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Old 13-02-2014, 06:33 PM
Aquaman Aquaman is offline
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Default Optimum droplet size?

Hello there-

Great forum you have here!

Does droplet size adhere to the law of diminishing returns?

Apparently the current industry standard of 160-200 psi with a properly designed nozzle without compressed air aeration atomization will result in 44-60 VMD micron size.

I also know that the evaporation rate is inversely proportional to the square of the diameter of the droplet.

Sooo... considering that you would prefer the evaporation to occur in the cylinder and not the compressor outlet would an equal sized water mass of 11 micron droplets which would evaporate 16 times faster than the 44 micron sample be advantageous or not? What about the alternative of a 176 micron droplet that would evaporate 16 times more slowly giving the water mass more time to achieve maximal cooling in the cylinder?

The reason I ask is that some system designs are using 15 psi of boost pressure alone while some of the big diesel setups are using 1000 psi.

Thanks for your thoughts!
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  #2  
Old 13-02-2014, 09:26 PM
Richard L Richard L is offline
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Default Re: Optimum droplet size?

Put this scenario to the extreme, it you are able to spray a single molecule of water, there are no cooling effect at all. The heat absorption capacity is defined by the breaking up of the molecules held together by the van de waal force.

As far as I know, the rate of evaporation is directly proportional to the surface area solely. A droplet of water at 10 microns may have a few thousands molecules stuck together. A droplet at 100 micron be bigger and more molecules but the surface area exposed of each molecule exposed to the heat source is actually smaller. This is where the expression of enthalpy becomes useful. A total energy of a thermodynamic system.

I don't quite understand your comment about 15psi and 1000psi, please explain.
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Old 17-02-2014, 07:18 PM
Aquaman Aquaman is offline
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Default Re: Optimum droplet size?

Richard-

Thanks for your response...

Without the aid of a compressed air source the droplet size for the most part (in my limited understanding), without an impaction needle tip, is determined by the pressure and the orifice size. Provided a given horsepower figure the CC/min of water needed is able to be calculated. Therefore at say 600 hp. A system that runs on boost pressure of 15 psi for example will not be able to provide the same vmd droplet size at the necessary flow rate as one that has 1000 psi of pressure to meet the required volume of 600 hp through a much smaller orifice.

It's been a few years since I had any physics or chemistry so bear with me but I had the understanding that the effectiveness of water injection was based on the high specific heat of water and the heat energy absorbed in the phase change from the latent heat of vaporization? So a single water molecule, if possible to practically isolate, would still absorb quite a lot of energy relative to other substances that could be injected but one molecule would not have the water mass to make a significant difference in the total heat of the system.

I would agree that extra energy would be obtained from the disruption of van der waal forces in a pure water injection system but wouldn't the polar hydroxyl group of the ionized methanol disrupt the majority of those forces immediately?

I also agree that the evaporation rate is solely proportional to the surface area exposed given the same altitude, boost/compression pressure and temperature. I think we are in agreement as well that a larger molecule solution of a given volume has less surface area exposed compared to a smaller molecule solution of the same volume due to there being so many more smaller spherical molecules. Isn't it 8 fold more molecules for a 1 fold reduction in diameter?

I think you are indicating to me is that the increased evaporation rate doesn't matter as the more water mass injected the better as the cylinder temperature will evaporate even a 500 micron rain droplet until the engine is hydrolocked or the oil is diluted too much?
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Old 17-02-2014, 11:58 PM
Richard L Richard L is offline
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Default Re: Optimum droplet size?

Thanks for the continuing, it is a very interest topic.

Agree on air assisted nozzle is needed in the absence of high line pressure. I much prefer low pressure line if I am able to regulate and control the flow as easy as a PWM valve. At present, I like to leave the effect of an impaction needle tip to narrow down the scope of this discussion.

The effect of specific heat is small compared to latent heat. When a water becomes a single molecule, it has no latent heat but still retains the specific heat property, as temperature increases, the molecule just expands and has minimal heat absorption properties. I think that is what you referring to earlier. Total agreement.

Adding methanol to water makes the discussion much complicated as steam table is readily available but methanol is not that common as the refrigerant CFC. So I like to leave methanol out of this discussion. You also mentioned the ionizing effect to the water molecules. I really do not have any idea how to tackle this, way beyond me.

No disagreement with the surface area is vital to rate of evaporation.

I was trying to say with a given mass of water, regardless how large or small the molecule is, it will extract the same amount of heat when it pass through the engine. The molecule size determines how much heat is absorbed at a given stage of the journey across the engine. Smaller molecule will cool the inlet stage more but less latent heat for the combustion chamber. It is unlikely the inlet tract has enough energy to break the the vapour into a single molecule. There are factors such as saturated vapour will stop further evaporation taking place.

