A numerical study of droplet motion/departure on condensation of mixture vapor using lattice Boltzmann method

Droplet motion/departure, which is governed by external force acceleration coefficient, droplet radius and surface wettability on solid surfaces under external forces such as gravitational force, play a significant role in characterizing condensation heat transfer, especially when high fractional non-condensable gases (NCG) present. However, due to the challenge in visualizing the vapor/steam velocity field imposed by droplet motion/departure, the detailed mechanism of droplet motion/departure on condensing surfaces has not been completely investigated experimentally. In this study, droplet motion/departures on solid surfaces under external forces and their interactions with steam flow are simulated using two dimensional (2D) multiphase lattice Boltzmann method (LBM). Large external force acceleration coefficient, droplet radius and contact angle, lead to large droplet deformation and high motion/departure velocity, which significantly shortens the droplet residual time on the solid surface. Our simulation shows that steam vortices (lateral velocity) induced by droplet motion/departure can greatly disturb the vapor flow and would be intensified by increasing external force acceleration coefficient, droplet radius, and contact angle. In addition, the location of vortex center shifts in the ascending direction with increase of these factors. The average lateral velocities induced by droplet motion/departure at various conditions are obtained. The mass transfer resistance is substantially reduced owing to the droplet motion/departure, leading to an enhanced heat flux. The experimental results are compared to validate the influence of droplet motion/departure on condensation heat transfer performance, especially for steam–air mixture with the presence of high fractional NCG.     

Low-Reynolds-number droplet motion in a square microfluidic channel

In this study, we investigate computationally the low-Reynolds-number droplet motion in a square micro-channel, a problem frequently encountered in microfluidic devices, enhanced oil recovery and coating processes. The droplet deformation and motion are determined via a three-dimensional spectral boundary element method for wall-bounded flows. The effects of the flow rate, viscosity ratio and droplet size on the interfacial dynamics are identified for droplets smaller and larger than the channel size and for a wide range of viscosity ratio. Owing to the stronger hydrodynamic forces in the thin lubrication film between the droplet interface and the solid walls, large droplets exhibit larger deformation and smaller velocity. Under the same average velocity, a droplet in a channel shows a significantly smaller deformation and higher velocity than in a cylindrical tube with the same size, owing to the existence of the corners’ area in the channel which permits flow of the surrounding fluid. A suitable periodic boundary implementation for our spectral element method is developed to study the dynamics of an array of identical droplets moving in the channel. In this case, the droplet deformation and velocity are reduced as their separation decreases; the reduction is influenced by the flow rate, viscosity ratio and more significantly the droplet size.     

Evaporation and condensation growth of a droplet in a vapor-gas medium for arbitrary knudsen numbers

In this paper, a study is made of the weak isothermal and nonisothermal evaporation of a particle, taking account of thermodiffusion, for arbitrary Kn numbers, evaporation-condensation coefficient , vapor concentration c, and arbitrary potential of intermolecular interaction.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 131–137, July–August, 1985.     

Design and performance of a rapid piston expansion tube for the investigation of droplet condensation

 A piston expansion tube (pex-tube) is described which has been developed for the investigation of homogeneous nucleation in a supersaturated vapor and subsequent droplet growth. The design of the tube with periphery is discussed as well as the underlying gasdynamics. Examples for the measurement of nucleation and growth rates in a binary mixture of n-propanol/water carried in nitrogen demonstrate performance and range. The focus of the paper is on the determination of the nucleation state. It is shown that this state may not be based on an adiabatic expansion when helium is used as carrier gas instead of argon or nitrogen. Received: 14 May 1997 / accepted: 24 August 1997     

Chemoconcentration capillary effect associated with the motion of a drop in a liquid

The effect of a surface chemical reaction involving a weak soluble surface active substance on the motion of a drop in a liquid is investigated. It is shown that as a result of the Marangoni effect the non-uniformity in the distribution of the substance along the surface associated with the proper motion of the drop and the chemical reaction has an important influence on the nature of the motion of the drop and on the force exerted by the surrounding Liquid. Under certain conditions this Leads to the development of a thrust proportional to the velocity of the drop (chemoconcentration capillary effect). The condition of occurrence of the thrust is obtained, together with its dependence on various physical parameters.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 147–154, January–February, 1988.     

Corona in a turbulent jet with condensation

The electrogasdynamic (EHD) effects associated with the introduction of corona discharge ions into a vapor-air jet with condensation are investigated. The electrical, acoustic and, moreover, integral and local optical (light scattered by condensate droplets) characteristics of the jet are measured. The time-dependent components of the recorded signals, which provide information about the characteristic fluctuations in the flow, are determined and processed. A new effect — the existence of a correlation between the electrical (Trichel frequency), acoustic and optical fluctuations in the flow — is detected and analyzed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.4, pp. 28–35, July–August, 1992.     

Collision of a vortex with a vaporizing droplet

Three-dimensional interactions between an advecting vortex tube and a vaporizing droplet, described by the Navier–Stokes, energy, and species equations, cause fluctuations in the droplet heating and vaporization, manifested by temporal and time-averaged variations in the droplet Nusselt and Sherwood numbers. Stefan flux not only inhibits the droplet heating, it also ‘blocks’ the influence of vortex collision on the droplet interface inhibiting Nusselt number perturbations. The Stefan flux has a primary effect on the Nusselt number and a secondary one on the Sherwood number. Fluctuations in Sherwood number can be significant in magnitude and exhibit self-similarity in both the temporal and time-averaged response. Derived correlations are demonstrated to be valid for at least three common fuel droplets (n-heptane, n-octane, n-decane). Furthermore, they quantify the effect of vortex collision on the droplet vaporization and compliment the accepted correlations for droplets in axisymmetric flows. It follows that, in spray combustion systems, vortical structures could significantly affect transport mechanisms, vaporization rates, and local mixture ratios.     

