Explanation of surge?

Surge is a type of abnormal vibration that occurs in vane compressors when the flow rate decreases to a certain level. A centrifugal compressor is a type of turbine compressor, and surge poses a serious threat to centrifugal compressors. In centrifugal compressors, surge occurs when the outlet pressure is too high and the inlet flow rate is too low; surge happens when the compressor operates below its minimum flow rate at a certain pressure. Surge is a phenomenon that occurs in the entire machine and pipeline system; therefore, it is related not only to the characteristics inside the compressor but also determined by the characteristics of the pipelines.

Surge phenomenon: When the speed remains constant and the amount of gas fed into the compressor decreases to a certain level, this leads to uneven gas velocities within the blade channels as well as backflow. Once this phenomenon spreads throughout the entire blade channel, the gas cannot flow out, resulting in a sudden drop in pressure within the compressor stage. The relatively higher pressure downstream forces the gas to flow back into the stage, after which the pressure within the stage returns to normal and the impeller resumes its normal operation, pushing the gas that has flowed back out again. Thereafter, the pressure in the stage dropped suddenly again, and the airflow flowed back; this phenomenon repeated itself, causing the compressor to operate unstably. This condition is known as surge. 　

From what the colleagues above have said, they are all explaining why surge occurs; so please explain to me why surge does not occur at high flow rates.

Surge occurs only when the compressor flow rate decreases to a certain level, causing the airflow to separate at the blades and thereby leading to a drop in outlet pressure; this, combined with the pressure in the outlet system, results in surge – as explained on page 4. No flow separation occurs at high flow rates, but excessive flow rates may cause blockage.

In flow-hour mode, the gas flow velocity at the compressor outlet is uneven, which can lead to a decrease in outlet pressure; this in turn causes local variations in outlet pressure, resulting in some of the gas exiting in reverse direction. An increase or decrease in exhaust flow causes compressor surge.

When the gas flow rate is high, the direction angle β1 of the relative velocity of the airflow entering the impeller is greater than β1A; the airflow strikes the non-working side of the blades, resulting in flow separation on the working side of those blades. Since the working side exerts pressure on the airflow, this flow separation does not spread. As the gas flow rate continues to increase until it reaches a certain maximum value, the flow velocity at the smallest cross-section within the blade channel will reach the speed of sound, after which the flow rate can no longer increase. At this point, all the work done by the impeller on the gas is used to overcome flow losses; the kinetic energy is converted into thermal energy, and the gas pressure does not increase. This condition is referred to as the \"stagnation condition\". This is the result of high traffic!

Reply to 7# hjh10010306: It seems that the explanation of surge using rotational separation is quite widely accepted. However, in practice, the damage caused by surge at low flow rates is much greater than that caused by high flow rates. The issue of airflow entering the impeller at an acute angle was mentioned in the book “Process Fluid Machinery,” but only in a general manner; it is the detailed force analysis results that are more widely recognized.

Put simply, it’s because the medium keeps changing places back and forth; why doesn’t this happen with liquids? That’s because gases can be compressed, haha