Impact of Multiple Inlet and Outlet Structures of Bipolar Plate Channel on the Mass Transport in ALK Electrolyzers

Abstract

The flow channel structure in alkaline electrolyzers critically impacts electrolyte distribution uniformity, influencing stagnant zones, gas bubble accumulation, and electrode reactions. Conventional concave–convex bipolar plates cause uneven flow and reduced current density. Therefore, a scaled-down-sized multiple inlet setup coupled with the bipolar plate channel of three typical concave–convex structures was designed to improve the uniformity of electrolyte. Three-dimensional computational fluid dynamics was employed to analyze the flow characteristics in the channels. The results indicated that in the single inlet/outlet model, the velocity near the center axis along the mainstream direction was higher than at the edge of the channels, resulting in a non-uniform flow distribution. The vorticity intensity gradually decreased along the flow direction, while the multiple inlet/outlet structure strengthened the intensity. The multiple inlet model allowed for the electrolyte flow across more areas along the channel and enhanced the velocity uniformity. According to the velocity uniformity evaluation criteria, the flow uniformity index of the three-inlet square concave–convex structure was the highest, reaching 0.80 at the middle cross-section normal to the incoming flow and 0.88 parallel to the flow. This study may help provide a useful guide for the design and optimization of efficient electrolyzer in alkaline water electrolysis.

1. Introduction

To combat climate change, reducing carbon dioxide emissions has become a global priority. Clean, decarbonized fuels like hydrogen are essential for future energy systems, as they produce only water when burned and can be generated using renewables (e.g., solar and wind) [1,2]. Current hydrogen production methods include steam reforming, biomass gasification, fermentation, and water electrolysis. While electrolysis accounts for just 4% of global hydrogen output, it offers ultra-pure hydrogen (>99.9%) and direct compatibility with renewable electricity, making it a critical long-term solution [3,4]. Alkaline water electrolysis is considered one of the most promising industrial processes for producing clean and sustainable “green hydrogen” with the advantage of low cost. It also has a longer stack lifespan and stability than other electrolysis methods because of the absence of costly membranes and noble catalysts. Moreover, ALK electrolyzes can be integrated into various energy conversion systems for hydrogen production, such as photovoltaic power, biomass power generation, wind power generation, and other hybrid systems [5]. However, alkaline water electrolysis exhibits higher overpotential than proton exchange membrane water electrolysis, and still presents a lower efficiency of approximately 60% [6]. As yet, the cost of this process is far higher than the conventional hydrogen technology. Therefore, it is imperative to improve the efficiency of alkaline water electrolysis so as to further decrease the hydrogen production cost and increase the market competition.

