Bubble Inception Research Articles

Improving boiling heat transfer with hydrophilic/hydrophobic patterned flat surface: A molecular dynamics study

The fast development of Internet of Things and MEMS/NEMS technologies has led the manufacture of chips and sensors towards micro/nano level, yet this poses huge challenges to dissipate heat in a limited space. Among various heat dissipation technologies, boiling heat transfer is a very promising method because of its rapid heat absorption caused by phase change. Though introducing different wettability to form a biphilic surface is recognized as a valid way to enhance boiling heat transfer, systematic and micro/nano scale investigations on the boiling process over patterned biphilic surface are still needed to meet the requirement of miniaturization. In this paper, a molecular dynamics study was adopted to investigate six biphilic surfaces with different hydrophilic/hydrophobic patterns. To the best of our knowledge, individual bubble behavior on biphilic surface was observed for the first time via molecular dynamics method. It was found that the bubble dynamics behaved differently when the wall temperature is 150 K and 200 K, respectively. In addition, the shape and volume of bubble could be controlled by designing the hydrophilic/hydrophobic patterns, and simulation results indicated that these patterns had significant effects on bubble inception temperature, film boiling temperature and evaporation rate. Also, an ever-present temperature and density difference were observed above biphilic surfaces, results shown that this difference helped to create a fluctuation and promoted horizontal heat and mass transfer, which in turn was considered to be favorable to bubble nucleation. In the end, different pattern designs were compared, results demonstrated that hydrophilic/hydrophobic pattern designing was important because heat transfer performance could be different even with the same hydrophilic to hydrophobic area ratio.

Pressure reconstruction of a planar turbulent flow field within a multiply-connected domain with arbitrary boundary shapes

Over the past two decades, it has been demonstrated that the instantaneous spatial pressure distribution in a turbulent flow field can be reconstructed from the pressure gradient field non-intrusively measured by Particle Image Velocimetry (PIV). Representative pressure reconstruction methods include the omnidirectional integration (Liu and Katz, 2006; Liu et al., 2016; Liu and Moreto, 2020), the Poisson equation approach (Violato et al., 2011; De Kat and Van Oudheusden, 2012), the least-square method (Jeon et al., 2015), and most recently, the adjoint-based sequential data assimilation method, which also essentially utilizes the Poisson equation to reconstruct the pressure(He et al., 2020). Most of these previous pressure reconstruction examples, however, were applied to simply-connected domains (Gluzman et al., 2017) only. None of these previous studies have discussed how to apply the pressure reconstruction procedures to a multiply-connected domain (Gluzman et al., 2017). To fill in this gap, this paper presents a detailed report for the first time documenting the implementation procedures and validation results for pressure reconstruction of a planar turbulent flow field within a multiply-connected domain that has arbitrary inner and outer boundary shapes. The pressure reconstruction algorithm used in the current study is the rotating parallelray omni-directional integration algorithm, which, as demonstrated in reference (Liu and Moreto, 2020) based on simply-connected flow domains, offers high-level of accuracy in the reconstructed pressure. While preserving the nature and advantage of the parallel ray omni-directional pressure reconstruction at places with flow data, the new implementation of the algorithm is capable of processing an arbitrary number of inner void areas with arbitrary boundary shapes. Validation of the multiply-connected domain pressure reconstruction code is conducted using the DNS (Direct Numerical Simulation) isotropic turbulence field available at the Johns Hopkins Turbulence Databases, with 1000 statistically independent pressure gradient field realizations embedded with random noise used to gauge the code performance. For further validation, the code is also applied for pressure reconstruction from the DNS pressure gradient in the ambient flow field of a shock-induced non-spherical bubble collapse in water (Johnsen and Colonius, 2009). The successful implementation of the parallel ray pressure reconstruction method to multiply-connected domains paves the way for a variety of important applications including, for example, experimental characterization of pressure field changes during the process of cavitation bubble inception, growth and collapse, non-intrusive unsteady aerodynamic force assessment for an arbitrary body shape immersed in flows, and multi-phase flow investigations, etc. In particular, as an immediate follow-up effort, the parallel ray pressure code will be used for the instantaneous pressure distribution reconstruction of the turbulent flow surrounding cavitation inception bubbles occurring on top of a cavity trailing corner based on high-speed PIV measurements.

