A Visualization Study of Bubble Behavior in Saturated Flow Boiling through a Vertical Mini-Tube

Forced convection saturated R-134a boiling experiments were conducted in a vertical mini-quartz tube coated with a transparent heater; the inner diameter of the tube was 1.33 mm and the heated length 235.5 mm. The dynamics of the saturated boiling process, bubble characteristics, and behavior were studied using a high-speed CCD camera at different mass fluxes in up-flow at 6.425 bar. The heat fluxes were 5 and 20 kW/m². The flow visualization results show that the bubble departure frequency generally increases as the heat flux increases. In addition, bubble lift-off diameter, growth rate, and velocity after bubble lift-off were determined by analyzing the images.     

Subcooled Flow Boiling in a Minichannel

It has been considered that dry-out occurs easily in boiling heat transfer for a small channel, a mini- or microchannel, because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric conditions for a horizontal rectangular channel for which the size is 1 mm height and 1 mm width, with a flat heating surface of 10 mm length and 1 mm width placed on the bottom of the channel. The heating surface has a top of copper heating block and is heated by ceramic heaters. In the high heat flux region of nucleate boiling, about 70–80% of the heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux as observed by high-speed video. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at liquid subcooling higher than 30 K. The maximum heat flux obtained was 8 MW/m² (800 W/cm²) at liquid subcooling of higher than 40 K and a liquid velocity of 0.5 m/s. However, the surface temperature was very much higher than that of a centimeter-scale channel. The high-speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.     

矩形窄缝流道内过冷沸腾汽泡行为的可视化

采用高速摄像仪,对矩形窄缝流道内过冷沸腾时的汽泡行为进行了可视化实验研究.分析了工况参数对汽泡成核起始点及其脱离直径的影响.结果表明:高过冷沸腾时,窄缝流道内加热面上产生了沿近壁面滑移的汽泡,这种滑移汽泡对窄缝流道内的换热产生了积极的作用,而且汽泡的滑移现象与主流流体的温度有着密切的关系,分析了产生这种滑移现象的原因.     

Experimental Study on Subcooled Flow Boiling in Horizontal Microtubes and Effect of Heated Length

ABSTRACT

In this study, subcooled flow boiling was investigated in horizontal microtubes. Experiments were conducted using deionized water as the working fluid over a mass flux range of 4000–7000 kg m^?2s^?1 in microtubes with inner and outer diameters of ～600 and ～900 μm, respectively. Microtubes with lengths of 3, 6, and 12 cm were tested to clarify the effect of heated length on flow boiling heat transfer and pressure drop characteristics. A force analysis related to two-phase flow was conducted to understand the effect of forces on bubble dynamics. Pressure drop and heat transfer data in flow boiling were acquired. Experimental heat flux data were compared with partial boiling heat flux correlations, and good agreements were obtained. Pressure drop was larger in longer microtubes in comparison to shorter ones, while higher heat fluxes were obtained in shorter microtubes at the same wall superheat. Two-phase heat transfer coefficient increased with the microtube length due to lower temperature difference between wall temperature and bulk fluid temperature in longer microtubes. Higher heat fluxes achieved in shorter microtubes at the same wall superheat imply higher critical heat fluxes in shorter microtubes.     

Numerical Simulation of Single Bubble Dynamics During Flow Boiling Conditions on a Horizontal Surface

Complete three-dimensional numerical simulations of single bubble dynamics during flow boiling conditions are carried out using the computational fluid dynamics code FLOW3D based on the volume-of-fluid method. The analyses include a numerically robust kinetic phase-change model and transient wall heat conduction. The simulation approach is calibrated by comparison with available experimental and theoretical data. It is found that the observed hydrodynamics (i.e., bubble shape, departure, and deformation) are simulated very well. The comparison with high-resolution transient temperature measurements during a heating foil experiment indicates that the modeling of the spatiotemporal heat sink distribution during bubble growth requires major attention. The simulation tool is employed for single bubble dynamics during flow boiling on a horizontal heating wall, and the agreement is excellent with published experimental data. The numerical results indicate how bulk flow velocity and wall heat transfer influence the bubble dynamics and heat transfer characteristics.     

窄矩形通道中欠热流动沸腾气泡图像分割方法

为快速且准确地计算气泡行为参数,本研究针对窄距形通道中欠热流动沸腾的气泡图像,提出了一种气泡分割方法.首先,对实验图像进行滤波,背景差值等操作消除背景噪声,提高两相之间对比度;然后,利用双阈值法获得气泡图像二值图;最后,依据气泡特征选择合适的结构元素,并利用形态学操作完成气泡图像分割.实验结果表明,该方法所得气泡图像分割结果有效消除复杂背景,气泡形状、大小及位置均与实验图像符合较好.因此,该方法能够对窄矩形通道中欠热流动沸腾的气泡图像进行快速有效的分割识别.     

