## 3 Results
### PVDF Filter  
Figure 11 shows images of the time-averaged velocity field for a flow rate in the channel of 960 L/h with the metal mesh with deposited PVDF nanofibrous filter. The time averaged velocity field was determined from about 50 individual measurements. Figure 12 presents the magnitude of velocity vector component perpendicular to the plane of mesh at different distances from the mesh downstream, determined from the flow field shown in Figure 11. Similarly, Figures 13 and 14 show images of the time-averaged velocity field for a flow rate in the channel of 650 L/h and the magnitude of velocity vector, respectively.  
Figure 12: Velocity component (in m/s) perpendicular to the filter plane at different distances (in mm) from the filter plane, determined from the velocity field shown Figure 11. Steel mesh with PVDF nonwoven fabric filter.  
Figure 13: Time-averaged flow velocity field in the channel (in m/s) with stainless steel mesh with PVDF nonwoven nanofibrous filter. Volume air flow rate through the channel 650 L/h. Time between laser pulses 150 μs: (**a**) velocity vector modulus (in color); (**b**) vector representation.  
Figure 11 shows the velocity field in the channel for the volume flow rate of air through the channel of 960 L/h. The time-averaged flow velocity in the channel determined from this flow rate was 1.19 m/s, and the Reynolds number about \(Re\) = 1260. The velocity vectors of seed particles downstream of the filter, are perpendicular to the plane of the filter and  
Figure 14: Velocity component (in m/s) perpendicular to the filter plane at different distances (in mm) from the filter plane, determined from the velocity field shown Figure 13. Steel mesh with PVDF nonwoven fabric filter.  
are almost symmetrical to the mid plane of the channel. The maximum velocity of about 1.1 m/s was at the center of the channel. However, at a distance of about of 1/2 of hydraulic diameter of the channel a vortex flow was formed due to small differences in the velocities caused probably by the inhomogeneity in the filter structure (thickness and porosity), and its clogging.  
Figure 12 shows the distribution of velocity vector component perpendicular to the filter plane in the channel at different cross sections, determined from the velocity field shown in Figure 11. The distribution of vertical component of velocity vector at these cross sections, close to the filter plane, was not uniform, but was distorted by the presence of the filter. Downstream of the filter, the component of velocity vector perpendicular to the filter plane reaches a value of about 1 m/s. in the mid-plane of the channel, and decreases to 0.2-0.3 m/s close to the channel walls. At a distance of about 8 mm from the filter, a negative velocity vector occurs due to the generated vortices.  
Figure 13 shows the velocity field in the channel for a volume flow rate of air through the channel of 650 L/h. The time-averaged flow velocity in the channel was about 0.8 m/s, and the Reynolds number \(Re\) = 853. The velocity vectors downstream of the filter close to its plane are perpendicular to the plane. The vortices in the channel are generated at a distance of about 2/3 of hydraulic diameter from the filter plane. The vortices are generated due to non-uniformity of the velocity vector magnitude at the filter outlet caused by filter inhomogeneity and non-uniform filter clogging.  
Figure 14 shows the distribution of the velocity vector component perpendicular to the filter plane in the channel at different cross sections determined from the velocity field shown in Figure 13. The distribution of this component of the velocity vector at different cross sections is not distributed uniformly at any distance from the filter. The maximal magnitude of the velocity vector component perpendicular to the filter plane at the center cross section reaches a value of about 0.73 m/s.