Trajectory of fine particles removal with diffusiophoresis and thermophoresis in a gas-liquid cross-flow array

Abstract

A gas-liquid cross-flow array (GLCA) system is proposed for fine particles (diameter between 0.1 μm and 2.5 μm, simplified as PM2.5) removal in exhaust gas, where the continuous and smooth wastewater films, providing huge specific surface area, each act as independent traps to remove PM2.5. The removal efficiency of PM2.5 is important for evaluating the performance of a GLCA, and the trajectory across the films determines the migration and ultimate fate of PM2.5. An analytical model based on a single film is developed to analyze the critical removal trajectory with diffusiophoresis (DP) and thermophoresis (TP) in the thermal boundary layer to calculate the efficiency, where the role of each force is examined. And experiments with a lab-scale GLCA are carried out with different vapor concentration and temperature gradients to verify the model. They both reveal that the removal efficiency can be increase sharply by increasing the humidity gradient between the bulk gas and film surface, while it increases slowly as temperature gradient increasing. Thus DP and TP have important effects on PM2.5 removal in the GLCA, and DP has a much more important effect than TP. A GLCA with appropriate humidity and temperature gradient can remove PM2.5 in a costly and efficient manner.

Fig. 1. GLCA system.

Fig. 2. Thermal boundary layer around a single film.

Fig. 3. Schematic of the model used for analysing particle trajectory.

Fig. 4. Critical removal trajectory of particle (d_p = 1 μm) in thermal boundary layer along a single film with or without DP and TP ((b) is the enlarge view of (a)).

Fig. 5. Critical removal trajectory of particle (d_p = 1 μm) in thermal boundary layer along a single film with different humidity gradient and same temperature gradient (T_g = 41.2 °C, T_w = 5 °C).

Fig. 6. Critical removal trajectory of particle (d_p = 1 μm) in thermal boundary layer along a single film with different temperature gradient and same humidity gradient (T_w = 5 °C).

Fig. 7. Critical removal trajectory in the thermal boundary layer along a single water film for different particle sizes (T_g = 41.2 °C, φ_g = 0.85, T_w = 5 °C).

Fig. 8. Experimental set up for PM2.5 removal: (a) flow chart of a GLCA, (b) picture of the experimental system.

Fig. 9. Particle number concentration density distribution dC_p/C_p at the inlet and outlet of the GLCA.

Fig. 10. Particle number concentration at the inlet and outlet of a GLCA with 100 films in row with different humidity gradient and same temperature gradient (T_w = 5 °C, T_g-in = 41.2 °C, Re_g = 72.2).

Fig. 11. Particle number concentration at the inlet and outlet of a GLCA with 100 films in row with different temperature gradient and same humidity gradient (T_w = 5 °C, Re_g = 72.2).

Fig. 12. PM2.5 removal efficiency by a GLCA after 100 films with different humidity gradient and same temperature gradient (T_w = 5 °C, T_g-in = 41.2 °C, Re_g = 72.2).

Fig. 13. PM2.5 removal efficiency by a GLCA after 100 films with different temperature gradient and same humidity gradient (T_w = 5 °C, Re_g = 72.2).