3-D numerical investigation of water/CuO nanofluid critical heat flux phenomenon in a PWR core channel during LOCA

Forced convection boiling and critical heat flux have been under considerable attention in variety of areas due to high heat removal capacity. However, once the heat flux exceeds a certain high level (CHF), the heated surface can no longer support continuous liquid contact, associated with substantial reduction in the heat transfer efficiency. One way to increase the level of the CHF is to add certain nanoparticles to the base fluid. The present paper investigates the effects of the addition of copper oxide nanoparticles on CHF phenomenon within the general-purpose computational fluid dynamics (CFD). The governing equations solved are generalized phase continuity, momentum and energy equations. Wall boiling phenomena are modeled using the baseline mechanistic nucleate boiling model developed in Rensselaer Polytechnic Institute (RPI). To simulate the critical heat flux phenomenon, the RPI model is extended to the departure from nucleate boiling (DNB) by partitioning wall heat flux to both liquid and vapor phases considering the existence of thin liquid wall film. It was shown that the presence of copper oxide nanoparticles in the base fluid, delays the dryout phenomenon dramatically and in specific concentration, CHF threshold would be enhanced, therefore, raising the upper limit of CHF could allow for higher safety margins.     

基于CFD方法的棒束通道内临界热流密度预测

为研究棒束通道内临界热流密度现象,采用基于对气、液两相分别建立基本守恒方程的欧拉两流体六方程模型和改进的壁面热流密度分配模型,利用CFD商用软件FLUENT 14.5对捷克大型水介质实验回路上开展的临界热流密度（CHF）实验进行数值模拟。通过计算获得CHF发生前、后计算域内重要热工水力参数的分布及CHF发生值,将CFD计算获得的CHF与实验测得值进行对比,结果表明,大多数工况的偏差在±30%以内,证明了欧拉两流体模型结合改进的壁面热流密度分配模型对CHF预测的准确性。本研究可为复杂结构的CHF预测提供依据。     

An investigation of transition boiling mechanisms of subcooled water under forced convective conditions

A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations.     

Subcooled flow boiling heat transfer of dilute alumina, zinc oxide, and diamond nanofluids at atmospheric pressure

A nanofluid is a colloidal suspension of nano-scale particles in water, or other base fluids. Previous pool boiling studies have shown that nanofluids can improve the critical heat flux (CHF) by as much as 200%. In a previous paper, we reported on subcooled flow boiling CHF experiments with low concentrations of alumina, zinc oxide, and diamond nanoparticles in water (≤0.1% by volume) at atmospheric pressure, which revealed a substantial CHF enhancement (∼40-50%) at the highest mass flux (G = 2500 kg/m² s) and concentration (0.1 vol.%) for all nanoparticle materials (Kim et al., 2009). In this paper, we focus on the flow boiling heat transfer coefficient data collected in the same tests. It was found that for comparable test conditions the values of the nanofluid and water heat transfer coefficient are similar (within ±20%). The heat transfer coefficient increased with mass flux and heat flux for water and nanofluids alike, as expected in flow boiling. A confocal microscopy-based examination of the test section revealed that nanoparticle deposition on the boiling surface occurred during nanofluid boiling. Such deposition changes the number of micro-cavities on the surface, but also changes the surface wettability. A simple model was used to estimate the ensuing nucleation site density changes, but no definitive correlation between the nucleation site density and the heat transfer coefficient data could be found.     

HFE-7100工质稳态临界沸腾传热实验研究

池沸腾临界热通量是沸腾相变传热的重要参数,决定了相变换热器件的推广应用。表面粗糙度和饱和压力对沸腾传热边界层分布、表面铺展润湿及工质动力学特性具有重要影响,进而对临界热通量作用显著。本文对HFE-7100工质在4种不同粗糙度的铜基表面（0.019、0.205、0.311和0.587 μm）条件及在不同饱和压力（0.07、0.10、0.15及0.20 MPa）工况下的池沸腾稳态临界状态下的传热及可视化实验进行了研究。对表面粗糙度及饱和压力对稳态临界沸腾的影响机制进行了分析,并考察了临界热通量预测模型对临界热通量的预测准确性。可视化研究表明,临界状态下的沸腾气液两相工质由小气泡、大气泡、气柱及蘑菇状气团组成,而在过渡状态下,沸腾表面会形成非平滑气膜,并不断分离出气泡。同时传热数据表明,表面粗糙度及饱和压力的增加能使表面临界热通量得到提升。相比而言,Bailey等建立的临界热通量预测模型能相对准确地预测HFE-7100工质沸腾临界热通量数据。为进一步提升预测准确度,建立了临界热通量无因次参数K预测经验关联式,其预测值与本实验及文献实验数据吻合较好。     

