Investigation of the Aerodynamic & Aeroacoustic Performance of Cross-flow Fans

D3 - Fan Performance (i)

For the air conditioner industry, cross flow fan (CFF) design is important to assure an efficient heat transfer from the heat exchanger and also to reduce the overall sound pressure level of the indoor air-conditioning unit. Theoretical and experimental studies about cross flow fans (CFF) are scarce in the literature, especially the ones where numerical or analytical calculations are validated with experimental measurements.
Air flow and noise generation mechanisms in a CFF are fundamentally different than axial or centrifugal fans. Air flows across the blades of a CFF, which causes that both ends of blades switch from leading to trailing edge throughout one rotation. The flow structure inside the fan consists of two regions: the through-flow region and the eccentric vortex region. Efficiency of a CFF is fundamentally limited by the unavoidable recirculation flow within the impeller at all fan speeds.
Design of the impeller effects acoustic and aerodynamic performance of the CFF dramatically. Our studies show that impellers, having the same outer diameter and operating in the same casing with the same rotational speed, result in a large variation both in the aerodynamic and acoustic performance. In the measurements overall sound power level changed in a range of 56 to 59 dB(A) and the flow rate changed in a range of 640 to 710 m3/h. This flow rate difference can change the seasonal coefficient of performance (SCOP) up to 0.5, which can result in a quieter air-conditioning unit in a higher energy class.
This study investigates the effect of the impeller design to the aerodynamic and aeroacoustic performance. The impeller is designed parametrically, where the effect of each parameter on noise generation and flow rate is tested via computational fluid dynamics. Results of the numerical simulations are evaluated to gain insight on the complex noise generation mechanisms and to design a CFF with high flow rate and low overall sound power level. Optimum geometries are produced via rapid prototyping. Their aerodynamic performance is measured via particle image velocimetry (PIV) and measurements in the psychrometric test chamber.