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J
jroth5-Regular Member
5-Regular Member
November 21, 2013
Solved

Solve block function says the 'This value must be a function'.

  • November 21, 2013
  • 3 replies
  • 5483 views

Hello,

I am unable to get the solve block function to work with what I"m trying to find. Essentially I'm starting a low level solve block that I hope to expand on. Currently I have a fluid that moves through a pipe, and has energy transfered to it. I am making an assumption that the surface temperature is constant.

I have no idea why the solve block is failing. Shouldn't it solve for my temperature out?

The expansion I'm wanting to do is solve for the Temperature lost in a fluid as it moves through a line. I'm wanting to use the ambient temperature only, and I won't know the inside or outside temperatures of the wall or the ending temperature. I will though, know that the convection on the inside of the pipe = conduction through the pipe = convection to ambient settings which should equal the temperature lost of the fluid.

I'm unable to get the first part of the solve block to work properly. Any help is much appreciated.

Best answer by Werner_E

Every calculation which is dependend on the searched for variable T.fo has to be turned into a function of this variable.

The result is far away from what you expected as in Rogers solution.

3 replies

R
RogerYeh1-Visitor
1-Visitor
November 21, 2013

I'm getting a pretty wacky answer, but it seems consistent if I were to plot Q_conv and Q_transport.

BTW, the original warning ("This value must be a function") was because of this term:

Untitled.png

J
jroth5-Regular MemberAuthor
5-Regular Member
November 22, 2013

Aw, I think I may now know what the problem is. I think I must define each single thing as a function. So in my original copy, I believe I needed to define Tavg as a function of Tfi and Tfo instead of just saying it was Tavg. As well, the reason for the weird answere is because the table is off. I fixed the table of values in your sheet, and on slight error, and I got the same answer then; so we are good.

I think that is why it wasn't working, is because I wanted mathcad to solve a condition that wasn't 'classified' as a function of the variables I needed to put in it. As a quick test, if you define Tavg in your sheet, the solve block will actually change the answer unless you define it as Tavg(Tfi,Tfo)

W
Werner_E25-Diamond IAnswer
25-Diamond I
November 22, 2013

Every calculation which is dependend on the searched for variable T.fo has to be turned into a function of this variable.

The result is far away from what you expected as in Rogers solution.

    J
    jroth5-Regular MemberAuthor
    5-Regular Member
    November 22, 2013

    I replaced the table and then fixed the slight error in the Q Transport function, and it works out. Thank you greatly for your help. This will allow me to set up my more complicated system. I'm basically looking to see if at a thick enough tube, or for various ambient conditions, if the fluid temperature gradient will be large enough to warrant a concern that I'd need to express the conduction temperature of the pipe as a function of radius and length z, thereby going into bessel equations.

    I have attached the fixed worksheet.

    Thank you greatly for your help.

    W
    Werner_E25-Diamond I
    25-Diamond I
    November 22, 2013

    You don't have to keep all that functions in the solve block but may define them before.

    So the solve block would consist of three lines only: guess value, constraint(Q...=Q...) and the line with find.

    J
    jroth5-Regular MemberAuthor
    5-Regular Member
    November 25, 2013

    I am wondering then how to fix this new problem. I have fixed the error for the basic file, but now I want to express the heat transfer not knowing the wall temperature but rather knowing the ambient temperature. The equations are not too bad to solve, but I'm having issues with the solve block. I would like to put my conditions in as a function and get the feedback for the correct outlet temperature (assuming the thermal gradient isn't too large that the wall temperature varies greatly in the z direction along the pipe axis).

    The issue is I would like to express the properties of any given substance as a function of temperature.

    The excel spreadsheet (minus updating the properties) gives a correct answer by iterating through. The user needs to press the buttons several times until all errors are zero.

    I know I can do this problem in excel or a programing language, but the outputs look messy. For my project homework I'd like to use Mathcad to express the given solution, that way it is more readily checked.

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