Optimum Droplet Size Research Articles

Heterogeneous Effects in the Propagation and Quenching of Spray Detonations

The steady propagation of the one-dimensional detonation wave in heptane/air sprays was computationally studied using detailed chemistry, with emphasis on the effects of droplet heating, vaporization, and drag, as well as momentum and heat loss on the detonation velocity and quenching. Results show strong coupling between these processes and the detonation response, leading to a variety of interesting phenomena such as drag-induced concentration of droplets; interference of thermal runaway by cooling due to droplet vaporization; one-stage vs two-stage ignition; chemical heat release controlled by either Arrhenius kinetics or droplet vaporization; internal vs external loss in causing quenching; nonlinear feedback between integrated loss, chemical heat release, and detonation velocity that is responsible for quenching; and the strong influence of the postshock specific heat of the fuel vapor. Consequently, we show that there exist optimum droplet sizes for higher detonation velocities and wider quenching limits and that, depending on the droplet size, higher droplet loading of the spray can also promote detonation propagation and extend its quenching limits.

Inhibition of premixed and non-premixed flames with fine droplets of water and solutions

Inhibition/extinction of premixed and non-premixed methane/air flames with fine droplets of water and solutions containing several chemical agents has been investigated experimentally. While solutions allow delivery of much higher concentrations of chemical agent to the flame front than otherwise possible, the non-premixed flame extinction results indicate saturation (or condensation) of the agent at some effective temperature below the flame temperature. Based on the chemical additives considered, on a molar basis, the following order of effectiveness is observed: KOH>NaCl>NaOH. The inhibition of premixed flames by similar size droplets indicates insensitivity to NaOH mass fraction in the water. This insensitivity was related to the shorter residence time of the droplets (13 μm median diameter) through the premixed flame structure. Detailed comparison of the premixed and non-premixed flame inhibition/extinction with pure water droplets supports the importance of droplet residence time and optimum droplet size in controlling the interaction of droplets with the flame front.

Control of Electro-Optic Modulation in Polymer-Dispersed Ferroelectric Liquid Crystals

Electro-optic (EO) modulation in polymer-dispersed ferroelectric liquid crystals (FLCs) has been studied as a function of the droplet size in a confined geometry. It is found that an optimum droplet size exists for the best alignment of molecules as well as smectic layers which produces the maximum EO modulation. The alignment quality depends critically on the shape and size of FLC dropIets. The stable structure formed inside the FLC droplets results from a competition between the elastic energy stored in a confined geometry and surface interactions at the FLC-polymer interface.

Determination of the Optimum Droplet Size and Operation Condition for Minimum Drift

Determination of the Optimum Droplet Size and Operation Condition for Minimum Drift

Wind-tunnel studies on the collection of airborne spray droplets by flying locusts

In view of the present environmental awareness, the reduction of spray wastage, which in turn lessens environmental contamination, can be achieved by reducing spray volumes and improvement on spray efficiency through the use of correct ranges of spray droplet sizes. Published data on droplets collected by some insects consistently suggest that the optimum droplet sizes lie below 60 µm. An experiment was conducted in a wind-tunnel to assess the locust collection efficiency when subjected to a spectrum of droplet size ranges with a view to determine the optimum droplet size. Locusts were induced to fly in a wind-tunnel and then exposed to spray droplets which were produced by a spinning disc atomiser. Droplets collected on antennae, head, abdomen, legs and wings were sized and counted. The collection efficiency at wind-speeds ranging from 2 m/sec to 6 m/sec was calculated for the different body parts in the droplet size range 10–40, 40–60 and 60–80 µm. The calculated collection efficiency for the 10–40 µm size range was comparatively higher for all locust body parts.