摇摆条件下竖直圆管内干涸型临界沸腾的数值研究

针对摇摆条件下竖直圆管内干涸型临界热流密度（Dryout CHF）进行了三维数值计算,研究了摇摆条件下竖直圆管内相态分布特性、圆管内临界热流密度（CHF）的位置以及最高壁面温度,同时对管壁沿程换热系数特性进行了分析。结果表明:在摇摆条件下,圆管内相分布呈现周期性变化,CHF的位置也会发生周期性变化;同时发现摇摆运动会导致壁面最高温度更高,因此摇摆条件会使沸腾临界现象更严重。随着流型转变和沸腾传热机制的变化,管壁换热系数沿流动方向也会显著变化。本研究可以为摇摆条件下Dryout CHF的数值预测提供参考。      

超亲水特性传热面上水喷流冷却时的沸腾换热特性实验研究

研究了在纳米材料涂层形成的超亲水材料传热表面上水喷流冷却时的沸腾换热特性,并与普通金属面上的喷流沸腾特性进行了比较.介绍了流动条件、加热方式、加热条件、超亲水材料涂层等条件对高温沸腾换热、临界热流密度的定量影响和一些基本换热规律.实验发现,超亲水传热表面对沸腾换热特性有十分显著的影响,超亲水传热面强化了喷流沸腾临界热流密度.     

Stage Efficiency for Mixer-Settlers Process with Chemical Reactions

The importance of heat transfer in a narrow gap has been pointed out in relation to the in-vessel retention of molten core under a severe accident condition of a light water reactor. In this study, heat transfer experiment has been carried out using internally heated annuli with a narrow gap and closed bottom. Effects of gap clearances (1.0, 2.0, 4.0 mm) on the cooling process were examined. The inner wall initially heated up to 800°C was cooled down by water supplied to the upper plenum, the transient rod temperature being measured. From the temperature history, transient boiling curves were obtained, which were compared with predictions by existing heat transfer correlations for nucleate boiling critical heat flux (CHF) and film boiling. In narrow gap channels water penetration to the gap was restricted by counter flowing vapor, which lead to a decrease of heat transfer coefficients.     

近临界压力区垂直内螺纹管临界热流密度实验研究

在近临界压力区,对垂直上升内螺纹管流动沸腾的偏离泡核沸腾(DNB)型临界热流密度(CHF)现象进行了实验研究。试验段采用ф35 mm×5.67 mm六头内螺纹管。实验参数范围为:压力18~21 MPa,质量流速500~1 000kg/(m~2·s),进口过冷度3~5℃,内壁热负荷40~960kW/m~2。实验得到了不同工况下的内壁温度和传热系数分布特性,分析了流动参数对内螺纹管中DNB型CHF的影响,并根据实验数据拟合出两相区的传热关联式与临界热流密度(qCHF)预测关联式。内螺纹管的qCHF实验数据被用于与光管的qCHF预测值进行对比,发现内螺纹管具有一定的CHF强化作用,但当压力越靠近临界压力时这种作用会被抑制甚至消失。实验结果表明:在近临界压力下,内螺纹管会在低干度区甚至过冷区发生DNB现象,压力的增大和质量流速的减小均会使DNB提前发生。qCHF随压力的减小和质量流速的增大而增大。在特定工况下,试验段不同截面会分别发生偏离泡核沸腾与蒸干。     

基于CFD方法的矩形窄缝通道内过冷流动沸腾模型研究

板状燃料元件中的矩形窄缝通道具有宽高比大的几何特征,高度方向速度梯度大、分布陡峭,发生过冷沸腾时,近壁面汽泡运动行为将受其影响而改变,其中汽泡滑移现象对沸腾换热影响较大。本文针对矩形窄缝通道中的汽泡滑移行为,构建了包含滑移热流的壁面热流分配模型,并建立机理性的汽泡受力模型和滑移模型计算汽泡脱离直径、浮升直径和滑移距离等辅助参数,开发了一套适用于矩形窄缝通道内向上流动沸腾的壁面沸腾模型。选用Nuthel窄缝通道沸腾实验进行数值模拟验证,结果表明：本文模型可以较好地预测1～4 MPa中低压工况窄缝通道向上流动沸腾的壁面过热度,最大误差相比RPI模型由80%降低至17%;蒸发热流份额和近壁面空泡份额相比RPI模型更低。     

