Needs["VectorAnalysis`"]

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

Div[λ*Grad[T[t,x]]]

mm=0.001;
ρ=8500;
cp=300;
ρel20=(1./(57*(10^6)));
αR0=0.02;
d=0.2mm;
o=Pi*d;
Skolmy=Pi*((d^2)/4);
λ=300;
proud=5;
ϵ=0.5;
σ=5.67*(10^(-8));
α=10;
To=20;
dPven=dS*(α*(T[x,t]-To)+ϵ*σ*((((T[x,t]+273))^4)-(((To+273))^4)));
dS=o*dx;
QvSO=((-dPven)/(Skolmy*dx));
Qvpravy=((((proud/Skolmy)))^2)*ρel20*(1+αR0*(T[x,t]-To));
QV=Qvpravy+QvSO;
rce=ρ*cp*D[T[x,t],t]==λ*D[T[x,t],{x,2}]+QV;
xmax=30mm;
poc=T[x,0]==To;
okr1=T[0,t]==To;
okr2=T[xmax,t]==To;
tmax=20;
res=NDSolve[{rce,poc,okr1,okr2},T[x,t],{x,0,xmax},{t,0,tmax},MaxStepSize->{0.01xmax,0.01tmax}][[1]];
Plot3D[T[x,t]/.res,{x,0,xmax},{t,0,tmax},AxesLabel->{"x [m]","t [s]","T [°C]  "}]