Study Of Hydrodynamic Behaviors Of Tubular Photobioreactor With Inserted Helical Rotors

The microalgae technology includes biological carbon sequestration,biological nitrogen fixation, as well as microalgae biofuel, such as bio-hydrogen, bio-methane and biodiesel. The microalgae technology gives every government a feasible way for combining protection of environment with production of biofuel. In the development of microalgae technology, the photobioreactors, as cultivation equipments, play significant roles, which must be provided with behaviors of high cells density, low cost and easy to enlarge.And it is widely recognized that the tubular photobioreactors hold great potential in these aspects. However, there are still some defects in tubular photobioreactors, for example, the mixing effect is not good, the cells are easy to settle and the photoinhibition is large. Therefore, it is important for tubular photobioreactors to ameliorate the configuration, improve the flow behaviors,promote the mixing effect and enhance the utilization of light.In this paper, the principle of microalgae technology, the classification and structures of tubular photobioreactors, as well as the primary factors affecting microalgae cultivation were introduced in detail. This paper described the principle of the tubular photobioreactor with inserted helical rotors which can rotate without electric motor driving. By using the visualization experiments and the computational fluid dynamics (CFD) method, the hydrodynamic behaviors, which included velocity field, circumferential velocity component of gas and liquid, swirling number, particles' trajectory and the maximum shear stress, of tubular photobioreactors with helical rotors were investigated.The results showed that the helical rotors got great potential in promoting mixing and light utilization of tubular photobioreactors.The influences of the rotor leads and the fluid inlet velocities on the hydrodynamic behaviors of tubular photobioreactors with two-blade helical rotors were studied. The results showed that both of the rotor leads and the fluid inlet velocities had enormous impacts. The smaller the rotor lead, the greater the effect of the helical rotors, and the higher the shear stress, which lead to the helical rotor with 200mm lead gaining more advantages on breeding microalgae. The higher the liquid inlet velocity, the more effective mixing and greater the light and dark cycle frequency, but the higher the shear stress, which resulted in liquid inlet velocities of 0.3?0.6m/s being preferred.The increasing of gas inlet velocity could elevate the gas holdup, while it was not obvious to promote the mixing.The hydrodynamic behaviors of U-shaped tubes which were essential connections of tubular photobioreactors under the action of helical rotors were analyzed. The study showed that the swirl generated by the helical rotors still existed when it reaching the U-shaped tube, and the bend construction of U-shaped tube could also cause a local swirl. What's more, for a U-shaped tube with an inner diameter of 24mm, increasing of the bending diameter could bring about a tendency of increasing first and then decreasing of the local shear stress.