Gas Droplet Research Articles

Effect of drainage channel dimensions on the performance of wave-plate mist eliminators

We investigated the effects of drainage channel dimensions on droplet removal efficiency and pressure drop of the gas droplet flow in a wave-plate mist eliminator. Droplet dispersion in turbulent gas flows is numerically simulated using eddy interaction model (EIM) and Eulerian-Lagrangian method. Reynolds stress transport model (RSTM) with enhanced wall treatment and shear stress transport (SST) k-ω model are used for simulating the turbulent airflow. Comparison between the numerical simulations and available experimental data shows that eddy lifetime constant (CL) can affect the results significantly, and by selecting suitable values of the eddy lifetime constant, both turbulence models give reasonable predictions of droplet removal efficiency. Simulations of gas droplet flow in the eliminators with various drainage channel dimensions show that the drainage channel length (LDC) has a greater effect on droplet removal effi- ciency than the drainage channel width (WDC).

Cell model of in-cloud scavenging of highly soluble gases

We investigate mass transfer during absorption of highly soluble gases such as HNO3, H2O2 by stagnant cloud droplets in the presence of inert admixtures. Thermophysical properties of the gases and liquids are assumed to be constant. Diffusion interactions between droplets, caused by the overlap of depleted of soluble gas regions around the neighboring droplets, are taken into account in the approximation of a cellular model of a gas–droplet suspension whereby a suspension is viewed as a periodic structure consisting of the identical spherical cells with periodic boundary conditions at the cell boundary. Using this model we determined temporal and spatial dependencies of the concentration of the soluble trace gas in a gaseous phase and in a droplet and calculated the dependence of the scavenging coefficient on time. We found that scavenging coefficient for gas absorption by cloud droplets remains constant and sharply decreases only at the final stage of absorption. In the calculations we employed a Monte Carlo method and assumed gamma size distribution of cloud droplets. It is shown that despite of the comparable values of Henry's law constants for the hydrogen peroxide (H2O2) and the nitric acid (HNO3), the nitric acid is scavenged more effectively by cloud droplets than the hydrogen peroxide due to a major affect of the dissociation reaction on HNO3 scavenging. It is demonstrated that scavenging of highly soluble gases by cloud droplets leads to strong decrease of soluble trace gas concentration in the interstitial air. We obtained also analytical expressions for the “equilibrium values” of concentration of the soluble trace gas in a gaseous phase and for concentration of the dissolved gas in a liquid phase for the case of hydrogen peroxide and nitric acid absorption by cloud droplets.

Experimental study of counter-current gas–droplet flow limitation in a 30 cm pipe

In the present study, gas–liquid two-phase flow at the counter-current flow limitation (CCFL) condition in a relatively large pipe is investigated analytically and experimentally. Droplet behavior and stable counter-current flow criteria are analyzed by studying the interaction between downward moving liquid droplets and upward gas flow. An analytical model based on the Drift Flux Model was developed to predict the onset of CCFL phenomena. The model takes into account various factors that may be encountered in the industry including gas velocity profile, droplet fraction profile, and velocity fluctuation factor. Analyses show that maintaining a uniform gas velocity profile and a stable gas flow rate is the key to prevent droplet carry-over and hence delaying the CCFL condition. For droplet–gas interaction, droplet size in the distorted particle regime is the major contribution. An experimental facility consisting of a 30cm test section pipe was designed and constructed to perform a fundamental study of counter-current droplet–gas flow. A dataset for the onset of CCFL at varying operating conditions was established. The proposed model showed the ability to predict the onset of CCFL in the counter-current two-phase flow.

CFD simulation of dust particle removal efficiency of a venturi scrubber in CFX

This research presents the computational fluid dynamics (CFD) simulation of dust particle removal efficiency of a venturi scrubber in ANSYS CFX. Venturi scrubber effectively encapsulates the dust particles from contaminated gas in petite droplets formed by disintegration of a liquid due to high kinetic energy of gas flowing into it. Eulerian–Lagrangian method is employed in ANSYS CFX to investigate the dust removal efficiency of a venturi scrubber for gas flow rate at 0.09, 0.115, and 0.14kgs−1 and liquid flow rate at 0.1, 0.13 and 0.16kgs−1. The hydrophobic dust particle titanium dioxide (TiO2) of diameter 1μm having density 4.23gcm−3 is used in the simulation and experimentation. Throat gas velocity, volume fraction, droplet size and removal efficiency are investigated to analyze the performance of venturi scrubber. Cascade atomization and breakup model (CAB) is used to predict the breakup of liquid in the venturi scrubber. The simulation results are verified from the experimental results of gas velocity at throat and dust removal efficiency. The simulation result predicts good with experimental results.

