Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

The mechanism of air bubble entrainment in self-aerated flow

The mechanism by which air bubbles are brought into a quick moving waterflow with free surface was studied. The processes taking place at the surface were visualized with a stroboscope. It shows, how the bubbles are generated by falling drops. The height of fall and the velocity with which a water drop must hit the surface in order to form an air bubble can be calculated by an energy balance. Measurements of diameter, height of fall and velocity of the drops as well as of the size of the bubbles coincide well with the obtained formulae. Results are valid for pure water and slightly dirty waste water. Such flows occur in steep smooth channels, spillways and strongly inclined partially filled conduits and pipelines.     

Convective and diffusive surfactant transfer in multiphase liquid systems

Laser interferometry was used to investigate diffusive and convective mass transfer in a multicomponent fluid mixture with a liquid–liquid or liquid–gas interface. For this purpose, an immobile gas bubble or insoluble fluid droplet, having the shape of a short cylinder with a free lateral surface, was inserted into a thin liquid layer. In the case of non-uniform distribution of the dissolved surfactant component, the Marangoni convection near the drop/bubble was initiated by the surface tension inhomogeneities, depending on the surfactant concentration. The applied experimental techniques allowed us to study the structure and evolution of the convective flows and concentration fields in a liquid layer, which due to its small thickness were nearly two-dimensional. Making use of both the vertical and horizontal orientation of the liquid layer, we investigated the mass transfer process at different levels of the interaction between gravity and capillary forces. During the experiments, we detected new solutocapillary phenomena, which were found to be caused by oscillatory regimes of solutal convection occurring around air bubbles and chlorobenzene drops in heterogeneous aqueous solutions of alcohol with a vertical surfactant concentration gradient. The role of the oscillatory instability in the processes of drop saturation by the surfactant from its water solution and an inverse process of surfactant extraction from the drop into the surrounding homogeneous fluid (water) was determined. A reasonable explanation for the driving mechanisms of the discovered effects has been proposed.     

Diffusional interaction of drops in a liquid

The method of combining asymptotic expansions (with respect to a large Peclet number) is used to investigate the three-dimensional problem of steady-state convective diffusion to the surface of drops, around which flows a laminar stream of a viscous incompressible liquid whose velocity field is assumed to be known from the solution of the corresponding hydrodynamic problem. It is shown that for large Peclet numbers the heat and mass transfer between drops is completely determined by the mutual arrangement of special (starting or ending at the surface of a drop) lines of flow; under these circumstances, in the flow there are chains of drops which have no mutual diffusional effect on one another, and the total diffusional flow to a drop is determined by diffusion to particles located upstream in the same chain. For the case where the distance between the drops in the chain is much leas than P^1/2 (P is the Peclet number), formulas for the distribution of the concentration and the total diffusional flow to the surface of each drop are obtained. It is shown that the total diffusional flow to the surface of a drop approaches zero in inverse proportion to its order number in a chain, which generalizes [1], in which the axisymmetric case is considered. A solution of the diffusional case is obtained for the case where there are critical lines at the surface of the drop. The problem is solved to the end if the singular flow lines are not closed and depart to infinity. With the presence of a region of closed circulation behind the drops, the problem is reduced to an integral equation.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika, Zhidkosti i Gaza, No. 2, pp. 44–56, March–April, 1978.The author thanks Yu. P. Gupalo and Yu. S. Ryazantsev for their interest in the work.     

Dynamics of a liquid with gas bubbles

The hydrodynamics and diffusion of an admixture near an isolated bubble, which simulates the rise of either a chain of identical bubbles or a system of regularly arranged bubbles of the same volume, are analyzed by solving the Navier-Stokes equations numerically. Data are presented for a specific liquid. It is shown that in both cases the maximum flow velocity on the surface of identical bubbles is practically the same, although in the former case the ascent velocity is considerably higher. The stationary admixture diffusion from a bubble also proves to be nearly the same.In relation to the bubbling of a gas through a liquid layer, it is shown that the total admixture diffusion is maximum for regularly arranged bubbles whose diameter is comparable with the liquids capillary constant. Although the flow past the bubble remains continuous, the values of the hydrodynamic parameters are no longer small.Novosibirsk. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 75–88, May–June, 1996.     

