Gas-droplet Flow Research Articles

Investigation of a promising method for spraying liquid hydrocarbons using the shadow photography method

A promising method for spraying liquid hydrocarbons by a high-speed jet of superheated steam was studied experimentally. Information on the disperse composition and structure of the gas-droplet flow during the spraying of used transmission automotive oil was obtained using the method of shadow photography. Size distributions of fuel droplets were obtained in a wide range of operating parameters (flow rate and temperature of steam, fuel consumption).

Investigating characteristics of a gas-droplet flow under different conditions of liquid dispersion by a pneumatic nozzle

An experimental setup has been created, and a laboratory sample of the pneumatic nozzle with a special design based on the use of the properties of cumulative jets, toroidal vortices and the Coanda effect has been tested. This nozzle may be used for dispersion of liquids and suspensions, in particular, coal-water slurry. The experimental setup allows obtaining a supersonic gas-droplet flow: with carrier phase velocity up to 500 m/s, at gas pressure up to 0.4 MPa, and liquid flow rate up to 500 kg/h. Shooting was used to determine a range of regime parameters, providing stable operation of the injector. Beyond this range, strong pulsations of the gas-droplet flow were observed. It has been established that in this range of modes the operating angle of a gas-droplet jet weakly depends on the liquid flow rate and gas pressure. The developed experimental and methodical base allows further measuring the dispersed composition of the generated gas-droplet flow with a view of optimizing the operation modes of the injector.

Effect of droplet evaporation on the flow structure and heat and mass transfer in a confined swirling gas-droplet flow downstream of a tube sudden expansion

The effect of flow swirl parameter and thermophysical properties of the droplet of water, ethanol, and acetone on the structure of turbulent flow and heat transfer in a gas-droplet flow was studied numerically. To describe the dynamics and heat and mass transfer of the two-phase flow, the Eulerian approach was used. The growth of the volume fraction of small particles on the tube axis is typical of a swirling flow because of their accumulation in the zone of reverse flows due to the turbophoresis force. It is shown that the addition of droplets leads to a signifi-cant increase in heat transfer (more than 2.5 times) at mass concentration of droplets ML1 = 0.1 in comparison with a single-phase swirling flow. Intensification of heat transfer with the use of ethanol droplets is higher than that for water droplets (approximately 10–20 %) and acetone (up to 65 %). When using the droplets of ethanol and acetone, the region of two-phase flow existence reduces, and the degree of suppression of carrier phase turbulence decreases. This is due to a more rapid evaporation of droplets of volatile liquids.

The effect of flow swirling on flow structure, turbulence and heat transfer in turbulent droplet-laden flow in a pipe sudden expansion

This paper presents the mathematical model to simulate the swirling turbulent gas-droplet flow in a sudden pipe expansion. The set of axisymmetrical steady-state RANS equations for the two-phase flow is used. The dispersed phase is modeled by the Eulerian approach. The flow swirl causes an increase in the intensity of heat transfer (more than 1.5 times in comparison with the non-swirling mist flow at other identical inlet conditions). Evaporation of the droplets leads to a significant increase in the heat transfer intensity in the swirling two-phase flow (more than 2.5 times in comparison with the single-phase flow). It is shown that ethanol droplets evaporate much faster than water droplets due to the lower heat of phase transition. The heat transfer enhancement by using ethanol droplets is higher than the corresponding value for water droplets (up to 20%).

Formation and Evaporation of Near-Wall Liquid Films Under Expansion from a Nozzle Into Vacuum

Expansion of near-wall liquid films with different physical properties accompanied by high-velocity co-current gas flow from a nozzle into vacuum is studied experimentally. Local parameters of liquid films inside the nozzle are measured. Gas-droplet flow structure behind the nozzle exit cross-section is examined. Limiting temperatures of liquid films formed on the outer surface of the nozzle are measured.

Effect of Flow Swirling on Heat Transfer in Gas-Droplet Flow Downstream of Abrupt Pipe Expansion

The effect the flow swirl parameter on heat transfer in a gas-droplet flow is numerically modeled by the Euler approach. The gaseous phase is described by a system of 3D RANS equations with consideration of the back effect of particles on transfer processes in the carrier phase. The gaseous phase turbulence is calculated according to the Reynolds stress transport model with consideration of the dispersed phase effect on the turbulent characteristics. A rapid dispersion of droplets over the pipe cross section is observed in a nonswirling gas-droplet flow downstream of an abrupt pipe expansion. A swirling flow is characterized by a growing concentration of fine particles at the pipe axis due to the accumulation of particles in the zone of flow recirculation and to the turbophoresis force. In a swirling flow, the separated-flow region becomes significantly shorter (by almost a factor of two as compared to that in a nonswirling flow). It is shown that addition of droplets results in a significant growth of heat transfer intensity (by more than a factor of 2.5) in comparison with single-phase swirling flow.

