def declare_material_streams(m, n_CH4f, n_H2Of, n_N2f, n_O2f, Uf):
    """
    Define known Material Streams and fix flows of methane and water to mix1,
    air and oxygen to splitter1,
    Optional keyword arguments:
    ---------------------------
    :param m: model
    :param n_CH4f: mole of methane feed
    :param n_H2Of: mole of water feed
    :param n_N2f: mole of nitrogen feed
    :param n_O2f: mole of oxygen feed
    :param Uf: Fuel ultilization, mole reductant consumed in FC per mole
               of reductant total
    :return: model after declaring material streams
    """
    # Define known Material Streams
    # Fix methane flow to Mix1:
    m.fs.Mix1.inlet_1.flow_mol.fix(n_CH4f)
    m.fs.Mix1.inlet_1.mole_frac_comp[0.0, :].fix(0.0)
    m.fs.Mix1.inlet_1.mole_frac_comp[0.0, "CH4"].fix(1.0)
    m.fs.Mix1.inlet_1.temperature.fix(25 + 273.15)
    m.fs.Mix1.inlet_1.pressure.fix(101325)

    # Fix water flow to Mix1:
    m.fs.Mix1.inlet_2.flow_mol.fix(n_H2Of)
    m.fs.Mix1.inlet_2.mole_frac_comp[0.0, :].fix(0.0)
    m.fs.Mix1.inlet_2.mole_frac_comp[0.0, "H2O"].fix(1.0)
    m.fs.Mix1.inlet_2.temperature.fix(25 + 273.15)
    m.fs.Mix1.inlet_2.pressure.fix(101325)

    # Fix air flow to Split1:
    m.fs.Split1.inlet.flow_mol.fix(n_N2f + n_O2f)
    m.fs.Split1.inlet.mole_frac_comp[0.0, :].fix(0.0)
    m.fs.Split1.inlet.mole_frac_comp[0.0, "O2"].fix(0.21)
    m.fs.Split1.inlet.mole_frac_comp[0.0, "N2"].fix(0.79)
    m.fs.Split1.inlet.temperature.fix(25 + 273.15)
    m.fs.Split1.inlet.pressure.fix(101325)

    # Fix O2 flow in Split1 outlet_1:
    m.fs.Split1.outlet_1.flow_mol.fix(n_CH4f * Uf * 2)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "CH4"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "CO"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "CO2"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "H2"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "H2O"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "N2"].fix(0.0)
    m.fs.Split1.outlet_1.mole_frac_comp[0.0, "O2"].fix(1.0)

    return m