Mean droplet diameter surface

D = mean droplet diameter (pm) s = surface tension (dyne- cm" ) d = density of liquid (g- cm" )... 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

For example, at a frequency of 1 MHz, the effect of ultrasound in water of surface tension 73 dyn-cm and density 1 g-cm- is to produce longitudinal waves of about 12 pm. The resulting mean droplet diameter (D) is given by Equation 19.2. 

In many applications, a mean droplet size is a factor of foremost concern. Mean droplet size can be taken as a measure of the quality of an atomization process. It is also convenient to use only mean droplet size in calculations involving discrete droplets such as multiphase flow and mass transfer processes. Various definitions of mean droplet size have been employed in different applications, as summarized in Table 4.1. The concept and notation of mean droplet diameter have been generalized and standardized by Mugele and Evans.[423] The arithmetic, surface, and volume mean droplet diameter (D10, D2o, and D30) are some most common mean droplet diameters ... 

Whereas few actual values of n for sprays from various fuel injectors are reported, it is usually possible to obtain a fairly reliable estimate of x. This is so because x is uniquely related to various mean droplet diameters solely in terms of n, and data for Sauter mean diameter are rather frequently reported. Sauter mean diameter (SMD) is that diameter representative of the surface area per unit volume which is characteristic of the actual spray. 

Many of the averages and mean droplet diameters in current use may be expressed in terms of the moments of the various weighted size distributions. For example, the first moment of the surface-weighted size distribution is given by the expression... 

This relationship is particularly useful as it allows one to calculate the total surface area of droplets in an emulsion, an important parameter that can be used to estimate the emulsifier concentration required to produce a kineticaUy stable emulsion. An appreciation of the various types of mean droplet diameter is also important because different experimental techniques used to measure droplet sizes are sensitive to different mean values (24). Consequently, it is always important to be clear about which mean diameter has been determined in an experiment when using or quoting droplet size data. 

It is important to note that lai e extents of mass and heat transfer take place near the liquid nozzle. Usually the operation of the venturi scrubber is almost isothermal, except for a few centimeters from the point of liquid injection. The effect of any solid phase in the liquid may be considered negligible the liquid residence time is so short that very little dissolution can take place. An interesting feature is that decreasing the surface tension results in an increase in a because of a decrease in the mean droplet diameter. 

A number of theoretical studies of venturi performance have been made to produce theoretical models that can predict performance from first principles. One of the key areas of uncertainty has been the droplet size formed by the venturi. Typically, this is estimated using the Nukiyama and Tanasawa equation to estimate the surface-mean droplet diameter ... 

Ultrasonic nebulisers make use of piezo technology to create an aerosol from a solution of active substance. In such nebulisers the high frequency mechanical vibration of a piezoelectric element is transmitted to a solution of the medicine which creates standing capillary waves on the surface of that solution. Small droplets break free from the crests of these waves and constitute the aerosol. The mean droplet diameter is a function of the frequency of the acoustic signal, the surface tension, density and viscosity of the... 

As demonstrated in Fig. 6 the brass plates had a thickness of 30 p,m and average distance between the conical pores of 100 or 500 p m. A syringe pump delivered the pressure at constant flow rates and we reduced the flow resistance using a polysiloxane coating at the surface of the brass plates. The obtained droplets showed average diameters between 15 and 35 pm (Fig. 7). The emulsion droplet size depended on several parameters like the surfactant type, the amphiphile concentration and the flow rate. With Dodecane as organic solvent and Lecithine as emulsifier the mean droplet diameter reached a minimum of 15 pm. 

An important measure of the droplet size distribution in spray appHca-tions is the Sauter mean diameter 32 = (d )/(d ). This measure is so important because during evaporative drying the mass transfer happens at the interface of the droplets and the surrounding air. To enhance the evaporation of a population of droplets, one has to maximize the active surface areas and minimize the internal volumes. The DSMC simulations showed that the Sauter mean diameter is a very nontrivial function of the axial and radial position in the spray. Figure 16 shows that at a given axial position, with increasing distance from the central axis the mean droplet diameter first increases, then decreases, and finally increases again. Exactly, the same trends... 

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. 

In order to estimate the specific surface area of the dispersed organic droplets, the mean droplet size (Sauter diameter 32) has to be determined, which can be calculated according to the Okufi equation (Eq. 5) ... 

Tables Mean bubble and droplet diameters and resulting specific surface areas of the dispersed gas and organic phase...

In all these tasks, the achievable (as narrow as possible) droplet size distribution represents the most important target quantity. It is often described merely by the mean droplet size, the so-called Sauter mean diameter J32 (Ref. 19), which is defined as the sum of all droplet volumes divided by their surfaces. Mechanisms of droplet formation are ... 

Another popular technique for obtaining an average diameter based on surface area consists of calculating the diameter of a hypothetical droplet whose surface area is equal to the total surface area of the spray divided by the total number of droplets in the spray. An average of this type may be expressed as a geometric mean of an appropriate set of distribution moments. 

The Sauter mean diameter has been found to be the most useful of the above definitions for characterizing the spray produced by a nozzle. It is a good indicator of a spray s performance in complex interactions with a droplet s surface and volume. Applications include spray drying, evaporative cooling, dry scrubbing, gas quenching, and gas absorption (Stavis, 1991). 

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