Initial Microlayer Thickness Research Articles

Predicting initial microlayer thickness in nucleate boiling using Landau–Levich theory

A phenomenological model is proposed to estimate the initial thickness of the liquid microlayer forming beneath a vapour bubble growing on a solid surface upon nucleate boiling. The model employs an analogy between the microlayer formation and the classic plate withdrawal problem. It calculates the microlayer thickness by considering it as a Landau–Levich film, where the thickness is a function of the meniscus speed and radius of curvature. Given the nearly hemispherical shape of the bubble during the early growth stage when the microlayer is first deposited, we assume that the meniscus speed can be approximated by the bubble expansion rate, and estimate the meniscus curvature using the Rayleigh equations. Unlike previous theories that assume that the bubble radius growth is proportional to the square root of time, the proposed model does not rely on any specific law of growth for vapour bubbles. The model is validated for predicting the microlayer thickness in water and ethanol, showing good agreement with experimental measurements and empirical correlations. Subsequent analyses of the microlayer interface profile address inconsistent reports – some described a wedge-like shape, whereas others reported a slight outward curvature with decreasing thickness in the outer region. This discrepancy is attributed to a reduction in the expansion rate of the microlayer's outer edge, particularly when the bubble reaches its maximum width. Our model provides insights into microlayer dynamics, essential to boiling heat transfer, as the evaporative heat flux through the microlayer is very sensitive to its initial thickness.

Comprehensive experimental thermal analysis of single bubble nucleation in vertical flow boiling: Whole field temperature and microlayer dynamics

Nucleate boiling, an important heat transfer phenomenon, holds significance in the thermal management of numerous engineering applications. Understanding and predicting its heat transfer characteristics are vital for system optimization. The formation of a microlayer, resulting from rapid bubble growth on a heater substrate, subsequently facilitates further bubble growth through evaporation playing a critical role in overall bubble dynamics. This study examines the intricate relationship between bubble and microlayer dynamics and their impact on heat transfer rates. Experimental work is conducted for varying heat fluxes using rainbow schlieren deflectometry (RSD) and thin-film interferometry (TFI) in tandem for simultaneous mapping of key parameters: bubble dynamics, microlayer dynamics, and two-dimensional temperature field in vertical flow boiling. The analysis of these parameters has provided significant insights into some fundamental aspects of boiling. Firstly, this study emphasizes that the bubble growth rate models need to take into initial microlayer thickness in accurately predicting bubble growth dynamics. The iteratively obtained constant Ceff (describing the initial microlayer thickness) for bubble growth models matches well the value of Ceff obtained experimentally from thin film interferograms. Additionally, this study also validates the applicability of the kinetic theory-based model in predicting the evaporative heat transfer coefficient during the initial stage of flow boiling, while demonstrating that the accommodation coefficient is ∼0.04 for the investigated range of heat fluxes. Subsequently, using the experimentally obtained evaporative heat transfer coefficient, further insights are provided into the dry patch dynamics. Experimental dry patch dynamics were in good agreement with theoretical predictions up to the rewetting phase. The results also indicate a substantial contribution of microlayer evaporation, constituting approximately 34 % of the total heat flux.

Modeling and study of microlayer effects on flow boiling in a mini-channel

The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel.

Experimental investigation of the microlayer and dry spot dynamic characteristics in a small channel flow boiling by laser interferometry method

It is well established that the microlayer beneath the bubble plays a pivotal effect in the heat transfer in nucleation boiling. However, experimental studies of microlayers in small channel flow boiling are scarce due to measurement difficulty. In this study, laser interferometry and high-speed camera techniques were used simultaneously to investigate the microlayer dynamic characteristics beneath boiling bubbles in a small channel. It is found that the microlayer's interference fringe is still not a complete "Newtonian ring" in small horizontal channels. The downstream of the bubble, the microlayer still shows a "wedge" structure, and the initial microlayer thickness distribution is similar to that of the pool boiling. The microlayer is not entirely evaporated, and the rewetting phenomenon occurs simultaneously with the evaporation of the microlayer. The newly dry spot area and the rewetting area show the characteristics of first increasing to a maximum and then decreasing.

Numerical simulation of nucleate pool boiling under high pressure using an initial micro-layer thickness model

Micro-layer evaporation is one of the main phase change mechanisms in nucleate pool boiling. However, the influence of pressure on micro-layer evaporation remains unclarified. The objective of this study is to establish an approach to calculate micro-layer evaporation at high pressure. Here, an analytical solution of the governing equations is applied to derive a semi-empirical correlation for the initial micro-layer thickness δ0. After the calibration with measurement under atmospheric pressure, δ0 becomes a function of fluid properties. A nucleation sites tracking method is then implanted in the micro-layer evaporation system for multi-bubble cases. In this study, two single bubble growing cases are employed to validate our initial micro-layer thickness model at high pressure (1.91 MPa, 4.47 MPa). The simulated bubble radius profiles show similar trends with experimental data. Furthermore, the entire multi-bubble evaporation system is also validated against a pool boiling experiment at high pressure (0.51 MPa). The simulated wall superheat is consistent with reported data, with the minimum deviation of about 20%. The contribution of evaporation during nucleate boiling from both the macro and micro region is demonstrated at various operating conditions. Micro-layer supplies about 70% vapor in nucleate pool boiling at 0.51 MPa.