CFD analysis on subcooled boiling phenomena in PWR coolant channel

Eulerian two-fluid model coupled with wall boiling model was employed to calculate the three dimensional flow field and heat transfer characteristics in a hot channel with vaned spacer grid in PWR. The heat transfer from pellet-gap-cladding to coolant was also taken into account by a system coupled code MpCCI. The wall boiling model utilized in this study was validated by Bartolomei experiment data, and a good agreement can be observed. By solving the governing equation in a two-way coupled method, the distribution of temperature in the pellet-gap-cladding region and the distribution of temperature, void fraction and velocity of two-phase flow in coolant channel can be obtained. The influences of spacer grid and mixing vane on the thermal-hydraulic characteristics were analyzed. The heat transfer capacity was strongly improved by the spacer grid and mixing vane, while the flow resistance was also enlarged. Localized volume fraction of vapor phase decreased due to mixing vane, which will decrease the possibility of the departure from nucleate boiling (DNB) and increase the critical heat flux (CHF). By analyzing the temperature and void fraction at cladding outer surface, the critical regions where hot spot may occur were determined.     

蒸汽发生器传热管一、二次侧耦合换热及管外过冷沸腾数值研究

采用两流体欧拉数学模型,结合气相和液相之间的界面传热、传质和动量交换封闭模型以及RPI壁面沸腾模型,利用ANSYS CFX 12.0对蒸汽发生器局部传热管束二次侧的过冷沸腾进行数值研究。数值研究结果与单管内过冷沸腾实验数据对比验证符合良好。结果表明,采用壁面沸腾模型能准确预测沸腾起始点的位置,同时梅花孔板的存在对二次侧流动换热特性影响显著。     

基于两相CFD的非均匀加热圆管CHF预测方法研究

为建立非均匀加热工况临界热流密度（CHF）预测方法，以对换热系统的安全分析提供新的辅助手段，本研究采用欧拉两流体模型和壁面沸腾模型，对非均匀加热圆管的CHF进行预测。通过数值计算得到不同热流密度下近壁面空泡份额和壁面温度的分布，将壁面温度出现二次峰值和此时近壁面空泡份额的峰值位置分别作为CHF发生的依据和CHF发生的点，并用此方法对2种不同功率分布圆管的CHF进行研究。研究结果表明，预测得到临界时的平均热流密度及临界发生的位置都与实验结果符合较好。因此，本研究建立的数值预测方法能够用于非均匀加热圆管CHF的预测。      

A new model for predicting the critical heat flux based on nucleation site density in downward-face boiling

ABSTRACT

Due to the important role critical heat flux (CHF) plays in the boiling field, it is of great significance to study CHF, especially the mechanism of CHF in the nucleate boiling. In this study, a new model to predict CHF both in pool boiling and flow boiling of downward-face was proposed and the relationship between CHF and nucleation site density (NSD) was studied. The model was based on the bubble interaction theory, which assumed that CHF happened due to the coalescing of the bubbles generated on the heating surface and prevented liquid to be supplied. The relationship between NSD and CHF was derived from previous observations in the experiments and simulations. To validate the relationship between NSD and CHF, several experiments with CHF and NSD were chosen and they all showed good agreement with our assumptions. Due to the rarity of experimental data on NSD and CHF, the numerical method was also used to validate. The results also showed an inverse relationship between CHF and NSD.     

基于流固共轭传热的两相流动数值模拟及燃料子通道CHF预测研究

为对过冷沸腾两相流动进行准确模拟,并探索临界热流密度（CHF）预测方法,本文基于共轭传热和两相CFD分析的方法,通过流固界面耦合,建立流固共轭传热两相流动耦合求解的数值模型。首先通过典型燃料棒栅元过冷沸腾两相流动的模拟,验证数值模型的正确性。随后对燃料子通道内两相流动进行模拟,并在两相流动模拟的基础上,通过准瞬态的方法,建立与CHF试验过程非常近似的CHF预测方法,将加热壁面的温度飞升作为CHF判定的标准,实现对燃料组件子通道CHF的数值预测。研究表明,本文建立的数值模拟方法,可为燃料组件或其他换热系统的CHF预测奠定基础,为燃料组件的设计提供新的辅助手段。     

