<<VectorAnalysis`

SetCoordinates[Cartesian[x,y,z]];

Div[λ[T[x]]*Grad[T[x]]]

Tin=1250;
Tpovrch=70;

datλ1={{1000,0.36},{1100,0.4}};
λ1=Fit[datλ1,{1,T},T]/.T->Tin

Tproλ2=((Tin+Tpovrch)/2);
Print["T prumerna izolace= ",Tproλ2," °C"]

datλ2={{400,0.14},{600,0.18}};
λ2=Fit[datλ2,{1,T},T]/.T->Tproλ2;

a=0.5;
S1=6(((a^2)+(((a+2d1))^2))/2);
Rt1=(d1/(λ1*S1));
S2=6(((((a+2d1))^2)+(((a+2d1+2d2))^2))/2);
Rt2=(d2/(λ2*S2));

Spec=6*(((a+2d1+2d2))^2);
σ=5.67*(10^(-8));
α=3;
Tok=25;
Tor=20;
Qk=α*Spec*(Tpovrch-Tok);
Qr=ϵ*σ*Spec*((((Tpovrch+273))^4)-(((Tor+273))^4));
rce=((Tin-Tpovrch)/(Rt1+Rt2))==Qk+Qr;

cm=0.01;
ϵ=0.8;
d1=3cm;
res=Solve[rce][[-1]]

vykon=Qk+Qr/.res

Pzvoleny=5000;
P1faze=(Pzvoleny/3.);
nclanku=12.;
P1clanek=(Pzvoleny/nclanku)
Uf=230;
Solve[((Uf^2)/R)==P1clanek]

(*dodelat topne clanky*)