Single Vapor Bubble Research Articles

Vapor bubble on a single nucleation site : Temperature and heat flux measurements

This work aimed at gaining a detailed understanding of heat transfer when boiling occurs on a heated substrate. More specifically, this work focuses on thermal transfers occurring at wall-fluid interfaces at the transition between natural convection and nucleate boiling regimes in pool boiling experiments. To avoid parasitic effects, the vapor bubble is created at a single artificial nucleation site. The boiling cell is instrumented with sensors for temperature measurement, pressure control, and parietal heat flux measurement. The fluid tested is Fluorinert FC-72. For providing a boiling surface and measuring heat flux, a boiling-meter consisting of a heating resistance, heat flux sensors and two thermocouples is used. This boiling-meter enables heat flux generation, the measurement of both temperature and heat flux, and can be rotated through 360°, enabling the influence of inclination to be studied. The boiling-meter is indented at its center to create a single vapor bubble. Thermal measurements are obtained and studied for different inclination angles between 0° and 180° at the saturation conditions. The results showed the nucleation site’s recurrent pattern of being active and inactive for whatever is the boiling surface inclination. Preliminary results are presented and discussed.

Experimental investigation to quantify the influence of single artificial indentations on vapour bubble dynamics and the associated heat transfer rates

Dependence of single vapour bubble dynamics and the associated heat transfer phenomena on the type of artificial indentations created on the heated substrate has been studied. Experiments under nucleate boiling regime have been carried out for conical indentations of varying cone angles (2γ = 30, 60 and 90o) and cylindrical cavity keeping the depth of features constant (hc = 500 μm). Rainbow schlieren deflectometry has been employed for simultaneous mapping of bubble dynamics and thermal field in a completely non-intrusive and real time manner. Direct comparison of the rainbow schlieren images shows a relatively large partition of thermal energy through natural convection for conical indentation of small cone angle and cylindrical cavity. For large-angled conical indentation, increased availability of thermal energy for bubble growth process results in the formation of relatively larger-sized vapour bubble at a relatively slower rate. In addition, due to the relative differences in the amount of vapour entrapped inside the cavities between any two bubbling cycles, the consistency of bubbling features was found to be much higher for larger cone angled cavity. The evidence for increased partition of thermal energy transfer through natural convection has been seen in the form of an increase in the portion of superheat layer in the vicinity of vapour bubble for conical cavity with small cone angles from the whole-field temperature reconstructions. For ΔTsup = 10 K, the heat transfer performance of surfaces with conical cavity with large cone angle (2γ = 90o) is significantly much better than the other conical and cylindrical cavities.

Numerical Study of Dielectric Fluid Bubble Behavior in Microchannel Evaporator in Presence of Diverging Electric Field

Herein, the diverging dielectrophoretic (DEP) electrode is designed to extract the vapor bubbles generated in a flow-boiling evaporator. The DEP and hydrodynamic forces acting on the undeformable single vapor bubble in the diverging electrode are estimated by the uncoupled flow and electrostatic simulation. The Maxwell stress, static pressure, and viscous shear stress on the bubble surface are considered. The diverging DEP electrode successfully generates a net force that acts on the bubble in the flow direction. The DEP force is dominant against drag at a low imposed mass flux and high applied electric field, where heat transfer enhancement could be expected. A large DEP force pushing the bubble toward the electrode is found when the bubble is close to the electrode.

