Industrial blowers, HVAC systems, cooling fans, and exhaust systems all make Vibration & Noise that can cause damage to the equipment ,discomfort or even a strong annoyance. For each of these products, the main source of Vibration & noise is often the turbulent flow producing acoustic waves, known as aeroacoustics and Flow induced Vibrations . Aeroacoustics noise and Flow induced Vibrations are complex and sensitive multi-disciplinary science involving airborne and structure borne acoustics, aerodynamics and structure vibration and deformation.
Flow-induced vibration can cause catastrophic failure of a structure if its natural frequencies “lock in” with the shedding frequencies of the flow. Short of catastrophic failure, flow-induced vibration can reduce equipment performance and lead to failure through fatigue. Engineers must understand the sources of this vibration, along with related amplitudes and frequencies, to produce designs that can withstand them.
Stall is aerodynamic phenomenon which occurs when a fan operates beyond its performance limits and flow separation occurs around the blade.
The angular relationship between the air flow impinging on the blade of a fan and the blade itself is known as “the angle of attack”. In axial flow fan, when this angel exceeds a certain limit, the air flow over the blade separates from the surface and centrifugal force then throws the air outwards, towards the rim of blades. This action causes a build up of pressure at the blade tip, and this pressure increases until it can be relived at the clearance between the tip and the casing. Under this condition the operation of the fan becomes unstable, vibration sets in and the flow starts to oscillate. The risk of stall increases if a fan is oversized or if the system resistance increase excessively.
Rotating Stall
This is a special case of stall that normally only occurs in backwardly inclined and airfoil centrifugal fans. Most observers also report that inlet box dampers are involved. Variable inlet vanes do a good job of preventing rotating stall because they provide a more stable flow path for the air through the wheel. These fans are encased in a scroll type housing that helps generate the fan’s pressure. The pressure around the periphery of the fan wheel varies relative to how near it is to the fan outlet (where it is highest). These fans have several blades, typically 9 to12.
Rotating stall typically occurs in fans which are severely throttled (inlet box damper typically less than 30% open).Most researchers have reported that the frequency of travel of this rotating stall occurs at about two-thirds of the fan rotational RPM(x). Some have observed two traveling cells at once generating a four-thirds rotational frequency. There are other reports of rotating stall ranging from two-thirds and even higher harmonics (2/3x, 4/3x, 6/3x, 8/3x, …). If these exciting frequencies coincide with the natural frequencies of the wheel or housing, resonance occurs and damage can result. This frequency will show up in both sound and vibration measurements. Rotating stall is among the most destructive of instabilities in the fan.
Surge
In concept, a system in surge is like an oscillator. The energy imparted to the air alternates between creating kinetic energy (high velocity in the duct) and potential energy (compressing the air in the plenum). The positive slope on the fan curve allows large amplification of this oscillation to occur. In extreme conditions, the air can temporarily blow back through the inlet.In a fixed system, the frequency of the surge is constant.
Usually the frequency is low enough that you can count the number of cycles per minute since it is quite audible. Most severe reports occur at a frequency below 300 cpm. One researcher reported that this effect seems to disappear at frequencies above 450 cpm.The frequency of surge can be be calculated for simple systems:
Frequency (Hz) = 175 * Square Root [Duct-area /(Plenum-volume * Duct-length)]
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