def helical_transition_Re_Kutateladze_Borishanskii(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]_,
    also shown in [2]_.

    .. math::
        Re_{crit} = 2300 + 1.05\\times 10^4 \\left(\\frac{D_i}{D_c}\\right)^{0.3}
        
    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 = 2300.
    Recommended for :math:`0.0417 < d_i/D_c < 0.1667`

    Examples
    --------
    >>> helical_transition_Re_Kutateladze_Borishanskii(1, 7.)
    7121.143774574058
    
    References
    ----------
    .. [1] Kutateladze, S. S, and V. M Borishanskiĭ. A Concise Encyclopedia of 
       Heat Transfer. Oxford; New York: Pergamon Press, 1966.
    .. [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 2300.0 + 10500.0 * (Di / Dc) ** 0.4
