Solving thermal flow without using simulation software 1

[MShineV]

Hi, it's me again,

I have a holder made out of basic 2mm sheet metal that a want to attach to a motor body. Can I somehow roughly aproximate (+-10C is ok) the steady state temperature in a particular point on the holder without using simulation software? Or is there some sort of online simulator for simple geometries? I don't want to install any new software on my work computer but repeating an experiment many times to find the right isolation material and holder shape is too time expensive.

Thank you in advance.

 

[AWloo]

Yes this is possible to do but it requires additional information. Before you do this, you need to define your problem better for an accurate solution.

If the plate is small and conductive (2mm thick) it may be that the plate takes the temperature of the motor it is in contact with. It depends greatly on the shape of the plate though. What is the application of the holder, is it holding an instrument which is heat sensitive?

-Does Lumped capacitance apply, where your Biot number is less than 0.1. (h*Lc/k <0.1) ? This will simplify your solution as the temperature in the solid will be spatially uniform.
-What are the dimensions of the plate? (affects the characteristic length in the Biot number)
-What is the material’s, thermal conductivity?
-What are the boundary conditions on the plate? Is the portion of the plate touching the motor which is at a constant temperature? Is the rest of the plate in still air or moving air, horizontal or vertical? What are the ambient conditions?

I will be glad to clarify further things.
AWloo

 

[MShineV]

Thanks for your reply.

The holder is holding a sensor. The sensors maximum ambient operating temperature is 70C.

The volume of the body is 0,000003m^3 and the surface area is 0,004005 m^2. The holder is quite small.
The material is steel so the value of thermal conductivity is 47 W.m^-2.K^-4.
The plate rests horizontaly in, lets say, still air. I quessed the convective heat transfer coefficient to be 5,7 W.m^-2.K^-1 (but i have no expirience in this matter, so it is seriously just a quess).
This gives me a Biot number of 0,00009.
A part of the motor touching the plate is at constant temperature, but I dont yet know the value (150C ?).


If you need any aditionl information, I will try to provide them.

 

[AWloo]

Since the plate is very small, has low volume (low thermal mass), the holder will take on the immediate surrounding temperatures. This is also supported by the very low Biot number which indicates lumped capacitance is a valid assumption. The plate will have a uniform temperature profile through out it. My understanding is that the plate will take on the temperature of the immediate surroundings.

How high does the sensor sit from the motor? It may be possible to install a standoff or spacer which will prevent direct contact with the higher temperature motor. I.e. will it take air temperature or if its directly mounted perhaps motor skin temperature.

What is the ambient air temperature?

As a side note when dealing with natural convection, the effects of radiation heat transfer cannot be ignored especially at higher temperatures.

Cheers,
AWloo

 

[MShineV]

Thank You a lot,
one important thing that I forgot to mentione is that the holder is supposed to be directly screw mounted to the aluminium side of the motor. This will probably be a conduction path right into the holder. We want to use plastic screws but I am not sure if we can find the right size.

The holder uses a part of the motor as support. There has to be contact between the motor an the holder.

What material and of what thickness would you recomend as isolation. I would like to install it in between the holder and the motor and the holder and the sensor.

The air temperature should not exceed 50C.

I am planing to counter the radiation heat transfer if it becomes a problem by simply covering the side of the sensor that is facing the motor, with insulation.

Thanks a lot.

 

[AWloo]

The thermal resistance is found by Rcond,t=L/k. Where L is the length of insulation and k, is the thermal conductivity. The heat transfer between this would be q=T1-T2/(L/k). T1 and T2 being the surfaces of the insulation.

A piece of foam or gasket should be sufficient. A radiation shield could be as simple as a piece of aluninum foil (think highly reflective).

 

[MShineV]

Today after doing an experiment on the motor we decidet to move the sensorto a different spot, which is much less heated.
I learned a lot, thank you.

Cheers
MShineV