In order to enhance the efficiency of alkaline water electrolysis, studies are mainly focused on the development of novel electrode materials, the optimization of operational parameters, and the improvement of the structure of electrolyze flow channel [7]. The electrolyte flow in the electrolyze plays an important role in enhancing the efficiency of the electrolyze as the flow pattern greatly affects the distribution of alkaline reactants, the gas bubble removal behavior, and electrochemical reaction process [5,6,7]. Especially for the industrial scale of electrolyzes, as they are long in the direction of gravity, the volume fraction of bubbles in the vicinity of the electrodes varies considerably depending on location, which makes the electrodes experience heterogeneity and reduce the effective conductivity of the electrolyte, thereby generating additional electric resistance to the alkaline water electrolysis process [6]. Moreover, gas bubbles prevent the reactants from getting to the reaction site, leading to a decrease in the effective reaction area and a generation of additional polarization overpotentials [7]. It is accepted that achieving a more uniform electrolyte flow can improve the local reaction rate and decrease the overpotential, resulting in the efficiency enhancement and energy consumption reduction of the electrolyzer. The anode and cathode channels always are fabricated into specific concave–convex structures. A well-designed structure can facilitate uniform electrolyte distribution, which is imperative for mass transport, ion and heat transfer, minimizing gas bubble accumulation, and reducing dead zones. Currently, research on electrolyze flow channel structures mainly focuses on designing specific shaped concave–convex structures and studying the disturbance effects of those structures on the flow mechanics, the electrochemical reaction, and bubble kinetics. As the spherical concave–convex combination structure has good characteristics in enhancing heat transfer and reducing flow resistance, it has been widely investigated. Zhang et al. [8] used the Euler–Euler RANS turbulence model to study the electrolysis process at the channel surface with a spherical concave–convex structure. They found that this structure could effectively improve the electrolyte distribution and bubble removal in the electrode chamber. Compared with the straight channels, the accumulation of hydrogen generated at the electrode surface was reduced, which improved the electrolysis efficiency of the electrolyze. Additionally, researchers have analyzed the effects of the arrangement type (e.g., the relative placement of spherical concave–convex surfaces) and structural parameters (e.g., concave depth and convex height) on the flow field in the channel by numerical simulations and experimental studies [9]. Other irregular structures, such as square, cylindrical, ellipsoidal, and teardrop-shaped concave–convex structures, have also attracted researchers’ attention. Ahmed et al. [10] systematically investigated the current density uniformity in differently shaped flow channels (rectangular, trapezoidal, and parallelogram), with the trapezoidal cross-section demonstrating superior current distribution homogeneity due to its graded geometric characteristics. Jong Myung et al. [11] conducted numerical simulations to study the flow and heat transfer characteristics with seven concave structures, including spherical, cylindrical, and triangular shapes. The results indicated that the spherical concave structure presented the best heat transfer result, leading to the most significant vortex diffusion coefficient in the electrolyte. There are also some research studies on the fluid flow uniformity inside electrolyzes. Wang et al. [12] established a three-dimensional laminar model to study the flow uniformity of liquid in the CCBP electrolyzes. They validated the simulation results with visualization experiments, revealing significant non-uniform flow in the CCBP electrolyze. Subsequently, they compared the flow uniformity inside the blank electrolyzes, CCBP electrolyzes, wedge-shaped electrolyzes, and rhomboid electrolyzes using a laminar model [13]. The results showed that the flow uniformity in the wedge-shaped and CCBP electrolyzes was improved by 19% and 28%, respectively.

Generally, a more uniform flow of electrolytes within the flow channel facilitates ion and heat transfer on the electrode surface, promotes gas separation, and reduces the energy consumption of the electrolyze. Except for the design of a concave–convex structure with variable shapes at the bipolar, the multiple inlet setup is another way to produce a more uniform fluid distribution in the electrolyze [14,15]. A single inlet/outlet structure always causes problems with inlet circulation and uneven fluid distribution. However, most studies about the optimization of flow channel structure in the electrolytic cell still focus on the flow and electrochemical behavior with variable concave–convex structures arranged in a linear distribution, with limited reports on the synergistic effects of multiple inlet setup and concave–convex structure on the flow field.

In this work, we establish a three-dimensional single-chamber channel structure model and select the concave–convex structures of spherical, square, and triangular types in a circumferential distribution as the bipolar plate. CFD methods are employed to explore the influence of a combination of concave–convex structure and multi-inlet setup on the flow distribution inside the electrolyze. A criterion evaluation based on flow velocity at specular positions is further introduced for quantitative analysis. As this paper emphasizes the effects of structural configuration on flow field and flow uniformity, only the single-phase model is adopted without coupling the electrochemical model. Additionally, a scaled-down-sized electrolytic cell is used in light to reduce the computation cost. The simulation study based on a single-phase model could eliminate the interference factors such as the interaction between gas production and electrolyte flow, and the heat transfer disturbance in the liquid phase from the electrochemical reaction. A single-phase model is also employed in some literature to evaluate the influence of the spherical convex structure on uniform flow patterns. This study has practical value in designing bipolar plate structure and configuration of inlet/outlet setup, although the electric field is neglected.

