Wood stove top fans (heat powered)

[Egon]

This thread has well exceeded my pay grade!

 

[Snobdds]

Your wood stove produces more than just radiant heat. The air molecules that touch your wood stove are heated through the process of conduction. Direct contact with each other. That causes the air molecules to expand. As they do, they move up and away from the stove, and cooler, denser molecules take their place. That's called convection.

That isn't quite how thermal radiant heat transfer works. In order for conduction to work, there has to be a temperature difference between two conductive bodies where heat can flow from hot to cold. It is not an instant thing either, there is a minimum time function for the transfer of energy to happen. Molecules, do not have enough matter, nor does the radiation waves have enough time to heat something with no density. The more dense an object, the more infrared heat waves it can absorb. Think cast iron vs aluminum.

I think what your thinking is infrared heat waves from a radiant heat source. They are the things that emit from a wood stove and travel until they hit an object, where the object then absorbs the heat and warms up. As a physics professor in college beat into us...radiant heat heats the objects in a room, not the air in the room. This flows nicely into confirming air molecules are not affected by radiant heat. Radiant heat does not need convection to warm things, think of a in slab radiant floor heat. There is no fan pushing those infrared heat waves, but the house is warmed.

 

[Snobdds]

Convection occurs within a medium based on the CHANGES in DENSITY attributed to TEMPERATURE.

The AIR molecules around your burning wood stove impacts the hot surfaces Caloric heat is transferred to the air molecule BY CONDUCTANCE, causing that molecule to INCREASE it's VELOCITY. The INCREASED VELOCITY requires MORE VOLUME for a given number of air molecules (mostly Nitrogen 2) at a constant PRESSURE.
Thus DECREASING the density of the warm air relative to the cold air that has NOT come in contact with the hot stove surface. Random Collisions of the fast moving molecule with other air molecules can result in conductive heat (energy as velocity?) transfer both ways depending on collision physics.
Lesser density will "float" on greater density, whether air , water or iron.
The Heat Transfer by Convection ignorant will say "heat rises".
Those in the know will say, HOT AIR rises. Hot water rises, hot air balloons rise aloft. Cold air sinks, the cellar is cold, the loft is warm.

Actually, Cream rises, but that IS a different topic.

ETA
I just now put wood in the stove. The ceiling fan is turning overhead,
I put on leather gloves to protect myself from the damaging effects of conductance while exposed to the hot coals.
I could feel the radiant heat of the stove dramatically increase when the glass door was opened. (Glass is essentially opaque to far infra red radiation. !0 micron wavelength is about the human body temp emission.)

Planck's Law​

Planck's Law can be generalized as such: Every object emits radiation at all times and at all wavelengths. If you think about it, this law is pretty hard to wrap your brain around. We know that the sun emits visible light (below left), infrared waves, and ultraviolet waves (below right), but did you know that the sun also emits microwaves, radio waves, and X-rays? OK... you are probably saying, the sun is a big nuclear furnace, so it makes sense that it emits all sorts of electromagnetic radiation. However, Plank's Law states that every object emits over the entire electromagnetic spectrum. That means that you emit radiation at all wavelengths -- so does everything around you!


It's all a matter of degree! ;-)

This was hard to follow...

 

[MossRoad]

You said your stove only produces radiant heat. And you said air molecules have no density. If that's the case, how does the air in the room get warm and why do we feel the wind when there's a breeze?

Air molecules have density. It's why we feel a breeze, and why there's air pressure. The weight of the molecules stacked on top of each other a gazillion high up into space. 14.7 pounds per square inch at sea level.

When the metal on a wood stove heats up, the air molecules that are being pressed against it with 14.7PSI are heated up through conduction. They get lighter and move off. Colder molecules take their place and the process repeats. It's conductive heat transfer to the molecules and then convective heat transfer (the transfer of heat from one place to another due to the movement of fluid (air)) in the air in the area of the stove.

The stove also produces radiant heat. That's why your skin gets hotter than the air when you're standing in front of the stove.

Your stove produces both radiant and conductive heat transfers, and causes convective heat transfer. All three.

 

[Snobdds]

You said your stove only produces radiant heat. And you said air molecules have no density. If that's the case, how does the air in the room get warm and why do we feel the wind when there's a breeze?

Air molecules have density. It's why we feel a breeze, and why there's air pressure. The weight of the molecules stacked on top of each other a gazillion high up into space. 14.7 pounds per square inch at sea level.

When the metal on a wood stove heats up, the air molecules that are being pressed against it with 14.7PSI are heated up through conduction. They get lighter and move off. Colder molecules take their place and the process repeats. It's conductive heat transfer to the molecules and then convective heat transfer (the transfer of heat from one place to another due to the movement of fluid (air)) in the air in the area of the stove.

The stove also produces radiant heat. That's why your skin gets hotter than the air when you're standing in front of the stove.

Your stove produces both radiant and conductive heat transfers, and causes convective heat transfer. All three.

where does an air molecule store heat energy to then pass on to another air molecule?

 

[MossRoad]

where does an air molecule store heat energy to then pass on to another air molecule?

Kinetic energy.

 

[MossRoad]

This explains it pretty well. It shows cold air passing over a warm surface, heating up, and causing natural convection (not forced air like a fan).

Understanding Convective Heat Transfer: Coefficients, Formulas & Examples

Heat transfer between a solid and a moving fluid is called convection. This is a short tutorial about convective heat transfer.

www.engineeringtoolbox.com

 

[Snobdds]

Kinetic energy.

LOL...motion is not thermal.

 

[MossRoad]

The air gets hotter, the molecules move faster. The whack into each other and things.

 

[MossRoad]

Let's face it. Your stove produces three kinds of heat transfer.