def helical_transition_Re_Seth_Stahel(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]_.

    .. math::
        Re_{crit} = 1900\\left[1 + 8 \\sqrt{\\frac{D_i}{D_c}}\\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 = 1900.

    Examples
    --------
    >>> helical_transition_Re_Seth_Stahel(1, 7.)
    7645.0599897402535
    
    References
    ----------
    .. [1] Seth, K. K., and E. P. Stahel. "HEAT TRANSFER FROM HELICAL COILS 
       IMMERSED IN AGITATED VESSELS." Industrial & Engineering Chemistry 61, 
       no. 6 (June 1, 1969): 39-49. doi:10.1021/ie50714a007.
    """
    return 1900.0 * (1.0 + 8.0 * (Di / Dc) ** 0.5)