2. Structure of Industrial ALK Electrolyze

The system of a typically industrial ALK electrolyze system is shown in Figure 1, which usually consists of the power, control, and reaction sections [16,17]. The most important part of the ALK electrolyze system is the reaction section, which comprises the electrolyze, pump, heater, separator, and piping. During the operation of the ALK electrolyze system, the pump adjusts the flow rate of the electrolyte and the heater controls the temperature of the electrolyte inside the electrolyze. The electrolyte is recycled in this process. The electrolyze is the core section of thee reaction section, in which hydrogen and oxygen are produced by the electrochemical reaction between the electrolyte and catalyst layer, which is further collected in the gas-liquid separator. In industrial alkaline water electrolysis, the electrolyze accounts for the largest part of the system cost.

An ALK electrolyze mainly comprises the electrodes, diaphragm, bipolar plates, and end plates, which are usually designed with a zero-gap structure to reduce the distance between the electrode plate and the diagram. OH- is transported through the electrolyte from the cathode to the anode, mainly 20~40% KOH or NaOH aqueous solution. Hydrogen is generated as bubbles at the cathode. Generally, bipolar plates are fabricated into the concave–convex structure by mold punching to enhance the disturbance of electrolyte flow and improve the multipoint contact between the plates. In this case, the space between the plates forms the flow channel for the electrolyte, as is shown in Figure 2. The channel construction of an ALK electrolyze is fundamental to affecting the turbulent flow patterns during the ALK electrolyze process. However, commercial electrolyzes generally face the issue of uneven electrolyte flow within the flow channels. The uneven flow of electrolytes has an important effect on facilitating the transfer of ions and heat on the electrode surface. Moreover, it also leads to the inconsonant distribution of electrolyte concentration and flow rate at the bipolar surface. Consequently, when the gas production reaction does not proceed at an appropriate flow rate, the gas bubbles would fail to escape in time and accumulate at the electrodes. In that case, the active surface area of the electrodes is reduced, the double electron layer is destroyed, the activation overpotential and ohmic losses are increased, and the energy consumption of the electrolyze is raised [18,19]. Therefore, it is essential to optimize the flow channel structure to improve the uniformity of electrolyte distribution and accelerate bubble removal, thereby enhancing the long-term operational stability of the industrial electrolyze [20].

3. Computational Model

3.1. Flow Governing Equations

The fluid flow within the flow channel is analogous to the flow around a cylinder array, consisting of a turbulent flow region influenced by the cylinder disturbances and a laminar flow region near the cylinder walls. Compared to the standard k-ε model, the RNG k-ε model is incorporated by additional turbulence source terms (such as C_1ε and K_g terms), which show improvements in the turbulence generation and dissipation processes. These modifications enable a more precise description of the turbulent behavior in various disturbed flow regions. Moreover, the RNG k-ε model can better handle flow processes under low Reynolds number conditions, particularly in the boundary layer and flow transition regions, where it accurately captures the turbulence generation and dissipation processes. Consequently, this study employs the RNG k-ε model, with its governing equations presented in Equations (1)–(3) [7,8,9,21].

The continuity equation is

$${\displaystyle \frac{\partial }{\partial x_i}}\left(\rho {\overline{u}}_i\right)=0$$

(1)

The momentum equation is

$${\displaystyle \frac{\partial }{\partial x_i}}(\rho {\overline{u}}_i{\overline{u}}_k)={\displaystyle \frac{\partial p}{\partial x_k}}+{\displaystyle \frac{\partial }{\partial x_i}}\left(\mu {\displaystyle \frac{\partial {\overline{u}}_k}{\partial x_i}}\right)$$

(2)

The k-ε equation is

$${\displaystyle \frac{\partial }{\partial t}}(\rho k)+{\displaystyle \frac{\partial }{\partial x_j}}(\rho k{u}_i)={\displaystyle \frac{\partial }{\partial x_j}}\left({\sigma }_k{\mu }_{\mathrm{eff}}{\displaystyle \frac{\partial k}{\partial x_j}}\right)+G_k+\rho \epsilon$$