Developing theory of probability density function for stochastic modeling of turbulent gas-particle flows

Turbulent gas-particle flows are studied by a kinetic description using a probability density function (PDF). Unlike other investigators deriving the particle Reynolds stress equations using the PDF equations, the particle PDF transport equations are directly solved either using a finite-difference method for two-dimensional (2D) problems or using a Monte-Carlo (MC) method for three-dimensional (3D) problems. The proposed differential stress model together with the PDF (DSM-PDF) is used to simulate turbulent swirling gas-particle flows. The simulation results are compared with the experimental results and the second-order moment (SOM) two-phase modeling results. All of these simulation results are in agreement with the experimental results, implying that the PDF approach validates the SOM two-phase turbulence modeling. The PDF model with the SOM-MC method is used to simulate evaporating gas-droplet flows, and the simulation results are in good agreement with the experimental results.

A mechanistic model for expansion loss coefficient in upward vertical annular flow

• Differential effect of flow deceleration on gas, droplets and liquid film postulated. • Developed semi-analytical expressions for velocity profile correction factor in pipe flow. • Carried out CFD simulations of gas-droplet flow in a rough-walled pipe expansion. • Effect of droplet deceleration on pressure variation elucidated. • Mechanistic correlation developed for expansion loss coefficient for annular flow. Gas–liquid annular flow draws different responses from its three constituents, namely, the liquid film, the entrained liquid droplets and the gas core, as they flow through a diverging section in a pipe. The resulting change in the pressure profile is a combination of several effects associated with the dynamic interactions among these three fields. Accurate simulation of the response using Eulerian–Eulerian computational fluid dynamics is not feasible because the processes are inherently complex and a framework of relevant and validated constitutive relations describing the physical processes is not yet available. In the present work, a simpler approach is adopted by studying the interactions individually in idealized settings, and bringing together the separate effects into a phenomenological model for pressure loss in upward vertical annular flow. The overall pressure change is expressed as a sum of three contributors: change in area of cross-section available for gas flow, change in the effective interfacial roughness leading to peaking of the velocity profile, and droplet-gas momentum exchange in the immediate downstream of the expansion. Using air–water experimental data from two expansion ratios and three half-angles of the diverging sections, a mechanistic correlation is proposed to evaluate the overall pressure loss coefficient.

Effect of various key factors on the law of droplet evaporation on the heated horizontal wall

Water evaporation in a wide range of initial droplet diameters and at different wall temperatures on structured surface was studied experimentally. With an increase in the wall temperature from 31 to 72°С and an increase in the initial droplet diameter, exponent n in the evaporation law increases from 1 to 1.37. Under the transitional regime, exponent n=1.6 reaches its maximum. Usually, while simulating droplet evaporation, a linear relationship of the evaporation rate of water vapor and droplet radius R is considered (dm/dt∼Rn, n=1). In this paper, it is shown that exponent n increases with a growth of the wall temperature. In generalization of droplet evaporation rate, the exponent for the Rayleigh number (Ra) is 0.457 due to the predominant role of gas convection. For large water drops (for air–vapor mixture over the droplet surface) high Reynolds numbers are achieved (Ra=2×105). A diffusion vapor layer on the droplet surface and boundary layer of air on the surface of the heated cylinder, whose diameter exceeds the droplet diameter, are formed. A neglect of free convection understates simulation results in comparison with experimental data more than by the factor of 10. The sequence of key factors, taking into account their influence on the rate of droplet evaporation, is as follows: 1) convection in a vapor–gas medium; 2) effect of wall roughness, wettability, and convection in a liquid; 3) thermal inertia of the metal wall. The calculation methodology for a sessile drop enables a qualitative analysis for a high-temperature gas-droplet flow.

Heat and mass transfer are in the interaction of multi-pulsed spray with vertical surfaces in the regime of evaporative cooling

Sprays with a periodic supply drop phase have great opportunities to control the processes of heat transfer. We can achieve optimal evaporative modes of cooling by changing the pulse duration and the repetition frequency while minimizing flow of the liquid phase. Experimental data of investigation of local heat transfer for poorly heated large surface obtained on the original stand with multi nozzle managed the irrigation system impact of the gas-droplet flow present in this work. Researches on the contribution to the intensification of spray options were conducted. Also the growth rate was integral and local heat. Information instantaneous distribution of the heat flux in the description of the processes have helped us. Managed to describe two basic modes of heat transfer: Mode “insular” foil cooling and thick foil with forming of streams. Capacitive sensors allow to monitor the dynamics of the foil thickness, the birth-belt flow, forming and the evolution of waves generated by “bombing” the surface with the droplets.