Influence of droplets in gas–metal arc welding: new modelling approach, and application to welding of aluminium

A new approach for calculating the influence of droplets on the arc and weld pool in gas metal arc welding is presented. A three-dimensional self-consistent model of the arc, workpiece and electrode that includes the influence of metal vapour is used to track the temperature, velocity and diameter of a droplet as it detaches and passes through the arc. The time averaged influence of the droplet on the arc and weld pool is then calculated. The approach allows very large savings of computational time compared to the usual volume of fluid method. The approach has been applied to one-drop per pulse welding of aluminium. Weld pool depth and shape were predicted accurately. The enthalpy transferred to the weld pool was found to have a strong influence on weld pool depth, while the flow in the weld pool depends on the momentum transferred by the droplet and the magnetic pinch force.

Emission studies from a CO2 capture pilot plant

We report on a detailed study of emissions from a pilot-plant for CO2 capture at Maasvlakte (in the Netherlands). Three contributions to emissions were identified and analyzed: Gas phase emission, aerosols (also referred to as mist or fog) and droplets of entrained solvents. For the emission campaign at Maasvlakte aerosols were found to be the major contributor to overall emissions. Entrainment was found to be a very small contributor to overall emissions. A Brownian demister unit (BDU) was tested as an emission reducing technology. It was found to be very efficient in reducing aerosol emissions.

Thermal effect of lubricating oil in positive-displacement air compressors

The isentropic efficiency of positive-displacement compressors may be improved in order to follow an increasing demand for energy savings. This work analyzes the thermal effect of the lubricating oil presence in the air during compression with the scope of exploiting it as a thermal ballast to mitigate both the gas temperature rise and its compression work. The bibliographic review shows that other authors suggested that oil can have positive effects if properly injected. Here an energy balance analysis is executed with the scope of deriving relations for the gas–liquid compression in analogy with those typical for the gas-only compression and of confirming that ideally the liquid presence may have beneficial effects, making the gas–liquid compression even better than 1- and 2-time intercooled gas compressions. Given these positive results, a heat transfer analysis is conducted to model the thermal interaction between gas and oil droplets within a mid-size rotary vane air compressor. A droplet diameter of the order of 100 μm leads to large reductions of both temperature increase and compression work: air can exit the discharge port at a temperature as low as 60 °C and compression work can be lowered by 23–28% with respect to conventional compressors. Finally, a test rig is constructed and operated to investigate a large-flow and large-angle oil nozzle taken from the market showing that, at the operating conditions of a compressor, oil breaks up into small droplets and undefined structures with large exchange surfaces.

The retention capacity for trace elements by the flue gas desulphurisation system under operational conditions of a co-combustion power plant

Water re-circulation to the scrubber and co-combustion of coal with petroleum coke are the most significant factors that affect the retention capacity for trace elements in a power plant equipped with a forced-oxidation flue gas desulphurisation (FGD) system. The water re-circulation favours the progressive saturation of most elements in the gypsum slurry with the subsequent increase in emission by entraining particles and droplets in the outgoing gas of FGD (OUT-FGD). The co-combustion increases the content of HCl and SO2 in the gaseous stream, which enhances the formation of gaseous chlorides species, especially of Hg, and the formation of condensed species of Cu, Cr, Ni, Na, Cd, and Zn OUT-FGD, respectively. High gaseous retention capacity (83–100%) for S, Cl, F, As, and B is attained by the whole plant with a subsequent reduction in their gaseous emissions below the limits established by the European directive 2001/80/EC for large combustion plants, and also below the Pollutant Release and Transfer Register (PRTR) threshold values. The retention of Se increases from 2007 (75%) to 2008 (97%). The emissions of gaseous Hg and particulate As, Zn, and Ni exceed the PRTR values. Remediation actions for the gaseous and PM emissions are relevant given the role of the filtered water re-circulation increasing the particles emissions OUT-FGD.