My point was, super-fine droplets will only be effective up to a certain point. Similarly the water molecular in the induced air during the day has little effect on cooling compared to the mist on a cool evening (larger molecule).
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Old 19-02-2014, 06:52 PM
Aquaman Aquaman is offline
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Default Re: Optimum droplet size?

Quote:
Originally Posted by Richard L View Post
My point was, super-fine droplets will only be effective up to a certain point. Similarly the water molecular in the induced air during the day has little effect on cooling compared to the mist on a cool evening (larger molecule).
Interesting point! Has anyone used ambient humidity sensors to modulate the flow volume of their water injection systems to maintain the "target" 15% water to fuel ratio without wasting precious reserves or over diluting the oil without benefit?

Is 165 degrees celsius/330 F an appropriate compressor outlet temperature for a 60% efficient turbo given 10 deg C/50 F ambient air?

I found an interesting website on evaporative cooling that is very similar to our discussion...

http://www.cheresources.com/cementkiln.shtml

if you knew the droplet size and boost pressure/charge pipe diameter to calculate air velocity couldn't you extrapolate from that information approximately how far a known mass of water would make it before being completely evaporated and where a second stage of water injection would be needed to make it into the combustion chamber? I guess port velocity would change based on throttle position but perhaps if calculations were based on WOT it would be more than adequate?

How would you know how much additional is needed in the combustion chamber? Wouldn't that cause more oil dilution and if temperature were dropped too much result in reduced efficiency? I am assuming that 15% target number builders shoot for has been derived from the NACA articles and those articles do mention excessive oil dilution if I remember correctly.
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Old 19-02-2014, 11:54 PM
Richard L Richard L is offline
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Default Re: Optimum droplet size?

When the temperature is raised, it will allow more evaporation to take place. When pressure is raised, the process of evaporation is reduced. The size of water droplet affects the rate of evaporation. These are two different processes.

Cooling towel has plenty of heat to support evaporation leading to temperature drop.

If you wish to calculate the effect of water, you can read the following:
http://www.aquamist.co.uk/rescr/faq/...s.html#results

It is not too complicated to understand if you think of the process as enthalpy change.
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Old 22-02-2014, 09:49 AM
Richard L Richard L is offline
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Default Re: Optimum droplet size?

Quote:
Originally Posted by Aquaman View Post

if you knew the droplet size and boost pressure/charge pipe diameter to calculate air velocity couldn't you extrapolate from that information approximately how far a known mass of water would make it before being completely evaporated and where a second stage of water injection would be needed to make it into the combustion chamber? I guess port velocity would change based on throttle position but perhaps if calculations were based on WOT it would be more than adequate?

How would you know how much additional is needed in the combustion chamber? Wouldn't that cause more oil dilution and if temperature were dropped too much result in reduced efficiency? I am assuming that 15% target number builders shoot for has been derived from the NACA articles and those articles do mention excessive oil dilution if I remember correctly.
Since you cannot separate the the effect of water during it's journey from the intake to the exhaust, the only control you have is the manipulating the droplet size. Although the enthalpy change at the inlet tract is minor compared to combustion chamber, it is significant. That said, it is not as easy as you think because of the large swing of conditions due to the varying environment change.

I believe a wmi system should track overall engine load rather than just boost, a single entity is subject to change depending on gearing and rpm. The aquamist system tracks fuel flow, has been extremely successful in keeping the flow from "over" and "under" cooling. At the same time, keeping the droplet as small as possible to maximise the effect of induction cooling. It is just speaking from past experiences.
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Old 24-02-2014, 12:09 AM
Dust Dust is offline
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Default Re: Optimum droplet size?

Something that I didn't see discussed yet. The larger the droplet gets, the more mass it has, and the less likely it is to flow with the air and more likely to fling itself against something. With a direct port setup it obviously wouldn't matter, but stepping farther away makes smaller droplets more suited for going with the flow.
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Old 24-02-2014, 12:29 AM
Richard L Richard L is offline
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Default Re: Optimum droplet size?

If distance is available, more time for the evaporation to take place resulting in smaller droplets. This is well proven at the N54 platform. There were two major suppliers and within a year, the jets were moved quite close to the IC exit flange. It is extremely easy to test and confirm because there are two WBO2 sensors from factory for each bank. Logging software proved this theory.
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Old 25-02-2014, 09:26 PM
Aquaman Aquaman is offline
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Default Re: Optimum droplet size?

Quote:
Originally Posted by Richard L View Post
If distance is available, more time for the evaporation to take place resulting in smaller droplets. This is well proven at the N54 platform. There were two major suppliers and within a year, the jets were moved quite close to the IC exit flange. It is extremely easy to test and confirm because there are two WBO2 sensors from factory for each bank. Logging software proved this theory.
I apologize for being dense but what was the theory that was proven?
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