docs: def Hughmark(m, x, alpha, D, L, Cpl, kl, mu_b=None, mu_w=None):
      r'''Calculates the two-phase non-boiling laminar heat transfer coefficient  
      of a liquid and gas flowing inside a tube of any inclination, as in [1]_  
      and reviewed in [2]_.
  
      .. math::
          \frac{h_{TP} D}{k_l} = 1.75(1-\alpha)^{-0.5}\left(\frac{m_l C_{p,l}}
          {(1-\alpha)k_l L}\right)^{1/3}\left(\frac{\mu_b}{\mu_w}\right)^{0.14}
      
      Parameters
      ----------
      m : float
          Mass flow rate [kg/s]
      x : float
          Quality at the specific tube interval []
      alpha : float
          Void fraction in the tube, []
      D : float
          Diameter of the tube [m]
      L : float
          Length of the tube, [m]
      Cpl : float
          Constant-pressure heat capacity of liquid [J/kg/K]
      kl : float
          Thermal conductivity of liquid [W/m/K]
      mu_b : float
          Viscosity of liquid at bulk conditions (average of inlet/outlet 
          temperature) [Pa*s]
      mu_w : float, optional
          Viscosity of liquid at wall temperature [Pa*s]
  
      Returns
      -------
      h : float
          Heat transfer coefficient [W/m^2/K]
  
      Notes
      -----
      This model is based on a laminar entry length correlation - for a 
      sufficiently long tube, this will predict unrealistically low heat transfer
      coefficients.
      
      If the viscosity at the wall temperature is not given, the liquid viscosity
      correction is not applied.
  
      Developed for horizontal pipes in laminar slug flow. Data consisted of the 
      systems air-water, air-SAE 10 oil, gas-oil, air-diethylene glycol, and
      air-aqueous glycerine.
      
      Examples
      --------
      >>> Hughmark(m=1, x=.9, alpha=.9, D=.3, L=.5, Cpl=2300, kl=0.6, mu_b=1E-3, 
      ... mu_w=1.2E-3)
      212.7411636127175
  
      References
      ----------
      .. [1] Hughmark, G. A. "Holdup and Heat Transfer in Horizontal Slug Gas-
         Liquid Flow." Chemical Engineering Science 20, no. 12 (December 1, 
         1965): 1007-10. doi:10.1016/0009-2509(65)80101-4.
      .. [2] Dongwoo Kim, Venkata K. Ryali, Afshin J. Ghajar, Ronald L. 
         Dougherty. "Comparison of 20 Two-Phase Heat Transfer Correlations with 
         Seven Sets of Experimental Data, Including Flow Pattern and Tube 
         Inclination Effects." Heat Transfer Engineering 20, no. 1 (February 1, 
         1999): 15-40. doi:10.1080/014576399271691.
      '''
      ml = m*(1-x)
      RL = 1-alpha
      Nu_TP = 1.75*(RL)**-0.5*(ml*Cpl/RL/kl/L)**(1/3.)
      if mu_b and mu_w:
          Nu_TP *= (mu_b/mu_w)**0.14
      return Nu_TP*kl/D