Hello engineers,

I have recent problem with solving the heat transfer from sand(hot) to air(ambient). Newton's law of cooling, convective heat flux is much bigger than sand heat conduction. First layers will get cooler much faster and temperature difference in convection comes so small that there is almost no heat transfer. Thats what my amateur CFD simulation shows, and cant solve the heat equation in 3 dimmensions and dependent on time. 

Whats your suggestion, is this simulation correct? Do I need to increase to heat transfer area according to sand heat diffusivity? 

How should I approach the problem?     

Attached photo (air flows through the small pipes)

Edited by Bojan332, 19 August 2022 - 01:44 AM.

No photo attached.  Edit your post / Use Full Editor / Attach picture under the text box.

See below.  It's easy.

Your description agrees with my intuition that sand cooling will take place where there is air movement and not much where there is no air movement. Hot sand tends to stay hot when packed. More description might help get useful answers. Quantity? Temperature? Purpose of the process? etc.

I coundt find how to attach the photos so here is the link

https://imgur.com/a/1GKpIqw

at initial conditions The sand Temperature is 800C

inlet Air temperature is 200C

Thermal energy storage is the purpose 

Attaching file is easy. Go back and follow the procedure you were given. We don't open links that are unknown.

Bobby

800C is pretty hot!  I suggest researching 'fluidized bed heat transfer' for a possible option. Must the air go through the internal tubes? Why not just have the air directly contact the sand through sieve holes, screen, or ceramic filter? Fish aquariums use porous stone for air distribution into the water. Maybe something like that to distribute the air directly into the sand bed.

Hi,

I think a direct contact solid gas should be an option , check for fluidization bed technology and heat transfer.

My 2 cents

Breizh 

Attaching file is easy. Go back and follow the procedure you were given. We don't open links that are unknown.

 

Bobby

There it is, apologies for that

800C is pretty hot!  I suggest researching 'fluidized bed heat transfer' for a possible option. Must the air go through the internal tubes? Why not just have the air directly contact the sand through sieve holes, screen, or ceramic filter? Fish aquariums use porous stone for air distribution into the water. Maybe something like that to distribute the air directly into the sand bed.

I have checked fluidized bed but I am sceptic about it. The air it doesnt have to go through the pipes but to put highly porous material such as ceramic filter the tank will be enormous for the same capacity.

To switch to fluidized bed i want to solve this problem first and get the positives and bad sides of it.

Putting something like sintered metal at each air entry point would keep the sand in and allow the air to pass https://www.cleanair...arger-diffuser/ You would need fewer air entry points and the current tubes would not be taking up the vessel volume so the sand volume would be about the same. Direct heat transfer between the air and sand is much more efficient and simpler/cheaper. I don't know your process requirements. If you must have air in tubes, then maybe circulate some sort of liquid through the sand to bring the heat to the tubes.

Thank you, this paper its really what I was looking for. As I thought temperature gradient its so big cuz of conductivity

Hi,

Your set up makes me thinking about fixed bed reactor . Let you find more using your search engine . I doubt about the heat transfer from a static bed of sand by conduction to air . 

You may find some interest in reading the doc attached :

https://journal.uob....e=1&isAllowed=y

Good luck

Breizh