Visualization study on preferential flow in highly saturated and super hydrophilic porous media by combining dye tracking and infrared imaging

• New method to track finger flow in highly saturated superhydrophilic porous media. • Preferential flow recurrence was observed at high saturation. • The preferential flow was found to impair desalination efficiency. • The temperature distribution was controlled by preferential flow. Air-gap diffusion distillation (AGDD) is a practical desalination technique. The highly saturated super hydrophilic porous medium as its evaporation surface has a decisive influence on the desalination efficiency of AGDD. In experiments, we found preferential flow in porous media. However, the preferential flow in highly saturated super hydrophilic porous media has not been studied, and the effect of preferential paths on evaporation from porous media surfaces remains poorly understood. The method of combining dye tracking and infrared imaging was used to visualize the process of thermal seepage in highly saturated super hydrophilic porous media. Under the condition of uniform and continuous liquid supply on the upper surface, the preferential flow characteristics of hot stream seepage in porous media are tracked; Point source infiltration experiments were carried out at room temperature to compare the difference between preferential flow in super hydrophilic porous media and general soil (sand). The experimental results of thermal seepage showed that the preferential flow greatly influences the uneven temperature distribution of the porous media, which directly impairs the desalination efficiency of AGDD. According to the coefficient of variation (CV), the preferential flow development process can be divided into three stages: stable development, exponential development, and high-speed growth. As the seepage flow is stabilized, the influence of preferential flow on the flow field and temperature field still exists and is affected by these three stages. Preferential flow characteristics were also captured in the saturated capillary rise zone at the bottom of the porous medium (below the phreatic level). The point source infiltration experiment found that the preferential flow in superhydrophilic porous medium will reappear at high saturation, which is completely different from the soil (sand) studied by previous studies. In addition, the effects of inlet temperature, flow rate, thickness, and hydrophilic, hygroscopicity on preferential flow were discussed. Increasing the flow rate and inlet temperature and reducing the thickness of the porous media can weaken the influence of preferential flow on the temperature field. But increasing the temperature only superficially weakens the preferential flow. Increasing hydrophilicity and hygroscopicity can also reduce the preferential flow.