search

atwhaley / cfast

Automatically exported from code.google.com/p/cfast
0 stars 2 forks source link

Clarification on boundary heat conduction #97

Closed GoogleCodeExporter closed 9 years ago

GoogleCodeExporter commented 9 years ago 
Hi,after reading the user's guide I am confused as to how this is handled:

1. When using VHEAT for vertical conduction through the ceiling and into the 
floor on upper levels, when it is written in the manual that the boundaries are 
merged, does it mean that the thickness and material properties of both 
boundaries are added together? The problem is that the ceiling of one 
compartment is more than likely part of the same structure as the floor of the 
above compartment, which in effect makes you have to designate material 
properties for half ceilings. I also miss the option of having three different 
materials in ceilings and walls in the current GUI, but I guess this means that 
you could have 6 materials? The same argument can be said for horizontal heat 
conduction through walls.

2. The manual is somewhat unclear on whether conduction through walls to the 
outside (ambient temp) is calculated without the use of HHEAT and VHEAT. I 
don't think it is, but this statement confuses me: "For each wall material 
turning HHEAT on leads to warmer calculated temperatures since the outside 
environment (where heat goes when HHEAT is off) is cooler than the adjoining 
room." My assumption however is still that one would have to turn on VHEAT and 
HHEAT with the approprate fraction in order to facilitate thermal conduction to 
the outside, through walls and ceilings.

3. Are vents and openings subtracted from the boundary area when calculating 
VHEAT and HHEAT? Also, is the assumption correct that when determining the 
fraction for inter-connected compartments, you should determine the fraction of 
the total vertical surface area of both compartments in order to find the 
correct fraction? Personally I think it would be simpler to specify the 
fraction for either the first or second compartment, rather than adding up the 
total surface area in order to find the fraction. Which again begs the question 
whether horizontal wall vents (openings) are subtracted from the total vertical 
surface area when you determine the surface fraction for HHEAT?

Sincerely, Mikael

Original issue reported on code.google.com by mikaelno...@gmail.com on 27 Oct 2012 at 11:38

GoogleCodeExporter commented 9 years ago 
VHEAT and HHEAT simply set the boundary conditions for the connected surfaces 
so that the "outside" surface of the two connected segment are at the same 
temperature. Material properties and thickness are separate for the two.

CFAST (the model) does still have the capability to use multiple materials in a 
surface lining, but the GUI and model inputs no longer support it.  This is a 
feature we would like to bring back at some point in the future.

Conduction through surfaces connected to the outside are not done through VHEAT 
and HHEAT.  By default, the outside surface of all compartments is exposed to 
the exterior ambient temperature and exchange heat to the exterior by 
convection and radiation.

Vent areas are not taken into account for conduction calculations in CFAST in 
general.  Again, this is something we would like to include in a future 
version, particularly radiation through openings, but the heat transfer through 
the walls by conduction would be impacted as well.

To be honest, both VHEAT and HHEAT are rarely-used features since they 
typically only apply for very conductive materials.

Original comment by cfastdev@gmail.com on 28 Nov 2012 at 8:54

GoogleCodeExporter commented 9 years ago

Original comment by cfastdev@gmail.com on 25 Mar 2013 at 12:36