Numerical simulation of sterilizing efficiency of ultraviolet disinfection reactors with different flow baffle types

Abstract: Nutrient solution recycling has become one of the essential techniques of soilless cultivation. But nutrient solutions are susceptible to be polluted by infectious diseases during the recycling process, so it is needed to disinfect nutrient solutions before recycling use. Compared to other disinfection methods of common nutrient solutions, ultraviolet (UV) disinfection has many advantages, such as high efficiency, low cost, not changing the physical and chemical properties of nutrient solutions, so UV disinfection is an environment-friendly technology of nutrient solution treatment．In general, experimental research and numerical simulation are the most common methods of UV disinfection. Although experimental results of the performance of UV disinfection reactor are credible, direct measurement is difficult and costly and thus seldom done. On the other hand, one can use numerical simulation techniques to model the UV disinfection. Computational fluid dynamics (CFD) has been widely used for simulating the UV disinfection. In previous studies, some researchers analyzed the performance of small-scale horizontal UV disinfection reactors with different flow baffle numbers and flow areas. They designed an alternately arranged flat-type flow baffle and analyzed the performance of the UV disinfection device for nutrient solutions using the CFD simulations and the measurements of biological bacterial disinfection. However, the effects of different flow baffle types on the performance of the UV disinfection reactor have seldom been investigated. In this study, we designed 5 different flow baffle types of the UV disinfection reactor. The main objectives were to increase disinfection efficiency by optimizing the flow baffle arrangement and to test the performance of the UV disinfection reactor with different flow baffle types. The 5 different flow baffle types were proposed: 2 circular channels and annulus alternation, 2 circular channels, 4 circular channels and annulus alternation, staggered arrangement of 4 circular channels and 4 circular channels, and 4 circular channels. Using the discrete phase model (DPM) of the software ANSYS Fluent 14.0, we simulated the motion of Tomato Fusarium oxysporum particle in nutrient solution in the UV disinfection reactors with different flow baffle types, and the inlet flow rate was set to 900 L/h. The UV irradiation dose was calculated and compared for different flow baffle types using the radial radiation model (RRM). The result indicated that for the flow baffle type with 4 circular channels, the lowest and average radiation doses received by the Tomato Fusarium oxysporum particle increased by 33.4% and 4.2% respectively, and the coefficient of variation (CV) decreased by 0.07, compared to the original type. Compared to other flow baffle types, the lowest radiation dose and the CV value for the type with 4 circular channels were respectively significantly greater and less. These results suggested that the UV disinfection reactor of the flow baffle type with 4 circular channels was the best scheme. Furthermore, the simulation results suggested that the combined baffle arrangement had a higher disinfection effect than the single baffle for 2 circular channels, but the combined baffle arrangement had a lower disinfection effect than the single baffle for 4 circular channels. Therefore, these results can provide a theoretical basis for optimizing the internal structure and enhancing the performance of the UV disinfection reactor.