Mixing (process engineering)

Résumé

In industrial process engineering, mixing is a unit operation that involves manipulation of a heterogeneous physical system with the intent to make it more homogeneous. Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, and the stirring of pancake batter to eliminate lumps (deagglomeration). Mixing is performed to allow heat and/or mass transfer to occur between one or more streams, components or phases. Modern industrial processing almost always involves some form of mixing. Some classes of chemical reactors are also mixers. With the right equipment, it is possible to mix a solid, liquid or gas into another solid, liquid or gas. A biofuel fermenter may require the mixing of microbes, gases and liquid medium for optimal yield; organic nitration requires concentrated (liquid) nitric and sulfuric acids to be mixed with a hydrophobic organic phase; production of pharmaceutical tablets requires blending of solid powders. The opposite

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https://en.wikipedia.org/wiki/Mixing_(process_engineering)

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Contained density currents with high volume of release reflect against the boundaries of the reception environment commonly leading to oscillatory flow. These flows exist in sediment clarifiers, compromising their operations, and deposited signatures of contained turbidity currents are found as part of the infill of sedimentary basins; for operation of the former and interpretation of the latter it is essential to understand the dynamic processes of these flows. Six lock-exchange experiments with different initial densities were made in a horizontal flume, where the volume of the saline mixture in the lock was equivalent to the volume of the ambient fluid. A further two tests, with a repeated initial density, were made: one with high volume of release and very long duration; and another with low volume of release. Firstly, the movement of the current is discussed, including the oscillations within the experimental tank involving the density current and an upper layer counter-current. It is shown that the cyclic behaviour is self-similar with the reduced gravity of the initial density in the lock. Secondly, entrainment and water mixing processes are characterized. The time evolution of mixing is characterized qualitatively by analysing the background potential energy of the density distributions to show that mixing occurs even in the earlier stages of the current, and mainly within the first cycle of the oscillation. Quantified analysis reveals that, in currents with high volume of release, entrainment discharge is one order of magnitude higher, mainly due to the larger interface between the ambient fluid and the current. A model for the evolution of the mixing process is proposed for density currents with high volume of release. Finally, the dynamics of the head of the current is analysed. The entrainment in the head, when compared to the entrainment in the remainder body of the flows, is less important for the configuration with a larger lock.

Wiley-Blackwell2016

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CFD Simulations Of Pollution Control Systems: DPF and SCR

Peter Loveday

New EU and US emissions regulations require the development of advanced after- treatment systems for non-road diesel engine exhausts. To help develop these at Liebherr Machines Bulle, the use of CFD has been adopted. The pro ject used two main models: one for the dosing of fuel for the regeneration of the Diesel Particulate Filter (HC Doser model) and one for the Selective Cataylitic Reduction system (SCR model). The HC Doser model was validated against published data and in-house experimental results. Good agreement was found in all cases. The SCR model was validated against published data only, showing good qualitative agreement -spatial distribution and spray formation- but the quantitative measures - N H3 and H N N C O concentrations- still need further adjustment. Validation against in-house experimental results was not possible as the necessary experiments have not yet been carried out. After validation, the DPF model was used to evaluate and improve specfic exhaust geometry configurations at LMB. Selected case studies are presented to show the appli- cability of the models to the design process, and the derived advantages. Using the DPF model, basic research was undertaken to investigate the ifluence of the static mixer's geometry on the performance of the system during regeneration. A new mixer geometry was designed which reduced the pressure drop and increased the uniformity of the HC distribution

2011

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Image analysis technique applied to lock-exchange gravity currents

Claudia Adduce, Helena Isabel Dos Santos Nogueira

An image analysis technique is used to estimate the two-dimensional instantaneous density field of unsteady gravity currents produced by full-depth lock-release of saline water. An experiment reproducing a gravity current was performed in a 3.0 m long, 0.20 m wide and 0.30 m deep Perspex flume with horizontal smooth bed and recorded with a 25 Hz CCD video camera under controlled light conditions. Using dye concentration as a tracer, a calibration procedure was established for each pixel in the image relating the amount of dye uniformly distributed in the tank and the greyscale values in the corresponding images. The results are evaluated and corrected by applying the mass conservation principle within the experimental tank. The procedure is a simple way to assess the time-varying density distribution within the gravity current, allowing the investigation of gravity current dynamics and mixing processes.

Institute of Physics2013

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