Droplet Collection Efficiency Research Articles

Performance of a Novel Rotating Gas-Liquid Separator

A novel gas separation process makes use of a rotating phase separator to separate micron-sized droplets from a gas stream. Based on an industrial scale design, a water/air separator is constructed and tested. The first experiment concerns the drainage of large fractions of separated liquid. During operation, drainage is observed via windows and a descriptive model is formulated. Because of the major influence on overall separation efficiency, liquid drainage is a key issue in the separator design. The second experiment comprises a droplet collection efficiency measurement using micron-sized droplets dispersed within the airstream. The separation efficiency of fine droplet removal is measured. This is an important factor in reducing capital costs.

Robust and Accurate Eulerian Multiphase Simulations of Icing Collection Efficiency Using Singularity Diffusion Model

:We develop a robust and accurate three dimensional Eulerian method to compute droplet impingement on an aircraft body, wings, and engines. One of the principal challenges in developing a robust Eulerian method is the existence of singularities in some analytic solutions. We have found that, particularly for higher order methods, that the non-linearity of the equations facilitates the creation of spurious singularities. These result in an unbounded growth of particle density in localized regions. We propose an automatic, physics based adaptive numerical diffusion model whereby the existence of singularities is detected and used to derive a numerical diffusion stabilization term. Presence of the numerical diffusion appears to significantly improve the stability of higher order simulations. The model of droplet collection efficiency is implemented in CHEM solver in which a traditional finite volume scheme is applied to discretize both droplet and gas flows. The governing equations for droplets are presented, along with the analysis and design of the singularity diffusion model. Test results on three dimensional bodies are presented in comparison with NASA experimental impingement results. The numerical diffusion model is shown to be effective and simple.

Single and two-stage electrostatic demisters for biomass pyrolysis application

Single-stage and two-stage tubular electrostatic precipitators were designed. A nitrogen stream containing very fine droplets of fogging oil was forced through the electrostatic precipitator chamber. It was found that 98.6 wt% of the oil droplets present in the turbulent jet were mechanically collected on the inner walls of the test chamber. When the electrode was energized at 13 kV, 92.37 wt% of the droplets that had not been mechanically separated were collected in single-stage mode. The collection efficiency was increased to 93.18 wt%, when the electrostatic precipitator was operated in two-stage mode. Voltage–current ( V– I) characteristics of the single-stage and two-stage electrostatic precipitators were studied in detail for different test conditions. Nitrogen impurities played a major role in determining the V– I characteristics. They became less relevant with the introduction of mist in the nitrogen stream, presumably due to the presence of water vapor in the gas. The two-stage tubular electrostatic precipitator was scaled up and tested on a fluidized bed pilot plant used for the pyrolysis of biomass. A droplet collection efficiency of 95 wt% was observed. Such demisters will extend, to the product recovery train, the process intensification gains of short residence time processes such as fast pyrolysis.

Ice accretion simulation on multi-element airfoils using extended Messinger model

In the current article, the problem of in-flight ice accumulation on multi-element airfoils is studied numerically. The analysis starts with flow field computation using the Hess-Smith panel method. The second step is the calculation of droplet trajectories and droplet collection efficiencies. In the next step, convective heat transfer coefficient distributions around the airfoil elements are calculated using the Integral Boundary-Layer Method. The formulation accounts for the surface roughness due to ice accretion. The fourth step consists of establishing the thermodynamic balance and computing ice accretion rates using the Extended Messinger Model. At low temperatures and low liquid water contents, rime ice occurs for which the ice shape is determined by a simple mass balance. At warmer temperatures and high liquid water contents, glaze ice forms for which the energy and mass conservation equations are combined to yield a single first order ordinary differential equation, solved numerically. Predicted ice shapes are compared with experimental shapes reported in the literature and good agreement is observed both for rime and glaze ice. Ice shapes and masses are also computed for realistic flight scenarios. The results indicate that the smaller elements in multielement configurations accumulate comparable and often greater amount of ice compared to larger elements. The results also indicate that the multi-layer approach yields more accurate results compared to the one-layer approach, especially for glaze ice conditions.

