Confined Bubble Growth Research Articles

Modeling of confined bubble growth and heat transfer characteristics in parallel microchannel heat sinks

The current study aims to present the modeling and numerical analysis of parallel microchannel heat sinks accounting for transient phenomena during confined bubble growth in one of the microchannels, considering dynamic aspects for the bubble growth and heat transfer characteristics to identify flow instabilities. The model incorporates the estimative of the inlet velocities and pressure drop due to parallel channels’ interaction. Therefore, the model can predict conditions that result in reversal flow, as well as the main trends and values of the main parameters during this process. A parametric analysis is performed numerically aiming to infer the effects of the number of channels, heat flux, inlet mass velocity, and location of the nucleation site on the occurrence of reversal flow.

Experimental investigation of bubble dynamics and flow patterns during flow boiling in high aspect ratio microchannels with the effect of flow orientation

Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFE−7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg m−2 s−1 and 42 kg m−2 s−1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW m−2 to 18.7 kW m−2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film.

On the Prediction of Pressure Fluctuations and Pressure Drop Caused by Confined Bubble Growth During Flow Boiling in a Rectangular Mini/Micro-Channel

Heat-sinks based on flow boiling in microchannels have the potential to mitigate temperature rise in high heat flux devices such as electronic equipment. One of the major challenges is the instability associated with pressure fluctuations caused due to the confined bubble growth during boiling in a microchannel. This paper presents a comprehensive model to estimate the dynamic pressure fluctuations in a channel of rectangular cross-section and compares with the experimental results. The time-averaged pressure drop obtained from the transient model has been verified with the experimental results and the steady two-phase flow pressure drop correlations. Effects of various parameters such as heat flux and mass flux on the time-averaged pressure drop and the transient pressure fluctuations are presented. The paper also provides an approximate methodology to estimate the nucleation frequency derived from the waiting period of the bubble. The calculated frequency is validated qualitatively with the experimental observation available in the literature. The current work also demonstrates that the transient pressure drop models, which, with suitable inputs, predict the local pressure fluctuations during confined bubble growth, can also be used to predict the steady- flow pressure drop.

Modeling of pressure drop and heat transfer for flow boiling in a mini/micro-channel of rectangular cross-section

In the present study, a one-dimensional model is proposed to estimate the pressure drop and heat transfer coefficient for flow boiling in a rectangular microchannel. The present work takes into account the pressure fluctuations caused due to the confined bubble growth and the effect of pressure fluctuations on the heat transfer characteristics. The heat transfer model considers five zones, namely, liquid slug, partially confined bubble, fully confined (elongated) bubble, partial dryout and full dryout. The model incorporates the thinning of liquid film due to shear stress at liquid-vapour interface in addition to evaporation. The transient fluctuations in pressure and heat transfer coefficient, along with the time-averaged ones, are verified with the experimental data available in the literature. Heat transfer characteristics with flow reversal caused by inlet compressibility are also presented.

Visualization research on confined bubble growth feature and heat transfer characteristic in ultra-shallow micro channel

Visualization study on flow boiling in ultra-shallow (aspect ratio=0.2) micro channels have been conducted under various operating conditions. The confined bubble behaviors are explored in detail with high-quality photos. Different from the traditional classifications on the growth process of the confined bubbles, an interesting but rationally new dividing criterion is proposed here, which is based on not only the objective bubble dynamics, but also the variations of the heat transfer mechanism. Two innovative definitions—“turning point” and “transition stage” are put forward for the first time, and their features are studied more thoroughly. In addition, the influences of the operating conditions on the bubble dynamics and the heat transfer characteristics have been summarized. The enhancement of heat transfer in ultra-shallow micro channels requires more harsh terms to be achieved, which brings about the risk of Departure from Nucleate Boiling (DNB) and Premature Critical Heat Flux (PCHF), dramatically reducing the heat transfer capability. Contents in present work help fill the gaps in the experimental research on the micro channels with extreme aspect ratios, and further guide the related utilization.