高热流密度微通道的周期性欠热沸腾

实验发现单微通道在高热流密度条件下会发生一种长周期/小幅度间歇性欠热沸腾现象.它发生在入口温度在室温到8℃的范围,出口温度及压差呈现非常有规律的小幅度脉动,且呈现反相.当热流密度略大于沸腾起始点时,脉动周期长达50～100 s.随着热流密度的增大,脉动周期变短.当热流密度接近临界值时,脉动周期在5～10 s.实验还发现在宽广的热流密度范围内.时均质量流速对于热流密度的变化不敏感,推断出在一个完整周期内,汽泡产生及逸出的时间相对较短.     

Experimental Study on Flow Boiling of Deionized Water in a Horizontal Long Small Channel

In this paper, an experimental investigation on the flow boiling heat transfer in a horizontal long mini-channel was carried out. The mini-channel was with 2 mm wide and 1 mm deep and 900 mm long. The material of the mini-channel was stainless. The working fluid was deionized water. The experiments were conducted with the conditions of inlet pressure in the range of 0.2~0.5 MPa, mass flux in the range of 196.57-548.96 kg/m²s, and the outlet vapor quality in the range of 0.2 to 1. The heat flux was in the range of 292.86 kW/m² to 788.48 kW/m², respectively. The influences of mass flux and heat flux were studied. At a certain mass flow rate, the local heat transfer coefficient increased with the increase of the heat flux. If dry-out occurred in the mini-channel, the heat transfer coefficient decreased. At the same heat flux, the local heat transfer coefficient would depend on the mass flux. It would increase with the mass flux in a certain range, and then decrease if the mass flux was beyond this range. Experimental data were compared with the results of previous studies. Flow visualization and measurements were conducted to identify flow regime transitions. Results showed that there were eight different kinds of flow patterns occurring during the flow boiling. It was found that flow pattern had a significant effect on heat transfer.     

Dynamic Analysis of Bubble Attachment and Sweeping on Microwire in Subcooled Nucleate Pool Boiling

With the development of industrial technology,heat transfer at the microscale has attracted more and more attention.In this work,200 μm platinum wire and 150 μm...     

Hysteresis Phenomena at the Onset of Subcooled Nucleate Flow Boiling in Microchannels

In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when a considerable rise in the wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis.” The impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.     

Subcooled Boiling Heat Transfer of CO2 in A Horizontal Tube at Low Temperatures

This article presents an experimental investigation on the subcooled flow boiling heat transfer characteristics of CO₂ in a horizontal tube with inner diameter of 6.16 mm below ?30°C. The effects of mass and heat fluxes and saturation temperature on the heat transfer coefficient are discussed. Large deviations are noted between the predictions from previous empirical correlations and the current CO₂ experimental data. Hence a new empirical correlation is developed, which agrees within ±30% with the current CO₂ experimental data. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO₂ at low temperatures.     

缸盖鼻梁区沸腾汽泡演化行为可视化试验研究

以某增压柴油机为研究对象,搭建了气缸盖鼻梁区内沸腾汽泡演化行为可视化试验平台,拍摄标定工况下鼻梁区域内汽泡成长、滑移、聚合等行为,开展了缸盖水腔内沸腾换热和汽泡演化行为特性的研究。研究结果表明,在成长开始阶段,汽泡成长平均直径的增长速度比较快,然后逐渐下降且有着稳定的趋势。在不同曲轴转角下,汽泡当量直径分布符合正态分布规律,且汽泡的聚合现象越多对整体汽泡行为的随机性影响就越强烈。     

非共沸混合制冷剂水平管内流动沸腾换热

近年来,非共沸混合制冷剂作为工质改善热泵和制冷设备工作效率和性能的潜力引起了广泛的重视。本文评述了非共沸混合制冷剂水平管内流动沸腾换热研究的主要问题,着重于讨论换热的计算,并提出了一些有益建议。     

Time and Frequency Characteristics of Pressure Fluctuations during Subcooled Nucleate Flow Boiling

Time and frequency analyses methods including PDF (Probability Density Function), CDF (Cumulative Probability Function), FFT (Fast Fourier Transform) and PSD (Power Spectral Density) were used to investigate the pressure fluctuation characteristics during subcooled nucleate flow boiling and the effects of various parameters. The experimental conditions were heating currents to a high frequency induction heater of 200–720 A, liquid temperatures of 50–85°C, and flow rates of 0–290 L/h. The CDF and PSD methods were more effective than the PDF and FFT methods for the time-frequency analyses of the pressure fluctuations. Higher heating currents, lower liquid temperatures and lower flow rates result in wider frequency distributions and larger pressure fluctuation amplitudes in the time domain and wider frequency distributions and higher PSD of the pressure fluctuations in the frequency domain. The effects of the operating conditions on the Onset of Nucleate Boiling (ONB) were analyzed qualitatively by correlating the pressure fluctuation signals with visual images obtained from the experiments. A pressure drop occurs at the ONB regardless of the conditions with the pressure drop appearing earlier at higher heating currents and liquid temperatures and lower flow rates.     

