Gas-droplet Flow Research Articles

CFD investigation of post-combustion CO2 capture using an industrial spray tower: Regulation effects of spray schemes

The use of spray towers for post-combustion CO2 capture has attracted growing industrial interest. However, its practical application is hampered by a lack of knowledge about CO2 spray absorption on an industrial scale. Herein, we developed a numerical model to explore the potential of the spray tower for CO2 removal at a 330 MW coal-fired power plant. The model was developed in the Euler-Lagrange framework integrated with interphase transfer models. The variations of gas-droplet flow, CO2 removal efficiency, CO2 loading of rich solution, and temperature distribution under different spray schemes were investigated. It was found that gas flow hydrodynamics and CO2 removal performance can be regulated by the spray scheme. Compared to the conventional downward spray scheme, the lower nozzle layer adopted an upward spray scheme can improve the uniformity of the gas flow and results in several advantages: (1) average droplet residence time is extended by 5.2 %; (2) CO2 removal efficiency increases from 79.1 % to 82.9 %. This improvement is attributed to two factors: one is the droplet’s re-distribution, lowering the gas flow resistance in the region below the first spray layer near the inlet; the other is the change in gas-droplet interaction whereby the upward-moving droplets entrain the gas to flow upwards. Moreover, we investigated the effects of the spray schemes under varying flue gas loads. Under these conditions, the upward spray mode adopted in the first layer shows the best CO2 removal performance. Further theoretical analysis reveals the mechanisms underlying the regulation effect. The findings in this work will benefit the design and optimization of the industrial-scale spray tower for CO2 removal.

Formation of a Spray of Conducting Liquid under High Voltage and Electrical Discharge Plasma

The joint impact of an electrical discharge and a gas–droplet flow of a conducting liquid created by a centrifugal nozzle is studied. In the experiment, the electrical parameters of the discharge are recorded, and the local characteristics of the spray, namely, the droplet diameter distributions of the number of droplets and the components of their velocity vector are measured. The measurements are carried out using double shadow microphotography. The effect of a spray of a conducting liquid on the time-averaged electrical parameters of the discharge, as well as changes in the properties of the spray under the influence of high voltage and current, are studied. Additionally, the spray characteristics are measured when a high voltage of pre-breakdown value is applied. High-speed photography of a discharge in the presence of droplets of conducting liquid is carried out.

Steady Gas-Droplet Flow Pattern and Heat Transfer Behind the Point of Incidence of a Shock Wave on a Flat Wall

Steady Gas-Droplet Flow Pattern and Heat Transfer Behind the Point of Incidence of a Shock Wave on a Flat Wall

Steady Gas-Droplet Flow Pattern and Heat Transfer Behind the Point of Incidence of a Shock Wave on a Flat Wall

The structure of a steady 2D gas-droplet flow in the near-wall region behind the point of incidence of a normal or an oblique shock wave on a plane wall is investigated. In this case, the normal wave corresponds to the Mach stem in the Mach reflection mode, and the oblique wave corresponds to the regular reflection mode of the incident oblique wave. The main aim of the study is to evaluate the effect of small liquid droplets present in the free stream on the equilibrium temperature of the adiabatic wall behind the point of wave reflection. The question is investigated: to what extent the presence of an oblique shock wave incident on the wall can enhance the effect of reducing the equilibrium wall temperature by small droplets present in the flow. The flow region is split into the outer region of “effectively inviscid flow” and the region of an asymptotic laminar boundary layer. Flow calculations in each region are based on a two-fluid model of a gas-droplet mixture, taking into account the phase transition (evaporation) on the droplet surface. The most interesting wave configurations from the point of view of heat transfer, corresponding to “fully and partially dispersed waves” with incomplete evaporation of droplets behind the reflected wave, are studied. A simple limiting scheme of the formation of a liquid film by droplets deposited on the wall is adopted, with the effects of film instability and spattering being ignored. Based on numerical calculations, the estimates are obtained for the possible decrease in the equilibrium temperature of the adiabatic wall behind the point of incidence of a shock wave in a steady supersonic gas flow containing a low concentration of liquid droplets.