Numerical Study of Growth of a Vapor Bubble in Superheated Seawater with Time-Varying Pressure

Flash evaporation is used in a wide range of applications from seawater desalination to nuclear and space applications. The fundamental mechanism in flash evaporation is mechanical and thermal non-equilibrium which results in liquid being superheated and subsequent bubble growth. Therefore, study of bubble growth is essential to accurately predict the vapor generation rate during flashing. Accordingly, this article provides a comprehensive numerical study of growth of a single vapor bubble during flash evaporation in seawater. Effects of system pressure, liquid superheat, salt concentration, and time-varying pressure during flash evaporation on bubble growth behavior are studied. The Rayleigh-Plesset equation with energy and salt concentration equations is solved numerically with 4th-order discretization schemes. Excellent agreement between the results of the present study and reported experimental results is found. It is observed that during bubble growth in seawater, salt continuously accumulates at the vapor-liquid interface which consequently reduces the growth rate and degrades thermal performance characterized by Nusselt number. However, it elongates the inertia-controlled regime during which the bubble radius is directly proportional to time. It is shown that properly handling the non-uniform distribution of solute in the liquid boundary layer is key to modeling bubble growth in seawater and binary solutions.

Superconducting source and sensor of single bubbles to study nucleate boiling of liquid helium

Joule heat generated by resistive elements of cryogenic micro- and nanodevices often originates boiling of the cooling cryogenic liquids (helium, nitrogen). The article proposes an experimental method to explore the dynamics of the formation and development of a single vapor bubble in cryogenic liquid by sensing the temperature change of a superconducting thin-film microbridge being in the resistive state with single phase slip center or line. It serves both the source of heat for generating single bubbles and the surface temperature sensor due to its temperature-dependent excess current. The average bubble detachment rate and the average single bubble volume were experimentally determined for nucleate helium boiling. The obtained values are in good agreement with the data of other authors found in literature.

Study of capillary-porous coatings used in power installations

This article has developed a physical model for the generation of single vapor bubbles in separate cells of a capillary-porous coating. The individual characteristics of bubbles are important for explaining the occurrence and development of cracks, damage to parts and assemblies of power plants. Solution of problem on evaporation of a clinoid microlayer under the vapor bubble growing in cell of porous structure covering metal heating surface is used. A model has been created for the development of the vapor phase in porous structures for cooling elements of power plants, melting units, which explains the mechanism of the generation, development and death of steam bubbles. The reliability of the cooling system of power plants is determined by the combined action of capillary and mass forces.

Experimental investigation on interface characteristics and heat transfer during bubble condensation in narrow rectangular channel

Direct steam contact condensation in subcooled water is widely used in various industries because of high efficiency in heat and mass transfer. A visual experimental is carried out to investigate the condensation process of single vapor bubble in narrow rectangular channel by a high-speed camera. Steam mass flux and water temperature are 0–1000 kg/(m2·s) and 57–93 °C, respectively. Four different condensation patterns are found in this study, such as chugging, ellipsoidal jetting, smooth grown bubble and rough grown bubble based on bubble behavior and surface feature. The bubble condensation period is positively correlated with initial bubble diameter. The bubble interface is smooth at low mass flow rates, while it becomes rough when the mass flow rate is high and the process of condensation is shorter. There is a bubble critical diameter during growth stage and each type of bubble will have distinctive phenomena when it reaches the critical diameter. The restriction of the narrow rectangular channel on vapor bubble is that an accelerated condensation process occurs when the bubble diameter changes from restricted to unrestricted. The factor of heat conduction is taken into account in the energy conservation equation, and the condensation heat transfer model is proposed. By deriving the energy conservation equation, a dimensionless parameter A related to the thermal conductivity of the wall and the width of flow channel is obtained. A predictive correlation for bubble diameter and a condensation Nusselt correlation are proposed, and the new correlations incorporate the dimensionless parameter A.

A numerical simulation of tube geometry effect on the performance of regenerators in Organic Rankine Cycle systems using the Volume of Fluid method

