Bubble growth characterization during fast boiling in an enclosed geometry

Microboiling is commonly used in thermal inkjet atomizers (TIJ) and microelectromechanical (MEM) devices. The TIJ and MEM devices performance is closely related to the dynamics of the bubble used to operate them; therefore, it is important to determine the conditions of input energy and power leading to specific bubble dynamics. The objective in this work is the characterization, in a confined space, of the bubble dynamics on a range of input conditions of energy and power and what is the effect of the input conditions on the bubble extractable mechanical efficiency. Mechanical efficiency is defined by the ratio of the integral of the mechanical work (work done by the bubble expansion due to the elevated internal pressure relative to atmospheric pressure minus the increase in bubble surface energy) to the total energy input to the microheater. Bubbles are generated with energies of 7–17 μJ under high heating rates and short pulses in deionized water. Resulting nucleation temperature measurements are consistent with homogeneous nucleation. The bubble lifecycle shows strong dependence on the input heater energy and input heating rate. This work presents new results in bubble growth where growth–shrink–growth derived from specific energy conditions. The bubble growth–shrink–growth may be due to subcooled fluid, local variation in the pressure field, and by the surface tension driven change in curvature of the bubble. Mechanical bubble efficiencies result in small values suggesting most of the energy applied to the heater is distributed in other processes which may include increasing the internal energy of the heater film and the fluid.     

Bubble dynamics at boiling incipience in subcooled upward flow boiling

Bubble dynamics in water subcooled flow boiling was investigated through visualization using a high-speed camera. The test section was a vertical rectangular channel, and a copper surface of low contact angle was used as a heated surface. Main experimental parameters were the pressure, mass flux and liquid subcooling. Although all the experiments were conducted under low void fraction conditions close to the onset of nucleate boiling, no bubbles stayed at the nucleation sites at which they were formed. Depending on the experimental conditions, the following two types of bubble behavior were observed after nucleation: (1) lift-off from the heated surface followed by collapsing rapidly in subcooled bulk liquid due to condensation, and (2) sliding along the vertical heated surface for a long distance. Since the bubble lift-off was observed only when the wall superheat was high, the boundary between the lift-off and the sliding could be determined in terms of the Jakob number. Based on the present experimental results, discussion was made for the possible mechanisms governing the bubble dynamics.     

Flow pattern transition instability during flow boiling in a single microchannel

We studied the unique characteristics of flow boiling in a single microchannel, including the periodic pressure drop, mass flow rate, and temperature fluctuations, in terms of a long time period. Experiments were conducted using a single horizontal microchannel and deionized water to study boiling instabilities at very small mass and heat flow rate conditions. A Polydimethylsiloxane (PDMS) rectangular single microchannel had a hydraulic diameter of 103.5 μm and a length of 40 mm. A series of piecewise serpentine platinum microheaters were fabricated on the inner bottom wall of the rectangular microchannel to supply thermal energy to the test fluid. Real-time flow visualizations of the flow pattern inside the microchannel were performed simultaneously with measurements of the experimental parameters. Tests were performed for mass fluxes of 170 and 360 kg/m² s and heat fluxes of 200–530 kW/m². The test results showed that the heated wall temperature, pressure drop, and mass flux all fluctuated with a long period and large amplitude. These periodic fluctuations exactly matched the transition of two alternating flow patterns inside the microchannel: a bubbly/slug flow and an elongated slug/semi-annular flow. Therefore, the flow pattern transition instability in the single microchannel caused a cyclic behavior of the wall temperature, pressure drop, and mass flux, and this behavior had a very long period (100–200 s) and large amplitude.     