Study of cavitation inception using multiphase lattice Boltzmann method with incorporating equations of state

The study of cavitation inception is crucial in several hydraulic machines, e.g. pumps, nozzles and sprays. In the present study, a multiphase lattice Boltzmann method (LBM) is implemented for simulation of the cavitation bubbles dynamics and characteristics of cavitating flows. The effect of employing various equations of state (EoS) is investigated on the computing of interaction forces and the phase separation between the liquid and its vapor in the cavitating flows. Herein, the cubic EoSs of Shan-Chen (SC) and Carnahan-Starling (CS) and the non-cubic EoS of Peng-Robinson (PR) are applied. The exact difference method is imposed to improve the numerical stability for simulation of two-phase flow systems. The accuracy and efficiency of the present method are examined by comparison of the results obtained for the homogeneous and heterogeneous cavitation with those of reported in the literature. Then, the implemented multiphase LBM is used for studying the inception and growth of the cavitation bubbles in the throat of a venturi. The effect of hydrophobicity and hydrophobicity of the nozzle wall on the cavitation dynamics is investigated and a detail discussion is made for the results from the numerical and physical point of view. Evaluation of the present results shows that the multiphase LBM with incorporating an appropriate equation of state has excellent capability for prediction of the bubble dynamics and cavitating flow characteristics in applied geometries.

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.

Effects of nozzle configuration on flash boiling fuel sprays of twin-orifice nozzle with aviation kerosene

To investigate the effects of nozzle configuration on flash boiling fuel sprays of the twin-orifice nozzle with aviation kerosene, five different two-dimensional transparent slit nozzles with various orifices’ diameter ratio and expansion chamber's aspect ratio are fabricated and studied. Both internal two-phase flow and near-nozzle fuel jet under subcooled and superheated conditions are studied via a high-speed camera with backlit measurement and image processing methods. The experimental results indicate that the bubbles nucleation from cavitation at the inlet-orifice wall can accumulate within the expansion chamber with enhanced bubble growth and aggregation under higher-level superheated conditions, which can significantly promote the fuel atomization and evaporation. In addition, a smaller orifices’ diameter ratio leads to larger in-nozzle bubble volumes within the twin-orifice nozzle, while the internal bubble inception may be inhibited by increasing the expansion chamber's aspect ratio. Furthermore, decreasing the orifices’ diameter ratio can promote the flash boiling process, while better-quality spray at flare flash boiling state may be facilitated by the near-nozzle mechanical breakup. The nozzle with a lower expansion chamber's aspect ratio has a larger spray cone angle and weaker jet instability, benefited from larger bubble volume and higher effective bubble rate, which accelerates the bubble disruption and fulfills the potential of in-nozzle flash boiling on near-nozzle jet disintegration.

Predicting laser‐induced cavitation near a solid substrate

AbstractThe asymmetric collapse of cavitation bubbles near a solid substrate generates large wall shear stresses, the precise magnitude of which is still not known with certainty. By comparing numerical simulations and experiments of a laser‐induced cavitation bubble near a solid substrate, we demonstrate that an accurate measurement of the pressure pulse emitted during bubble inception and of the maximum bubble radius allow a unique initialisation of the simulation. This allows an accurate reproduction of the asymmetric collapse, with reliable predictions of the shear stress and pressure generated at the substrate.

Numerical Investigation of Nucleate Pool Boiling Heat Transfer for Different Superheat Conditions

Nucleate boiling has been a topic of great interest due to its heat transfer enhancement capability. In the present work, a highly resolved numerical model is developed combining two submodels, a microlayer evaporation submodel and phase change sub model, to get insights on various sub-processes like bubble inception, growth, departure, waiting time and scavenging effect. Here, a new approach in contact angle modeling is implemented by using a variable contact angle which is pre-defined using curve-fitting of the experimental data with respect to bubble cycle time. For the performance evaluation of the model, two fluids, water and a refrigerant (HFE7100) under two superheat conditions are studied and results are validated with existing experimental data. The simulations are carried out to understand the individual contribution of microlayer and other sub-processes toward overall heat transfer and such individual contributions are rare in the available literature. It was concluded that microlayer heat transfer is the second most important contributing component of heat transfer for both superheat conditions and contributes ∼30% for both the fluids. Moreover, the influence of superheat over bubble dynamics is analyzed. Overall, the comparison of present simulation with existing test data indicates a satisfactory agreement.

