msteckou=0.3;
c=4186;
ΔT=32;
vykon=msteckou*c*ΔT;
Print["vykon= ",(vykon/1000)," kW"]

Needs["VectorAnalysis`"]

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

v={vx,0,0};
v.Grad[T[x]]

Div[λ*Grad[T[x]]]

gammaPraha[T_]=Fit[{{10,(1/35.)},{50,(1/10.)}},{1,T},T];
gammaMB[T_]=Fit[{{20,(1/72)},{75,(1/30.)}},{1,T},T];

ro=1000;
d=5mm;
U=230;
m=400mm;
Vsteckou=(msteckou/ro);
vx=(Vsteckou/(d*m));
mm=0.001;
rce={ro*c*vx*T'[x]==gammaPraha[T[x]]*((((U/d)))^2),T[0]==8};
res=DSolve[rce,T,x][[1]]
Plot[{T[x]/.res,40},{x,0,1}]
delka=x/.FindRoot[(T[x]/.res)==40,{x,0.4}]

rce={ro*c*vx*T'[x]==gammaMB[T[x]]*((((U/d)))^2),T[0]==8};
res=DSolve[rce,T,x][[1]]
Plot[{T[x]/.res,40},{x,0,1}]

Print["prazsky ohrivac da v MB> ",(T[x]/.res)/.x->delka," °C"]

Plot[{(ksi^2)-1,5Log[ksi]},{ksi,1,5},AxesLabel->{"(r2/r1)","Rth,cena"},PlotStyle->Thick]

ClearAll[delka];
Rthcelkem=(1/(2*Pi*delka*λ))*Log[(r2/r1)]+(1/(α*2*Pi*delka*r2));
D[Rthcelkem,r2]//Simplify;
Solve[Numerator[%]==0,r2]

(λ/α)/.{λ->0.04,α->15}

(λ/α)/.{λ->0.2,α->6}