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hankwang writes:
"For the past couple of days, China's capital has been suffering under severe smog, leaving the sun and skyscrapers barely visible. The highest concentration of small airborne particles (PM2.5) was 0.50 mg per cubic meter, a factor 20 above the Word Health Organization's safe limit. Scientists went as far as comparing this to a nuclear winter. The worst seems to be over for now: today, the monitors are reading 0.180 mg/m3, only a factor 7 above the WHO limit.
The Chinese smog seems to behave differently from the smog in Europe and the US. Existing scientific models developed in the West do not work well. To improve the models and understanding, plans are underway to build a 600 cubic-meter (that's 21,000 cubic ft or 160,000 US gallon) transparent dome as a smog chamber."
[NOTE TO EDITOR: I can't get slashcode to display the mu symbol, so I converted to milligrams] [Ed's Note: Thankyou - but what is it in firkins?]
(Score: 5, Interesting) by anubi on Monday March 03 2014, @06:00AM
I have had some interesting observations regarding high voltage negative DC ( air ionizers ) and particulates.
I note once I have charged particulates suspended in air, they head for ground. Whatever is grounded. I have experimented with a device much like one of those Sharper Image Ionic Breezes, and note it is extremely effective in removing crap out of the air. Actually, mine was just charged tin cans configured like a "lifter" [jnaudin.free.fr] whose "thrust" was to move air like a ceiling fan. I thought it would be a great thing to have in the pet store down the street, as all the animals in there could give that end of the mall a distinctive odor. It did not remove odors all that well. However everything needed dusting very frequently when it was in use.
I note China is building high-voltage DC transmission lines at the Three Gorges Dam [nationalgeographic.com]. Would it be feasible to tap some of that power and run it up to a tethered dirigible covered in some fine spiky material ( like stainless steel wool) with the intent to charge it up to make a very large area air ionizer? Before I get nailed for frankenstein-thinking, consider the very same thing happens during thunderstorms with so much voltage gradient being created from air rubbing against air that the electricity literally jumps out of the sky.
The voltage levels look in the 500KV range. What I do not know is this may make too much ozone, as I know my little experimental box makes ozone if I crank it up too high. With mine, if I heard it hissing, it was making ozone.
I am thinking if they can get enough surface area up there, with enough ionic emission, they may be able to charge that crap in the air to make it plate itself to anything grounded. And this is not a fix-all solution - it oughta get it out of the air, but I have a strong idea it will also make a dusty mess plating everything outdoors like a blanket of fine dust, which will have to be cleaned off, but at least it will be onto the ground instead of in everyone's lungs.
I really feel for the Chinese in this dilemma. If I wasn't so damned old, I would love to go over there and experiment with this thing. To me, that is what Engineering is all about. I can only imagine the frustration the Chinese go through having a cloud of hardly breathable air. In this day and age of technology, no-one should have to put up with that.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 4, Informative) by hankwang on Monday March 03 2014, @07:24AM
A device that sprays electrons in order to charge particles is certainly going to make a lot of ozone. Ozone is actually part of the smog problem, even though the smog reports focus on the PM2.5 particle readings, so the device had better not create too much ozone indeed.
Basically you are proposing an gigantic electrostatic air filter, similar to the ones you can buy for use at home. I'm not sure how this would scale up. The electrostatic force on a charged particle is proportional the electric field strength (volts per meter) and must be strong enough to overcome the mixing due to wind turbulence. In a small electrostatic dust filter, the air may be passing through electrode plates spaced 1 cm apart, at 1 m/s velocity, which gives a Reynolds number [wikipedia.org] of 70. That is going to be pretty much laminar flow without turbulence. However, wind outside with building-sized obstacles is very much turbulent.
Apart from that, you'll have the problem of occasional sparks. Although being killed by lightning strikes is a pretty rare way of dying, that may very much change if you put a high-voltage system like you propose above a metropolian area...
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(Score: 2, Interesting) by anubi on Monday March 03 2014, @08:27AM
Yes... I do not think I would get many sparks from a dirigible several hundred feet up; as its far higher up than power lines. Anything getting near it would charge up to the local gradient anyway just as crows do when they roost on overhead power distribution cables.
However its the field gradient you illustrated may be insufficient to do the job.
I do not know how well this would scale up either. That's why I throw this out here for others which may have a better feel for this to comment on.
I was hoping the turbulence would mix the ionization with the city air sufficiently to charge a wide swath of atmosphere, causing its particulate load to be attracted to the ground. I was considering keeping the dirigible charged via an electron flow in its tether, which would be connected into the DC power grid. If this DC were negative with respect to earth, then it looks to me like this might work.
I wonder if the dirigible were positive, it would then attract the airborne crap, but how long could it stay aloft as I consider the Chinese problem as having tons of airborne soots of various types.
My stuff was powered by a television flyback transformer with rectification. I could get either polarity by reversing the diode and the drive to the transformer. Being I worked as a TV repairman in my earlier days, I was quite aware of how high positive voltages would attract all sorts of things out of the air. A TV repair for someone who smoked was not a fun job; the HV cage would be full of tarry dust.
I guess what I am trying to do is inject electrons into the air only to have them charge any airborne particle they encounter... once charged, the particles are attracted to ground.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 3, Informative) by hankwang on Monday March 03 2014, @10:39AM
I see two issues here.
