K (thermal conductivity) value question.

Click For Summary

Discussion Overview

The discussion revolves around the relationship between thermal conductivity (k value) and temperature, particularly in the context of walls and materials like concrete. Participants explore the implications of temperature dependence for practical applications, including simulations and heating calculations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants propose that the k value is generally temperature dependent, with most materials exhibiting this characteristic, although exceptions may exist.
  • One participant notes that thermal conductivity values are often listed at room temperature, with some materials providing multiple values at different temperatures.
  • Another participant mentions that for simple calculations, the k value can be treated as constant, particularly in building heating and cooling load calculations.
  • A participant inquires specifically about concrete and its thermal conductivity changes when heated, indicating a need for precise information for a simulation program.
  • There is a discussion about the acceptable accuracy for specifying the k value, with some suggesting that small temperature ranges may allow for a constant value to be used.
  • One participant points out that small temperature changes (from 55 to 110 degrees Fahrenheit) might not significantly affect the k value in engineering terms.
  • Another participant references ASHRAE guidelines, indicating that multiple k values are not typically listed for HVAC purposes.

Areas of Agreement / Disagreement

Participants generally agree that the k value is temperature dependent, but there is no consensus on the extent of this dependence or its practical implications for specific applications. Multiple viewpoints on how to approach the use of k values in calculations remain present.

Contextual Notes

Limitations include the lack of specific material data for concrete at varying temperatures and the potential for differing interpretations of acceptable accuracy in calculations.

lawtonfogle
Messages
159
Reaction score
0
Does the k value have any relationship with the temperature of the wall (use of the word wall because that is what the k value is mostly used with).
 
Science news on Phys.org
lawtonfogle said:
Does the k value have any relationship with the temperature of the wall (use of the word wall because that is what the k value is mostly used with).

Almost all materials' thermal conductivity coefficients are temperature dependent (I say almost to avoid absolutes, although I don't think I have heard of any materials that aren't). The thermal conductivity you can look up on www.matweb.com is usually at room temperature (25 degrees C), but there are exceptions (Some stainless steels, for example, may have a few tempertuare values listed). Heat transfer textbooks will tend to have temperature dependent properties of solids listed in their appendices.

Usually, a simple problem will approximate the conductivity coefficient as constant, but several highly complex problems I have solved in FEA must take this into account, especially with thermal conductivity, and electrical resistivity. Is there any particular material you are inquiring about?

As a point of fact, almost all material properties are to some extent temperature dependent.
 
Concrete, and how does it change when heated. I need the information for a simulation program, but all I have is the normal k value.
 
If you are just interested in calculating building heating/ac loads, you can consider it constant.
 
lawtonfogle said:
Does the k value have any relationship with the temperature of the wall (use of the word wall because that is what the k value is mostly used with).
What kind of accuracy do you need to specify k to (allowed % error) and what temperature range do you expect the wall to experience?

Yes, in general there is a dependence, but it may be small enough that you don't have to worry about it (but not if you are doing a very precise calculation and your tolerances are 0.01% or somesuch).
 
The wall will be experencing a temperature range (in F) from no less than 55 to no more than 110. Mainly, I need to calculate is the difference enough to try to heat the concrete as an effective way to keep warmth in a house, and so I am looking for the equation to see if there is.
 
lawtonfogle said:
The wall will be experencing a temperature range (in F) from no less than 55 to no more than 110. Mainly, I need to calculate is the difference enough to try to heat the concrete as an effective way to keep warmth in a house, and so I am looking for the equation to see if there is.

Those are pretty small temperature changes in engineering terms. You should be able to use just one value for the conductivity.
 
AFAIK, ASHRAE doesn't even list multiple k/r/u values for HVAC purposes, and the software I use doesn't calculate it. You may want to try it... www.elitesoft.com
 

Similar threads

Undergrad Generate heat conduction curves at different time steps
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad How to measure average thermal conductivity of a metallic material?
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Physical meaning of thermal conductivity tensor
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Average temperature in a greenhouse
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Need help to explain the peak of thermal conductivity
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Possible error sources in thermal conductivity experiment?
  • · Replies 16 ·
Replies
16
Views
14K
Undergrad Interseasonal heat storage / sand battery insulation?
  • · Replies 27 ·
Replies
27
Views
6K
Undergrad Ideal working gas for a Stirling engine
  • · Replies 1 ·
Replies
1
Views
3K
Undergrad Max Heat Transfer w/ Low Conductivity Brick & Copper
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Can Anisotropic Materials Have Asymmetric Thermal Conductivity Tensors?
  • · Replies 14 ·
Replies
14
Views
15K