Question about Nusselt Number

[sun wenning]

Hi,

I am running a thermal break system stimulation.

If I don`t subdivide the frame cavity Nusselt number all below 1.2, I got the following result. 

If I subdivide the frame cavity so that all the frame cavity elements have a Nusselt number below 1.2, I can get much lower U-value for Frame and SHGC. As below.

Can anyone briefly explain to me why there is a big difference between dividing and no dividing?

[yalin uluaydin]

Sun,

When you divide the cavity up so many times the program treats each one of the Cavity Elements as an individual air gap with individual resistances or conductivity. Because the software calculates the conductivity of the air gap for each one separately based on temperature differences and gap width it results in a better u-value. For example to simplify it - if each gap has calculated resistance of R1 then you would have 5 times more thermal resistance in the air gap when compared to one air gap with a resistance of R1. There are many factors that affect the resistance of air gaps like the gap width and temperature difference across the gap but you are affecting these by dividing up the gap and artificially improving the thermal performance.

The nusselt number is the proportion of convection to conduction of an air cavity. So a value more than 1.0 has added heat transfer due to convection effects. In large air gaps with large temperature difference between the walls will have some convection effects and you are eliminating this by division of gaps.

I would recommend following the NFRC or ISO 15099 modeling procedure. You should not divide up cavities unless they are separated by a throat. If there is no throat then you leave the cavity as is even if the Nusselt number is more than 1.2.

Yalin

[sun wenning]

Thanks very much for your reply. It is really useful because after I subdivide the throat I still have large air gap Nusselt number above 1.2. I will leave them as what it is. Thanks a lot!

在 2019年12月4日星期三 UTC+11上午7:46:18，yalin uluaydin写道：