Confined bubble growth and heat transfer characteristics during flow boiling in microchannel

Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5mm×1mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid–vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less.

Bubble confinement and deformation during flow boiling in microchannel

The bubble dynamics during subcooled flow boiling were investigated in a single rectangular cross-section (0.5mm×1mm) microchannel. Degassed deionized water was used as the working fluid with the confined bubble behavior observed using a high-speed CCD camera. The microchannel wall confines the bubble growth in the bubble height direction when the bubble departure diameter was larger than the microchannel cross section, resulting in strong deformation of the bubble top interface even before the liquid–vapor interface touched the microchannel wall. The microchannel confinement effect works by exerting a wall confining force on the top interface of the growing bubble. The negative feedback between the velocity of the top interface and the wall confining force is the main reason for the fluctuations in the increasing maximum local void fraction and the bubble aspect ratio during the confined bubble growth period.

Unsteady-state fluctuations analysis during bubble growth in a “rectangular” microchannel

Boiling in microchannels shows great potential for cooling systems and compact heat removal applications. However for confidence in this cooling technique, it is essential that any excursions from typical flow boiling are understood and predicted. Confined bubble growth can cause pressure fluctuations which interfere with bubble nucleation and growth and can also lead to flow reversal and instances of temperature excursions. Boiling experiments are performed in a single rectangular microchannel of hydraulic diameter 771 μm, using n-Pentane as the working fluid. A heating technique was incorporated on the exterior walls of the microchannel; a transparent, metallic, conductive deposit, which allows simultaneous uniform heating and visualisation to be achieved. In conjunction with obtaining high-speed imaging, an infrared camera is used to record the temperature profile at the microchannel wall, and sensitive pressure sensors are used to record the pressure drop across the microchannel over time. During flow boiling in the microchannel periodic and non-periodic fluctuations in both the channel pressure drop and channel temperature profile over time are apparent. In this paper we provide a full analysis of the temperature measurements and pressure data obtained during the growth of a vapour bubble in the microchannel. An augmentation of the heat transfer coefficient of over 216% has been achieved during periodic two-phase flow boiling in the microchannel. However overpressure (over 410% increase) in the microchannel occurs at corresponding instances to the heat transfer enhancement. The two time steps during the periodic bubble dynamics, namely the bubble expansion time period and the waiting time period in-between the bubble expansion fluctuations, are also investigated and modelled. It was determined that both the bubble dynamics and the channel wall heating time period are responsible for the pressure and temperature fluctuation time periods observed.

Confined bubble growth during flow boiling in a mini/micro-channel of rectangular cross-section Part I: Experiments and 1-D modelling

Heat sinks using evaporation in arrays of parallel microchannels have potential for the removal of high heat fluxes from small areas. They suffer from flow instabilities and uneven distribution between channels that may cause local dryout and overheating. The current state of the art is reviewed critically. A simple 1-D model for bubble growth in a single channel with a compressible volume in its upstream plenum is developed as a tool for the rational design of measures known to reduce flow instabilities, namely inlet resistance and enhanced nucleation in every channel. The model considers two stages of partially and fully confined bubble growth in a single channel of rectangular cross-section, suggested by experimental observations, followed by venting of vapour to the downstream plenum. The experiments also show the influence of apparently minor changes in rig design and operation on upstream compressibility and flow reversal. The model considers upstream compressibility due to subcooled boiling in a preheater or trapped non-condensable gas and the reduction of flow reversal by inlet resistance. The feasibility of measuring transient axial variations in pressure within small channels using inexpensive transducers is demonstrated.

