def helical_turbulent_fd_Prasad(Re, Di, Dc,roughness=0):
    r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
    pipe such as a helical coil under turbulent conditions, using the method of 
    Prasad [1]_, also shown in [2]_.
        
    .. math::
        f_{curv} = f_{\text{str,turb}}\left[1 + 0.18\left[Re\left(\frac{D_i}
        {D_c}\right)^2\right]^{0.25}\right]
        
    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]
    roughness : float, optional
        Roughness of pipe wall [m]        

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

    Notes
    -----    
    No range of validity was specified, but the experiments used were with 
    coil/tube diameter ratios of 17.24 and 34.9, hot water in the tube, and
    :math:`1780 < Re < 59500`. At very low curvatures, converges on the
    straight pipe result.

    Examples
    --------
    >>> helical_turbulent_fd_Prasad(1E4, 0.01, .2)
    0.043313098093994626

    References
    ----------
    .. [1] Prasad, B. V. S. S. S., D. H. Das, and A. K. Prabhakar. "Pressure 
       Drop, Heat Transfer and Performance of a Helically Coiled Tubular 
       Exchanger." Heat Recovery Systems and CHP 9, no. 3 (January 1, 1989): 
       249-56. doi:10.1016/0890-4332(89)90008-2.
    .. [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.
    '''
    fd = friction_factor(Re=Re, eD=roughness/Di)
    return fd*(1. + 0.18*(Re*(Di/Dc)**2)**0.25)