When reading about the action of opening a volume at vacuum submerged in a fluid at pressure, the fluid is said to "surge", i.e. to fill the void space (the vacuum). Per Google, surge is defined as a sudden powerful forward or upward movement, especially by a crowd or by a natural force such as the waves or tide. Another source says
Transient flow is usually referred to as surge or water hammer, and
the terms are commonly interchanged. But strictly speaking surge is
where the mass oscillation of the fluid is the dominating force and
the compressibility of fluid is not significant, for example two
connecting reservoirs oscillating up and down
So it seems in the study of fluid mechanics, "surge" has a more defined meaning and is related to a force. Can someone tell me another example of fluid surge and what equations that explain/demonstrate the forces producing/(causing by?) surge?
In the above description, it is said that the compressibility of fluid is not significant, and then describe fluid (presumably "incompressible" liquid) flowing back and forth between containers. But I am still having trouble understanding the difference caused by compressibility. Can someone give an example of a compressible fluid in situation that would cause surge flow for an incompressible fluid and show how the compressibility of fluid limits the surge?
Best Answer
Surge is a highly complex, nonlinear behavior that can occur in pumps and blowers. It occurs in centrifugal pumps, for example, at specific states of flow pressure; usually higher pressure at low flow where the vanes spend more of their energy compressing the fluid in the downstream compliance rather than moving the gas through the pump. So there is a kind of tug of war between the gas trying to expand back into the pump and the pump trying to push gas downstream.
Surge may manifest itself and be observed as solitary transients, but can also occur as a sustained limit cycle or chaotic oscillation, and at the low flow actual cause transient flow reversals in the pump. In liquids such as water for example, pressures may be reduced below the vapor pressure causing water vapor or dissolved gases to precipitate, and this multi-phase state can lead to surge.
Surge is very difficult to predict since it requires 3 dimensional rotational (CFD)modeling of the fluid flow, and this can even be a poor predictor. Designing a pump that has a low tendency to surge involves allot of trial and error in shaping the blower vanes, pump plenum, and inlet/outlet flow areas, and design relies mostly on experimental and empirical results.
Surge can suddenly drop or raise pressure on the rotating impeller, and so unless the pump has closed loop speed and torque control, the pressure fluctuations may cause sudden racing or delay in the rotor rotational speed - a 'surging' of the rotor from which the name comes I believe.