Showing papers on "Convective mixing published in 2006"

Convective mixing and chemical reactions in microchannels with high flow rates

Abstract: This work presents a theoretical and experimental investigation of convective micromixing in various mixer structures and combinations with the aim of high mixing intensity and a high throughput. Different mixing elements are integrated on a silicon chip to achieve a device for high flow rates above 20 kg/h. Test structures are fabricated and characterized according to their flow behavior and mixing performance. Flow measurements with pH neutralization and indication by bromothymol blue confirm the numerical simulations of the flow characteristics and concentration fields. The integral mixing quality in the micromixer is measured with the iodide–iodate reaction (Villermaux–Dushman) and shows excellent values for high Re numbers. This offers the potential to use microstructures for new applications in the production of chemicals. With the help of the obtained experimental and theoretical results, a new class of dimensionless numbers is proposed which characterizes the effectiveness of a mixing device and of the mixing process and compares different mixing times.

Stability of a fluid in a horizontal saturated porous layer: effect of non-linear concentration profile, initial, and boundary conditions

Abstract: Carbon dioxide injected into saline aquifers dissolves in the resident brines increasing their density, which might lead to convective mixing. Understanding the factors that drive convection in aquifers is important for assessing geological CO2 storage sites. A hydrodynamic stability analysis is performed for non-linear, transient concentration fields in a saturated, homogenous, porous medium under various boundary conditions. The onset of convection is predicted using linear stability analysis based on the amplification of the initial perturbations. The difficulty with such stability analysis is the choice of the initial conditions used to define the imposed perturbations. We use different noises to find the fastest growing noise as initial conditions for the stability analysis. The stability equations are solved using a Galerkin technique. The resulting coupled ordinary differential equations are integrated numerically using a fourth-order Runge–Kutta method. The upper and lower bounds of convection instabilities are obtained. We find that at high Rayleigh numbers, based on the fastest growing noise for all boundary conditions, both the instability time and the initial wavelength of the convective instabilities are independent of the porous layer thickness. The current analysis provides approximations that help in screening suitable candidates for homogenous geological CO2 sequestration sites.

Seasonal variability of the mixed layer in the central Bay of Bengal and associated changes in nutrients and chlorophyll

Abstract: Hydrographic data from National Oceanographic Data Center (NODC) and Responsible National Oceanographic Data Centre (RNODC) were used to study the seasonal variability of the mixed layer in the central Bay of Bengal (8–20°N and 87–91°E), while meteorological data from Comprehensive Ocean Atmosphere Data Set (COADS) were used to explore atmospheric forcing responsible for the variability. The observed changes in the mixed-layer depth (MLD) clearly demarcated a distinct north–south regime with 15°N as the limiting latitude. North of this latitude MLD remained shallow (∼20 m) for most of the year without showing any appreciable seasonality. Lack of seasonality suggests that the low-salinity water, which is perennially present in the northern Bay, controls the stability and MLD. The observed winter freshening is driven by the winter rainfall and associated river discharge, which is advected offshore under the prevailing circulation. The resulting stratification was so strong that even a 4 °C cooling in sea-surface temperature (SST) during winter was unable to initiate convective mixing. In contrast, the southern region showed a strong semi-annual variability with deep MLD during summer and winter and a shallow MLD during spring and fall intermonsoons. The shallow MLD in spring and fall results from primary and secondary heating associated with increased incoming solar radiation and lighter winds during this period. The deep mixed layer during summer results from two processes: the increased wind forcing and the intrusion of high-salinity waters of Arabian Sea origin. The high winds associated with summer monsoon initiate greater wind-driven mixing, while the intrusion of high-salinity waters erodes the halocline and weakens the upper-layer stratification of the water column and aids in vertical mixing. The deep MLD in the south during winter was driven by wind-mixing, when the upper water column was comparatively less stable. The deep MLD between 15 and 17°N during March–May cannot be explained in the context of local atmospheric forcing. We show that this is associated with the propagation of Rossby waves from the eastern Bay. We also show that the nitrate and chlorophyll distribution in the upper ocean during spring intermonsoon is strongly coupled to the MLD, whereas during summer river runoff and cold-core eddies appear to play a major role in regulating the nutrients and chlorophyll.

Silicon microstructures for high throughput mixing devices

Abstract: Convective mixing in microstructures gives good mixing results in a very short time. In this work, a theoretical and experimental study is performed on convective micro mixing in different mixing structures and their combinations. Various mixing elements had been integrated on a silicon chip to achieve a device for a high mass-flow rate above 15 kg/h. These test structures are fabricated and tested concerning their flow behaviour and mixing characteristics. Flow measurements with pH neutralization and indication by bromothymol blue confirm the numerical simulations of the flow characteristics and mixing behaviour. The integral mixing quality in the micromixer is measured with the iodide–iodate-reaction (Villermaux–Dushman) and shows excellent values for high Re numbers. This opens the potential of microstructures for new applications in the production of chemicals.

