Hydrodynamics of a Large-scale Mixed-Cell Raceway (MCR): Experimental studies

Abstract

The Mixed-cell Raceway (MCR) is a design that intends to combine the best characteristics of circular tanks and linear raceways in a single production system. The conceptual idea is to convert traditional linear raceways into a series of hydraulically separated cells, each of which behaves as an individual circular tank. The MCR can take advantage of the solids removal ability of circular tanks and can be managed as either a partial reuse or intensive recirculation system. This study investigated the hydrodynamics of a large-scale (90m3) MCR composed of three in-series 5.5m×5.5m mixed-cells (∼1m water depth). Water velocity measurements of the entire tank were used to generate velocity-magnitude contours and vector plots, investigate the distribution of water velocities, and evaluate the self-cleaning characteristics and related management issues of an MCR.The grand mean of the water velocities of the three MCR in-series cells was 16.5cm/s (16.1, 15.5, and 17.8cm/s for cell 1, cell 2, and cell 3, respectively). Results showed that water velocities decreased somewhat in a linear manner from the tank bottom to the top, i.e., 18.9, 15.8 and 14.7cm/s, and in the same way from the periphery (21.9cm/s) to the center of the cells (3.7cm/s). Analyses indicated that these water velocities were in the optimum range to promote fish health and condition as well as to achieve tank self-cleaning. For a water exchange rate of 1.7 volumes per hour and an operating head of 1.36m in the jet port manifolds, the power requirements of the MCR reached 8.9W/m3. Contour and vector velocity plots showed that the mixed-cells develop a well-defined rotational pattern around the center drain. Also, strategically located water jets directed across the width of the MCR were able to limit the rotational flow to each cell and create the required counter-rotational pattern between adjacent cells. Velocity vectors showed a relatively low turbulence in the corners of the cells, even in the middle cell (cell 2) that had solid-walls on only two sides. Velocity vectors and contour plots also suggested an absence of dead volumes or short-circuiting within the cells, indicating that adequate mixing was being attained in the MCR.