Heat transfer from hot liquid, through wall to cold liquid below. 1

[Mariusz001]

Hi,

Could anyone help with following, I'm calculating total heat flux from liquid heated by heaters to exterior. Walls of the tank containing heated liquid are surrounded by air, only bottom wall is connected to another tank containing constant (lower) temperature liquid. I've calculated heat transfer coefficients (h) for top and vertical walls using known correlations for natural convection. The problem is that, I'm not sure, how should I treat bottom wall. Should I treat the heat transfer from heaters to lower temperature liquid as a conduction? Through layer of fluid and bottom wall?

Kind regards,
Mariusz

 

[TD2K]

Heat loss through the floor is a combination of convection and conduction.

You have a convective heat transfer from the hot fluid to the tank floor, conduction through the tank floor plus convective heat transfer to the cold liquid. Fouling, if there is any, would be another factor on one or both sides of the tank wall.

 

[Mariusz001]

Convective heat transfer is a mechanism of transporting energy with movement of fluid. In terms of natural convection movement of fluid occurs with density differences. In this case, the floor is colder than above fluid. So the fluid closer the bottom is denser! So what convection are You talkin about?

 

[Compositepro]

Heat flow downwards will be by conduction only, not convection. There will be a short distance below the heaters where there will be some effects caused by turbulence effects due to natural convection. A shroud around the heaters that extends to the bottom can create convection to the bottom.

 

[Mariusz001]

Compositepro, thanks. That's just what I've suspected.

 

[rmw]

I read a statement in a Heat Transfer text book once that stated "it is considered that if Re is <2000, no heat transfer takes place. Take your liquid properties and see how much velocity would be required to get Re above 2000 and ask yourself if you think you have that type of movement in the tank(s).

rmw

 

[IRstuff]

That seems to be counterintuitive. If Re is low, say 0, then there's still conduction.

TTFN
faq731-376

 

[rmw]

Well, Re refers to fluid flow so I thought I didn't have to state that the quote referred to convection heat transfer. Yes, you would be correct - there is always conduction.

rmw

 

[IRstuff]

OK, just making sure ;-)

TTFN
faq731-376