Mechanistic considerations on the release of electrolytes from multiple emulsions stabilized by BSA and nonionic surfactants

The stability of w/o/w emulsions has been significantly improved by using a blend of nonionic surfactant (Span 80) and bovine serum albumin (BSA) as an interfacial complex in the inner phase. Improved stability was obtained by replacing the common nonionic hydrophilic monomeric emulsifier by BSA in the outer phase. Optimum stability and droplet size of double emulsion was achieved with 0.2 wt% BSA+10 wt% Span 80 as emulsifier I and 5.0 wt% Span 80:Tween 80 (2:3) or 0.1 wt% BSA as emulsifier II. Significant slow release of NaCl was obtained. The results were examined in view of Higuchi mechanism and it was found that one can account for the thickness of the inner interfacial complex from the plot of B (diffusion parameter correlated to the fraction of electrolyte release) vs. time. It can be clearly seen that in the inner phase the surfactant and the BSA act synergistically (active interfacial complex), and enhance stability and reduce release. In the outer phase BSA has mainly a stabilizing effect with a limited release retardation effect. Effective diffusion coefficients for each BSA concentration in the inner and outer phase have been calculated and evaluated in view of the diffusion controlled mechanism and the thickness of the viscoelastic film which is formed.

IGNITION BEHAVIOR OF A FUEL SPRAY FLOWING IN A TUBE

The ignition behavior of a spray flowing in a tube is numerically investigated. The ignition process is initiated by a localized heat source or a flame kernel. An hybrid finite-difference scheme is used to solve the two-phase equations. A consistent ignition criterion is obtained, which is shown to provide a sufficient ignition condition for all the cases considered. Results indicate the existence of an optimum droplet size and an optimum equivalence ratio for the minimum ignition energy. The optimum values vary with the mixture conditions such as droplet size, equivalence ratio, and fuel volatility. This is a significant result since the conclusions of earlier studies for the initially quiescent sprays are validated for the flowing situations.

Droplet size and equivalence ratio effects on spark ignition of monodisperse N-heptane and methanol sprays

The effects of the droplet size and the equivalence ratio on the ignition of monodisperse n-heptane and methanol sprays at atmospheric pressure were investigated using a capacitive discharge spark ignition system. The minimum ignition energy E min of the sprays was measured over a range of droplet diameters, D = 30–57 μm, and equivalence ratios, φ = 0.44 – 1.8. As expected, results showed that E min decreased with decreasing droplet size, with increasing equivalence ratio, and with increasing fuel volatility. While the same trends have been observed previously for polydisperse sprays, this is the first study to quantify these effects for monodisperse sprays in this size range. The minimum ignition energy was also measured for fully prevaporized n-heptane and methanol. An optimum gas phase equivalence ratio for prevaporized n-heptane ignition was found between 1.5 < φ < 2.0. No corresponding optimum was obtained for prevaporized methanol due to difficulties in generating fuel-rich methanol mixtures. comparison of the spray and prevaporized ignition results indicated the existence of an optimum droplet size, D < 30 μm, for the ignition of fuel-lean sprays for both fuels. Extension of the lean prevaporized ignition limit, φ = 0.55, was also observed for all sprays. Experimental ignition results were compared to the predictions of two existing ignition models for quiescent sprays: the characteristic time model for ignition of Peters and Mellor and the general ignition model of Ballal and Lefebvre. Both models, using a characteristic time approach, predicted the experimentally determined ignition energies accurately for most conditions. Model performance deteriorated, however, for leaner ration, φ < 0.7, and for smaller droplet size, D < 35 μm, with both models increasingly underpredicting E min.

Spark ignition of aerosols

An experimental investigation was carried out to determine the minimum spark ignition energy of monodisperse tetraline aerosols in a laminar gas flow. Results, presented in terms of spark ignition frequency, and for droplet sizes between 6.7 μm and 40 μm, showed: a relatively weak dependence on spark duration, an optimum spark gap width between 3 and 4 mm, an increase in minimum spark ignition energy with decreasing oxygen concentration in the gas, a decrease in spark ignition energy as the gas to fuel ratio decreased from lean to stoichiometric, a maximum ignition frequency depending on droplet size, and an optimum droplet size between 22 and 26 μm for minimum spark ignition energy. A model of droplet motion and vaporization allowed an estimation of the non-reactive fuel vapor concentration and droplet motion due to the spark discharge and was used for qualitative discussion of the experimental results.