Bubble jet flow formation during boiling of subcooled water on fine wires

Both visual technology and numerical simulation were employed to investigate bubble jet flow formation during boiling of subcooled water on ultrathin platinum wires. Experimental observations and measurements indicated that bubble jet flow formation could be typically divided into three stages: waiting, burst and stably developing stage. A transient numerical model was proposed to simulate the formation process accounting for the thermocapillary force existing at the bubble interface. The dynamical temperature variation in the liquid region, including the regions of the mushroom head and root stem, were numerically analyzed, showing a very good agreement with the experimental observations. The simulation very well explored the physical nature of the dynamical evolution of bubble jet flows and explained the experimental phenomena.     

Bubble lift-off size in forced convective subcooled boiling flow

Forced convective subcooled boiling flow experiments were conducted in a BWR-scaled vertical upward annular channel. Water was used as the testing fluid, and the tests were performed at atmospheric pressure. A high-speed digital video camera was applied to capture the dynamics of the bubble nucleation process. Bubble lift-off diameters were obtained from the images for a total of 91 test conditions. A force balance analysis of a growing bubble was performed to predict the bubble lift-off size. The dimensionless form of the bubble lift-off diameter was formulated to be a function of Jacob number and Prandtl number. The proposed model agreed well with the experimental data within the averaged relative deviation of ±35.2%.     

Bubble growth and transient heat transfer at the onset of boiling

An experimental study was conducted to investigate transient local heat transfer around a bubble at onset of boiling on a thin glass heating plate immersed in saturated n-hexane at low pressure. Eight rapid response Cu-Ni thermocouples consisting of a vacuum deposited thin film were used to measure the temperature change of the heating surface. Simultaneous high-speed video photographs were also obtained. The surface temperatures near a nucleation site decreased rapidly owing to the evaporation of a thin layer (microlayer) of liquid formed beneath the bubble in the early period and the rate of bubble growth increased with increasing incipient boiling superheat (ΔT_IB). The thickness of the microlayer decreased markedly with increasing ΔT_IB. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(7): 484–492, 1997     

Bubble sweeping and interactions on wires during subcooled boiling

Multi-bubble dynamics on a heated wire are modeled with a phenomenological model of the temperature distributions around the bubbles and the forces acting on the bubbles. The model calculates the heat transfer from the wire to the bubble and the temperature distribution in the wire. The model balances the Marangoni force, the drag force, and the contact line force acting on the bubble to predict the bubble velocities. The predicted velocities and the predicted interactions agree well with experimentally observed bubble dynamics. The predictions show that when a moving bubble approaches a stationary bubble at moderate superheats, the reduced wire temperatures around the stationary bubble cause the moving bubble to slow and reverse direction before colliding with the stationary bubble. At higher superheats, the bubbles coalesce. The model also shows that when two bubbles approach each other from opposite directions, they will collide and coalesce at lower superheats than when only one bubble is moving because the temperature gradients in front of the moving bubbles are much steeper than in front of a stationary bubble; thus, moving bubbles do not slow much before coalescing.     

Numerical study of bubble growth and wall heat transfer during flow boiling in a microchannel

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier–Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. Experiments have been conducted to validate the numerical model. The bubble growth rate and shapes show good agreement between numerical and experimental results. The numerical results show that the wall heat transfer increases with wall superheat but stays almost unaffected by the liquid flow rate. The liquid vapor surface tension value has little influence on bubble growth and wall heat transfer. However, the bubble with the lowest contact angle resulted in the highest wall heat transfer.     

Development of a diabatic two-phase flow pattern map for horizontal flow boiling

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. Based on new flow pattern data for three different refrigerants covering a wide range of mass velocities, vapor qualities and heat fluxes. The new flow pattern map includes the prediction of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters and is an extension to the flow pattern map model of Kattan et al. [J. Heat Transfer 120 (1998) 140]. The proposed modifications allow an accurate prediction of the flow pattern for very different fluids which are the substitute refrigerants (HFC-134a and HFC-407C) and the natural refrigerant R-717 (ammonia).     

