The steps below explain how to convert relative humidity value into mole fraction, which can be entered as a boundary condition value 1) Relative humidity = 100 * Partial pressure of vapor in air / partial pressure of vapor in air at saturation 2) Partial pressure of vapor in air at saturation = saturation pressure of water at the cell temperature 3) Using 1) & 2), you will get partial pressure of vapor in air 4) As per Dalton’s law of partial pressures, partial pressure of vapor in air / total pressure of air = mole fraction of vapor 5) Mole fraction can be specified as an input at the inlet BC in the species tab.