(3a)

$${\displaystyle \frac{\partial }{\partial t}}(\rho \epsilon )+{\displaystyle \frac{\partial }{\partial x_j}}(\rho \epsilon u_i)={\displaystyle \frac{\partial }{\partial x_j}}\left({\sigma }_{\epsilon }{\mu }_{\mathrm{eff}}{\displaystyle \frac{\partial \epsilon }{\partial x_j}}\right)+C_{1\epsilon }{\displaystyle \frac{\epsilon }{k}}{G}_k-C_{2\epsilon }^{*}\rho {\displaystyle \frac{{\epsilon }^2}{k}}$$

(3b)

Besides the streamline analysis, the index of vortices intensity is also used for the investigation of the flow field distribution. The vorticity transport equation is presented here, as shown in Equation (4) [21].

The vorticity transport equation is

$${\displaystyle \frac{\partial \overrightarrow{\omega }}{\partial t}}+\left(\overrightarrow{u}\cdot \nabla \right)\overrightarrow{\omega }=\left(\overrightarrow{\omega }\cdot \nabla \right)\overrightarrow{u}+\nu {\nabla }^2\overrightarrow{\omega }$$

(4)

3.2. Computational Parameters

Under the conditions studied in this paper, H₂O represents electrolyte. The boundary conditions include the velocity inlet and pressure outlet with the atmospheric pressure (1.013 × 10⁵ Pa); the flow velocity for a single inlet setup is set to 2.1 m/s while 0.7 m/s is set for the multiple inlet setup. The inlet electrolyte temperature is maintained at 300 K. Considering the complexity of turbulence near the wall, an enhanced wall function model is implemented at the wall to capture the turbulent characteristics, particularly the influence of the interaction between the fluid and the solid wall on the turbulence structure. Using wall functions also achieves good convergence speed and relatively low memory requirements.

The SIMPLE algorithm is utilized for pressure–velocity coupling. To ensure the stability and accuracy of the numerical solution, a second-order upwind scheme is applied to discretize the momentum, turbulent kinetic energy, and turbulence dissipation rate equations. The specific computational simulation parameters are provided in

Table 1. Parameters in the simulation.

Turbulence Model	Fluid Medium	Inlet/Outlet Boundary Conditions	Wall Boundary Conditions	Pressure-Velocity Coupling	Discretization Scheme
RNG k-ε	H2O	Velocity inlet 2.1 m/s Pressure outlet	No-slip condition	SIMPLE	Second-order upwind scheme

.

3.3. Single-Chamber Geometry Model and Mesh Generation

3.3.1. Geometric Modelling

Industrial electrolytic cells generally possess a compact, stacked structure. Each cell comprises the bipolar plates, the cathode, the anode, and the membrane. Given the substantial scale of industrial electrolyzes, this study focuses on simulating a laboratory-scale single cell to reduce computational complexity. Therefore, a scaled-down model of the single-chamber electrolyze is developed with a reduced electrode chamber diameter of 100 mm while preserving key model features [22]. The reduced size of the electrolyze still ensures the capture of typical fluid characteristics and flow patterns in the bipolar plate channels [17]. Since the anode and cathode chambers have identical structures, the computation domain is simplified by selecting a single electrode chamber. The concave–convex structures with different shapes within the electrode plates are arranged uniformly. The primary geometric parameters are introduced using the nipple structure as an example: the cylindrical electrode chamber is established with a diameter of 100 mm, and the nipple structure consists of spheres with a diameter of 6 mm (the side length of both the square and triangular structures is 6 mm).

The inner diameters of both the inlet and outlet are 2.8 mm. The specific geometric structure and dimensions are shown in Figure 3. To investigate the effects of the concave–convex structure type and the configuration of inlets and outlets on the flow field, six structural constructions are designed, labelled as Case A through Case F. Specifically, Case A represents a spherical concave–convex structure coupled with a single inlet/outlet, Case B represents a square concave–convex structure coupled with a single inlet/outlet. Case C represents a triangular concave–convex structure coupled with a single inlet/outlet. Cases D through F correspond to the concave–convex structures coupled with multiple inlets and outlets. The detailed structural configurations are summarized in

Table 2. The shape of concave–convex structure and configuration of the inlet/outlet.