低压下水欠热流动沸腾的两相CFD数值模拟研究

采用两流体(汽相和液相)基本数学模型,结合汽相和液相之间的界面传热、传质和动量交换封闭模型、汽泡平均直径模型、汽泡脱离直径模型、汽泡成核模型、汽泡脱离频率模型、欠热沸腾起始点模型和壁面热流密度分配模型,在CFD软件CFX4.4中采用用户自定义函数将相变引起的传热、传质和动量交换作为源项分别添加到汽相和液相的能量、质量和动量守恒方程中,对低压下内管加热外管绝热的环形通道内的欠热沸腾进行了数值研究,得到了欠热流动沸腾下汽相体积份额、液相速度、汽相速度分布等。采用Lee等的环形通道内低压下欠热沸腾体积份额实验数据对计算结果进行了验证,吻合良好。     

Experimental observations of flow modifications in nanofluid boiling utilizing particle image velocimetry

In order to understand the effects of nanofluids upon boiling, pool boiling experiments involving both pure water and water based nanofluids were investigated. Point temperature and full velocity field measurements were obtained during pool boiling conditions using state-of-the-art dynamic particle image velocimetry (DPIV). Specifically, results of nanofluid's effect upon nucleate boiling heat transfer are investigated, in an effort to find engineering solutions in critical applications such as nuclear energy. A change in the hydrodynamic behavior of bubbles was observed with the introduction of nanoparticles. The measured velocity fields exhibited different fluid behavior for the various cases investigated which in turn highlights the complexity of the fluid flow in the pool boiling experiment. The results show a relation between wall temperature and nanofluid concentration.     

An experimental investigation on the critical heat flux enhancement by mechanical vibration in vertical round tube

In order to gain an understanding of the relationship between critical heat flux (CHF) and flow-induced vibration (FIV), an experimental investigation was carried out with vertical round tube at the atmosphere. In the both condition of departure from nucleate boiling (DNB) and the liquid film dryout (LFD), CHF increases up to 12.6% with vibration intensity, represented by vibrational Reynolds number (Re_v). CHF enhancement by tube vibration seems to come from the reinforced flow turbulent mixing and the increment of deposition of droplet into the liquid film. Based on the experimental results, an empirical correlation is proposed for the prediction of CHF enhancement ratio. The correlation predicts the CHF enhancement ratio (E_n) with reasonable accuracy, with an average error rate of 4.5 and 26.5% for RMS. Vibration is an effective method for heat transfer enhancement as well as CHF. Nonetheless, the risk of system failure by FIV has made it very difficult to take advantage of vibration in heat transfer facilities. Therefore, it is necessary to find out optimal fuel design enhancing the CHF but preventing FIV damage in an acceptable vibration range.     

微细化沸腾传热实验研究

气泡微细化沸腾是沸腾到达某个临界热负荷后,加热面温度升高不大,与该临界热负荷相比,热流密度大幅提高的沸腾现象。本文在设计完成一可视化实验装置的基础上,通过高速摄影仪观察并结合采集的壁温数据,对常压下直径为10 mm铜加热面上的池式气泡微细化沸腾现象进行了研究,并讨论了液体过冷度对其的影响。实验发现,气泡微细化沸腾状态下,加热面上生成1层极其不稳定的气膜,气液交界面上不停地有大量微小气泡生成并以极高速度射入过冷液体中。随加热面热流密度的增大,气膜厚度波动周期缩短,气膜最大厚度减小,所生成微小气泡的直径也明显减小。实验中获得的最高热流密度达9 MW/m²。     

Critical Heat Flux and Near-Wall Boiling Behaviors in Saturated and Subcooled Pool Boiling on Vertical and Inclined Surfaces

The mechanism of the critical heat flux (CHF) where the departure from nucleate boiling (DNB)-type boiling transition takes place has not been fully elucidated. In this paper, we examine the trigger mechanism of the CHF for saturated and subcooled pool boiling on vertical and inclined surfaces based on measurements of the liquid-vapor behaviors near heating surfaces by using a conductance probe. The angle of inclination was varied from 90° (vertical) to 170° (facing almost horizontally downwards). The probe signals and the void fraction distributions showed that a liquid layer remains beneath the vapor masses moving upward along the heating surface at high heat fluxes near the CHF. The thickness of the liquid layer was determined from the location where the probe signals corresponding to the vapor masses disappeared. The thickness of the liquid layer formed on the vertical surface increased with increasing degree of subcooling, which may be the cause of the increases in CHF with increasing degree of subcooling. The measurements of saturated boiling on the inclined surface confirmed that the orientation of the heating surface greatly affects the period it takes for vapor masses to pass, but it negligibly affects the liquid layer thickness. This suggests that the decrease in CHF with increasing angle of inclination is primarily caused by the lengthening of the duration of vapor mass passage.