Due to the retention of solute by the semipermeable membrane, the concentration gradient of solute is formed from the membrane surface to the main body of the stock solution, that is, the concentration polarization phenomenon is formed. Under the cross flow operation mode, the flow pattern of the raw liquid in the flow channel can be divided into three types: the laminar layer near the film surface, the turbulent layer in the middle of the flow channel, and the transition layer between the laminar layer and the turbulent layer. Driven by the pressure difference on both sides of the membrane, some solvents form an osmotic flow through the membrane, and solutes are trapped and accumulated on the membrane surface. The solute concentration c in the whole channel changes in the linear direction of the membrane surface. Since turbulence can effectively realize the reverse diffusion of solute from the membrane surface to the mainstream, the solute concentration cf in the turbulent layer can be regarded as uniform. If the solute concentration cp on the liquid side of the membrane permeation is constant, there is a gradient in the solute concentration c in the laminar layer, and the maximum value cm is reached on the surface. The so-called concentration polarization β is the ratio of the solute concentration cm on the surface of the laminar layer film to the solute concentration cf in the turbulent layer: β=cm/cf.