Study on boiling heat transfer mechanism based on microlayer evaporation theory

Based on dynamic microlayer evaporation model. Heat transfer mechanism of nuclear boiling and microlayer thickness under single bubble on heated surface. It has a lot to do with the temperature of the wall. Therefore, this paper studies the mechanism of bubble microlayer and makes comparative analysis with its experiments. It is helpful to master nuclear boiling heat transfer more comprehensively. In the study of individual bubble microlayer thickness. The effects of time and radius on wall temperature are considered in this paper. The theoretical formula of initial thickness of microlayer is deduced. The theoretical formula is compared with the experimental data of microlayer evaporation under the nucleation bubble. It was found that the experimental data fell within ±25% of the formula range. It is consistent with the previous experimental research.

Comprehensive simulations of boiling with a resolved microlayer: validation and sensitivity study

The dynamics of the microlayer beneath a growing bubble in nucleate boiling significantly impacts the heat-transfer characteristics of the process. The minute thickness of the microlayer motivates the use of direct numerical simulation (DNS) to model its behaviour if empirical models are to be avoided. In this work, we develop a computational strategy for utilising DNS to model nucleate boiling by resolving explicitly the microlayer, directly coupling, in a stable manner, the mass, momentum and energy conservation equations with the conjugate heat transfer between the solid and fluid domains. To this end, closure models for the treatment of interfacial heat transfer and the dynamic contact angle are introduced and substantiated. The computational procedure is validated against relevant experimental data recently measured at the Massachusetts Institute of Technology; it is shown that the main observed growth features and surface heat-transfer characteristics are well reproduced using our model. We go on to perform a sensitivity study of the dependence of the initial microlayer thickness distribution on the applied superheat and fluid properties. The results indicate that an equation derived from lubrication theory captures the observed trends well. Finally, a first demonstration of DNS of boiling with an explicitly resolved microlayer in three-dimensional Cartesian coordinates is presented in one of the appendices.

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

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

Hydrodynamic formation of a microlayer underneath a boiling bubble

The hydrodynamic formation of a microlayer at the base of a vapor bubble growing on a heated wall was experimentally and theoretically studied. Single bubble nucleate boiling experiments were conducted in a pool of saturated water under atmospheric pressure. A complete picture of the bubble geometry was obtained, including the three-phase contact line, microlayer and macroscopic bubble. This was performed using integrated laser interferometry, infrared thermometry and shadowgraph techniques. Existing models of the initial microlayer thickness that use an idealized hemispherical bubble shape and neglect surface tension force significantly overestimate microlayer thickness measured in experiment. The visualization results revealed that the non-hemispherical shape and surface tension of the bubble play a critical role in determining initial microlayer thickness. Theoretical analysis also indicated that the short-lived residual flow could play a key role in microlayer formation. Finally, a sophisticated model of initial microlayer thickness, developed to take into account the three identified mechanisms, provides predictions in agreement with experiments for different fluids.

Detailed structure of microlayer in nucleate pool boiling for water measured by laser interferometric method

During nucleate pooling boiling, a microlayer forms on the heat transfer surface beneath a growing bubble. In previous studies, the existence of a microlayer was confirmed, and its structure was investigated by using several different experimental methods. However, the characteristics and mechanism of the formation of a microlayer have not been thoroughly elucidated to date. In this study, in order to obtain further understanding on the microlayer structure, a laser interferometric method was adopted for a more detailed observation. As a result, it was observed that the special bended shape of the microlayer forms near its outer margin when the microlayer radius is close to its maximum value. By analyzing the variation of the ratio of bubble height to diameter, it can be inferred that the detachment of a bubble causes a decrease of the initial microlayer thickness when the microlayer thickness is close to the maximum radius. This effect explains the formation of the bended shape of initial microlayer.