Stochastic modeling approaches based on neural network and linear–nonlinear regression techniques for the determination of single droplet collection efficiency of countercurrent spray towers

This paper presents a new mathematical model and a two-layer neural network approach to predict the single droplet collection efficiency (SDCE), hd, of countercurrent spray towers. SDCE values were calculated using MATLAB \ algorithm for 205 different artificial scenarios given in a large range of operating conditions. Theoretical results were com- pared with outputs obtained from a two-layer neural network and DataFit \ scientific software. The predic- ted model developed from linear-nonlinear regression analysis and network outputs agreed with the theoret- ical data, and all predictions proved to be satisfactory with a correlation coefficient of about 0.921 and 0.99, respectively. By using the proposed model, iterations between Reynolds number (Re), drag coefficient (CD) and terminal velocity values (vT) were neglected for a large range of operating conditions. SDCE values were also obtained speedily and practically for five main operating inputs used in the model.

Analytic formulation and parametrization of the kinetic potential theory for drizzle formation

The kinetic potential of nucleation theory is extended to describe cloud droplet growth processes that can lead to drizzle formation. In this model drizzle formation is identified as a statistical barrier crossing phenomenon that transforms cloud droplets to much larger drizzle size with a rate dependent on turbulent diffusion, droplet collection efficiency, and properties of the underlying cloud droplet size distribution. Closed-form expressions for the kinetic potential, critical drop volume, barrier height, and both steady-state and transient barrier crossing drizzle rates are obtained in terms of measurable cloud properties. In an analogy with the theory of phase transformation, clouds are classified into two regimes: an activated metastable regime, in which there is a significant barrier and drizzle initiation resembles nucleation, and an unstable regime where kinetics dominates analogous to the spinodal regime of phase transformation. Observational evidence, including the threshold behavior of drizzle formation and the well-known effect that aerosols have on drizzle suppression, is shown to favor drizzle formation under activated conditions (more similar to nucleation than spinodal decomposition) and under transient conditions rather than steady state. These new applications of the kinetic potential theory should lead to more accurate parametrizations of aerosol-cloud interaction and improved algorithms for weather forecasting and climate prediction.

The Role of Drainage Channels in the Performance of Wave-Plate Mist Eliminators

Numerical simulation of the performance of an idealized wave-plate mist eliminator is used to assess the influence of drainage channels in the mist eliminator passages. The effect of the drainage channels on the primary gas flow and on the evolution of the liquid droplet distribution is calculated for a range of gas speeds, wave-plate spacing, channel sizes and inlet droplet size distributions. The ability of drainage channels to increase the droplet collection efficiency is confirmed and several features of the flow are predicted which have implications for the design of wave-plate mist eliminators.

Numerical Study of Particle Collection by Single Water Droplets

Particle collection efficiencies of water droplets were calculated by considering thermophoresis, diffusiophoresis, inertial impaction, wake capture, and effects of drop deformation. The computatio...

AN IMPROVED SINGLE DROPLET COLLECTION EFFICIENCY FOR THE INTERMEDIATE FLOW REGIME

ABSTRACT An improved angle droplet collection efficiency model for the intermediate flow regime is presented in this paper, taking into account both inertial impaction and interception mechanisms. This model uses the equations of motion that has been derived by performing a force balance on a particle interacting with the flow field of a spherical collector. The fluid flow field around the collector is assumed to be the approximate solution as developed by Hamielec and Johnson for Reynolds numbers ranging between 10 and 80 and Tomotika and Aoi for Reynolds numbers less than 1.0. The results of this work indicate that the collection efficiencies calculated by using potential flow conditions may have overestimated the overall collection of particulate matter. It was identified that the transition from intermediate to potential flow occurs when the Reynolds number is about 80. The interpolation scheme for the single droplet collection efficiency proposed in this work can be used from Stokes flow to potential flow conditions including intermediate flow regime.

Collection Efficiency of Spray Droplets on Vertical Targets

A computational fluid dynamics program (FLUENT) was used to determine the effects of several variables on collection efficiencies of spray droplets directed horizontally toward vertical targets in turbulent air. Variables were: droplet diameter (10 to 600 mm), wind velocity (0.5 to 8.0 m/s), turbulence intensity (5 to 80%) and target width (6.35 to 50.8 mm). An equation (r2 = 0.94) was developed to predict the collection efficiencies of water droplets for the variables. Collection efficiency increased with increasing droplet size, wind velocity, and target width, but decreased with increasing turbulence intensity. Collection efficiencies calculated with FLUENT were very close to those determined experimentally in a wind tunnel. The maximum difference between experimental and calculated collection efficiencies was 11.1% for all experimental conditions.

