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3-Visitor

## Can someone help me with this resolution Sketch ?

Can someone help me with this? I am trying to replicate this resolution on MathCad but I am doing something wrong or not using the right functions to solve it. Can someone tell me what am I doing wrong? Thank you in Advance.

1 ACCEPTED SOLUTION

Accepted Solutions
24-Ruby V
(To:Rodyxd)

I can't open the rar-archives you attached, being told that they are corrupt.

You have a system of two equations in L and Ti. To solve this system symbolically, you would have to put the two equations in a 2 x 1 matrix and then uses "solve,L,Ti".

But Prime would choke on the units, because the symbolics are not unit aware and treats them as unknown variables. Bad, because the "unit" °C is not a simple factor but a conversion function.

Better you use a numeric solve block as shown below:

11 REPLIES 11
24-Ruby IV
(To:Rodyxd)

3-Visitor
(To:ValeryOchkov)

3-Visitor
(To:ValeryOchkov)

Can you look at the file down in the next answer, please? I sent the file in the wrong reply box. Thanks

23-Emerald I
(To:Rodyxd)

Be careful with temperature units--there are two types: temperature degrees Kelvin/Centigrade and Rankine/Fahrenheit and delta temperature.  Some of your values need to be deltas.

See if the attached file helps.  I couldn't see a value for Ti, so I couldn't solve for L, but you can create a function.

3-Visitor
(To:Fred_Kohlhepp)

Thanks for the Tip! The question doesn't give the Ti. It seems that can be solved simultaneously using both equations as shown in the paper.

24-Ruby V
(To:Rodyxd)

I can't open the rar-archives you attached, being told that they are corrupt.

You have a system of two equations in L and Ti. To solve this system symbolically, you would have to put the two equations in a 2 x 1 matrix and then uses "solve,L,Ti".

But Prime would choke on the units, because the symbolics are not unit aware and treats them as unknown variables. Bad, because the "unit" °C is not a simple factor but a conversion function.

Better you use a numeric solve block as shown below:

24-Ruby V
(To:Werner_E)

Here is a way to do it with the symbolic solve command.

The trick is to get rid of the °C conversion function by providing all temperatures in Kelvin. This is achieved by using an inline evaluation after the assignment (yellow shaded areas)

3-Visitor
(To:Werner_E)

Amazing! Thank you!

24-Ruby V
(To:Werner_E)

As for the question you asked by PM (use a forum/thread reply for this to keep the thread public and consistent): The guess values were chosen arbitrarily. Mathcads numeric solver needs values to start with. If a (non-linear) system has more than one set  of solutions, the guess values may decide which set the solver will finally arrive at. And some systems are very sensible wrt the guesses and solving will fail if you don't provide guesses close enough to the solutions.

And while your system isn't linear, its quite well behaved and you will get your solutions using any guess values. Of course the guesses must have the correct units and you can't chose L=0 (divison by zero in your expressions). Otherwise you may use any guesses you like - give it a try.

23-Emerald III
(To:Werner_E)

To prevent "choking on units" (and even if no units are involved):

24-Ruby IV
(To:Rodyxd)

May be this book will be any help to you

Thermal Engineering Studies with Excel, Mathcad and Internet

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