Heterogeneous bubble nucleation dynamics

Abstract

Schlieren-based simultaneous mapping of bubble dynamics and temperature gradients in nucleate flow boiling regime: Effect of flow rates and degree of subcooling

Bubble dynamics and the local thermal field associated with single nucleating bubble under subcooled flow boiling conditions have been investigated in a one side-heated rectangular channel with water as the working fluid. A real-time non-intrusive technique, namely rainbow schlieren deflectometry has been employed to simultaneously map the full bubble cycle and temperature gradients field near the heated surface as well as around the bubble. Influence of varying subcooled conditions and Reynolds number on bubble dynamic parameters, various sub-processes associated with subcooled flow boiling phenomenon and the resultant temperature gradients have been studied. Schlieren images for each data set have first been qualitatively interpreted to develop an understanding of sub-processes such as bubble inception, bubble departure/sliding, bubble lift-off, bubble reattachment and their effects on the thermal and flow fields. Experimental observations recorded in the form of color schlieren images clearly highlighted the possible influence of process parameters on a range of phenomena that include enveloping of the superheated liquid layer during bubble inception, micro-layer formation and scavenging of the superheated liquid layer during bubble lift-off. The bubble dynamic parameters observed under different experimental conditions have been quantified in terms of equivalent bubble diameter, aspect ratio and contact angle and bubble trajectory in the field of view. The equivalent bubble diameter is found to be decreasing with increasing Reynolds number and degree of subcooling. The results show that for all the experimental conditions, the bubble grew in a flattened shape, gradually became more spherical during sliding and elongates after lift-off. Regardless of the experimental conditions, the occurrence of bubble lift-off was found in the aspect ratio range of 0.75–0.85. Local thermal field observed over the full bubble cycle has also been captured and presented in the form of temperature gradients field in quantitative terms.

Performance of SiO2-water nanofluids for single bubble-based nucleate pool boiling heat transfer

Experiments on single bubble-based pool boiling have been performed under saturated bulk conditions to understand the influence of suspended nanoparticles on bubble dynamics and the associated temperature gradients field. Water and two concentrations of silica-water nanofluid (0.005 and 0.01% (V/V)) have been subjected to isolated nucleate pool boiling on a ITO-coated borofloat heater by supplying constant heat flux. Experimental measurements have been made in a purely non-intrusive manner by employing refractive index-based rainbow schlieren deflectometry technique. Bubble dynamic parameters, such as bubble departure diameter and departure frequency, wait and growth times etc. and the temperature gradients in the bulk fluid have been mapped simultaneously in the form of schlieren images. Results on bubble dynamic parameters revealed that these parameters show more variability and assume skewed normal distribution in the case of nanofluids whereas water experiments showed insignificant variations in these parameters. Of the significant changes, the bubble departure diameter reduced, departure frequency and growth rate increased with increasing concentration of nanofluids. Schlieren images corresponding to 0.005 and 0.01% (V/V) nanofluids showed more spreading out of the superheat layer adjacent to the heater substrate than that observed in the case of water experiments. Time-averaged values of natural convection and evaporative heat transfer rates associated with single bubble-based boiling indicated that both these rates decrease in the presence of dispersed silica nanoparticles. Reduction in the overall heat transfer rate in the case of nanofluids has been explained on the basis of direct experimental measurements. In this direction, the observed phenomenon has been attributed to the role played by the suspended nanoparticles in diffusing the thermal gradients (reflected in the form of the broadening of the superheat layer) adjacent to the heater substrate along with substantially increasing the wait time of bubble inception in the case of nanofluids as compared to water-based experiments conducted under same wall superheat conditions.

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.

