Aeration performance of vortex flow regulators: Pilot scale experiments

P. Wójtowicz*, M. Szlachta*

* Faculty of Environmental Engineering, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland, e-mail: patryk.wojtowicz@pwr.edu.pl

Abstract: Vortex flow regulators (VFRs) are used in wastewater and storm water collection systems for flow throttling and control. Vortex regulators are not only very efficient energy dissipators but also atomizers, which are beneficial for sewer aeration. In this study, we present the experimental results and modelling of the aeration capacity of cylindrical type of vortex regulators (CVR). This study compares basic and optimised for flow throttling CVR with two new innovative designs – vortex regulators modified to enhance aeration performance with the minimum impact on the hydraulic performance. Pilot scale devices were examined and compared in a closed-circuit test rig. The oxygen mass transfer coefficient, standard oxygen transfer rate and standard aeration efficiency were determined for a wide range of tested geometrical configurations. We developed formulas which enable prediction of CVR aeration capacity.

Keywords: aeration; urban drainage system; flow throttling; atomization; pressure-swirl

Introduction Vortex flow regulators (VFR) are used in urban drainage systems as a replacement for traditional flow throttling devices in gravitational as well as pressurised systems. From a practical point of view - vortex regulators are compact, efficient in throttling and reliable. In order to efficiently throttle the flow, they do not reduce pipe cross-section, do not require power supply or moving parts. Vortex devices are fascinating with their simple design and straightforward operational principle contrasting with a highly complex vortex motion, and that has defined the complete solution for over 100 years now. The deficit of dissolved oxygen can be a problem in both natural waters and sewerage. Hydrodynamic regulators can boost oxygen concentration preventing putrefaction and improving treatment of stormwater and wastewater. Oxygen absorption is driven mainly by the atomisation of liquid discharged at a high speed by vortex regulator – a side effect of energy dissipation.

We investigated the aeration efficiency of semi-commercial scale cylindrical vortex flow regulators in order to determine the potential of their application in environmental engineering and to propose modification to enhance aeration capacity of basic designs. Different device geometries and arrangements of active outlets for both single and double discharge vortex regulators were tested. The experimental program included tests of three types of vortex regulators shown in Figure 1. The first type (CVR – Fig. 1a) are well-known, basic designs originally developed to maximise throttling performance. They discharge only through the bottom outlet. Using CVR type as a baseline, we proposed two new types with enhanced aeration capability. First modification was the addition of second outlet in the top plate (CVR-2). In the next modification (CVR-U), we were only using the top outlet, although the bottom outlet was open and used to drain device during low flows.

Material and Methods The semi-commercial scale models of hydrodynamic flow regulators were tested in a recirculating system. The absorption tests were performed in compliance with the

standard unsteady state clean water test procedure. The aeration efficiency was measured by means of the overall oxygen transfer coefficient (kLa20, h-1) adjusted to the standard conditions (20ºC, 101.325 kPa), standard oxygen transfer rate (SOTR, kgO2/h) and standard aeration efficiency (SAE, kgO2/kWh).

Results Flow rate vs. oxygen transfer coefficient and SOTR. We found that the oxygen transfer coefficient and oxygen transfer rate are in direct proportion to the flow rate (Figure 2a-2c). This result can be explained by increased turbulence and renewal rate of the liquid film available for absorption with increasing flow rate. Effect of active outlets: one bottom (CVR) vs. two active outlets (CVR-2): In type CVR-2 by adding second outlet we were expecting to double the aeration efficiency. Indeed, experiments proved that there was an increase, but not proportional to the number of active outlets. The oxygen transfer coefficient and standard oxygen transfer rate were improved on average by 15%. Better results were obtained for standard aeration efficiency, where the improvement was about 50%. The addition of second outlet divided the outflow between top and bottom one - this reduced the effective flow per area and the quality of atomisation. Also, the splashing and formation of air bubbles in downstream basin were decreased. On the other hand, the second spray cone was formed and contact time was longer. Of course, this effect was limited by the local saturation of droplets as the turbulence and consequently renewal rate of the liquid film contacting with air was relatively low. We think that the major effect, which limited the expected gain in aeration efficiency, was the quality of atomisation.

Effect of active outlets: one bottom (CVR) vs. one top outlet (CVR-U): Comparing two modes of operation with outlets located on the opposite sides (CVR vs. CVR-U) - the oxygen transfer coefficient and standard oxygen transfer rate were improved on average by 20% in favour of regulators discharging with the top outlet (CVR-U). The hydraulics was similar but in CVR-U type of regulators, the fountain-like discharge combined with inevitable gravity improved the disintegration of liquid film and increased the contact time of droplets. On the other hand, splashing and air entrainment in downstream basin was reduced, but the effect was not substantial.

Empirical formulas: We studied parameters that influence the aeration capacity of hydrodynamic flow regulators. Using all collected data, we derived empirical formulas describing their aeration efficiency. In our previous research, we also developed formulas for hydraulic parameters. By combining this set of equations we can predict throttling and aeration performance of any vortex flow regulator. Conclusions

The oxygen absorption in vortex regulators depends on several competing phenomena - the end result is always a superposition of these effects. We found that the best aeration efficiency can be obtained for regulator discharging through the top outlet only. Generally, the aeration capability of basic devices can be increased up to 20% with a negligible effect on throttling performance. The aeration efficiency can be increased even further by combining two chambers and adding more active outlets. The aeration performance is constant and on a par with mechanical aerators and diffuser systems.

Acknowledgments This investigation was supported by the Polish Ministry of Science and Higher Education grant N N523 450036.

Figures

a) b) c) Figure 1. Photos of three types of cylindrical vortex regulators tested – a) CVR, b) CVR-2 and c) CVR-U.

Figure 2a. Flow rate vs oxygen transfer coefficient for CVR type of vortex regulator

Figure 2b. Flow rate vs oxygen transfer coefficient for CVR-2 type of vortex regulator

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Figure 2c. Flow rate vs oxygen transfer coefficient for CVR-U type of vortex regulator

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