def laminar_T_const():
    """Returns internal convection Nusselt number for laminar flows
    in pipe according to [1]_, [2]_ and [3]_. Wall temperature is assumed
    constant.
    This is entirely theoretically derived and reproduced experimentally.

    .. math::
        Nu = 3.66

    Returns
    -------
    Nu : float
        Nusselt number, [-]

    Notes
    -----
    This applies only for fully thermally and hydraulically developed and flows.

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry’s Chemical Engineers’ Handbook,
       Eighth Edition. New York: McGraw-Hill Education, 2007.
    .. [2] Bergman, Theodore L., Adrienne S. Lavine, Frank P. Incropera, and
       David P. DeWitt. Introduction to Heat Transfer. 6E. Hoboken, NJ: Wiley, 2011.
    .. [3] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd ed. 2010 edition.
       Berlin ; New York: Springer, 2010.
    """
    return 3.66
