Signs of rotating aerodynamic breakaway in axial pit ventilator fans
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In aerodynamic tests on fans, either in test beds with input bends simulating pit installations or in pit conditions, rotating aerodynamic breakaway may cause variations in the fan's output, which can be heard as characteristic changes in the fan noise. These variations are usually very weak, but if there is an accidental resonance in the acoustic system formed by the fan and internal test-bed cavity, they may become stronger and appear similar to “pumpage.”
This undesirable effect can be avoided by appropriate choice of the test-bed chamber volume or the fan rotation speed. The acoustic mass of the ventilator and the resonance frequency can be calculated from Eqs. (1) and (3).
Equation (4) for the relative rotation speed of the breakaway zone, derived for axial compresors at the TsAGI[Central Aerodynamics] Institute, also gives good agreement with the experimental results for mine ventilator fans.
By measuring the pressure in a hole in the fan casing in front of the rotor by means of an inertialess device, we found that there are warning signs of the onset of breakaway, and these can be used in the construction of automatic anti-breakaway devices.
KeywordsPressure Characteristic Pumpage Frequency Acoustic System Explosion Physics Aerodynamic Test
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