Confined bubble growth during flow boiling in a mini-/micro-channel of rectangular cross-section part II: Approximate 3-D numerical simulation

This Part II of the paper reports the three-dimensional (3-D) numerical modelling on bubbly flow in confined mini-/micro-channels using the volume of fluid (VOF) method in commercial CFD code FLUENT. The numerical simulation aims to provide detailed information of the fields of velocity, temperature and pressure so as to further understand the effect of bubble growth on the flow field and heat transfer from the channel wall. In Part I, the experiment of flow boiling in a mini-/micro-channel of rectangular cross-section was carried out and a simple one-dimensional (1-D) model for the interaction of the pressure fluctuations during the growth of a confined bubble with various kinds of upstream compressibility was developed as an aid to the rational specification of inlet resistance. In Part II, the experimental observers and the theoretical model developed in Part I are tested by performing the 3-D numerical simulation of bubble growth from nucleation to full confinement. The simulation involves some approximations based on a concept of pseudo-boiling to avoid the well-known difficulties of modelling bubble generation and growth. During the simulation, the volumetric growth rate of the bubble is defined to match the experimental observations. At small times prior to bubble detachment, a vapour flow was injected through a small hole in the wall to simulate nucleation. Following partial confinement, vapour injection was stopped and growth was driven by the generation of vapour at a defined rate at the contact area between the bubble and the superheated wall. The 3-D simulation reproduces the experimental observations of the distorted profile of the bubble and its trajectory during partially confined growth and provides information about flow and heat transfer in the bulk liquid outside the thin film region. The 3-D and 1-D predictions of the development of axial pressure distributions during partially and fully confined growth are in satisfactory agreement.

Bubble confinement in flow boiling of FC-72 in a “rectangular” microchannel of high aspect ratio

Boiling in microchannels remains elusive due to the lack of full understanding of the mechanisms involved. A powerful tool in achieving better comprehension of the mechanisms is detailed imaging and analysis of the two-phase flow at a fundamental level. Boiling is induced in a single microchannel geometry (hydraulic diameter 727 μm), using a refrigerant FC-72, to investigate the effect of channel confinement on bubble growth. A transparent, metallic, conductive deposit has been developed on the exterior of the rectangular microchannel, allowing simultaneous uniform heating and visualisation to be achieved. The data presented in this paper is for a particular case with a uniform heat flux applied to the microchannel and inlet liquid mass flowrate held constant. In conjunction with obtaining high-speed images and videos, sensitive pressure sensors are used to record the pressure drop across the microchannel over time. Bubble nucleation and growth, as well as periodic slug flow, are observed in the microchannel test section. The periodic pressure fluctuations evidenced across the microchannel are caused by the bubble dynamics and instances of vapour blockage during confined bubble growth in the channel. The variation of the aspect ratio and the interface velocities of the growing vapour slug over time, are all observed and analysed. We follow visually the nucleation and subsequent both ‘free’ and ‘confined’ growth of a vapour bubble during flow boiling of FC-72 in a microchannel, from analysis of our results, images and video sequences with the corresponding pressure data obtained.

Bubble-Induced Water Hammer and Cavitation in Microchannel Flow Boiling

While microchannel flow boiling has received much research attention, past work has not considered the impact of acoustic waves generated by rapidly nucleating bubbles. The present work provides a theoretical framework for these pressure waves, which resembles classical “water hammer” theory and predicts a strong influence on bubble nucleation rates and effective convection coefficients. These pressure waves result directly from confinement in microchannel geometries, reflect from geometrical transitions, and superimpose to create large transients in the static liquid pressure. Feedback from the pressure waves inhibits bubble growth rates, reducing the effective heat transfer. Pressure depressions generated by the propagating pressure pulses can cause other bubbles to grow at lower than expected wall temperatures. The additional nucleation enhances heat transfer over short times but increased flow instability may inhibit heat transfer over longer periods. The limited quantitative measurements available in the literature indicate confined bubble growth rates in microchannels are significantly lower than those predicted by the classical Rayleigh–Plesset equation. The present model predicts confined bubble growth rates to within ± 20%. A nondimensional number indicative of the relative magnitude of the water hammer pressure to bubble pressure is proposed to characterize the transitions from conventional to microchannel flow boiling.