| Microalgae is of great potential and value in bio-energy, environmental protection, food, health and pharmaceutical fields. However, microalgae cultivating reactor of poor performance has been a bottleneck of the microalgae industry development for a long time. Traditional design, optimization and amplification of photobioreactor took more time, larger cost, and gave out little flow information inside the reactor. In recent years, computational fluid dynamics (CFD) was applied in more and more algae culture photobioreactor studies. Although CFD takes many advantages over traditional methods, there remain loopholes. Based on the former research, multilevel baffle reactor with multistage intakes was designed. The mix, mass transfer and optical transfer properties of the new reactors were systematically researched using CFD technology. Results were as follows.Firstly, calculation model of multistage intake multilevel baffle reactor was built in commercial software ANSYS Fluent, and the reactor was simulated using coupled equation of Eulerian-Eulerian multiphase flow model and the PBM model. Compared calculated results with experimental results, the simulation model was proved to be feasible and reliable. Partial performance parameters of multistage intake reactor results showed that, compared to the lower stage, local gas holdup upper, liquid velocity vector of upper and middle stages in this reactor were affected largely by aeration rate. Turbulent kinetic energy in airlift region was bimodal distribution, and in the downcomer region its distribution was more gentle.Then, CFD simulation of ordinary reactor(A), Multilevel Baffle reactor(B), Multistage intake Multilevel Baffle reactor(C) were executed, flow field and mass transfer of the three reactors were compared. The results showed, in certain aeration rate, gas holdup, mean liquid velocity, turbulent kinetic energy, turbulent kinetic energy dissipation rate, dead zone, mass transfer coefficient of the multilevel intake photobioreactor have been greatly improved compared to the other two photobioreactors. When the ventilation rate at 0.8 vvm, gas holdup of reactor C enhanced by 52.63% and 39.11% than reactor A and B respectively. When ventilation rate were 0.4 vvm and 0.6 vvm, kLa value was increased by 36.16% and 11.27%, compared to reactor B.Furthermore, using multiphase flow model and PBM model coupling method, bubble characteristics were studied further, which has great influence on reactor transmission and mass transfer performance. Results showed, at height 390 mm, Bin-0 volume fraction of reactor C was reduced by 32.79%, compared to reactor B. The volume fraction of 6 mm-diameter bubble is 8.58% in Reactor B, while in reactor C it was 0.54%. These indicated that, in Reactor C bubble coalescence was reduced remarkably, and bubble distribution of reactor C was favorable, which helped interphase mass transfer.Light transmission characteristics of the reactor C was analyzed by CFD simulation. The results showed, illumination radially mixing intensity of reactor C increased, light district radial velocity of reactor C was increased by 28.25%, average residence time of reactor C has been shortened, cycle of light and dark cycle was shortened 19.63%, and light-dark cycle frequency was accelerated. Resultingly, microalgae flash effect was speeded up, which helped microalgae cultivation.Finally, internal structural parameters (baffle positions (Ar/Ad), distance from baffle top to liquid surface (ho) and the baffle gap (D)) of multistage intake reactor (Reactor C), were studied and analyzed in according to mixing, mass transfer and light transfer properties. The results showed that the radial velocity of light district and liquid volumetric mass transfer coefficient was decreased with Ar/Ad rising. The overall reactor achieved a good mixed result at Ar/Ad 0.5. When ho rose, light district radial velocity and overall mixing performance increased. At ho 40 mm, reactor got a good mass transfer performance. As the distance from the reactor baffle gap increased, ATK and radial velocity of light district increased. When the baffle gap reached 15 mm, mass transfer performance was satisfied.In summary, reactor C had greatly improved mix, mass transfer and light transfer performance, which include reducing bubble coalescence phenomenon, optimizing bubble distribution, increasing flash effect. The influences of internal structure parameters on mix, mass transfer and light transfer performance were analyzed. The research provided new ideas and direction for the design and optimization of flat panel bioreactor. |
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