Sprayed Water Droplets Research Articles

Preparation of TiO 2 hollow microparticles by spraying water droplets into an organic solution of titanium tetraisopropoxide

Hollow, spherical TiO 2 microparticles several tens of micrometers in diameter were prepared by spraying water into an organic phase containing titanium tetraisopropoxide (TTIP) as a titanium source. The rapid hydrolysis of TTIP at the water–oil interface resulted in the formation of a TiO 2 shell covering the water droplet. Hexane and cyclohexane were better solvents than isopropanol for fabricating hollow spherical microparticles, suggesting the importance of immiscibility of the solvent with water in this synthesis method. The average particle size increased as the distance from the nozzle to the surface of the TTIP solution was increased. The shell thickness was reduced by the addition of ethanol to the sprayed water droplet. These results demonstrate the controllability of the structure of TiO 2 hollow microparticles, including the diameter and the shell thickness.

Study on a Novel Semidry Flue Gas Desulfurization with Multifluid Alkaline Spray Generator

The advantages and disadvantages of the typical semidry flue gas desulfurization (FGD) processes are analyzed, and a novel semidry FGD process with multifluid alkaline spray generator is first proposed to improve the colliding contact efficiency between sorbent particles and spray water droplets, and to form a large amount of aqueous lime slurry. The experimental results show that the colliding contact efficiency between lime particles and water droplets in the prefix alkaline spray generator may reach about 70%, which is significantly higher than the colliding contact efficiency of 25% in duct sorbent injection. The SO2 removal efficiency can reach 64.5% when the Ca/S molar ratio is 1.5, the approach to the saturation temperature is 10.3 °C, and the flue gas residence time is 2.25 s. It is higher than that of in-duct sorbent injection under similar conditions, and the sorbent utilization is improved to 43%. Therefore, the FGD process with a prefix alkaline spray generator can greatly improve SO2 removal efficiency and sorbent utilization and it will be a new, simple and efficient semidry FGD process for industrial application in the future.

Modeling study on the impaction and humidification process in desulfurization activation reactor

In a semi-industrial desulfurization activation reactor, the high-CaO coal ash is directly used as desulfurization sorbent and activated by spray water. As a result of the inertial impactions between ash particles and water droplets, concentrated slurry droplets are formed. In order to numerically investigate the process, a new model of particle-flow-passage is deduced to simulate the impaction humidification process between particles and droplets. Computation results show that the spray water flow rate and droplets size have obvious influence on droplets catching efficiency, and the ash particles are caught primarily in the vicinity of the atomizer nozzle. With the models of gas phase turbulence, heat and mass transfer, and chemical reactions being considered, a comprehensive 3D model for FGD process (Flue Gas Desulfurization) is established, and several operation conditions are simulated by the model. The predicted results such as SO 2 absorption efficiency and the flue gas temperature at the outlet are in good agreement with experimental results.

Evaporative spray cooling of plain and microporous coated surfaces

Experiments were performed on air and evaporative spray cooling of plain and microporous coated surfaces on flat and cylindrical heaters. Micron-size aluminum particles were used to build the microporous structures on the heated surfaces. To analyze the evaporative cooling, heat transfer curves were obtained in the form of the wall temperature difference versus heat flux. The heat transfer coefficients were also determined as a function of heat flux. Three water flow rates (1.25, 1.75 and 2.40 ml/min) were tested for the flat heater and one rate (3.0 ml/min) for the cylindrical heater, maintaining the air pressure of 7 psig (48 kPa) at the inlet of the nozzle. The effect of different particle sizes in the coating was also tested to optimize the microporous coating technique. Spraying water droplets on the microporous coating surface enhanced the heat removal due to the capillary pumping phenomenon through the microporous cavities connecting each other. The evaporative spray cooling increased the heat transfer coefficient by up to 400% relative to that of the uncoated surface cooled by dry air, and this enhancement was maintained at high heat fluxes by using microporous surfaces.