CFD analysis of the spray droplet parameters effect on gas-droplet flow and evaporation characteristics in circulating fluidized bed desulfurization tower

The circulating fluidized bed flue gas desulfurization (CFB-FGD) process is a promising semi-dry flue gas desulfurization technology. The gas-droplet flow characteristics and droplets evaporation characteristics are critical to improving the desulfurization efficiency of the FGD tower. The effects of droplet parameters, including spray droplet diameter, spray droplet velocity, spray droplet mass flow rate and spray angle are investigated. Two evaluation indices, the coefficient of variation of droplet concentration (CV) and droplet mass-weighted life, are adopted to evaluate mixing degree of gas-droplet and droplet evaporation time, respectively. Results show that as the spray droplet diameter increases from 40 µm to 80 µm, the amount of water droplets trapped by tower wall increases from 3% to 21%. Increasing the spray droplet velocity and spray droplet mass flow rate could effectively increase the turbulent kinetic energy, which could improve the turbulent mixing ability. Moreover, droplet with smaller diameter and high velocity are beneficial to improving the temperature distribution uniformity. When the spray droplet mass flow rate exceeds 2.7 kg/s, the temperature in the tower would be lower than the dew point temperature, resulting in corrosion of the tower wall. Furthermore, the evaporation rate of droplets decreases with the increase of droplet diameter, spray droplet velocity and spray mass flow rate, while increases with the increase in the spray angle. This study would provide a better reference for the optimum operating scheme in practical applications.

Investigation of Gas-Droplet Flow in Presence of Electrical Discharge

Investigation of Gas-Droplet Flow in Presence of Electrical Discharge

Numerical simulation on gas-droplet flow characteristics and spray evaporation process in CFB-FGD tower

Nozzle arrangement in the nozzle spray system has a significant impact on the gas-droplet flow characteristics and the temperature distribution within the circulating fluidized bed flue gas desulphurization (CFB-FGD) tower, which is critical to the SO2 removal efficiency. The effects of spray direction, nozzle number and nozzle spray angle on gas-droplet distribution and temperature distribution inside the FGD tower are investigated with numerical simulation based on a Eulerian-Lagrangian mathematical model. An optimal nozzle arrangement scheme is proposed to improve the contact between gas and water droplets and the flue gas temperature distribution. Results show that upward spray direction is beneficial to the interaction between water droplets, improving gas-droplet flow characteristics and spray evaporation process, and water droplets number trapped by tower wall could be reduced in the water droplets evaporation. With the increase in nozzle number, it is conducive to the contact between flue gas and water droplets to increase the evaporation efficiency of water droplets, as well as the uniformity of temperature distribution inside the tower. With nozzle spray angle increases from 30° to 120°, flue gas velocity decreases, water droplets number trapped by the tower wall increases. The temperature distribution at different cross-section is the most uniform when the nozzle spray angle is 60°.

Моделирование процесса обледенения корпуса рыболовецкого судна на поверхности воды с учетом влияния волнения

Studying the icing of ships is an urgent task. The paper considers the problem of modeling the flow of a model vessel with a gas-droplet flow and the occurrence of the icing process. Initially, the simulation was performed using the interDyMFoam solver, taking into account the assignment of the Stokes wave of the first kind to determine the position of the droplets. Further modeling was carried out using the iceFoam solver, which is based on the Euler-Lagrangian method for describing the gas-droplet flow. The considered model of a fishing vessel had a scale of 1:10. The position of the droplets was set at the entrance to the calculated rectangular domain. The estimated grid had from 1.5 to 10 million. cells. With the help of calculations, the trajectories of droplet movement around the hull of the vessel, the distribution of the air velocity field, the position of the water film and the thickness of ice on the deck surface were obtained. The mass of the overgrown ice was estimated. The simulation was performed on the computing cluster of the ISP RAS. One typical calculation was run on 48-96 computing cores and lasted no more than three days.

