[TRNSYS-users] Insert polynomial equation

[David BRADLEY]

Anas,
   It isn't quite as simple as introducing a new formula. You would need 
to modify the Fortran source code of all of the affected models. For one 
thing, you would need to change those input variables (such as specific 
heat and density) from TRNSYS parameters to inputs because parameters 
are set once at the beginning of the simulation and are not updated 
after that. However, the problem goes far deeper than that. The 
assumption of constant properties is implicit in the Types' algorithms 
in many cases. When we write the equation q = m*Cp*dT/dt, we are writing 
a simplification of the actual energy balance of q = d(m*Cp*T)/dt which 
expands to: q = m*Cp*dT/dt + m*T*dCp/dt + T*Cp*dm/dt. The simplification 
comes from assuming that dCp/dt and dm/dt are both zero over a time 
step. You would need to rederive all of the algorithms in the affected 
Types and then recode them in order to allow all of the properties to 
vary with time. Fortunately most of the fluids that we deal with have 
relatively constant properties over the temperature range at which they 
are used. One exception is the working fluids in high temperature solar 
thermal applications and you will find some components available in the 
High Temperature Solar TESS Library that do not make the simplifying 
assumptions above.
regards,
  David


On 4/27/2015 3:44 AM, Anas Majdouli wrote:
> Hi TRNsys community,
>
> I hope you are doing very well.
>
> For TRNsys simulations, usually when we are using types, density 
> and/or Fluid Specific Heat are among the parameters to specify. The 
> problem is sometimes, these parameters vary and depend on temperature, 
> pression...
>
> So I looked for a polynomial equation that gives for example 
> density=f(Temperature) and I found one [formula 1 in 
> http://metgen.pagesperso-orange.fr/metrologiefr19.htm]. 
> <http://metgen.pagesperso-orange.fr/metrologiefr19.htm%5d.> My 
> question is : how to introduce this formula in TRNsys so that this 
> latter will use it automatically ? I don't want to create an 
> 'Assembly/Insert new equation' component each time...
>
> Thank you in advance,
>
> Anas MAJDOULI
>
>
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-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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