EXPERIMENTAL SECTION

2.1 Materials and Setup

The average pore size of single channel ceramic membrane (Zhishen Fluid Membrane Separation Technology Operation Department, China) is 0.2 μm. The inner and outer diameters are 8 mm and 12 mm separately. The ceramic membrane in the core of CMGD disperses the gas into micrometer-sized bubbles, and the schematic of the experimental setup is shown in Figure 1. The peristaltic pump and mass flow controller (MFC, D07-19B, Beijing Sevenstar Flow Co. Ltd.) are used to feed the two-phase fluids and control their flow rates. In the work, the volume flow rates of gas (nitrogen, N2) was varied from 5 ml/min to 50 ml/min by an MFC and the volume flow rates of deionized water (Reverse osmosis water purification equipment purchased from Sichuan Xixi Science and Technology Co., Ltd., China) was varied from 0.5 ml/min to 5 ml/min by a peristaltic pump (LM40B, Nanjing Runze Fluid Control Equipment Co., Ltd.). The visualization system was assembled by a high-speed camera (NPX-GS6500UM, Dongguan Qiansheng Electronic Technology Co., Ltd.) with a 12 mm macro len (NPX0412M14IR-2M, Dongguan Qiansheng Electronic Technology Co., Ltd.), a telecentric len of 110 mm object distance (Dongguan Qiansheng Electronic Technology Co., Ltd.), a quartz glass tube (outer diameter, 16 mm; inner diameter, 13 mm), and a LED light source (white light, 150 W). The rising dynamics of the bubbles were captured at 790 frames per second (fps) and 1 ms exposure time in a field of view of 640 × 480 pixels. The camera position was set in front of the exit of the ceramic membrane to capture bubbles in the two-phase flow, and the camera was mounted on a slide to capture bubbles rising up to 5 cm and 10 cm. Additionally, internals with different numbers of baffles (as shown in Figure 2A-C) were designed using UG NX 12.0. The internals with a length of 10 cm were printed by a 3D printer (ANYCUBIC Photon Ultra) and placed in ceramic membrane channels to intensify the stability of bubbles.