Numerical modeling of turbulent flow structure and heat transfer in a droplet-laden swirling flow in a pipe with a sudden expansion

ABSTRACTThis contribution presents the mathematical model to simulate the swirling turbulent gas-droplet flow in a sudden pipe expansion. The set of axisymmetrical steady-state Reynolds averaged Navier–Stokes equations (RANS) for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. The flow swirl causes an increase in the intensity of heat transfer (more than 1.5 times compared with the nonswirling mist flow at other identical inlet conditions). Evaporation of the droplets leads to a significant increase in the heat transfer intensity in the swirling two-phase flow (more than 2.5 times compared with the single-phase flow).

Cooling the vertical surface by conditionally single pulses

You Sprays with periodic supply of the droplet phase have great opportunities to control the heat exchange processes. Varying pulse duration and frequency of their repetition, we can achieve the optimal conditions of evaporative cooling with minimization of the liquid flow rate. The paper presents experimental data on studying local heat transfer on a large subcooled surface, obtained on the original setup with multinozzle controlled system of impact irrigation by the gas-droplet flow. A contribution to intensification of the spray parameters (flow rate, pulse duration, repetition frequency) per a growth of integral heat transfer was studied. Data on instantaneous distribution of the heat flux value helped us to describe the processes occurring on the studied surface. These data could describe the regime of “island” film cooling.

Investigating characteristics of liquid hydrocarbon spraying by a steam jet

Using the shadow photography method, the dispersed composition of a gas-droplet flow has been investigated at diesel fuel spraying by a superheated steam jet. The distributions of droplet sizes have been obtained in a wide range of parameters (steam and fuel consumption). Dependencies of droplets size on steam consumption have been found. The optimum values of steam and fuel consumption, at which the smallest prevailing droplet size (below 30 μm) is reached, have been found.

Characteristics of the behavior of a liquid film in a pulsed gas-droplet flow

The results of the investigation of the influence of a multi-jet impact air flow on the process of formation of a liquid film from a drop flow on the vertical surface of a heat exchanger with a low temperature are presented. Characteristics of the film flow and the effect of this flow on heat transfer, shown in the paper, are related to the interaction of the boundary layer of the wall air flow and the perturbation induced by the droplets under the pulsed regime of introducing the cooling mass into the flow of impact air jets. The paper determines the contribution of the impact air flow to the heat transfer during the cooling of the vertical surface.

Predictions of the gas–liquid flow in wet electrostatic precipitators

Based on Computational Fluid Dynamics (CFD), the present paper aims to simulate several important phenomena in a wet type ESP from the liquid spray generation to gas-droplet flow in electric field. A single passage between the adjacent plates is considered for the simulation domain. Firstly, the electric field intensity and ion charge density are solved locally around a corona emitter of a barbed wire electrode, which are applied to the entire ESP using periodic conditions. Next, the Euler–Lagrange method is used to simulate the gas-droplet flow. Water droplets are tracked statistically along their trajectories, together with evaporation and particle charging. Finally, the deposition density on the plate is taken as the input for the liquid film model. The liquid film is simulated separately using the homogenous Eulerian approach in ANSYS-CFX. In the current case, since the free surface of the thin water film is difficult to resolve, a special method is devised to determine the film thickness.As parametric study, the variables considered include the nozzle pressure, initial spray spreading patterns (solid versus hollow spray) and plate wettability. The droplet emission rate and film thickness distribution are the results of interest. Main findings: electric field has strong effect on the droplet trajectories. Hollow spray is preferred to solid spray for its lower droplet emission. The liquid film uniformity is sensitive to the plate wettability.

Interaction of a shock wave with an array of particles and effect of particles on the shock wave weakening

Two-phase systems that involve gas-particle or gas-droplet flows are widely used in aerospace and power engineering. The problems of weakening and suppression of detonation during saturation of a gas or liquid flow with the array of solid particles are considered. The tasks, associated with the formation of particles arrays, dust lifting behind a travelling shock wave, ignition of particles in high-speed and high-temperature gas flows are adjoined to safety of space flight. The mathematical models of shock wave interaction with the array of solid particles are discussed, and numerical methods are briefly described. The numerical simulations of interaction between sub- and supersonic flows and an array of particles being in motionless state at the initial time are performed. Calculations are carried out taking into account the influence that the particles cause on the flow of carrier gas. The results obtained show that inert particles significantly weaken the shock waves up to their suppression, which can be used to enhance the explosion safety of spacecrafts.