Research on Spray Cooling Within Finite Volume Based on Eulerian-Lagrangian Method

The two cooling styles of spraying cooling and water jacket cooling were studied for exhaust gas of engine within water-collecting box (WCB). Based on Eulerian-Lagrangian method, the heat and mass transfer between exhaust gas and water droplets were solved. By comparing the general characteristic index K/ΔP of six models with different tubes, the model with bended tubes is defined as the best model in which the length of outer tube is less than inner one. With the increase of water spraying flux, the gas temperature and the evaporation rate of water droplets tend to decrease. The decrease of gas temperature can be restrained by the phenomenon of “water assembled”, after the spraying flux rise to a certain value with continued increase. Compared with the water jacket cooling, the spraying cooling has a high heat transfer coefficient and a low cooling water cost, as well as reduces the resistance lost of exhaust gas.

Modeling of droplet traffic in interconnected microfluidic ladder devices

The problem of controlling the droplet motion in multiphase flows on the microscale has gained increasing attention because the droplet-based microfluidic devices provide great potentials for chemical and biological applications. It is critical to understand the relevant physics on droplet hydrodynamics and thus control the generation, motion, splitting, and coalescence of droplets in complex microfluidic networks. Numerical simulations using the volume of fluid algorithm are conducted to investigate the time-dependent dynamics of droplets in gas-liquid multiphase devices. An analytical model based on the electronic-hydraulic analogy is developed to describe the hydrodynamic behavior of the droplets in interconnected microfluidic ladder devices. It is found that the pressure drop caused by the droplets plays a critical role in the droplet synchronization. A fitted formula for pressure drops in the presence of surfactant is achieved by using numerical simulations. Both the numerical and the theoretical results agree well with the corresponding experimental results.

A Computational Study of Sprays Produced by Rotary Cage Atomizers

This article describes a computational study of poly-dispersed droplet spray plumes produced by Micronair AU4000 and AU6539 rotary cage atomizers that are commonly used in agriculture and forestry management. It combines experimental measurements of the droplet size spectrum and computational fluid dynamics (CFD) methods to develop a comprehensive understanding of droplet dispersion and spatial segregation in the near wake of the atomizer. The results will assist the development of improved measurement methodologies of pesticide droplet spectra. A parametric study of the Micronair AU4000 atomizer examines the effects of air speed, atomizer speed, liquid flow rate, and power source on spatial segregation of the plume. The RANS model uses a combined Lagrangian (droplet phase) and Eulerian (gas phase) procedure and includes the sprayer with blades and a large portion of the wind tunnel geometry. The computational results are compared to full-scale experimental measurements of pressure, gas phase velocity, turbulence intensity, and droplet size spectra measured using phase Doppler interferometry (PDI).

Numerical Simulation of the Twin-Wire Arc Spraying Process: Modeling the High Velocity Gas Flow Field Distribution and Droplets Transport

During the twin-wire arc spraying, a high velocity gas stream is used to accelerate the arc-melting materials and propel the droplets toward the substrate surface. This study is aimed at investigating the gas flow formation and droplets transport processes using numerical simulation method. Results from the 3-D gas flow field model show that the distribution of the gas flow velocity on the twin-wire intersection plane is quite different from that on the twin-wire vertical plane. Based on the 3-D model, the convergence amplitude of the high velocity zone in the jet center is improved by modifying the gun head design. It is also observed that a flat substrate existed downstream from the gas nozzle exit results in decreasing close to zero in velocity of the gas jet near the substrate. In addition, the predicted droplet trajectories and velocity distributions exhibited good agreement with experimentally observations.