1. Assuming that the home electrostatic filter needs about 1 kV over 1 cm (E=100 kV/m field strength) to catch small particles within the 50 ms travel time along the electrode plates, it appears that the drift velocity at that field strength is about 0.2 m/s. Atmospheric turbulence involves air velocities that are much higher than that.
2. Moreover, if you scale up this concept to atmospheric dimensions, then having a 500 kV potential over 100 m distance is actually only 5 kV/m of field strength - i.e. only 0.01 m/s of particle drift velocity. So, this is not going to work. If you did increase the voltage, the risk of causing lightning bolts would go up considerably.
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(Score: 1) by anubi on Tuesday March 04 2014, @02:39AM
Thanks, Hank.
I appreciate the feedback on the technical aspects. I am trying to structure this as an electrostatic precipitator with the earth itself as the positive plate and the dirigible as the negative ion emitter. I was hoping maybe it would not be so windy at ground level that the precipitate could drop out at ground level. If it were blowing over the ground, the particle drift velocity, like you say, would be so great that the kinetic energy of its motion would exceed the forces attracting it to the plate and the deposition would not happen.
I sure hope they try to precipitate at the coal plants themselves as you indicated in another post. When I worked at the Chevron Oil Refinery in Pascagoula, Mississippi, I remember walking into huge "buildings" with electrical plates hung from the ceiling whose function was to charge emissions from the refining process and drop them out before they reached the atmosphere.
I know you already know all about this, but I will link it for the casual readers of this thread what this is all about...its known as a "Cottrell precipitator". [google.com].
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 2) by hankwang on Tuesday March 04 2014, @08:53AM
I found some lecture notes on electrostatic precipitators with harder numbers: http://ce.udel.edu/~dentel/434/Lecture6-ESP.pdf [udel.edu] . My hand-waving estimate based on what I think a home-ESP looks like (100 kV/m, 20 cm/s drift velocity) were a bit off; on page 7 it shows an example calculation: 300 kV/m and 3 cm/s drift velocity for typical 1 micron particles.
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(Score: 1) by anubi on Wednesday March 05 2014, @01:56AM
Thanks. Hank!
I find these precipitators an interesting thing to study, being its so very much easier to clean up a mess before its made ;) .
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 4, Informative) by hankwang on Monday March 03 2014, @10:49AM
P.S. FYI Electrostatic precitipator [wikipedia.org]. Apparently, they are commonly used at the exhaust of coal plants. Given that coal plants are suspected to be a large contributor to China's smog problem, it would be better to install them there rather than floating above the city...
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(Score: 1) by ls671 on Monday March 03 2014, @10:56AM
"I guess what I am trying to do is inject electrons into the air only to have them charge any airborne particle they encounter... once charged, the particles are attracted to ground."
That's what I understood the first time ;-)
Lets pretend your idea could work perfectly: You would have to carefully estimate the impact of those concentrated pollutants, now in a solid form, around big cities where there are more grounded structures to catch the pollutants. For example, think of the rain diluting those now solid pollutants and distributing them deep in the ground.
Right now, since the particles do not fall to the ground as fast as they would, they tend to distribute on a wider area, some going around the planet. The cities would end up with a higher concentration share than what they have right now.
Everything I write is lies, including this sentence.
(Score: 1) by anubi on Tuesday March 04 2014, @02:25AM
Thanks. Exactly why I posted here. I am of the belief I do not have a solution to the problem; rather I can mutate one mess into another. The question is: Do I gain anything by trading mess#1 for mess#2?
And that's assuming this method will work, which I am dubious of.
HankWang has been posting some very insightful and informative replies to the technical aspects of this.
I feel for the Chinese and wish them the best with dealing with the mess. They are caught up in a socioeconomic situation where they are being led into trading off their environment for economic matters - matters that seem to make a few individuals filthy rich while leaving the rest simply filthy.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 0) by Anonymous Coward on Tuesday March 04 2014, @03:12AM
It's a matter of doing things in order. What I suggested you to look at comes before any prototyping or any physical implementation thus, technical details about said implementation.
What I suggested you to look at could be fairly simulated with software. IMHO, this is where you should start.
Intuitively, if this is ever going to work, I feel like the positively charged ones would make it much easier to pick up and control the mess but that's just intuition.
Peace.
(Score: 1) by anubi on Tuesday March 04 2014, @06:11AM
Often intuition and past experience is most of what I have to go on.
I love to simulate, but all too often my models, hence anything I get from simulation, is not up to snuff.
God knows how many Spice simulations of mine have given me misleading results due to bad models.
So I often try empirical approaches to see how well reality matches my models.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 2) by hankwang on Tuesday March 04 2014, @09:38AM
The question is: which software? And then: which settings? Typically, computational fluid dynamics software has many model parameters that need to be set in order to capture the relevant physical effects for the problem at hand, without wasting processing time on effects that do not make a difference. Doing so correctly requires that you have a good grasp of the underlying physics, otherwise it will be garbage in, garbage out. Moreover, once you have this good grasp, it is likely that you can do back-of-the-envelope estimates to see whether what you want to achieve is at all plausible, or likely to be orders of magnitude off.
Part of my job is doing, outsourcing, and reviewing simulations (gas flow, diffusion, thermal, deformation, particle-gas transport, and others) for engineering purposes (not electrostatic gas filtering, though), and have seen many times that simulations produced nonsense results because the elephant in the room was missed, i.e., some dominant physical effect that was not accounted for.
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