Need explanation on Chracteristic Length in heat convection

In summary, the conversation discusses constructing a thermal circuit using natural convection from a vertical hot plate in the shape of a DVD. The characteristic length is needed to calculate the Grashoff number, and it is recommended to use the largest length scale of the fluid. The concept behind characteristic length is to use the largest vortex in the flow. A sketch of the problem is requested, and it is clarified that the heat source is the cylinder itself due to copper loss.
  • #1
thiru muruga
3
0
Hi,

I am working on constructing a thermal circuit where I am using the concept of natural convection from a vertical hot plate in the shape of a DVD. In order to calculate the Grashoff number, I have to use the characteristic length. What should I take as my characteristic length? The disc OD or ID or average of OD and ID?

Could you also explain the concept behind characteristic length?
 
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  • #2
Can you upload a sketch of the problem?
 
  • #3
upload_2015-1-19_11-41-50.png


The cylinder is hollow and isothermal. I need to calculate convection coefficient for the heat transfer taking place from the two ends of the cylinder to ambient when the cylinder is placed horizontally. I assume that the ends are similar to circular disc (like a DVD) and need to calculate the Grashoff no. for which I need to know what is the characteristic length.
 
  • #4
What/where is the heat source? For charictaristic length I would use the largest length scale of the fluid, which would correspond to the largest vortex in the flow. If the heat source was inside the cylinder, for example, then it would be best to use the ID. If it was from outside, then I'd say either the OD or length depending on the direction of the flow. You can always write down the governing equations (mass, momentum, energy) and nondimensionalize yourself to determine the proper length.
 
  • #5
See the energy equation of Navier Stokes using temperature, not enthalpy
 
  • #6
The cylinder itself is the source, cause I m trying to model a copper winding here which is cylinderical. The source is heat generated due to copper loss.
 

Related to Need explanation on Chracteristic Length in heat convection

1. What is "characteristic length" in heat convection?

Characteristic length refers to the characteristic size or length scale of a system or object that is experiencing heat convection. This can vary depending on the geometry and dimensions of the object, but it is typically defined as the length of the object in the direction of heat flow.

2. Why is characteristic length important in heat convection?

Characteristic length is important because it is used in equations that describe heat transfer through convection. It helps to determine the heat transfer coefficient and plays a role in the overall heat transfer rate of a system.

3. How is the characteristic length calculated?

The characteristic length can be calculated in different ways depending on the specific system or object. For simple shapes like a cylinder or sphere, it is the diameter of the object. For more complex shapes, it may involve finding the average of multiple dimensions or using specific equations for that shape.

4. How does the characteristic length affect heat convection?

The characteristic length affects heat convection because it is used in equations that determine the heat transfer rate. A larger characteristic length means a larger surface area for heat transfer and can result in higher heat transfer rates. On the other hand, a smaller characteristic length may lead to lower heat transfer rates.

5. Can the characteristic length change during heat convection?

Yes, the characteristic length can change during heat convection if the object or system experiences a change in geometry or dimensions. For example, if a cylindrical object heats up and expands, the characteristic length will also increase, affecting the heat transfer rate. However, in most cases, the characteristic length is assumed to be constant for simplicity in calculations.

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