Structural Configuration	Concave–Convex Structure Shapes	Number of Inlets	Number of Outlets	Number of Elements
Case A	Spherical	1	1	1,397,530
Case B	Square			2,836,298
Case C	Triangular			2,778,067
Case D	Spherical	3	3	1,490,483
Case E	Square			3,147,665
Case F	Triangular			2,854,809

.

3.3.2. Establishment of Numerical Model and Grid Independence

This study generates a polyhedral unstructured mesh for the computational model using Fluent Meshing software of Version 2016. Unstructured meshes are flexible and well-suited for complex geometries like ours. In particular, an unstructured mesh generation can ensure computational efficiency while maintaining the accuracy of the results [23]. To capture the characteristics of near-wall flow, particularly the turbulent viscous effects in the boundary layer region, mesh refinement is applied at the concave–convex structures near the channel walls and the sphere. The initial mesh layer’s thickness near the wall is set to 0.1 mm, with a mesh growth factor of 1.2, comprising six layers. Taking the nipple structure as an example, the final mesh configuration is shown in Figure 4.

A grid independence test ensures that the simulation results are reliable and independent of grid resolution. For the nipple structure, three different maximum mesh sizes are used, i.e., 0.6, 1, and 1.2, resulting in total mesh elements of 1,141,034, 1,397,530, and 1,546,366, respectively. The results show negligible disparity for the velocity at the outlet when the mesh element counts are 1,397,530 and 1,546,366, meaning that the computational accuracy stabilizes upon reaching 1,397,530 mesh elements. The mesh independence validation is depicted in Figure 5. For the other five cases, the same independence tests are also conducted. The final adoptive mesh elements are given in Table 2.

3.4. Analysis Method of Flow Field Uniformity

In order to provide quantitative information about the flow uniformity within the flow field with six different concave–convex structures, an evaluation criterion based on the statistical deviation of flow velocity is introduced. The indicator of the flow uniformity is expressed in Equation (5) [24].

$${\gamma }_v=1-{\displaystyle \frac{1}{2n}}{\displaystyle \sum _{j=1}^n\sqrt{\frac{{(v_j-\overline{v})}^2}{\overline{v}}}}$$

(5)

where ${\gamma }_{\mathrm{v}}$ is between 0 and 1, with a higher ${\gamma }_{\mathrm{v}}$ indicating better flow uniformity, and 1 represents ideal uniform flow and 0 indicates that the fluid passes through only one measurement point; $v_j$ and $\stackrel{—}{v}$ are the velocities at the measurement point and the average velocity on the measurement cross-section, respectively. Velocity samples are taken within the flow channel at the Y-Z (X = 0) and X-Z (Y = 0) cross-sections. The detailed sampling locations are shown in Figure 6.

4. Results and Discussion

An analysis of the velocity and streamline distribution and vorticity distribution within the scaled-down-sized electrolyze flow channel is performed to investigate the influence of concave–convex structure types on the flow field distribution under multi-inlet conditions. The observation planes are the Y-Z plane at X = 0 mm, X-Y plane at Z = 0 mm, and X-Z plane at Y = 0 mm. The specific cross-section and the origin are shown in Figure 6.

