def helical_turbulent_fd_Guo(Re, Di, Dc):
    """Calculates Darcy friction factor for a fluid flowing inside a curved 
    pipe such as a helical coil under turbulent conditions, using the method of 
    Guo [1]_, also shown in [2]_.
        
    .. math::
        f_{curv} = 0.638Re^{-0.15}\\left(\\frac{D_i}{D_c}\\right)^{0.51}
        
    Parameters
    ----------
    Re : float
        Reynolds number with `D=Di`, [-]
    Di : float
        Inner diameter of the coil, [m]
    Dc : float
        Diameter of the helix/coil measured from the center of the tube on one
        side to the center of the tube on the other side, [m]

    Returns
    -------
    fd : float
        Darcy friction factor for a curved pipe [-]

    Notes
    -----    
    Valid for :math:`2\\times10^4 < Re < 1.5\\times10^{5}`. Does not use a 
    straight pipe correlation, and so will not converge on the
    straight pipe result at very low curvature.

    Examples
    --------
    >>> helical_turbulent_fd_Guo(2E5, 0.01, .2)
    0.022189161013253147

    References
    ----------
    .. [1] Guo, Liejin, Ziping Feng, and Xuejun Chen. "An Experimental 
       Investigation of the Frictional Pressure Drop of Steam–water Two-Phase 
       Flow in Helical Coils." International Journal of Heat and Mass Transfer 
       44, no. 14 (July 2001): 2601-10. doi:10.1016/S0017-9310(00)00312-4. 
    .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
       Correlations for Convection Heat Transfer and Pressure Losses in 
       Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
       (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
    """
    return 0.638 * Re ** -0.15 * (Di / Dc) ** 0.51
