g = 9.81;
koefrozt[T_] := (1/(T + 273));
datany = {{0. , 13.7}, {20. , 15.7}, {40. , 17.6}, {60. , 
            19.6}, {80. , 21.7}, {100, 23.8}, {120, 26.2}};
pomfce = Interpolation[datany];
\(\(ny[T_] := pomfce[T]*10^\(-6\);\)
 (*Plot[ny[T], {T, 0, 80}, 
        PlotRange -> All, 
        AxesLabel -> {T[°C], ν[T]}]*) \)
dataPr = {{0. , 0.718}, {100. , 0.706}, {200. , 0.693}, {300. , 
            0.686}, {400, 0.681}, {500, 0.680}, {600, 0.680}};
Pr[T_] := Interpolation[dataPr][T];
datalambda = {{0. , 2.366}, {20. , 2.17}, {40. , 2.28}, {60. , 
            2.41}, {80. , 2.52}, {100, 2.64}, {120, 2.75}};
\(\(lambda[T_] := 0.01*\(Interpolation[datalambda]\)[T];\)
\)
Gr[T1_, T2_, 
          delka_] := ((koefrozt[((T1 + T2)/2)]*(delka^3)*Abs[(T2 - T1)]*g)/((ny[((T1 + T2)/2)])))^2;
Nu[T1_, T2_, delka_] := 
        1.18*(Gr[T1, T2, delka]*Pr[((T1 + T2)/2)])^(1/8) /; 
          10^-3 ≤ Gr[T1, T2, delka]*Pr[((T1 + T2)/2)] < 
            500;
\(\(Nu[T1_, T2_, delka_] := 
        0.54*\((Gr[T1, T2, delka]*Pr[\(T1 + T2\)\/2])\)^\((1/4)\) /; 
          500 ≤ Gr[T1, T2, delka]*Pr[\(T1 + T2\)\/2] < 
            2*10^7;\)
\)
\(\(Nu[T1_, T2_, delka_] := 
        0.135*\((Gr[T1, T2, delka]*Pr[\(T1 + T2\)\/2])\)^\((1/3)\) /; 
          2*10^7 ≤ Gr[T1, T2, delka]*Pr[\(T1 + T2\)\/2] < 
            10^13;\)
\)
alfa[T1_, T2_, 
          delka_] := ((Nu[T1, T2, delka]*lambda[((T1 + T2)/2)])/delka);