**1. Introduction**

318 Mass Transfer - Advanced Aspects

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water electrolysis: A review of the research programme of the Commision of

Considering the variety of environment problems, including global warming, that call for a reduction in emission gases, increasing the fuel efficiency and reducing exhaust gases from automobiles has become an important issue for the automobile industry. Compared to traditional automobiles, fuel cell vehicles have many advantages, including high efficiency, low emissions, and diversification of fuel supply. Therefore, fuel cell vehicles are expected to become a viable means of transportation in the 21st century. Consequently, extensive research is being conducted to develop fuel cell vehicles that use polymer electrolyte membrane fuel cells as a power source.

Fuel cell systems for vehicles are composed primarily of a fuel and air supply unit, a humidifier, a cooling device, and the polymer electrolyte membrane fuel cell (PEMFC). Since the degree of ion conduction in an electrolyte film in the fuel cell is determined by the water content of the film, some water content is necessary in order to maintain ion conduction in the film. Generally, the film is humidified through gas diffusion layer (GDL) using high humidity work gases. The research on the humidifying methods and the influence of the humidity of work gases on the performance of fuel cells have been reported by Nguyen & White (1993) and Yoshikawa et al. (2000), respectively. Buchi & Scherer (2001) investigated the effects of the water content and the membrane thickness on the resistance of Nafion membranes in PEMFC.

From the point of view of saving space, it is desirable to recover and reuse the humidity in the exhaust gas using the supply air. In the present study, a method involving a thin porous plate for air dehumidification(Asaeda et al., 1984, 1986),in which direct recovery of the moisture of the exhaust gas to the supply air through a thin porous plate or membrane, is considered. In this case, the following phenomena may occur: 1) mass and heat transport and an accompanying phase change inside the porous plate, 2) water evaporation from the surface of the porous plate and moisture diffusion around the surface of the plate on the supply air side, and 3) condensation of moisture on the porous plate surface on the exhaust gas side. Analysis is difficult because of the complex interaction between these phenomena. Therefore, in order to simplify our investigation, as a first step, we focus on the heat and mass transport characteristics on the supply gas side and inside the porous plate. In order to fix the heat and mass transfer characteristics of the exhaust side, we assume that the

Moisture Transport Through a Porous Plate with Micro Pores 321

present authors have also investigated the effect of thermal conductivity of the porous plate on the moisture transport (Wang et al., 2009). Here,we will summarize the work done in

Figure 1 shows a schematic diagram of the experimental apparatus, which is composed of a constant-temperature water circulation system and an airflow loop. The constanttemperature water system consists of a circulation water tank, a water transport pump, a water filter, and ion-exchange equipment. The water used in the experiment is generally maintained in a pure state, using both a water filter that removes particles larger than 0.1 μm and the ion-exchange equipment. Air is pumped to the flow loop and is dehumidified by cooling with water at approximately 0°C. The dehumidified air is heated to an established temperature and absorbs moisture from the constant-temperature water that is in contact with the bottom of the porous plate when supplied to the test device. Highhumidity air is discharged to the atmosphere from the test device. The flow rate of air is adjusted by a valve installed at the exit of the air pump and is measured by a flow meter installed after the valve. Thermo-hygrometers are installed at the entrance and exit of the test device to measure the temperature and humidity of the air. In order to prevent the formation of dew at the thermo-hygrometer, a heater was installed around the duct, including the thermo-hygrometer. The heater was also used to control the air temperature in the duct and to maintain the temperature to be consistent with the air temperature in the channel outlet. The temperature of the water was measured by a thermocouple installed on the undersurface of the porous plate in contact with the constant-temperature water. All measurement signals, for example, temperature, humidity, and flow rate, were converted to

digital signals by an A/D converter and were recorded by a personal computer.

