Study on the surface structure effect of anode flow channel in proton exchange membrane fuel cell

The paper mainly discusses the transport behavior of hydrogen gas in the anode flow channel of the Proton Exchange Membrane Fuel Cell. The flow pattern of hydrogen gas at the anode is affected by the surface structure of flow channel. In order to accurately simulate the transport characteristics of the hydrogen gas in the anode flow channel, a structured 3-D multi-block, staggered grid system is used for spatial discretization in numerical methods, and the finite volume method is used to sequentially iteratively solve the continuous and momentum conservation equations. The coupling between velocity and pressure in the flow field is calculated by the PISO algorithm. Two shapes design of three-dimensional smooth straight flow channel and three-dimensional bump straight flow channel are adopted in this paper. The hydrogen gas with Reynolds number 200 enters the smooth straight flow channel. The flow field inside the flow channel has been developed about 0.3 ms. At the same inlet Reynolds number and straight flow channels of three kind of bump height ratio (h/H=0.25, h/H=0.5, h/H=0.75), these calculation results are discovered that the height ratio of the bump increases and the backflow effect becomes more obvious. The current density of the Proton Exchange Membrane Fuel Cell is further analyzed by the flow field variation in the bump flow channel.


Introduction
Proton Exchange Membrane Fuel Cell (PEMFC) is a chemical reaction device that converts chemical energy into electrical energy. Fuel gas (hydrogen) enters the flow channel at the anode and dissociates into hydrogen ions and electrons through the diffusion layer and catalyst layer. The electrons move to the collector plate and the hydrogen ions move to the cathode through the proton exchange membrane. Therefore, the transmission pattern of hydrogen gas at the anode will be affected by many factors such as flow channel structure, temperature effect and fuel concentration. These factors will change the important key of PEMFC performance. In the past, Le et al. [1] described the influence of fuel gas diffusion on electrodes in PEMFC. Amphlett et al. [2] and Costamagna et al. [3] used an electrochemical model to simulate fuel cell conditions with relevant conditions, gas pressure, and output results. Dutta [4] developed a theoretical analysis of gas flow, voltage and current, so as to understand the influence of gas flow on voltage and current. Hohlein [5] and Amphlett et al. [6] used methanol in the anode channel to decompose to obtain sufficient hydrogen as fuel gas, but the decomposition process did not affect fuel cell performance. Argyopoulose et al. [7,8] conducted experiments on the hydrogen gas flow in the flow channel, and matched the electrode plate design to obtain the best conditions. Scott et al. [9] used the amount of produced hydrogen by methanol to discuss the relationship between the energy and power output by the fuel cell. Chen and Tsai [10] added bumps to the anode flow channel of PEMFC, and simulated the transport 2 behavior of fuel gas by two-dimensional steady-state numerical calculation model with periodic boundary conditions. The bump height of the anode flow channel affected the current density of the fuel cell from these results. Based on the above research, this paper will simulate the transient transmission process of hydrogen gas in the three-dimensional anode flow channel. By the variation of surface structure in the flow channel, the influence of the current density will be discussed through different flow characteristics.

Formulation
The schematic diagram of anode flow channel of PEMFC is shown in Figure 1.The lower wall of the flow channel is the electrode plate. The inner, outer and upper walls are the separator plates. By assuming incompressible three-dimensional laminar flow, gravity and surface tension are neglected. Two shapes design of three-dimensional smooth straight flow channel and three-dimensional bump straight flow channel are presented in the paper Momentum equation: All symbols of Equations 1-2 are defined, u is flow velocity, ρ is fluid density, p is pressure, μ is viscosity coefficient, g is gravity, F is surface tension source.

Numerical method
Upon inspection of the governing equations of the physical problem, it is recognized that finding the analytical solutions of the equations is impossible, owing to their nonlinearity and coupling. Instead, one has to develop a numerical method to simulate the physical phenomena. In the physical problem, these boundary conditions are assumed that hydrogen gas flow has an inlet velocity, with the pressure boundary of flow channel outlet and no slip of flow channel wall. A structured 3-D multi-block, staggered grid system is used for spatial discretization in numerical methods, and the finite volume method is used to sequentially iteratively solve the continuous and momentum conservation equations. The coupling between velocity and pressure in the flow field is calculated by the PISO Pressure-Implicit with Splitting of Operators) algorithm. PISO is a pressure-velocity calculation procedure for the Navier-Stokes equations developed originally for non-iterative computation of unsteady compressible flow, but

Results and discussions
In order to understand the transmission behavior of hydrogen gas in the anode flow channel, two shape designs of three-dimensional smooth straight flow channel and three-dimensional bump straight flow channel are shown in Figure 2 and Figure 3. All geometric dimensions and grid systems are in Table 1 and Table 2.    ). Initially, there is no hydrogen gas in the flow channel. With the increase of time, due to the fast inlet speed and the small geometric shape length, the flow field inside the flow channel has been developed about 0.3 ms.

Straight flow channel with bump height
All dimension and grid system of straight flow channel with bump height are obtained from Table 1 and  Table 2. In straight flow channel with 0.25 height ratio of bump, similarly, hydrogen gas has an inlet velocity of 4 m/s (Re=200). At the beginning, there is no hydrogen gas in the flow channel. During simulation time, the calculation results are known that the backflow effect between the bumps is not obvious due to the lower height of bumps (Figure7a-7c).   The pressure distribution in flow channel at different times is shown in Figure 8a-8c. These results can be discovered that pressure variation between the bumps is not very strong. So, the backflow effect is not obvious. As the height ratio of bump is 0.5 and 0.75, in initial time, the recirculation area between the bumps has appeared. With increasing time, the recirculation effect between the bumps becomes more obvious as   In addition to the smooth straight flow channel, the cf value has the periodic variation due to the same interval distance between the bumps with different heights or lengths in straight flow channel. Under the same length of bump, the cf value of bump with height ratio 0.75 is the largest, and cf value of bump with height ratio 0.25 is the smallest. The cf value of bump with height ratio 0.5 is almost the same regardless of whether the bump length is increased. So, the height of the bump in flow channel increases, the current density increases too. The bump length of flow channel hardly affects the current density.

Conclusions
Numerical method is used to calculate the hydrogen gas transmission characteristics in the anode flow channel. In order to discuss the influence of surface structure of the anode flow channel, two shape designs of three-dimensional smooth straight flow channel and three-dimensional bump straight flow channel are adopted. In the simulation results of this study, for the increased height ratio under same length of bump, the backflow effect becomes more obvious, and the cf value also increases, which means the current density increases. For the increased length under same height ratio of bump, the cf value changes little, which means the variation of bump length of flow channel hardly affects the current density.