Структура течения и теплоперенос в газокапельном течении за внезапным сужением канала

The numerical study of the local flow structure and heat transfer in a gas-droplet turbulent flow behind a forward-facing step in the two-phase gas-droplet flow is carried out. Two-dimensional steady-state Reynolds-averaged Navier-Stokes (RANS) equations are used for the numerical solution. They were written taking into account the presence of a dispersed phase. The Eulerian two-fluid approach is used to describe the flow dynamics and heat and mass transfer in the gaseous and dispersed phases. The turbulence of the carrier phase was described using an elliptical model for the transport of Reynolds stress components, taking into account the presence of droplets. The effect of evaporating droplets flowing after the forward-facing step on the local flow structure, turbulence, distribution of the dispersed phase, and heat transfer inetensification is analyzed.

Convection in the liquid at droplet squeezing out of the capillary

Experimental studies of droplet geometry and instantaneous velocity field in the liquid at a rapid change in the droplet shape are conducted. A droplet is squeezed out of the capillary in the presence of an external air flow. This article for the first time shows that when a droplet surface changes, impurities from the ambient air accumulate on the free surface within 10 s rather than instantly. Consequently, the average velocity in the drop varies from 90 mm/s to 3 mm/s. As the air temperature increases, the liquid in the drop heats up and the dynamic viscosity of both the drop and the air changes. Therefore, with an increase in air temperature from 20 to 100 ° C, the convection velocity in the drop increases almost two times from 0.1 to 0.2 ms. In practice we often have to deal with droplets and gas-droplet flows when fast-flowing processes are implemented. The lifetime of the droplet, after its formation from the nozzle, is a short time interval (0.001 s – 1 s). For correct modeling of the gas droplet flow, it is necessary to take into account the change in the convection velocity in the droplet, which changes with time.

Modeling of Turbulent Heat-Transfer Augmentation in Gas-Droplet Non-Boiling Flow in Diverging and Converging Axisymmetric Ducts with Sudden Expansion

The effect of positive (adverse) and negative (favorable) longitudinal pressure gradients on the structure and heat transfer of gas-droplet (air and water) flow in axisymmetric duct with sudden expansion are examined. The superimposed pressure gradient has a large influence on the flow structure and heat transfer in a two-phase mist flow in both a confuser and a diffuser. A narrowing of the confuser angle leads to significant suppression of flow turbulence (more than four times that of the gas-drop flow after sudden pipe expansion without a pressure gradient at φ = 0°). Recirculation zone length decreases significantly compared to the gas-droplet flow without a longitudinal pressure gradient (by up to 30%), and the locus of the heat-transfer maximum shifts slightly downstream, and roughly aligns with the reattachment point of the two-phase flow. Growth of the diffuser opening angle leads to additional production of kinetic energy of gas flow turbulence (almost twice as much as gas-droplet flow after a sudden pipe expansion at φ = 0°). The length of the flow recirculating region in the diffuser increases significantly compared to the separated gas-droplet flow without a pressure gradient (φ = 0°), and the location of maximum heat transfer shifts downstream in the diffuser.

Compressible gas-droplet flow and heat transfer behind a condensation shock in an expanding channel

A two-fluid mathematical model of a supersonic gas-droplet mixture flow with account of droplet growth due to vapor condensation behind a condensation shock in a plane expanding channel is constructed. The flow structure in both the inviscid core and the two-phase boundary layer assumed to be laminar is studied numerically. The range of flow parameters is considered in which the spontaneous condensation zone (“condensation shock”) is fairly narrow, and just behind this shock thermal parameters of the phases almost reach thermodynamic equilibrium. Downstream, the droplet radius continues to grow due to the flow expansion and further vapor condensation. Of primary concern is the flow region where the droplets become sufficiently inertial to travel with a velocity slip, and hence to model adequately the velocity and temperature fields in both phases a two-fluid model is required. In addition to the Stokes drag, the Saffman lifting force exerted on the droplets in the boundary layer is taken into account. The Saffman force results in droplet deposition and the formation of a thin liquid film on the channel walls. A parametric numerical analysis of the two-phase flow structure in the inviscid flow region and in the near-wall boundary layer is performed, and the effects of phase transitions (droplet condensation and evaporation, as well as the film formation) on the reduction of the adiabatic channel wall temperature are analyzed.