Microlayer formation characteristics in pool isolated bubble boiling of water

Investigation of microlayer formation characteristics is important for developing a reliable nucleate boiling heat transfer model based on accurate physical mechanisms. Although formation mechanisms of the thin liquid film in two-phase flow of confined spaces, such as micro-tubes and closely positioned parallel plates, have been thoroughly studied, microlayer formation mechanisms of pool boiling have been sparsely studied. In a previous study (Yabuki and Nakabeppu in Int J Heat Mass Transf 76:286–297, 2014; Int J Heat Mass Transf 100:851–860, 2016), the spatial distribution of initial microlayer thickness under pool boiling bubbles was calculated by transient heat conduction analysis using the local wall temperature measured with a MEMS sensor. In this study, the hydrodynamic characteristics of microlayer formation in pool boiling were investigated using the relationship between derived initial microlayer thickness and microlayer formation velocity determined by transient local heat flux data. The trend of microlayer thickness was found to change depending on the thickness of the velocity boundary layer outside the bubble foot. When the boundary layer thickness was thin, the initial microlayer thickness was determined by the boundary layer thickness, and the initial microlayer thickness proportionally increased with increasing boundary layer thickness. On the other hand, when the boundary layer was thick, the initial microlayer thickness decreased with increasing boundary layer thickness. In this thick boundary layer region, the momentum balance in the dynamic meniscus region became important, in addition to the boundary layer thickness, and the microlayer thickness, made dimensionless using boundary layer thickness, correlated with the Bond number.

On heat transfer and evaporation characteristics in the growth process of a bubble with microlayer structure during nucleate boiling

We have previously observed and experimentally measured a microlayer formed beneath a growing bubble during nucleate pool boiling. The initial microlayer thickness was of micrometer order and increased linearly with distance from the bubble inception site. The quantitative degree of contribution of the microlayer evaporation to bubble growth was still not elucidated, although a large number of experimental studies have been conducted on the distribution and evaporation characteristics of the microlayer. To clarify the heat transfer characteristics, especially the contribution of microlayer evaporation in nucleate pool boiling, numerical simulations are performed here for two phase vapor–liquid flow induced by the growth of a single bubble using the volume of fluid method. Furthermore, a special model is proposed to combine the microlayer and bulk liquid regions, the scales of which are extremely different in the simulations. The microlayer was neglected in the volume fraction calculation, while the evaporation from the microlayer was included by applying the source terms of the basic equations for the presence of a virtual microlayer. Similar tendencies were observed between the calculated and experimental results on the variations in the microlayer radius and bubble volume. The proportion of microlayer evaporation to total bubble volume was generally in agreement with the previous results, and the ratio of microlayer evaporation to the change in the total bubble volume was approximately 40 percent.

Heat transfer mechanisms in isolated bubble boiling of water observed with MEMS sensor

Heat transfer characteristics of isolated bubble saturated pool boiling of water were investigated by local wall temperature measurement using an original micro-electro-mechanical systems (MEMS) sensor and a wall heat transfer evaluation. The measured temperature clearly indicated dynamic heat transfer phenomena below an isolated bubble including microlayer evaporation, dry-out of the microlayer, and rewetting of the dry-out region. Local heat removal due to microlayer evaporation exceeded 1MW/m2. Microlayer evaporation was found to have a central role in the wall heat transfer. The contribution of microlayer evaporation to bubble growth was about 50%, and did not vary significantly with wall superheat. The remaining 50% of the latent heat in the bubble was transferred from the superheated liquid surrounding the bubble. Finally, the spatial distribution of the initial microlayer thickness was calculated from the local heat flux of the wall.

Numerical simulation on the heat transfer characteristics during the bubble growth considering microlayer evaporation in nucleate boiling

In the previous studies, it was confirmed that there is a microlayer that forms between a growing bubble and the heat transfer surface in nucleate pooling boiling. The initial microlayer thickness was found to increase linearly with distance from the bubble inception site. Although a large amount of heat transport results from the evaporation of microlayer during the growth process of bubble, the quantitative degree of contribution of microlayer evaporation was not elucidated. In this study, numerical simulation was carried out on the two phase vapor-liquid flow induced by the growth of a single bubble in nucleate boiling by using the VOF (Volume of Fluid) method. The objective of this study is to clarify the heat transfer characteristics, especially the effect of evaporation of microlayer in nucleate pool boiling. A special model for simulating the microlayer evaporation was proposed, in which the microlayer was not considered in the calculation of flow field while its amount of evaporation was considered by applying the source term of the basic equations as an existence of virtual microlayer. Similar tendencies were shown between the calculation and experimental results on the variations of microlayer radius and bubble volume. The proportion of the amount of evaporation from microlayer to the volume of bubble is generally agreed with the previous results, which showed the ratio of microlayer evaporation to the total vapor volume change was approximately 40 percent.