Theoretical analysis of polydisperse water spray extinction of opposed flow diffusion flames

We present a new mathematical analysis of the influence of a polydisperse spray of water droplets on the extinction of opposed flow diffusion flames. The current interest in the use of fine-water mists for fire extinguishment provides the motivation for our research. Previous experimental and numerical work in the literature used the opposed flow configuration as a convenient paradigm for extracting the relative importance of the various mechanisms at play in extinction of diffusion flames. In the current work we present an analytical approach to describe the problem in which a spray of water droplets of arbitrary polydispersity is present in the oxidant stream that opposes the fuel stream, the latter two being responsible for the formation of a diffusion flame. Transport effects are included through (possibly non-unity) Lewis numbers. The analysis permits arbitrary water spray polydispersity. Our level of modeling currently focuses on the effect of the water mist's endothermicity as the mechanism that primarily determines conditions for extinction. Here, we examine how the initial water droplet size distribution produced by the spray influences the spatial distribution of heat loss, and how the extinction of the diffusion flame is consequently effected. We consider six initial droplet size distributions, three having the same Sauter Mean Diameter and three having the same D 20. In both cases the distributions have rather different forms: (1) a quasi-monodisperse spray, (2) a bimodal spray and (3) a “normal”-spray. It is shown that the different sprays lead to quite different extinction characteristics despite having the same SMD or D 20. Under certain circumstances it is shown that conditions can be found that delineate between effective and ineffective use of the water droplets for suppression. The results strongly underscore the sensitivity of flame suppression to the water mist's actual droplet size distribution.

Removal of domestic airborne dust particles by naturally charged liquid sprays

Exposure to indoor particulate pollutants and allergens has been associated with recent increases in the prevalence of allergic diseases like asthma. Reducing exposure to these particles can play an important role in prevention and abatement of asthma symptoms. A spray of fine water droplets is a well-known method for removing airborne particles. This paper presents evidence to show that the fine sprays produced by domestic pressure-pack dispensers deplete the concentration of dust particles, and when these sprays are charged, particle depletion is significantly improved. Using an air particle counter to quantify particle concentrations, comparisons were made of the rate of particle depletion by liquid aerosols with the rate at which settling occurred through natural processes alone. A standard spray from a pressure-pack dispenser with a charge-to-mass ratio ( q/ m) of 14 μC/kg was compared with a naturally charging equivalent with a q/ m of 140 μC/kg. The standard aerosol removed up to 62% of dust particles, while the charged equivalent removed up to 78% within 90 s of spraying. The sprays differed slightly in their droplet size distributions, the charged spray comprising droplets with a volume median diameter (VMD) of 42 μm and the standard spray having a VMD of 35 μm. Comparing uncharged sprays we also measured a trend for small droplets (VMD=26 μm) to deplete particle concentration more efficiently than a spray with larger droplets (VMD=36 μm). Thus, despite the charged spray having slightly larger droplets, the electrostatic charge resulted in more effective airborne particle removal than uncharged sprays. The use of domestic pressure-pack sprays, of the type used as room fragrance sprays, offers an inexpensive and convenient method of removing airborne dust and associated allergens in the home. This would be of particular benefit following domestic activities that entrain airborne dust.

Numerical Simulation and Experiment on Air Washer with One Row Water Spray

The main purpose of the present work is to acquire further insight into the procedure of heat and mass transfer between sprayed water droplets and air stream in a direct evaporative air cooler used in air-conditioning systems in textile mills. The thermodynamic models of the two-phase flow in such an air treatment system have been developed for both the one row parallel flow spray and one row counter flow spray. The fields of temperature and relative humidity in the spray chamber have been obtained by calcula-tion. A series of experiments aimed at quantifying the system performance and its influence factors have been conducted. This indicates that the increases of air velocity and water/air ratio, together with the decrease of nozzle density are favorable. Comparison of these two different spray types have been carried out by means of humidity efficiency. Finally, numerical calculations have been compared with experimental results. Satisfactory agreements have been found regarding outlet air temperature while a maximum error of 10% has been observed regarding relative air humidity.

Analysis of extinction of counterflow polydisperse spray diffusion flames by a polydisperse water spray

A new mathematical analysis of the influence of a polydisperse spray of water droplets on the extinction of counterflow polydisperse spray diffusion flames is presented. Previous experimental and numerical work in the literature examined the use of fine water droplets to extinguish counterflow gaseous diffusion flames. In the current work, we present an analytical approach to describe the problem in which a polydisperse spray of water droplets is present in the oxidant stream that opposes the fuel stream that carries both fuel vapor and a polydisperse spray of fuel droplets. Transport effects are included through (possibly non-unity) Lewis numbers. Characteristics of the steady-state spray flames are investigated using outer region analytic solutions. It is found that the endothermicity of evaporation of both the fuel and water droplets strongly influences the flame temperature and location. Conditions for flame extinction are also examined. The analysis shows that the water spray's initial polydispersity has a profound impact on spray-flame suppression. In particular, it is shown that there exists a set of conditions that delineates between effective and ineffective use of the water droplets for suppression. For the data used here, these conditions favor use of an initially quasi-monodisperse water spray rather than a bimodal spray having the same Sauter mean diameter (SMD). This indicates the difficulty in utilizing the SMD to characterize a polydisperse spray and for drawing conclusions therewith in the current context.