Improvements to the Construction of Bubble Inception Formulae for Use With Transformer Insulation

Moisture in transformer insulation is a topic of major interest to power system equipment operators. Moisture plays a large role in the degradation of insulation within the transformer, impacting on the longevity and the failure rate of this crucial asset. Therefore, understanding of how moisture behaves in the transformer insulation has been the focus of many studies. One particular matter that can occur during high-temperature events is the formation of bubbles of water vapor which are released from the cellulosic paper wrapped on the transformer windings. During such events, the presence of these bubbles could lead to an electrical failure of the transformer, and so it is desirable to be able to set operational criteria which prevent such occurrences. A formula which can predict the temperature at which bubbles will form from the paper insulation can allow operators to restrict transformer operation in order to avoid such situations. However, it has proven difficult to generate a formula with global applicability that is simplistic enough for generic use. This paper assesses the current formula against existing data in literature and compares a suggested alternative, drawing on fundamental science to assist in development of both formulae. The alternative formula is based on a better representation of bubbling activity and requires fewer input variables. Finally, adjustments are suggested to the formulae which allow them to be used more generally throughout the transformer fleet, including on service-aged transformers and with alternative insulation materials.

Acoustically induced bubble jets

A detailed experimental study on acoustically induced bubble jets produced far from any solid wall is presented. This particular type of bubble jet is observed when a laser-induced gas cavity is seeded in a liquid by an optical dielectric breakdown while applying a strong ultrasonic field. This generation method allows a high degree of control over jet dynamics, achievable by changing critical experimental parameters such as the laser pulse energy (i.e., the size of the bubbles), the driving signal frequency, the seeding phase (relative to the sound field), the acoustic pressure amplitude, and, in particular, the pressure gradient at the bubble inception location. The temporal evolution of the characteristic dimensions of the bubble jets is analyzed by means of numerical simulations of spherical bubble dynamics and their dependence on the local acoustic field. The jet strength is presented in terms of the jet elongation ratio, defined as the axial length over the maximum width. The most relevant parameters related to the jet strength are the maximum radius reached by the bubble prior to the jet formation and the pressure gradient during the bubble collapse phase. The existence of optimum driving parameters and the bubble seeding position to obtain the strongest jets achievable in a given acoustic pressure distribution is discussed. This study is a first step to shed some light on the jet formation process due to known time-variable pressure gradients. The work can be extended to further understanding the role of bubble jets in sonochemical reactions and biological or medical applications.

Investigations on the effects of heater surface characteristics on the bubble waiting period during nucleate boiling at low subcooling

In nucleate boiling the ‘bubble waiting period’, that is, the time duration between the departure of a grown bubble and the start of the formation of a new bubble from a cavity, plays a crucial role for the total heat transfer. Experiments were performed to study the influence of the heater surface characteristics on this parameter. A femtosecond pulsed laser was used to produce nano- and micro-patterned surfaces with roughness in the range of micrometers on stainless steel heater surfaces. Boiling experiments were conducted on a vertically oriented heater at atmospheric pressure and with degassed deionized water. Bubble generation, departure, sliding, detachment and inception of the next bubble have been recorded by high-resolution optical shadowgraphy. Bubble waiting periods were found to be longer for low-wettability smooth and rough surfaces. High-wettability rough surfaces showed a shorter bubble waiting period. The shortest (approximately 3 ms) and the longest (approximately 30 ms) bubble waiting periods were found for well-wetting surfaces with Sq = 0.18 µm and for low-wetting surfaces with 0.12 µm, respectively. These corresponding roughness heights are denoted as ‘optimal roughness heights’.

Vapor bubble and streamer initiation in water under long duration impulses

The generation of vapor bubbles in water under high voltage impulses of long duration (500 μs) is investigated experimentally. In most cases, the appearance of bubbles is followed by the inception of discharges occurring within the vapor, eventually transforming the bubble into a propagating streamer. Time delays to bubble and discharge inception are measured versus conductivity and voltage, using fast visualization and optical detection of bubbles. Before bubble inception, the liquid heating and the onset of an electro-hydro-dynamic (EHD) motion can be observed by Schlieren visualization. Electrical measurements show a decrease of the gap resistance versus time, attributable to the heating of water. When conductivity and voltage are changed, the onset of bubble inception is correlated to a nearly constant resistance drop, suggesting that a constant temperature is present when bubbles appear. At short times, the energy dissipation required to observing bubbles is independent of conductivity and applied voltage. Experiments show that vapor bubble generation results from a competition between liquid heating by Joule dissipation, and forced cooling induced by the EHD motion.

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.

