Relative velocity influence

Most studies indicate that air velocity has a profound influence on mean droplet size in twin-fluid atomizers. Generally, the droplet size is inversely proportional to the atomizing air velocity. However, the relative velocity between the Hquid and air stream is more important than the absolute air velocity. 

The regulation of axial fan blade angle also influences the inlet and exit velocity triangles in such a way that the axial velocity and thus the volume flow change. When the relative velocity remains parallel to the blade, the efficiency remains high (Fig. 9.52). 

Most theoretical studies of heat or mass transfer in dispersions have been limited to studies of a single spherical bubble moving steadily under the influence of gravity in a clean system. It is clear, however, that swarms of suspended bubbles, usually entrained by turbulent eddies, have local relative velocities with respect to the continuous phase different from that derived for the case of a steady rise of a single bubble. This is mainly due to the fact that in an ensemble of bubbles the distributions of velocities, temperatures, and concentrations in the vicinity of one bubble are influenced by its neighbors. It is therefore logical to assume that in the case of dispersions the relative velocities and transfer rates depend on quantities characterizing an ensemble of bubbles. For the case of uniformly distributed bubbles, the dispersed-phase volume fraction O, particle-size distribution, and residence-time distribution are such quantities. 

Various correlations for mean droplet size generated by plain-jet, prefilming, and miscellaneous air-blast atomizers using air as atomization gas are listed in Tables 4.7, 4.8, 4.9, and 4.10, respectively. In these correlations, ALR is the mass flow rate ratio of air to liquid, ALR = mAlmL, Dp is the prefilmer diameter, Dh is the hydraulic mean diameter of air exit duct, vr is the kinematic viscosity ratio relative to water, a is the radial distance from cup lip, DL is the diameter of cup at lip, Up is the cup peripheral velocity, Ur is the air to liquid velocity ratio defined as U=UAIUp, Lw is the diameter of wetted periphery between air and liquid streams, Aa is the flow area of atomizing air stream, m is a power index, PA is the pressure of air, and B is a composite numerical factor. The important parameters influencing the mean droplet size include relative velocity between atomization air/gas and liquid, mass flow rate ratio of air to liquid, physical properties of liquid (viscosity, density, surface tension) and air (density), and atomizer geometry as described by nozzle diameter, prefilmer diameter, etc. 

The influence of liquid density on the mean droplet size is relatively small but complex. An increase in liquid density may reduce the mean droplet size due to a decrease in sheet thickness at the atomizing lip of a prefilming atomizer, or due to an increase in the relative velocity between liquid and air for a plain-jet atomizer. However, increasing liquid density may also increase the mean droplet size because a liquid sheet may extend further downstream of the atomizing lip of a prefilming atomizer so that the sheet breakup may take place at lower relative velocity between liquid and air. 

The release location influences the vertical distribution of the time-averaged concentration and fluctuations. For a bed-level release, vertical profiles of the time-averaged concentration are self-similar and agreed well with gradient diffusion theory [26], In contrast, the vertical profiles for an elevated release have a peak value above the bed and are not self-similar because the distance from the source to the bed introduces a finite length scale [3, 25, 37], Additionally, it is clear that the size and relative velocity of the chemical release affects both the mean and fluctuating concentration [4], The orientation of the release also appears to influence the plume structure. The shape of the profiles of the standard deviation of the concentration fluctuations is different in the study of Crimaldi et al. [29] compared with those of Fackrell and Robins [25] and Bara et al. [26], Crimaldi et al. [29] attributed the difference to the release orientation, which was vertically upward from a flush-mounted orifice at the bed in their study. 

As noted, two principles of heat transfer are involved evaporation and convection. The rate of heat transfer by both convection and evaporation increases with an increase in air-to-water interfacial surface, relative velocity, contact time and temperature differential. Packing and fill in a tower serve to increase the interfacial surface area the tower chimney or fans create the relative air-to-water velocity and contact time is a function of tower size. These three factors all may be influenced by the tower design. 

