Unsteady Pressure Field Investigation of an Axial Fan—Blade Unsteady Pressure Field Measurement

Abstract

A 1.829 m (6 ft) diameter industrial large flow-rate axial fan operated with 1770 rpm was studied experimentally in laboratory conditions. The flow characteristics near the fan blade surfaces were investigated by measuring the pressure distributions on the blade suction and pressure surfaces and the results were discussed by comparing with analytical formulations. Flow visualizations were also performed to validate the flow characteristics near the blade surface and demonstrated that the flow characteristics near the fan blade surface were dominated by the centrifugal force of the fan rotation, which result in strong three-dimensional flows. The time-dependent pressure measurement showed that the pressure oscillations on the fan blade were significantly dominated by vortex shedding from the fan blades. It was further demonstrated that the pressure distributions during the fan start-up were severely unsteady, and the main frequency variation of the static pressure was much smaller than the fan rotational frequency. The time-dependent pressure measurement when the fan operated at a constant speed showed that the magnitude of the blade pressure variation with time and the main variation frequency was much smaller than the fan rotational frequency. The pressure variations that were related to the vortex shedding were smaller than the fan rotational frequency. The complete set of blade-surface pressure measurements obtained can be used as a guide for performance improvements, vibration analysis, and CFD code validation. Detailed experimental research in the axial fans has been one of the major concerns of HVAC industries. This is because a significant improvement in the axial fan performance can be achieved if the aerodynamic and vibration losses are reduced within the fan. Moreover, a better understanding of axial fan blade flow and its pressure characteristics help lead to improved fan performance, thereby improving the fan design, can contribute to reduced operating costs. The prediction of the fan flow characteristics using Computational Fluid Dynamics (CFD) technology is becoming available in industries. The development of the unsteady three-dimensional CFD codes for fan design has become more important. The validation of the CFD codes needs detailed pressure measurements, especially the pressure information on the fan blade surfaces.Fan industries have been developing large flow-rate fans for many years. Nevertheless, they still face many difficulties in constructing efficient and stable fan units. This is primarily because the increase of flow-rate sacrifices the fan stability. To solve these problems, an extensive unsteady aerodynamic study is in high demand. Because the fan efficiency and corresponding loading are caused by an aerodynamic behavior which relates to the fan performance, the underlying fluid dynamics must be well understood to accurately determine the structures and to improve fan efficiency. Earlier such research on fans appeared almost three decades ago. However, due to the limitations of the...