**2. Experimental apparatus and method**

320 Mass Transfer - Advanced Aspects

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 constant-

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).

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

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

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

transfer on the surface of the porous media plate are not reported.

φ

54×254 mm and 40×99×29 mm,

temperature water through a porous plate.

methane.

The sizes of the porous media used in these studies were

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.

Fig. 1. Experimental system

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

Moisture Transport Through a Porous Plate with Micro Pores 323

pore diameter 2μm plate thickness 1mm

pore diameter 2μm

plate thickness 3.5mm

pore diameter 14μm plate thickness 3.5mm

 vacuum impregnation non-vacuum impregnation

10 20 30

Air volumetric flow *V* 10 m /s

effects of the flow condition of air, the physical properties of the porous plate, and the geometrical size of the channel on the moisture transport. Considering the conditions of practical use, experiments were performed under an air volumetric flow of 3.3 ~ 24.7×10-5 m3/s. To examine the effect of the heat conductivity of the porous plate on the moisture transport, in present study, we used porous media having heat conductivities of 1.7 W/(mK) and 20.2 W/(mK). For reasons related to material manufacture, the experiment to investigate the effect of pore diameter was performed using the porous media with low heat conductivity (1.7 W/(mK))and three average pore diameters *D* of 2, 5, and 14 μm, and the experiment regarding the effect of porosity was performed mainly using the porous media with high heat conductivity (20.2 W/(mK))and average porosities of 12%, 20%, and 30%.

thermal conductivity 1.7 W/(mK) porosity 30% channel height 1 mm

(a)

(b)

(c)

3-5

2

*1*

1

0

Fig. 3. Effect of impregnation on mass flux

The condition of detail is shown Table 1.

2

3

4

5

Mass flux

*2*

*3*

*m* 10 kg/(m s)


*4*

*5*

 2 4

6

8

10

order to prevent heat loss from the sides of the metal frame. The air temperature in the channel above the porous plate and the temperature in the upper surface of the porous plate were measured by ten K-type thermocouples (±0.1°C) of 0.25 mm in diameter that were installed in the channel and the plate along the path of the airflow, respectively. Holes in the porous plate for the insertion of the thermocouples were 0.3 mm in diameter and 15 mm in depth. In Figure 2, the symbols ○ and ● represent the thermocouples, which measure the temperatures of the air in the channel and the upper surface of porous plate, respectively. The temperature of the porous media plate measured here is used as the air-side plate temperature. In addition, the temperature of the plate measured by the thermocouples attached to the bottom of the plate contacting the liquid is used as the liquid-side plate temperature. In the present study, since the temperature of the constant-temperature-water is adjusted to have a small range, the experiments were carried out under an approximately constant liquid-side plate temperature.

Fig. 2. Schematic diagram of the test section

Alternatively, in present study, in order to remove the effect of the air that is trapped in the porous plate on the experiment result, the porous plate, as a specimen, was impregnated by a vacuum impregnation method before beginning the experiments. In other words, air was evacuated from a closed vessel containing the porous plate in water in order to remove the air contained within the porous plate. Figures 3(a) ~ 3(c) show the variation in the mass flux of the moisture transport through the porous media plate with respect to the air volumetric flow under the conditions of both vacuum impregnation and non-vacuum impregnation. This graph indicates that the variation of the mass flux caused by the vacuum impregnation depends on the thickness and the pore diameter of the porous plate. In other words, the effect of vacuum impregnation appears to be more remarkable for porous plates having smaller pore diameter and greater thickness. The reason for this is thought to be that almost all of the air inside the pores of the porous plate was removed by the impregnation, so that the water transposition in the porous plate driven by capillary forces becomes easier. Therefore, the vacuum impregnation was carried out for the porous plate used in the present study before the experiments.