Techniques for generating centimetric drops in microgravity and application to cavitation studies

This paper describes the techniques and physical parameters used to produce stable centimetric water drops in microgravity, and to study single cavitation bubbles inside such drops (Parabolic Flight Campaigns, European Space Agency ESA). While the main scientific results have been presented in a previous paper, we shall herein provide the necessary technical background, with potential applications to other experiments. First, we present an original method to produce and capture large stable drops in microgravity. This technique succeeded in generating quasi-spherical water drops with volumes up to 8 ml, despite the residual g-jitter. We find that the equilibrium of the drops is essentially dictated by the ratio between the drop volume and the contact surface used to capture the drop, and formulate a simple stability criterion. In a second part, we present a setup for creating and studying single cavitation bubbles inside those drops. In addition, we analyze the influence of the bubble size and position on the drop behaviour after collapse, i.e., jets and surface perturbations.     

Self-similar thinning of a liquid film separating liquid phases or a liquid and a solid phase

The article discusses the thinning of a thin flat film of liquid between coalescing bubbles or coalescing drops due to motion of the liquid under the action of capillary forces, under drag conditions of the surfaces of the film. A study was also made of the outflow of a liquid from a film separating a bubble (drop) from a solid surface. The problem of the decrease in thickness and subsequent breakaway of thin liquid films is of interest in investigations of the kinetics of coalescence and coagulation and in the theory of the stability of foams and lyophobic colloids in the presence of surfactants or electrolytes, as well as in the theory of heterogeneous boiling. Information on some work and on special characteristics of the statement of the problem may be found in [1].Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 2, pp, 39–48, March–April, 1973.     

Droplet production from disintegrating bubbles at water surfaces. Single vs. multiple bubbles

We experimentally determine the droplet production rate at a water surface where either single or multiple bubbles (bubbly flow) with similar mean diameters disintegrate and produce film and jet droplets. A detailed assessment of film drop production from bubbly flow is important, since most presently used correlations are based on single-bubble measurements. Moreover, jet drops––even though they contain a much larger fluid volume––are de-entrained into the water surface in most technical and geophysical applications. Detailed phase Doppler anemometry (PDA) measurements are performed in the vicinity of the water surface with long sampling times. For a considered mean diameter of approximately 3 mm, the size distribution of the non spherical bubbles is determined from photographic images. From single-bubble measurements we find, consistent with literature data, a narrow size distribution of the jet drops with a mean diameter of 477 μm. For bubbly flow, the maximum is shifted to somewhat smaller jet drop diameters (425 μm) and the production of film droplets increases significantly. We relate this increase to the coalescence of bubbles prior to their disintegration at the surface. Our results therefore show that for a fixed bubble size and gas flow rate the number of film drops entrained from a bubbly flow is underestimated, if the estimate is based on single-bubble data.     

Dissipation of the energy of sound waves in small gas bubbles

One of the main factors affecting the dynamics of homogeneous solution type pulse reactors is the formation of gas bubbles on the fission-fragment tracks [1, 2]. The behavior of the reactor depends very considerably on the size (10^?5 cm) and growth rate of these bubbles [2], and it is, accordingly, a very important matter to study these properties. One convenient means of doing this lies in the acoustic method. The behavior of gas bubbles in the field of a sound wave has been studied in a large number of papers and reviews [3, 4]. In this paper we shall see the approximation of a sound wave of small amplitude to consider the dissipation of sound-wave energy in a gas bubble, at the same time allowing for inertia, surface tension, viscosity, heat transfer, and the diffusion of gas through the surface of the bubble.     

Gravity-driven motion of a deformable drop or bubble near an inclined plane at low Reynolds number

The creeping motion of a three-dimensional deformable drop or bubble in the vicinity of an inclined wall is investigated by dynamical simulations using a boundary-integral method. We examine the transient and steady velocities, shapes, and positions of a freely-suspended, non-wetting drop moving due to gravity as a function of the drop-to-medium viscosity ratio, λ, the wall inclination angle from horizontal, θ, and Bond number, B, the latter which gives the relative magnitude of the buoyancy to capillary forces. For fixed λ and θ, drops and bubbles show increasingly pronounced deformation in steady motion with increasing Bond number, and a continued elongation and the possible onset of breakup are observed for sufficiently large Bond numbers. Unexpectedly, viscous drops maintain smaller separations and deform more than bubbles in steady motion at fixed Bond number over a large range of inclination angles. The steady velocities of drops (made dimensionless by the settling velocity of an isolated spherical drop) increase with increasing Bond number for intermediate-to-large inclination angles (i.e. 45° ? θ ? 75°). However, the steady drop velocity is not always an increasing function of Bond number for viscous drops at smaller inclination angles.     

