ODESOLVE too few equations

[rguerrero]

Hi all,

This is my first post here on the PTC Community. I have a question regarding an ordinary differential equation solver.

I just simply do not understand why it can not solve the equations with the constraints provided.

I did not miss any initial conditions.

The document is about a membrane distiller, where the temperature is variable along the length of the membrane. The change of length is stated as x where the temperature becomes a function of x. Also the mass flow and flux will be a function of the length of the membrane since the mass flow at the seawater section loses mass that goes to the demiwater section.

The literature references for this problem are these:

https://www.dropbox.com/s/na4xr1ko7jgzvm2/Modelling%20heat%20and%20mass%20transfers%20in%20DCMD%20using%20compressible%20membranes.pdf

http://web.mit.edu/seawater/Seawater_Property_Tables.pdf

Can someone take a look at it? If you need more information, just ask me.

Thanks in advance, Rowan

[Fred_Kohlhepp]

I think you're too complex!

You really have only three functions to solve: T_sw, T_dw, and phi. (phi_sw = -phi_dw, and H and J are combinations of the other three.

Try recasting the problem for those three, see how you do.

[rguerrero]

It do sounds like a problem where it would terminate the calculation. Though my second task is to plot the functions J_m(x), phi_sw(x), phi_dw(x), T_sw(x), T_dw(x), Psat_dw(T_dw(x)) and Psat_sw(T_sw(x)).

Would it still be possible to plot these since these are not solved in the odesolver?

After deleting the combinations out of the odesolver, it comes up with a new error:

"dae%_initial%_condition%_error"

If I rewrite the inital conditions of all functions like in the .mcdx file, but keep the odesolver like in the image above, the problem persists.

[Fred_Kohlhepp]

Took a closer look:

You have two functions Psw(T) and Pdw(T) that aren't defined, and you need them defined. You need to write your equations in T and phi, remove the J's and H's.

[rguerrero]

I have now removed the fluxes J and enthalpy's H, substituted them for the whole equations and defined the missing functions Cp_sw(T), Cp_dw(T), Psat_sw(T) and Psat_dw(T).

Can you tell me if I did this right? I still do have the same "dae%_initial%_condition%_error" error in the odesolver.

If I take all Psat's and Cp's as constant, and delete their initial conditions, it again comes up with the error "There are too few equations".

This problem is annoying me haha

EDIT: I tried to change some of the T_sw into T_dw, where the error message changed. I think mathcad does not have enough equations to determine T_dw, but does for T_sw.

[Werner_E]

Sorry, you file continously crashes Prime for me with the folling error window

It happens quite often and its a shame that a version 3 is still that unstable.

Prime often fails with labelling the variables - so look if the labels of all functions (definitions and calls) are the same.