Heat conductivity and Grashofs number

In summary: Consequently, you will find all the required data in the NIST database.In summary, the conversation was about finding values for heat resistance and conductivity for different types of building materials. The suggestion was made to check engineering handbooks or websites such as matweb.com or the NIST database for this information. Additionally, the second part of the conversation discussed the Grashof number and the \beta term, which is the volume expansion coefficient used in natural convection flow calculations. The approximate equation for \beta was provided.
  • #1
TSN79
424
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1)
I'm in need of some values (tables) about heat resistance, conductivity, and such for different types of building materials. Any good sites out there?

2)
I have the following equation for calculating Grashofs number:

[tex]
Gr = g \cdot \beta \cdot \Delta t \cdot {{l^3 } \over {v^2 }}
[/tex]

All well and good, but the [itex]\beta[/itex] is described as being the "volume expansion coefficient", something I have never heard of before. The unit is 1/C, where C is Celsius. This wouldn't be a problem if I just knew what this number actually is, or where to find it. Any ideas?
 
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  • #2
1) Don't know of any sites. If your school has access to them, engineering handbooks are usually your best bet. If you have specific materials and properties you are looking for post them and someone should be able to pass some along to you.

2.) In free convection, the Grashof number is akin to the Reynolds number for forced convection. The [tex]\beta[/tex] term comes from the variation in density required for natural convection flow.

[tex]\beta = - \frac{1}{\rho} (\frac{\partial \rho}{\partial T})_p[/tex]

it can be approximated by

[tex]\beta \approxeq - \frac{1}{\rho} (\frac{\rho_\infty - \rho}{T_\infty - T})[/tex]

[tex](\rho_\infty - \rho) \approxeq \rho \beta (T_\infty - T)[/tex]

The RHS should look very similar to the first three terms of the Grashof number.
 
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  • #3
#1)
matweb.com lists properties for most metals, polymers and ceramics. This is a valuable resource for all kinds of property searches.

The more relevant resource for you is the NIST database for Heat Transmission Properties of Insulating and Building Materials : http://srdata.nist.gov/insulation/insul_search_menu_12.asp
 
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Related to Heat conductivity and Grashofs number

1. What is heat conductivity?

Heat conductivity, also known as thermal conductivity, is a measure of a material's ability to conduct heat. It is the rate at which heat energy is transferred through a material, typically measured in watts per meter-kelvin (W/mK).

2. How is heat conductivity related to Grashof's number?

Grashof's number is a dimensionless quantity that is used to determine the dominant mode of heat transfer in a fluid. It is calculated by dividing the product of the fluid density, acceleration due to gravity, and the cube of the characteristic length by the product of the fluid viscosity and thermal diffusivity. A higher Grashof's number indicates a greater influence of buoyancy-driven convection, which can significantly affect heat transfer. Therefore, heat conductivity is an important factor in determining Grashof's number.

3. What is the significance of Grashof's number in heat transfer?

Grashof's number is used to determine the mode of heat transfer in a fluid, which can greatly impact the rate of heat transfer. In natural convection, where fluid motion is driven by density differences due to temperature variations, a high Grashof's number indicates a stronger buoyancy force and therefore a higher rate of heat transfer. In forced convection, where fluid motion is induced by external forces, a high Grashof's number may indicate a transition from laminar to turbulent flow, which can greatly increase heat transfer.

4. How does heat conductivity affect thermal insulation performance?

The heat conductivity of a material is an important factor in determining its ability to provide thermal insulation. Materials with low heat conductivity, such as aerogels, are often used in insulation applications because they can significantly reduce the amount of heat transfer through a material. On the other hand, materials with high heat conductivity, such as metals, are poor insulators and are not suitable for insulation purposes.

5. How is heat conductivity measured?

Heat conductivity is typically measured using a device called a thermal conductivity meter. This instrument consists of a heat source, a temperature sensor, and a material sample to be tested. The heat source is placed on one side of the sample, while the temperature sensor is placed on the other side. By measuring the temperature difference across the sample and the amount of heat being transferred, the thermal conductivity can be calculated.

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