Elsevier

Water Research

Volume 45, Issue 13, July 2011, Pages 3855-3862
Water Research

UV reactor flow visualization and mixing quantification using three-dimensional laser-induced fluorescence

https://doi.org/10.1016/j.watres.2011.04.041Get rights and content

Abstract

Three-dimensional laser-induced fluorescence (3DLIF) was applied to visualize and quantitatively analyze mixing in a lab-scale UV reactor consisting of one lamp sleeve placed perpendicular to flow. The recirculation zone and the von Karman vortex shedding that commonly occur in flows around bluff bodies were successfully visualized. Multiple flow paths were analyzed by injecting the dye at various heights with respect to the lamp sleeve. A major difference in these pathways was the amount of dye that traveled close to the sleeve, i.e., a zone of higher residence time and higher UV exposure. Paths away from the center height had higher velocities and hence minimal influence by the presence of sleeve. Approach length was also characterized in order to increase the probability of microbes entering the region around the UV lamp. The 3DLIF technique developed in this study is expected to provide new insight on UV dose delivery useful for the design and optimization of UV reactors.

Highlights

► Flow in a lab-scale UV reactor was visualized for the first time using 3DLIF. ► Mixing in the UV reactor was highly unsteady not only spatially but also temporally. ► Flow past a UV lamp was characterized by the presence of recirculation zone and vortex shedding. ► Trajectory of tracer strongly depended on upstream point of entry. ► Sufficient downstream length would be required to obtain homogenous water sample at the outlet.

Introduction

UV disinfection has been gaining popularity in drinking water treatment over the past decade due to the discovery of the efficient inactivation of Cryptosporidium parvum oocysts and Giardia lamblia cysts at relatively low doses (Clancy et al., 1998, Linden et al., 2002) with much less concern on the formation of disinfection by-products as compared to chemical disinfectants (Bellar et al., 1974, Glaze et al., 1993). However, a spatial and temporal assessment of the UV dose delivered and the reactor performance has been severely limited for utilities practicing UV disinfection. The commonly used validation method, biodosimetry, treats the UV reactor as a “black-box” and hence cannot account for the dependence of the dose delivery on the complex hydrodynamics and the spatial variation in UV intensity. Development of proper reactor design would be ideally pursued through an understanding of the fluid behavior that determines how microorganisms accumulate UV dose as they spend varying amounts of time in regions of fluctuating light intensity (Lawryshyn and Cairns, 2003).

The unsteadiness of flow in UV reactors arises mainly due to the placement of cylindrical UV lamps perpendicular to flow that leads to separation forming unsteady large-scale vortices and consequently significant fluid mixing (Williamson, 1996, Zdravkovich, 1997). These so called von Karman vortices (repeating pattern of alternating vortices produced downstream of a bluff body) compound the complexity and unsteadiness in the flow region behind the lamp, as they involve the interactions of three shear layers, i.e., a boundary layer around the sleeve, a separating free shear layer, and a highly turbulent wake (Williamson, 1996). The presence of multiple lamps in a staggered configuration further complicates the flow, rendering the prediction of unsteady hydrodynamics significantly difficult. Furthermore, inlet configuration (Sozzi and Taghipour, 2006), upstream pipe bends (Zhao et al., 2009), and the presence of modifications such as baffles (Blatchley et al., 1998, Wols et al., 2010a), rings (Janex et al., 1998), or “wave-like” walls (Chiu et al., 1999a) have been found to significantly affect the hydrodynamics and reactor performance.

Due to the complexity of the flow, computational fluid dynamics (CFD) has been increasingly used to model the hydrodynamics in UV reactors based on the time-averaged Reynolds Averaged Navier Stokes (RANS) approaches (Sozzi and Taghipour, 2006, Alpert et al., 2010, Wols et al., 2010b). However, the velocity distributions in the RANS simulations have been found to differ from the experimental results typically obtained using particle image velocimetry (PIV). Alpert et al. (2010) and Wols et al. (2010b) determined that RANS simulations under-predicted the flow complexity in dynamic wake regions and dead zones due to the poor capture of large vortices and turbulent motions. These studies concluded that resolving the unsteady turbulent motions is essential to provide an accurate representation of the microorganism trajectories and more significantly the UV dose received by each microbe.

Dose distributions consist of both spatial and temporal components. While many past studies have focused on the former, the authors are unaware of any experimental or computational studies that considered the temporal component in the reactor. In this study, a three-dimensional laser-induced fluorescence (3DLIF) (Tian and Roberts, 2003) was applied for the first time to examine the hydrodynamics in a lab-scale model UV reactor both spatially and temporally. In the 3DLIF system, a planar monochromatic laser sheet is created and scanned across the width of the reactor to obtain three-dimensional images. The laser causes a tracer dye to fluoresce which is captured by a high-speed CCD camera (Guiraud et al., 1991). Using this technique, instantaneous, 2D and 3D mixing characteristics in a model UV reactor were visualized and quantitatively examined.

Section snippets

3DLIF system

Detailed information regarding the 3DLIF system used in this study is given in Kim et al. (2010). Briefly, a laser beam generated by an argon ion laser (Innova 90, Coherent®, Palo Alto, CA) at a wavelength of 514 nm and an intensity of 1.5 W was used to excite a fluorescent dye tracer, Rhodamine 6G (Sigma–Aldrich, St. Louis, MO). The laser beam was first directed toward a mirror which oscillated vertically at high frequency to produce a 2D laser sheet that passed through the center width of a

Flow visualization using 3DLIF

Fig. 2 shows how three-dimensional flows in a UV reactor can be visualized using 3DLIF at a high resolution (corresponding to millions of sampling points), which is not possible with traditional dye tracer test techniques. These images were obtained from a 3DLIF experiment performed with dye injected at the point in the center of the y-z plane. The region from the sleeve to the outlet is dissected in the stream-wise (x-z plane), span-wise (x-y plane), and cross-stream (y-z plane) directions.

Conclusions

This study applied the 3DLIF technique for the first time to visualize and quantitatively analyze the flow across a UV lamp in a model reactor used for drinking water treatment. In addition to three-dimensional mixing, the technique successfully visualized the two-dimensional, transient mixing behaviors within the reactor, which has not been possible with traditional tracer test techniques. It is also noteworthy that the 3DLIF technique is non-intrusive, i.e., there is no disturbance in the

Acknowledgments

This research was partially funded by Water Research Foundation (Project No. 4134) and Korea Water Resources Corporation (Kwater).

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