THERMODYNAMICS ON CAVITATION

Many studies on cavitation phenomena were based on the theoryof single bubble motion which was first put forward by Rayleighin his1917 article and later developed by plesset et al.Bythis theory,only some effects of forces were taken into conside.ration from hydrodynamics leaving out any thermodynamical effectssuch as matter interchange between liquid and gaseous phases.Strictly speaking,the theory may be suitable for discussing ex-pansion or/and contraction motion of a bubble formed in liquid,but this theory does not cpoe with cavitation behaviors in general.In this paper,the cavitation conditions and similarity problemsare discussed with thermodynamic effects taken into considerationin addition to the hydrodynamic ones.     

Experimental Study on Bubble Movement Characteristics during Underwater Pyrotechnic Combustion

High-speed photography was used to study bubble movement characteristics during underwater pyrotechnic combustion. The results show that bubble behaviors include bubble formation at the nozzle, departure from the nozzle, bubble coalescence, and bubble breakup. Compared with cavitation bubbles and fluidization bubbles, the nozzle bubbles formed during underwater pyrotechnic combustion feature larger diameters, up to centimeters, and darker, and more irregular shapes. During large bubble coalescence, two bubbles approach each other, generate a channel for transfer of mass and heat, and finally coalesce. The bubbles contain high-temperature gases and solid residues generated during pyrotechnic combustion, which lead to non-uniform forces on the bubble surface and make the bubbles more prone to breakup. Because of the high-temperature solid grains, the surrounding liquid vaporizes to form bubbles.     

Investigation of surfactant effect on the bubble shape and mass transfer in a milli-channel using high-resolution microfocus X-ray imaging

In this paper we present an experimental study on the influence of surface active agents (surfactants) on Taylor bubble flow in a vertical millimeter-size channel. Moreover we give a short review on the subject and previous investigations. We investigated the shape and dissolution rate of individual elongated carbon dioxide Taylor bubbles, which were hydraulically fixed in a downward flow of water. Bubble shape and dissolution rate was determined from microfocus X-ray radiographs. From the shrinking rate we calculated the liquid side mass transfer coefficient.The results show that the presence of surfactants causes a change of the bubble shape and leads to a slight increase of the liquid film thickness around the bubble and as a result the elongation of contaminated bubbles. In addition, the comparison of clean and contaminated bubbles indicate that presence of surfactant has a more significant impact on the dissolution rate of small bubbles. Furthermore, applying different concentrations of surfactant reveals that in our case, where surface coverage ratio of surfactant on the bubbles is high, increase of contamination does not have a noticeable influence on the mass transfer coefficient of bubbles.     

Effect of homogeneous condensation on the dynamics of a hot vapor bubble in a cold liquid jet

The problem of initiating cavitation bubbles in a cold liquid jet by injecting hot steam into high-pressure zone specially organized at the nozzle outlet is considered. Previously, in [1], a plane flowfield in which vapor bubbles were formed at the cusp of the cavity (high-pressure zone) and propagated together with the liquid along the axis of symmetry was considered. In certain cases, in the bubble expansion process the vapor temperature drops below the saturation temperature. In the present paper, vapor condensation in the bubble volume (homogeneous condensation) is also taken into account.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 56–61, November–December, 1996.     

Influence of vertical internals on a bubbling fluidized bed characterized by X-ray tomography

An ultra-fast X-ray tomographic scanner is applied to study the hydrodynamics in a bubbling fluidized bed with and without vertical internals (e.g., heat exchanger tubes). The objective of this study is to understand the influence of vertical internals on hydrodynamic properties such as bubble volume, size and velocity and to provide measurement data for the design and scale-up of catalytic bubbling fluidized bed reactors with vertical internals. With these new measurements, correlations of bubble properties can be developed to reliably scale-up bubbling fluidized beds with vertical internals. For the investigated reactor with Geldart A/B particles, no relation between bubble size and velocity was observed for individual bubbles, i.e.; smaller bubbles tend to rise with higher velocities. A significant reduction in bubble size and sharpening of the bubble size distribution was generally obtained for a bed with vertical internals.     

The Fokker–Planck Equation for Bubbly Flows and the Motion of Gas Bubble Pairs

A method is proposed to calculate the bubble distribution function in a bubbly flow. First a review is given of the equations of motion and the dynamic behaviour of a pair of bubbles moving through a liquid at moderate Reynolds number. Subsequently, a Fokker–Planck type transport equation is derived for the bubble distribution function. It is assumed that the interaction is primarily by frequent and binary encounters, each with weak hydrodynamic interaction between the bubbles. The bubble collision cross-section, which needs to be known for the transport coefficients, is presented. A comparison with PDF-methods for fluid particles in turbulent reacting flows is made.