In this work, the tube geometry effect on the performance of regenerators in ORC systems is numerically studied using the Volume of Fluid (VOF) method. Three tube geometries are investigated: normal, divergent, and convergent tubes. The divergent tube offers increasing space, which is beneficial to bubble growth and delay the tube clogging induced by the increasing bubble size. The delay of tube clogging in the divergent tube can benefit the boiling heat transfer performance. The decreasing flow velocity due to the increasing space can also reduce the friction pressure drop. As a result, the divergent tube induces smaller total pressure drop as well as less pressure fluctuation, which are good for pump efficiency. It is found that the pressure drop can be up to 34.5% lower in divergent tube while the averaged wall heat flux is 2.94% higher comparing with the normal one. The features of enhancing heat transfer performance and reducing total pressure drop with less pressure fluctuation of the divergent tube can be used as a novel design for the liquid side of regenerators. The convergent tube will induce higher pressure drop with higher heat flux before the tube is clogged by the bubbles. However, the heat flux decreases as the tube clogging occurs. Due to the higher pressure drop induced, the convergent tube might not be suitable for the design of the liquid side of regenerators. It is the first time to study the heat transfer problem with a single vapor bubble considering the geometric effect, and this work provides physical insights on heat transfer performance and pressure drop for a two-phase boiling flow. The optimized geometry can increase the energy efficiency, whch could profoundly impact many industrial applications.

Simulation of Single Vapor Bubble Condensation with Sharp Interface Mass Transfer Model

Pure numerical simulation of phase-change phenomena such as boiling and condensation is challenging, as there is no universal model to calculate the transferred mass in all configurations. Among the existing models, the sharp interface model (Fourier model) seems to be a promising solution. In this study, we investigate the limitation of this model via a comparison of the numerical results with the analytical solution and experimental data. Our study confirms the great importance of the initial thermal boundary layer prescription for a simulation of single bubble condensation. Additionally, we derive a semi-analytical correlation based on energy conservation to estimate the condensing bubble lifetime. This correlation declares that the initial diameter, subcooled temperature, and vapor thermophysical properties determine how long a bubble lasts. The simulations are carried out within the OpenFOAM framework using the VoF method to capture the interface between phases. Our investigation demonstrates that calculation of the curvature of interface with the Contour-Based Reconstruction (CBR) method can suppress the parasitic current up to one order.

Numerical investigation of vapor bubble condensation in subcooled quiescent water

The vapor bubble condensation process is numerically investigated via three-dimensional (3D) simulation using Volume of Fluid method (VoF). Two phase change models, i.e. Lee model and Tanasawa model, are implemented and assessed with experimental data. Since no mesh-independent results can be obtained with the Lee model, the Tanasawa model is adopted for the study of bubble condensation process. The single vapor bubble condensation in subcooled quiescent water is analyzed. Different influencing factors on bubble condensation, i.e. diameter size, subcooling and liquid properties, are studied. With the increase of the bubble diameter, the condensation rate increases due to increase of the shear stress between the vapor bubble and the cold bulk, resulting in the turbulence inside the bubble being more intensive and the thermal field around the bubble more unstable. High subcooling increases the mass flux across the interface, which brings about the interface instability on the bubble surface and bubble breakup is observed. The increase of the liquid viscosity restricts the bubble rising and deformation, which reduces the convective heat transfer and condensation rate. The occurrence of bubble central breakup due to the decrease of surface tension enlarges the effective condensation area and increases the condensation rate.

A New Method for Estimating Bubble Diameter at Different Gravity Levels for Nucleate Pool Boiling

Abstract Nucleate boiling has significant applications in earth gravity (in industrial cooling applications) and microgravity conditions (in space exploration, specifically in making space applications more compact). However, the effect of gravity on the growth rate and bubble size is not yet well understood. We perform numerical simulations of nucleate boiling using an adaptive moment-of-fluid (MoF) method for a single vapor bubble (water or Perfluoro-n-hexane) in saturated liquid for different gravity levels. Results concerning the growth rate of the bubble, specifically the departure diameter and departure time, have been provided. The MoF method has been first validated by comparing results with a theoretical solution of vapor bubble growth in superheated liquid without any heat-transfer from the wall. Next, bubble growth rate, bubble shape, and heat transfer results under earth gravity, reduced gravity, and microgravity conditions are reported, and they are in good agreement with experiments. Finally, a new method is proposed for estimating the bubble diameter at different gravity levels. This method is based on an analysis of empirical data at different gravity values and using power-series curve fitting to obtain a generalized bubble growth curve irrespective of the gravity value. This method is shown to provide a good estimate of the bubble diameter for a specific gravity value and time.

