Local Drop Size Distribution Research Articles

Effects of secondary breakup, collision dynamics, gravity and evaporation on droplet size distribution in a pressure-swirl JET A-1 spray

Size and velocity of droplets in pressure-swirl sprays vary with distance downstream the nozzle. This study aims to explain the phenomena contributing to size-resolved spatial variation of the drop size distribution for Jet A-1 spray from a small pressure swirl atomizer injected into quiescent ambient air at atmospheric conditions. The effects of secondary breakup, droplet collisions, gravity, drag-driven spray dispersion, and evaporation are evaluated. Phase Doppler anemometer (PDA) was used to resolve the velocity and size of the droplets in radial profiles at several axial distances (Z) from the nozzle exit at injection pressures between 0.5 and 1.5 MPa. The droplet motion and collision dynamics were qualitatively characterised by high-speed imaging. The analysis focused on areas along 1) the spray axis and 2) the liquid sheet direction. The first area covers small droplets with marginal evolution, while the mean drop size in the second area significantly increases downstream. The local drop size distribution and its axial evolution results from a combined effect of ballistic filtering (drifting of small droplets from periphery to spray centre and large droplets vice versa), centrifugal and turbulent droplet dispersion, and droplet collisions (increasing drop size with distance). The relative droplet-gas velocity was found at investigated Z positions too small for drag-driven secondary droplet breakup to occur. Evaporation of Jet A-1 sprayed at room temperature and pressure into still atmosphere reduces the drop size marginally. Trajectory of all drop sizes is insignificantly altered by gravity. The smallest droplets are strongly drag-driven to the spray axis, while large ones disperse centrifugally and have sufficient momentum to neglect the gravity. Combining results from imaging and laser diagnostics, various collision outcomes were identified, with a conclusion that these depend on the size and velocity of colliding droplets. Coalescing collisions dominate and strongly increase the mean drop size downstream the spray at off-axis positions while insignificantly contribute along the spray axis. The drop size span factor reduces with Z as separative collisions between large droplets and mixed-type collisions between small and large droplets narrow the drop size distribution. The above effects can be most easily amplified for drop size reduction via modification of physical properties of sprayed liquid by its heating.

Evaluation of the Polarimetric-Radar Quantitative Precipitation Estimates of an Extremely Heavy Rainfall Event and Nine Common Rainfall Events in Guangzhou

The development and application of operational polarimetric radar (PR) in China is still in its infancy. In this study, an operational PR quantitative precipitation estimation (QPE) algorithm is suggested based on data for PR hydrometeor classification and local drop size distribution (DSD). Even though this algorithm performs well for conventional rainfall events, in which hourly rainfall accumulations are less than 50 mm, the capability of a PR to estimate extremely heavy rainfall remains unclear. The proposed algorithm is used for nine different types of rainfall events that occurred in Guangzhou, China, in 2016 and for an extremely heavy rainfall event that occurred in Guangzhou on 6 May 2017. It performs well for all data of these nine rainfall events and for light-to-moderate rain (hourly accumulation <50 mm) in this extremely heavy rainfall event. However, it severely underestimated heavy rain (>50 mm) and the extremely heavy rain at stations where total rainfall exceeded 300 mm within 5 h in this extremely heavy rainfall event. To analyze the reasons for underestimation, a rain microphysics retrieval algorithm is presented to retrieve Dm and Nw from the PR measurements. The DSD characteristics and the factors affecting QPE are analyzed based on Dm and Nw. The results indicate that compared with statistical DSD data in Yangjiang (estimators are derived from these data), the average raindrop diameter during this rainfall event occurred on 6 May 2017 was much smaller and the number concentration was higher. The algorithm underestimated the precipitation with small and midsize particles, but overestimated the precipitation with midsize and large particles. Underestimations occurred when Dm and Nw are both very large, and the severe underestimations for heavy rain are mainly due to these particles. It is verified that some of these particles are associated with melting hail. Owing to the big differences in DSD characteristics, R(KDP, ZDR) underestimates most heavy rain. Therefore, R(AH), which is least sensitive to DSD variations, replaces R(KDP, ZDR) to estimate precipitation. This improved algorithm performs well even for extremely heavy rain. These results are important for evaluating S-band Doppler radar polarization updates in China.

