Blade natural frequency

The blade natural frequencies shall not coincide with any source of excitation from 10% below minimum governed speed to 10% above... 

Effect of Vibration. Tfll objects have a natural frequency of vibration when stmck sharply and fan rings, blades, stmcture, etc are no exception. Vibration is usually siausoidal and its frequency measured ia Hertz. The travel or displacemeat of the vibratioa is measured ia mils (1 /1000 of an iach) ia the United States but ia micrometers elsewhere. Another measuremeat is velocity (mm/ s) of movement. 

A fan blade is continuously vibrating millions of cycles up and down ia operatioa over a short period of time. Each time a blade tip moves past an obstmction it is loaded and then unloaded. If forced by virtue of tip speed and number of blades to vibrate at its natural frequency, the ampHtude is greatly iacreased and internal stresses result. It is very important when selecting or rating a fan to avoid operation near the natural frequency. The most common method of checking for a resonance problem is by usiag the relatioa ... 

A requirement of the standards is that the fundamental natural frequency of the blade should be at least two times the maximum continuous speed, and at least 10% away from the passing frequencies of any stationary parts. Experience has shown that the natural frequency should be at least four times the maximum continuous speed. Care should be exercised on units where there is a great change in the number of blades between stages. 

This speed becomes critical when the frequency of excitation is equal to one of the natural frequencies of the system. In forced vibration, the system is a function of the frequencies. These frequencies can also be multiples of rotor speed excited by frequencies other than the speed frequency such as blade passing frequencies, gear mesh frequencies, and other component frequencies. Figure 5-20 shows that for forced vibration, the critical frequency remains constant at any shaft speed. The critical speeds occur at one-half, one, and two times the rotor speed. The effect of damping in forced vibration reduces the amplitude, but it does not affect the frequency at which this phenomenon occurs. 

Blade stall causes Karman vortices in the airfoil wake. Whenever the frequency of these vortices coincides with the natural frequency of the airfoil, flutter will occur. Stall flutter is a major cause of compressor blade failure. 

Although the effect of the mass of the impeller is easy to take account of in air, little notice has been paid to m, , the associated added mass of liquor, which is the amount of liquid that effectively vibrates with the impeller, so increasing its inertia and reducing its natural frequency (and thus critical speed). The general subject of added mass is researched in, and applied to impellers in. The added mass of an impeller has been determined as a coefficient multiplied by the mass of liquid contained within a cylinder of diameter equal to the impeller diameter and of length equal to the projected blade width ... 

The heart of the eompander is the design of the impellers. A high performance aerodynamic shape for eaeh expander and compressor impeller is defined using Computational Fluid Dynamics (CFD). Finite Element Analysis (FEA) verified acceptable stresses due to speed and blade loading, and defined an operating zone free from destructive natural frequencies. This work was verified by outside consultants. 

Large mixers running at less than 150 rpm usually operate below the first critical speed. Small mixers operating above 250 rpm usually operate between first and second critical, 1.2Nc to 0.8Nc2, where Nc2 is the second lateral natural frequency. Other frequencies, such as a blade-passing frequency, four times the operating speed for a four-blade impeller with four baffles, can cause mechanical excitations. Structural vibrations at certain fractions of operating speed can also contribute to natural frequency problems. 

Piezoelectric fans are small, low-power, relatively low-noise, solid-state devices that provide viable thermal management solutions for a variety of portable electronic appliances, including laptop computers and cellular phones. In these fans piezoceramic patches are bonded onto thin, low-frequency flexible blades driven at resonance frequency, thereby creating an air stream directed at the electronics components. Thereby, up to 100% improvement over natural convective heat transfer can be achieved (Acikalin et al. 2004). 

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