def Guo_Sun(dp, voidage, vs, rho, mu, Dt, L=1):
    """Calculates pressure drop across a packed bed of spheres using a
    correlation developed in [1]_. This is valid for highly-packed particles 
    at particle/tube diameter ratios between 2 and 3, where a ring packing 
    structure occurs. If a packing ratio is so low, it is important to use this
    model because in some cases its predictions are as low as half those of 
    other models!

    .. math::
        f_v = 180 + \\left(9.5374\\frac{d_p}{D_t} - 2.8054\\right)Re_{Erg}^{0.97}
        
    .. math::
        f_v = \\frac{\\Delta P d_p^2}{\\mu v_s L}\\frac{\\epsilon^3}{(1-\\epsilon)^2}

    .. math::
        Re_{Erg} = \\frac{\\rho v_s  d_p}{\\mu(1-\\epsilon)}

    Parameters
    ----------
    dp : float
        Particle diameter of spheres [m]
    voidage : float
        Void fraction of bed packing [-]
    vs : float
        Superficial velocity of the fluid (volumetric flow rate/cross-sectional 
        area)[m/s]
    rho : float
        Density of the fluid [kg/m^3]
    mu : float
        Viscosity of the fluid, [Pa*s]
    Dt : float
        Diameter of the tube, [m]
    L : float, optional
        Length the fluid flows in the packed bed [m]

    Returns
    -------
    dP : float
        Pressure drop across the bed [Pa]

    Notes
    -----
    Developed with data in the range of:

    .. math::
        100 < Re_{m} <33000\\\\
        2 < d_t/d_p < 3 1\\\\
        0.476 < \\epsilon <0.492

    Examples
    --------
    >>> Guo_Sun(dp=14.2E-3, voidage=0.492, vs=0.6, rho=1E3, mu=1E-3, Dt=40.9E-3)
    42019.529911473706
    
    References
    ----------
    .. [1] Guo, Zehua, Zhongning Sun, Nan Zhang, Ming Ding, and Jiaqing Liu. 
       "Pressure Drop in Slender Packed Beds with Novel Packing Arrangement." 
       Powder Technology 321 (November 2017): 286-92.
       doi:10.1016/j.powtec.2017.08.024. 
    """
    Rem = dp * rho * vs / mu / (1 - voidage)
    fv = 180 + (9.5374 * dp / Dt - 2.8054) * Rem ** 0.97
    return fv * (mu * vs * L / dp ** 2) * (1 - voidage) ** 2 / voidage ** 3
