“Pressure Blocking” Effect in the Growing Vapor Bubble in a Highly Superheated Liquid

Journal of Engineering Physics and Thermophysics - The problem on the growth of a vapor bubble in a liquid whose superheating enthalpy exceeds the phase transition heat has been considered. A...     

Bubble spreading during the boiling crisis: modelling and experimenting in microgravity

Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.     

Explosive collapse of vapor film under conditions of intensive heat fluxes

The stability is investigated (linear and nonlinear analysis) of the interface between a thin vapor film and a layer of liquid in the presence of a steady heat flux from a metal surface heated to a high temperature to the vapor film and then from vapor to subcooled liquid. In view of thermal disequilibrium which takes into account the temperature dependence of saturation pressure, boundary conditions on the vapor-liquid interface are derived, which generalize the known correlations on the free surface of liquid in the gravity field. A number of new effects arise in the problem under consideration, as distinct from the classical problem. The thermal processes, which occur on the phase boundary and are possible in the absence of the force of gravity as well, lead to the generation of weakly decaying periodic waves of low amplitude, whose velocity may exceed significantly that of gravity waves. The heat flux through the interface may cause on this surface periodic waves of small length (ripple) which are not capillary. The processes of phase transition on the interface are capable of providing for the stability of vapor film under the layer of liquid in the gravity field. Along with periodic waves and solitons, the mode of explosive instability may arise in the nonlinear stage because of a weak variation of the film thickness, where the amplitude of an initially low-amplitude plane wave rises to infinity during a finite period of time.     

Thermal Response of a Two-Phase Near-Critical Fluid in Low Gravity: Strong Gas Overheating as Due to a Particular Phase Distribution

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF₆ in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase ³He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.     

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

The theory of the growth of vapor bubbles in a metastable liquid

The growth of a single bubble and a system of vapor bubbles in the initial state in thermal and mechanical equilibrium with a liquid has been considered. Linear and nonlinear solutions have been derived, and the effects of the radial inertia, viscosity of the liquid, interphase heat and mass exchange, and the conditions of the tightness of bubbles on the development of the process have been studied on their basis.     

Bubble growth rate and phenomenon of degenerate boiling of a fluid in the form of film vaporization

An analysis of results of an investigation of vaporization of a thin fluid film under vacuum is presented. The vaporization process has basic features characteristic of bubble boiling of a fluid, except for formation of bubbles. This makes it possible to classify the phenomenon as degenerate boiling of a fluid in the form of film evaporation during which local thinning of the layer takes place and funnel- and crater-shaped structures are formed. Funnels and craters are vapor sources with different powers. Dependences of the bubble growth rate above vapor sources of both types that generalize available experimental data are obtained.     

Breakaway of a bubble from a heating surface

In this paper we solve the problem of determining the breakaway, diameter of the vapor bubble in the case of a large number of active vapor-forming centers. The motion of the liquid is discussed. The dynamic pressure of the liquid surrounding the vapor bubble and the radius of the bubble on breakaway from the heating surface are determined.     

A modified golden gate attenuated total reflection (ATR) cell for monitoring phase transitions in multicomponent fluids at high temperatures

A new continuous flow method using attenuated total reflection infrared (ATR-IR) spectroscopy has been developed for monitoring phase transitions in multicomponent fluids at high pressures and temperatures. Our approach uses Fourier transform infrared (FT-IR) and a modified Golden Gate attenuated total reflection (ATR) cell and exploits the fact that the absorbance of a vapor is much lower than that of the corresponding liquid to monitor the phase transition between vapor and liquid. We demonstrate that this method can provide quantitative measurements on both the dew point and the bubble point. We have validated our approach using three single-component systems (EtOH, MeOH, and H(2)O) and a binary system of EtOH + H(2)O, monitoring phase transitions at temperature up to 300 °C and pressure up to 10 MPa.     

Theoretical investigation of modes of compression of a spherical vapor bubble using a simplified model

The evolution of a single spherical vapor bubble in unbounded liquid is considered. The pressure in the bubble is related to its volume by the polytropic relation. The minimal value of polytropic exponent is taken to be zero, which corresponds to isothermal vapor bubble. It is demonstrated that diverse modes of compression of bubble are possible in the case of polytropic exponent of less than unity. Special features of evolution of bubble in these modes and the boundaries between the modes are investigated.     

