Heat radiation in vacuum

[ravindrar]

TL;DR

I have a question about heat transfer in a vacuum chamber.

Suppose I have a vacuum chamber made entirely of stainless steel and it is completely enclosed on all sides. Inside the chamber, fruit slices are placed on a tray for vacuum drying.

Assume the chamber is not externally heated. Instead, its outer surface is exposed only to ambient air at about 30°C, so the stainless steel walls gradually reach approximately the ambient temperature.

In a vacuum, there is essentially no convective heat transfer. My understanding is that the chamber walls, having absorbed heat from the surroundings, will emit thermal radiation into the chamber.

My questions are:

1. Will all the inner stainless steel surfaces (side walls, bottom, and top lid) radiate heat toward the product?
2. Will the top lid, being directly above the tray of fruit slices, also radiate heat downward onto the product?
3. Is the thermal radiation from the chamber walls sufficient to transfer a meaningful amount of heat to the fruit slices, or would the effect be negligible at an ambient temperature of only 30°C?

I would appreciate an explanation based on the principles of thermal radiation and heat transfer in a vacuum.

 

[Matterwave]

Is the tray just floating there? If not, conductive heat will flow through whatever support is holding up the tray.

As far as radiative heat transfer, you might be interested in:

https://en.wikipedia.org/wiki/Black-body_radiation

and

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

Note that you should also consider the thermal properties of the fruit to figure out the actual heat transfer that will happen.

 

[Bandersnatch]

Are the fruit slices significantly below ambient temperature?

 

[pines-demon]

TL;DR: I have a question about heat transfer in a vacuum chamber.

Suppose I have a vacuum chamber made entirely of stainless steel and it is completely enclosed on all sides. Inside the chamber, fruit slices are placed on a tray for vacuum drying.

Assume the chamber is not externally heated. Instead, its outer surface is exposed only to ambient air at about 30°C, so the stainless steel walls gradually reach approximately the ambient temperature.

In a vacuum, there is essentially no convective heat transfer. My understanding is that the chamber walls, having absorbed heat from the surroundings, will emit thermal radiation into the chamber.

My questions are:

1. Will all the inner stainless steel surfaces (side walls, bottom, and top lid) radiate heat toward the product?
2. Will the top lid, being directly above the tray of fruit slices, also radiate heat downward onto the product?
3. Is the thermal radiation from the chamber walls sufficient to transfer a meaningful amount of heat to the fruit slices, or would the effect be negligible at an ambient temperature of only 30°C?

I would appreciate an explanation based on the principles of thermal radiation and heat transfer in a vacuum.

1. yes 2. yes, note that there is no difference between the top lid and the other lids as in this problem gravity is mostly irrelevant 3. it depends on the temperature of the fruits, large temperature differences will result in more meaningful heat transfer.

 

[Roberto Pavani]

As noted, the initial temperature of the fruit is the key parameter. Heat transfer depends on the temperature difference. Note that the air inside follows $PV = nRT$, but the pressure is reduced because $n$ is reduced by pumping, so there is not necessarily any temperature change. The fruit, however, may cool due to evaporation (latent heat loss). If the process is slow enough, thermal radiation from the chamber walls can compensate for this cooling, and the system may remain close to thermal equilibrium at about 30°C.

 

[kuruman]

Here is what I noticed when I lived in a northern US state and parked my car in my driveway on a cold, clear, still, winter night: In the morning, the windshield (front or rear) facing away from the house was covered with frost but the windshield facing towards the house was frost-free. I attributed this to radiation heat transfer from the house to whichever windshield was closer.

 

[Matterwave]

Here is what I noticed when I lived in a northern US state and parked my car in my driveway on a cold, clear, still, winter night: In the morning, the windshield (front or rear) facing away from the house was covered with frost but the windshield facing towards the house was frost-free. I attributed this to radiation heat transfer from the house to whichever windshield was closer.

Why radiative heat transfer? Although radiative heat loss does occur, conduction or convection would probably do the job here? I say this because thermos exist and those can only shield the hot water inside from conduction and convection, not radiation. And they work pretty well~

The earth has an atmosphere after all. :)

 

[Roberto Pavani]

I say this because thermos exist and those can only shield the hot water inside from conduction and convection, not radiation.

