Failure to recognise when an aerosol isn't a droplet 26

[LittleInch]

FacEngrPE dropped this in an obscure post and it reads very well.

Basically the whole epidemiological world though various dieses and viruses were spread by "droplets" which landed on surfaces and then infected people or were sneezed at you.

And a lot of times they are probably right.

But there was a magic 5 micron cut off between droplets and aerosols. Why? Read on

https://www.wired.com/story/the-tee...t-helped-covid-kill/?utm_source=pocket-newtab

So is this a disaster - Well you tell me.
But it goes to show that just because a lot of people write something, it doesn't mean that they originally got the wrong end of the stick and then the error repeats itself until it becomes fact.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Because IMO it (mass = volume) happens to be a coincidence. True only under the right conditions. Mass is what is being heated (ΔT = Q x Mass x Cp , Cp = 1 F/Btu-lbm). Volume is irrelevant to reaching the approximate answer we were looking for, estimated dropet lifetime. If we want to consider additional geometric factors to improve our model for more accurate evaporation effects, then I would suggest that surface area would be far more appropriate to consider than the volume. That would also allow us to easily examine shapes other than perfect spheres, subject to varying temperatures, all kinds of shapes ... if we wanted, without introducing additional complications of volumes or densities. It just seemed wrong to me to talk about anything but mass and heat in the context of gravity, trajectory and expected lifetime of droplets.

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[1503-44]

Compositepro, exactly what I've been trying so hard to say. Except for one thing. IMO it is not the volume, its the mass to surface ratio (i hesitate to mention anything about ratios), the heat flux across the surface and, specific heat of that mass that drives the increase in temperature that eventually causes it to reach the phase boundary. Did I say that correctly? Are we good?

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[TugboatEng]

There is no need to bring in mass and specific heat when making comparison between the same substance. We are talking water droplets here. You could use mass to surface area and multiply by density to represent volume but you're bringing in unnecessary units when you can make the same calculation using only one of the fundamental units, length.

 

[1503-44]

I thought I said that.
" without introducing additional complications of volumes or densities."

But you're right about trajectory is velocity and inclination angle, so if that's flat and all have the same velocity, yeah. Mass cancels for gravity, but still does not for lifetime. All particles hit the same spot, but some still evaporate faster or slower than others.

OK, I guess we can set Cp = 1 for water. But the large drops still live longer, just because there's more mass.

Thats what I said a long time ago, but wasn't going to work that out to any more level of detail. Have at it.

I'm outta' here.
BTW Lighten up.
Super major PIA for nothing.
Are you guys in it for the arguing?
Seems like it.
If this forum isn't enjoyable, what's the point?
Ratios?




If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[LionelHutz]

Because IMO it (mass = volume) happens to be a coincidence. True only under the right conditions.

Well, mass may not be exactly proportional to volume, but it's a pretty damn good approximation for the conditions of interest in this thread. Anyone suggesting so certainly isn't warranted the belittling you have been throwing at them.

Some still evaporate faster or slower than others.

As engineers, we're discussing why this happens, and how fast it happens. Mass doesn't explain it. Only mass mattering isn't the answer. Just because there is more mass or less mass isn't the answer. Like many subjects, it's much more complex than that.

 

[1503-44]

Yeah. Lots of good times here.

You started it. BS, engineers talking about constants.
Get over yourselves.
Mass never equals volume. Thats just wrong.


If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[electricpete]

fwiw I'm in agreement with Lionel. But I think it's just a matter of perspective and maybe the relativistic effects of beertonian fluids ....the mass of beer in my fridge mysteriously decreases whenever my relatives visit!
Maybe it's time to move on...

=====================================
(2B)+(2B)' ?

 

[IRstuff]

One of the links posted today by Eufalconimorph answers the question; evaporation time is proportional to droplet diameter, as would be expected

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Alistair_Heaton]

Well actually this bun fight has made me think and better understand some of my work stuff on fog and icing.... So its not wasted.

Wait to you see the stuff on super cooled droplets and ice formation and they start quoting tons per min on your airframe.

