Observing convective effects

In Chapter 6.4, J. Chomiak and J. Jarosinski discuss the mechanism of flame propagation and quenching in a rofating cylindrical vessel. They explain the observed phenomenon of quenching in ferms of the formation of fhe so-called Ekman layers, which are responsible for the detachment of flames from the walls and the reduction of fheir width. Reduction of the flame speed with increasing angular velocity of rofation is explained in terms of free convection effects driven by centrifugal acceleration. 

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

Koutecky [57] evaluated Ax = 34.7 and A2 = 100, while Matsuda [58] found = 31.1 and A2 = 294. However, for typical values of D and m, these second-order terms can be neglected. Nevertheless, early in the drop life, the observed currents are lower than those predicted and later on they are higher (possibly due to convection effects). 

Experimental Observations. Whereas many experiments have been performed on single-droplet vaporization (21,22), most of them are conducted under the influence of either natural and/or forced convection, which not only distorts spherical symmetry, but also produces unwanted temporally varying convective effects on the vaporization process as the droplet size diminishes. The observations on the droplet surface area variations, however, do agree qualitatively with the predicted behavior of the transient models shown in Figure 4. 

To counteract the (vexing) convection effects on kinetic experiments, Aogaki and co-workers, having developed a special electrode assembly to separate mass transport and kinetic effects, report a marked decrease in the exchange current density (about 25%) in magnetic fields imposed on a copper deposition cell. Virtually no effect on the transfer coefficient (a 0.44) was observed. Experimental results obtained in nickel-phosphorus alloy deposition, cupric ion reduction in ethylenediamine solutions, and the electrolytic reduction of acetophenone " are further demonstrations of the interaction of the magnetic fields with polarization characteristics, and point to the difficulty of fully eliminating the effect of convection and/or diffusion on electrode kinetics. 

Two-dimensional experiments with optical microscopy are also affected by other negative factors. Laser confocal microscopy has a relatively slow imaging speed and is not suitable for real-time electrodeposition studies. Furthermore, hydrogen bubbles interfere with optical microimaging making it difficult to observe their effects. Why, then, have two-dimensional studies so far dominated this domain The answer is that they are considerably simpler than those in 3D since they are not affected by complications like convection and, indeed, hydrogen bubbles. It is time,... 

N.S. Reejhsinghani, W.N. Gill, A.J. Barduhn, Laminar dispersion in capillaries. 3. Experiments in horizontal tubes including observations on natural convection effects, AIChE J 12 (1966) 916. 

Stirring and convection effects have both been observed when CuO particles were trapped, using a doughnut beam, in water to which a little detergent had been added. This greatly reduced the tendency of particles to stick to the slide and generally freed up their motion so that Brownian motion was more evident. 

As early as 1815 it was observed qualitatively that whenever a gas mixture contains two or more molecular species, whose relative concentrations vary from point to point, an apparently natural process results which tends to diminish any inequalities in composition. This macroscopic transport of mass, independent of any convection effects within the system, is defined as molecular diffusion. 

The observed stabilizing effect of surfactants toward convection induced by surface tension has been confirmed theoretically in a recent paper by Berg and Acrivos (B13), in which the stability analysis technique and the physical model were the same as Pearson s except that the free-surface boundary condition [(iii) of Table III] took into account the presence of surface active agents. Critical values for the Thompson number were computed as functions of two dimensionless parameters, one embodying the surface viscosity and the other the surface elasticity. ... 

One might have hoped that, following the modifications to the stability analyses for liquids with free surfaces, quantitative comparisons between experiment and theory could have been made, but such is not the case. Further refinement in both theory and experiment is still required. Nevertheless it is indeed encouraging that all of the experimentally observed qualitative effects regarding convective stability appear to be in agreement with the predictions of hydrodynamic stability theory. 

Here, in Equation 31.24, the first two terms in the right-hand side represent the convective and diffusive mass flow, while the third term denotes the transfer of material from the aqueous phase. The model was validated by the experimental results and was found in good agreement. However, the authors suggested further rigorous treatment of the model for observing the effect of various experimental and system parameters on the overall working profile of hollow-fiber modules. 

At low scan rates, the tacit assumption that no convection takes place in the cell breaks down. For example, in a dichloromethane/0.1 M NBU4PF6 electrolyte at room temperature, convection effects are already observable at t <= 10 mV s . This defines a lower limit of the applicable scan rates (see also edge effects below). 

At the polarized water/DCE interface, the photoisomerization of DBA has been studied by Naujok et al. [38]. In this study, it is observed that nearly a full mono-layer of trans-DBA may be converted to a full monolayer of cis-DBA, with no appreciable thermal conversion back to the trans-DBA form. Also, the achievement of the conversion of the complete monolayer on a short time scale was attributed to surface mixing and surface tension driven convection effects. Interestingly, the conversion of the monolayer back to the trans-DBA form could easily be performed by proper illumination of the interface. 

To determine if convective effects can be observed using this technique the sample cell was placed in various orientations. Figures 6 and 7 show the comparison in surface polymerization irradiated from above and below when the sample is in the horizontal orientation. (See Figures 2b and 3b.) The capacitances for both measurements are identical except for differences at longer times. This suggests that gravitational effects do not influence surface polymerization at early times. However when convection is dominant (irradiated from bottom) the capacitance does decrease at longer times. Since the reaction has slowed at this point, the decrease in capacitance is probably due to the removal of polymer from the surface by convection. 

The movement of the gas surroimding the sample due to convection may cause noise and affect the observed mass. This may be reduced by introducing baffles within the sample enclosure. Usually experiments are conducted in flowing atmosphere to reduce convective effects. 

Under these conditions enhanced currents due to convection and thinning of the Nernst layer are observed. The effects of vibrating electrodes depend on electrode shape, frequency of vibration, and amplitude. Vibrating electrodes are employed in industrial processes [i] and in electroanalysis [ii]. 

This test consists of deflecting the specimen under specified conditions of time and temperature and observing the effect on the thickness of the specimen. The test procedure requires the specimen to be compressed between two or more parallel plates to a specified deflection thickness. The entire assembly is placed in a mechanically convected air oven for a specified time and temperature. Following this exposure, the specimen is removed from the apparatus and the recovered thickness is measured. The constant deflection compression set, expressed as a percentage of the original thickness, is calculated. 

James et al. [26] applied the commercial software of computational fluid d5mamics (CFD) in their strategy to simulate the heat and moisture diffusive and convective transport as well as effect of sweating to predict the performance of chemical and steam/fire protective clothing. Kothari et al. [27] simulated the convective heat transfer through textiles with the help of CFD to observe the effects of convection on the total heat transfer of the fabric. The software tools like CFD provide a possible pathway for the user to simulate the heat and fluid distribution in the clothing. However, these tools do not take into account the structural features of the... 

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