Injecting prior to turbo comp' impellers

[Greenv8s]

Originally Posted by hotrod

Interesting side note is that the damage is confined to the extreme outer blade tips, probably a combination of the swirl that develops ahead of the compressor inlet centrifuging the dust to the outside of the tube and the higher relative velocity of the blade tips vs the portion of the blades closer to the hub.

Way off topic, but I wonder would it be feasible to use this effect to create a centrefugal air cleaner? Vacuum cleaners using this effect claim no loss of suction as the 'bag' fills, presumably in our case that would translate to no increase in pressure drop across the filter with age? For this to work it would be necessary to recover the static pressure drop across the vortex, in an ideal world it would be possible to recover this by slowing and expanding the air to convert dynamic pressure back to static, as long as there was not too much drag within the vortex. Also, straying slightly closer to topic, by passing a water mist through a vortex of this sort, fine droplets might tend to be drawn through with the air while larger droplets woud be held back until they have evaporated, broken up or settled out on the walls. Probably a crazy idea, but do you reckon it would work? How much of a swirl would be required to separate out the dirt and water droplets of interest, and how would you go about testing it?

Pete 'crazy ideas' Humphries (and a green V8S)

[b_boy]

I want to say, I'm perfectly willing to accept that Hotrod's impeller wear is from dragging with no filter. At 150,000 RPM, that's 2500 RPS, so 2 seconds of dust, dirt and you have 5000 turns of the impeller. You could do a lot of damage with 6 runs.

I'm only trying to express the possibility of wear, erosion. I would not want anyone reading these threads to go into pre-impeller injecting without this as a concern.

[hotrod]

would it be feasible to use this effect to create a centrefugal air cleaner?

They already exist, industrial equipment (cat tractors diesel trucks etc.) use them as pre-filters ahead of the normal filter element to extend its service life.


Larry

[Richard L]

This may be a dumb question. Isn't the main heat generated by the turbo is friction between the air/impeller surface? The same can be said with moisted air.

Shouldn't really compare it with the space shuttle entering the earth's atmosphere and picks up speed and heat rises as the craft speeds up, but pretty close to the air sliding passes the impeller at 150,000rpm.

[SaabTuner]

I'd say that probably constitutes a big portion of it.

Also when the air enters the scroll, or vane, diffuser section the velocity pressure is converted to static pressure by slowing the air back down. This involves a lot of small eddie currents in the air associated with fluid vorticity. Generally vorticity ends up dispersing into heat.

Adrian~

[Richard L]

Originally Posted by SaabTuner

I'd say that probably constitutes a big portion of it.

Also when the air enters the scroll, or vane, diffuser section the velocity pressure is converted to static pressure by slowing the air back down. This involves a lot of small eddie currents in the air associated with fluid vorticity. Generally vorticity ends up dispersing into heat.

Adrian~

"eddie current" of the air is another word for the air molecules rubbing against each other in a circular fashion and the friction between the molecules generate heat?

Sorry to look at things in a more simplistic description.

[Greenv8s]

Originally Posted by Richard L

This may be a dumb question. Isn't the main heat generated by the turbo is friction between the air/impeller surface? The same can be said with moisted air.

Some of the heating corresponds to inefficiency in the compressor, and I imagine most of this comes from small scale local variations in velocity around the compressor which are eventually damped out by friction in the air, as suggested previously. However, turbo compressors are relatively efficient so this effect would only account for a small part of the heating effect. The rest of it comes from adiabatic heating. The mechanism causing this occurs at the molecular level, where gas molecules bouncing off the inner surface of a container rebound with increased speed when the container shrinks (think of tennis balls bouncing off a moving racket). This results is increased speed and momentum in the molecules, which is essentially what heat is.

[b_boy]

Turbos are rated in terms of efficiency of compression, but they should really say efficiency compared to adiabatic compression. At best a turbo in a narrow RPM range is 80% efficient with respect to adiabatic compression. So, there is 20% of the heat that is not merely the result of compression, but comes from somewhere else--friction, turbulance, heating of the turbo materials. As the turbo moves toward stall speed (the speed of sound) the efficiency drops off exponentially. As mach 1 is reached, not just heat but castistrophic sound waves disrupt the whole stability of the spinning wheel.

All the talk above in this thread about isothermal compression, is compression without the generation of any heat. Isothermal is a theoretical idea that cannot be achieved. However, if we can cool the air with water droplets as it is being compressed we may achieve a quasi-isothermal compression. At the outer limits of the impeller speed (near mach 1) the efficiency can drop to 50% or less of the adiabatic value.

