[salkynbekov]

I am modeling PCM using solidifcation/melting module in ANSYS FLUENT.
(laminar, transient simualtion)

to define properties of the PCM, I use UDF.

and for specific heat I have following:

DEFINE_SPECIFIC_HEAT(cell_cp,T, Tred,h,yl)
{
    real cp;
    
    if(T < T_m) {cp = f(T);}
    else {cp = 2150;}
    *h = cp*(T-Tref);

    return cp;	
}

But it errors as
"Warning: invalid cp (-1.15454545 J/kgK) for PCM at temperature 230.5151 K"

As you see in udf there can not be negative specific heat.

best regards.

[MKuhn]

Check your polynome. For 230 K I get a negative value as well. Anyway, you do not have to define the specific heat with an UDF. The possibilities in the material data panal are sufficient, as you can define even stepwise polynomes for temperature depend data.