Two-phase flow characteristics of liquid nitrogen in vertically upward 0.5 and 1.0 mm micro-tubes: Visualization studies

Application of liquid nitrogen to cooling is widely employed in many fields, such as cooling of the high temperature superconducting devices, cryosurgery and so on, in which liquid nitrogen is generally forced to flow inside very small passages to maintain good thermal performance and stability. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical two-phase flow patterns of liquid nitrogen in vertically upward micro-tubes of 0.531 and 1.042 mm inner diameters. It was found from the experimental results that the flow patterns were mainly bubbly flow, slug flow, churn flow and annular flow. And the confined bubble flow, mist flow, bubble condensation and flow oscillation were also observed. These flow patterns were characterized in different types of flow regime maps. The surface tension force and the size of the diameter were revealed to be the major factors affecting the flow pattern transitions. It was found that the transition boundaries of the slug/churn flow and churn/annular flow of the present experiment shifted to lower superficial vapor velocity; while the transition boundary of the bubbly/slug flow shifted to higher superficial vapor velocity compared to the results of the room-temperature fluids in the tubes with the similar hydraulic diameters. The corresponding transition boundaries moved to lower superficial velocity when reducing the inner diameter of the micro-tubes. Time-averaged void fraction and heat transfer characteristics for individual flow patterns were presented and special attention was paid to the effect of the diameter on the variation of void fraction.     

Growth of a bubble of noncondensing gas injected into a liquid

The solution of the problem of growth of a gas-vapor bubble injected into a liquid is obtained. The growth of the bubble depends on mass transfer in the gas phase and heat transfer in the liquid phase. Experimental and theoretical data are compared.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 32, No. 6, pp. 978–989, June, 1977.     

Variant of a volume-of-fluid method for surface tension-dominant two-phase flows

The capabilities of the volume-of-fluid method for the calculation of surface tension-dominant two-phase flows are explained. The accurate calculation of the interface remains a problem for the volume-of-fluid method if the density ratios of the fluids in different phases are high. The simulations of bubble growth is performed in water at near critical pressure for different degrees of superheat using combined level-set and volume-of fluid (CLSVOF) method. The effect of superheat on the frequency of bubble formation was analyzed. A deviation from the periodic bubble release is observed in the case of superheat of 20 K in water. The vapor-jet-like columnar structure is observed. Effect of heat flux on the slender vapor column has also been explained.     

Dynamics of phase transition in H2 under high frequency vibrations

Can vibrations act in space as an artificial gravity? We investigate here the role of high frequency vibrations to accelerate the dynamics of phase transition of gas and liquid in space. Hydrogen is studied near its critical point (T_c =33 K). Gravity effects are compensated in a high magnetic field gradient as provided by a 10 T superconducting coil. The experiments are performed in the temperature range [0.08 ? 1.1] mK from T_c, at critical and off-critical densities. The pattern shows up as interconnected gas-liquid domains or bubbles. When the domain size becomes larger than the viscous boundary layer, growth is accelerated and the domains eventually elongate in the direction perpendicular to the vibration (interconnected pattern case) or align in periodic planes in the same direction perpendicular to vibration (bubble pattern case). We explain the experimental findings by the presence of inertial velocity gradients between the vapor and liquid domains, which favor coalescence and fast domain growth.     

Properties of adsorption microlayers and the kinetics of bubble growth on a solid surface

The article contains an analysis of the kinetics of the growth of a vapor bubble on a solid surface in dependence on the thermophysical and capillary properties of the microlayer, which makes it possible to determine the parameters of the microlayer on the basis of experimental data on the kinetics of boiling.     

Dynamic behaviour of the CO<Subscript>2</Subscript> bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO₂) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO₂ bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO₂ concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO₂ bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eötvös number and the behaviour of the CO₂ bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO₂ bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO₂ concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO₂ transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.     

Study on Local Heat Transfer in the Vicinity of the Contact Line under Vapor Bubbles at Pool Boiling

The results of an experimental study of the dynamics of local heat transfer at nucleate pool boiling of liquids are presented. Experimental data on the nucleation site density and the evolution of the temperature field underneath individual vapor bubbles were obtained by high-speed infrared thermography with high spatial and temporal resolutions. Deionized water and ethanol at the saturation line under atmospheric pressure were used as working liquids. Evolution of the distribution of the local heat flux rate in the region of an individual nucleation site has been constructed based on numerical simulation. It has been shown that the maximum rate of the local heat flux is observed in the region of the liquid microlayer during the period of vapor bubble growth and reaches a value exceeding the average heat flux rate by 15–20 times. Based on the results, the thickness of the microlayer underneath the vapor bubble during the period of the bubble growth was estimated. The estimates satisfactorily agree with experimental literature data obtained with the use of laser interferometry.