ClearAll[Evaluate[Context[]<>"*"]]

proud=15;
delkaDratu=240mm;
mm=0.001;
d=0.2mm;
obvod=Pi*d;
prurez=((Pi*(d^2))/4);
ϵ=0.5;
α=15;
σ=5.67*(10^(-8));
Tokoli=20;
dS=obvod*dx;
dV=prurez*dx;
dPkonvekci=α*dS*(T[x,t]-Tokoli);
dPsalani=dS*ϵ*σ*((((T[x,t]+273))^4)-(((Tokoli+273))^4));
QVfiktivni=-((dPkonvekci+dPsalani)/dV)//Simplify;
αR=4*(10^(-3));
γ=((57*(10^6))/(1+αR*(T[x,t]-20)));
Jecko=(proud/prurez);
QVpravy=((Jecko^2)/γ);
hustota=8900;
cp=400;
λ=400;
rce=hustota*cp*D[T[x,t],t]==λ*D[T[x,t],{x,2}]+QVfiktivni+QVpravy;
poc=T[x,0]==Tokoli;
okr1=T[0,t]==Tokoli;
okr2=T[delkaDratu,t]==Tokoli;
tmax=1;
res=NDSolve[{rce,poc,okr1,okr2},T[x,t],{x,0,delkaDratu},{t,0,tmax}][[1]];
Plot3D[T[x,t]/.res,{x,0,delkaDratu},{t,0,tmax},AxesLabel->{"x [m]    ","   t [s]","   T [°C]"}]

teplotaVPuli[t_]=(T[x,t]/.res)/.x->0.5*delkaDratu;
Plot[{teplotaVPuli[t],1084},{t,0,tmax}]
FindRoot[teplotaVPuli[casTaveni]==1084,{casTaveni,0.8}]