Effect of formulation properties on ground and foliar deposits of two insecticides in flushed and one year‐old balsam fir needles following aerial application

AbstractA study was conducted in 1985 in Newfoundland to investigate the interrelationships between the physical properties of spray formulations, droplet‐size spectra and deposits on ground sampling units, and deposits on flushed and one‐year‐old balsam fir foliage following aerial application of aminocarb and fenitrothion at different dosage rates, but using the same volume rates. The fenitrothion formulations were pseudoplastic, because of the presence of polymeric ingredients, and provided higher deposits on the ground sampling units and on foliage than the aminocarb formulations, which were nearly Newtonian. At similar dosage rates, the ground deposits of fenitrothion were about twice those of aminocarb, whereas the corresponding foliar deposits were about 6 to 8 times higher than those of aminocarb, suggesting possible advantages of pseudoplastic formulations in providing optimum droplet sizes for enhanced deposition on balsam fir needles.The foliar deposits generally showed an increase when the dosage rates of the two insecticides were increased. Deposits in flushed foliage were slightly higher than those in mature foliage, probably due to the location of the flushed needles at the extreme periphery of the tree canopy.

Power Plant Spray Cooling: Design and Performance Studies

A new three-dimensional model for determining the flow and thermal characteristics of spray units is utilized in an effort to predict and improve the cooling efficiency of spray units. In a marked departure from present theories, which are based on the behavior of an average droplet in a uniform average environment, the model examines the local variation in properties of both the air and the droplets throughout the flow field encompassing the spray umbrella. Model predictions of droplet return temperature along the spray perimeter, downwind wet-bulb temperature, and downwind air-velocity were compared with field data on a commercial spray. Good agreement was observed. A parametric study was then performed to determine the optimum droplet size and discharge angle for a spray unit. It was found that substantial improvements in the cooling obtained for a given spray power input can be achieved over currently used floating spray units.

Droplet Size Effects on NOx Formation in a One-Dimensional Monodisperse Spray Combustion System

A one-dimensional monodisperse aerosol spray combustion facility is described and experimental results of post-flame NO/NOx emissions are presented. Four different hydrocarbon fuels were studied: isopropanol, methanol, n-heptane, and n-octane. The results indicate an optimum droplet size in the range of 43-58 microns for minimizing NO/NOx production for all of the test fuels. This NOx behavior is associated with droplet interactions and the transition from diffusive type of spray burning to that of a prevaporized and premixed case. Decreasing the droplet size results in a trend of increasing droplet interactions, which suppresses temperatures and reduces NOx. This trend continues until prevaporization effects begin to dominate and the system tends towards the premixed limit. The occurrence of the minimum NOx point at different droplet diameters for the different fuels appears to be governed by the extent of prevaporization of the fuel in the spray and is consistent with theoretical calculations based on each fuel’s physical properties.

Formation of Oxides of Nitrogen in Monodisperse Spray Combustion of Hydrocarbon Fuels

Experimental results of exit plane NO/NO(x) emissions from atmospheric monodisperse fuel spray combustion are presented. Six different hydrocarbon fuels were studied: isopropanol, n-propanol, n-octane, iso-octane, n-heptane and methanol. The results indicate an optimum droplet size for minimizing NO/NO(x) production for all of the test fuels. At the optimum droplet diameter, reductions in NO/NO(x) relative to the NO(x) occurred at droplet diameters of 55 and 48 microns respectively, as compared to a 50-micron droplet size for isopropanol. The occurrence of the minimum NO(x) point at different droplet diameters for the different fuels appears to be governed by the extent of prevaporization of the fuel in the spray, and is consistent with theoretical calculations based on each fuel's physical properties. Estimates are also given for the behavior of heavy fuels and of polydisperse fuel sprays in shifting the minimum NO(x) point compared to a monodisperse situation.