Note that a thermos does not only suppress conduction and convection. The inner walls are usually highly polished or silvered specifically to reduce radiative heat transfer. Otherwise radiation would remain an important heat-loss mechanism across the vacuum gap.

 

[Matterwave]

Note that a thermos does not only suppress conduction and convection. The inner walls are usually highly polished or silvered specifically to reduce radiative heat transfer. Otherwise radiation would remain an important heat-loss mechanism across the vacuum gap.

Ah, my mistake. :)

 

[Roberto Pavani]

The earth has an atmosphere after all. :)

Sure, but on a clear night, looking at the stars often feels colder because of radiative heat loss to the sky.

 

[Matterwave]

Sure, but on a clear night, looking at the stars often feels colder because of radiative heat loss to the sky.

You mean by clear night, not windy? I think the wind bites hardest haha.

 

[Roberto Pavani]

I meant clear sky, not no wind. A clear sky is an efficient radiative heat sink. That's why sleeping under the stars is often colder than sleeping under a tent, even with the same air temperature.

It's also why frost can form even when the air temperature is a few degrees above freezing.

 

[Matterwave]

Got it. From personal experience I would say the wind (convection) makes me feel colder and so I suspect it is more efficient at moving heat away from me.

 

[Roberto Pavani]

Here is what I noticed when I lived in a northern US state and parked my car in my driveway on a cold, clear, still, winter night: In the morning, the windshield (front or rear) facing away from the house was covered with frost but the windshield facing towards the house was frost-free. I attributed this to radiation heat transfer from the house to whichever windshield was closer.

There are many possible factors. The house may provide wind shelter, radiate heat toward the car, or partially block the sky, reducing radiative cooling. Depending on the geometry, sunlight shortly after sunrise could also play a role. So I'm not sure radiation from the house is the only explanation. If it is, your heating bill may be trying to tell you something about the insulation.

 

[Roberto Pavani]

Got it. From personal experience I would say the wind (convection) makes me feel colder and so I suspect it is more efficient at moving heat away from me.

Sure, but I mean on a clear windless night. Standing under a gazebo often feels warmer than standing under the open sky because the roof above you is roughly at air temperature and is much warmer, radiatively speaking, than the night sky.

 

[kuruman]

There are many possible factors. The house may provide wind shelter, radiate heat toward the car, or partially block the sky, reducing radiative cooling. Depending on the geometry, sunlight shortly after sunrise could also play a role. So I'm not sure radiation from the house is the only explanation. If it is, your heating bill may be trying to tell you something about the insulation.

My heating bill was fine.

Sure, but I mean on a clear windless night. Standing under a gazebo often feels warmer than standing under the open sky because the roof above you is roughly at air temperature and is much warmer, radiatively speaking, than the night sky.

Precisely my argument. The roof of the gazebo is not heated, yet it keeps what's below it warmer "radiatively speaking." The house fulfills the same role without needing to be heated: when I look out the windshield facing the house, I see house, I don't see the starry night sky.

 

[pines-demon]

Here is what I noticed when I lived in a northern US state and parked my car in my driveway on a cold, clear, still, winter night: In the morning, the windshield (front or rear) facing away from the house was covered with frost but the windshield facing towards the house was frost-free. I attributed this to radiation heat transfer from the house to whichever windshield was closer.

Yes but also outer space (in the sky) is at 3K, the windshield facing away from the house is cooling by emitting heat away. This is the principle of radiative cooling.

 

[Roberto Pavani]

pines-demon and kuruman, I see your explanations as complementary. The house partially blocks the view of the sky, so it reduces the net radiative heat loss to space.

This discussion also reminds me of the sudden temperature drop often observed during a solar eclipse.

 

[kuruman]

Yes but also outer space (in the sky) is at 3K, the windshield facing away from the house is cooling by emitting heat away. This is the principle of radiative cooling.

I don't see this as a "yes but" issue regarding what I said. If the car were sitting in the open field across the street, both windshields will be frosted by radiative cooling. The presence of the house inhibits frost formation only on the windshield facing it.