Help setting up a heat transfer equation

[ecihan]

Hi,

Please see attached Mathcad Prime 3.1 worksheet and advise if I am setting up the definitions and equation up properly.  The correct answer should be around 307, but I'm pretty sure I need to set up the equation a different way.

Should I be setting the equation equal to Ts?

Thank you

[MJG · Accepted solution]

It appears you typed the constraint equation outside of the solve block and then moved it into the solve block.  When you do that, Prime fails to recognize that the equation is part of the solve block.  Instead, Prime treats it as two unrelated math areas that happen to overlap each other.

To correct this:

- Move the solve block away from your constraint equation (guess value & "find" equation should move with solve block).

- Cut the constraint equation.

- Paste it into the solve block.

Prime then calculates a value of 307.845.

[ecihan]

This works.

Thank you very much.

[ValeryOchkov]

1. It will be good to use units of temperature, length etc in your calculation!

2. Better use root not Given/Fimd

[ValeryOchkov]

[ecihan]

Are you sure the units for thermal conductivity are correct?

This is what I have:

[ValeryOchkov]

Erman Cihan написал(а):

Are you sure the units for thermal conductivity are correct?

The unit of thermal conductivity is W*m^2/(m * K) or W*m/K but this simplification is not quit correct for the physic! 

[-MFra-]

[ValeryOchkov]

F.M. написал(а):

[ValeryOchkov]

[-MFra-]

On some physics textbooks a distinction is made between the law of Stefan-Boltzmann (e=σT^4), and the constant σ, called Kurlbaum constant.

[ecihan]

Attached is my updated worksheet where I have used both the root and given/find applications.  I noticed that the variable, L=0.003 m, not 0.0003 m, therefore the answer of Ts changes (extra zero, sorry!).

The given/find solver gives Ts= 306.627 Kelvin.

The root solver gives Ts= 308.852 Kelvin.

The application of this problem is to find the surface temperature of the human skin (Ts), due to the internal temperature (Ti=308 Kelvin).  Therefore, the heat loss due to conduction of the skin, convection to the outside air (air temperature is 297 Kelvin), and emissivity of thermal radiation, Ti has to be greater than Ts.  I have go to with the answer of Ts= 306.627 Kelvin.

Thank you for all the help.

[ValeryOchkov]

Erman Cihan написал(а):

The given/find solver gives Ts= 306.627 Kelvin.

The root solver gives Ts= 308.852 Kelvin.

Sorry - it was my error