ПРИМЕНЕНИЕ АЭРОЗОЛЬНОГО МЕЛКОДИСПЕРСНОГО СПРЕЯ ДЛЯ ПОВЫШЕНИЯ ЭФФЕКТИВНОСТИ ОХЛАЖДЕНИЯ ОРЕБРЕННЫХ ТРУБ

The article presents an overview of works on the experimental and computational study of the processes of heat removal from heated surfaces using a water-air mixture. A sharp increase in the heat transfer coefficient is shown even when adding water with a mass content of 0.1 % to the air flow. The factors that determine the efficiency of the heat exchange process are listed: the shape of the water spray jet, the distance from the outlet point of the water-air flow to the heat exchange surface, the characteristics of the heat exchange surface, the method of jet formation, the size of droplets and the location of the outlet points of the gas-droplet flow. The weak applicability of the data available in the literature is shown for calculating the cooling of finned tubes of industrial heat exchangers using a water-air mixture. It is substantiated that in order to establish the most optimal cooling modes for each for a separate heat exchange surface, it is necessary to conduct experimental studies, due to the complexity of heat transfer processes and the presence of a large number of influencing factors. It is shown that the finned tubes of heat exchangers of the passive heat removal system of WWER-1200 have a number of important features that affect the efficiency of their cooling using a water-air mixture. An experimental setup has been developed and a technique has been proposed for performing experiments to study these processes as applied to heat exchangers of a passive WWER safety system.

Ultra-dispersed particles of water-soluble and water-insoluble substances formed from gas-droplet flows

The work is devoted to the experimental study of the ultra dispersed particles formation process, which is essentially a fundamental problem of heat and mass transfer in gas-droplet flows under phase transformation conditions. Various methods of generating and controlling the gas-droplet flow parameters are considered. The description of the created equipment and diagnostic methods for studying gas-droplet flows is given. The particle size distribution functions are obtained using an aerosol particle spectrometer. The possibility of controlling the parameters of ultrafine particles of water-soluble and water-insoluble substances (medications) formed under evaporation of microdroplets of solutions and suspensions is shown experimentally. An attempt to reconstruct the distribution functions of microdroplets and their average dimension from measurements of the parameters of ultrafine particles formed after complete evaporation of the droplets is made.

The Mechanism of Droplet Levitation in Gas–Droplet Flows Past Bodies

The Mechanism of Droplet Levitation in Gas–Droplet Flows Past Bodies

Interaction of a liquid jet with a co-current gas flow inside the nozzle and under ejection into vacuum

The structure of a gas-droplet flow arising under gas outflow with liquid jet injected into it from a supersonic nozzle into vacuum is studied experimentally. Possibility of the flow structure control in order to obtain droplets of a certain size, composition, and velocity, is considered. The liquid was injected into the co-current gas flow in the prechamber of the supersonic nozzle and then flowed out into the vacuum chamber in the form of a gas-droplet jet. Using the developed technique of droplet deposition on paper substrates, the effect of the Reynolds number of the gas and the pressure in the vacuum chamber on the angular distribution of droplet phase behind the nozzle exit is investigated.

