Aerodynamic instability

Compressor surge is a phenomenon of considerable interest, yet it is not fully understood. It is a form of unstable operation and should be avoided in both design and operation. Surge has been traditionally defined as the lower limit of stable operation in a compressor and involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually a part of the compressor is the cause of the aerodynamic instability, although it is possible that the system arrangement could be capable of augmenting this instability. Figure 6-36 shows a typical... 

The following parameters are monitored in a typical predictive-maintenance program for fans aerodynamic instability, running speeds, and shaft mode shape, or shaft deflection. 

In addition, this type of compressor tends to have both the suction and discharge ports on the same side of the compressor s housing. As a result, there is a potential for aerodynamic instability within the compressor. Orientation of the primary (X-axis) radial measurement point should be opposite the discharge port and oriented toward the discharge. The secondary (Y-axis) radial point should be in the direction of shaft rotation and 90° from the primary radial point. 

The Spencer blower in this example provides air to a drying process in a metal-coating line. Its configuration includes an end-suction inlet that is in-line with the shaft and a horizontal discharge that is perpendicular to the shaft. In this particular example, the source of the shaft deflection observed in the mode plot is aerodynamic instability. 

Other In fact, all failure modes create some form of imbalance in a machine, as do aerodynamic instability. 

As we discussed, fans and blowers are prone to aerodynamic instability. The indication of abnormal vanepass suggests that this may be contributing to the problem. The additional data provided by the narrowband readings help to eliminate many of the possible failure modes that could be affect the blower. However, we still cannot confirm the specific problem. 

As mentioned in the previous section, a major drawback of the simplex atomizer is the poor atomization quality at the lowest flow rate due to too-low pressure differential if swirl ports are sized to allow the maximum flow rate at the maximum injection pressure. This problem may be resolved by using dual-orifice, duplex, or spill-return atomizers. Alternatively, the atomization processes at low injection pressures can be augmented via forced aerodynamic instabilities by using air or gas stream(s) or jet(s). This is based on the beneficial effect of flowing air in assisting the disintegration of a liquid j et or sheet, as recognized in the application of the shroud air in fan spray and pressure-swirl atomization. 

In practical fan sheet breakup processes, sheet thickness diminishes as the sheet expands away from the atomizer orifice, and liquid viscosity affects the breakup and the resultant droplet size. Dombrowski and Johns[238] considered these realistic factors and derived an analytical correlation for the mean droplet diameter on the basis of an analysis of the aerodynamic instability and disintegration of viscous sheets with particular reference to those generated by fan spray atomizers ... 

Hollow-Cone Sprays. In swid atomizers, the liquid emerges from the exit orifice in the form of a conical sheet. As the liquid sheet spreads radially outward, aerodynamic instability immediately takes place and leads to the formation of waves which subsequently disintegrate into ligaments and droplets. Figure 3 illustrates the breakup process in an annular liquid sheet. 

Dombrowski N, Johns WR, The aerodynamic instability and disintegration of viscous liquid sheets, Chem, Eng. Sci. 18, 203-214, 1963. 

Clark CJ, Dombrowski N, Aerodynamic instability and disintegration of inviscid liquid... 

The sheet perforation process is not well understood. Several different causes are suggested for the onset of perforation, including impingement of small droplets on the surface of the sheet, turbulent fluctuations inside the sheet, and small bubbles in some cases, and disturbances in the air core of the nozzle [65-67]. The main mechanism that is currently used to model swirling sheet atomization is based on the aerodynamics instability. 

The electrical instability in the electrospinning process was compared to aerodynamic instabilities where partial differential equations of the aerodynamics of the fluid jet with electric field equations are used to describe the process. Many theoretical models have been used to describe the combination of a viscoelastic fluid driven by an electrical field. On the basis of Taylor studies and with the use of high-velocity cameras, it was proposed that the electrospinning... 

Aerodynamic instability is the most common failure mode for centrifiigal compressors. Variable demand and restrictions of the inlet-air flow are common sources of this instability. Even slight variations can cause dramatic changes in the operating stability of the compressor. 

Most of the problems that limit fan performance and reliability are either directiy or indirectiy caused by improper application, installation, operation, or maintenance. However, the majority are caused by misapplication or poor operating practices. Table 13.4 lists failure modes of centrifugal fans and their causes. Some of the more common failures are aerodynamic instability, plate-out, speed changes, and lateral flexibility. 

Theodorsen, F., "General Theory of Aerodynamic Instability and the Mechanism of Flutter," National Advisory Committee for Aeronautics, Technical Report 496, U.S. Government Printing Office, Washington, D.C., 1934. 

The problems of wind-induced buffeting and aerodynamic instability of long flexible bridges have, over some half century, received considerable empirical treatment, notably via wind tunnel experiments on models. Such treatment has, overall, been quite... 

The first cable-stayed bridge in North America proved prone to aerodynamic instability. 

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