Spherical-shaped ice particle production by spraying water in a vacuum chamber

A theoretical and experimental study was performed to examine the water spray evaporation method for ice particle production. The conditions for the formation of ice particles were investigated theoretically by the diffusion-controlled evaporation model. The prediction by the model was proved to agree relatively well with experiments. The production of cold storage heat will increase almost proportionally to the number of spray nozzles because no substantial difference was found in the mean droplet size of the overlapped sprays from twin nozzles. Finally, based on the results, the vacuum chamber was designed, and spherical ice particles of size below 300 μm were experimentally obtained by spraying water droplets of ambient temperature in the vacuum chamber where pressure is maintained below the freezing point of water. From the experiment for producing ice particles, it was found that the spray flow rate influences the performance of the system more than the position of spray nozzle.

The effects of free stream air velocity on water droplet size and distribution for an impaction spray nozzle

Fog generation for ingestion by gas turbine engines is a method by which the amount of work done during compression can be reduced by evaporative cooling of the inlet air and can improve the engine's efficiency by as much as 15 per cent. Generating fog produces problems if the droplet size is too high which can result in erosion and possibly corrosion damage to the compressor and stator blades. This study examines the water droplet spray characteristics of an impaction-type atomizer over a range of free stream air velocities, water pressures and fluid viscosities.

A Eulerian/Lagrangian model to calculate the evolution of a water droplet spray

AbstractWe introduce a Eulerian/Lagrangian model to compute the evolution of a spray of water droplets inside a complex geometry. To take into account the complex geometry we define a rectangular mesh and we relate each mesh node to a node function which depends on the location of the node. The time‐dependent incompressible and turbulent Navier‐Stokes equations are solved using a projection method. The droplets are regarded as individual entities and we use a Lagrangian approach to compute the evolution of the spray. We establish the exchange laws related to mass and heat transfer for a droplet by introducing a mass transfer coefficient and a heat transfer coefficient. The numerical results from our model are compared with those from the literature in the case of a falling droplet in the atmosphere and from experimental investigation in a wind tunnel in the case of a polydisperse spray. The comparison is fairly good. We present the computation of a water droplet spray inside a complex and realistic geometry and determine the characteristics of the spray in the vicinity of obstacles.

Studies on the Water Cooling of Hot Surfaces : Analysis of Spray Cooling in the Region Associated with Film Boiling

An analytical study has been performed on the spray cooling of a hot surface in the region associated with film boiling under the assumption that there will be two parallel conductances of heat transfer. One is of radiation from the hot surface to the environment, the correlation of which is cited from the authors' preceding report(1). The other is heat transfer to the sprayed water droplets by heat conduction through the hypothetical uniform vapor layer underneath the droplet which is in the spheroidal state with a flat bottom. First, we deduce analytically an expression for heat transfer per droplet by solving the equations of momentum in the vapor layer, energy balance at the interface and balance of static forces exerting on a droplet simultaneously. Then a semiempirical correlation of the mean lifetime of the droplets is estimated using the experimental data with surface superheating below 500K at atmospheric environmental pressure, mass velocity of water between 0.0162 and 0.174kg/(m2·s), volume mean diameter of droplets between 130 and 550μm and Weber number between 10 and 120(1). The procedure we propose predicts the heat transfer data of spray cooling within the experimental range mentioned above fairly well.

High speed micro-cinematography of aerosols

Many existing techniques for aerosol characterisation are based on the utilisation of their optical properties, but have a major disadvantage in so far as the results tend to be dependent on the signal processing techniques employed. A direct measurement technique has many advantages particularly in terms of ease of validation and verification of results. An innovative method for the characterisation of aerosols has been developed using a synchronised metal vapour laser light source and a high speed cine camera. Data has been collected in the form of size distribution, speed and trajectory. Example studies have related to the characterisation of water spray droplets, aerosols from bioprocessing and two-phase releases following catastrophic failure of containment vessels.