Structure of the Detached Gas–Droplet Flow and Heat Transfer in an Axisymmetric Confuser

Structure of the Detached Gas–Droplet Flow and Heat Transfer in an Axisymmetric Confuser

Numerical study on the mixing and evaporation process of a liquid kerosene jet in a scramjet combustor

The mixing and evaporation process of a liquid kerosene jet in a scramjet combustor with dual-cavity is investigated by two-phase Large Eddy Simulation. The gas-droplet flow system is solved through a two-way coupled Eulerian-Lagrangian method. The primary and secondary breakup processes of the liquid fuel jet are taken into account. The Langmuir-Knudsen evaporation model, which considers the non-equilibrium effect, is used to calculate the evaporation process of the droplets. The penetration depth of the jet is in good agreement with the experimental value. The dynamic process of the liquid jet breakup, spray evaporation, and fuel transportation within the combustor are well described by the numerical results. The distribution and evaporation characteristics of the spray are analyzed and most droplets are evaporated within 30 mm downstream of the jet orifice. However, a small portion of droplets with high velocity and low temperature can survive for a long distance. The influence of injection pressure on the mixing process near the cavity is also analyzed, and it is found that the injection pressure affects the total quantity of fuel entrained into the cavity by affecting the fuel distribution in the near-wall region and the total fuel mass flow rate. The local distribution of the liquid and gas phase fuels, local temperature and turbulence characteristics in the cavity are analyzed, and the ignition environment on the development path of the flame kernel is discussed. The results indicate that the injection pressure and the distance between the cavity and the orifice have a critical effect on the quantity of the kerosene entrained into the cavity, which then have a great influence on the ignition environment in the cavity.

Two-phase Interactions between Propagating Shock Waves and Evaporating Water Droplets

One-dimensional numerical simulations based on hybrid Eulerian-Lagrangian method are performed to study the interactions between propagating shocks and dispersed evaporating water droplets. Two-way coupling for exchanges of mass, momentum, energy and vapour species is adopted for the dilute two-phase gas-droplet flows. Interphase interactions and droplet breakup dynamics are investigated with initial droplet diameters of 30, 50, 70 and 90 {\mu}m under an incident shock wave Mach number of 1.3. Novel two-phase flow phenomena are observed when droplet breakup occurs. First, droplets near the two-phase contact surface show obvious dispersed distribution because of the reflected pressure wave that propagates in the reverse direction of the leading shock. The reflected pressure wave grows stronger for larger droplets. Second, spatial oscillations of the gas phase pressure, droplet quantities (e.g., diameter and net force) and two-phase interactions (e.g., mass, momentum, and energy exchange), are observed in the post-shock region when droplet breakup occurs, which are caused by shock / droplet interactions. Third, the spatial distribution of droplets (i.e., number density, volume fraction) also shows strong oscillation in the post-shock region when droplet breakup occurs, which is caused by the oscillating force exerted on the droplets.

The Eulerian–Lagrangian Approach for the Numerical Investigation of an Acoustic Field Generated by a High-Speed Gas-Droplet Flow

This paper presents the Eulerian–Lagrangian approach for numerical modeling of high-speed gas-droplet flows and aeroacoustics. The proposed hybrid approach is implemented using the OpenFOAM library and two different methods. The first method is based on a hybrid convective terms approximation method employing a Kurganov–Tadmor and PIMPLE scheme. The second method employs the regularized or quasi-gas dynamic equations. The Lagrangian part of the flow description uses the OpenFOAM cloud model. Within this model, the injected droplets are simulated as packages (parcels) of particles with constant mass and diameter within each parcel. According to this model, parcels moving in the gas flow could undergo deceleration, heating, evaporation, and breakup due to hydrodynamic instabilities. The far-field acoustic noise is predicted using Ffowcs Williams and Hawking’s analogy. The Lagrangian model is verified using the cases with droplet evaporation and motion. Numerical investigation of water microjet injection into the hot ideally expanded jet allowed studying acoustic properties and flow structures, which emerged due to the interaction of gas and liquid. Simulation results showed that water injection with a mass flow rate equal to 13% of the gas jet mass flow rate reduced the noise by approximately 2 dB. This result was in good coincidence with the experimental observations, where maximum noise reduction was about 1.6 dB.