def helical_transition_Re_Kubair_Kuloor(Di, Dc):
    """Calculates the transition Reynolds number for flow inside a curved or 
    helical coil between laminar and turbulent flow, using the method of [1]_,
    as shown in [2]_.

    .. math::
        Re_{crit} = 12730 \\left(\\frac{D_i}{D_c}\\right)^{0.2}
        
    Parameters
    ----------
    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
    -------
    Re_crit : float
        Transition Reynolds number between laminar and turbulent [-]

    Notes
    -----
    At very low curvatures, converges to Re = 0.
    Recommended for :math:`0.0005 < d_i/D_c < 0.103`

    Examples
    --------
    >>> helical_transition_Re_Kubair_Kuloor(1, 7.)
    8625.986927588123
    
    References
    ----------
    .. [1] Kubair, Venugopala, and N. R. Kuloor. "Heat Transfer to Newtonian
       Fluids in Coiled Pipes in Laminar Flow." International Journal of Heat 
       and Mass Transfer 9, no. 1 (January 1, 1966): 63-75. 
       doi:10.1016/0017-9310(66)90057-3. 
    .. [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 12730.0 * (Di / Dc) ** 0.2