4.1. Velocity and Streamline Distribution Within the Flow Field

Figure 7 and Figure 8 illustrate the velocity and streamline distribution characteristics in the Y-Z plane where X = 0 mm and the X-Y plane where Z = 0 mm under different structural configurations. As shown in Figure 7, in Cases A to F, the flow within the channel is predominantly longitudinal (in the X-direction). It is found that the presence of concave–convex structures significantly alters the distribution characteristics of the fluid in the transverse (Y-direction), as shown in Figure 8. Specifically, by obstructing the fluid flow, the concave–convex structures enhance the transverse liquid distribution effect, leading to significant changes in both flow velocity and distribution in the transverse direction. This modification contributes to improving the yield of gas products within the electrolyze. However, it may also influence overall energy consumption due to the increased flow resistance. In all configurations, the flow velocity in the central region of the channel is generally higher, indicating that the fluid retains significant kinetic energy in the main flow area. In contrast, the flow velocity is lower in the peripheral regions, which results in a higher degree of flow field non-uniformity, leading to uneven local distributions of reactant concentration and temperature. Moreover, in the single inlet/outlet setups, the velocity near the center axis along the mainstream direction is higher than at the edge of the channels. The low velocity and circulation region accounts for a large proportion of the anode/cathode area as shown in Figure 8. Generally, the circulation has a negative effect on the removal of gas bubbles. Also, a low velocity reduces the heat and ion transfer, decreasing the electrolyze efficiency. For the multiple inlet/outlet model, however, the electrolyte flows across more areas along the channel. Those setups enhance the high velocity regions and mitigate the electrolyte circulation.

The comparison results of different structures in Figure 7 (Cases A–C) demonstrate that the concave–convex structures significantly influence the flow velocity distribution. The flow velocity in the square concave–convex structure is markedly lower than in the spherical and triangular concave–convex structures. This can be attributed to the larger surface area obstructing the flow in the square concave–convex structure, which hinders the passage of the fluid, leading to longer residence times within the structure and a reduction in flow efficiency. In contrast, with their streamlined geometries, the spherical and triangular concave–convex structures facilitate better fluid flow, thereby reducing fluid retention. Furthermore, the obstructive effect of the square concave–convex structure is more likely to create localized low-velocity regions, which are detrimental to the effective diffusion of hydrogen during the electrolysis process. Compared to multi-inlet configurations, the single-inlet structure exhibits significant low-velocity zones, particularly near the longitudinal sides of the flow channel (as indicated by the red dashed box in Figure 7). The presence of low-velocity regions not only prolongs the residence time of hydrogen in the electrolyte but also increases local resistance. The multi-inlet configuration notably reduces the area of low-velocity regions and enhances the overall flow process. The streamlined distribution results further illustrate that the fluid distribution in the multi-inlet configuration is more uniform, which helps to suppress the formation of low-velocity zones within the flow channel and reduces the generation of large vortices. As a result, the hydrogen residence time is shortened, significantly improving the flow conditions of the electrolyte.

As illustrated in Figure 8, the multi-inlet configurations under Case D, Case E, and Case F demonstrate improved flow velocity distribution characteristics. Compared to the single-inlet design, the multi-inlet configuration significantly reduces the extent of large vortex regions and mitigates the occurrence of excessively low flow velocities at the channel boundaries. This optimized design reduces gas retention within the electrolyte and diminishes the recirculation zones on the electrode surface, thereby enhancing both ion and heat transfer efficiency. Further analysis of the streamline distribution reveals that the concave–convex structures facilitate the development of vortices within the flow channel. The generation of vortices plays a crucial role in disturbing the flow field, promoting adequate fluid mixing, and increasing the diffusion rate of reactants. However, giant vortices may lead to localized gas retention. Specifically, in Case A, Case C, and Case D, the strong vortices induced by the concave–convex structures cause an expansion of the fluid recirculation regions, resulting in prolonged gas residence times within the electrode chamber. This is unfavorable for hydrogen evacuation and may contribute to increased energy consumption.

In summary, the multi-inlet configuration effectively reduces low-velocity flow zones and suppresses the formation of large vortices, thereby facilitating the flow of gas and increasing the velocity of the electrolyte. This design also decreases gas content on the electrode surface and within the electrode chamber, leading to enhanced reaction efficiency. Furthermore, reducing the gas content within the electrolyte contributes to a decrease in its electrical resistance, ultimately reducing energy consumption.