Power and duration of impact flashes on the Moon: Implication for the cause of radiation

Meteoroid falls on the Moon produce transient luminous events usually named impact flashes. These emissions have been reported by several independent observers using ground-based telescopes over the last decade. We present here a compendium of these observations for the period 1999–2007, including apparent flash magnitudes, durations and the origin of the bolide (meteor shower or sporadic impact). Impact flashes appear on 1–10 camera frames corresponding to durations ranging from ∼10ms to ∼1s. The analysis of these data reveals a correlation between duration and intensity, with the exception of Leonid meteors. The difference between Leonids and other meteoroids are likely explained by the higher velocity of this swarm. For the other events, the observed trend implies that impact flash detections are at present limited by the frame rate which is generally equal or less than 60 frames par second (f/s). The durations of these transient events are typically longer than predictions based on expanding plasma-gas clouds. We thus argue that these luminous events correspond to radiation emitted by a cloud composed of gas and small ejected melt droplets. A simple model considering the black body radiation of cooling droplets provides a time scale commensurable with the observations. In addition, such modeling is useful for optimizing the specifications of monitoring equipment. In particular, the inferred range of effective temperatures implies that near-infrared observations would efficiently increase the number of detections, whereas multi-spectral observations are essential to progress in the understanding of the nature of these luminous events.

A high order moment method simulating evaporation and advection of a polydisperse liquid spray

In this paper, we tackle the modeling and numerical simulation of sprays and aerosols, that is dilute gas–droplet flows for which polydispersity description is of paramount importance. Starting from a kinetic description for point particles experiencing transport either at the carrier phase velocity for aerosols or at their own velocity for sprays as well as evaporation, we focus on an Eulerian high order moment method in size and consider a system of partial differential equations (PDEs) on a vector of successive integer size moments of order 0 to N, N > 2, over a compact size interval. There exists a stumbling block for the usual approaches using high order moment methods resolved with high order finite volume methods: the transport algorithm does not preserve the moment space. Indeed, reconstruction of moments by polynomials inside computational cells coupled to the evolution algorithm can create N-dimensional vectors which fail to be moment vectors: it is impossible to find a size distribution for which there are the moments. We thus propose a new approach as well as an algorithm which is second order in space and time with very limited numerical diffusion and allows to accurately describe the advection process and naturally preserves the moment space. The algorithm also leads to a natural coupling with a recently designed algorithm for evaporation which also preserves the moment space; thus polydispersity is accounted for in the evaporation and advection process, very accurately and at a very reasonable computational cost. These modeling and algorithmic tools are referred to as the Eulerian Multi Size Moment (EMSM) model. We show that such an approach is very competitive compared to multi-fluid approaches, where the size phase space is discretized into several sections and low order moment methods are used in each section, as well as with other existing high order moment methods. An accuracy study assesses the order of the method as well as the low level of numerical diffusion on structured meshes. Whereas the extension to unstructured meshes is provided, we focus in this paper on cartesian meshes and two 2D test-cases are presented: Taylor–Green vortices and turbulent free jets, where the accuracy and efficiency of the approach are assessed.

THE GAS PRESSURE INFLUENCE ON COMBUSTION CHAMBER FLOW CHARACTERISTICS

The atomization process influences combustion. The Sauter Mean Diameter (SMD), the pressure, and the temperature of the combustor play an important role in the progress of the combustion. The objective of this study is to develop a mathematical model to evaluate the effect of the gas pressure on the combustion chamber flow characteristics. Based on this model a computer program has been designed taking into account the effects of droplet internal circulation, change in combustion chamber gas temperature, the non-unitary Lewis number, the changing gas film thickness surrounding the droplet, the slip, the variable thermo-physical properties. The effect of gas pressure on Liquid Reynolds number, gas Reynolds number, non-dimensional radii, Lewis number, droplet velocity and droplet surface temperature is established. Increasing the gas pressure increases the vapor mass fraction, which affects all other parameters (thermal conductivity, diffusivity, specific heat of liquid and gas, mass and heat transfer number) and leads to a decrease in combustion time. The droplet surface is found to reach its maximum temperature in a shorter time if the pressure is higher. By increasing the gas pressure the droplet-evaporating rate increases, which leads to a decrease in evaporating time. Consequently the combustion chamber volume is decreased. Iterations on the chamber dimensions are done until the program converges.