Phoretic phenomena in tar vapor‐particulate mixture separation from fuel gas streams

AbstractAnalytical models were developed to predict the performance of a spray scrubber for separation of tars from gasifier off‐gas streams. The models included heat transfer, mass transfer, condensation, nucleation, temperature and flow fields, and various phoretic phenomena involved in droplet‐particle and droplet‐vapor interaction and collection. The models indicate that for the tar particulates, both Brownian diffusion and inertial impaction, rather than diffusiophoresis and thermophoresis, are the more important phoretic forces causing the collection. For the limiting case of all tars being present only as vapor, the efficiency of their removal by condensation on water spray droplets could be extremely high.Scrubbing experiments were carried out on the hot gas/tar stream evolved from the simulated devolatilization section of a laboratory fixed bed gasifier. The combined overall collection efficiency for a mixed particulates and vapor stream compares satisfactorily with the model predictions.

Free Convection Diffusion Flames With Inert Additives

An analysis of laminar, free-convection, diffusion flames burning on a vertical fuel surface is presented. The study is directed towards determining the effects of inert additives on the boundary layer structure and burning rates. Results are presented for several classes of inert additives using polymethyl methacrylate burning in air as the base system. The use of a fuel-phase additive to release noncombustible gases is modeled by varying the parameter YFT, the mass fraction of fuel in the gaseous products of pyrolysis. The effects of an additive which promotes char formation is modeled by varying the heat of pyrolysis per unit mass of gaseous pyrolysate released for combustion. The effects of a spray of water droplets on a wall fire is modeled by a combination of the release of a noncombustible gas and a surface heat sink which results from evaporation of water droplets at the fuel surface. The water spray calculations predict an extinction limit which is within a factor of two of experimental results.

Measurements related to the fundamental processes of aerosol electrification

With the object of illuminating the fundamental processes responsible for the electrification of aerosols, a number of measurements have been made to compare the initial distribution of charges carried by various aerosols with their final equilibrium state. Measurements have been made on freshly condensed water droplets and sulfur particles, on both freshly dispersed and aged silica dust particles, and on sprayed water droplets; all lying within the 1- to 2-μ radius range. The measurements show that when numerous ions are formed within the aerosol space the particle charges are not completely discharged but rather the ions greatly hasten the attainment of an equilibrium distribution. Moreover, it is found that irrespective of the initial measured distribution of charges, the final equilibrium distribution approaches a Gaussian curve whose width is dependent upon the mean particle radius and is usually symmetrical about the axis of zero charge. Because the infrequent collision of water droplets can hardly produce appreciable electrification by friction, and because the equilibrium charge on water droplets and that on silica particles of the same size are indistinguishable, it is concluded that the maintained charges on aerosols are a direct consequence of the diffusion of the ambient atmospheric ions onto the particles. The equilibrium distributions approach the theoretical form recently worked out by Ross Gunn.

Measurements related to the fundamental processes of aerosol electrification

With the object of illuminating the fundamental processes responsible for the electrification of aerosols, a number of measurements have been made to compare the initial distribution of charges carried by various aerosols with their final equilibrium state. Measurements have been made on freshly condensed water droplets and sulfur particles, on both freshly dispersed and aged silica dust particles, and on sprayed water droplets; all lying within the 1- to 2-μ radius range. The measurements show that when numerous ions are formed within the aerosol space the particle charges are not completely discharged but rather the ions greatly hasten the attainment of an equilibrium distribution. Moreover, it is found that irrespective of the initial measured distribution of charges, the final equilibrium distribution approaches a Gaussian curve whose width is dependent upon the mean particle radius and is usually symmetrical about the axis of zero charge. Because the infrequent collision of water droplets can hardly produce appreciable electrification by friction, and because the equilibrium charge on water droplets and that on silica particles of the same size are indistinguishable, it is concluded that the maintained charges on aerosols are a direct consequence of the diffusion of the ambient atmospheric ions onto the particles. The equilibrium distributions approach the theoretical form recently worked out by Ross Gunn.

Underwater Sound from the Splashes of Water Droplets and Sprays of Water Droplets

The underwater sound radiated from the splashes of individual water droplets is correlated with the development of the splash. The observed instantaneous sound pressure and the spectrum of the sound energy radiated into the water are compared with theory. The spectra of the sound energy from a large number of individual splashes of droplets of various sizes and velocities are plotted in dimensionless form and compared with a similar dimensionless spectrum obtained from data on the rms sound pressure from a spray of water droplets. An expression is given for the sound pressure spectrum level of the sound to be expected from sprays on the surface in terms of the intensity of the spray, the size and velocity of the droplets in the spray, and the configuration of the spray area on the surface. Estimates are made of the spectrum levels of the underwater sound from the splashing of rain on the surface of a sea in terms of the rate of rainfall.