docs: def Nu_Kitoh(Re, Pr, H=None, G=None, q=None):
      r'''Calculates internal convection Nusselt number for turbulent vertical
      upward flow in a pipe under supercritical conditions according to [1]_, 
      also shown in [2]_, [3]_ and [4]_. Depends on fluid enthalpy, mass flux,
      and heat flux.
          
      .. math::
          Nu_b = 0.015Re_b^{0.85} Pr_b^m
          
          m = 0.69 - \frac{81000}{q_{dht}} + f_cq
          
          q_{dht} = 200 G^{1.2}
          
          f_c = 2.9\times10^{-8} + \frac{0.11}{q_{dht}} \text{ for } 
          H_b < 1500 \text{ kJ/kg}
          
          f_c = -8.7\times10^{-8} - \frac{0.65}{q_{dht}} \text{ for } 
          1500 \text{ kJ/kg} < H_b < 3300 \text{ kJ/kg}
          
          f_c = -9.7\times10^{-7} + \frac{1.3}{q_{dht}} \text{ for } 
          H_b > 3300 \text{ kJ/kg}
  
      Parameters
      ----------
      Re : float
          Reynolds number with bulk fluid properties, [-]
      Pr : float
          Prandtl number with bulk fluid properties, [-]
      H : float, optional
          Enthalpy of water (if the fluid is water), [J/kg]
      G : float, optional
          Mass flux of the fluid, [kg/m^2/s]
      q : float, optional
          Heat flux to wall, [W/m^2]
  
      Returns
      -------
      Nu : float
          Nusselt number as explained below, [-]
  
      Notes
      -----
      The reference point for the enthalpy values is not stated in [1]_. The 
      upper and lower enthalpy limits for this correlation are 4000 kJ/kg and 
      0 kJ/kg, but these are not enforced in this function.
      
      If not all of H, G, and q are provided, the correlation is used without
      the correction.
      
      This correlation was ranked 6th best in [3]_, and found 4th best for 
      enhanced heat transfer in [2]_ with a MAD of 12.3%.
      
      For the data used to develop the correlation, G varied from 100-1750 
      kg/m^2/s, q varied from 0 to 1800 kW/m^2, and bulk temperature varied from 
      20 to 550 decrees Celsius. 
  
      This correlation does not have realistic behavior for values outside those
      used in the study, and should not be used.
  
      Examples
      --------
      >>> Nu_Kitoh(1E5, 1.2, 1.3E6, 1500, 5E6)
      331.80234139591306
      
      References
      ----------
      .. [1] Kitoh, Kazuaki, Seiichi Koshizuka, and Yoshiaki Oka. "Refinement of 
         Transient Criteria and Safety Analysis for a High-Temperature Reactor 
         Cooled by Supercritical Water." Nuclear Technology 135, no. 3 
         (September 1, 2001): 252-64.
      .. [2] Chen, Weiwei, Xiande Fang, Yu Xu, and Xianghui Su. "An Assessment of
         Correlations of Forced Convection Heat Transfer to Water at 
         Supercritical Pressure." Annals of Nuclear Energy 76 (February 2015): 
         451-60. doi:10.1016/j.anucene.2014.10.027.
      .. [3] Yu, Jiyang, Baoshan Jia, Dan Wu, and Daling Wang. "Optimization of 
         Heat Transfer Coefficient Correlation at Supercritical Pressure Using 
         Genetic Algorithms." Heat and Mass Transfer 45, no. 6 (January 8, 2009): 
         757-66. doi:10.1007/s00231-008-0475-4.
      .. [4] Jäger, Wadim, Victor Hugo Sánchez Espinoza, and Antonio Hurtado. 
         "Review and Proposal for Heat Transfer Predictions at Supercritical 
         Water Conditions Using Existing Correlations and Experiments." Nuclear 
         Engineering and Design, (W3MDM) University of Leeds International 
         Symposium: What Where When? Multi-dimensional Advances for Industrial 
         Process Monitoring, 241, no. 6 (June 2011): 2184-2203. 
         doi:10.1016/j.nucengdes.2011.03.022. 
      '''
      if H and G and q:
          qht = 200.*G**1.2
          if H < 1.5E6:
              fc = 2.9E-8 + 0.11/qht
          elif 1.5E6 <= H <= 3.3E6:
              fc = -8.7E-8 - 0.65/qht
          else:
              fc = -9.7E-7 + 1.3/qht
          m = 0.69 - 81000./qht + fc*q
      else:
          m = 0.69
      return 0.015*Re**0.85*Pr**m