Bubble separation and collision on thin wires during subcooled boiling

An experimental investigation was conducted to observe bubble separation and collision phenomena during subcooled boiling of water on heating wires, and a theoretical model was proposed to describe the associated dynamical phenomena and understand the physical significance. Both experimental and theoretical evidences indicate that interfacial thermocapillary force induced by temperature difference and their interaction between neighbor bubbles was the most important forces during bubble separation and collision. The bubble separation process with positive effectual viscosity can be divided into two stages, accelerating and decelerating stage. The collision with an immobile bubble was concluded as an elastic collision, and the two equivalent bubbles just exchanged their momentum during their collision. Bubbles coalescence characteristics were also analyzed using the collision dynamics. The theoretical conclusions were compared with experimental results, showing a reasonable agreement with each other.     

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m² s, a heat flux range of 1–83 kW/m² and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.     

Bubble growth rates in nucleate boiling of water at subatmospheric pressures

The growth rate of vapour bubbles has been investigated experimentally up to departure in water boiling at pressures varying from 26·7 to 2·0 kPa (the corresponding Jakob number increasing from 108 to 2689).Comparison of the data with existing theory shows the substantial influence of liquid inertia during initial growth, in agreement with previous results of Stewart and Cole [1]on water boiling at 4·9 kPa, the Jakob number varying from 955 to 1112. As an extreme case, at a pressure of 2·0 kPa, large “Rayleigh” bubbles are observed during the entire adherence time. During advanced growth, bubble behaviour is gradually governed by heat diffusion, especially at relatively high (subatmospheric) pressures.Experimental bubble growth in the investigated pressure range is in quantitative agreement with the van Stralen, Sohal, Cole and Sluyter theory [10]. This model combines the Rayleigh solution with a diffusion-type solution, which accounts for the contributions to bubble growth due to both the relaxation microlayer (around the bubble dome) and the evaporation microlayer (beneath the bubble).Finally, a curious bubble cycle is observed at the lowest investigated pressures, which is attributed to the combined action of a high-velocity liquid jet (originating in the wake following a large primary bubble) and a succeeding secondary vapour column (generated at the adjacent dry spot at the heating wall beneath the primary bubble).     

Bubble growth and horizontal coalescence in saturated pool boiling on a titanium foil,investigated by high-speed IR thermography

Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 μm thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1 ms and 40 μm, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions.     

Bubble behavior and heat transfer during pool boiling of binary mixtures

Heat transfer coefficients were measured during pool boiling of binary mixtures on a heated wire hung horizontally and bubble behavior was simultaneously captured with a high‐speed video camera. The experiment was carried out at a pressure of 0.4 and 0.7 MPa for the whole range of mass fractions in a binary mixture of R22/R11. We clarified the change in bubble behavior and heat transfer by measuring the bubble departure diameter, frequency and growth rate on the basis of the video images. Furthermore, we discussed the relationship between the bubble behavior and the boiling heat transfer coefficient in the binary mixtures. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 449–459, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20087     

Bubble sweeping and jet flows during nucleate boiling of subcooled liquids

An experimental investigation was conducted to investigate nucleate boiling on a very fine heating wire. By using zoom routine and CCD camera system, the dynamical process of nucleate boiling was visually observed. Sweeping bubbles and several modes of jet flows were described and discussed. For some cases, big bubbles, small bubbles, sweeping bubbles and jet flows coexisted in boiling system, and greatly enhanced heat transfer. These phenomena are quite different from usual observation of nucleate boiling. In this paper, the process of bubble sweeping phenomenon is described in detail and the effect induced by sweeping bubbles is argued. And also, several jet flows are illustrated and discussed, as well as the interaction between bubble sweeping and jet flows.     