Utilization of Specific Attenuation for Tropical Rainfall Estimation in Complex Terrain

Abstract To improve the accuracy of quantitative precipitation estimation (QPE) in complex terrain, a new rainfall rate estimation algorithm has been developed and applied on two C-band dual-polarization radars in Taiwan. In this algorithm, the specific attenuation A is utilized in the rainfall rate R estimation, and the parameters used in the R(A) method were estimated using the local drop size distribution (DSD) and drop shape relation (DSR) observations. In areas of complex terrain where the lowest antenna tilt is completely blocked, observations from higher tilts are used in radar QPE. Correction of the vertical profile of rain rate estimated by the R(A) algorithm (VPRA) is applied to account for the vertical variability of rain. It has been found that the VPRA correction improved the accuracy of estimated rainfall in severely blocked areas. The R(A)–VPRA scheme was tested for different precipitation cases including typhoon, stratiform, and convective rain. Compared to existing rainfall estimation algorithms such as rainfall–reflectivity (R–Z) and rainfall–specific differential phase (R–KDP), the new method is able to provide accurate and robust rainfall estimates when the radar reflectivity is miscalibrated or significantly biased by attenuation or when the lower tilt of the radar beam is significantly blocked.

Investigation of Rain Attenuation in Equatorial Kuala Lumpur

This letter investigates rain attenuation in Kuala Lumpur, Malaysia, by exploiting local drop size distribution (DSD) measurements. Coefficients for the well-established power-law model relating rain rate and specific attenuation are derived for frequencies in the Ku-, Ka-, and Q/V-bands based on three years of disdrometer data. We analyze the diurnal variation of rainfall rate for four time intervals and, moreover, we present statistics of rain attenuation for slant-path Earth-space links estimated by means of a new model (Stratiform-Convective SST) that combines the advantages of the Dual-Layer Synthetic Storm Technique (SST) and the SC EXCELL model. The predicted statistics are in good agreement with those obtained from beacon measurements (MEASAT-1 satellite at 12 GHz). Finally, the diurnal variation of the slant-path rain attenuation is presented to provide system operators and radio communication engineers with useful information on the quality of service (QoS) that can be achieved during a typical day on an Earth-space link.

Effects of Mass Transfer on the Steady State and Dynamic Performance of a Kühni Column − Experimental Observations

Experimental data obtained in a pilot-scale Kuhni liquid-liquid extraction column were used to obtain local drop size distributions, dispersed-phase hold-up profiles, and solute concentrations. To obtain information about the dynamic behavior of the column, several experiments were planned and performed, under both steady and controlled transient conditions, with and without mass transfer. The analysis of the effects of the presence of a transferring solute (not of the mass transfer flux itself) on the dispersion's steady state and dynamic behavior was the main objective. For this, we imposed forced step variations in the operating conditions - agitation intensity and continuous and dispersed-phase flowrates.

Water Mist Spray Characterization and Its Proper Application for Numerical Simulations

A phase Doppler particle analyzer (PDPA) was used to screen candidate water mist nozzles for use in a scaling validation aimed to allow scaled-down testing of water mist systems. Proper configuration of the PDPA remains somewhat of an art due to a lack of definitive guidelines for the many user-defined setup and data regression options. To validate the setup, a custom-designed iso-kinetic sampling probe (Probe) was developed to independently measure water mist fluxes at the same locations as the PDPA measurements. Using the Probe as a guide, the optimal PDPA operation parameters can be determined expeditiously for reliable measurements. To provide input to numerical modeling of water mist sprays, gross drop size distributions in terms of cumulative volume fraction were conventionally derived from local drop size distributions and water flux measurements and fitted using a composite of Rosin-Rammler and log-normal distributions. However, it was found that the accuracy of the data fit in the lower drop size range can substantially affect the conversion accuracy to the gross drop number distribution required by numerical modeling. This issue can be resolved by deriving the gross drop number distribution directly from measurements, or by ensuring a sufficiently accurate data fit in the lower drop size range if converted from the cumulative volume fraction.

Application of the maximum entropy technique in tomographic reconstruction from laser diffraction data to determine local spray drop size distribution

This work proposes a new deconvolution technique to obtain local drop size distributions from line-of-sight intensity data measured by laser diffraction technique. The tomographic reconstruction, based on the maximum entropy (ME) technique, is applied to forward scattered light signal from a laser beam scanning horizontally through the spray on each plane from the center to the edge of spray, resulting in the reconstructed scattered light intensities at particular points in the spray. These reconstructed intensities are in turn converted to local drop size distributions. Unlike the classical method of the onion peeling technique or other mathematical transformation techniques that yield unrealistic negative scattered light intensity solutions, the maximum entropy constraints ensure positive light intensity. Experimental validations to the reconstructed results are achieved by using phase Doppler particle analyzer (PDPA). The results from the PDPA measurements agree very well with the proposed ME tomographic reconstruction.

