docs: def Nu_KTA(Re, Pr, voidage):
      r'''Calculates Nusselt number of a fluid passing over a bed of particles
      using a correlation shown in [1]_ and also cited in the review of [2]_.
      
      .. math::
          Nu = 1.27\frac{Pr^{1/3}}{\epsilon^{1.18}}Re^{0.36}
          + 0.033\frac{Pr^{0.5}}{\epsilon^{1.07}}Re^{0.86}
  
      Parameters
      ----------
      Re : float
          Reynolds number with pebble diameter as characteristic dimension, [-]
      Pr : float
          Prandtl number of the fluid [-]
      voidage : float
          Void fraction of bed packing [-]
  
      Returns
      -------
      Nu : float
          Nusselt number for heat transfer to the packed bed [-]
  
      Notes
      -----
      100 < Re < 1E5;
      0.36 < ε < 0.42;
      D/d > 20 with D as bed diameter, d as particle diameter;
      H > 4d with H as bed height.
      
      Examples
      --------
      >>> Nu_KTA(2000, 0.7, 0.4)
      102.08516480718129
  
      References
      ----------
      .. [1] Reactor Core Design of High-Temperature Gas-Cooled Reactors Part 2: 
         Heat Transfer in Spherical Fuel Elements (June 1983).
         http://www.kta-gs.de/e/standards/3100/3102_2_engl_1983_06.pdf
      .. [2] Abdulmohsin, Rahman S., and Muthanna H. Al-Dahhan. "Characteristics 
         of Convective Heat Transport in a Packed Pebble-Bed Reactor." Nuclear
         Engineering and Design 284 (April 1, 2015): 143-52. 
         doi:10.1016/j.nucengdes.2014.11.041.
      '''
      return (1.27*Pr**(1/3.)*Re**0.36/voidage**1.18 
              + 0.033*Pr**0.5/voidage**1.07*Re**0.86)