docs: def Nu_sphere_Churchill(Pr, Gr):
      r'''Calculates Nusselt number for natural convection around a sphere
      according to the Churchill [1]_ correlation. Sphere must be isothermal.
  
      .. math::
          Nu_D=2+\frac{0.589Ra_D^{1/4}} {\left[1+(0.469/Pr)^{9/16}\right]^{4/9}}
          \cdot\left\{1 + \frac{7.44\times 10^{-8}Ra}
          {[1+(0.469/Pr)^{9/16}]^{16/9}}\right\}^{1/12}
  
      Parameters
      ----------
      Pr : float
          Prandtl number [-]
      Gr : float
          Grashof number [-]
  
      Returns
      -------
      Nu : float
          Nusselt number, [-]
  
      Notes
      -----
      Although transition from laminar to turbulent is discrete in reality, this
      equation provides a smooth transition in value from laminar to turbulent.
      Checked with the original source.
  
      Good for Ra < 1E13. Limit of Nu is 2 at low Grashof numbers.
  
      Examples
      --------
      >>> Nu_sphere_Churchill(.7, 1E7)
      25.670869440317578
  
      References
      ----------
      .. [1] Schlunder, Ernst U, and International Center for Heat and Mass
         Transfer. Heat Exchanger Design Handbook. Washington:
         Hemisphere Pub. Corp., 1987.
      '''
      Ra = Pr*Gr
      Nu = 2 + (0.589*Ra**0.25/(1 + (0.469/Pr)**(9/16.))**(4/9.)*(
           1 + 7.44E-8*Ra/(1 + (0.469/Pr)**(9/16.))**(16/9.))**(1/12.))
      return Nu