Visualization and quantification of hydrodynamics and dose in UV reactors by 3D laser-induced fluorescence

A novel technique for the spatial and temporal assessment of the hydrodynamics and the UV dose delivered and the link between these two factors in a lab-scale UV reactor using three dimensional laser induced fluorescence (3DLIF) is developed in this study. This tool can also be utilized for the optimization of UV reactors and to provide data for validation of CFD-based simulation techniques.;3DLIF allows real time characterization of mixing conditions in a small-scale UV disinfection reactor by capturing fluorescence images emitted from a laser dye, Rhodamine 6G, using a high speed CCD camera. The 3DLIF system analyzed the hydrodynamics and dose delivery in a single lamp UV reactor placed perpendicular to flow. In addition to three-dimensional mixing, the technique successfully visualized the two-dimensional, transient mixing behaviors and dose delivery within the reactor, which has not been possible with traditional tracer test techniques.;Conservative tracer tests revealed unsteady turbulent flow characteristics such as the recirculation zone and the von Karman vortex street that are normally observed in flows around cylinders. Paths above the cylinder center, characterized by higher velocities and less influence of the cylinder, were also analyzed. The results demonstrated that a major difference between the paths at and above the cylinder center was the decreasing amount of dye entering the recirculation zone, which has a higher residence time, as the injection height increased. The results also suggested that a longer approach length was beneficial to increase the probability of microbes entering the region around the lamp sleeve irrespective of their entrance height into the reactor. This is especially important when elbows are placed upstream of the UV reactor. Lastly, the length of the outlet i.e., distance from the last lamp array to the reactor exit, was examined as mixing at the outlet was determined to drastically vary over time with an increase in injection height. A well-mixed outlet, i.e., when the concentration profile across the reactor height is within 10% as in the center injection case, would be desirable to improve the accuracy of the biodosimetry results. These inlet and outlet analyses were the first of its kind and aid in the optimization of the reactor design.;Furthermore, evaluating the performance of typical water treatment UV reactors is challenging due to the complexity in assessing spatial and temporal variation of UV fluence, resulting from highly unsteady, turbulent nature of flow and variation in UV intensity. Mapping the spatial and temporal fluence delivery and MS2 inactivation revealed the highest local fluence in the wake zone due to longer residence time and higher UV exposure while the lowest local fluence in a region near the walls due to short-circuiting flow and lower UV fluence rate. The location of tracer injection, varying the height and upstream distance from the lamp center, was found to significantly affect the UV fluence received by the tracer. A Lagrangian-based analysis was also employed to predict the fluence along specific paths of travel, which agreed with the experiments. The 3DLIF technique developed in this study provides new insight on dose delivery that fluctuates both spatially and temporally and is expected to aid design and optimization of UV reactors as well as validate computational fluid dynamics models that are widely used to simulate UV reactor performances.;Finally, the correlation between the hydrodynamics and cumulative local fluence delivery in the reactor was determined using a technique called the proper orthogonal decomposition (POD). Since this method is mainly applied to velocity and vorticity fields, the decomposition of the spatial concentration data, obtained using laser-induced fluorescence (LIF), for the flow around a confined circular cylinder (Re = 4,900) was compared with their corresponding velocity and vorticity decompositions to validate the use of POD on concentration data. The velocity, vorticity and concentration modes identify the dominating coherent structures, i.e. shear layer roll-up, instantaneous recirculation and von Karman vortex shedding. In the velocity and vorticity decomposition, instantaneous recirculation and von Karman vortex shedding are represented in the highest energy modes, modes 1 and 2. However, in the concentration decomposition, mode 1 represents regions with strong concentration intermittency, i.e., where no dye was present in alternative cycles, while mode 2 represents the coherent structures, which are identified in modes 1 and 2 in the velocity and vorticity decomposition. The concentration based decomposition identifies the shear layer around the cylinder that was only partially captured by their vorticity counterpart. This validated decomposition method was then applied to the fluence results revealing similar coherent structures showing the dependency of fluence delivery on the reactor flow.