10807 沸騰におけるミクロ液膜の形成機構と蒸発特性(OS6-1【熱流体工学のフロンティア(1)】)

The wall temperature under a boiling bubble in the mini-channel was measured at high speed with the MEMS thermal sensor. The surface heat flux was evaluated by a one-dimensional transient heat conduction analysis in the sensor substrate with the measured data as a surface boundary condition. The initial microlayer thickness was calculated from amount of heat transferred until the dryout of the heating wall. The calculated initial microlayer thickness was a few μm and was found to become thicker with a bubble forefront velocity. The non-dimensional initial microlayer thickness showed a correlation to Capillary number corresponding to the ratio of the viscous force and surface tension. Superheat of liquid-vapor interface was observed during the evaporation of the microlayer by the transient heat conduction analysis in the microlayer.

Microlayer structure in nucleate boiling of water and ethanol at atmospheric pressure

This study investigates the structure of the microlayer that forms between growing bubbles and the heat transfer surface for nucleate pool boiling in the isolated bubble region. The microlayer thickness was measured for water and ethanol by a specially designed measurement system that employs the laser extinction method. A thin (diameter: 94μm) optical fiber in a thin stainless-steel tube was inserted in a liquid pool. Gas flow was used to prevent the fiber, which was made of fluorinated glass, from becoming wet and to remove the pool liquid from the light path. A method for determining the initial microlayer thickness was developed based on numerical calculations. The microlayer thickness decreased over time due to evaporation. The thickness of the initially formed microlayer was found to be uniquely determined. For water, it was in the approximate range of 0–9μm and it increased linearly with increasing distance between the incipient bubble site and the measurement position; this relation was independent of the heat flux. The initial microlayer structure for ethanol was similar to that for water, but its thickness was about twice that for water.

Formation Mechanism and Characteristics of a Liquid Microlayer in Microchannel Boiling System

Experiments were performed using the laser extinction method to measure the thickness of the liquid film formed by growing flattened bubbles in a microchannel for gap sizes of 0.5 mm, 0.3 mm, and 0.15 mm. Water, ethanol, and toluene were used as test fluids. A high-speed camera was also used to simultaneously measure the bubble growth process. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of the interface was divided into two regions: region I, where the velocity is small and the thickness increases linearly with increasing velocity, and region II, where the thickness is almost constant or decreased slightly with increasing velocity. Furthermore, a nondimensional correlation for investigating the effects of test materials and gap sizes on microlayer thickness is presented. An analysis of the results showed that the boundaries of the two regions correspond to a Weber number of approximately 110, and in the region where the Weber number was smaller than 110, the thickness of the microlayer was thinner for the liquid whose value of ρ0.62ν0.42σ−0.62 was relatively small. However, for the region where Weber number was larger than 110, the smaller the kinematic viscosity of the liquid, the thinner the microlayer became.

MNM-P3-2 沸騰熱伝達研究へのMEMSセンサの応用(P3 デバイスを用いた熱・流体計測)

This paper describes a novel approach method to investigate heat transfer mechanism of nucleate boiling by use of the MEMS sensor including micro thermocouples, trigger electrodes and thin film heaters. In the experiments, the local temperature variation beneath the isolated bubble was measured with the MEMS sensor at six wall superheats. In addition, the wall heat transfer was quantitatively evaluated by the transient heat conduction simulation in the heating wall with the measured temperature data. Features of used MEMS sensor, measurement results, wall heat transfer characteristics of the isolated bubble boiling and the initial microlayer thickness are demonstrated.

21001 MEMSセンサを用いた核沸騰熱伝達機構の研究 : 孤立気泡下のミクロ液膜厚さの評価(OS10 マイクロ・ナノ熱流体工学(1),オーガナイズドセッション)

The objective of this study is to investigate heat transfer mechanism of nucleate boiling with MEMS (Micro-Electro-Mechanical systems) sensor fabricated for boiling research. In the experiment Temperature variation beneath an isolated bubble under nucleate boiling condition of saturated water was measured with the sensor. Measured temperature data presents microlayer evaporation and expansion of dry-out area in bubble growth process and rewetting of the dry-out area in bubble departure process. Additionally, heat transfer from the heated surface was evaluated by calculating local heat flux by transient heat conduction analysis with the measurement data as a boundary condition. Spatial distribution of initial microlayer thickness was also computed with the derived local heat flux.

Behaviors of micro-layer in micro-channel boiling system applying laser extinction method

To elucidate the mechanism and characteristics of boiling heat transfer in a micro-channel vaporizer, the experimental investigation of the micro-layer thickness that formed between the heating surface and vapor generated was important. The micro-layer thickness was measured applying the laser extinction method for channel gap sizes of 0.5, 0.3, and 0.15 mm. It was clarified that the gap size, the rate of bubble growth, and the distance from the incipient bubble site have an effect on the micro-layer thickness in a micro-channel boiling system. The initial micro-layer thickness grew with an increase of the velocity of bubble forefront to moderate the value of the velocity. In the region of greater velocity, the thickness was constant for each gap. The distributions of the initial thickness of micro-layer on the heat transfer surface were shown. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 35–46, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20096