In the field of water treatment, electrocoagulation technology has become a hot topic in the fields of water treatment and river dredging due to its simple operation, no secondary pollution, high efficiency and low cost. However, there is still a lack of systematic research on the mechanism of the influence of bubble movement and flow field disturbance on flocculation effect.
Recently, a joint research team of Qinghai University and Tsinghua University conducted an experimental study based on the particle image velocimetry system (PIV system), revealing the coupling law between bubble dynamics and flow field evolution during electrocoagulation.
For details, please refer to the paper "Characteristics and Control Factors of Electrocoagulation Flow Field and Sediment Settling Effect".
It consists of a Qianyanlang Revealer PIV system and a flocculation system.
1. PIV system: Qianyanlang Revealer PIV high-speed cameras × 4, with a resolution of 2560×1920, a frame rate of 2000 fps, and a PIV cross-frame time of 300 ns, which are used to capture the transient movement of bubbles. Pulsed laser, with an energy of 30 mJ and a frequency of 1 kHz, is used to vertically irradiate the experimental observation area and clearly image the tracer particles. The synchronization controller is used to coordinate the timing of the high-speed camera and the laser, as shown in Figure 1.
Figure 1: Revealer PIV system
2. Electrocoagulation system: flocculation tank, size 180 mm × 110 mm × 130 mm, aluminum electrode plate, spacing 40 mm, adjustable DC power supply 0~30 A, resolution 0.01.
1.1 The experimental solution is composed of 1.3 L of pure water and 1.3 g of NaCI, and the solution temperature is ambient temperature (20°C). In the experiment, the sediment sample was taken from the upper reaches of the Yellow River. After cleaning and drying, 2.6 g of sediment particles were added to the experimental solution. The sediment concentration of the solution was 2 g/L. After stirring evenly, it was placed for 60 s to start the experiment.
1.2 Set 9 groups of current density (10~50 A/m², step size 5 A/m²) to simulate the electrocoagulation process under different working conditions.
2.1 Set the laser pulse frequency to 200 Hz, the intensity of lasers A and B to 10 A, adjust the light guide arm so that the laser irradiation surface is parallel to the camera observation surface, and adjust the focal length so that the camera image is focused on the laser irradiation surface.
2.2 The PIV system was used to continuously observe the movement of bubbles under 9 different current densities, and a total of 5500 frames of image data were collected. The results showed that with the increase of current density, the bubble generation rate on the electrode surface increased, and the bubble distribution expanded from the surface layer of the solution to the deep layer, as shown in Figure 2.
Figure 2: Bubble images at 15s electrolysis under 9 current densities
2.3 PIV flow field calculation and visualization research, based on the cross-correlation algorithm, the average displacement and velocity of particles in the window are calculated by fast Fourier transform FFT. The Revealer RFlow4 flow field measurement software is further used to calculate the vortex structure distribution in the flow field using the Omega criterion, and the vortex boundary is identified with Ω=0.52, and the vortex cloud map is drawn to accurately quantify the number and intensity distribution of vortices, as shown in Figure 3.
Figure 3: Omega distribution cloud map under 9 current densities
The experiment measured the changes in sediment particle size and solution turbidity under different current densities. When the current density is 40 A/m², D/V (the ratio of average particle size to average velocity) reaches the maximum, and the removal efficiency is the highest, indicating that floc formation and stability are best under this condition. Air flotation entrainment and flocculation sedimentation are the main factors affecting the reduction of turbidity, and the boundary particle size is 37.2 μm. Sediment particles smaller than this particle size are mainly removed by air flotation entrainment, and particles larger than this particle size are mainly removed by flocculation sedimentation.
The high-speed continuous observation capability and particle image processing capability of the Qianyanlang Revealer PIV system provide strong support for in-depth research on the electroflocculation process. By accurately capturing the original image of bubble movement and processing and presenting the vortex cloud map, it provides a scientific basis for optimizing the electroflocculation effect, helps solve complex flow field problems, and promotes technological innovation in environmental protection, chemical industry and other fields.
