vyr = 
        2*((x - 1)/(Log[x]*(x + 1)));
Plot[vyr, {x, 0.25, 4}, PlotStyle -> {Hue[0], Thickness[0.008]}, 
      PlotRange -> All]

<< Miscellaneous`Units`
Off[General::"spell1"];
ConvertTemperature[50, Centigrade, Fahrenheit]

Ts = 80;
To = 20;
Tdef = 0.5*(Ts + To);
Pr = 0.7;
β = (1/(Tdef + 273));
g = 10;
ΔT = Ts - To;
ν = 18.6*(10^(-6));
Ddef = 0.1016;
Gr = ((β*ΔT*g*(Ddef^3))/(ν^2));
GrPr = Gr*Pr;
Print["Gr*Pr= ", GrPr]
c = 0.535;
n = 0.25;
Nu = c*(((Gr*Pr))^n);
Print["Nu= ", Nu];
λ = 0.0234*(4186. /3600);
\(\(Print["λ= ", λ, \*"" W.(m^(-1)).(K^(-1))""];\)\)
α = ((Nu*λ)/Ddef);
\(\(Print["α= ", α, \*"" W.(m^(-2)).(K^(-1))""];\)\)

S = Pi*Ddef*1;
P1m = S*α*ΔT;
Print["z 1 m teče= ", P1m, " W"];

mm = 0.001;
ts = 3.6  mm;
Dvnitrni = Ddef - 2*ts;
QV = 0.1*(10^(-3));
prurez = ((Pi*(Dvnitrni^2))/4);
v = (QV/prurez);
Tdefvoda = 80;
Prvoda = 2.23;
λvoda =  .58*(4186. /3600);
νvoda = 0.366*(10^(-6));
Rey = ((v*Dvnitrni)/ν)voda;
Nuvoda = 0.023*(Rey^0.8)*(Pr^0.4);
αvoda = ((Nuvoda*λvoda)/Dvnitrni);
\(\(Print["αvoda= ", αvoda, \*"" W.(m^(-2)).(K^(-1))""];\)\)