def Nu_Achenbach(Re, Pr, voidage):
    """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]_.
    
    .. math::
        Nu = [(1.18Re^{0.58})^4 + (0.23\\left(\\frac{Re}{1-\\epsilon}
        \\right)^{0.75})^4]^{0.25}

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

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

    Notes
    -----
    Claimed value for Re/ε < 7.7E5
    Developed with tests performed in a wind tunnel at conditions up to 30 bar.
    
    Examples
    --------
    >>> Nu_Achenbach(2000, 0.7, 0.4)
    117.70343608599121

    References
    ----------
    .. [1] Achenbach, E. "Heat and Flow Characteristics of Packed Beds." 
       Experimental Thermal and Fluid Science 10, no. 1 (January 1, 1995): 
       17-27. doi:10.1016/0894-1777(94)00077-L.
    .. [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 ((1.18 * Re ** 0.58) ** 4 + (0.23 * (Re / (1 - voidage)) ** 0.75
        ) ** 4) ** 0.25