Comparison of model results of collection efficiency of aerosol particles by individual water droplets and ice crystals in a subsaturated atmosphere

The aerosol collection efficiencies of water droplets and ice crystals are compared based on the concept of equivalent geometrical kernel K ∗ which is the geometrical sweep-out volume per unit time by the collector. It is thought that the comparison based on this quantity reveals the real difference of the aerosol collecting abilities of different collectors and sheds lights on the precipitation scavenging mechanisms. The collection efficiencies are taken from theoretical model results computed by us previously at relative humidities of 95% for water droplets, columnar and hexagonal plate ice crystals. It is shown that the efficiencies are rather insensitive to collector shape for aerosol particles smaller than 0.01 μm. The shape factor becomes more important for larger aerosol particles, especially in the Greenfield-Gap size range.

Estimating cloud water deposition to subalpine spruce-fir forests—I. Modifications to an existing model

A previously published steady-state model for computing cloud water deposition to a subalpine balsam fir ( Abies balsamea) forest was modified for more generalized application to spruce-fir forests. One modification provided options for describing the cloud droplet size spectrum using observed relationships, in the vicinity of high elevation forests, between cloud liquid water content and the distribution of droplet size. Another modification implemented an optional experimental droplet collection efficiency parameterization scheme. This scheme computes collection efficiency for the most dense portion of a tree by treating it as a bulk collector rather than a multi-component (stems, branches, etc.) structure for which collection efficiency is determined by the individual collection efficiencies of its components. A study of model sensitivity to various physical parameterizations revealed that computations of gross cloud water flux to a canopy are most sensitive to canopy inhomogeneity, the relationship between cloud liquid water content and droplet size spectrum, and droplet collection efficiency. As expected, sensitivity test results also indicated that computed evaporation (and hence, the computed net cloud water flux) of intercepted cloud water from a canopy is strongly dependent on net radiation. The model modifications and sensitivity studies described here are the prelude to a test of the model described in a companion paper.

Estimating cloud water deposition to subalpine spruce-fir forests—II. Model testing

A modified version of a previously published steady-state model for computing cloud water deposition to a subalpine balsam fir ( Abies balsamea) forest was tested using water throughfall data collected in a red spruce ( Picea rubens) forest on Whitetop Mountain, Virginia. Detailed wind data were collected in two distinctly different spruce stands to define airflow conditions within the forest canopy. Other meteorological and canopy structure data were also collected for use as inputs to the deposition model. Model simulations of cloud deposition during 11 cloud events in the two forest stands revealed that the model performed best when site-specific wind speed profiles and droplet size spectra were used along with an experimental droplet collection efficiency scheme that treats the densest portions of trees as bulk collectors (as opposed to modelling collection efficiency for individual tree components). An analysis of residuals indicated that model errors were most strongly correlated with cloud liquid water content ( W), a model input. It is speculated that the correlation with W was due to a combination measurement bias when clouds were thin or intermittent and a model computational bias when the potential (defined by the model) for the vertical turbulent flux of cloud water was high. Overall, computed values (using the optimally-configured model) of net cloud water flux tended to exceed measured throughfall rates by 20–30%, and the model explained 38–68% of the variance in throughfall rate. A comparison between the mechanistic model and a simpler empirical model indicated that the mechanistic model performed no better than its empirical counterpart.

A comparison of two cloudwater/fogwater collectors: The rotating arm collector and the caltech active strand cloudwater collector

A side-by-side comparison of the Rotating Arm Collector (RAC) and the Caltech Active Strand Cloudwater Collector (CASCC) was conducted at an elevated coastal site near the eastern end of the Santa Barbara Channel in southern California. The CASCC was observed to collect cloudwater at rates of up to 8.5 ml min −1. The ratio of cloudwater collection rates was found to be close to the theoretical prediction of 4.2:1 (CASCC:RAC) over a wide range of liquid water contents (LWC). At low LWC, however, this ratio climbed rapidly, possibly reflecting a predominance of small droplets under these conditions, coupled with a greater collection efficiency of small droplets by the CASCC. Cloudwater samples collected by the RAC had significantly higher concentrations of Na +, Ca 2+, Mg 2+ and Cl − than those collected by the CASCC. These higher concentrations may be due to differences in the chemical composition of large vs small droplets. No significant differences were observed in concentrations of NO 3 −, SO 4 2− or NH 4 + in samples collected by the two instruments.

