Injecting prior to turbo comp' impellers

[SaabTuner]

If the "choke line" is the point at which the boundary layer next to the vanes breaks down, then water injection will surely make the compressor more efficient.

The water droplets will make the air denser. Denser fluids have less vorticity. Vorticity is responsible for turbulence and the breakdown of the boundary layer.

Therefore denser air makes the turbo more efficient by reducing vorticity and thus stabilizing the boundary layer of air.

Makes sense, right?

Adrian~

[hotrod]

If you have the means to log a couple data streams it should be pretty simple to prove.

On the WRX, you have a MAF (Mass Air Flow) meter that reads directly in grams/second ( if you have the proper logging software) it is a 0-5 volt output so even a peak hold volt meter could be used. Just set the boost to a known value and log the mass air flow at a known rpm. Then turn on the WI and adjust until you hit the same boost setting at the same rpm and compare the new MAF numbers.

Larry

[b_boy]

So, if choke occurs later, the compression could continue at higher wheel speed. The speed of sound increases in denser mediums, the factor of which I do not know.

I'm not familiar enough with vorticity other than I can see that disrupting of either the boundary layer or added sonic turbulance would lower compression by preventing laminar flow, and add heat to that air (turbulance) without compression. For the layman, is that what you are saying Adrian?

Choke occurs when the small diameter (inlet) of the impeller reaches sonic speeds. You think that the added density with water injection will slow the wheel to under Mach1, yes? Or, that the speed of sound will increase, permitting extension (more wheel speed) of compression until choke?

[SaabTuner]

Originally Posted by b_boy

So, if choke occurs later, the compression could continue at higher wheel speed. The speed of sound increases in denser mediums, the factor of which I do not know.

I'm not familiar enough with vorticity other than I can see that disrupting of either the boundary layer or added sonic turbulance would lower compression by preventing laminar flow, and add heat to that air (turbulance) without compression. For the layman, is that what you are saying Adrian?

Choke occurs when the small diameter (inlet) of the impeller reaches sonic speeds. You think that the added density with water injection will slow the wheel to under Mach1, yes? Or, that the speed of sound will increase, permitting extension (more wheel speed) of compression until choke?

I'm not sure choke has as much to do with compressor speed as it does flow Remember that compressor speed is more closely tied to boost than flow. As an example 150,000 on a compressor with a 2" small side is nowhere near the choke line at low flow. Despite that, the tips of the compressor blades (on the small side) are travelling at roughly mach 1.37 even when only flowing half of the maximum "choke" flow. The outer tips of the compressor blades are travelling at well over mach 1 even at just one or two psi of boost pressure on most turbos.

One of the mechanisms for generating vorticity in compressable flows (like air) is the baroclinic generation term.

"The baroclinic mechanism for vorticity generation is frequently invoked to explain the existence and direction of off-shore and on-shore breezes and the generation of vorticity in compressible flows. The baroclinic generation mechanism will be non-zero whenever the density and pressure gradients are not aligned." Taken from Navier-Stokes.net

Since the evaporation of water is easiest in low pressure regions, vorticity would inherantly be absorbed. This would be because any time the pressure gradients were un-alligned by a low pressure region, some water would evaporate there and bring the pressure up to the same level as the higher pressure region. Which would make the pressure/density gradients uniform and thus reduce vorticity.

Worth a read: http://www.navier-stokes.net/nsvcr.htm

Does that make sense? In lower pressure regions the water would evaporate and raise the pressure to equal the other regions more or less. Which would stabilize the flow in the compressor and allow it to flow more easily, and with less heat generated from vorticity. That would also stabilize the boundary layer.

Adrian~

[Greenv8s]

Its clear that you guys have a far better understanding of this than I do! Your comments have stirred up a couple of thoughts though.

Gut feeling :roll: is that with WI just upstream of the compressor relatively little evaporation will occur outside the compressor, most evaporation would occur as the air is heated and shredded inside the compressor. Since the heat will be generated during compression, and the compression occurs progressively, and there will be some latency between the air being heated and the resulting water evaporation, it may be that water evaporation also occurs relatively late within the compression process. I can sort of imagine a plot of temperature against time for a typical molecule passing through the compressor, and seeing the temperature rise quickly for the dry case and slower for the wet case IYSWIM. If so, I expect the impact on the compressor entry conditions would be relatively small.

