Characteristics of flow field near membrane surface based on particle image velocimetry technology

Abstract: This study built the test system of membrane surface flow field. The system was built under the condition of photo-metric optical fiber instead of hollow fiber membrane filaments. The paper analyzed the affection regularity of aeration intensity to fluid mechanics characteristics of hollow fiber membrane surface under the condition of clear water and three different aeration pore sizes of 1, 1.5, 2.5 mm based on the particle image velocimetry (PIV) technology. Further experiments were carried out on the variation of turbulence intensity values of hollow fiber membrane surface and liquid velocity values under the condition of different aeration intensities for three different aeration pore sizes. The movement of continuous phase (liquid) in PIV technology was marked by fluorescent particles to identify the velocity field of liquid. Under the conditions of this experiment, the trajectory of the bubble itself could reflect its velocity vector substantially, and the velocity field of each phase can be obtained after the phases were separated when we used the image processing techniques of each single phase. The results indicated that the increasing range of the average liquid velocity on the surface of the membrane appeared to be the highest when the aeration intensity was 140 L/h; and when over 140 L/h, it decreased to be much lower, so under the condition of this experiment, the optimum aeration intensity is 140 L/h, and more optimal turbulence intensity values are corresponding to this aeration intensity. In the project, according to optimized parameters, we can not only save the cost of system operating, but also eliminate the cake layer on the membrane surface and reduce the concentration polarization on both sides of the membrane surface. Under the condition of three different aeration pore sizes, the average speed of the liquid near the membrane surface is basically consistent with the aeration intensity; when the aeration is at 1 mm aperture, the velocity vectors of liquid near the surface of the membrane increase with the increase of aeration intensity, when the aeration intensity is over 140 L/h, the liquid velocity vectors near the membrane surface are basically the same; at lower aeration intensity, the liquid velocity of the membrane surface near the aeration pore is significantly higher than the ones which locate other places, and the liquid appears the phenomenon of rocking after affected by the bubble rising trajectory; at higher aeration intensity, liquid velocity value increases significantly in the horizontal direction; with the increase of the liquid velocity, the ranges of membrane filaments influenced correspondingly increase; and when the aeration intensity reaches 140 L/h, 6 membrane filaments are influenced at different degrees, and the shaking of the bubbles causes the rocking of the membrane filaments, which can play an active role in controlling the pollution of the membrane. This research provides a theoretic foundation for the optimization of flow field in membrane bioreactor so as to improve the membrane fouling problem.