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

Quiet@Remove["Global`*"];
Ts=80;
Ta=20;
Tdef=((Ts+Ta)/2);
Print["Tdef= ",Tdef," °C"]
λ=28.08*(10^(-3));
Pr=0.705;
ν=17.88*(10^(-6));
w=1;
Rey=((Dchar*w)/ν);
Dchar=0.1;
Print["Re= ",Rey," -"]
Nu=0.193*(Rey^0.618)*(Pr^(1/3));
Print["Nu= ",Nu," -"];
α=Nu*(λ/Dchar);
Print["α= ",α," W.(m^(-1)).(K^(-1))"];
delka=1;
plocha=Pi*Dchar*delka;
vykonKonvekci=α*plocha*(Ts-Ta);
Print["vykon 1m trubky konvekci= ",vykonKonvekci," W"]

Quiet@Remove["Global`*"];
Ts=80;
Ta=20;
Tdef=((Ts+Ta)/2);
Print["Tdef= ",Tdef," °C"]
λ=28.08*(10^(-3));
Pr=0.705;
ν=17.88*(10^(-6));
Dchar=0.1;
g=9.81;
ΔT=Ts-Ta;
β=(1./(Tdef+273));
Gr=((β*ΔT*g*(Dchar^3))/(ν^2));
Print["Gr= ",Gr," -"];
Ra=Gr*Pr;
Print["Ra= ",Ra," -"];
Nu=0.48*(Ra^0.25);
Print["Nu= ",Nu," -"];
α=Nu*(λ/Dchar);
Print["α= ",α," W.(m^(-1)).(K^(-1))"];
delka=1;
plocha=Pi*Dchar*delka;
vykonKonvekci=α*plocha*(Ts-Ta);
Print["vykon 1m trubky konvekci= ",vykonKonvekci," W"]
ϵ=0.9;
σ=5.67*(10^(-8));
Psalani=plocha*ϵ*σ*((((Ts+273))^4)-(((Ta+273))^4));
Print["vykon 1m trubky salanim= ",Psalani," W"]