Understanding of bubble growth at nucleation site using energy based non-dimensional numbers and their impact on critical heat flux condition in microchannel

Two phase flow initiates at Onset of Nucleate Boiling, where the first bubble emerges in the downstream flow direction. Bubble nucleation and growth in microchannel heat sinks is very conspicuous phase as the growing bubble can completely block the flow cross-section area at high heat flux. Hence, microchannels are more susceptible to flow boiling instability. In this paper effort has been put to study the bubble dynamics during bubble growth at nucleation site for microchannel in terms of non-dimensional energy ratio numbers and their variation from bubble inception until departure. New non-dimensional energy ratio is also proposed, which helps in differentiating inertia controlled and thermal diffusion controlled region during bubble growth at nucleation site. Further, possible impacts of these non-dimensional energy terms on critical heat flux are discussed.

Combined numerical and experimental investigations of heat transfer of a highly loaded low-pressure turbine blade under periodic inlet flow condition

This paper experimentally and numerically investigates heat transfer characteristics of a low-pressure turbine blade under steady/unsteady flow conditions. Generally, the low-pressure turbine blades are not exposed to excessive temperatures that require detailed heat transfer predictions. In aircraft engines, they operate at low Re-numbers causing the inception of large separation bubbles on their suction surface. As documented in previous papers, the results of detailed aerodynamic simulations have shown significant discrepancies with experiments. It was the objective of the current investigation to determine the discrepancies between the experimental and numerical heat transfer results. It is shown that small errors in aero-calculation results in large deviations of heat transfer results. The characteristics of the blades mentioned above, make low-pressure turbine blades suitable candidates for evaluating the predictive capability of any numerical method. Documenting the scope of these discrepancies defines the framework of the current paper. The periodic flow inside the gas turbine engine was simulated using the cascade facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. In this study, the wakes that originate from stator blades were simulated by moving rods. The instrumented blade was covered with a liquid crystal sheet and it was used to measure heat transfer coefficient. Reynolds-averaged Navier–Stokes equations were used for numerical investigation purposes. Measurements and simulations were conducted at three different Reynolds numbers (110,000, 150,000, and 250,000). Furthermore, for unsteady flow condition, reduced frequencies of the incoming wakes were varied. The current paper includes a comprehensive heat transfer assessment of the predictive capability of Reynolds-averaged Navier–Stokes based tools. The effect of the separation bubbles on heat transfer is thoroughly discussed in this paper. Comparisons of the experimental and numerical results detail the differences and identify the sources of error that leads to in accurate calculations in terms of predicting heat transfer calculation results.

Rainbow schlieren-based investigation of heat transfer mechanisms during isolated nucleate pool boiling phenomenon: Effect of superheat levels

Experimental investigation of various heat transfer mechanisms associated with isolated nucleate pool boiling have been presented. Measurements have been made in a complete non-intrusive manner using rainbow schlieren deflectometry technique. Boiling experiments have been performed for two levels of superheat with the bulk fluid maintained under saturated conditions. The rainbow schlieren images have first been subjected to qualitative interpretation wherein various sub-processes associated with the boiling phenomenon, such as development of thermal boundary layer on the substrate surface, inception of single bubble, growth of the vapor bubble till it departs, and scavenging of the superheat layer following the bubble departure have been discussed. Contributions of individual sub-processes towards the overall heat transfer rates achieved for a given superheat level have been determined through quantitative analysis of the images. Schlieren observations revealed the effect of varying superheat levels on parameters such as bubble diameter and departure time. Detailed heat transfer analysis revealed the dominance of evaporative heating in contributing towards the overall heat transfer rates. On the other hand, the contribution of natural convection from the heated substrate was found to be relatively small. In quantitative terms, the evaporative heating was seen to have an individual contribution as high as ≈66% to the overall heat transfer and ≈88% to the growth of the vapor bubble in the case of superheat level of 7 °C.

On the Inception of Financial Representative Bubbles

In this work, we aim to formalize the inception of representative bubbles giving the condition under which they may arise. We will find that representative bubbles may start at any time, depending on the definition of a behavioral component. This result is at odds with the theory of classic rational bubbles, which are those models that rely on the fulfillment of the transversality condition by which a bubble in a financial asset can arise just at its first trade. This means that a classic rational bubble (differently from our model) cannot follow a cycle since if a bubble exists, it will burst by definition and never arise again.