Needs["VectorAnalysis`"]

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

Div[λ[T[x]]*Grad[T[x]]]

D[λ[T[x]]*D[T[x],x],x]

datλPlastizol={{900,0.39},{1000,0.41},{1100,0.44}};
proklad=Fit[datλPlastizol,{1,T,(T^2)},T];
pl2=ListPlot[datλPlastizol];
Plot[proklad,{T,800,1300},PlotStyle->Red,AxesLabel->{"T[°C]","λPl[W.(m^(-1)).(K^(-1))]"}];
Show[%,pl2]
λPl=proklad/.T->1300

<<PhysicalConstants`

σ=StefanConstant[[1]];

Tpec=1300;
TokoliKonvekce=25;
TokoliSalani=25;
lambda=.2;
a=0.4;
cm=0.01;
dPlastizol=2cm;
Sstr=6*0.5*((((a+2dPlastizol))^2)+(((a+dPlastizol+diz))^2));
Svnejsi=6*(((a+dPlastizol+2*diz))^2);
α=5;
ϵ=0.9;
rceUstalene={P==((Tpec-Tplast)/(diz/(lambda*Sstr))),P==Svnejsi*(α*(Tplast-TokoliKonvekce)+ϵ*σ*((((Tplast+273))^4)-(((TokoliSalani+273))^4)))};

rce[dizz_]:=rceUstalene/.diz->dizz
res[diz_]:=FindRoot[rce[diz],{P,1000},{Tplast,100}];
vykon[diz_]:=P/.res[diz];
teplotaPlaste[diz_]:=Tplast/.res[diz];
Plot[teplotaPlaste[diz],{diz,0.1,0.5},PlotStyle->Red,AxesLabel->{"diz[m]","Tpl[°C]"},AxesOrigin->{0.1,0}];
Plot[vykon[diz],{diz,0.1,0.5},PlotStyle->{Red,Thick},AxesLabel->{"diz[m]","P[W]"},AxesOrigin->{0.1,0},GridLines->Automatic]

xmax=0.15;
ro=100;
c=500;
rceFK=ro*c*D[T[x,t],t]==lambda*D[T[x,t],{x,2}];
poc=T[x,0]==TokoliKonvekce;
okrIn=-lambda*D[T[x,t],x]==((Pzvoleny*0.5(1-Tanh[((T[x,t]-Tpec)/10)]))/Sstr)*(t/(t+1))/.diz->xmax/.x->0;
okrOut=-lambda*D[T[x,t],x]==((α*(T[xmax,t]-TokoliKonvekce)+ϵ*σ*((((T[xmax,t]+273))^4)-(((TokoliSalani+273))^4))))*(t/(t+1))/.x->xmax;

Pzvoleny=4000;
res=NDSolve[{rceFK,poc,okrIn,okrOut},T[x,t],{x,0,xmax},{t,0,5*3600}][[1]];
Plot3D[T[x,t]/.res,{x,0,xmax},{t,0,2*3600},AxesLabel->{"x (m) ","t (s)","T (°C)    "}]

Plot[0.5(1-Tanh[((T-Tpec)/10)]),{T,0,1500},PlotRange->All];

(*navrh topnych clanku*)

Pzvoleny

nClanku1Faze=3;
nClankuCelkem=3.*nClanku1Faze;
U1clanek=(230./nClanku1Faze);
p1Clanek=(Pzvoleny/nClankuCelkem);
R1cl=R1clanek/.Solve[p1Clanek==((U1clanek^2)/R1clanek)][[1]];
TmaxDrat=1320;
Sclanek=S1Clanek/.Solve[p1Clanek==0.8ϵ*σ*S1Clanek*((((TmaxDrat+273))^4)-(((Tpec+273))^4))][[1]];
cmKvadratNaOhm=((Sclanek*(10^4))/R1cl);
Print["potrebuji ",cmKvadratNaOhm]

(*volim drat prumer 1.18mm*)
R1m=1.326;(*Ω/m*)
delkaDrat1clanek=(R1cl/R1m);
Print["potrebuji ", delkaDrat1clanek,"  m dratu"]