Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: scientizscht on 24/05/2019 21:04:44
-
Hello!
If we have a nozzle that releases combustible fuel and we maintain a flame at the tip of the nozzle as we keep the flow of fuel through the nozzle:
This flame consumes oxygen at a high rate as lots of fuel can be burned per second.
At the same time, the oxygen is replenished around the area of the flame through passive diffusion in the atmosphere.
My question is, given that the oxygen diffusion coefficient in the atmosphere is approx 1 cm2/s, how does it replenish so far the very high rate of oxygen consumption by the flame?
What would be the rate of oxygen consumption that would surpass the rate of oxygen replenishment from diffusion in the atmosphere?
-
A candle flame creates a thermal draught that sucks fresh air into the base of the combustion area. Most nozzle flame guns have a premix chamber where a high velocity of fuel gas entrains excess air (like the vent on a Bunsen burner).
For hydrocarbon fuels generally you need somewhere between 10:1 and 15:1 air/fuel mix for efficient combustion, so diffusion through a static air mass won't sustain continuous burning. A gas cooker relies on venturi premixing but the high-output burner of a hot air ballooon squirts preheated liquid fuel into the flame volume and relies partly on convection to sustain the air supply. Turbo-Diesel and jet engines use mechanical compressors to ram air into the combustion chamber.
-
If we have a nozzle that releases combastible fuel, then
... we do not have a diffusion limited system.
-
Are you sure that your cited oxygen diffusion coefficient (of 1cm2/s) is accurate - that seems high to me. THe figures I've seen are closer to 0.2 [http://compost.css.cornell.edu/oxygen/oxygen.diff.air.html]
-
"How does a burning flame replenish its oxygen supply sufficiently fast?"
because "sufficiently fast" is determined by how quickly oxygen can get to the flame.
Incidentally, a diffusion flame is rather different from the ones you generally see.
-
Are you sure that your cited oxygen diffusion coefficient (of 1cm2/s) is accurate - that seems high to me. THe figures I've seen are closer to 0.2 [http://compost.css.cornell.edu/oxygen/oxygen.diff.air.html]
I read that CO2 has 0.8cm2/s so I assumed oxygen would be a bit higher, but it's surprising that it's actually lower.
Btw what does 1cm2/s really mean? In terms of speed? I could understand a speed of 1cm/s but 1cm2/s?
-
Are you sure that your cited oxygen diffusion coefficient (of 1cm2/s) is accurate - that seems high to me. THe figures I've seen are closer to 0.2 [http://compost.css.cornell.edu/oxygen/oxygen.diff.air.html]
I read that CO2 has 0.8cm2/s so I assumed oxygen would be a bit higher, but it's surprising that it's actually lower.
Btw what does 1cm2/s really mean? In terms of speed? I could understand a speed of 1cm/s but 1cm2/s?
When you ask a lot of questions, and you ask a lot of questions, then you have to be prepared to honestly assess the answers you are given. You seem to want to straw man everything.
The positive outcome is that a lot of science gets discussed in a way that informs the general membership.
Thank you for being such a help in encouraging other members to share their knowledge. Without members such as yourself we might not have such an excellent learning environment.
-
Are you sure that your cited oxygen diffusion coefficient (of 1cm2/s) is accurate - that seems high to me. THe figures I've seen are closer to 0.2 [http://compost.css.cornell.edu/oxygen/oxygen.diff.air.html]
I read that CO2 has 0.8cm2/s so I assumed oxygen would be a bit higher, but it's surprising that it's actually lower.
Btw what does 1cm2/s really mean? In terms of speed? I could understand a speed of 1cm/s but 1cm2/s?
When you ask a lot of questions, and you ask a lot of questions, then you have to be prepared to honestly assess the answers you are given. You seem to want to straw man everything.
The positive outcome is that a lot of science gets discussed in a way that informs the general membership.
Thank you for being such a help in encouraging other members to share their knowledge. Without members such as yourself we might not have such an excellent learning environment.
Thanks it's indeed the opposite of most universities.
-
Fick's Law of Diffusion is
J = − D δφ /δx
where
J is the diffusion flux, of which the dimension is amount of substance per unit area per unit time, so it is expressed in such units as mol m-2 s-1. J measures the amount of substance that will flow through a unit area during a unit time interval.
D is the diffusion coefficient or diffusivity. Its dimension is area per unit time, so typical units for expressing it would be m2/s.
φ (for ideal mixtures) is the concentration, of which the dimension is amount of substance per unit volume. It might be expressed in units of mol/m3.
x is position, the dimension of which is length. It might thus be expressed in the unit m.
hence the rather strange dimensions of D.