def Steiner(x, rhol, rhog, sigma, m, D, g=g):
    r'''Calculates void fraction in two-phase flow according to the model of 
    [1]_ also given in [2]_ and [3]_.
    
    .. math::
        \alpha = \frac{x}{\rho_g}\left[C_0\left(\frac{x}{\rho_g} + \frac{1-x}
        {\rho_l}\right) +\frac{v_{gm}}{G} \right]^{-1}
        
    .. math::
        v_{gm} = \frac{1.18(1-x)}{\rho_l^{0.5}}[g\sigma(\rho_l-\rho_g)]^{0.25}

    .. math::
        C_0 = 1 + 0.12(1-x)
        
    Parameters
    ----------
    x : float
        Quality at the specific tube interval []
    rhol : float
        Density of the liquid [kg/m^3]
    rhog : float
        Density of the gas [kg/m^3]
    sigma : float
        Surface tension of liquid [N/m]
    m : float
        Mass flow rate of both phases, [kg/s]
    D : float
        Diameter of the channel, [m]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    alpha : float
        Void fraction (area of gas / total area of channel), [-]

    Notes
    -----
    [1]_ has been reviewed. 
        
    Examples
    --------
    >>> Steiner(0.4, 800., 2.5, sigma=0.02, m=1, D=0.3)
    0.895950181381335

    References
    ----------
    .. [1] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
       Berlin; New York:: Springer, 2010.
    .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
       Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
       1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
    .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
       Void Fraction Models on the Two-Phase Friction Factor of R134a during 
       Condensation in Vertical Downward Flow in a Smooth Tube." International 
       Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
       921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
    '''
    G = m/(pi/4*D**2)
    C0 = 1 + 0.12*(1-x)
    vgm = 1.18*(1-x)/rhol**0.5*(g*sigma*(rhol-rhog))**0.25
    return x/rhog*(C0*(x/rhog + (1-x)/rhol) + vgm/G)**-1