Effects of the particle residence time and the spray droplet size on the particle removal efficiencies in a wet scrubber

A new type of scrubbing system equipped with air-atomized spray nozzles, full cone type spray nozzles and the maze shape channels has been developed and the mass transfer mechanism to remove sub-micron particles is analyzed. There is a minimal time duration for the mixture of air and sprayed water droplets should remain in the scrubbing zone for the sub-micron particles and hydrogen fluoride (HF) gas to diffuse and be captured by water droplets. The grown water droplets enter the maze shape channels which have sharp corners and bends to eliminate the water droplets by collision with the walls. As a result of applying the developed design methodology, the sub-micron particle removal efficiencies of the scrubber are found to be above 99% for the particles of 0.5–1 μm, 96% for those of 0.3–0.5 μm, and 86% for those smaller than 0.3 μm in diameter under the optimum operating condition.     

Effect of non-condensable gas on condensation in a highly-rotating drum with a scraper

The effect of non-condensable gas on condensation in a highly rotating drum with a scraper is explored analytically. The formulation is first carried out generally, and then the application of the general formulation to the steam-air system gives heat transfer results for a wide range of operating conditions. It is found that a very small amount of non-condensable gas can induce a significant reduction in heat transfer. The reduction is accentuated at larger temperature differences, thinner condensate thickness and, to one's surprise, at higher operating pressures. By comparison, it is indicated that the present situation is more sensitive to non-condensable gas than forced convection condensation along a horizontal plate. Received on 14 August 1997     

Precursor waves associated with the motion of sources of variable intensity in a stratified fluid

When a source of variable intensity moves in a stratified fluid, several types of waves, possibly including waves that outstrip the source (precursor waves), are generated. By analyzing the expressions for the mean energy losses due to internal wave radiation per unit time it is shown that in fluids with convex wave dispersion curves up to four types of waves per mode are possible. One of these types, which vanishes under supercritical conditions, is related to the precursor waves. The angle dependence of these waves and their conditions of excitation with respect to source velocity are established. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.2, pp. 97–103, March–April, 1994.     

Numerical investigation of droplet pre-dispersion in a monodisperse droplet spray dryer

Monodisperse droplet spray dryers have great advantages in particle formation through spray drying because of their ability to produce uniform sized particles. Experimental analyses of this system have shown that droplets atomized through the piezoceramic nozzle need to be sufficiently well dispersed before entering the drying chamber to achieve sufficiently dried particles. However, the dispersion dynamics cannot be readily observed because of experimental limitations, and key factors influencing the dispersion state currently remain unclear. This study carried out numerical simulations for droplet dispersions in the dispersion chamber, which allow this important process to be visualized. The systematic and quantitative analyses on the dispersion states provide valuable data for improving the design of the dispersion chamber, and optimizing the spray drying operation.     

Longitudinal dispersion associated with the motion of an adsorbed mixture in a column filled with porous granules

The effect of the radial nonuniformity of the linear flow velocity on the radial nonuniformity of the adsorbate concentration distribution over the cross section of a column and on the longitudinal dispersion is estimated for the diffusion model, and the legitimacy (for relatively small adsorber radii) of using a one-dimensional diffusion model for describing the interphase mass transfer in cylindrical adsorbers is thereby determined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 93–101, July–August, 1987.     

Transport processes in a diffusion chamber with photoinduced condensation

The photonucleation-accompanied transport processes in a diffusion chamber are analyzed. Monomolecular condensation theory [15] is used to substantiate one of the possible mechanisms leading to photonucleation by physical rather than chemical means. In order to eliminate the difficulties associated with considering binary nucleation (the formation of complexes of molecules of different species), simple systems of the CS₂-He and H₂O-Ar types, for which the absence of radicals and heterogeneous complexes in the photonucleation process has been demonstrated [9, 10], are considered.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 95–104, November–December, 1991.     

Integral analysis of condensation in a paper drum with a scraper

An integral analysis is made for laminar film condensation in a rotating paper drum with a scraper. The analysis includes the nonlinear inertial terms in the equation of motion, with the interface shear stress being considered. The results of numerical solutions show that the inertial terms cannot be neglected except in the cases of a very thin condensate film or a very large Froude number, that the condensation heat transfer will be, to some extent, enhanced by the inertial effect, and that the vapor shear at the vapor-liquid interface proves to be negligible. Distributions of the interface velocity and film thickness as well as heat transfer results are obtained and discussed. Included also in this paper is the investigation for transition of the condensate flow from steady state to unsteady one. Received on 6 December 1996     

Forced-convection film condensation on a horizontal cylinder with wavy surface structure

The forced-convection film condensation on a horizontal cylinder with wavy surface structure was performed by boundary-layer-approximation. The local/mean heat fluxes were obtained for the effects of tube temperature, wave number, and wave amplitude. The mean heat flux increases with decreasing wavy amplitude and tube temperatures. Furthermore, when β = 20 and α = 0.005, the mean heat flux slightly increases from 1.1 to 3.6% compared with that of smooth tube, depending on tube temperature.