def Nu_vertical_helical_coil_Ali(Pr, Gr):
    """Calculates Nusselt number for natural convection around a vertical
    helical coil inside a tank or other vessel according to the Ali [1]_ 
    correlation.

    .. math::
        Nu_L = 0.555Gr_L^{0.301} Pr^{0.314}
    
    Parameters
    ----------
    Pr : float
        Prandtl number of the fluid surrounding the coil with properties 
        evaluated at bulk conditions or as described in the notes [-]
    Gr : float
        Prandtl number of the fluid surrounding the coil with properties 
        evaluated at bulk conditions or as described in the notes
        (for the two temperatures, use the average coil fluid temperature and
        the temperature of the fluid outside the coil) [-]

    Returns
    -------
    Nu : float
        Nusselt number with respect to the total length of the helical coil
        (and bulk thermal conductivity), [-]

    Notes
    -----
    In [1]_, the temperature at which the fluid surrounding the coil's
    properties were evaluated at was calculated in an unusual fashion. The 
    average temperature of the fluid inside the coil
    :math:`(T_{in} + T_{out})/2` is averaged with the fluid outside the coil's
    temperature.
    
    The correlation is valid for Prandtl numbers between 4.4 and 345, 
    and tank diameter/coil outer diameter ratios between 10 and 30.

    Examples
    --------
    >>> Nu_vertical_helical_coil_Ali(4.4, 1E11)
    1808.5774997297106

    References
    ----------
    .. [1] Ali, Mohamed E. "Natural Convection Heat Transfer from Vertical
       Helical Coils in Oil." Heat Transfer Engineering 27, no. 3 (April 1,
       2006): 79-85.
    """
    return 0.555 * Gr ** 0.301 * Pr ** 0.314