TH

P

Porous Plate Glass Plate

Figure 2 shows a cross-section of the test device. The surface of the porous plate is 100×28 mm. To observe the surface state of the porous plate, the top of the test device is constructed of a transparent material. A space for vacuum thermal insulation exists at the top of the test device, and insulation is accomplished by the drawing of a vacuum pump. In addition, 1-mm Teflon sheets were installed as insulating material on two sides of the channel in

Flow Adjust Valve Water Pump

Humidifier

Heater

A/D Converter PC

To Vacuum Pump

TH

To Atmosphere

*T*<sup>h</sup> Circulation Water Tank

Micro Filter

these previous investigations.

*T*<sup>l</sup> Circulation

Air pump Air Flow Meter F

Fig. 1. Experimental system

Water Tank Cooler

**2. Experimental apparatus and method** 

moisture supply capacity of the exhaust side is sufficiently high so that constanttemperature water can be used rather than the exhaust gas. Thus, the subject of the examination becomes the heat and mass transport between dry air and constanttemperature water through a porous plate.

A number of studies have examined the heat and mass transport accompanied by a phase change in porous media. For example, the gas-liquid two-phase flow, driven by capillary force in the porous media and accompanied by the evaporation of water has been experimentally and theoretically investigated by Udell (1983, 1985) and Zhao & Liao (2000). The sizes of the porous media used in these studies were φ54×254 mm and 40×99×29 mm, respectively. In addition, the diameter of the particles that composed the porous media were 0.1 ~ 0.8 mm and 1.09 mm, respectively, and the corresponding pore diameters were 0.05 ~0.3 mm and 0.46 mm, respectively. Wang et al. (1993a, 1993b, 1996) introduced a multiphase mixture model for the heat and mass transport of multiphase and multicomponent mixtures, including the phase change in the porous media, based on a separated flow model in which various phases are regarded as distinct fluids. Simulations were performed employing this multiphase mixture flow model. The infiltration and transport of non-aqueous phase liquids in the unsaturated subsurface were investigated by Cheng & Wang (1996), and the mass transport in the cathode of a PEMFC under isothermal conditions was investigated by You & Liu (2002). Vafai & Whitaker (1986) applied a volume averaging technology to analyze the accumulation and migration of moisture in an insulation material, and, based on a previous study (Vafai & Whitaker, 1986), Vafai & Tien (1989) reduced the number of assumptions and simulated the same problem. Using the network method, Prat (1993) presented a model to investigate drying in porous media under the condition whereby the media was initially saturated with water. Plourde & Prat (2003) studied the influence of a surface tension gradient induced by thermal gradients on the phase distribution within a capillary porous media by developing the model described in Reference (Prat, 1993). Furthermore, Usami et al. (2000, 2001) conducted a quantitative evaluation of the controlling factors, both experimentally and via numerical analysis, for the heat and mass transport in the reforming catalyst bed of a steam reforming fuel cell using methane.

In summarizing the above studies, we observed the following. 1) Several theoretical studies have been performed. 2) The dimensions of the porous media used as an experimental object in previous studies (e.g., the size of the porous media and the diameter of the particles that comprise the porous media) were relatively large. 3) Few studies have examined the influencing factors or mechanism of heat and mass transport in porous media. Therefore, it is difficult to apply the results of the above-mentioned studies in the present study. In particular, research regarding the moisture transport through porous media plate depends of the heat and mass transport inside the porous media and the conditions of heat and mass transfer on the surface of the porous media plate are not reported.

In order to clarify the characteristics of moisture recovery from the exhaust gas of fuel cell vehicles with a porous plate, it is necessary to determine experimentally both the mechanism of heat and mass transport in a thin porous plate having very small pores and the influence of various factors on heat and mass transport in this process. As a first step towards this goal, we evaluate the factors that influence heat and mass transfer from constant-temperature water to dry air through a porous plate. The present authors have investigated moisture transport through a porous plate having a thermal conductivity of 1.7W/(mK) to dry air from constant-temperature water (Wang et al., 2005, 2006). And the present authors have also investigated the effect of thermal conductivity of the porous plate on the moisture transport (Wang et al., 2009). Here,we will summarize the work done in these previous investigations.
