In a fuel cell vehicle, the water content of the gas supply within certain ranges plays a key role in improving the performance of a proton exchange membrane. The lower limit of water content in the air supply is to avoid the problem of drying-out, while the upper prevents flooding. Water management can be accomplished by a membrane humidifier which allows water vapor to permeate the mixture from the side having the higher water concentration, moving to the other side of the membrane. The mass transfer rate in perm-selective membrane depends on membrane properties, types of transported components, driving force and operating conditions. In fuel cell, operating variables such as temperature, humidity and flow rates are varied, and this poses challenges to humidifier designs and operation.

In this study, the variation in water content collected at the outlet of a hollow fiber membrane humidifier is investigated with a one-dimensional mass exchanger model and various operating variables. The vapor concentration of outlet flows is affected by operating temperature and relative humidity of the membrane humidifier. Relative humidity of the dry side at the point of outlet flow, to be supplied to the fuel cell module, is the key characteristic. In modeling of the humidifier, the membrane module is considered as a counter-flow mass exchanger that effectiveness-NTU method approaches. The analogy of the effectiveness-NTU approach for heat transfer is used to analyze the characteristics of the mass exchanger. Mass flux through the membranes is estimated with an overall mass transfer coefficient which represents vapor transport characteristics moving through the membrane module. This coefficient has a similar role to the overall heat transfer coefficient in heat exchanger analysis. This parametric study is conducted to understand the effects of different variables. The effectiveness-NTU methodology of mass transfer uses the overall mass transfer coefficient and the mass transfer rate, as evaluated experimentally. Simulink software is then employed to deliver outcomes of the model for different operating conditions.

The results shows significant effects upon outflow characteristics of operating temperature and humidity. Higher operating temperature improves the transport of water particles, and therefore generates higher water content flow into the dry side. In addition, the greater relative humidity rate on the wet side boosts the mass transfer rate, the relative humidity, and the specific humidity of the streams. Remarkably, condensation occurs if the wet air relative humidity exceeds 80% and may block the water transport, this should be avoided to improve the performance of the vehicular humidifier.   


Modeling Vapor Transport in a Hollow Fiber Membrane Humidifier Using \u0190-NTU Approach

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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