 

[1503-44]

IRsttuff Not so fast. Since apparently mass can equal volume here at eng-tips and volume is PROPORTIONAL to diameter, why do you not suspect that perhaps the graph is showing diameter in error, when that should really be dependent on mass. If you agree that mass = volume, neither do you have any basis to citizens that opinion.

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[1503-44]

Now its getting interesting again.

OK Alistair, pop test. When supercool liquid water droplets impact your aircraft, freeze and stick, is it the mass or the volume that was undergoing the phase change. [pre][/pre]

IR, that graph literally shows us that when "diameters are boiling", they get smaller at a linear rate much faster than when the diameters are not boiling, right?.

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[RedSnake]

Maybe it's just me who's super tired and my brain not really working properly, but it seems strange to me that if Dc or is it De stands for diameter that a droplet at a diameter 1,75 mm should evaporate in 35 sekunds at +200 C.
I am not saying it's wrong, I am just a bit surprised.
Do you have a link.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[1503-44]

the paper link is

https://www.ht.energy.lth.se/fileadmin/ht/Kurser/MVK160/2012/Holger.pdf

I have to admit that I do not understand the minimal explanation given in the paper for figure 4, nor the reason it is opposite to other observations.


I read it differently too. But is it 35 or 0 seconds?
A droplet of D = 2mm at 100 C, evaporates in 0 seconds???

Start on the left with D = 1.5mm, go right to 100 C, go down to 58 seconds.
for D = 1mm, go right to 100 C, down to 120 s

Smaller drops take longer to evaporate.

RedSnake, its a Swedish study. Maybe some translation difficulty???
IRStuff is usually pretty good with heating, and most things really, so I'm sure he can tell us what's going on with that chart.
I can't even seem to interpret it correctly.

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[Alistair_Heaton]

Pop quiz answer

Depends on the airframe temp and size and will dictate the type of icing formed.

Volume instant freeze will produce rime icing which has a huge effect roughing up the aero dynamic surfaces. This is fog type icing it hugely increases drag.

And mass takes longer and the water will splat and run giving clear ice. This usually occurs with bigger size than we are discussing and is a feature of freezing rain and flying through CB's. This adds significantly to the weight of the aircraft.

As someone else mentioned surface tension comes into play so it's not going to be anything like what we would accept as logical from experience from boiling pans of water.

 

[Alistair_Heaton]

I am confusing myself now... I am in the sim for the next 6 hours so will look forward to the discussion on my survival of everything but icing.... That's only because it's the summer session so everything will be hot and humid temperature limited.

 

[1503-44]

"mass" Correct! At least 'till further notice.

Have a good one.


If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[Alistair_Heaton]

The freezing fog though is very different. And the ice is granular and will take your skin off if brushed against.

But we are now into Swedish territory. They more than likely have 400 different words for different types of freezing water.

 

[RedSnake]

Yes the Sami have that, because it's a way of referencing the structure of the snow and ice in relation to how easy or difficult it is for the reindeers to find food and water and also the people.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[LionelHutz]

"It is interesting that, in opposite to papers [1] and [2], the droplet diameters were found to decrease linear with time in paper [3]. The droplet diameters over time for an air velocity of 1m/s and for varying temperatures are shown in Fig. 4."

Same drop. The diameter reduces linearly with time, not surface area, nor volume, nor mass.

 

[RedSnake]

RedSnake, its a Swedish study. Maybe some translation difficulty???

Figure 2 [1], 3 [2] and 4 [3] isn't made in Sweden they are taken from 3 different other studies.
They have very different approaches and are a mix of calculations and actual experiments.

[1] Kinetics and evaporation of water drops in air, H.J. Holterman, IMAG report 2003-12, July 2003
[2] LABORATORY STUDIES OF WATER DROPLET EVAPORATION KINETICS, A.M. Moyle, P.M. Smidansky and D. Lamb, Department of Meteorology, The Pennsylvania State University, University Park, PA 16802
[3] The Evaporation of Water Droplets. A Single Droplet Drying Experiment, D.E. Walton, Drying Technology: An international journal, 22:3, pp. 431-456, 2004

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein