Single Vapor Bubble Research Articles

Whole field measurements to quantify the thermal impact of single vapor bubble under nucleate flow boiling regime

The work reports on simultaneous measurement of bubble dynamics and thermal field to understand the local effects of bubble motion on thermal boundary layer behavior and heat transfer characteristics under subcooled flow boiling conditions. Single vapor bubble was generated using a specially designed ITO glass heater to avoid the phenomenological complexities and to obtain reproducible data. Flow boiling experiments were carried out in a vertically oriented rectangular channel with water for varying Reynolds numbers (Re = 600–4800), keeping constant subcooling level and power input. The thermal field around the vapor bubble along with the dynamics has been visualized using a non-intrusive optical technique; rainbow schlieren deflectometry, which was configured with a high-speed color camera to acquire high spatial and temporal resolution. The recorded experimental data have been analyzed qualitatively as well as quantitatively and the obtained results have been presented in the form of whole field temperature distribution, the dynamics of thermal boundary layer, local heat transfer coefficients etc. It was observed that the effect of bubble formation on the wall temperature variation, thermal boundary layer and heat transfer coefficient was relatively higher during the growth period (until the bubble lift-off) compared to the waiting period after the lift-off of the bubble. The experimental results showed that the bubble-induced heat transfer significantly enhance the overall heat transfer rate from the heated surface while the degree of enhancement was observed to be reducing with increasing Reynolds number.

Flow behavior around single nucleate boiling bubble quantitatively grasped by particle tracking visualization

In this study, a single nucleation site was made on the upper surface of heated substrate, and small tracer particles suspended in water for flow visualization. Single vapor bubble dynamics and particle movements were recorded by high speed camera, then the flow field was quantitatively grasped by particle tracking visualization. Resulting of the visualization, in earlier stage of nucleate boiling, rapid growth motion of bubble pushed liquid surrounding bubble interface outwardly. There was no retracting flow into root of boiling bubble though it needed to feed water into root of boiling bubble due to evaporation in case of lower heat flux condition. Liquid flow pushed by rapid bubble growth was explained by combining thermal diffusion equation with mass conservation law, moreover the pressure increase caused this liquid flow was estimated. In bubble departure process, liquid near substrate was horizontally pulled to nucleation site, but the eddy to enhance a vertical mixing of superheated liquid near substrate was not observed. After bubble departure, it was found that a hot liquid plume under a rising bubble was not mainly caused by entrainment of superheated liquid layer formed on heated substrate, but by a relative descending flow around a rising bubble induced by mass conservation law.

Understanding the growth mechanism of single vapor bubble on a hydrophobic surface: Experiments under nucleate pool boiling regime

Growth and departure characteristics of single vapor bubble on hydrophobic surfaces have been investigated under saturated bulk conditions. Nucleate pool boiling experiments, with water as the working fluid, have been carried out on two sets of hydrophobic surfaces; (1) black painted ITO-coated glass substrate, and (2) ITO-coated sapphire substrate subjected to two different levels of constant heat flux conditions. High speed camera was employed to characterize the bubble dynamics. In conjunction with the high speed camera, an IR thermal imaging camera was used to simultaneously map the spatio-temporal temperature distribution of the substrate underneath the growing vapor bubble. Compared to the black paint coated glass substrate, significantly higher transmissivity of sapphire substrate in the mid-IR spectral range ensured the authenticity of the spatio-temporal accuracy of temperature measurements of the bubble nucleation site using the employed IR camera. Results on the bubble growth clearly showed that, irrespective of the substrate employed, the bubble exhibits almost zero waiting time, leaving a small residue of vapor on the substrate after the bubble departure, which further acts as a nucleus for the next bubble. Moreover, the bubble growth process was observed to be extremely slow. The IR camera-based temperature field of the hydrophobic substrates revealed that the presence of vapor bubble does change the local temperature of the substrate underneath the vapor bubble. However, it was observed that the temperature field of the nucleation site does not change with bubble growth and its departure, which, in turn, indicated towards the possibility that the microlayer ceases to exist underneath the vapor bubble. This is in contrast to the microlayer evaporation-driven growth of single vapor bubble on a hydrophilic substrate. Based on these direct experimental observations, plausible bubble growth mechanism(s) on the given hydrophobic surfaces have been predicted and discussed.

Experiments on pool boiling regimes and bubble departure characteristics of single vapor bubble under subcooled bulk conditions

An experimental study for understanding the effects of subcooling of bulk fluid on the phenomenon of nucleate boiling heat transfer was carried out. The whole field experimental data was recorded in a purely non-intrusive manner using rainbow schlieren deflectometry as the optical diagnostic tool. Boiling experiments were conducted over a range of heat flux conditions (1⩽q′′⩽40 kW/m2) and for varying levels of subcooling (0 K ≤ ΔTsubcooling ≤ 10 K). The recorded images were first been interpreted to understand the effect of subcooling on various regimes of boiling phenomenon, which include convection, isolated nucleate boiling and vertical coalescence of the departing bubbles. Observations on convection regime showed an increasing influence of natural convection heat transfer. Based on the formation of vapor bubble and its departure characteristics, the isolated nucleate boiling regime of the classical boiling curve was further segmented into three sub-regimes namely, stable vapor bubbles, oscillating vapor bubbles and oscillating-departing vapor bubbles. A mechanism explaining the oscillations of vapor bubbles under subcooled condition was proposed that takes into account the relative dominance of condensing apex region and evaporating microlayer region. Various bubble dynamic parameters such as equivalent bubble diameter, departure frequency, aspect ratio and oscillation frequency were determined quantitatively to understand the effect of subcooling on the bubble departure characteristics. Based on the experimental results, an empirical correlation for predicting the bubble departure diameter and evaporative heat flux was developed and proposed.

Performance Analysis and Cavitation Prediction of Centrifugal Pump Using Various Working Fluids

Background: The application of centrifugal pumps is found in domestic and petrochemical industries. Industrial centrifugal pumps are designed and tested using water as working fluid before supplied to industries, as water is commonly available. However, centrifugal pumps are used in industries for various applications, which involve the handling of fluids other than water- like saline-water, crude oil, gasoline, etc. Consequently, hydraulic performance of the pump differs from the designed and tested values and pump performance becomes unpredictable. Cavitation characteristics of the pump handling different fluids other than water are also changed and many a time, cavitation starts prematurely. As a result, the operating cost of pump is increased. Objective: A CFD based computational analysis of a single-stage, single-entry industrial centrifugal pump having double-volute casing is carried out to compare the performance and cavitation characteristics for various working fluids, namely water, saline water with varying salinity, gasolene and crude oil. Methods: Multiple Reference Frame method (MRF) available in Reynolds-Averaged Navier-Stokes (RANS) equations based CFD solver Ansys-CFX is used in the present study. CFD simulation is carried out for five flow rates with Standard k-ε turbulence model. Rayleigh-Plesset equation describing the growth of a single vapor bubble in a liquid is used for predicting the cavitation flow behaviour. Results: Minimum static pressure is computed at the suction side of saline water as compared to the other working fluids studied here. Hydraulic efficiency of crude oil is found to be the lowest as compared to other fluids. Supercavitation (excessive formation of vapor bubbles and sudden drop in head up to 3%) starts early for saline water with 40g/kg salinity. Conclusion: The results show little variation in pump efficiency when water and saline water are used as working fluids. However, cavitation characteristics differ considerably with the working fluids. Recent patents filed/published in this area revealed that efforts are needed to develop effective cavitationresistant centrifugal impellers and pumps.

Experiments to compare the dynamics and thermal impact of single vapor bubble subjected to upward and downward flow boiling configurations

ABSTRACT Experiments on subcooled flow boiling have been carried out to investigate the dynamical parameters and thermal impact of a growing vapor bubble under vertically upward and downward flow configurations with water as the fluid medium. Bubble growth and detachment phenomena have been visualized in real-time using high-speed rainbow schlieren technique. Schlieren-based imaging technique simultaneously maps both the bubble dynamics and thermal gradients field along the heater surface. Experimental data have been compared for a range of Reynolds number (Re = 3600–7200) at a constant subcooling level of ΔTsub = 4°C. The thermal aspects have been identified in the form of real-time visualization of thermal boundary layer and its scavenging as the bubble detaches/lifts-off from the heater surface. Time variation of various bubble dynamic parameters have been determined from the quantitative processing of the schlieren images. Significant differences in the bubble trajectory after departure from the nucleation site were observed for downward and upward flow configurations. For Re ≤ 6000, comparatively higher evaporative heat flux dissipation was obtained for upward flow, while at Re ≥ 6000, the evaporative heat flux dissipation was found to be more or less similar for both upflow and downflow configurations.

Measuring the activation energy barrier for the nucleation of single nanosized vapor bubbles

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes that has received broad interest from diverse fields of physics and chemistry. While most studies focused on large microbubbles, here we employed a surface plasmon resonance microscopy to measure the nucleation rate constant and activation energy barrier of single nanosized embryo vapor bubbles upon heating a flat gold film with a focused laser beam. Image analysis allowed for simultaneously determining the local temperature and local nucleation rate constant from the same batch of optical images. By analyzing the dependence of nucleation rate constant on temperature, we were able to calculate the local activation energy barrier within a submicrometer spot. Scanning the substrate further led to a nucleation rate map with a spatial resolution of 100 nm, which revealed no correlation with the local roughness. These results indicate that facet structure and surface chemistry, rather than geometrical roughness, regulated the activation energy barrier for heterogeneous nucleation of embryo nanobubbles.

Single vapour bubble growth under flash boiling conditions using a modified HLLC Riemann solver

Common fuel-oxidizer combinations in orbital manoeuvring systems consist of toxic substances but will be replaced in the future. Liquid oxygen (LOX) is one potential oxidizer but it rapidly superheats under the initial low-pressure conditions in the combustion chamber. Bubble nucleation and growth dominate the efficient disintegration of the liquid jet, which is called flash boiling. An investigation of the small scale bubble dynamics will help to improve existing models for the break-up of the LOX jet and the mixing with the fuel. Direct numerical simulations (DNS) can be used to analyse the underlying processes if the solver handles phase transition effects at extreme ambient conditions. In this paper we use a fully compressible discontinuous Galerkin solver combined to a level-set equation and to a modified HLLC Riemann solver. The main goal of our investigation is to assess closure conditions for the mass transfer models which accurately represent the physics of vapour bubble growth. We couple three models for the estimation of vaporisation mass fluxes to the interface Riemann solver. The Hertz-Knudsen relation and a kinetic relation predict the volumetric expansion well but cannot represent the instantaneous mass flux. A sub-grid scale heat flux model predicts the mass flux qualitatively better but the volumetric bubble expansion matches only at late time intervals. The dependency of the calibrated coefficients on the physical conditions is similar for the different vaporisation models.

Numerical simulation of liquid film formation and its heat transfer through vapor bubble expansion in a microchannel

The evaporation of vapor bubbles inside a microchannel is important to realize a device with high cooling performance. The liquid film formed on the solid surface is essential for evaporative heat transfer from solid to fluid; its formation process and heat transfer characteristics need to be investigated. The expansion process of a single vapor bubble via evaporative heat transfer in microchannels was evaluated via a numerical simulation in this study. In the calculation model, the working fluid used was saturated FC-72 at 0.1013 MPa and the channel diameter was 200 µm. The superheat of the initial temperature field and wall were considered as parameters. To evaluate the heat transfer characteristics, the time variation of liquid film thickness was evaluated. The averaged liquid film thickness had a correlation with the capillary number. Additionally, the dominant heat transfer mode was estimated by decomposing the heat transfer rate into the heat-transfer rate through the liquid film, rear edge, and wake. When the superheat was low, the heat transfer mostly occurred via liquid film evaporation; the heat flux through the liquid film could be predicted using the liquid film thickness. On the other hand, in cases of higher superheat, owing to rapid expansion of the vapor bubble, no evaporative heat transfer occurred through the liquid film around the bubble head. It could be inferred from this study that the relationship between the thickness of the thermal boundary layer of the bubble and liquid film thickness is important for predicting the cooling effect of this phenomena. When the vapor bubble grows in the high superheat liquid, the rapid growth makes the liquid film thick, and the thick liquid film prevents the heat transfer between the liquid–vapor interface and heated wall.

Experiments on the effects of varying subcooled conditions on the dynamics of single vapor bubble and heat transfer rates in nucleate pool boiling regime

Dependence of bubble dynamic parameters and the associated heat transfer mechanisms on varying levels of sub-cooled conditions has been experimentally studied. Isolated nucleate pool boiling experiments have been performed to generate a single vapor bubble on an ITO (Indium Tin Oxide) glass substrate for three degrees of sub-cooling (ΔTsub-cooled = 2 °C, 5 °C and 0 °C). Field measurements have been made using gradient-based rainbow schlieren deflectometry. Various sub-phenomena associated with a bubble cycle, for instance, bubble inception and its growth, bubble oscillation and its departure from the heated substrate, have been visualized in real time for the range of bulk conditions employed. The recorded rainbow schlieren images have been analyzed to determine the transient variations of bubble dynamic parameters such as equivalent bubble diameter, bubble base diameter and aspect ratio. Effects of sub-cooled level on bubble oscillations have been estimated based on the dynamic variations in the hue values in the vicinity of the oscillating bubble. Spectral analysis of the hue variations showed an increasing trend in the oscillation frequency with sub-cooling conditions (foscillation = 52.8 Hz and 47.2 Hz for ΔTsubcool = 5 °C and 2 °C respectively). Based on the whole-field temperature distribution retrieved through the quantitative analysis of the schlieren images, individual contributions of the various heat transfer mechanisms (natural convection, bubble growth and superheat layer) towards the overall heat transfer rates have been determined. Results revealed that the contribution of natural convection heat transfer increases from 54% under saturation conditions to up to 99% for ΔTsub-cooling = 5 °C.

Simultaneous mapping of single bubble dynamics and heat transfer rates for SiO2/water nanofluids under nucleate pool boiling regime

Dependence of single vapor bubble dynamics and heat transfer rates on varying concentration of SiO2 nanoparticles for a range of subcooled conditions (0–9 °C) has been experimentally studied under nucleate pool boiling configuration. Non-invasive measurements have been carried out using rainbow schlieren deflectometry. Results on bubble dynamics showed that the bubble diameter and aspect ratio decrease with increasing subcooling levels as well as concentration of nanofluids. The frequency of bubble oscillations was found to increase first and then decrease with increasing subcooling levels while it decreases monotonically with increasing nanofluid concentration. Bubble departure frequency increased significantly for nanofluids, while it decreased with increasing subcooling levels. Condensation effects at the bubble interface were reflected in the form of redistribution of colors around it. Schlieren images clearly revealed a spread in the spatial extent of the thermal boundary layer region caused by the suspended nanoparticles around the vapor bubble as well as near the heated substrate. This phenomenon has been considered as one of the factors that tends to alter the condensation effects and, in turn, affects the bubble dynamics. Quantitative analysis of schlieren images revealed that the natural convective heat flux increases with increasing subcooling levels, while it decreases with increasing nanoparticle concentration. Deterioration in the natural convection phenomenon in the presence of suspended nanoparticles has been attributed to the reduced strength of thermal gradients adjacent to the heater substrate. On the other hand, evaporative heat flux was observed to decrease with increasing subcooling levels and increase with increasing concentration of nanofluids.

A laser interference/high-speed photography method for the study of triple phase contact-line movements and lateral rewetting flow during single bubble growth on a small hydrophilic heated surface

The laser interferometric method and high-speed camera techniques are used simultaneously to study dynamic characteristics of a single vapor bubble growth and departure from an ITO heater surface submerged in ethanol. Two high-speed cameras are used simultaneously in this experiment: one is used to record interference fringes of the microlayer at the bubble's bottom and the other is used to record the growth and detachment of the single bubble synchronously from the side view. By comparing these images frame by frame, it is found that the bubble's growth time can be divided into two periods sequentially: the microlayer period and the lateral rewetting period. Variations of dry spot radius, bubble contact radius, bubble volume and diameter, dynamic contact angles during these two periods are illustrated.

Numerical simulation of single vapor bubble dynamics in a liquid in an intense acoustic field

The possibility of increasing the calculation efficiency by the joint use of two models of the dynamics of a single weakly-nonspherical vapor bubble under its strong collapse in liquid is studied. In both models the motion of liquid and vapor is split into a spherical component and its small nonspherical perturbation. The models differ in the description of the spherical component. In the first (simplified) model, it is described by a system of ODE together with partial differential equations in temperature, derived under the assumption of weak compressibility of liquid and bubble homobaricity. In the second model, one-dimensional gas dynamics equations are applied. The advantage of the simplified model consists in determining a numerical solution with much-less computer time costs in comparison with what is required for the numerical integration of gas dynamics equations. The assumptions used in the simplified model in the final stage of collapse become incorrect, and, as a result, the numerical solution errors increase. Therefore, the simplified model is applied at the beginning of bubble collapse, whereas the gas dynamics equations are used at its end. Within this approach, the numerical solution in the final stage of collapse is dependent on the moment of transition to the gas dynamics equations. It is shown that satisfactory description of evolution of bubble sphericity distortion is achieved when the transition is made under the condition that the Mach number M of vapor in the vicinity of the bubble surface is less than 0.4. Satisfactory resolution of the shock wave in the bubble is attained when the transition is performed at M<0.2.

Numerical investigation of bubble dynamics and heat transfer in subcooling pool boiling under low gravity

Abstract The numerical simulation of a single vapor bubble growth in subcooling liquid under different gravity conditions has been carried out. In the numerical model, a thin superheated layer and the thermocapillary convection caused by the surface tension variation along the surface are considered. The continuity equation and energy equation are modified to allow for the phase change. In addition, the thermocapillary convection effect has been included in the momentum equation. The vapor-liquid interface is captured by the phase field method. The results show that the bubble behavior in the numerical model agrees well with previous experiments conducted in high subcooling liquid under microgravity. The effects of gravity level, contact angle and wall superheat on the bubble growth, critical subcooling (the liquid subcooling under the condition that the evaporation rate of a bubble is equal to its condensation rate), together with heat transfer have been investigated. The growth period and departure radius both reduce with the increase in gravity level, while the critical subcooling increases slightly. Large contact angle at the three-phase contact line augments the departure radius. However, the critical subcooling decreases as contact angle increases. With the wall superheat increasing, the growth period reduces rapidly, while the departure radius and the critical subcooling increase. What’s more, the non-departing bubble adhering to the surface would prevent heat transfer with a dry spot, which may damage the heating element in application.

Numerical Study of a Vapor Bubble Collapse near a Solid Wall

Cavitation is the reason for degradation of the performance of technical devices that involve high speed liquid flows. Detailed information on structure of cavitating liquid flow is of key importance for design of such devices. But experimental study of these flows is a severe problem due to extreme flow parameters and very small spatial and temporal scales. That is why numerical simulation became an efficient tool of investigation of the detailed flow structure. Modelling of the collapse of a bubble located in the proximity of a solid wall is of particular interest from the point of view of cavitation erosion analysis. A quasi-homogeneous model of cavitating flows is formulated based on the assumptions of mechanical and thermodynamic equilibrium between the liquid and vapor phases. Numerical investigation is carried out for the unsteady compressible flow developing in the process of single vapor bubble collapse. The proposed algorithm demonstrates adequacy and robustness.

Rainbow schlieren-based direct visualization of thermal gradients around single vapor bubble during nucleate boiling phenomena of water

Real time non-intrusive diagnostics of thermal gradients around a single vapor bubble in isolated nucleate pool boiling with water as the working fluid have been presented. Direct visualization of transient evolution of temperature gradients in the vicinity of the heated substrate and around the single bubble as it undergoes various sub-processes has been performed using rainbow schlieren deflectometry. Boiling experiments have been conducted under saturated conditions. Results have been presented in the form of two-dimensional rainbow schlieren images wherein the degree of color re-distribution gives a direct measure of the strength of thermal gradients. Through the real time experimental images, various sub-processes/phenomena associated with boiling heat transfer such as development of superheat layer in the vicinity of the heated substrate, inception and further growth of the vapor bubble followed by scavenging of the superheat layer as the vapor bubble departs into the bulk fluid have been discussed. The profiles of hue distribution near the triple contact line brought out the presence of near stagnant fluid zone in which the heat transfer phenomenon was seen to be primarily diffusion-dominated. Beyond this narrow region, significant bulk fluid movement was observed on the basis of the spatial distributions of hue values recorded in the form of real time schlieren images. The experiments further revealed an instantaneous localized bending of the superheat layer as the bubble leaves the heater surface and subsequent shedding of the wake vortices from the edges of the departing bubble as it moves upwards in the bulk fluid.

A numerical simulation of the water vapor bubble rising in ferrofluid by volume of fluid model in the presence of a magnetic field

Multiphase flow is one of the most complicated problems, considering the multiplicity of the related parameters, especially the external factors influences. Thus, despite the recent developments more investigations are still required. The effect of a uniform magnetic field on the hydrodynamics behavior of a two-phase flow with different magnetic permeability is presented in this article. A single water vapor bubble which is rising inside a channel filled with ferrofluid has been simulated numerically. To capture the phases interface, the Volume of Fluid (VOF) model, and to solve the governing equations, the finite volume method has been employed. Contrary to the prior anticipations, while the consisting fluids of the flow are dielectric, uniform magnetic field causes a force acting normal to the interface toward to the inside of the bubble. With respect to the applied magnetic field direction, the bubble deformation due to the magnetic force increases the bubble rising velocity. Moreover, the higher values of applied magnetic field strength and magnetic permeability ratio resulted in the further increase of the bubble rising velocity. Also it is indicated that the flow mixing and the heat transfer rate is increased by a bubble injection and applying a magnetic field. The obtained results have been concluded that the presented phenomenon with applying a magnetic field can be used to control the related characteristics of the multiphase flows. Compared to the previous studies, implementing the applicable cases using the common and actual materials and a significant reduction of the CPU time are the most remarkable advantages of the current study.

Modeling aspects of vapor bubble condensation in subcooled liquid using the VOF approach

ABSTRACTDirect contact condensation of a vapor bubble during subcooled boiling flow scenario has diverse applications in many fields such as thermal, chemical, and nuclear energies. The present work aims at the exploration of the underlying physics of single vapor bubble condensation in subcooled water following the volume of fluid method approach using ANSYS FLUENT 14.5. This work emphasizes on the modeling of the mass transfer rate using interfacial jump conditions for investigating the effect of various parametric conditions on the collapse phenomenon. A comparative study is also performed between the interface jump approach and the models based on the proposed empirical correlations to assess the condensation heat transfer (in terms of the collapse rate and Nusselt number) and bubble shape. The mass transfer model based on the interfacial jump condition is found to be the more realistic among all models for capturing the collapse rate as well as its shape.

Numerical investigation of the bubble growth in horizontal rectangular microchannels

ABSTRACTTo explore the mechanism of flow boiling in microchannels, the processes of a single-vapor bubble evaporating and two lateral bubbles merging in a 2D microchannel are investigated. The temperature recovery model based on volume of fluid method is adopted to perform the flow boiling phenomena. The effects of wall superheat, Reynolds number, contact angle, surface tension, and two-bubble merger on heat transfer are discussed. Wall superheat dominates the bubble growth and is roughly proportional to wall heat flux. The update of velocity and temperature fields’ distribution in the channel increases with increasing inflow Reynolds number, which improves the wall heat flux markedly. Besides, the area of thin liquid film between the wall and the bubble is enlarged by reducing the contact angle, thus, expanding the wall heat flux several times compared with the single-phase cross section. However, variation of surface tension (0.0589, 0.1 N/m) is found to be insignificant.

Numerical simulation of superheated vapor bubble rising in stagnant liquid

In present study, the rising of superheated vapor bubble in saturated liquid is simulated using volume of fluid method in OpenFOAM cfd package. The surface tension between vapor–liquid phases is considered using continuous surface force method. In order to reduce spurious current near interface, Lafaurie smoothing filter is applied to improve curvature calculation. Phase change is considered using Tanasawa mass transfer model. The variation of saturation temperature in vapor bubble with local pressure is considered with simplified Clausius–Clapeyron relation. The couple velocity–pressure equation is solved using PISO algorithm. The numerical model is validated with: (1) isothermal bubble rising and (2) one-dimensional horizontal film condensation. Then, the shape and life time history of single superheated vapor bubble are investigated. The present numerical study shows vapor bubble in saturated liquid undergoes boiling and condensation. It indicates bubble life time is nearly linear proportional with bubble size and superheat temperature.