Destratification by Mechanical Mixers: Mixing Efficiency and Flow Scaling D. F. Hill 1 ; A. M. Vergara 2 ; and E. J. Parra 3 Abstract: Experiments have been performed to assess the efficiency of mechanical mixers in destratifying water bodies. The effects of Reynolds number, physical scale, and Richardson number were all considered. The results demonstrated that the variation of mixing efficiency with Richardson number was well described by two different power-law regimes. At low Richardson numbers, the efficiency increased with increasing Richardson number. Above a critical value, however, the efficiency decreased rapidly with further increases in Richardson number. The experiments further showed that, over the range of Reynolds numbers considered, the results collapsed fairly well. Finally, experiments at different physical scales showed reasonable agreement. The main conclusion of the present technical note is that the destratification efficiency of mechanical mixers can be well parameterized by an overall Richardson number. The results, however, are specific to the particular geometric configuration studied. It is hoped that the present technical note will help to guide future studies of destratification systems in small lakes and reservoirs and fluid mixing in the chemical and process industries. DOI: 10.1061/ASCE0733-94292008134:121772 CE Database subject headings: Stratified flow; Reservoirs; Stratification; Water quality; Turbulence; Mixing. Introduction Mixing processes are of interest in hydraulic engineering as they redistribute tracers including heat, contaminants and nutrients, and fine sediments. Given the large length and velocity scales typical of hydraulic flows, turbulent diffusion is the primary mechanism for this redistribution. The presence of vertical den- sity gradients, or stratification, has a strong impact on mixing in turbulent flows. Excellent reviews on the topic are provided by Linden 1979 and Fernando 1991. Conceptually, stratification serves to reduce, or completely suppress, turbulent mixing as a portion of the turbulent kinetic energy must be expended in over- coming gravitational effects. Stratification in water bodies is a matter of significant practical importance. Stephens and Imberger 1993 discuss how stratifica- tion, often driven by strong solar heating of near-surface waters in the summer, limits the transport of oxygen to hypolimnetic wa- ters. This, in turn, will lead to anoxic conditions in these lower waters. There are two popular methods for destratifying water bodies—bubble plumes and mechanical mixers. Antenucci et al. 2003 point out that bubble plumes have the advantages of 1 being able to cover a greater horizontal extent and 2 having shore-based mechanical parts. Mechanical mixers, on the other hand, have the advantage of being able to directly access and entrain oxygen-rich surface waters. Regarding mechanical mixers, there have been numerous labo- ratory and field studies in the past that have assessed their perfor- mance and have considered how to predict prototype performance Busnaina et al. 1981; Robinson et al. 1982; Vandermeulen 1992; Stephens and Imberger 1993; Kirke and Gezawy 1997; Milstein et al. 2001. The work of Stephens and Imberger 1993 is the most relevant to the present technical note as it represents the first systematic and thorough laboratory investigation of the destratifi- cation by mechanical mixers process. Their experiments consid- ered both two-layer and linear density profiles and their results were parameterized by an “impeller number.” Maximum mixing efficiencies were found to compare favorably with bubble plume destratification systems. The goal of the present technical note is to more thoroughly investigate the mixing efficiency of impeller-driven turbulence in stratified flows. The objectives are to understand the effects of the overall Richardson number, the Reynolds number, and the physi- cal scale of the experiment. To help isolate these relationships, the present study is restricted to a single geometrical configuration. It is recognized that the obtained results are specific to the particular configuration being studied. Of greatest interest, therefore, are the general trends and the identification of the relative importance of the above-mentioned parameters. Experimental Facilities and Procedures The present experiments were carried out in a series of unbaffled cylindrical tanks, a schematic of which is shown in Fig. 1a. The inner diameters D T  of the tanks were 29.1, 44.3, and 59.7 cm. Two-layer stratification in the tanks was created by first filling the tank with fresh water to a depth of D T / 2. Then, using a bottom- mounted diffuser plate, salt water was introduced until the total fluid depth was equal to D T . Typically, the interface between the 1 Associate Professor of Civil Engineering, Dept. of Civil and Environmental Engineering, Pennsylvania State Univ., 212 Sackett, Blvd., University Park, PA 16802. E-mail: dfh4@psu.edu 2 Student, Dept. of Civil and Environmental Engineering, Pennsylvania State Univ., University Park, PA 16802. E-mail: amv5003@ psu.edu 3 Project Engineer, Camp, Dresser, and McKee, 3050 Post Oak Blvd., Ste. 300, Houston, TX 77056. E-mail: parraej@cdm.com Note. Discussion open until May 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this technical note was submitted for review and possible publication on October 11, 2007; approved on April 2, 2008. This technical note is part of the Journal of Hydraulic Engineering, Vol. 134, No. 12, December 1, 2008. ©ASCE, ISSN 0733-9429/2008/12-1772–1777/$25.00. 1772 / JOURNAL OF HYDRAULIC ENGINEERING © ASCE / DECEMBER 2008 Downloaded 01 Dec 2008 to 130.203.206.163. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright