def Nu_coil_Xin_Ebadian(Pr, Gr, horizontal=False):
    """Calculates Nusselt number for natural convection around a vertical
    or horizontal helical coil suspended in a fluid without 
    forced convection. 
    
    For horizontal cases:
    
    .. math::
        Nu_D = 0.318 Ra_D^{0.293},\\; 5 \\times {10}^{3} < Ra < 1 \\times {10}^5

    For vertical cases:
    
    .. math::
        Nu_D = 0.290 Ra_D^{0.293},\\; 5 \\times {10}^{3} < Ra < 1 \\times {10}^5

    Parameters
    ----------
    Pr : float
        Prandtl number calculated with the film temperature -
        wall and temperature very far from the coil average, [-]
    Gr : float
        Grashof number calculated with the film temperature -
        wall and temperature very far from the coil average,
        and using the outer diameter of the coil [-]
    horizontal : bool, optional
        Whether the coil is horizontal or vertical, [-]

    Returns
    -------
    Nu : float
        Nusselt number using the outer diameter of the coil
        and the film temperature, [-]

    Notes
    -----
    This correlation is also reviewed in [2]_.

    Examples
    --------
    >>> Nu_coil_Xin_Ebadian(0.7, 2E4, horizontal=False)
    4.755689726250451
    >>> Nu_coil_Xin_Ebadian(0.7, 2E4, horizontal=True)
    5.2148597687849785
    
    References
    ----------
    .. [1] Xin, R. C., and M. A. Ebadian. "Natural Convection Heat Transfer 
       from Helicoidal Pipes." Journal of Thermophysics and Heat Transfer 10, 
       no. 2 (1996): 297-302. 
    .. [2] Prabhanjan, Devanahalli G., Timothy J. Rennie, and G. S. Vijaya
       Raghavan. "Natural Convection Heat Transfer from Helical Coiled Tubes."
       International Journal of Thermal Sciences 43, no. 4 (April 1, 2004): 
       359-65.
    """
    Ra = Pr * Gr
    if horizontal:
        return 0.318 * Ra ** 0.293
    else:
        return 0.29 * Ra ** 0.293