The second thought is that a centrefugal compressor is highly sensitive to density. As well as the 'each shovel holds twice as much air' analogy, it has been pointed out that the denser air will generate a higher outlet pressure for the same RPM or the same pressure at lower RPM. Does this reduced RPM (and presumably reduced air friction drag on the tubine) mean that the compressor will also (a) be more efficient and (b) spool up more quickly (in terms of flow and pressure ratio, even if it may be slower in rotor rpm terms)?

There are too many unknowns to speculate, but I would be extremely interested to see what difference it does make to have WI upstream rather than downstream.

[b_boy]

From Stealth316:

The wheel rotational speed, in revolutions per minute (rpm), at various values is shown on the flow map as a function of air flow rate and pressure ratio. When air flow is held constant (a vertical line on the flow map), faster rotation means a higher pressure ratio. When the PR is constant (a horizontal line on the flow map), faster rotation generally means more air flow. However, air flow does not appreciably increase after the outer tips of the compressor wheel are moving faster than the speed of sound. When the air flow reaches sonic speeds, the diffuser becomes choked and only very small increases in flow rate are possible even with large increases in wheel speed. Larger compressor wheels have maximum rotational speeds less than smaller wheels because of this limitation.

On the flow map above, the air flow regime to the right of the dotted line marking maximum wheel speed is called the choke area. The choke area is almost never noted on a flow map. To determine the choke area, you can drop a vertical line from about where the fastest wheel speed curve ends on the right side of the map. This vertical line is the approximate maximum air flow the compressor is capable of, regardless of efficiency or pressure ratio.

The key phrase here is just as you said, SaabTuner, it's not the wheel speed it's "when air flow reaches sonic speed." Big difference.

I also like the sentence about how the choke area is almost never noted on a compressor map (the map usually stops at 60% efficiency). That's great for us, we have some room to spare.

I wrote to a "turbomachinary" professor in England. His take is as we have noted, the gain in work with "wet compression" over and above cooling the air after compression (intercooling) is that the more air is compressed with the same amount of work energy applied--it's the shovels of air analogy. To recover, the same mass of compressed air without water injection, your need to turn the compressor more to make up for the loss in efficiency. He also said there is no combustion penalty up to 5% water:air mass. Unfortunately, he did not give much of a molecular take on things in fluid dynamics terms.

[ride5000]

what a great read.

thanks to all the posters.

ken

[Richard L]

Originally Posted by slowMX5

I have been watching this thread with some interest. I'm beginning to become inspired to do a little testing. Currently I am only using one of the TEC3r's (www.getfuelinjected.com) mapable outputs to map WI (using Aquamists 2C system), with 2 nozzles post IC. I have another spare mapable output - and I'm beginning to think that placing another HSV in a line supplying a pre-compressor nozzle might be a worthwhile experiment. This would enable me to play with the water delivery and compare results of varying switch on points and delivery amounts independently of water that is being delivered post IC.

The only thing making me think twice at present is I am running a Garrett Disco Potato turbo (GT28RS, 0.86 AR compressor 76 trim, 0.64 AR turbine 62 trim) - so it is if anything oversized for a 1.6L engine making 280bhp (sized for future plans).

Worth making the effort? I have full data logging software that will allow repeatable power/torque graphs to be plotted if using the same stretch of road.

Slow MX5 has now all the tools to do whatever you consider to be a real-life test on the theory, must need some careful planning how to set it up properly so that the results can be analysed. It is so lucky thart he has data logging controller.

Do you think a temperature probe is a good addition?

[ryan]

Well, if the shoveling more are per turn is correct, I guess you would get a good power increase due to turbine does not need so much pressure to supply same air mass/pressure, less backpressure on motor will increase power output too.

this all sounds good, let's see the results, I would try on my car but I have twin turbo v6, so would need more hardware ie atleast 1 more hsv, but really 2 would be the one to make sure misting is o.k., - turbos are on oppersite sides of engine bay.

also, would injecting straight arfter air cleaner/maf sensor, not just before compressor be even more benaficial, ie longer time to cool the air down makeing it even more dense before it enters the compressor ie shoveling even more air per turn, or is the most benerfit from cooling the compression stage. maybe that could be tested also.

Cheers
Ryan

[hotrod]

A have a friend that has a small number of dual probe temperature gauges that can display delta T between the probes or either probe temp.

I will be buying one from him soon. I think its the best way to confirm the WI is running (ie if no temp drop, means back off the boost).

They are a bit expensive but if anyone wants to try one, let me know and I can get you contact info.

Larry