Quiet@Remove["Global`*"];
$HistoryLength=2;
SetDirectory[NotebookDirectory[]];

Needs["VectorAnalysis`"]
SetCoordinates[Cartesian[x,y,z]]

proud=20;
mm=0.001;
d=0.2mm;
Sk=((Pi*(d^2))/4);
J=(proud/Sk);
γ=57*(10^6);
αR=0.004;
Qvel=(((J^2)*(1+αR*(T[x,t]-20)))/γ);
dS=Pi*d*dx;
dV=Sk*dx;
ϵ=0.2;
α=12;
σ=5.67*(10^(-8));
To=20;
Qkonv=α*(T[x,t]-To)*dS;
Qsal=ϵ*σ*((((T[x,t]+273))^4)-(((To+273))^4))*dS;
QVztraty=((Qkonv+Qsal)/dV)//Simplify;
ρ=8940;
c=(24.440/0.064);
λ=401;
rce=ρ*c*D[T[x,t],t]==Div[λ*Grad[T[x,t]]]+Qvel-QVztraty;
tmax=0.4;
poc=T[x,0]==To;
xmax=20mm;
okr1=T[0,t]==To;
okr2=T[xmax,t]==To;
res=NDSolve[{rce,poc,okr1,okr2},T[x,t],{x,0,xmax},{t,0,tmax}][[1]]
Plot3D[T[x,t]/.res,{x,0,xmax},{t,0,tmax}]

Plot[{T[x,t]/.res/.x->xmax*0.5,1100},{t,0,tmax}]