def Nu_Wakao_Kagei(Re, Pr):
    """Calculates Nusselt number of a fluid passing over a bed of particles
    using a correlation shown in [1]_ and also cited in the review of [2]_.
    Relatively rough, as it has no dependence on voidage.
    
    .. math::
        Nu = 2 + 1.1Pr^{1/3}Re^{0.6}

    Parameters
    ----------
    Re : float
        Reynolds number with pebble diameter as characteristic dimension, [-]
    Pr : float
        Prandtl number of the fluid []

    Returns
    -------
    Nu : float
        Nusselt number for heat transfer to the packed bed [-]

    Notes
    -----
    Fit for Re from 3 to 3000; claimed reasonableness of fit to to 1E6.
    
    Examples
    --------
    >>> Nu_Wakao_Kagei(2000, 0.7)
    95.40641328041248

    References
    ----------
    .. [1] Wakao, Noriaki, and Seiichirō Kagei. Heat and Mass Transfer in 
       Packed Beds. Taylor & Francis, 1982.
    .. [2] Abdulmohsin, Rahman S., and Muthanna H. Al-Dahhan. "Characteristics 
       of Convective Heat Transport in a Packed Pebble-Bed Reactor." Nuclear
       Engineering and Design 284 (April 1, 2015): 143-52. 
       doi:10.1016/j.nucengdes.2014.11.041.
    """
    return 2 + 1.1 * Pr ** (1 / 3.0) * Re ** 0.6
