Thread Content
Rotational stall is a common fault that occurs during the operation of centrifugal compressors. Primarily, when the operating point is far from its designed conditions, flow separation occurs within the flow channels, leading to compression of the airflow and unstable flow patterns. This results in pressure fluctuations in the flow channels and pipes of the machine, causing fatigue damage to the machine’s components or pipes. In severe cases, this can progress to surge, posing serious damage to the machine. 1. Mechanism of rotational stall: When the operating conditions of a centrifugal or axial flow compressor change, and the volume of gas flowing through the compressor decreases to a certain extent, the direction of the airflow entering the impeller diffuser channels changes. The airflow strikes the convex side of the blades (known as the working surface), resulting in the formation of numerous air vortices near the concave side of the blades. As these vortices increase in number, the effective flow area of the channels gradually decreases. Of course, the distribution of the airflow entering the compressor across the various flow channels is not very uniform, and the number of airflow vortices also varies. If there are many air vortices in a certain flow channel (such as flow channel 2 in Figure 1), the amount of air flowing through that channel will decrease; the excess air will then be directed to other channels, before returning to the preceding channel (flow channel 1). Because the incoming gas hits the concave surface of the blade, pushing away a large amount of the airflow that was originally on that surface, the airflow in this flow channel becomes somewhat smoother. The airflow directed toward the rear flow channel (flow channel 3) impacts the convex surface of the blade, causing more vortices to form in the concave area of the blade; this blocks the effective flow area of the channel and forces the airflow there to turn toward an adjacent flow channel. If this development continues, air flow blockages composed of vortices (referred to as stall clusters or stall zones) will appear alternately in various flow channels in the opposite direction to the rotation of the impeller. Since the propagation speed of the stall region in the reverse direction is lower than the rotation speed of the impeller, from a fixed point outside the impeller, the stall region still rotates in the direction of the impeller’s rotation; this is the mechanism behind rotational stall. 2. Basic characteristics of rotational stall (1) The process of rotational stall formation can be of either gradual or sudden type. Progressive stall occurs as the compressor airflow decreases, causing the area occupied by the flow blockage zone to gradually increase. Specifically, this is manifested as the pressure ratio decreasing gradually as the flow rate decreases, with no discontinuities on the constant speed line ； The number of separation zones gradually decreases as the air flow rate decreases, and the separation zones expand progressively in the direction of blade height ； The moving speed of the separation zone does not change as the number of separation zones increases. Mutated stall occurs when the gas flow rate decreases to a certain level; as the stall region expands rapidly and covers a larger area, it tends to cause significant gas pressure fluctuations, which have a substantial impact on the compressor’s performance and vibration. Specifically, this is manifested in the fact that the number of separation zones is generally not high, with only one or two ； The pressurization coefficient drops sharply at stall, with discontinuities on the constant-speed line ； The characteristic lines are clearly divided into the upper left and lower right, with hysteresis occurring. (2) The rotating stall process also has a lag effect. That is, as the gas volume decreases, the compressor enters the rotational stall range, causing the discharge pressure to drop suddenly. However, after increasing the flow rate again, the compressor’s performance curve does not follow the original path; instead, there is a delay in the flow rate response. That is, when the flow rate rises to the original stall point, the compressor cannot immediately return to its previous pressure level, and an additional increase in flow rate is required for the pressure to rise. 3. Research progress on the mechanism and characteristics of rotational stall. The formation of the stall region is a rather complex hydrodynamic process. Among them, the propagation speed of the rotating stall region and the stall frequency are two important issues, as they are crucial for determining whether the vibrations of a compressor are caused by rotating stall. In addition to conducting extensive theoretical research, scientific institutions at home and abroad have also carried out numerous practical tests in laboratories. 4 Diagnostic Example: In a factory, when the process conditions during production changed and the gas flow rate dropped from 29.6 to 28 in normal operating conditions, the compressor began to vibrate abnormally, reaching a dangerous level; its operating speed was 13,825. Diagnostic analysis: Fundamental vibration frequency of the unit: 13825/60 = 230.4 Hz. Conclusion: Rotational stall is one of the common faults that occur during the operation of compressors. Only by understanding its mechanism of occurrence and fault characteristics, conducting proper diagnostic analysis, and taking appropriate measures can it be prevented from progressing to surge, thus ensuring the normal and stable operation of the unit.