Dear Javed are you talking about absolute vaccum or the condition that can be approximated as vaccum.

If you are talking about a hypothetical case where their is no air, what molecular mass you are talking about, I mean of which substance if their is no air.Otherwise in any other condition the molecular mass of air will not change.

Raza,

It depends on how you pump out the air.

If your vessel has been exposed to air, then adsorbed water will appear as a larger fraction - it desorbs slowly from stainless steel and at pressures ~1mbar, it may be the dominant gas in your chamber.

Mechanical pumps (such as turbo pumps) cannot pump light gases well - so at even harder vacua, you may find that hydrogen will be a larger fraction than is normally the case in air.

Syed Mughees Ali Thank you for your reply.

As I am doing Openfoam simulations, and I need to see the heat radiations in vacuum.

In Openfoam I can only find the thermophysical properties of Air (like molar mass, heat capacity, viscosity, prandl number etc.)

Now, as we know that the pressure in the vacuum is near to zero, so I have to find that If I put pressure considerably small then what would be the thermophysical properties.

For that, I used the software REFPROP, where I selected the dry air and then changed its pressure to a very low value, and it gave me all of thermophysical parameters except molar mass.

So, Now I want to know what would be molar mass in this case?

I don't know my question makes sense or NOT. but you can guide me if I am wrong somewhere.

Thank you

Dear Raza Javed,

As i understand that you need to define heat transfer from radiation in vaccum. For radiation, thermo-physical properties are not required (If you neglect the radiation by the gases which is very small value in your case).

See the lectures in the link below. I will be helpful.

https://nptel.ac.in/courses/119/108/119108004/

Syed Mughees Ali Thank you and I will surely check those lectures.

I have one general question:

If we have two walls: wall1 and wall2.

wall1 is at higher temperature (say 500K), and wall2 is at lower temperature(300K).

And we are assuming that there is only radiation heat transfer.

No Conduction

No Convection

Now we take two cases:

Case 1:

There is normal air between these two walls

Case 2:

There is a vacuum between these two walls

Now, In which case, the heat transfer from wall1 to wall2 would be fast OR the rate of heat transfer would remain same? because radiation doesn't need any medium, so it doesn't matter there is air or vacuum.

Thank you

Raza Javed It would be same for both the cases. However, to be very precise it can be negligibly higher in case 2. The reason can be, if some of the gas molecules are present these molecules can absorb the photons and re-emit them in randomly. Also i have no idea if we assume no conduction and convection how this kind of phenomena can be modeled.

Syed Mughees Ali I am not sure if this type of scenario can be modelled. I have one question:

1. Does opaque solid not absorb any radiation that are made to fall on its surface?

2. If an Opaque object is kept in hot air, and we assume that this object doesn't absorb any radiation, and the hot air in the room is also not moving, then through which phenomenon the heat will transfer from air to object? OR how the temperature of that object will rise?

Actually I am asking this question because, I am modelling a case, in which an opaque object is placed in a room. and the object is dissipating 5 joules of energy per second i.e. the power dissipation of that object is 5W.

Now, can I calculate the temperature of that object using boltzman law? knowing the emissivity and surface area of that object?

When I run the simulation, the temperature of the object increases from the value I get from hand calculation.

I don't know why the temperature of the body increasing continuously?

Dear Javed,

Question 1. Generally opaque objects absorb more radiation in comparison of transparent objects.

Question 2. In this case the heat transfer will take place through conduction.

Question 3. How are you simulating. There is one possibility in which the temperature of opaque object can increase.

Syed Mughees Ali Thank you for your reply.

I try to explain my problem:

I am simulating the following case:

There is one region (say one empty box). There is one another body (say a small cubic solid), that is present inside the big empty box, and this small body is not touching any side of the empty box. (picture attached)

Now,

I want to simulate two cases:

1. The empty box is filled with normal air

I am putting power of 5W on the small body. (actually I am calling it small body for simulation point of view, in reality it is some electronics equipment that is disspating a power of 5W in the form of heat). Now, I want to find two things here.

1. The temperature of the small body due to power dissipation of 5W? How to find this temperature of the body?

2. I want to model the radiation in the air, and want to check how fast the heat will be radiated in air?

2. The empty box is filled with Vacuum

same operations as above to be done here as well.

What I did up till now:

I made the geometry as attached picture:

1. I declared the small body made of aluminium, and an opaque object with absorptivity=0, and emissivity=0.5.

2. For air, the radiation model I used is viewFactor.

3. I put the velocity of air=0.

4. But what happens is that, when I RUN the simulation, the temperature of the small body continues to increasing limitlessly even if I put power of 1W.

According to my understanding:

1. As one region is air and the other is solid, so there should be NO Conduction because conduction happens between solids.

2. There should be NO Convection because the velocity of air is 0.

3. So, there would be just radiation.

Please correct me where I am wrong.

I would be happy to provide further information if required.

Thank you

@ Javed

Case 1 The empty box is filled with normal air

The heat transfer can take place by CONDUCTION, CONVECTION as well as by RADIATION.

I am not sure what does bigger box represent. If it represents the atmosphere, then the body will loose heat by all the three processes. The energy balance will be given as:-

Rate of change of internal energy of the body (m*c*DeltaT) = Heat generation inside the body (5 W) - Heat loss/gain by the body (all three processes).

But, if it represents the a closed casing completely enclosing the body, then the body can loose heat/gain only by convection and conduction.

Case 2 The empty box is filled with Vacuum

No convection or conduction in this case. The body can loose heat only by radiation. Apply above energy equation to find the final temperature (function of time in this case and reach steady state after certain time).

According to my understanding:

1. Conduction happen in all solids, liquids and gas.

2. Read about natural onvection

3. I am not sure this statement is correct.

Syed Mughees Ali Thank you so much for clarifying most of my doubts.

I have one question from CASE II, is it possible to use Boltzman law to find the temperature of the body?

P=e*sigma*A*T^4

Where,

e= emissivity of material

sigma=boltzman constant

A= surface area of the body

T= Temperature

P=Power(5W in our case)

Thank you