For the sake of arguement let's say we could "recover" all the energy lost to heat at these near maximum speed. We could realize a 50% increase in compression with no heat loss. Effectively, our turbo would be 50% larger. That is a major accomplishment for the injection of a little water.

Practically, we will never cool compression this much, but even a 10% or 20% gain would make a huge difference in the amount of air mass available for combustion. For a 300hp engine, that's 30-60 hp gain. Many folks would kill for a gain like that.

Ideally, I'd like to experiment with the turbo off of the car. Compare the compression with and without water injection pre-turbo and see if compression is increased (i.e. air flow) and if post turbo temperature is reduced. Also in this ideal set up, we would want not liquid water (droplets) to survive traversing the turbo (not that that "overspray" as it is called is a bad thing, we are just not measuring the change in compression/heat only).

In the real world the best we can do is measure post turbo pressure and boost with and without injection of water. My hope is that I'll see more compression (higher boost) and lower temp (adiabatic and "friction losses" absorbed my water evaporation).

We are basically talking about running the turbo outside of it's efficiency range, using water to compensate for adiabatic and turbo losses. Like, the fear of water shut down that may destroy the engine, similarly the turbo may spin out of control if water were suddenly removed.

Engine management to prevent catastrophic damage is essential, since the computer can react much faster than we can. All attempts to operate at the edge of tuning involve some form of risk. Sensors, redundant pumps, auto shut off fuel or boost blowoff, would all lead to a safer system. This is one of the reasons I was so excited about the now indefinitely postponed EcuTek-Aquamist offering. For now, I'll be using the Element Hydra to design a system as best I can (one I'm sure will not be as good as a professional product).

[SaabTuner]

"As the turbo moves toward stall speed (the speed of sound) the efficiency drops off exponentially. As mach 1 is reached, not just heat but castistrophic sound waves disrupt the whole stability of the spinning wheel."

I think what you are referring to is termed the "choke line".

As a rule boost pressure is related to the square of the compressor speed. The ultimate boost limit is not related to the speed of sound, rather by the heat at the compressor discharge. Aluminum compressors become weak above 300F degrees, too much boost and the wheel is weakened significantly.

Also, remember that the speed of sound is an entirely localized phenominon. The speed of sound on the backside of the compressor blades is much less than the speed on the front side. Even at a relatively low speed the backside surface of the compressor blades exceeds the local speed of sound.

Furthermore, a few simple calculations will yield that most compressors operate at a speed which exceeds the speed of sound at 1 atmosphere of pressure.

The choke line is more likely related to cavitation on the backside of the impellor intake side fins. Air cannot flow inside the cavitation. As the compressor speeds up the cavitation size grows for any given pressure ratio. Eventually the increase in flow generated by a higher compressor velocity does not make up for the decrease in area due to the expansion of those cavities.

Cavitation is worsened with increased flow because the faster the air flows through the compressor at any given static pressure ratio, the lower the internal pressure ratio vector in the direction of the cavitation as the flow is perpendicular to the direction of cavitation.. (Bernuelli Principle essentially.)

edit: Another possibility is that the choke limit is when the boundary layer between air and compressor is no longer stable. This is probably the most likely reason. Increased speed and flow both contribute to a destabilized boundary layer of air. Vorticity will become immense when the boundary layer is disturbed and the air will get very hot, and not flow well.

Anyhow ... sorry for that topic tangent.

Adrian~

[b_boy]

Don't be sorry for the diversion, I truely enjoyed it, and it was intimately related to the topic of tuning. I did imply and desire to turn back to tuning, but all your comments and others are welcome information.

The extent lf my fluid dynamics knowledge is weak at best, so I am aprreciative also of the more thorough explication of choke (I always confuse the two sides of the compressor maps, my apologies). I find all of your explanations highly probable, cavitation, heat, and laminar distruption--all probably play a role in destablizing air flow.

Please let us know if you think the argument still stands, that water/methanol will stretch the choke line by reducing heat and possibly slowing the compressor. I would think that heat radiating from the impeller would be a profound cause of disruption of the boundary layer in particular.

I'm putting a thermocouple in after the turbo to measure temp, a pressure transducer would be nice too, but they are too much dough and I already have the boost gauge and engine MAP sensor to follow pressure.

Thanks again for your detailed input and data!