K (thermal conductivity) value question.

In summary, the k value is a measure of a material's thermal conductivity and is often used in conjunction with other properties to calculate the amount of heat a material will transfer. Most materials' thermal conductivity coefficients are temperature dependent, so depending on the application, you may need to take this into account. 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. For a simple problem approximation, the conductivity coefficient
  • #1
lawtonfogle
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Does the k value have any relationship with the temperature of the wall (use of the word wall becuase that is what the k value is mostly used with).
 
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  • #2
lawtonfogle said:
Does the k value have any relationship with the temperature of the wall (use of the word wall becuase 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.
 
  • #3
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.
 
  • #4
If you are just interested in calculating building heating/ac loads, you can consider it constant.
 
  • #5
lawtonfogle said:
Does the k value have any relationship with the temperature of the wall (use of the word wall becuase 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).
 
  • #6
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.
 
  • #7
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.
 
  • #8
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
 

1. What is the definition of thermal conductivity?

Thermal conductivity (K value) is a measure of a material's ability to conduct heat. It is the rate at which heat energy is transferred through a material per unit distance, under a temperature gradient.

2. How is thermal conductivity measured?

Thermal conductivity is typically measured using a device called a heat flow meter, which measures the amount of heat flowing through a material over a given time period. It can also be calculated using the material's specific heat capacity, density, and thermal diffusivity.

3. What factors affect the thermal conductivity of a material?

The thermal conductivity of a material is affected by several factors, including its chemical composition, density, temperature, and moisture content. Generally, materials with high thermal conductivity have strong bonds between atoms, low density, and low moisture content.

4. Why is thermal conductivity an important property to consider?

Thermal conductivity is an important property in many applications, such as insulation, building materials, and electronic devices. It can greatly affect the efficiency of heat transfer and can impact the overall performance and safety of a system.

5. How does thermal conductivity differ from thermal resistance?

Thermal conductivity and thermal resistance are related but different measures. Thermal conductivity measures the ability of a material to transfer heat, while thermal resistance measures the resistance to heat flow. In other words, thermal resistance is the inverse of thermal conductivity and is used to measure the insulation properties of a material.

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