Convective movement

Reactions in bulk are used commercially but careful control of temperature is required. Polymerisation in a suitable solvent will dilute the concentration of reacting material and this together with the capability for convective movement or stirring of the reactant reduces exotherm problems. There is now, however, the necessity to remove solvent and this leads to problems of solvent recovery. Fire and toxicity hazards may also be increased. 

After detachment of the flame from the wall and reduction of its width, three zones develop in the vessel parallel to each other a flame and burned gas zone and two zones (adjacent to the sidewalls), where no flame is present and where the gas temperatures are lower than behind the flame. All this happens in a field of very high centrifugal acceleration, which induces a free convection movement of the flame and the product zone behind it toward the... 

Both pH and the availability of nutrient ions in soil play important roles in rhizo-sphere dynamics and are often dependent on one another. Nutrient ions move in soil toward plant roots either by mass flow with the soil water or by diffusion. Mass flow is the result of bulk convective movements of the soil solution toward roots, whereas diffusion occurs in response to a concentration gradient for a particular ion, which results from its absorption by the root and depletion from the... 

Oddson solved an Initial value problem that described the convective movement of an organic down from the soil surface for the following specific conditions ... 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Convection is the movement of a species under the action of a mechanical force (a gradient of pressure). The convective movements can be fortuitous (resulting from collisions or vibrations of the electrochemical cell) or intentionally forced (through controlled stirring). 

Apart from a limited number of cases (laminar flows around a rotating disk or through a tubular electrode), it is very difficult to make a rigorous treatment of the convective movements. 

Temperature and concentration gradients are absent inside an ideal stirred reactor. Mixing can be achieved by forced or free-convective movement of a material. For such a reactor the energy balance equation can be written as ... 

Alternative hypotheses about the geodynamic evolution of the Tyrrhenian basin advocate convective movements of the modified upper mantle... 

First, the different components of the bulk solution must be brought up to the electrode surface by the processes summarized under the heading mass transfer. Convection (movement of the solution relative to the electrode) takes molecules and ions to the boundary of the Nemst layer (Section 2), through which diffusion takes them up to the electrode surface. Diffusion occurs due to the... 

The three primary mechanisms responsible for mixing are convective movement of relatively large portions of the bed, shear failure that primarily reduces the scale of segregation, and diffusive movement of individual particles. 

Heat is transferred from or to a region by the motion of fluids and the phenomenon of convection. In natural convection, the movement is caused by buoyancy forces induced by variations in the density of the fluid these variations are caused by differences in temperature. In forced convection, movement is created by an external agency such as a pump. 

The transport of heat in a water-saturated porous medium is governed by conduction in the solid matrix, transport by the groundwater (convective movement of heat) and heat exchange between the solid and the groundwater, which depends on their temperature difference (e.g. De Marsily, 1986). 

Rainfall, temperature, salinity, pH, luminosity, nutrients, metals and chelants, growth promoters, water-column stability, winds, converging phenomena, convection movements.)... 

We shall see later that a typical dc polarography result takes the form of a wave, (Fig. 1.3f). The normal result is shown together with anomalous maxima superimposed. Both of these maxima are due to an enhancement in the rate of mass transport of the analyte to the electrode caused by convective movements at the Hg/HzO interface. 

Maxima of the second kind. These appear as relatively small rounded enhancements in the current on the wave plateau. They occur in solutions of high electrolyte concentration and are most common when a high flowrate of mercury is used. It is thought that the actual disturbance within the mercury drop generates the convective movement of the electrolyte next to the drop. 

Your sketch should resemble Fig. 1.3f. There are two kinds of maxima and they are both due, in differing ways, to an enhancement of the required diffusion-controlled mass transport mechanism by convective movements at the Hg/H20 interface. Both types of maxima may be prevented by the addition to the solution of a small amount of surfactant, eg Triton X-100. 

Convective gas phase transport in natural unsaturated soil systems may be induced by variations in temperature, air pressure and groundwater, by wind pressure or by seepage water movement after rain events. All these processes are of minor importance in lab column experiments and have not been implemented in DiffModV. The only convective movement in the gaseous phase considered in DiffMod is the ground air recharge due to gas volume decrease caused by oxygen consumption. The effects of this type of convection have to be taken into consideration as they result in an increase of the initial oxygen penetration depth or the thickness of the pyrite oxidation zone and thus induce an increase of the total pyrite decomposition rate on the column by 10 to 15 %. 

FORCED CONVECTION - Movement of fluid by mechanical force such as fans or pumps. 

NATURAL CONVECTION - Movement of a fluid caused only by temperature differences (density changes). 

Thermosyphon - The natural, convective movement of air or water due to differences in temperature. In solar passive design a thermosyphon collector can be constructed and attached to a house to deliver heat to the home by the continuous pattern of the convective loop (or thermosyphon). 

The atmosphere is subject to convective movements which transport the pollutants both horizontally and vertically. These convective movements are due to the thermal gradients which themselves result from the surface heating of the land and the oceans by the sun. 

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