def wall_factor_fd(mu, mu_wall, turbulent=True, liquid=False):
    r'''Computes the wall correction factor for pressure drop due to friction
    between a fluid and a wall. These coefficients were derived for internal 
    flow inside a pipe, but can be used elsewhere where appropriate data is 
    missing. 
    
    .. math::
        \frac{f_d}{f_{d,\text{constant properties}}} 
        = \left(\frac{\mu}{\mu_{wall}}\right)^n

    Parameters
    ----------
    mu : float
        Viscosity (or Prandtl number) of flowing fluid away from the wall,
        [Pa*s]
    mu_wall : float
        Viscosity (or Prandtl number) of the fluid at the wall, [Pa*s]
    turbulent : bool
        Whether or not to use the turbulent coefficient, [-]
    liquid : bool
        Whether or not to use the liquid phase coefficient; otherwise the gas
        coefficient is used, [-]
        
    Returns
    -------
    factor : float
        Correction factor for pressure loss; to be multiplied by the friction
        factor, or pressure drop to obtain the actual result, [-]

    Notes
    -----
    The exponents are determined as follows:
        
    +-----------+--------+---------+---------+
    | Regime    | Phase  | Heating | Cooling |
    +===========+========+=========+=========+
    | Turbulent | Liquid | -0.25   | -0.25   |
    +-----------+--------+---------+---------+
    | Turbulent | Gas    | 0.1     | 0.1     |
    +-----------+--------+---------+---------+
    | Laminar   | Liquid | -0.58   | -0.5    |
    +-----------+--------+---------+---------+
    | Laminar   | Gas    | -1      | -1      |
    +-----------+--------+---------+---------+
    
    Examples
    --------
    >>> wall_factor_fd(mu=8E-4, mu_wall=3E-4, turbulent=True, liquid=True)
    0.7825422900366437

    References
    ----------
    .. [1] Kays, William M., and Michael E. Crawford. Convective Heat and Mass 
       Transfer. 3rd edition. New York: McGraw-Hill Science/Engineering/Math,
       1993.
    '''
    params = wall_factor_fd_defaults[(turbulent, liquid)]
    return wall_factor(mu=mu, mu_wall=mu_wall, **params)