Ice monster (Jyuhyo) in Mt. Zao

Ice monster are the product of a peculiar icing phenomenon that is seen only in certain mountain areas in Tohoku Distrct, including Mt. Zao, Mt. Hatimanntai.The ice-snow accretion (Ice monster) in Mt. Zao results from both icing and snow accretion and is a complicated compound phenomenon which requires wide-ranged experiments and observations to be explained and overcome. In this study, attention was directed to the environmental conditions that facilitate the development of ice monster, based on the topography of Mt. Zao, meteorological conditions, depth of the snow cover, and observations of the growth process of this peculiar phenomenon.Several types of models for the Abies mariesii were prepered in order to determine the collection efficiency of snow particles and supercooled water droplets by these models. The observations were carried out on the development of ice-snow accretion. They shed light on unknown factors in the growth of the ice monster. Experiments were conducted at a low-temperature laboratory and a field station in Mt. Zao. The process of growth, rates of growth, adhering state of snow, etc., were measured ; and the data obtained were compared and discussed.

Predicted and observed droplet deposition on tsetse flies (Glossina morsitans) following aerosol application from aircraft

AbstractA non‐toxic formulation containing fluorescent tracer was dispersed as an aerosol over savannah woodland in stable meteorological conditions, with only light wind, shortly after sunrise. Droplet deposition on wild caught tsetse flies (Glossina morsitans) was measured by means of a fluorescent tracer technique and compared with the deposit levels predicted from rotary sampler measurements using droplet collection efficiency data for samplers and flies derived from a laboratory wind tunnel study. Good correlation was found between the predicted and observed dose. An additional comparison, based only on rotary sampler data, between a typical spray (measured v.m.d. 27 μm) and a spray with smaller drop size (measured v.m.d. 21 μm) suggested that the latter might be significantly more effective as a means of control.

Droplet size distribution effects on aircraft ice accretion

The impinging mass flux distribution which determines aircraft ice accretion rate is shown to be related to the atmospheric droplet size distribution through the droplet collection efficiency of the body. Collection efficiency is studied by means of a two-dimensional droplet trajectory code which includes the effect of nonspherical droplet shape due to hydrodynamic deformation. The simulation was found to agree with wind tunnel photographic studies of droplet kinematics. The results of the simulation are used to generate impinging mass flux distributions for typical cloud and precipitation size distributions. The impinging mass approach is also used to determine relative icing rates from several supercooled cloud characterizations including the intermittent maximum icing envelope of Federal Aviation Regulation, Part 25.

Collection and coalescence efficiencies for accretion

The coalescence efficiency for accretion was assumed to be governed by a collisional Weber number. This concept was used with the aid of theoretical collision efficiences to organize collection efficiency data from several sources into a unified set. The resulting collection efficiencies show a maximum of 71% for a collector drop of 115 μm and a cloud droplet of 12‐μm radius. Accretion calculations were made for a monodisperse cloud to illustrate the effects of cloud droplet sizes and collection efficiencies on drop growth. The time average collection efficiency for growth from 50 to 500 μm by accretion of 30‐μm droplets is reduced by 50% when compared to the time average collision efficiency. A maximum of 65% in the time average collection efficiency occurs for accretion of 10‐μm droplets. The contribution to accretion by cloud droplets smaller than 5 μm appears to be negligible.

The effects of thermophoresis and diffusiophoresis on the collection of charged sub-μm particles by charged droplets

The collection efficiency of charged water droplets of 500 μm diameter for charged particles of 0.2 μm diameter is examined as a function of the nondimensional Coulombic parameter. The present paper provides an experimental technique to determine the contribution of the phoretic forces in particle collection by comparing the collection efficiencies of water droplets to that of oil droplets for which the vapor pressure and the phoretic forces are negligible. Reasonable agreement is obtained between the experimental data and the values predicted by a collection efficiency model considering only Coulombic force, thermophoresis and diffusiophoresis.

COLLECTION OF INERTIALESS PARTICLES ON SPHEROIDS AND SPHERES WITH ELECTRICAL FORCES AND GRAVITATIO

Collection efficiencies are predicted for the capture of fine, incrtialess, charged particles on a single spheroidal collector in a gaseous flow field by the action of coulombic and external electric field forces and gravity. With the flow and external fields parallel to the axis of symmetry, collection efficiencies for spheroidal collectors are found by determining particle trajectories. For three-dimensional nonsymmetric systems resulting from the flow being at an arbitrary angle to both the axis of symmetry and the external fields, collection efficiencies are found by determining particle fluxes to the collector. For single force cases, particle deposition is independent of collector geometry and, for point panicles, is the same for all stationary incompressible flows. Using a cellular model of flow around a sphere, droplet density in wet scrubbers is shown to have little hydrodynamic effect on single droplet collection efficiencies.