## 3 Results and Discussion
### Pints in Inlet Channel
#### 3.3.1 Symmetrical Arrangement  
The pins are distributed symmetrically in three, four, and five groups above and below the inlet channel, respectively, as seen in Figure 8a. Each pin has a length of 0.5 mm and a height of 0.2 mm. In the porous medium region corresponding to the pins, there is a sudden change in permeation velocity due to the coverage of the porous medium, which influences the size of the permeation velocity at the same radial position. However, its impact on flow heterogeneity can be negligible because the coverage area is limited. Additionally, for the convenience of observation and statistical calculation, the permeation velocity in this area is excluded from variance calculation.  
Figure 8b demonstrates that the permeation velocity increases most near the inlet and proportionally with the increasing number of pins. When pins are introduced into the inlet channel, there is a varying degree of reduction in the corresponding position's permeation velocity as the gas passes through each pin. Nevertheless, throughout the entire axial porous medium region, the permeation velocity exhibits an increase relative to the initial condition, potentially attributable to the pressure increase in the inlet channel induced by pins (Figure 8c).  
In conjunction with the contour and streamlines of velocity in the inlet channel in Figure 8a, the presence of pins can be attributed to multiple groups of converging-diverging nozzles in the inlet channel [40]. When the gas flows past the pin, the pressure at the front side of the pin increases due to the contraction of the channel diameter, leading to increased gas flow through the porous medium in this place. As the gas flows between two pins, the velocity increases suddenly, causing a drop in permeation velocity in this position. After this place, the velocity in the inlet channel returns to normal, and the pressure reduces slightly. Consequently, the velocity of permeation exhibits variations at a lower magnitude.  
Figure 7: Structural diagram of GPF with pins.  
Furthermore, the pins' presence also influences the gas flow direction in the inlet channel, subsequently impacting the axial permeability velocity. In front of the pins, part of the gas is obstructed and redirected around. Upon passing through the region occupied by the pins, fluid disperses laterally, resulting in an increased gas flow toward the porous medium. Simultaneously, low-speed eddy recirculation zones form in front of and behind the pins, altering fluid direction and converting a portion of gas kinetic energy into pressure potential energy, thereby enhancing the regional permeation rate. Moreover, the greater the kinetic energy present near the pins, the more obvious the effect of eddy motion and pressure on kinetic energy reduction [47].  
Figure 8: (**a**) Contour and vector of axial inlet channel velocity, (**b**,**c**) curve of permeation velocity difference and pressure distribution, and (**d**) variance corresponding to different numbers of pins arranged symmetrically.  
The variance in axial permeation velocities among the three conditions with different numbers of pins is compared to investigate their impact on the axial permeation velocities. The findings are presented in Figure 8d. The addition of pins can partially enhance the axial permeation velocity, exhibiting an enhancement in uniformity.