Phase structure and its temporal evolution under growing single vapor bubble

A vapor bubble's footprint on a heated surface is composed of dry and wet regions, and their temporal evolution is governed by several forces, such as gravity, vapor inertia, viscous friction, and capillarity. In this Letter, we derive three types of relations that depict the dynamic variations in the length scales for the bubble base, dry area, and thin liquid film under a single vapor bubble. The proposed relations are validated using measurement results from clear images of the phase structure captured by high-speed visualization. Here, we reveal that the viscous friction is a dominant force acting on the bubble expansion process; however, it has been previously ignored for simplicity in dealing with the classical Rayleigh equation. Additionally, the critical role of the liquid wetting character in the vicinity of the moving triple phase line is illuminated to quantify the dry area dynamics. Finally, we find an analytic expression to obtain the thickness profile of the thin liquid film under a bubble based on the well-known Landau and Levich liquid coating theory.

Microlayer dynamics during the growth process of a single vapour bubble under subcooled flow boiling conditions

The phenomena of microlayer formation and its dynamic characteristics during the nucleate pool boiling regime have been widely investigated in the past. However, experimental works on real-time microlayer dynamics during nucleate flow boiling conditions are highly scarce. The present work is an attempt to address this lacuna and is concerned with developing a fundamental understanding of microlayer dynamics during the growth process of a single vapour bubble under nucleate flow boiling conditions. Boiling experiments have been conducted under subcooled conditions in a vertical rectangular channel with water as the working fluid. Thin-film interferometry combined with high-speed cinematography have been adopted to simultaneously capture the dynamic behaviour of the microlayer along with the bubble growth process. Transients associated with the microlayer have been recorded in the form of interferometric fringe patterns, which clearly reveal the evolution of the microlayer beneath the growing vapour bubble, the movement of the triple contact line and the growth of the dryspot region during the bubble growth process. While symmetric growth of the microlayer was confirmed in the early growth phase, the bulk flow-induced bubble deformation rendered asymmetry to its profile during the later stages of the bubble growth process. The recorded fringe patterns have been quantitatively analysed to obtain microlayer thickness profiles at different stages of the bubble growth process. ForRe = 3600, the maximum thickness of the almost wedge-shaped microlayer was obtained asδ ~ 3.5 μm for a vapour bubble of diameter 1.6 mm. Similarly, forRe = 6000, a maximum microlayer thickness ofδ ~ 2.5 μm was obtained for a bubble of diameter 1.1 mm.

Experimental analysis of the evaporation of a thin liquid film deposited on a capillary heated tube: estimation of the local heat transfer coefficient

The aim of this work is to estimate the local heat flux and heat transfer coefficient for the case of evaporation of thin liquid film deposited on capillary heated channel: it plays a fundamental role in the two-phase heat transfer processes inside mini-channels. In the present analysis it is investigated a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. The estimation procedure here adopted is based on the solution of the inverse heat conduction problem within the wall domain adopting, as input data, the temperature field on the external tube wall acquired by means of infrared thermography.

On the identification and mapping of three distinct stages of single vapor bubble growth with the corresponding microlayer dynamics

