Thermal Conductivity and Insulation
Thermal Conductivity and Insulation
(OP)
Hey people, i need some assistance.
I have a system that is exposed to a hot airflow.
We have tested the system and know the thermal time constant.
We now want to use a insulative paint to increase the time that the system can be exposed to the airflow.
I tried to approach this problem using transient conduction theory but couldn't figure out how to manipulate the time constant to take into account the paint and the paint thickness.
Has anyone has any ideas or websites ect that could help me!!
Thanks people
I have a system that is exposed to a hot airflow.
We have tested the system and know the thermal time constant.
We now want to use a insulative paint to increase the time that the system can be exposed to the airflow.
I tried to approach this problem using transient conduction theory but couldn't figure out how to manipulate the time constant to take into account the paint and the paint thickness.
Has anyone has any ideas or websites ect that could help me!!
Thanks people





RE: Thermal Conductivity and Insulation
the following thread on this forum has a similar discussion...it should help.
Thread391-92345
daveleo
RE: Thermal Conductivity and Insulation
Something like a ceramic coating with some thickness to it might be more in order.
TTFN
RE: Thermal Conductivity and Insulation
I've got one more question.
How would you deal with a multiple material layer where the conductivity was very different.
For example
Hot Air
IIIIIIIIIIIIIIIIIIIIIIIIIIII
CCCCCCCCCAAAAAAAACCCCCCCCCCC
Cold Air
Where I = Insulation
A = Aluminium
C = Composite (such as fiberglass)
The Aluminium only accounts for approximately a third of the surface area.
I've done some calculations assuming only one of the materials is present but would like to be able to take into account both materials.
Also, whilst I'm asking questions - what would be the best approach to the following
Hot Air
IIIIIIIIIIIIIII
CCCCCCCCCAAAAAAAACCCCCCCCCCC
Cold Air
Again, thanks people for your assistance.
RE: Thermal Conductivity and Insulation
corus
RE: Thermal Conductivity and Insulation
if this is true, you can treat each region separately as 1D. for example....
hot air
IIIIIIIIIII
CCCAAAAA
cold air
becomes
hot air
IIIIIII
CCC
cold air
AND
hot air
IIIIIIIII
AAAAA
cold air
treat each as 1D case and calculate heat flux and layer temperatures.
i've seen HVAC people do similar approach in calculating heat loss through walls and windows of a building....each is treated separately.
after you get answers from this....back up and calculate the in-plane conduction heat transfer between the CCC and AAAA materials......if its significant....do what corus says....get serious about 2D analysis.
daveleo
RE: Thermal Conductivity and Insulation
I'm going to get started on that type of analysis imediately
If anyone can suggest a good FREE fe 2d thermal programs it would be appreciated.
Thanks people.
RE: Thermal Conductivity and Insulation
Essentially, for identical thickness and area of the aluminum and fiberglass, the aluminum dominates by a factor of 5000:1. This means that all the heat must try to get through the aluminum. There will be some crowding in the insulator, particularly if the aluminum area is small. This would mean that the apparent thermal conductivity of the insulator is reduced from the parallel flow condition.
TTFN
RE: Thermal Conductivity and Insulation
The 1D heat flow across the layers is very similar to the Ohm's law: heat flux (W/m2) is a "currrent", temperature drop is like voltage drop, thermal resistance (thickness divided by thermal conductivity) is like electric resistance.
Transient processes inside the layer relax (i.e. the temperature profile becomes linear) when the Fourier number Fo=at/x2 becomes >>1, where a is thermal diffusivity (m2/sec), t is time, x is thickness (or half-thickness).
So the thin layer of paint shouldn't change significantly the inertia of the system, it just adds some additional thermal resistance.