An entropy‐based measure of mixing at the tropopause

Abstract: Qualitative analysis of tracer correlations is now a well established technique for investigating mixing in the tropopause transition layer. Generally, these studies rely on in situ data retrieved over short distances and time-scales. Using satellite data from HALOE, we introduce a simple quantitative analysis technique, based on entropy, to provide a climatology of mixing, from H2O:O3 tracer correlations. Results compare well with previous studies using Lagrangian-based estimates of effective diffusivity and equivalent lengths and also climatologies of tropopause folding. Further analysis techniques reveal characteristics of the transition layer, including its depth and position, leading to the distinction of three primary forms of mixing. Within our analysis domain, we estimate that half of total entropy can be attributed to subtropical ‘Rossby-driven’ tropopause folding events. The majority of the remaining entropy is associated with tropical convective mixing and a third form, possibly linked to shear-induced mixing at subtropical jet streaks. Copyright © 2006 Royal Meteorological Society

The Role of diapycnal mixing in coupled atmosphere-ocean general circulation models

Abstract: The value of ocean diapycnal diffusivity (v) sets the rate at which dense bottom water can be mixed up through the stratified water column and thus plays an important role in the meridional overturning circulation (MOC). Previous idealised experiments and simplified theory suggest that the strength of the MOC and the ocean heat transport scale with the v. This study investigates the dependence of the MOC and other parameters on v using atmosphere-ocean general circulation models (AOGCM). Firstly, the dependence of the MOC strength on v is studied using a low resolution AOGCM with realistic geometry, FORTE, with spatially constant v values ranging from 0.1 cm2/s to an unrealistic high value of 5 cm2/s. At the cyclostationary state, global MOC strength is found to scale with v (in agreement with previous studies) according to a power law of 0.5. No power law is found for the MOC in the individual basins. The increase in MOC strength in the Atlantic and Pacific Oceans is associated with an increase in the ocean heat transport. The atmosphere responds to the change in the ocean state by a decrease of its energy transport and surface winds. Only a partial compensation is found between the ocean and atmosphere energy transport. The strength of v is found to have a strong impact on coupled phenomena, such as a cessation of El Nino at high v. Secondly, similar experiments are conducted with a state-of-the-art AOGCM, ECHAM5/ MPIOM. In this model, v is derived from a constant background diapycnal diffusion (b), wind induced mixing, the Richardson number and the convective adjustment. A set of 3 coupled experiments is conducted, with b = 0.1, 0.25 and 1 cm2/s. The scaling law from simple theory and the previous experiments with FORTE is not observed with this coupled model. At the cyclostationary state, the MOC strength weakens by 16% as b increases from 0.1 to 1 cm2/s. This behavior is not found when the experiments are repeated with an ocean-only model. The reduction in MOC in the coupled model is linked to a strong reduction in the convective mixing at high latitudes. The convective mixing is reduced by a continuous strong freshening in the Arctic region due to an increase in surface air temperature and melting of the sea-ice in the coupled experiments, which is not observed in the ocean-only experiments. The responses of the two coupled models show many similarities as b increases. Both models show convection in the Pacific for high values of b. The main difference is the response of the MOC in the Atlantic is linked to the different locations of the deep convection and their relative changes in the models. I conclude that the diapycnal mixing and the ocean-atmosphere interactions both control the strength of the MOC, and their influences cannot be considered separately.

Research on Mixing Character of Fluidized Bed Particles with Image Processing Technique

Abstract: By measuring the velocity distribution in a two-dimension small scale gas-solid fluidized bed with image processing techniques based on cross-correlation, an uneven index was proposed to estimate the velocity fluctuation and to evaluate the mixing property in fluidized bed The fluctuation of velocity plays an important role to the diffused and convective mixing of particles in fluidized bed At the bottom of the dense phase, the velocity of particles is smaller than those near the free space, but the velocity fluctuation is bigger than those of the latter The velocity uneven index at the bottom is changed with the passing of bubbles, which is beneficial to blend of materials Mixing at the bottom is mainly composed of diffuseness and convection, while in place near free space, convection plays key role in blending Increasing the gas velocity will be of benefit to mixing in fluidized bed According to these conclusions, different feeding schemes for fluidized bed incinerator were introduced, which is important for the designing of new fluidized bed waste incinerator

Reactive mixing in a pipeline with a side-tee

Abstract: Fast reactions in pipelines with side-tees are experimentally investigated. The temperature rise caused by reacting sodium hydroxide with hydrochloric acid is measured at a number of monitoring points downstream of the side-tee. The distance required for the reaction to be completed is indicated by reaching a constant temperature. The experimental results are compared with results obtained using a computational fluid dynamics (CFD) model of non-reactive mixing in exactly the same set-up and under the same flow conditions. Results showed that convective mixing and not micromixing plays a limiting role in fast reactions in pipelines with side-tees. The length of the tube required for non-reactive mixing is found to be very similar to the reactive case and a function of the ratio of the main to the side velocity. It is concluded that factors such as the tee angle which enhanced non-reactive mixing in pipelines with side-tees do have a similar impact on reactive mixing in a similar geometry.

Parameterization of the depth of the upper quasi-homogeneous layer based on measurements in the North Atlantic (Section along 53° N)

Abstract: The dependence of the variation in the depth of the upper mixed layer (MLD) on the governing parameters (the momentum flux, the buoyancy fluxes at the ocean surface, and the density gradient in the pycnocline) is considered. It is shown that, in the spring storm season, wind mixing dominates over convective mixing. In this case, the MLD is linearly correlated with the Ekman scale calculated from the friction velocity observed approximately 12 h before the measurement of the MLD.

Isotopic abundances of Carbon and Oxygen in Oxygen-rich giant stars

Abstract: 16O/17O and 12C/13C ratios in 23 M giants are determined from high resolution IR spectra. The results are confronted with the current models on the convective mixing.