Effects of Multiple Reflections and Refractions at Interfaces and Water Temperature on Disinfection Efficacy in UV Reactors

Computational Fluid Dynamic (CFD) simulation plays an important role in the prediction of Ultraviolet (UV) disinfection performance in UV disinfection systems. Simulation of efficacy of UV water disinfection requires the specific setup of parameters at the interfaces between air, quartz and water in the photoreactors. The reflections and refractions at the air-quartz and quartz-water interfaces will directly impact the simulation results of the UV radiation field in the water domain. The simulation results here have demonstrated that over 35% difference had been found whether considering reflections and refractions at the above mentioned interfaces or neglecting them. Six models have been created for comparison, which are the model including air annulus and quartz sleeve (named as all include model ), the model including air annulus only (named as air only model ), the model neglecting both air annulus and quartz sleeve (named as all exclude model ), the model combining air and quartz sleeve into quartz (named as quartz only model ), the models changing air annular thickness or quartz sleeve thickness to 0.5 mm (named as all include 0.5mm air layer model” and all include 0.5 quartz layer m odel ). These six models have been applied in simulations of a typical annular single-lamp UV disinfection photoreactor by Discrete ordinate (DO) Model in ANSYS FLUENT commercial package. This study has investigated the impact of the reflections and refractions resulting from the different refractive indices at air- quartz and quartz-water interfaces. A mathematical model for multiple reflections and total reflections in the air annulus and quartz sleeve has been developed in this work. The results have demonstrated that the multiple reflections and refractions result in significant impact on the UV intensity distribution field in the longitude direction and at the end of the photoreactor. Another contribution of this thesis is the experimental investigation of the effect of water temperature on the UV output efficiency in photoreactors. Low pressure UV lamp had been widely applied in drinking water, wastewater disinfection and food processing because it emits wavelength at mostly 254 nm UV and owns high UVC output efficiency. UVC output efficiency of low pressure UV lamps is seriously impacted by the temperature of lamp surface that is related to the water temperature in the system. This study has established the relationship between the low pressure UV lamp surface temperature and the UVC output efficiency through experiments. Such results will provide useful guidance for UV disinfection industries to design low pressure UV lamps with optimal UVC output efficiency according to water temperature