An experimental study to establish a one-on-one correspondence between microlayer dynamics and growth process of single vapor bubble under nucleate pool boiling regime has been presented. The transient evolution of microlayer and the corresponding vapor bubble growth have been simultaneously recorded using a thin film interferometer and one of the gradients-based imaging techniques, namely rainbow schlieren deflectometry. Based on interferometry measurements, the microlayer is observed to undergo three distinct stages. Each of these stages is identified by distinct microlayer structure namely, inverted conical, flattened peripheral region and truncated conical structure during the initial, transitional and diffusion-controlled growth phases of the vapor bubble. Contribution of microlayer towards bubble growth has been established by quantifying the time rate of change of net mass of the superheated fluid entrapped in the microlayer region and the corresponding rate of change of mass of the growing bubble. The analysis showed that the ratio of mass of the superheated fluid in the microlayer region to the mass of the vapor bubble does not exceed a maximum of ~ 15%. For any given heat flux level, the growing vapor bubble is seen to acquire its maximum size in the transitional regime of the growth process during which the microlayer exhibits a flattened peripheral region. Possible impact of varying heat flux level on microlayer dynamics and movement of the apparent contact line of the vapor bubble has also been discussed. A reasonably good agreement of the experimental data with some of the well-established bubble growth models for initial microlayer thickness has been observed. Based on the experimental results, critical value of empirical parameter ‘Cslope’ (parameter correlating the bubble base radius and the maximum microlayer thickness) for transition from inertia-driven to diffusion dominated growth has been identified to be ~ 0.0042.

Numerical Investigation of Thermal Performance of Key Components of Electric Vehicles Using Nucleate Boiling

Abstract An efficient thermal management system is desirable for improving the performance of key components of electric vehicle (EV), such as battery packs and insulated-gate bipolar transistors (IGBTs). This paper investigates the application of single bubble nucleate boiling heat transfer in battery and IGBT component cooling pack. A semi-mechanistic flow boiling model, which combines four main submodels i.e., phase change model, micro-region model, Marangoni model, and contact angle model, is developed to get the insight of various subprocesses like bubble inception, growth, departure, scavenging effect while the bubble departs, and condensation. For model validation, simulations are carried out for single bubble flow boiling in a vertical rectangular channel and compared against the experimental data available in the literature. Thereafter, simulations are carried out for the battery and IGBT cooling pack to understand the physical phenomena associated with nucleate boiling in such systems. The choice of a single vapor bubble vis-à-vis multiple bubbles has been based on the objective of validating the developed numerical model. An enhancement of ∼30% in heat transfer is achieved for both battery and IGBT components when the system is subjected to a nucleate boiling cooling regime as compared to a conventional single-phase convection cooling system. Nusselt number variation due to the single bubble movement along the coolant path is studied in detail for both serpentine-shaped cooling path in a battery and straight flow path in an IGBT. Moreover, the influence of Reynolds number over bubble dynamics is analyzed.

Theoretical investigation of a single vapor bubble during Al2O3/H2O nanofluids in power-law fluid affected by a variable surface tension

This article analyzes the growth of the vapor bubble in a novel model of power-law nanofluids (Al2O3/H2O) under a new effect of variable surface tension. The governing equations of the rising vapor bubble flow model are formulated and converted to a single equation describing the bubble dynamics behavior. By employing the model of Plesset and Zwick method, we investigate a new model of equations within power-law nanofluids to examine the effect of different physical parameters such as initial superheating liquid, critical bubble radius, and thermal diffusivity on the vapor bubble formation. Furthermore, the effects of surface tension behavior with the initial bubble radius, time, and initial rate of bubble radius are examined. It is found that the growth of the vapor bubble radius increases with the increase of initial superheating liquid, critical bubble radius, and thermal diffusivity. In addition, the connection between shear stress and shear rate is analyzed in detail. Using appropriate values for the physical parameters, the behavior of solutions of the vapor bubble is discussed. Based on the conducted simulation analysis, the behavior of the solutions is found to be more accurate than those in the previous studies. Besides, the obtained results demonstrate that the vapor bubble in power-law nanofluids grows slower than in pure water.

