My CFD simulation requires a humidified (mixed species H2 & H2O) hydrogen supply at a boundary in which the mass flow (without humidification) of hydrogen to be supplied is predetermined.
I have to calculate the mass flow of the mixed species (H2 and H2O) with the fixed mass flow of hydrogen and different mass fraction of water species in order to get a range (i.e. 0.5, 0.7, 0.9 RH) of humidified hydrogen flow.
Should I go through any standard chart or have to use any formula to get the above stated thing?
Dear Prince:
Note that P denotes the thermodynamic (or hydrostatic) pressure, while the saturation vapour pressure (Pvs) can be taken as if contributed over a plane surface of a large mass of pure liquid water, supposed to be in contact with the gas, at the same temperature (in thermal equilibrium) and in liquid-vapour equilibrium with the gas, except for the fact that the mass of water is considered to be so large that the water can be thought as pure. This is certainly a reasonable approximation for hydrogen, as it would be for air, considering its relatively low solubility in water. Therefore, Pvs can be taken as the same saturation vapour pressure that would apply to the case of air in contact with (pure) water, or even to a rarefied (vacuum) atmosphere, down to a thermodynamic pressure equal to the saturation vapour pressure of water, as it would be the case if the atmosphere was constituted only by water vapour. For gas highly soluble in water, it could be preferable, in principle, to account for the effect of the equilibrium solubility of the gas in water. The vapour pressure of liquid water is readily available from Perry's Chemical Engineers' Handbook, or from other reliable source.
The relative humidity (Hr) of the humidified hydrogen gas stream would be given by the ratio of the water vapour pressure in the gas stream (Pv) to the saturation water vapour pressure at the gas stream temperature (Pvs): Hr = (Pv / Pvs)·100%. The humidity ratio (r), would be the ratio between the mass of water vapour (mv) and the mass of dry hydrogen (mH2): r = mv / mH2 = (nv / nH2)·MH2O/MH2. Here nv / nH2 and MH2O/MH2 denote the corresponding molar ratio and molar mass ratio, respectively. It follows that r = [Pv / (P - Pv)]·MH2O/MH2 with Pv = Pvs·Hr / (100%) and being P the (total) pressure; i.e.: r = [(Pvs·Hr / (100%)) / (P - (Pvs·Hr / (100%)))]·MH2O/MH2. For the special case of Hr = 100 % we would have a saturation humidity ratio of: rs = [Pvs / (P - Pvs)]·MH2O/MH2. The dry H2 gas stream was taken as pure and no condensed water was accounted for. For possible calibration concerns if a Mass Flow Controller (MFC) is involved, you may want to check my answer at: https://www.researchgate.net/post/Is_it_possible_to_use_Nitrogen_MFC_of_200_bar_for_Hydrogen_flow_of_the_same_pressure_What_order_of_error_is_expected
Thank you so much Carlos. Your answer is very clear.
r = [(Pvs·Hr / (100%)) / (P - (Pvs·Hr / (100%)))]·MH2O/MH2
So if we want to calculate the mass of H2 by using the above relation, we need to have the following datas.
1. Relative humidity (Hr)- Its known in my case.
2. Molar Mass ratio (MH2O/MH2)- Can be easily calculated by the molar masses.
3. Total pressure (P)- Instead, i have a mass flow rate. How could i convert to pressure?
4. Saturation vapor pressure of H2 at given temp. (Pvs)- Is there any standard table to find the vapor pressure of hydrogen at 323K?
Could you please help me by answering the 3rd and 4th points?
Dear Prince:
Note that P denotes the thermodynamic (or hydrostatic) pressure, while the saturation vapour pressure (Pvs) can be taken as if contributed over a plane surface of a large mass of pure liquid water, supposed to be in contact with the gas, at the same temperature (in thermal equilibrium) and in liquid-vapour equilibrium with the gas, except for the fact that the mass of water is considered to be so large that the water can be thought as pure. This is certainly a reasonable approximation for hydrogen, as it would be for air, considering its relatively low solubility in water. Therefore, Pvs can be taken as the same saturation vapour pressure that would apply to the case of air in contact with (pure) water, or even to a rarefied (vacuum) atmosphere, down to a thermodynamic pressure equal to the saturation vapour pressure of water, as it would be the case if the atmosphere was constituted only by water vapour. For gas highly soluble in water, it could be preferable, in principle, to account for the effect of the equilibrium solubility of the gas in water. The vapour pressure of liquid water is readily available from Perry's Chemical Engineers' Handbook, or from other reliable source.
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