Droplet studies

Measurements from synthetic fuel spray flames and laboratory droplet reactors indicated the extent to which fuel properties and combustion conditions influenced particulate yields. A series of seven fuels were tested in a 21 kW spray combustor for total particulate by gravimetric sampling and soot by Bacharach smoke number. Variations in total particulate were dominated by the tendency of the fuel to form ceno-spheres while smoke number correlated with the C H ratio of the fuel. The laboratory droplet studies were performed in a gas flame supported reaction environment. These results confirmed the correlation between soot yield and C H ratio. In addition, two distinct forms of disruptive droplet combustion were observed. 

McMahon studied the effect of waxes on emulsion stability as monitored by the separation of water over time (46). The size of the wax crystals showed an effect in some emulsions but not in others. Interfacial viscosity indicated that the wax crystals form a barrier at the water/oil interface which retards the coalescence of colliding water droplets. Studies with octacosane, a model crude oil wax, show that a limited wax/asphaltene/resin interaction occius. A wax layer, even with absorbed asphaltenes and resins, does not by itself stabilize an emulsion. McMahon concludes that the effect of wax on emulsion stability does not appear to be through action at the interface. Instead, the wax may act in the bulk oil phase by inhibiting film thinning between... 

Table IV summarizes a number of SO2 oxidation rates measured in the laboratory and the atmosphere. The rates vary from a low of 0.1 /hr for photooxidation of SO2 in clean air to over 2 /min measured in water droplets. Studies reflecting both homogeneous and heterogeneous processes are presented in Table IV. In the next subsections we consider the elements of both homogeneous and heterogeneous processes in an attempt to estimate the contribution of each to the atmospheric oxidation of SO2.

While Eq. III-18 has been verified for small droplets, attempts to do so for liquids in capillaries (where Rm is negative and there should be a pressure reduction) have led to startling discrepancies. Potential problems include the presence of impurities leached from the capillary walls and allowance for the film of adsorbed vapor that should be present (see Chapter X). There is room for another real effect arising from structural peiturbations in the liquid induced by the vicinity of the solid capillary wall (see Chapter VI). Fisher and Israelachvili [19] review much of the literature on the verification of the Kelvin equation and report confirmatory measurements for liquid bridges between crossed mica cylinders. The situation is similar to that of the meniscus in a capillary since Rm is negative some of their results are shown in Fig. III-3. Studies in capillaries have been reviewed by Melrose [20] who concludes that the Kelvin equation is obeyed for radii at least down to 1 fim. 

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

An important approach to the study of nucleation of solids is the investigation of small droplets of large molecular clusters. Years ago, Turnbull showed that by studying small droplets one could eliminate impurities in all except a few droplets and study homogeneous nucleation at significant undercoolings [13]. 

Some studies have been made of W/O emulsions the droplets are now aqueous and positively charged [40,41 ]. Albers and Overbeek [40] carried out calculations of the interaction potential not just between two particles or droplets but between one and all nearest neighbors, thus obtaining the variation with particle density or . In their third paper, these authors also estimated the magnitude of the van der Waals long-range attraction from the shear gradient sufficient to detach flocculated droplets (see also Ref. 42). 

There have been some studies of the equilibrium shape of two droplets pressed against each other (see Ref. 59) and of the rate of film Winning [60, 61], but these are based on hydrodynamic equations and do not take into account film-film barriers to final rupture. It is at this point, surely, that the chemistry of emulsion stabilization plays an important role. 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

The structure of microemulsions have been studied by a variety of experimental means. Scattering experiments yield the droplet size or persistence length (3-6 nm) for nonspherical phases. Small-angle neutron scattering (SANS) [123] and x-ray scattering [124] experiments are appropriate however, the isotopic substitution of D2O for H2O... 

The term microemulsion was introduced by Schulman, who studied surfactant solutions as eady as 1943 (22). At that time it was widely accepted that "oil and water do not mix," and Schulman understood that an emulsion scatters light because it contains droplets whose diameters are large compared to the wavelength of light (see Emulsions). Thus, the term y /mJemulsion implies a system which (like an emulsion) contains droplets of oil or water, but in which the droplets are too small to scatter light. 

Hollow Sprays. Most atomizers that impart swid to the Hquid tend to produce a cone-shaped hoUow spray. Although swid atomizers can produce varying degrees of hoUowness in the spray pattern, they aU seem to exhibit similar spray dynamic features. For example, detailed measurements made with simplex, duplex, dual-orifice, and pure airblast atomizers show similar dynamic stmctures in radial distributions of mean droplet diameter, velocity, and Hquid volume flux. Extensive studies have been made (30,31) on the spray dynamics associated with pressure swid atomizers. Based on these studies, some common features were observed. Test results obtained from a pressure swid atomizer spray could be used to iUustrate typical dynamic stmctures in hoUow sprays. The measurements were made using a phase Doppler spray analyzer. 

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. 

Divinylbenzene copolymers with styrene are produced extensively as supports for the active sites of ion-exchange resins and in biochemical synthesis. About 1—10 wt % divinylbenzene is used, depending on the required rigidity of the cross-linked gel, and the polymerization is carried out as a suspension of the monomer-phase droplets in water, usually as a batch process. Several studies have been reported on the reaction kinetics (200,201). 

The study of the combustion of sprays of Hquid fuels can be divided into two primary areas for research purposes single-droplet combustion mechanisms and the interaction between different droplets in the spray during combustion with regard to droplet size and distribution in space (91—94). The wide variety of atomization methods used and the interaction of various physical parameters have made it difficult to give general expressions for the prediction of droplet size and distribution in sprays. The main fuel parameters affecting the quaHty of a spray are surface tension, viscosity, and density, with fuel viscosity being by far the most influential parameter (95). 

Experimental techniques used for studying the combustion of single droplets can be divided into three groups suspended droplets, free droplets, and porous droplets, with ongoing research in all three areas (98). 

The use of multiple tube banks as a droplet collector has also been studied by Calvert (R-12). He reports that collection efficiency for... 

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