Surge is the phenomenon whereby laminar flow off the wheel is separated, and
reverse flows through the compressor.
This can accompany any of the above phenomenon. But surge is when the airflow stops and reverses. It can snap off blades. Blow piping joints. Make for an emergency service call worth $275,000!!!
The above discription gets into the forces but is 'muddy' on what it really is.
Centrifugal compressors are VERY influenced by speed of the wheel. The curves change dramatically. In industrial compressors as little as a 2% change in input voltage to the driver can slow the compressor enough to cause massive surging.
It's easier for me to draw a picture of the compressor curve and show on a Pressure versus Flow graph what happens in each instance. This is how I conduct training.
Imagine if you will flow on the horizontal axis, and pressure on the vertical. Zero for each is at the origin, pressure rises as you ascend the vertical axis, flow increases as you traverse the horizontal axis to the right.
The simplest surge line is a slope starting at origin and raising (for argument) at a 30 degree angle to the right. The line represents where the compressor will surge if crossing that line. It is a line denoting the minimum stable flow for the pressure and flow given.
Now, somewhere to the right of that surge curve there will be an 'eyebrow' showing the design curve of the wheel being used. Design point is on this curve. Below that point, you will get more flow for imparted horsepower to a point. Until you reach stonewall, a phenomenon where the wheel, due to the physical size, can not move any more air and flow will stagnate. It is still flowing, but regardless of horsepower applied, the pressure won't rise, and you can't get any more flow. This is in the nebulous region somewhere in or off the lower right of this graph. It is also a curve...but lets skip that!
In a drop throttle condition (assume no blowoff valve), your flow moves to the LEFT rapidly(Instantly) along the horizontal axis while simultaneously RISING along the vertical axis.... This is because the flow requirement from the engine is STOPPED, and the pressure rises simultaneously due to this. Bad Surge that continues to decrease in severity as air is discharged back
through the compressor to the intake ductwork and the pressure reaches a point where the flow that exists lets the air reattach itself to the wheel and flow smoothly again through the compressor.
In a lift throttle conditon-say to half throttle, the movement to the left does not move all the way to the vertical origin line, there is still a flow demand, but the compressor will continue making air at high speed till the torque slip mentioned above equalizes. This condition will cause the same pressure rise and it may be at a higher pressure---so the surge is louder due to higher pressure, but because flow is still occuring it usually is only a couple of times and quieter---'honking' is usually what people refer to it as...
On WOT, and hammering max boost you may reach a natural surge point---that is sufficient flow to stay stable but due to increasing turbine speed the pressure keeps climbing higher and higher. You are a boostaholic so you have wired your wastegate closed. The pressure reaches a point where somewhere almost vertically from design point you will cross the surge line. WHAM! This one is LOUD. It can be repetitive and in quick succession. You lift and then it starts 'honking'...damned if you do damned if you dont---take a look at what you did when you drop throttled afte a surge: You has a pressure situation where you were ABOVE the surge line to begin with, and then moved HARD LEFT along the horizontal axis while ALREADY in surge. Compound this, the wastegate pops open and SLOWS the compressor wheel---MOVING THE SURGE LINE AS WELL!
The whole thing to understand is the way to stop surge is to throw FLOW to the compressor. FLOW may result in stonewall, but that won't break things.
This is why bypass valves should open on partial throttle IMMEDIATELY! Even LIGHT lifts of the throttle should result in a 'sigh' from the turbo piping. This drops pressure on the graph towards the horizontal, and moves the flow along the vertical to the RIGHT.
When in doubt: induce FLOW! On hard drop throttle the bypass should open and let that flow off. You set the spring 'hard' and what you see when you graph it is a slight spike towards that surge line, in addition to the same quick move to the left, before the valve opens to vent pressure and move the flow-pressure point down towards the horizontal axis and to the right towards stonewall. With a 'hard set ricer woosh spring' it is VERY possible that the toy you are using for sound effects is INDUCING a surge before doing what it was SUPPOSED TO DO: PREVENT SURGE!
Most industrial compressors will sense the pressure differentials, or the amperage change of a surge, and IMMEDIATELY pop open an unloading valve to get flow stable. They will keep in this condition for a few seconds to let flow stabilize, and then close the blowoff (unloading) valve slowly to bring the pressure in the system back up. There is more to it than this with PID loops controlling the inlet and unloading valves independently, but generally it opens WIDE to induce stable flow. Some units using older control systems may sense a surge and then just unload until there is an operator input.
If you wonder why Indy Cars never seemed to surge...they controlled boost through blowing off excess pressure to atmosphere off the top of the plenum. The big BOOM you would hear when they manually shifted (I hate the sequential boxes and paddle shifters!) was that valve opening after they dropped throttle and it had to open and vent the plenum (throttle rotors in the head...) SSSS BOOM SSSSSSSSS BOOM SSSSSSSS BOOM. It's almost like a wired close wastegate on a stock Z and running off the emergency relief valve. But this has the thing spooled and running at maximum flow almost all the time! Now, the F1 Engines of the early 80's... oooooh... gaaaaarrrrgh! F1 Turbo Cars.......gaaagggaaaargggrrrrgggghhhh!
Did I make that clear as mud, or what? It's better with graphic aids...
My photo is decidely non-corporate! WONK WONK, GEEK GEEK!
http://www.fs-elliot...catid=0&subid=0
I searched around Hybridz for an explanation, but nothing came up regarding explaining the phenomenom.
