def Huq_Loth(x, rhol, rhog):
    """Calculates void fraction in two-phase flow according to the model of 
    [1]_, also given in [2]_, [3]_, and [4]_.

    .. math::
        \\alpha = 1 - \\frac{2(1-x)^2}{1 - 2x + \\left[1 + 4x(1-x)\\left(\\frac
        {\\rho_l}{\\rho_g}-1\\right)\\right]^{0.5}}

    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]

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

    Notes
    -----
    [1]_ has been reviewed, and matches the expressions given in the reviews
    [2]_, [3]_, and [4]_; the form of the expression is rearranged somewhat 
    differently.

    Examples
    --------
    >>> Huq_Loth(.4, 800, 2.5)
    0.9593868838476147

    References
    ----------
    .. [1] Huq, Reazul, and John L. Loth. "Analytical Two-Phase Flow Void 
       Prediction Method." Journal of Thermophysics and Heat Transfer 6, no. 1 
       (January 1, 1992): 139-44. doi:10.2514/3.329.
    .. [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.
    .. [4] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
       Fraction Correlations for Different Flow Patterns in Horizontal and 
       Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
       no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
    """
    B = 2 * x * (1 - x)
    D = (1 + 2 * B * (rhol / rhog - 1)) ** 0.5
    return 1 - 2 * (1 - x) ** 2 / (1 - 2 * x + D)
