heat and mass transfer analogy, fan blade evaluation, velocity distribution, wall shear stress
A mass transfer method has been employed to characterize tangential flow velocity around fan blades. In the current research, two commercial air-conditioning fan blade samples have been investigated. The blade surfaces were covered with a 10 μm yellow coating by a spray method. The fan and a camera are driven by a motor and shaft system that can synchronize the rotational speed. Therefore, a relative stay still image of the fan blade during rotation can be obtained. Subsequently, a small amount (50 ppmv) of ammonia has been injected into an environmental chamber that has the fan installed and rotating. The coating material on the blade surfaces absorbs the ammonia from the airflow and responds with a color change from yellow to blue. At the same time, the color change was recorded by the camera. The surface color change can be quantified by image processing which represents the local mass transfer. According to the fluid mechanics and analogy between mass transfer, heat transfer, and Colburn’s relation for turbulent flow, local tangential flow velocity distribution at the external of the boundary layer on the blade surface can be quantified. Thereafter, friction factor and wall shear stress can be calculated accordingly. Comparing to other experimental and computational methods, this new experimental method provides a robust way to evaluate turbulent flow velocity around the rotational fan blades. The results show this method is promising to be employed to evaluate and optimize fan systems to improve efficiency and reduce noise.