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Mixing Rate (mixing + rate)

Distribution by Scientific Domains

Chemistry	37%

Selected Abstracts

On the effect of the local turbulence scales on the mixing rate of diffusion flames: assessment of two different combustion models

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 10 2002
Jose Lopes
Abstract A mathematical model for the prediction of the turbulent flow, diffusion combustion process, heat transfer including thermal radiation and pollutants formation inside combustion chambers is described. In order to validate the model the results are compared herein against experimental data available in the open literature. The model comprises differential transport equations governing the above-mentioned phenomena, resulting from the mathematical and physical modelling, which are solved by the control volume formulation technique. The results yielded by the two different turbulent-mixing physical models used for combustion, the simple chemical reacting system (SCRS) and the eddy break-up (EBU), are analysed so that the need to make recourse to local turbulent scales to evaluate the reactants' mixing rate is assessed. Predictions are performed for a gaseous-fuelled combustor fired with two different burners that induce different aerodynamic conditions inside the combustion chamber. One of the burners has a typical geometry of that used in gaseous fired boilers,fuel firing in the centre surrounded by concentric oxidant firing,while the other burner introduces the air into the combustor through two different swirling concentric streams. Generally, the results exhibit a good agreement with the experimental values. Also, NO predictions are performed by a prompt-NO formation model used as a post-processor together with a thermal-NO formation model, the results being generally in good agreement with the experimental values. The predictions revealed that the mixture between the reactants occurred very close to the burner and almost instantaneously, that is, immediately after the fuel-containing eddies came into contact with the oxidant-containing eddies. As a result, away from the burner, the SCRS model, that assumes an infinitely fast mixing rate, appeared to be as accurate as the EBU model for the present predictions. Closer to the burner, the EBU model, that establishes the reactants mixing rate as a function of the local turbulent scales, yielded slightly slower rates of mixture, the fuel and oxidant concentrations which are slightly higher than those obtained with the SCRS model. As a consequence, the NO concentration predictions with the EBU combustion model are generally higher than those obtained with the SCRS model. This is due to the existence of higher concentrations of fuel and oxygen closer to the burner when predictions were performed taking into account the local turbulent scales in the mixing process of the reactants. The SCRS, being faster and as accurate as the EBU model in the predictions of combustion properties appears to be more appropriate. However, should NO be a variable that is predicted, then the EBU model becomes more appropriate. This is due to the better results of oxygen concentration yielded by that model, since it solves a transport equation for the oxidant concentration, which plays a dominant role in the prompt-NO formation rate. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Design modifications to SMX static mixer for improving mixing

AICHE JOURNAL, Issue 1 2006
Shiping Liu
Abstract Laminar mixing in SMX static mixers and the effect of geometry on mixing are studied using computational fluid dynamics. A frame-indifferent parameter is used to classify the flow types in the SMX mixer. All three typical flows (simple shear, pure elongation, and squeezing) appear within the flow field of the SMX mixer. The strain rate distribution in the SMX mixer is observed to be very nonuniform. A mixing element with 10 crossbars shows the best mixing quality, followed closely by the standard SMX mixing element with eight crossbars. The improved designs of the mixing element increase the average and peak strain rate, and provide a more uniform strain rate distribution and faster mixing rate compared to those of the standard SMX mixer. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source]

The evolution of the mixing rate of a simple random walk on the giant component of a random graph

RANDOM STRUCTURES AND ALGORITHMS, Issue 1 2008
N. Fountoulakis
Abstract In this article we present a study of the mixing time of a random walk on the largest component of a supercritical random graph, also known as the giant component. We identify local obstructions that slow down the random walk, when the average degree d is at most O(), proving that the mixing time in this case is ,((n/d)2) asymptotically almost surely. As the average degree grows these become negligible and it is the diameter of the largest component that takes over, yielding mixing time ,(n/d) a.a.s.. We proved these results during the 2003,04 academic year. Similar results but for constant d were later proved independently by Benjamini et al. in 3. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008 [source]

Photosynthetic efficiency of Chlorella sorokiniana in a turbulently mixed short light-path photobioreactor

BIOTECHNOLOGY PROGRESS, Issue 3 2010
Anna M. J. Kliphuis
Abstract To be able to study the effect of mixing as well as any other parameter on productivity of algal cultures, we designed a lab-scale photobioreactor in which a short light path (SLP) of (12 mm) is combined with controlled mixing and aeration. Mixing is provided by rotating an inner tube in the cylindrical cultivation vessel creating Taylor vortex flow and as such mixing can be uncoupled from aeration. Gas exchange is monitored on-line to gain insight in growth and productivity. The maximal productivity, hence photosynthetic efficiency, of Chlorella sorokiniana cultures at high light intensities (1,500 ,mol m,1 s,1) was investigated in this Taylor vortex flow SLP photobioreactor. We performed duplicate batch experiments at three different mixing rates: 70, 110, and 140 rpm, all in the turbulent Taylor vortex flow regime. For the mixing rate of 140 rpm, we calculated a quantum requirement for oxygen evolution of 21.2 mol PAR photons per mol O2 and a yield of biomass on light energy of 0.8 g biomass per mol PAR photons. The maximal photosynthetic efficiency was found at relatively low biomass densities (2.3 g L,1) at which light was just attenuated before reaching the rear of the culture. When increasing the mixing rate twofold, we only found a small increase in productivity. On the basis of these results, we conclude that the maximal productivity and photosynthetic efficiency for C. sorokiniana can be found at that biomass concentration where no significant dark zone can develop and that the influence of mixing-induced light/dark fluctuations is marginal. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010 [source]

A STATISTICAL ANALYSIS OF CREAMING VARIABLES IMPACTING PROCESS CHEESE MELT QUALITY

JOURNAL OF FOOD QUALITY, Issue 4 2003
T.A. GLENN III
To simulate commercial processing, a pilot scale 10-gallon (0.04m3), dual ribbon blender was equipped with a thermal control system and a 0.75 hp (559.27 W) electrical motor. An experimental design consisted of three temperatures (75, 80, 85C), three mixing rates (50, 100,150 RPM), and six durations (1, 5, 10, 15, 25, 35 min). Quantified process variables included: process strain and thermal history, and total, instantaneous, and change in mechanical energy. The Schreiber melt test was used to examine the relationship between the processing parameters and melt performance. A statistical analysis revealed significant parameter estimates (P < 0.0001) for each quantified variable in a general linear model. The process cheese industry will gain insight into controlled manufacturing conditions to deliver desired melt functionality. [source]

Simulation and experiments of mixing and segregation in a tote blender

AICHE JOURNAL, Issue 3 2005
O. S. Sudah
Abstract Experimental and computational investigation of mixing and segregation of granular material in a tote blender was carried out. The discrete element method (DEM) was used to simulate flow of spherical, free-flowing particles where the results of the computations were compared to blending. Computational results are compared to blending experiments of monodisperse and bidisperse systems using spherical glass beads in a 1:1 scale. Although some discrepancies were observed, DEM simulations illustrated good agreement with experimentally measured mixing and segregation rates for different fill levels and loading conditions. The effects of blender geometry on particle velocities and flow patterns were examined using DEM. The presence of a hopper and bin section, as well as the axial offset proved to introduce greater axial mixing rates that would be expected from pure dispersion. Vibrated experiments showed better agreement than not-vibrated experiments, indicating that modeling of friction forces needs to be further improved to enhance the accuracy of DEM methods. © 2005 American Institute of Chemical Engineers AIChE J, 51: 836,844, 2005 [source]