#### **3. Experimental results and discussion**

Since there are many factors that affect moisture transport from the constant-temperature water to the dry air through the porous plate, in the present study, we first examine the

order to prevent heat loss from the sides of the metal frame. The air temperature in the channel above the porous plate and the temperature in the upper surface of the porous plate were measured by ten K-type thermocouples (±0.1°C) of 0.25 mm in diameter that were installed in the channel and the plate along the path of the airflow, respectively. Holes in the porous plate for the insertion of the thermocouples were 0.3 mm in diameter and 15 mm in depth. In Figure 2, the symbols ○ and ● represent the thermocouples, which measure the temperatures of the air in the channel and the upper surface of porous plate, respectively. The temperature of the porous media plate measured here is used as the air-side plate temperature. In addition, the temperature of the plate measured by the thermocouples attached to the bottom of the plate contacting the liquid is used as the liquid-side plate temperature. In the present study, since the temperature of the constant-temperature-water is adjusted to have a small range, the experiments were carried out under an approximately

> 100 160

Air In Air Out

Constant Temperature Water

Alternatively, in present study, in order to remove the effect of the air that is trapped in the porous plate on the experiment result, the porous plate, as a specimen, was impregnated by a vacuum impregnation method before beginning the experiments. In other words, air was evacuated from a closed vessel containing the porous plate in water in order to remove the air contained within the porous plate. Figures 3(a) ~ 3(c) show the variation in the mass flux of the moisture transport through the porous media plate with respect to the air volumetric flow under the conditions of both vacuum impregnation and non-vacuum impregnation. This graph indicates that the variation of the mass flux caused by the vacuum impregnation depends on the thickness and the pore diameter of the porous plate. In other words, the effect of vacuum impregnation appears to be more remarkable for porous plates having smaller pore diameter and greater thickness. The reason for this is thought to be that almost all of the air inside the pores of the porous plate was removed by the impregnation, so that the water transposition in the porous plate driven by capillary forces becomes easier. Therefore, the vacuum impregnation was carried out for the porous plate used in the

Since there are many factors that affect moisture transport from the constant-temperature water to the dry air through the porous plate, in the present study, we first examine the

4 0

To Vacuum Pump

constant liquid-side plate temperature.

Polycarbonate

Fig. 2. Schematic diagram of the test section

present study before the experiments.

**3. Experimental results and discussion** 

Glass

Vacuum Duct

Porous Plate

Fig. 3. Effect of impregnation on mass flux

effects of the flow condition of air, the physical properties of the porous plate, and the geometrical size of the channel on the moisture transport. Considering the conditions of practical use, experiments were performed under an air volumetric flow of 3.3 ~ 24.7×10-5 m3/s. To examine the effect of the heat conductivity of the porous plate on the moisture transport, in present study, we used porous media having heat conductivities of 1.7 W/(mK) and 20.2 W/(mK). For reasons related to material manufacture, the experiment to investigate the effect of pore diameter was performed using the porous media with low heat conductivity (1.7 W/(mK))and three average pore diameters *D* of 2, 5, and 14 μm, and the experiment regarding the effect of porosity was performed mainly using the porous media with high heat conductivity (20.2 W/(mK))and average porosities of 12%, 20%, and 30%. The condition of detail is shown Table 1.

Moisture Transport Through a Porous Plate with Micro Pores 325

thermal conductivity 1.7 W/(mK) pore diameter 5 μm channel height 1 mm plate thickness 1 mm

(a)

(b)

(c)

(d)

Air volumetric flow *V* 10 m /s

Fig. 4. Basic characteristics of moisture transport from water to air through a porous plate process calculated from the airflow and the change in enthalpy of the air between the entrance and exit of the channel. The variation in the heat flux showed the same tendency as that for the above mass flux, because the heat flux is primarily caused by the transport of


10 20 30

0

30

10

80

40

1

0.5

0

latent heat accompanying the mass transport in this process.

Relative humidity at outlet

50

60

Air outlet temperature

*RH*

*T*

℃

70

0

20

2

Mass flux

Heat flux *q* kw/m 2

*m* 10 kg/(m s) 2


4

6

8

10


Air volumetric flow: 3.3-24.7×10-5 m3/s, temperature of air at the inlet: 32ºC, relative humidity of air at the inlet: 15%, temperature of constant-temperature water: 69ºC.

Table 1. Specifications of the porous plate and experimental conditions
