Tsurface=70;
Tambient=20;
Tdef=((Tsurface+Tambient)/2);
Print["T def= ", Tdef," °C"]

β=(1/(Tdef+273.));
ΔT=Tsurface-Tambient;
g=9.81;
Dchar=0.4;
TdefinK=Tdef+273;
ν=(10^(-6))Fit[{{300,15.68},{350,20.76}},{1,T},T]/.T->TdefinK;
Gr=((β*ΔT*g*(Dchar^3))/(ν^2));
Print["ν= ",(10^6)ν," μ (m^2).(s^(-1))"]
Print["Gr= ",Gr,"  -"]
Pr=0.7;
Ra=Gr*Pr;
Print["Ra= ",Ra,"  -"]

Nu=0.15*(Ra^(1/3));
Print["Nu= ",Nu, "  -"]

λ=Fit[{{40,0.0271},{60,0.0285}},{1,T},T]/.T->Tdef;
Print["λ= ",λ,"  W.(m^(-1)).(K^(-1))"]

α=((Nu*λ)/Dchar);
Print["α= ",α,"  W.(m^(-2)).(K^(-1))"]

S=2*0.6*0.4;
Print["Area S= ",S," (m^2)"]

Q=α*S*ΔT;
Print["Q due to convection= ",Q,"  W"]

σ=5.67*(10^(-8));(*W.(m^2).(K^4)*)
ϵ=0.9;
Qr=σ*S*ϵ*((((Tsurface+273))^4)-(((Tambient+273))^4));
Print["Q due to radiation= ",Qr,"  W"]

(*Heat Exchangers*)
Qexch=kS*((Δ1-Δ2)/(Log[((Δ1)/(Δ2))]));
Limit[Qexch,Δ2->Δ1]

expr=((ξ+1)/(2*(ξ-1)))*Log[ξ];
Plot[100(1-expr),{ξ,0.25,4},AxesLabel->{"((Δ1)/(Δ2))","error (%)"},PlotStyle->Thick,GridLines->Automatic]

Volume=1000.;
cp=1000;
Vflow=(Volume/(3*3600));
Print["Volume flow= ",Vflow," (m^3).(s^(-1))"]
Tin=20;
Tout=-10;