To find the influence function rj(s), we shall consider shear deformation of the system at velocity gradient 7y, while two macromolecular coils, separated by a distance dj, move beside each other at velocity 7ijdj. We add to the sum the contributions of every coil, apart from the chosen one, and find the density distribution of the energy dissipation for the chosen coil. The proportionality coefficient depends only on the concentration of the Brownian particles, if an assumption is made that local dissipation is determined by relative velocities of macromolecular coils,... 

Combined with densities, molecular weights, and transference numbers (fractions of the current carried by the various ionic constituents), the conductivity yields the relative velocities of the ionic constituents under the influence of an electric field. The mobilities (velocity per unit electric field, cm2 s-1 V-1) depend on the size and charge of the ion, the ionic concentration, temperature, and solvent medium. In dilute aqueous solutions of dissociated electrolytes, ionic mobilities decrease slightly as the concentration increases. The equivalent conductance extrapolated to zero electrolyte concentration may be expressed as the sum of independent equivalent conductances of the constituent ions... 

Reactions within a van der Waals (vdW) complex of calcium with hydrogen halides (HC1 and HBr) lead to electronically excited calcium halides. These reactions have been quite extensively studied in full collisions of excited calcium beams (Brinckmann et al. 1980 Brinckmann and Telle 1977 Rettner and Zare 1981, 1982 Telle and Brinckmann 1990). The electronic excitation of the calcium atom results in a strong chemiluminescence under collisional conditions. The efficiency of this chemiluminescence depends upon the electronic state and the fine structure component, and the final product state is influenced by the preparation conditions of the collision. In the reaction Ca(4s4p1P1) + HC1, the direction of the polarization of the P orbital with respect to the collision relative velocity (pK or pff) has an effect on the branching ratio to the products CaCl, A2n or B2X+ (Rettner and Zare 1981, 1982). 

Factors that influence growth of sucrose crystals have been listed by Smythe (1971). They include supersaturation of the solution, temperature, relative velocity of crystal and solution, nature and concentration of impurities, and nature of the crystal surface. Crystal growth of sucrose consists of two steps (1) the mass transfer of sucrose molecules to the surface of the crystal, which is a first-order process and (2) the incorporation of the molecules in the crystal surface, a second-order process. Under usual conditions, overall growth rate is a function of the rate of both processes, with neither being rate-controlling. The effect of impurities can be of two kinds. Viscosity can increase, thus reducing the rate of mass transfer, or impurities can involve adsorption on specific surfaces of the crystal, thereby reducing the rate of surface incorporation. 

A comparison of the present theoretical model with the experimental observations of Okajima and Kumagai (8) is shown in Figure 7 for the combustion of a 1300-fi ethyl alcohol droplet under the influence of weak forced convection as a result of relative motion with respect to the gas phase in a gravity-free environment not disturbed by free convection. For these experiments Okajima and Kumagai (8) used a freely falling chamber within which a free droplet was combusted while the droplet moved at a relative velocity of 1.29 cm/sec in a standard air atmosphere with an initial temperature of 300 K. Since the droplet was spark-ignited... 

The relatively considerable influence of the pressure on the diffusion coefficients has a major consequence when chromatography is carried out imder very high pressures, in the range of 0.5 to several kbar [24]. The mass transfer resistances are all direct functions of the diffusivity, except the adsorption/desorption kinetics (see Sections 5.2 and 5.2.7 for the case of surface diffusion). Accordingly, the efficiency of colimms packed with very fine particles dp of the order of 1 jim) decreases rapidly with increasing velocity, far more rapidly than anticipated from the small particle diameter. Since it seems highly improbable that this range of pressure will ever be used in preparative HPLC, there will be no elaboration here on the reasons for this fact. 

Further, fibre orientation in the plane of the web is influenced by the relative velocities of the mix flowing through the nozzle and the fast-moving wire. Generally the veloeity of the mix should eorrespond very closely to the velocity of the wire. 

Effect of flow velocity. An increase in flow rate through the column also increases the product throughput with a resulting payoff in decreased separation efficiency. A change in the flow velocity influences all contributions to the variance except the contribution of the feed band because the time scale for flow relative to time scales for dispersion. 

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