Abstract
This paper presents a critical overview on worst-case design scenarios for which low-speed axial flow fans may exhibit an increased risk of blade resonance due to profile vortex shedding. To set up a design example, a circular-arc-cambered plate of 8% relative curvature is investigated in twofold approaches of blade mechanics and aerodynamics. For these purposes, the frequency of the first bending mode of a plate of arbitrary circular camber is expressed by modeling the fan blade as a cantilever beam. Furthermore, an iterative blade design method is developed for checking the risky scenarios for which spanwise and spatially coherent shed vortices, stimulating pronounced vibration and noise, may occur. Coupling these two approaches, cases for vortex-induced blade resonance are set up. Opposing this basis, design guidelines are elaborated upon for avoiding such resonance. Based on the approach presented herein, guidelines are also developed for moderating the annoyance due to the vortex shedding noise.
Highlights
Vortex shedding (VS) from low-speed axial flow fan rotor blades has become of engineering relevance in the past decades
(8), and compared to the eigenfrequency calculated based on Equations (5), (7) and (8), and compared to the eigenfrequency obobtained by means of Finite Element Method (FEM)
If the rotor diameter and the nominal rotor speed are fixed for further defining a specific case study, the resultant, nominal utip value can be compared to the aforementioned critical one. It can be judged whether a risk of blade resonance may occur by changing the rotor speed, e.g., via a frequency converter
Summary
Vortex shedding (VS) from low-speed axial flow fan rotor blades has become of engineering relevance in the past decades. PVS is referred to as a laminar-boundary-layer VS, because it can only occur if the boundary layer is initially laminar at least over one side of the blade profile In this case, the initially laminar boundary layer, being separated near or after mid-chord position, reattaches in the vicinity of the TE— resulting in a separation bubble—and undergoes a laminar-to-turbulent transition. [2] published a detailed investigation about tonal TE noise radiated by low Reynolds number airfoils. They observed that a precondition for tonal noise emission is the formed separation bubble being sufficiently close to the TE. When PVS is discussed for low-speed fans, as in the present paper, incompressible flow is considered by implying a Mach number of ≤0.3.
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