Droplet Deposition Rate Research Articles

Numerical Heat Transfer Modelling in Spray Formed IN718 Billets

A numerical finite difference model has been developed to describe the transient heat flow inside Ni superalloy IN718 billets manufactured by spray forming. This model described the progressive build-up and solidification of the billets, and accounted for the latent heat of solidification, convective and radiation heat loss, and heat removal through the substrate, coupled with transient heat transfer in the substrate. The model has been used to predict critical values of important process parameters: average droplet arrival temperature, deposition rate, and billet surface convective heat transfer coefficient, in terms of key microstructural features by correlation of quantitative predictions with qualitative microstructural investigations of the as-sprayed billets. On a micro scale, repeated re-heating/remelting of deposited layers because of subsequent deposition was predicted and has been suggested to be beneficial in reducing porosity and microsegregation, as well as in playing an important role in the formation of the characteristic equiaxed spray formed microstructure.

OS8-06 環状気液二相流中の液滴輸送量に関する実験的研究(OS8 軽水炉・新型炉・核燃料サイクル)

The deposition rate of droplets in annular two-phase flow in a small vertical tube was measured using the double film extraction method. Based on the present experimental data and available ones, new phenomenological models for the deposition rate and entrainment rate were developed. It was also shown experimentally that the deposition rate is markedly increased if a small flow obstacle is placed in the test section tube. An empirical correlation to account the influence of flow obstacle on deposition rate was proposed.

Effects of a Flow Obstacle on the Deposition Rate of Droplets in Annular Two-Phase Flow

Double film extraction technique was used to elucidate the effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow. In the present experiments, the test section was a round tube of 5 mm in inside diameter, air and water were used as test fluids and the flow direction was vertical upward; a short tube that was 2 mm in inside diameter, 3 mm in outside diameter and 20 mm in length was used as a flow obstacle. It was shown that the deposition rate of droplets was markedly increased if the present flow obstacle was concentrically placed in the flow channel. In all the experimental conditions tested, the deposition rates when the flow obstacle was placed were approximately 1.5 times larger than the deposition rates when no obstacle was placed. An empirical correlation was developed to elucidate the influence of flow parameters on the enhancement of deposition rate by the present flow obstacle of simple geometry.

二重管流路の液滴付着率評価(原子炉伝熱流動解析の革新(2),原子炉伝熱流動解析の革新)

To predict thermal-hydraulic performance of BWR fuel bundle, such as the critical power, the subchannel analysis is usually performed. In the subchannel analysis code, many empirical correlations are included. The one of the important correlations is that for the rate of droplet deposition. However, most available correlations for deposition rate were deduced from experimental data for simple cross-sectional geometry such as round tube and rectangular duct. Thus, the cross-sectional geometry effect on the droplet deposition rate was studied numerically using the Eulerian-Lagrangian analysis method. As the first step, the droplet deposition rate onto the inner and outer walls in annulus channel were estimated. It was shown numerically that the droplet deposition rate onto the outer surface is larger than that onto the inner surface.

Effects of a Flow Obstacle on the Deposition Rate of Droplets in Annular Two-Phase Flow

Effects of a Flow Obstacle on the Deposition Rate of Droplets in Annular Two-Phase Flow

真空アーク蒸着法によるダイヤモンド様炭素成膜における巨視的プロセス特性の陰極素材依存性

Macroscopic process parameters of arc voltage, cathode erosion rate, droplet emission rate, ion current, and deposition rate were measured in T-shape filtered cathodic arc deposition using various kinds of carbon cathode materials. Strong correlation between deposition rate and ion current was found. When the ion current was higher, the deposition rate was higher. However, the higher cathode erosion rate did not always brought the higher ion current, and the droplet emission rate was mildly tended to increase as the ion current increased. As a comprehensive evaluation, the optimum cathode material in the present study was considered to be pure graphite. However, the materials of carbon graphite and that including Si hardly emitted the droplets, suggesting the future possibility of the development of droplet-free carbon cathode material.

