def Morimoto_Hotta(Re, Pr, Dh, Rm):
    """Calculates Nusselt number for flow inside a spiral heat exchanger of
    spiral mean diameter `Rm` and hydraulic diameter `Dh` according to [1]_,
    also as shown in [2]_ and [3]_.
    
    .. math::
        Nu = 0.0239\\left(1 + 5.54\\frac{D_h}{R_m}\\right)Re^{0.806}Pr^{0.268}
        
    .. math::
        D_h = \\frac{2HS}{H+S}
        
    .. math::
        R_m = \\frac{R_{min} + R_{max}}{2}
    

    Parameters
    ----------
    Re : float
        Reynolds number with bulk properties, [-]
    Pr : float
        Prandtl number with bulk properties [-]
    Dh : float
        Average hydraulic diameter, [m]
    Rm : float
        Average spiral radius, [m]

    Returns
    -------
    Nu : float
        Nusselt number with respect to `Dh`, [-]

    Notes
    -----
    [1]_ is in Japanese.

    Examples
    --------
    >>> Morimoto_Hotta(1E5, 5.7, .05, .5)
    634.4879473869859

    References
    ----------
    .. [1] Morimoto, Eiji, and Kazuyuki Hotta. "Study of Geometric Structure 
       and Heat Transfer Characteristics of Spiral Plate Heat Exchanger." 
       Transactions of the Japan Society of Mechanical Engineers Series B 52, 
       no. 474 (1986): 926-33. doi:10.1299/kikaib.52.926.
    .. [2] Bidabadi, M. and Sadaghiani, A. and Azad, A. "Spiral heat exchanger 
       optimization using genetic algorithm." Transaction on Mechanical 
       Engineering, International Journal of Science and Technology,
       vol. 20, no. 5 (2013): 1445-1454. 
       http://www.scientiairanica.com/en/ManuscriptDetail?mid=47.
    .. [3] Turgut, Oğuz Emrah, and Mustafa Turhan Çoban. "Thermal Design of 
       Spiral Heat Exchangers and Heat Pipes through Global Best Algorithm." 
       Heat and Mass Transfer, July 7, 2016, 1-18. 
       doi:10.1007/s00231-016-1861-y.
    """
    return 0.0239 * (1.0 + 5.54 * Dh / Rm) * Re ** 0.806 * Pr ** 0.268