Whole Field Measurements to Understand the Role of Varying Depths of Nucleation Site on Vapor Bubble Dynamics and Heat Transfer Rates

Abstract This work studies the possible effects of varying depths of cavity on bubbling features and the associated heat transfer rates in nucleate pool boiling regime. A single vapor bubble has been generated on a substrate with a cylindrical cavity at its center that acts as the nucleation site. Experiments have been conducted for three cavity depths (250, 500, and 1000 μm), while keeping its throat diameter constant at 200 μm. With the bulk fluid maintained under saturated conditions, for each cavity depth, surface superheat level has been varied in the range of ΔTsuperheat = 8, 10 and 12 °C. A gradient-based visualization technique, coupled with a high speed camera, has been employed to simultaneously map the changes in thermal gradients during the formation of the vapor bubble as well as bubble dynamic parameters. The image sequence obtained has been qualitatively and quantitatively analyzed to elucidate the dependence of bubbling features and various heat transfer processes on cavity depth. With an increase in the depth of cavity, the net effect of reduction in the available thermal energy due to the increased convection effects and significant depletion of superheated layer are identified as the dominant heat transfer processes that influence the bubbling features. Furthermore, based on the statistics of bubble departure characteristics, the cavity with higher depth (1000 μm) showed a much stable bubble formation with minimal variation in the bubble departure frequency as compared to the bubbling features from a cavity with smaller depth (250 μm). Evaporative heat transfer process has been identified as the primary cause for increased inconsistency of bubbling features at high superheat conditions for experiments performed for low cavity depths.

On the Development of Correlations for Bubble Liftoff Parameters During Subcooled Nucleate Flow Boiling Using Nonintrusive Dynamic Measurements

Abstract Accurate prediction of bubble dynamic parameters is essential to improve boiling heat transfer models. Considering the complexities and challenges associated with performing a large number of boiling experiments, researchers have realized the importance of experimental correlations for predicting bubble dynamic parameters. In this direction, we report an experimental work concerned with the development of correlations for various bubble liftoff parameters during nucleate flow boiling regime. As a definite advancement, the experimental measurements have been performed in a purely nonintrusive manner, thereby minimizing the errors arising due to the interaction of any external probe with the process under study. The measurement approach makes use of a gradient-based imaging technique to simultaneously map the bubbling features and thermal field around a single vapor bubble generated under subcooled flow boiling conditions. Experiments have been performed in a rectangular channel for a wide range of heat fluxes (q" = 20–50 kW/m2), subcooling level (ΔTsub = 2–9 K), and Reynolds numbers (Re = 600–6000) with water as the working fluid. Results show a strong dependence of bubble liftoff parameters on Reynolds number, subcooling level, and applied heat flux. Based on the experimental measurements, empirical correlations have been developed for various bubble liftoff parameters as a function of Jacob number and Reynolds number. Predictions made through the developed correlations are found to be in good agreement with the measured values as well as with the values reported in the available literature. Of all the bubble parameters, maximum deviation between the predicted and measured values (≈23%) was found to be in bubble release frequency.

A mass-preserving interface-correction level set/ghost fluid method for modeling of three-dimensional boiling flows

We present an efficient method for the direct numerical simulation of three-dimensional (3D) boiling flows. The liquid-vapor interface dynamics is captured using an interface-correction level set method, modified to account for the interface heat and mass transfer due to phase change. State-of-the-art computational techniques, such as the fast pressure-correction and ghost-fluid methods, are implemented to accurately solve the coupled thermo-fluid problem involving large density contrasts and jump conditions. The solver is thoroughly validated against four benchmark cases with increasing complexity, which show better mass conservation properties than traditional level set methods, thus allowing for coarser grid resolutions and lower computational costs. We further demonstrate our method by simulating two realistic 3D boiling flows in greater details. In the first case, a saturated film boiling of water vapor at near critical conditions over a horizontal hot flat plate is considered. The results are analyzed by comparing the transient evolution of the interface morphology, temperature distribution, space and time averaged Nusselt numbers obtained from numerical simulations with the semi-empirical correlation of Berenson and existing numerical literature. In the second case, we simulate the condensation and buoyancy-driven motion of a single spherical water vapor bubble at different subcooled liquid temperatures and saturation pressures. We find opposite trends of the condensation rate and the bubble rising velocity when the degree of subcooling is increased, and an increase of the condensation rate at lower saturation pressures, due to variation of the thermophysical properties.