Assessment of Water Based Controlled Droplet Application of Insecticides on Lowland Rice

Abstract An optimum droplet size for water based controlled droplet application (c.d.a.) on lowland rice was chosen by measuring the deposition rate for a 54 to 127 μm volume median diameter (vmd) range of droplet sizes on the canopy of the rice crop. The optimum size was 127 μm and this size was compared with knapsack spraying by measuring the deposition rate, at two growth stages, on the canopy and leaf sheath of the rice plant. The c.d.a. technique gave a predicted 30 to 40% reduction in applied dosage rate when spraying on the canopy, but no reduction was predicted for applications against the planthopper species which feed at the basal leaf sheath. The savings in labour and chemical for c.d.a. spraying were quantified in monetary terms, as well as the purchase price of the sprayers, using Philippine prices, c.d.a. gave a saving of US $8/ha over knapsack spraying and a locally manufactured sprayer could be made for US $25 cheaper than a knapsack sprayer.

ULV/CDA optimum spray droplet size for control of the eastern spruce budworm in Canada

The post-World War II availability of aeroplanes and pilots and synthetic insecticides provided Canadian forest managers with the means of controlling devastating plague outbreaks of spruce budworm. Subsequent adoption of ULV spray rates, definition of the biologically optimum droplet size, and development of aerial spray generators such as the Micronair and Beecomist, have since given rise to an effective and environmentally acceptable forest protection practice.

C.d.a.–Controlled Droplet Application

Abstract Efficiency of pesticide application can be improved if more consideration is given to where and when the active ingredient is most needed, and the optimum droplet size selected for a given target. Even when minimal volumes of spray are applied concentrate sprays are not justified as a greater proportion of the spray is collected on the target. Sprays with a narrow range of droplet size can be produced from ligaments thrown from centrifugal energy nozzles; droplet size being inversely proportional to the rotational speed.

The dose-toxicity of chlorpyrifos and endosulfan insecticides on the house fly by topical, vapor, and spray treatments as estimated by gas chromatography.

Gas chromatography was used for chemical analysis of amounts of chlorpyrifos and endosulfan reaching Musca domestica L. by vapor and spray. The dose-toxicity data obtained by these methods of treatment were analyzed by the probit method. The LD50 values thus derived were compared with those of topical applications of known concentrations, LD50 values for chlorpyrifos, in ng/female fly, were: 43±6.5, 57±27, 35±7.5, and 46.8±10.5, for topical, spray, vapor, (Experiment 1), and vapor (Experiment 2) treatments, respectively. Experiments with endosulfan gave LD50 values of 167±20, 216±30, and 143±47 ng/female for topical, spray, and vapor applications, respectively. These LD50, values obtained from different methods of delivery were not statistically different based on the overlapping of their confidence limits. They show that the toxicity of externally administered chlorpyrifos and endosulfan is not significantly influenced by differences in external methods of delivery. If true for all insecticides then the optimum spray droplet size for any target insect can be calculated from physical parameters related to diffusion and impingement.

A contribution towards the reduction of ice fog caused by humid stack gases at Alaskan power stations

The humid power station gases at U.S. Army and Air Force Bases in Alaska can cause considerable ice fog visibility problems. This phenomenon occurs when the ambient temperature is such that water vapour in the gases nucleates and freezes on grit particles to form minute ice crystals that have no appreciable settling velocity, thus forming a dense blanket of ice fog in the temperature range − 20 to − 40°C. Various alternatives are considered for drying the stack gases and counter current droplet scrubbing is chosen as the most promising method. Theoretical moisture removal predictions are made and confirmed experimentally. A spray tower is then designed for the conditions at a typical air base, with consideration given for optimum droplet size to accomplish both grit and moisture removal from the gases.