docs: def Knott(m, x, D, rhol, rhog, Cpl=None, kl=None, mu_b=None, mu_w=None, L=None,
            hl=None):
      r'''Calculates the two-phase non-boiling heat transfer coefficient of a 
      liquid and gas flowing inside a tube of any inclination, as in [1]_ and 
      reviewed in [2]_.
  
      Either a specified `hl` is required, or `Cpl`, `kl`, `mu_b`, `mu_w` and 
      `L` are required to calculate `hl`.
  
      .. math::
          \frac{h_{TP}}{h_l} = \left(1 + \frac{V_{gs}}{V_{ls}}\right)^{1/3}
              
      Parameters
      ----------
      m : float
          Mass flow rate [kg/s]
      x : float
          Quality at the specific tube interval [-]
      D : float
          Diameter of the tube [m]
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      Cpl : float, optional
          Constant-pressure heat capacity of liquid [J/kg/K]
      kl : float, optional
          Thermal conductivity of liquid [W/m/K]
      mu_b : float, optional
          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]
      L : float, optional
          Length of the tube [m]
      hl : float, optional
          Liquid-phase heat transfer coefficient as described below, [W/m^2/K]
  
      Returns
      -------
      h : float
          Heat transfer coefficient [W/m^2/K]
  
      Notes
      -----
      The liquid-only heat transfer coefficient will be calculated with the 
      `laminar_entry_Seider_Tate` correlation, should it not be provided as an
      input. Many of the arguments to this function are optional and are only
      used if `hl` is not provided. 
      
      `hl` should be calculated with a velocity equal to that determined with 
      a combined volumetric flow of both the liquid and the gas. All other 
      parameters used in calculating the heat transfer coefficient are those
      of the liquid. If the viscosity at the wall temperature is not given, the 
      liquid viscosity correction in `laminar_entry_Seider_Tate` is not applied.
      
      Examples
      --------
      >>> Knott(m=1, x=.9, D=.3, rhol=1000, rhog=2.5, Cpl=2300, kl=.6, mu_b=1E-3, 
      ... mu_w=1.2E-3, L=4)
      4225.536758045839
  
      References
      ----------
      .. [1] Knott, R. F., R. N. Anderson, Andreas. Acrivos, and E. E. Petersen. 
         "An Experimental Study of Heat Transfer to Nitrogen-Oil Mixtures." 
         Industrial & Engineering Chemistry 51, no. 11 (November 1, 1959): 
         1369-72. doi:10.1021/ie50599a032. 
      .. [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.
      '''
      Vgs = m*x/(rhog*pi/4*D**2)
      Vls = m*(1-x)/(rhol*pi/4*D**2)
      if not hl:
          V = Vgs + Vls # Net velocity
          Re = Reynolds(V=V, D=D, rho=rhol, mu=mu_b)
          Pr = Prandtl(Cp=Cpl, k=kl, mu=mu_b)
          Nul = laminar_entry_Seider_Tate(Re=Re, Pr=Pr, L=L, Di=D, mu=mu_b, mu_w=mu_w)
          hl = Nul*kl/D
      return hl*(1 + Vgs/Vls)**(1/3.)