Development of a Multi-Dimensional Fluid Dynamics Code and Its Benchmarking for the Subcooled Boiling Flow

In a two-phase flow analysis, the interfacial area concentration (IAC) is a dominant factor governing the interfacial transfer of the momentum or energy. For a dynamic analysis with the implementation of IAC transport equation, a multi-dimensional computational fluid dynamics code was developed. The code is based on the two-fluid model and the simplified marker and cell algorithm by using the finite volume method, where the conventional approach for a single-phase flow has been modified in order to consider the term for a phase change. As benchmark problems of a single-phase flow and two-phase flow, a natural convection in a rectangular cavity and a subcooled boiling in an annulus channel were selected, respectively. In the calculation for the single-phase flow, the developed code predicted a reasonable behavior for a buoyancy-driven flow depending on the Rayleigh number. In the analysis of the subcooled boiling, the calculation results showed the robustness of code for the analysis of the boiling phenomena and void propagation, where they represented limitations of the one-dimensional IAC model. To conduct a multi-dimensional analysis for the two-phase flow, it is confirmed that the implementation of an IAC transport equation into the code is essential.     

High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 μm-square fins separated by 50 μm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel micro-channel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.     

Critical Heat Flux (CHF) Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor,for which an accurate critical heat flux(CHF)correlation is necessary.Up to now,there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures.However,the CHF correlations’accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor.The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for some database.In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure,the present paper developed a new CHF correlation.Compared with other existing CHF correlations,the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices,such as those in the fusion reactor divertor.The validation results show that the new correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new CHF correlation is 7.4%less than that of the best existing one(W-2 correlation),further confirming its superior prediction accuracy and reliability.Besides,the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.     

Bubble Dynamics,Flow, and Heat Transfer during Flow Boiling in Parallel Microchannels

Significant efforts have recently been made to investigate flow boiling in microchannels, which is considered an effective cooling method for high-power microelectronic devices. However, a fundamental understanding of the bubble motion and flow reversal observed during flow boiling in parallel microchannels is lacking in the literature. In this study, complete numerical simulations are performed to further clarify the boiling process by using the level-set method for tracking the liquid–vapor interface which is modified to treat an immersed solid surface. The effects of contact angle, wall superheat, and the number of channels on the bubble growth, reverse flow, and heat transfer are analyzed.     

Numerical Simulation of Bubble Growth and Heat Transfer During Flow Boiling in a Surface-Modified Microchannel

Flow boiling in a microchannel without or with surface modifications, such as fins, grooves, and cavities, has received significant attention as an effective cooling method for high-power microelectronic devices. However, a general predictive approach for the boiling process has not yet been developed because of its complexity involving the bubble dynamics coupled with boiling heat transfer in a microscale channel. In this study, direct numerical simulations for flow boiling in a surface-modified microchannel are performed by solving the conservation equations of mass, momentum, and energy in the liquid and vapor phases. The bubble surfaces are determined by a sharp-interface level-set method, which is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip and contact-angle conditions on immersed solid surface of microstructures. This computation demonstrates that the surface-modified microchannel enhances boiling heat transfer significantly compared to a plain microchannel. The effects of various surface modifications on the bubble growth and heat transfer are investigated to find better conditions for boiling enhancement.     

Falling Liquid Film Evaporation in Subcooled and Saturated Water Over Horizontal Heated Tubes

ABSTRACT

This paper presents an experimental study of heat transfer and film thickness behavior in falling liquid water film evaporation technology over horizontal tubes. Liquid distribution systems have also been evaluated. The experimental setup consisted of two horizontal 0.019-m OD stainless-steel tubes, 0.194 m in length. Reynolds numbers in the 160–940 range were tested in both subcooled and saturated liquid regimes. For the liquid distribution system study, several distributor geometries were tested in order to develop the least disturbed film over the tubes. An intrusive method was used for measuring the liquid film thickness in the laminar regime and the measured values were compared with the theoretical prediction computed from the Nusselt equation. An experimental heat transfer correlation was obtained and compared with previous ones obtained by other authors. In addition, the local heat transfer coefficient was observed to be always higher at the horizontal tube top region for all operational conditions (on the order of 14 kW/m²-°C). Finally, the use of a liquid storage distribution system along with the installation of a wire mesh to obtain an uniform liquid distribution for all Reynolds numbers tested.