def Bejan_L(dP, L, mu, alpha):
    """Calculates Bejan number of a length or `Be_L` for a fluid with the
    given parameters flowing over a characteristic length `L` and experiencing
    a pressure drop `dP`.

    .. math::
        Be_L = \\frac{\\Delta P L^2}{\\mu \\alpha}

    Parameters
    ----------
    dP : float
        Pressure drop, [Pa]
    L : float
        Characteristic length, [m]
    mu : float, optional
        Dynamic viscosity, [Pa*s]
    alpha : float
        Thermal diffusivity, [m^2/s]

    Returns
    -------
    Be_L : float
        Bejan number with respect to length []

    Notes
    -----
    Termed a dimensionless number by someone in 1988.

    Examples
    --------
    >>> Bejan_L(1E4, 1, 1E-3, 1E-6)
    10000000000000.0

    References
    ----------
    .. [1] Awad, M. M. "The Science and the History of the Two Bejan Numbers."
       International Journal of Heat and Mass Transfer 94 (March 2016): 101-3.
       doi:10.1016/j.ijheatmasstransfer.2015.11.073.
    .. [2] Bejan, Adrian. Convection Heat Transfer. 4E. Hoboken, New Jersey:
       Wiley, 2013.
    """
    return dP * L ** 2 / (alpha * mu)
