Showing papers on "Complete mixing published in 2012"

Numerical modelling of supercritical submerged water jets in a subcritical co-flow

Abstract: Among the different applications of supercritical fluids, the use of water submerged jets is common in different processes such as the production of nanoparticles or combustion applications. This paper presents a computational methodology for the modelling and simulation of subcritical and supercritical injection of water in a subcritical co-flow. In this work, we focus on numerical simulation of the process, emphasizing mixing modelling. Subcritical to supercritical submerged water jets in a subcritical co-flow are analysed and validated against experimental data with good accuracy (below 5% of absolute average deviation). The main results show that mixing zones are longer at subcritical temperatures than well above the critical point. Besides, due to the low dissipation rate found, the system is poorly micromixed and complete mixing at the molecular scale is never achieved. A methodology based on the evaluation of a macro-mixing length has been used. Using this method, the presence and location of mixing zones are better identified, showing that when pressure is well above the critical point, fluid-dynamic behaviour is more similar to subcritical conditions. Implications of these results are commented and analysed.

Study of Active Micromixer Driven by Electrothermal Force

Abstract: Biochemical applications of microchips often require a rapid mixing of different fluid samples. At the microscale level, fluid flow is usually a highly ordered laminar flow and diffusion is the primary mechanism for mixing owing to the lack of disturbances, yielding inefficiency for practical biochemical analysis. In this work, we design a prototype active micromixer by employing the electrothermal effect. We apply to the flow microchannel a non-uniform AC electric field, which can generate an electrothermal force on the fluid flow and induce vortex pairs for enhancing mixing efficiency. The performance of this active micromixer is studied and compared, under the same mixing quality, with that of a conventional passive micromixer of the same size with obstacles in the flow channel by three-dimensional finite element simulations. The numerical results show that the pressure drop between the inlet and the outlet for the active micromixer is much less than (only 3000th) that for the passive micro-mixer with the same mixing quality. To obtain an optimal mixing quality, we have systematically studied the mixing quality by varying the geometrical arrangements of the electrodes. An almost complete mixing can be obtained using a specific design. Moreover, the temperature increases around the electrodes are lower than 3 K, which does not adversely affect the biochemical analysis. It is suggested that the prototype active micromixer designed is promising and effective and useful for biochemical analysis.

Confocal Microscopic Evaluation of Mixing Performance for Three-Dimensional Microfluidic Mixer

Abstract: We developed a confocal microscopic method for a quantitative evaluation of the mixing performance of a three-dimensional microfluidic mixer. We fabricated a microfluidic baker’s transformation (MBT) mixer as a three-dimensional passive-type mixer for the efficient mixing of solutions. Although the MBT mixer is one type of ideal mixers, it is hard to evaluate its mixing performance, since the MBT mixer is based on several cycles of complicated three-dimensional microchannel structures. We applied the method developed here to evaluate the mixing of water and a fluorescein isothiocyanate (FITC; diffusion coefficient, 4.9 × 10−10 m2 s−1) solution by the MBT mixer. This method enables us to capture vertical section images for the fluid distributions of FITC and water at different three-dimensional microchannel structures of the MBT device. These images are in good agreement with those of mixing images based on numerical simulations. The mixing ratio could be calculated by the fluorescence intensity at each pixel of the vertical section image; complete mixing is recognized by a mixing ratio of more than 90%. The mixing ratios are measured at different cycles of the MBT mixer by changing the flow rate; the mixing performance is evaluated by comparisons with the mixing ratio of the straight microchannel without the MBT mixer.

The influences of typhoon‐induced mixing in a shallow lake

Abstract: Vertical mixing affects the vertical transport of biochemical materials and modifies the characteristics of basin-scale internal waves in subtropical, subalpine Yang Yuan Lake (YYL), in the north central region of Taiwan. Vertical mixing in YYL is generally caused by typhoons (strong winds and heavy rainfalls) during the spring, summer and early fall, or by cooling of the water column during late fall and winter. Vertical mixing caused by typhoons significantly affects internal thermal dynamics and biochemical processes, with basin-scale internal waves enhanced before complete mixing occurs, and temporal variations of water temperature dramatically changing after typhoon events. This study quantitatively determines when the complete mixing (defined as homogenous temperature in a water column) occurs on the basis of the balance between kinetic energy and potential energy and also shows what meteorological conditions contribute to the vertical mixing. Data on water column thermal profiles and meteorological variables were collected by a wireless, instrumented buoy in the deepest location of YYL and from a nearby meteorological station, from spring 2004 to summer 2006. This study also investigated how a physical process (internal waves) is affected by the mixing associated with typhoons. Signal processing and a two-layer model of the water column are used to understand the characteristics of basin-scale internal waves.

Method and an apparatus/universal combine for agitation of liquids

Abstract: A method of mixing together liquids or liquid/solid combinations, and mixing apparatus/universal combine utilizing a vertical spinning container or vessel having a rib, or a cross rib in its bottom wall. The container is spun about a vertical axis with no wobbling component to the motion. Meshed elements are used for high shear mixing. Start/stop routines, and variable acceleration/speed values are used, to facilitate complete mixing. Use of ‘impeller’ blade stirrers is completely eliminated.