Experimental Study of Low Liquid Loading Gas-Liquid Flow in Near-Horizontal Pipes

Gas-liquid two-phase flow exists extensively in the transportation of hydrocarbon fluids. A more precise prediction of liquid holdup in near-horizontal, wet-gas pipelines is needed in order to better predict pressure drop and size downstream processing facilities. The most important parameters are pipe geometry (pipe diameter and orientation), physical properties of the gas and liquid (density, viscosity and surface tension) and flow conditions (velocity, temperature and pressure). Stratified flow and annular flow are the two flow patterns observed most often in near-horizontal pipelines under low liquid loading conditions. Low liquid loading is commonly referred to as cases in which liquid loading is less than 1,100m3/MMm3 (200 bbl/MMscf). Low liquid loading gas-liquid two-phase flow at −1° downward pipe was studied for air-water flow in the present study. The measured parameters included gas flow rate, liquid flow rate, pressure, differential pressure, temperature, liquid holdup, pipe wetted perimeter, liquid film flow rate, droplet entrainment fraction and droplet deposition rate. A new phenomenon was observed with air-water flow at low superficial velocities and with a liquid loading larger than 600m3/MMm3. The liquid holdup increased as gas superficial velocity increased. In order to investigate the effects of the liquid properties on flow characteristics, the experimental results for air-water flow are compared with the results for air-oil flow provided by Meng. (1999, “Low Liquid Loading Gas-Liquid Two-Phase Flow In Near-Horizontal Pipes,” Ph.D. Dissertation, U. of Tulsa.)

Droplet-based Freeform Fabrication of Aluminum Objects through Internet.

Three-dimensional aluminum objects were fabricated by depositing molten Al droplets layer by layer. The effects of aluminum droplet temperature, droplet size, droplet deposition rate and the initial velocity of droplets on the micro joining of aluminum beads were investigated. The relative density of fabricated parts increased with an increase in the initial velocity of Al droplets. The relative density reached 92% when the velocity increased to 8.1m/s. Compared their CAD models, the length deformation of fabricated objects was 3-8%, and the diagonal deformation was -3%. Microstructure of the above samples was observed.

Experimental Study of Low-Liquid-Loading Gas-Liquid Flow in Near-Horizontal Pipes

Summary Low-liquid-loading flow in wet-gas pipelines is a common occurrence in the transport of raw gas. The most important parameters governing the flow behavior are pipe geometry (inclination angle and diameter), operating conditions (flow rate, pressure, and temperature), and physical properties of the gas and liquid (density, viscosity, and surface tension). In this study, extensive experiments were conducted with a test loop made of 50.1 mm diameter acrylic pipe with inclination angles from the horizontal of −2°, −1°, 0°, 1°, and 2°. Gas and liquid superficial velocities ranged from 5 to 25 m/s and from 0.001 to 0.053 m/s, respectively. In-situ liquid loading ranged from 150 to 1800 m3. The flow patterns studied were stratified (smooth and wavy) and annular. Measured parameters included gas and liquid volumetric flow rates, liquid-film flow rate, pressure drop, temperature, liquid holdup, and droplet deposition rate. The experimental results show that there is a broad gas velocity range for the transition from stratified to intermittent flow at low liquid loading. Entrainment can occur in the gas core at relatively low velocities, and droplet deposition occurs simultaneously with entrainment. An increase in gas velocity did not increase the liquid entrainment fraction over a relatively broad range of velocities. However, an increase in the liquid flow rate did increase the liquid entrainment fraction. In the annular flow region, a new phenomenon was observed at certain superficial gas velocities; an increase in the liquid flow rate decreased the liquid-film flow rate and liquid holdup and increased the entrainment flow rate. This phenomenon may have a significant impact on operations to sweep liquids from wet-gas pipelines. Seven correlations of the liquid entrainment onset point and the entrainment fraction in the gas core are evaluated. The interfacial friction-factor closure relationship should be different for upward flow when compared to horizontal and downward stratified flow, owing to the existence of counter-current flow in upward-inclined pipes.

Mechanistic modeling for ring-type BWR fuel spacer design: 1. Drift flow model

The fuel spacer is one of the components of a fuel rod bundle and its role is to maintain an appropriate rod-to-rod clearance. Since the fuel spacer influences liquid film flow on fuel rods in BWR core, its specification has a strong effect on thermal hydraulics in the core such as critical power and pressure drop. Spacers have been developed through empirical modifications, so that a large amount of test data are required for optimum design of the spacer. It is, therefore, important to develop a mechanistic model of the spacer for future design of BWR fuel bundles. The authors considered that enhancement of droplet deposition is induced downstream of the spacer where velocity profiles split by a spacer are recovered into one velocity profile; higher velocity in the core and lower velocity in a gap clearance between the rod surface and the spacer. In this paper, the effect of spacer geometry on the droplet deposition rates downstream of it was investigated experimentally in a open rectangular channel using air and water as test fluids. A Phase Doppler Anemometer (PDA) was used for this measurement. In addition, velocity profile equations were obtained from the drift flow model. From these results, a mechanistic model of enhancing droplet deposition rate downstream of the spacer was proposed.

