def helical_transition_Re_Srinivasan(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]_ and [3]_. Correlation recommended in [3]_.

    .. math::
        Re_{crit} = 2100\\left[1 + 12\\left(\\frac{D_i}{D_c}\\right)^{0.5}\\right]        
        
    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 = 2100.
    Recommended for :math:`0.004 < d_i/D_c < 0.1`.
    
    Examples
    --------
    >>> helical_transition_Re_Srinivasan(1, 7.)
    11624.704719832524
    
    References
    ----------
    .. [1] Srinivasan, P. S., Nandapurkar, S. S., and Holland, F. A., "Pressure
       Drop and Heat Transfer in Coils", Chemical Engineering, 218, CE131-119,
       (1968).
    .. [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.
    .. [3] Rohsenow, Warren and James Hartnett and Young Cho. Handbook of Heat
       Transfer, 3E. New York: McGraw-Hill, 1998.
    """
    return 2100.0 * (1.0 + 12.0 * (Di / Dc) ** 0.5)