A combined viscous-vortex, thermal-blob and Lagrangian method for non-isothermal, two-phase flow modelling

A meshless method for modelling of 2D transient, non-isothermal, gas-droplet flows with phase transitions, based on a combination of the viscous-vortex and thermal-blob methods for the carrier phase with the Lagrangian approach for the dispersed phase, is developed. The one-way coupled, two-fluid approach is used in the analysis. The method makes it possible to avoid the ‘remeshing’ procedure (recalculation of flow parameters from Eulerian to Lagrangian grids) and reduces the problem to the solution of three systems of ordinary differential equations, describing the motion of viscous-vortex blobs, thermal blobs, and evaporating droplets. The gas velocity field is restored using the Biot–Savart integral. The numerical algorithm is verified against the analytical solution for a non-isothermal Lamb vortex and some asymptotic results known in the literature. The method is applied to modelling of an impulse two-phase cold jet injected into a quiescent hot gas, taking into account droplet evaporation. Various flow patterns are obtained in the calculations, depending on the initial droplet size: (i) low-inertia droplets, evaporating at a higher rate, form ring-like structures and are accumulated only behind the vortex pair; (ii) large droplets move closer to the jet axis, with their sizes remaining almost unchanged; and (iii) intermediate-size droplets are accumulated in a curved band whose ends trail in the periphery behind the head of the cloud, with larger droplets being collected at the front of the two-phase region.

Correct numerical simulation of a two-phase coolant

Different models used in calculating flows of a two-phase coolant are analyzed. A system of differential equations describing the flow is presented; the hyperbolicity and stability of stationary solutions of the system is studied. The correctness of the Cauchy problem is considered. The models’ ability to describe the following flows is analyzed: stable bubble and gas-droplet flows; stable flow with a level such that the bubble and gas-droplet flows are observed under and above it, respectively; and propagation of a perturbation of the phase concentration for the bubble and gas-droplet media. The solution of the problem about the breakdown of an arbitrary discontinuity has been constructed. Characteristic times of the development of an instability at different parameters of the flow are presented. Conditions at which the instability does not make it possible to perform the calculation are determined. The Riemann invariants for the nonlinear problem under consideration have been constructed. Numerical calculations have been performed for different conditions. The influence of viscosity on the structure of the discontinuity front is studied. Advantages of divergent equations are demonstrated. It is proven that a model used in almost all known investigating thermohydraulic programs, both in Russia and abroad, has significant disadvantages; in particular, it can lead to unstable solutions, which makes it necessary to introduce smoothing mechanisms and a very small step for describing regimes with a level. This does not allow one to use efficient numerical schemes for calculating the flow of two-phase currents. A possible model free from the abovementioned disadvantages is proposed.

Structure of the nonisothermal swirling gas-droplet flow behind an abrupt tube expansion

Flow structure and heat and mass transfer in a swirling two-phase stream is numerically modeled using the Reynolds stress transport model. The gas phase is described by the 3DRANS system of equations with account for the inverse influence of particles on the transport processes in the gas. The gas phase turbulence is calculated using the Reynolds stress transport model with account for the presence of disperse particles. The two-phase nonswirling flow behind an abrupt tube expansion contains a secondary corner vortex which is absent from the swirling flow. The disperse phase is redistributed over the tube cross-section. Large particles are concentrated in the wall region of the channel under the action of the centrifugal forces, while the smaller particles are in the central zone of the chamber.

The boundary layer on a flat plate in a supersonic gas-droplet flow: Influence of evaporating droplets on the temperature of an adiabatic wall

In the framework of the two-continuum model of a compressible gas-droplet boundary layer in a supersonic two-phase flow on a plane wall, the fields of phase parameters, droplet trajectories, and mass and energy fluxes of the dispersed phase on the streamlined surface are calculated. In the interphase momentum exchange, in addition to the aerodynamic drag force, the Saffman lift force resulting in droplet deposition on the solid wall is taken into account. The mass concentration of the dispersed phase is assumed to be small, and the wall temperature exceeds the temperature of droplet evaporation. The effect of evaporating droplets on the equilibrium temperature of a heat-insulated (adiabatic) wall is studied in a wide range of dimensionless governing parameters. It is shown that the presence of even a very low concentration of droplets can lead to a considerable decrease in the temperature of the adiabatic wall, which makes use of the evaporating condensed phase promising in different schemes of energy separation of gas flows.