def Nu(Ra, Pr):
    """
    Calculation of Nusselt number for natural convection. See eq. 4.7-4  and Table 4.7-1 in
    C. J. Geankoplis Transport Processes and Unit Operations, International Edition,
    Prentice-Hall, 1993

    Parameters
    ----------
    Ra : float
        Raleigh number
    Pr : float 
        Prandtl number

    Returns
    ----------
    Nu : float
        Nusselt numebr
    """
    if Ra >= 1e9:
        NNu = 0.13 * Ra ** 0.333
    elif Ra < 1e9 and Ra > 1e4:
        NNu = 0.59 * Ra ** 0.25
    else:
        NNu = 1.36 * Ra ** 0.20
    return NNu