Relation of deposition to drop size when the rate law is nonlinear

Measurements of drop size distributions have been obtained by photography for air-water annular flow in a 4.2 cm vertical pipe. Results are presented for a constant superficial gas velocity (36 m/s) for different liquid flows. It is shown that droplet deposition rates can be interpreted by relating the deposition constant directly to the root-mean square of the velocity fluctuations of the drops. The observed decrease of the deposition constant with increasing liquid flow cannot be explained by an increase in drop size. It is suggested that droplet-droplet interactions cause a decrease in drop turbulence.

ＢＷＲ燃料集合体の沸騰遷移現象スペーサ形状による限界出力への影響

A thorough understanding of the thermal-hydraulic phenomena near fuel spacer is necessary for the accurate prediction of the critical power of BWR fuel assemblies, and is thus essential for effective developments of a new BWR fuel assembly. The main purpose of this study is to develop an accurate method for predicting the effect of spacer shapes on critical power. Tests have been conducted under actual BWR operating conditions, using an annulus flow channel consisting of a heated rod and circular-tube channel, and BWR simulated 4×4 rod bundles with heater rods unheated just upstream of spacer. The effect of spacer shapes on critical power was predicted analytically based on the droplet deposition rate estimation. The droplet deposition rate for different spacer shapes was calculated using a single-phase flow model. The prediction results were compared with the test results for the annulus flow channel using ring-type spacers. Analytical results of critical power agreed with measured critical power from point of the effects of changes in the rod-spacer clearance and the spacer thickness on critical power.

三流体モデルに基づく環状噴霧流の予測手法に関する研究 第三報 液滴付着率の構成方程式の検討

The correlation of droplet deposition rate is one of the most important constitutive equations in predicting the thermohydrodynamic behavior of annular dispersed flow based on the three fluid model. In view of this, quantitative comparison and evaluation of typical correlations of droplet deposition coefficient were made. The result indicated that the correlations show different dependencies on gas flux and droplet concentration and the considerable scattering in predicting the droplet deposition coefficient particularly for the higher gas fluxes and lower pressures.

Droplet deposition and momentum transfer in annular flow

AbstractEntrainment and deposition in gas‐liquid annular upflow are known to account for as much as 20% of the pressure gradient, through droplet acclelerations in the core refion. Momentum is transerred from the core when droplets decelerate upon impactwith the liquid film. It is usually assumed that all of this momentum is transferred to the film, essentially driving the film upwardin conjunction with interfacial friction. New data, abtained for annular gas‐liquid upflow in a 5.08‐cm‐ID tube, are used in a momentum balance analysis to determine the mechanismof momentum transfer from depositing droplets. Measurements include the liquid film thickness, wall shear stress, pressure gradient, entrained liquid fraction, droplet deposition rate, droplet centerline axial velocity, and mass‐average drop size for two gas‐liquid systems. This analysis supports the idea that large droplets displace the film locally and decelerate primarily at the wall, effectively transferring negligible momentum to the liquid film.

95/04158 Evaluation and quantification of the impact of cooling tower emissions on indoor air quality

Assessment of the potential impact of outdoor pollutant sources on indoor air quality through the reentrainment of pollutants vis-a-vis air-handling units, doorways, and windows has mainly focused on the evaluation of fume hood, boiler, diesel generator, and vehicular pollutant emissions. In recent years, however, gaseous and waterborne pollutants emitted from cooling towers have become an increasing source of concern. Chemicals such as biocides and corrosion and scale inhibitors are used to reduce and/or eliminate algae blooms, decrease bacterial and fungal growth, and reduce the corrosion of equipment. When added to the water used in cooling towers, these chemicals are emitted in both the gaseous phase and as pollutants dissolved in or suspended in water droplets. A qualitative evaluation of exhaust dispersion and droplet deposition rates associated with cooling towers is necessary when conducting an overall review of the environmental impact on indoor air quality. This paper identifies source emission rates to be used in assessing emissions of chemical additives in cooling towers, presents provisional design criteria for evaluating the impact of the chemical additives, and evaluates alternative methodologies for quantifying impact concentrations. These alternative assessment methodologies include numerical models, physical wind tunnel simulations, and computational fluid dynamics (CFD) simulations. Parameters more » used in comparing the methodologies include relative accuracy (order of magnitude) and modeling and simulation limitations. « less