4.2. Vorticity Distribution

Figure 9 illustrates the vorticity distribution in the X-Z plane where Y = 0 mm. As is shown, distinct differences in vorticity distribution near the electrolyte inlet region are observed for the three concave–convex structures (spherical, triangular, and square). Both the spherical and triangular concave–convex structures significantly enhance vorticity near the inlet, which aligns with the streamline analysis results presented in Figure 7. This enhancement is attributed to the streamlined geometries of the spherical and triangular shapes, which induce more muscular local disturbances during the flow process, resulting in higher vorticity intensities. Such effects promote improved liquid mixing near the inlet, optimizing the initial distribution of the electrolyte [25]. In contrast, the planar design of the square concave–convex structure restricts fluid slip, resulting in weaker flow disturbances and lower vorticity levels, thereby limiting local mixing effectiveness. Nevertheless, the spherical and triangular structures can sustain adequate disturbance effects over a more extended section of the flow channel, thereby enhancing overall flow uniformity.

Compared to the single-inlet configuration, the multi-inlet structure significantly increases the vorticity intensity in both the inlet region and the central portion of the electrode chamber. In the inlet region, the multi-inlet configuration promotes the formation of more muscular disturbances and vortices, effectively enhancing the uniformity of fluid distribution and reducing the potential for localized fluid retention. As the flow progresses, these disturbances propagate into the central region of the electrode chamber, effectively preventing the formation of local low-velocity zones. This, in turn, improves the bubble detachment rate from the electrode surface, enhancing the surface activity of the electrode and ultimately increasing the circulation efficiency of the electrolyte [26].

4.3. Velocity Distribution Uniformity

The velocity distribution uniformity index for the six configurations is computed, with the uniformity indices in the X-Z and Y-Z planes and the corresponding velocity distribution profiles presented in Figure 10. As illustrated, the multi-inlet square concave–convex structure exhibits the best overall uniformity index among the six configurations, resulting in a more homogeneous velocity field within the channel. The characteristics of the square concave–convex structure enhance local disturbances, further improving the flow’s uniformity. For the single-inlet triangular concave–convex structure, the lower inlet velocity of the multi-inlet triangular structure leads to a lower uniformity index in the X-Z plane. However, the single-inlet triangular concave–convex structure can still somewhat enhance local disturbances, improving the velocity distribution. Nonetheless, the single-inlet design is constrained regarding its overall flow field coverage. Due to the streamlined features of the spherical concave–convex structure, this configuration reduces flow resistance but fails to introduce sufficient disturbances, resulting in relatively poorer overall flow field uniformity.

5. Conclusions

Enhancing the energy efficiency of the ALK electrolyzer requires a well-design fluid field structure to improve the uniformity of electrolyte. In this study, simulation research is conducted to investigate the flow characteristics of a scaled-down-sized single-chamber flow channel incorporating three concave–convex structures (spherical, square, and triangular) arranged in a circumferential distribution. The primary focus was to study the impact of concave–convex structure types on the flow field distribution within the alkaline electrolyzer flow channel under multi-inlet configurations. The following conclusions were drawn:

(1) The flow within the channel is predominantly longitudinal, with concave–convex structures significantly enhancing the transverse liquid distribution. The single-channel configuration exhibits non-uniform velocity distribution and low-velocity zones, which may hinder gas bubble removal. Overall, the multi-inlet configuration reduces low-velocity zones, suppresses large vortex formation, facilitates gas flow, and reduces energy consumption;

(2) The spherical and triangular concave–convex structures promote vortex development. The vorticity distribution becomes more uniform as the flow progresses under both configurations. The multi-inlet arrangement further intensifies the vorticity within the flow channel align with the flow direction;

(3) A flow field velocity uniformity evaluation criterion based on statistical deviation is employed, revealing that the multi-inlet square concave–convex structure exhibited the highest overall uniformity index, attaining 0.80 at the middle cross-section normal to the incoming flow and 0.88 at the middle cross-section parallel to the flow, followed by the single-inlet triangular concave–convex structure. While the spherical concave–convex structure offers favourable drag-reduction characteristics, its velocity distribution uniformity is relatively poor.