CFD-based process optimization of a dissolved air flotation system for drinking water production

Dissolved air flotation (DAF) has received more attention recently as a separation technique in both drinking water as well as wastewater treatment. However, the process as well as the preceding flocculation step is complex and not completely understood. Given the multiphase nature of the process, fluid dynamics studies are important to understand and optimize the DAF system in terms of operation and design. The present study is intended towards a comprehensive computational analysis for design optimization of the treatment plant in Kluizen, Belgium. Setting up the modelling framework involving the multiphase flow problem is briefly discussed. 3D numerical simulations on a scaled down model of the DAF design were analysed. The flow features give better confidence, but the flocs escape through the outlet still prevails which is averse to the system performance. In order to improve the performance and ease of maintenance, design modifications have been proposed by using a perforated tube for water extraction and are found to be satisfactory. The discussion is further reinforced through validating the numerical model against the experimental findings for stratified flow conditions

Studies on convective cooling of cryogenic fluids towards superconducting applications

peer reviewedTo understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nuave). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nuave ~ Ra^0.374. Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results.ARC 11/16-0

Study of buoyancy driven heat transport in silicone oils and in liquid nitrogen in view of cooling applications

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural convection. In one recent experimental investigation (Dubois et al., 2016) carried on silicone oils and liquid nitrogen, we have demonstrated that the velocity field satisfies specific scaling laws with respect to the temperature increase in the liquid pool. In this work, we pursue the analysis by modeling the heat transfer in a parallelepiped enclosure for a steady laminar flow regime. The Navier-Stokes equations are solved using a finite volume approach to obtain the detailed three-dimensional flow and heat transfer characteristics. A quantitative analysis of the velocity field over the temperature field shows that the experimental power laws are reproduced in simulations. Following Dubois and Berge (1978), a theoretical law originally introduced in the context of the classical Rayleigh-Bénard experiment is shown to be satisfied in the simulations over a wide range of Rayleigh numbers (Ra), assuming the definition of the characteristic convection length is adapted to the investigated geometry. Moreover, the simulations are shown to correctly reproduce the main features of the flow, including the characteristic convection length, for different heater lengths.ARC 11/16-0

Studies of heat transport in cryogenic fluids toward superconducting applications

To meet the increasing global energy demand with minimal green house effect, electrical energy is a viable option. But, it requires an efficient electrical power system with optimal loss. Towards this, superconducting materials represent a promising way of improving the existing systems. However, the efficiency may be affected by a small temperature increase due to electromagnetic losses induced by time-varying magnetic fields. To minimize this, two approaches can be considered (i) reduce the losses and (ii) improve the heat exchange with the cooling environment. Although experimental and theoretical studies of the superconducting losses have been extensively pursued [1], few attempts have been made to combine the 'losses' and the 'cooling' aspects in the current scenario. Such studies require a detailed characterization of magneto-thermal interaction of the superconductors, as well as fundamental aspects of heat draining by the cryogenic fluid. Towards this, numerical studies on natural convection in an enclosed cavity, interacting with a constant heat source (heater) are carried out to understand the cooling efficiency of the cryogenic fluids (viz., liquid nitrogen, silicon oil and water). Natural convection heat transfer in an enclosure continues to be an active research area, due to its significance for both fundamental interests and engineering applications. Majority of the published studies can be classified into two groups: enclosure heated from below and cooled from above (Rayleigh–Bénard problem) and differentially heated enclosures [2]. Considerable attention has been given to natural convection from vertical/horizontal enclosures specified either with constant temperature or heat flux, while limited studies has been done with constant volumetric heat source [3]. The resulting characteristics of fluid flow and heat transfer are quite interesting and deserve a detailed investigation, which motivates for the present work. As a priori, three dimensional steady forms of incompressible Navier-Stokes equations are solved through finite volume approach [4]. Detailed flow features are presented in terms of velocity, temperature and path line profiles for different heater lengths. Comparative analyses are further reinforced in terms of average Nusselt number (Nu) for different fluids. It shows that, effective heat transfer enhancement occurs for liquid nitrogen with the shortest heater length, resulting in an efficient cooling. The profile of temperature and velocity magnitude distributions agrees well with the in-house experiment. Benchmark validation is found to be satisfactorily against the reported result [5]. Complete descriptions on geometrical details, governing equations, boundary conditions and solution methodology adopted for the numerical solution will be provided in full length paper. This study will be a gateway for further magneto-thermal analysis in an electromagnetic environment.ARC 11/16-0