Simulated and Experimental Dispersed-Phase Breakage and Coalescence Behavior in a Kühni Liquid−Liquid Extraction ColumnSteady State

Given the difficulties normally associated with direct experimentation in liquid-liquid extraction processes, their direct computer simulation has acquired increased relevance and utility, especially when dealing with some of the details of the dispersed-phase hydrodynamics. The possibility of testing very complex theoretical models of such behavior is increasingly attractive to the researcher, as a result of both the power and the availability of personal computing resources. Experimental data obtained in a pilot-scale Kiihni liquid-liquid extraction column and simulated data generated by means of a drop population-balance model and algorithm were used to describe and compute the local drop-size distributions and dispersed-phase holdup profiles. In this work, the applicability of this algorithm to describe the steady-state behavior of a Kiihni liquid-liquid extraction column is illustrated. Data generated using this algorithm exhibit reasonable agreement with experimental data, with physically meaningful model parameter values.

Hydrodynamic Behavior of a Rotating Disc Contactor Under Low Agitation Conditions

ABSTRACT The hydrodynamics of a rotating disc contactor (76.2 mm diameter, 19 stages, and operating height of 815 mm) has been investigated at limiting operational conditions of low holdup (φ ≤ 3%) and low agitation (Re R ≤ 22,000) for the water/n-heptane system in the absence of mass transfer. Water was chosen as the continuous phase, and it was kept stagnant inside the column. Local drop size distribution, local mean drop diameter, and total holdup have been measured at different flow rates of the dispersed phase (70 to 640 mL/min) and rotor speeds (0 to 700 rpm). Drop size distributions along the column were suitably described using the log-normal probability density function, and empirical expressions were derived to predict the parameters of the distribution curve as a function of operating conditions. An empirical expression was also proposed to predict the Sauter mean drop diameter along the column with a mean deviation of 3.4%. Statistical analysis has revealed no significant effect of the rotor speed on the total holdup, which changed solely with the dispersed phase flow rate for the operating conditions investigated.

Experimental Study of Gas Absorption with a Spray Column.

In order to investigate spray properties, measurements of local fluxes of the sprayed liquid, local number fluxes of drops and local drop size distributions were made for wide ranges of liquid and gas flow rates. An empirical correlation for the volume mean diameter of sprayed drops at the nozzle exit was obtained. Experimental studies of gas absorption with a spray column were carried out for carbon dioxide-air-water and carbon dioxide-air-aqueous sodium hydroxide solution systems over wide ranges of liquid flow rates, feed gas concentrations and initial alkaline concentrations. The observed data for dimensionless rates of absorption of carbon dioxide were compared with the values predicted by a solid sphere penetration model.

Effect of liquid viscosity on internal flow and spray characteristics of Y-jet atomizers

Internal flow characteristics within Y-jet atomizers and the local drop size distribution and cross-sectional averaged drop size at the outside were investigated with the liquid and air injection pressures, mixing port length of atomizers, and the liquid properties taken as parameters. To examine the effect of the liquid viscosity, glycerin-water mixtures were used in this study. The liquid viscosity plays only a minor role in determining the internal flow pattern and the spatial distribution shape of drops, but the drop sizes themselves generally increase with increasing of the liquid viscosity. An empirical correlation for the liquid discharge coefficient at the liquid port was deduced from the experimental results; the liquid discharge coefficient strongly depends on the liquid flow area at the mixing point which is proportional to the local volumetric quality(.betha.), and the volumetric quality was included in the correlation. Regardless of the value of the liquid viscosity, the compressible flow through the gas port was well represented by the polytropic expansion process(k=1.2), and the mixing point pressure could be simply correlated to the aspect ratio( / ) of the mixing port and the air/liquid mass flow rate ratio( / ) as reported in the previous study.udy.udy.y.

Drop breakage in counter current flow liquid-liquid extraction columns

A mathematical model of the drop breakage process is applied to countercurrent flow liquid-liquid extraction columns. The resulting drop population balance equations have been solved to predict rates of drop breakage and ensuing local drop size distribution changes with height. Predictions have been compared with data obtained from a Rotating Disc Contactor.