If temperature gradients cause wind in air, how about in zero gravity?

[Buzzworks]

If temperature gradients cause wind in air, how about in zero gravity??

Hey guys, I'm aware that atmospheric temperature gradients causes changes in bouyancy so hot air goes up and cold air goes down and immediately fills in the 'void' the hot air left, thus causing wind.

What about in zero/micro gravity, how does temperature gradients in air behave? Will the differences in kinetic molecular speeds in temperature gradients still cause wind or none at all? How strong or how weak the wind will be if there are any and what direction (hot to cold or vice versa)?

 

[Andy Resnick]

There is no convective flow in orbit (there is a little; it's not zero gravity, it's microgravity). This makes fluid mechanics very interesting, both for experiments and for on-orbit cooling systems.

 

[Buzzworks]

Thanks for the answer! Perhaps to have convective flow, fluids had to be rotated or spun into a vortex by using rifled pipes!

 

[QuantumPion]

There is no convective flow in orbit (there is a little; it's not zero gravity, it's microgravity). This makes fluid mechanics very interesting, both for experiments and for on-orbit cooling systems.

I'm pretty sure convective flow still occurs, it is just directionally independent due to no buoyancy force. E.g. a candle flame would be spherical.

 

[Darryl]

Convection requires a fluid, and a force acting on it, that could be a fan or gravity, in forced convection or 'natural' convection.

So in space, or zero gravity, as long as the air is moving (somehow), there will be heat transferred by convection heat conduction.

trying to remember the 4 types of heat transfer, got 3 out of 4..

conduction
convection
radiation
state phase change (turning a solid to a liquid, or liquid to gas) takes heat away.

I guess now days, you could also include laser techniques for slowing down molicules, therefore removing heat from the system. If that can be considered a form of heat transfer.

 

[Andy Resnick]

I'm pretty sure convective flow still occurs, it is just directionally independent due to no buoyancy force. E.g. a candle flame would be spherical.

What do you think drives convective (or bouyancy-driven) flow?

Not only are candle flames spherical, but unless fuel and oxygen are actively supplied, the flame extinguishes vary rapidly.

And if boiling occurs in microgravity, the bubble sits on the solid surface, which can lead to burnout of the heating element.

 

[QuantumPion]

What do you think drives convective (or bouyancy-driven) flow?

Not only are candle flames spherical, but unless fuel and oxygen are actively supplied, the flame extinguishes vary rapidly.

And if boiling occurs in microgravity, the bubble sits on the solid surface, which can lead to burnout of the heating element.

Diffusion due to temperature gradient?

I've seen pictures/videos of flames in zero-g that were not actively supplied by oxygen.

 

[Andy Resnick]

Diffusion due to temperature gradient?

I've seen pictures/videos of flames in zero-g that were not actively supplied by oxygen.

Diffusion occurs due to a *concentration* gradient. Thermocapillary (Marangoni) flow is a process that allows flow to develop due to temperature gradients, but requires an interface.

Here's what flame in microgravity looks like when you don't add reactants:





Diffusion-limited transport also affects crystal growth in microgravity, often leading to dendritic crystals.

 

[QuantumPion]

Diffusion occurs due to a *concentration* gradient. Thermocapillary (Marangoni) flow is a process that allows flow to develop due to temperature gradients, but requires an interface.

Here's what flame in microgravity looks like when you don't add reactants:





Diffusion-limited transport also affects crystal growth in microgravity, often leading to dendritic crystals.

As shown in those videos, the ball of flame does expand outwards spherically before it self-extinguishes, what causes this then?

 

[Andy Resnick]

As I mentioned earlier, unless fuel and oxidizer are actively supplied, the flame will extinguish itself.

 

[inflector]

As shown in those videos, the ball of flame does expand outwards spherically before it self-extinguishes, what causes this then?

Heat still causes expansion of the gases even in zero gravity.

 

[94JZA80]

Hey guys, I'm aware that atmospheric temperature gradients causes changes in bouyancy so hot air goes up and cold air goes down and immediately fills in the 'void' the hot air left, thus causing wind.

What about in zero/micro gravity, how does temperature gradients in air behave? Will the differences in kinetic molecular speeds in temperature gradients still cause wind or none at all? How strong or how weak the wind will be if there are any and what direction (hot to cold or vice versa)?

i tend to agree with Andy if we stick to your specific interpretation of convection as the movement of air molecules differing in temperature directly toward or away from a source of gravity. that is, there is very little convective flow in orbit b/c there is very little gravity, and that there is no convective flow in absolute zero gravity. simply b/c the convection of air relies on both the existence of temperature gradients AND gravity.

but even if there is no convection (air movement directly toward or directly away from some source of gravity) to induce wind in a zero-gravity environment, wind can be induced via temperature gradients in other ways. remember that atmospheric temperature gradients also cause atmospheric pressure gradients. and these pressure gradients not only result in air movement directly toward or away form the center of the earth, but also air movement more or less parallel to the surface of the earth. this is why air from high-pressure systems spreads out and flows in the direction of adjacent low-pressure systems. so not only is wind on Earth caused by the movement of air perpendicular to the Earth's surface (convection), but also by the movement of air parallel to the Earth's surface (pressure differentials). for a specific example, imagine that there is a high-pressure system over the eastern U.S. and a low-pressure system over the western U.S. folks all over the U.S. can then be expected to feel a westerly wind (wind moving from east to west) until the pressure in the east decreases and the pressure in the west increases to a point of equilibrium. think "entropy."

now transfer this train of thought to a zero-gravity environment. imagine a mass of air floating freely in space, free of any gravitational influences (or imagine the mass of air enclosed in a box that's floating freely in space, and just take the gravitational pull of the walls of the box to be negligible...either example is suitable). even if there is a temperature gradient in the mass of air, it will not convect b/c there is no source of gravity. i.e. the air molecules won't align themselves in layers of decreasing temperature/increasing density in the direction of some gravitational pull b/c we are in a hypothetical zero-gravity environment. any areas of high pressure (and thus denser air) though will spread out to fill in areas of lower pressure, causing air movement, and thus wind...at least up until the point that all the high-pressure and low-pressure areas have reached equilibrium and the pressure is constant throughout the entire mass of air.

so yes, wind can exist in a zero-gravity environment so long as there is a temperature gradient in the fluid.

...notice though that I'm holding to your definition of convection as the movement of air directly toward or away from a source of gravity, and it differs slightly from say, Darryl's definition. but if we define convection generally as buoyancy-driven flow (as Andy described it), then perhaps what i just described above is in fact convection. but either way, you can see how wind can occur in a gravity-free environment.