Hello Everyone,
As we know that:
Thermal conductivity=k=qL/dT with units (W/m K)
where
q = heat flow rate(W/m2)
L = width of the material (m)
dT = change in temperature
Now, some people mention that thermal resistance like this
R = L/k with units (K.m2/W)
and some people mention like this
R = L/(k.A) with units (K/W)
I am confused that which definition is correct?
I shall be very thankful for your help.
Thank you
Dear Raza,
The thermal resistance R (in m2.K / W) depends on the insulation thickness (e expressed in meters) and the thermal conductivity (λ) of the material: R = e / λ.
Regards
Bachir ACHOUR
Dear Raza Javed,
Both definitions are correct for thermal resistance, but the first one in the units (K.m2/W) is used to define the thermal resistance of for example building materials which is found in markets in standard thickness L, (L in the definition is the thickness which is the length of the path the heat flows), ex. concrete blocks, bricks, tiles, glass sheets, ... etc.Using this definition the heat flow q= U A (t2-t1) & U=1/R.
The second definition can be used to calculate the thermal resistance of any thing of nonstandard dimensions or thickness. here q= R*(t2-t1) here R= L/k.A.
in both cases A is the surface area normal to the direction of heat flow.
thank you.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Heat Transfer