact design and a low noise. This is a result of a high specific pressure ratio and the expanded flow rate. Cross-flow fans are used in air conditioning and ventilation technology, applications which require a high and wide flow rate. A cross-flow fan consists of a cylindrical rotor, forward curved blades, and a casing with suction side inflow, rear wall contour, vortex wall and pressure side outlet. The air flows from the suction into the blades radially, passes through in a direction transverse to the rotor axis for discharge in another direction. The flow traverses the blade row twice. Due to the vortex wall the so called steering vortex is produced. The center of the vortex is inside the rotor near the vortex wall. This affects a transient flow in the blade channels. Since fan geometry and flow structures are complex, the aerodynamic modeling of the cross-flow fan has been pursued with some limits.Recently, the aeroacoustic noise reduction of the cross-flow fan has become an important issue beside its energy savings and size compactness, since they are mainly used in closed spaces. The aerodynamic noise can be divided into a discrete tonal noise induced by the periodic interactions between the rotating blade and the fan casing and a broadband noise due to the turbulent fluctuation.In the present study, a computational method is proposed for the aerodynamic and aeroacoustic modeling of the cross-flow fan are directly calculated by time-accurately solving the two-dimensional incompressible Navier-Stokes equation in moving coordinates. The structured quadrangle meshes are used to model the rotating impeller blades and a sliding mesh technique, is employed to allow the unsteady interaction through the interface between the rotating mesh and the stationary one. The sound pressure at far field is then predicted by the Ffows-Williams-Hawking equation, based on unsteady pressure data acquired at the surfaces of 36 rotating blades and vortex wall. The aerodynamic noise sources of the cross-flow fan are identified and the performance and aeroacoustic noise characteristics of the uniform and random pitch fans are investigated.The fundamental cause for the turbine discrete noise is its equally spaced blades. So the most efficient way to reduce the discrete noise is to break the equally spacing fashion of the blades. This paper discusses and analyses the different inner field and aeroacoustic performance when the unequal spaced blades are applied according to the Ewald design theory.As well-known that casing is an important component of the fans, which makes effects on the performance of the fans. Therefore, this paper also discusses the inner field and aeroacoustic performance when different distances between the blades and the vortex wall are employed. Beside that, this paper also briefly discusses the performance of the fan when a shape of the casing is used.