Liquid CO 2 droplet extraction from gases

Efficient mechanical separation of CO 2 from combustion effluent affects the utilization potential of high CO 2 producers such as coal. Novel mechanical separations of condensing CO 2 from gas flows need to be able to capture the small condensed droplets below the cyclone cut-off limit of 20 μm. We describe the thermodynamics, the energy costs and droplet formation of CO 2 phase separation from combustion effluent and natural gas. We report the first measurements of condensing CO 2 droplet sizes from gas. This shows that application of homogeneous condensation of CO 2 yields much smaller droplets in flue gas (N 2/CO 2) than from contaminated natural gas (CH 4/CO 2). These small droplets can only be efficiently removed at high throughputs using the novel centrifugal method we describe. Such mechanical separations are preferable to the current standard chemical methods because of the much lower environmental footprint.

Evaporation of liquid fuel droplet at supercritical conditions

A comprehensive analysis of liquid fuel droplet evaporation at supercritical conditions is performed. The numerical model is based on complete time-dependent conservation equations, with a full account of variable thermophysical properties and vapor-liquid interfacial thermodynamics. And the model employs the Peng-Robinson (PR) equation of state (EOS). As a specific example, problems involving n-heptane droplet in nitrogen gas are investigated. The results indicate that the increase of ambient pressure and temperature results in the increase of surface temperature rise rate and surface regression rate. The transition from subcritical state to supercritical state can occur at the droplet surface when the droplet evaporates in a strongly supercritical environment.

Numerical study of droplet impact and coalescence in a microline patterning process

The droplet impact and coalescence on a substrate, which is applicable to the manufacture of microlines, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid–gas interface or droplet shape is tracked by a sharp-interface level-set method which is modified to account for the change between advancing and receding contact angles at the liquid–gas–solid interline. The numerical results show that the droplet impact and merging pattern depends significantly on the advancing and receding contact angles. Also, the effect of droplet spacing on the droplet motion is investigated to find the optimal conditions in manufacturing microlines.

Tornillos at Vulcano: Clues to the dynamics of the hydrothermal system

The number of tornillo events has recently increased at the Vulcano Island, Italy. While only 15 tornillos were recorded during 2004–2006, 584 events occurred in 2007–2008. They were located just below La Fossa Crater at depths ranging between 0.1 and 1 km b.s.l. During two intervals in 2007–2008 increases in the number of tornillos took place at the same time as temperature and geochemical anomalies were observed. The spectral content of the tornillos, generally characterized by one–two dominant spectral peaks near 6 and 10 Hz, varied over time, with changes also noted in the quality factors. The simplest source mechanism proposed for tornillos is the free eigenvibration of a fluid volume within a crack or a conduit. Based on this model, we propose a causal relationship between the temperature and geochemical anomalies and the increases in numbers of tornillos. As the amount of hydrothermal fluids increases during the anomalies, the upward flux of fluids grows. The consequent changes in the pressure, temperature and dynamics of the system of cracks and conduits result in the generation of tornillos. Based on the fluid-filled crack/conduit model, the shallow depths of the sources and the values of the quality factors, the fluid within the resonant crack/conduit was inferred to be an ash–gas or water droplet–gas mixture. Moreover, the observed variations in the wavefield can be caused by small changes in the location of the source, in the source mechanism, or in the medium in between the source and the seismic station. Finally, another peculiar feature of tornillos is the amplitude modulation that can be explained as a result of a beating phenomenon.

The effect of gas and liquid properties and droplet size ratio on the central collision between two unequal-size droplets in the reflexive regime

A numerical investigation of various parameters affecting the collision process of two unequal sized droplets in a gaseous environment in the reflexive regime is presented. The investigation is based on the finite volume numerical solution of the Navier–Stokes equations, in their axial-symmetric formulation, expressing the flow field of the two phases, liquid and gas, coupled with the volume of fluid method (VOF) for tracking the liquid–gas interfaces; a recently developed adaptive local grid refinement technique is used in order to track more accurately the liquid–gas interface with reduced computational cost. A colour function is used in order to follow the mixing process of the two droplets after collision. The reliability of the procedure is first established by comparing predictions with available experimental data and important details as regards the time evolution of the deformed shapes, the ligament formation, the maximum deformation of the droplets and the penetration of one droplet into the other are presented. The collision process and its outcome are investigated having the liquid properties (Ohnesorge number based on the liquid properties), the gas phase properties (Reynolds number based on gas properties) and droplet size ratio as the controlling parameters.