def py2(h, kr, rho, cp, r):
    """
    Calculate the pyrolysis number.

    Parameters
    ----------
    h = heat transfer coefficient, W/m^2K
    kr = rate constant, 1/s
    rho = density, kg/m^3
    cp = heat capacity, J/kgK
    r = radius, m

    Returns
    -------
    py = pyrolysis number, -
    """
    py = h / (kr * rho * cp * r)
    return py