Numerical analysis on flow pattern and heat transfer characteristics of flow boiling in the mini-channels

Abstract

Flow pattern and heat transfer of flow boiling for different flow orientation, mini-channel width and height were presented in this work. The data were obtained by the numerical simulation with the coolant of R141b flow in a vertical mini-channel. Orientation includes upward and downward. A constant heat flux was loaded at the wall of the channel, of which the width ranges from 1?mm to 3?mm, and a length changes from 200?mm to 400?mm. Subsequently the impact of those parameters that referred to heat flux, inlet temperature and inlet temperature of the coolant could be investigated by carrying out the numerical calculation. In addition, a validation for the model was illustrated in comparison with the previously experimental data.     

The effect of inlet constriction on bubble growth during flow boiling in microchannels

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow mal-distribution that leads to reversed flow in certain channels. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present study, a nucleating vapor bubble placed near the restricted end of a single microchannel is numerically simulated. Placing restrictions at channel inlet increased the incoming liquid velocity for the same flow rate that prevented explosive bubble growth and reversed flow. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.     

Oscillation of a liquid-vapor interface during flow film boiling

For both pool and flow film boiling, the liquid-vapor interface is rarely stationary, stable film boiling exhibits a continuously wavy interface with an amplitude of oscillation dependent on parameters such as liquid saturation temperature, amount of superheat, amount of subcooling, liquid velocity, and heater geometry. The present analysis treats from a theoretical point of view stagnation point flow film boiling on a sphere and on a cylinder immersed in a subcooled liquid. The effects of system parameters on the dynamic behavior as well as the response of a liquid-vapor interface to changes in the temperature and velocity fields are investigated.     

不同纳米流体水平管内沸腾流型的模拟

对以纯水为基液的不同纳米流体管内流动沸腾进行了模拟。利用UDF编程定义流体的沸腾相变源项,将其导入FLUENT软件中分别模拟这四种流体在水平管内的流动沸腾过程。得到了四种流体流动沸腾的速度云图,以及流型分布云图。对比分析了四种流体的速度分布云图、四种流体从初始状态沸腾到1s时的流型分布云图以及四种流体层状流流型的特点。结果表明,四种流体在水平管内沸腾都会出现泡状流、弹状流、层状流以及波状流四种流型,并且沿管长方向含气量逐渐增加。在相同的时间内,纳米流体的沸腾比纯水更剧烈,而且,不同的纳米流体沸腾程度也不同。在相同的体积分数下,纳米颗粒的导热系数越大,其对应的纳米流体沸腾越剧烈。说明纳米颗粒的导热系数是影响纳米流体沸腾传热的主要因素。     

Hydrodynamics and heat transfer during flow boiling instabilities in a single microchannel

Boiling in microchannels is widely considered as one of the front runners in process intensification heat removal. Flow boiling heat transfer in microchannel geometry and the associated flow instabilities are not well understood, further research is necessary into the flow instabilities adverse effect on heat transfer.Boiling is induced in microchannel geometry (hydraulic diameter 727 μm) to investigate several flow instabilities. A transparent, metallic, conductive deposit has been developed on the exterior of rectangular microchannels, allowing simultaneous heating and visualisation.Presented in this paper is data for a particular case with a uniform heat flux of 4.26 kW/m² applied to the microchannel and inlet liquid mass flowrate, held constant at 1.13 × 10^?5 kg/s. In conjunction with obtaining high-speed images, a sensitive infrared camera is used to record the temperature profiles on the exterior wall of the microchannel, and a data acquisition system is used to record the pressure fluctuations over time. Various phenomena are apparent during the flow instabilities; these can be characterised into timescales occurring at 100’s seconds, 10’s seconds, several seconds and finally milliseconds. Correlation of pressure oscillations with temperature fluctuations as a function of the heat flux applied to the microchannel is possible.From analysis of our results, images and video sequences with the corresponding physical data obtained, it is possible to follow simultaneously particular flow, pressure and temperature conditions leading to nucleate boiling, flow instabilities and transition regimes during flow boiling in a microchannel. The investigation allowed us to quantify and characterise the timescales of various observed instabilities during flow boiling in a microchannel. High speed imaging revealed some of the controlling physical mechanisms responsible for the observed instabilities.