Spray deposition of a Sn-40 Wt Pct Pb alloy with uniform droplets

An apparatus capable of producing and depositing uniform droplets (100 to 200 μm in diameter) was developed and used to study the relationship between spray deposition parameters and the microstructures of Sn-40 wt pct Pb alloy spray deposits. The sprays used in the study consisted of uniform droplets, either 103 or 178 μm in diameter, that were in identical thermal and solidification states as they impacted the substrate. The thermal and solidification states of the uniform droplets were determined as a function of the flight distance (the distance from the metal pouring orifice) by model calculations and calorimetric measurements assuming equilibrium solidification. Although a fair agreement was noted between the model and the calorimetric measurements at small flight distances, corresponding to large liquid fractions, the calorimetric measurements indicated 10 to 20 pct higher liquid fractions at larger flight distances. The resultant microstructures comprised either a mixture of the Pb-rich and Sn-rich phases, both in an equiaxed morphology, or a lamellar eutectic structure with a small amount of the Pb-rich primary phase in a coarse spherical morphology. The mostly lamellar eutectic structure resulted from an excessive enthalpy flux and/or slow heat extraction from the deposit. Fine, equiaxed, two-phase microstructures and high deposit density resulted from optimal combinations of droplet enthalpy, deposition rate, droplet size, and deposit cooling rate which gave short local solidification times.

Fuel Atomization and Spray Behavior in an Intake Manifold of Gasoline Engine. 2nd Report. In a Pulsating Air Flow.

The newly developed single-point injector was tested. The spray characteristics of the new twin -fluid injector and the spray behavior in a pulsating air flow were investigated. The mean drop size changes according to the spacial position in the spray clump. The droplet deposition rate is influenced by the pulsation of the air flow in the intake manifold of a gasoline engine. The rate decreases with the increase of the air velocity in the manifold. The temporal changes of the number density of the spray droplets, mean drop size and drop velocity are greatly influenced by the pulsation of the air flow at low atomizing air velocity and at high air velocity in the intake manifold. The smaller droplet follows the air flow more easily, and this difference results in the position change in the spray clump.

On the Droplet Deposition and Mist Supercooling in a turbulent channel flow

AbstractA combined experimental and theoretical study of the droplet deposition and mist supercooling in a turbulent channel flow has been performed. The measurement of droplet deposition on the unheated and heated channel wall was carried out using a particle‐sizing two‐dimensional reference‐mode laser‐Doppler anemomentry technique in a 41 mm × 41 mm vertical square channel at Re = 1.0 × 104 − 6.05 × 104. The dimensionless deposition coefficient kd/uZ. H. Yang Prof. S. L. Lee Department of Mechanical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794 (USA) is analyzed theoretically based on a new unified theoretical approach for the entire transverse flow region from the turbulent core to the quasi‐laminar region next to the boundary wall. The proposed analytical model shows satisfactory agreement with the results of present experimental measurement. In the mist supercooling, the heated surface is cooled by the evaporation on the outside surface of an extra thin continuous liquid film which is maintained by the continuous deposition of extremely small droplets of an optimal size determined by a selection process on the droplets transverse migration due to the dynamic interaction between the phases in a parallel turbulent mist flow. The heat transfer enhancement coefficient depends on droplet deposition rate, droplet size, concentration and the Reynolds number.

Investigation of droplet deposition on a plate with two-phase flow separation at the leading edge

The shape of the leading edge of the plate and the Stokes and Reynolds numbers are shown to have a decisive influence on the droplet deposition rate.

Effect of a ambient air flow rate on deposition rate of droplets on spray chamber.

噴霧塔壁への液滴の付着量が, 微粒化用空気 (Ma) および液流量 (Ml), 周囲空気流量 (Ms) そして噴霧塔長を変えて測定された.塔壁付着量はMs/ (Ma+Ml) <50において微粒化用空気流量と共に増加するが, Ms (Ma+Ml) ≧50ではMaに依らないことがわかった.塔壁付着量は, k-ε方程式に基づく乱流モデルと粒子の作用を考慮したPSIモデルを用いて計算された.この際, 計算の初期値として次の量の半径方向分布がノズル近傍において実験的に与えられた;噴霧滴の分散分布, 滴径および周囲空気および噴霧流の速度成分.この結果, 塔壁付着量に関する計算と実験値とが良好に一致した.