steam properties in a DAE system

Hi guys,

Suppose that someone wants to model a dynamic thermal energy system . He will have to write down the differential equations (energy balances etc) and the algebraic supporting formulas (steam properties and others). It is the steam tables that I want to ask about. There are several tools (XSteam by Magnus Holmgren or the IAPWS_IF97sth by Mark Mickowski) but these are functions and we can't have them in our list of equations that will be solved with say ode15 for example. Am I right? We need some simple algebraic approximations.

I have seen around in the literature that there can be an approximation with a quadratic function (see drum boiler dynamics from Astrom and Bell) but they don't provide the equations. Has anyone any idea how this is done?

Thanks guys,

Warm regards,

Chris

[dpb]

Virtually always in numerical simulations the steam properties are computed by calls to implementations of these. There are at least a couple of implementations in Matlab on the File Exchange.

I've been forced to do some other things in some analog computer-based simulators that had very limited digital interfaces, but that's not an issue w/ Matlab/Simulink. There you just treat the steam properties as variable coefficients in the state variables and go on--the values will be substituted for by the function call in whatever solver you choose via your system equations that reference them.

Answer 1

[Christos]

Hi dpd, Thank you for your answer. You mean that I can have a call to an external function inside my list of equations? ie say that my DAE system is the following (just indicative):

dPdt=Q-hA(Tf-Ts)

Re=f()

Pr=f()

Nu=f()

h=f()

h(P,t)=call to an external function with P as an input

ts(h,P)=call to an external function with P as an input

and the ode solver could solve it? I thought that it would be better if I had sth like: h=a*P+B+C yeah whatever anything algebraic would do.

Thank you for your time ;)