Continuous mathematical models of airlift bioreactors: Families, affinity, diversity and modelling for single-substrate kinetics

Abstract: This paper presents a method of describing an airlift bioreactor, in which biodegradation of a carbonaceous substrate described by single-substrate kinetics takes place. Eight mathematical models based on the assumption of liquid plug flow and axial dispersion flow through the riser and the downcomer in the reactor were proposed. Additionally, the impact of degassing zone with assumed complete mixing on the obtained results was analyzed. Calculations were performed for two representative hydrodynamic regimes of reactor operation, i.e. with the presence of gas bubbles only within the riser and for complete gas circulation. The conclusions related to the apparatus design and process performance under sufficient aeration of the reaction mixture were drawn on the basis of the obtained results.

Process for production of polymer composition

Abstract: PROBLEM TO BE SOLVED: To provide a process for production of a polymer composition able to more effectively obtain the polymer composition with high quality.SOLUTION: The process for production of the polymer composition includes a first polymerization step of supplying a raw material monomer and a polymerization initiator through a supply port (11a) of a first complete mixing type reactor (10) to successively polymerize it in the first complete mixing type reactor (10) under thermal insulation conditions and extracting the resulting intermediate composition from an extraction port (11b) situated at the top of the first complete mixing type reactor (10), and a second polymerization step of supplying the intermediate composition through a supplying port (21a) of a second complete mixing type reactor (20) to further successively polymerize it in the second complete mixing type reactor (20) under thermal insulation conditions and extracting the resulting polymer composition from an effluent port (21b) situated at the top of the second complete mixing type reactor (20), wherein the first polymerization step is carried out at 125-170°C and the second polymerization step is carried out at 130-180°C.

Modelling Mixing and Particle Formation in Supercritical Antisolvent (SAS) Processes

Abstract: The fluid properties at supercritical conditions such as small thermal diffusivity and close-to-zero surface tension favour complete mixing between solution and antisolvent and thus facilitate the production of ultra-fine nano-sized particles with a narrow size distribution. The supercritical antisolvent (SAS) process exploits these properties by injecting a liquid jet with a solute into a fluid at supercritical conditions, usually CO2, that acts as antisolvent for the solute. The SAS process is an inherently complex system, that involves the interplay of equilibrium/nonequilibrium thermodynamics/transport properties of supercritical fluids with real gas effects as well as the hydrodynamics for turbulent mixing. In addition, the particle dynamics including nucleation and growth coupled with supercritical thermodynamic properties and mixing at molecular scale need to be carefully examined and modelled. Of all these processes, the mixing is pivotal in the SAS process. It is of primary importance for the formation of supersaturation conditions that lead to particle nucleation and thus controls the dynamics of particle production and evolution. A Large-Eddy Simulation (LES) method is used for the simulation of the flow and mixing fields. Compressibility effects cannot be neglected, and a compressible Navier-stokes formulation employing an AUSM + -up scheme needed to be implemented for the description of the compressible but low Mach number flows predominant in SAS processes. The Peng-Robinson equation of state is used to describe real gas effects, and the particle evolution and size distribution are modelled by the population balance equation using the method of moment approach. In LES only the large scale processes flow structures are spatially and temporally resolved. The small, sub-grid processes, in particular the micro-mixing that locally affects supersaturation and particle nucleation, and the interactions between turbulence and particle dynamics need to be modelled. We introduce a conditional moment closure (CMC) method as a sub-grid model to account for the unresolved effects of turbulence on particle nucleation and growth. CMC is a sub-grid model for non-premixed combustion [1] where chemical reaction is strongly dependent on micro-mixing, and the same modelling ideas can be used for the prediction of the nucleation term. Preliminary investigations of particle formation of paracetamol in the mixture of ethanol and CO2 at T=314K and P=16MPa have been carried out. Computations with varying mole fractions of CO2 using the equilibrium thermo-hydrodynamics assumption show good agreement with experimental results from [2]. The poster will present preliminary simulation results from a ternary system of paracetamol-ethanol-CO2, where ethanol and the solute are injected into the mixing chamber by a jet. The LES of the hydrodynamics is fully coupled with the solution of the transport equations of the conditionally averaged moments of the particle dynamics and size distribution. The effects of sub-grid micro-mixing (and essentially turbulence) on particle nucleation and growth can be quantified, and the results can be compared with earlier experiment by Bristow et. al. [2].

A new approach for complete mixing by transverse and streamwise flow motions in micro-channels

Abstract: Mixing control is an important issue in micro-fluid chip applications, such as μTAS (Micro-Total Analysis System) or LOC (Lab-on-Chip) because the flow at micro-scale is highly laminar. Several flow control schemes had been developed for complete mixing in the micro-channels in the past decades. However, most of the mixing control schemes are performed by utilizing specific excitation devices, such as electrokinetic, magnetic or pressure drivers. This paper investigates a new control scheme which is composed of a series of flow manipulation by changing the pressure at the two inlets of the micromixer as the external excitation. The fluids from two inlets are introduced to a square mixing chamber, which provides a space where the streamwise and transverse flow motions take place. The results show that the micromixer can be used to produce a large recirculation zone with series of small transverse fringes under external excitations. It is found that this new flow pattern enhances mixing processes at the micro-scale. A complete mixing can be achieved under appropriate flow control with the corresponding design.