Selection And Optimization Of Static Mixers In Tubular Microalgal Photobioreactors

Tubular photobioreactor has been considered to be the most potential reactor for mass cultivation of microalgae,excepting its poor mixing performance.It has been found that adding static mixer to the reactor can effectively improve the mixing of algae.Due to the diverse structures of static mixer,it is very important to select the suitable one.In this paper,the influence of four tyes of static mixers on the mixing performance of the reactor was studied by the method of computational fluid dynamics(CFD).A way of promoting the movement of algal cells by forming multi-longitudinal vortex was proposed.The blade-type static mixer was introduced and optimized to improve the average light/dark cycle(L/D)frequency of algal cells without greatly increasing the energy consumption.First of all,the rib arrangement angle of the discrete inclined ribs was optimized.The average velocity in the lighting direction(U_y),average L/D cycle frequency(f)and the efficiency of L/D cycle enhancement(?)were introduced to evaluate the improvement of the mixing performance of the reactor.One discrete inclined rib could form one longitudinal vortex,and the multi-longitudinal would promote the reciprocating movements of algal cells in the lighting direction and significantly improve f.The results showed that as the rib arrangement angle increases,f increases monotonously,while?first increases and then decreases.Taking into account the evaluation indices of f and?,?=30°was supposed to be the best angle for rib arrangement.Secondly,the blade-type static mixer,which could also induce multi-longitudinal vortex,was introuced to the tubular photobioreactor.The effect of the blade-type static mixer on the mixing performance of the reactor was compared wit h that of the SK type,spiral type and discrete inclined rib static mixer.The results showed that the traditional SK type and spiral type static mixer generated swirl to promote the mixing of algae,while the discrete inclined rib mixer and blade-type static mixer induced multi-longitudinal vortex to promote the mixing of algae,which could improve f without significantly increasing energy consumption,demonstrating the superiority of the multi-longitudinal vortex structure.On the one hand,the reactor with the blade type static mixer had a higher f compared with the one with the discrete inclined ribs,and the process manufacture of the blade type static mixer was simpler than that of the discrete inclined ribs.Therefore,the blade type static mixer was the best type among the four mixer types studied in this thesis.Finally,four main structural parameters(the number of blades N,central angle?,inclination angle?and the pitch p)of the blade-type static mixer have been optimized.Based on the idea of synergy,the optimal number of blades N=3 was determined according to the relative locations of the vortex cores to that of the L/D boundary.The orthogonal test was used to optimize the blade structure further.It was found that,when f was taken as the test index,the optimal structural parameters were?=60°,?=45°and p=40mm.With this combination,f was 2.19 Hz,which is the highest of 25tests;When?was taken as the test index,the optimal structural parameters were?=20°,?=15°and p=80 mm,and the corresponding highest?was 5.45.