Al2O3/H2O纳米流体球形下封头表面气泡脱离行为模拟

压力容器内滞留（IVR）策略可在反应堆发生严重事故后,有效地将堆内熔融物滞留在压力容器内,是防止放射性物质外泄的关键技术。纳米流体是将粒径小于100 nm的固体颗粒加入到基液中以提高换热特性的稳定悬浮液,其热物性以及换热特性与传统固液悬浮液相比有较大区别,适宜的纳米流体种类及配比可强化换热。本文采用引入了无网格对流格式的移动粒子半隐法（MPS方法）研究了体积份额为1.0%的Al₂O₃/H₂O纳米流体和纯水中加热面朝下时气泡在加热面上的成长、脱离以及附着形成气膜的过程,探索了气泡脱离的临界角度及其影响因素,为加速纳米流体的工业应用、增强IVR能力提供理论基础。     

Effects of water based Al2O3, TiO2, and CuO nanofluids as the coolant on solid and annular fuels for a typical VVER-1000 core

In this paper, the effect of nanofluids as the coolant on solid and annular fuels for a typical VVER-1000 core is analysed. The considered nanofluids are various mixture composed of water and particles of Al₂O₃, TiO₂, and CuO. The fuel rod is modeled using a CFD code. To validate the calculated results, the present results of solid fuel with nanofluid and pure water are compared with other studies which have been done with visual FORTRAN language, DRAGON/DONJON code, COBRA-EN code and the mentioned analytical approaches have been validated by comparing with the final safety analysis report (FSAR). The comparison of the calculated results shows that the results are in good agreement with other studies. Thus, the accuracy of the validation is satisfactory. Radial and axial temperature distributions in various components of fuel are illustrated. Moreover, the temperature distributions of the fuel, clad and coolant are described for water based Al₂O₃, TiO₂, and CuOnanofluids in solid fuel and annular fuel. The results are compared with base fluid and it is concluded the nanoparticles of Al₂O₃have good properties in comparison with other nanoparticles. By using the nanofluids, the central fuel temperature is reduced and the temperature of the coolant is increased. In addition, by increasing the heated surfaces in annular fuel, the heat flux on these surfaces is reduced, the minimum departure from nucleate boiling ratio (MDNBR) margin is increased, and therefore the critical heat flux can be increased. Finally, it is concluded the use of the annular fuel instead of solid fuel and also the use of the nanofluids as coolant in the core of the reactor, security and efficiency of the nuclear power plant will be increased.     

Neutronic simulation of water-based nanofluids as a coolant in VVER-1000 reactor

The main goal of this study is to perform the neutronic simulation of nanofluids application to reactor core. The variation of the Bushehr VVER-1000 reactor primary neutronics parameters is investigated with using different nanofluids as coolant. In the present neutronic simulation, water-based nanofluids containing various volume fractions of Al₂O₃, Si, Zr, TiO₂, CuO, Ti, Cu and Ag nanoparticles are investigated. Optimization of type and volume fraction of nanoparticles affects the reactor neutronic characteristics. The results achieved by using WIMS and CITATION codes, show that below 0.1 percent volume fraction of Al₂O₃ is the optimum nanoparticle for normal operation and Ag/water nanofluid is suggested to use as a reactor safety enhancement tool.     

The quenching behavior of aqueous nanofluids around rods with high temperature

The quenching behavior of aqueous nanofluids containing various volume fractions of Al₂O₃, SiO₂, TiO₂ and CuO nanoparticles is experimentally investigated around high temperature brass rod (diameter 20 mm × 75 mm). The experiments are performed at saturated conditions under atmospheric pressure. The results show that the quenching process is strongly dependent on the kind of nanoparticle, as well as its volume fraction. Although it is not observed from the first run in nanofluids, the quenching time is considerably shortened during the repetitive quenching tests. After the repetition tests in nanofluids, a nanoparticles porous layer occurs on the quenched surface and thus, the film boiling vanishes. The surface contact angles and the surface roughness of the quenched surfaces are measured. The results show that contact angles decreases and the surface roughness increases. It can be concluded that the primary reason of critical heat flux enhancement is the change of the surface characteristics due to the porous layer. In addition, the results also showed that there is no significant change in the nucleate boiling heat transfer.     

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.     

Thermal–hydraulic modeling of water/Al2O3 nanofluid as the coolant in annular fuels for a typical VVER-1000 core

In this paper, a thermal–hydraulic analysis of nanofluid as the coolant is performed in a typical VVER-1000 reactor with internally and externally cooled annular fuel. The fuel assembly for annular case with 8 × 8 arrays is considered for annular pin configuration. The considered nanofluid is a mixture composed of water and particles of Al₂O₃ with various volume percentages. The fuel rod is modeled using a CFD code. To validate the calculated results, the present results of solid fuel with nanofluid and pure water are compared with other studies which have been done with visual FORTRAN language, DRAGON/DONJON code, COBRA-EN code and the mentioned analytical approaches have been validated by comparing with the final safety analysis report (FSAR). The comparison of the calculated results shows that the results are in good agreement with other studies. Thus, the accuracy of the validation is satisfactory. Moreover, the temperature distributions of the fuel, clad and coolant are described for water/Al₂O₃ nanofluid in solid fuel and annular fuel. It is observed that as the concentration of Al₂O₃ nanoparticles increases, due to higher heat transfer coefficient of Al₂O₃ nanofluid, the temperature of the coolant is increased and the central fuel temperature is reduced. Thus, it improves margin from peak fuel temperature to melting. Finally, it is illustrated the use of the annular fuel instead of solid fuel in core of the reactor, security and efficiency of the nuclear power plant will be increased.     

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。     

A review of nanofluid heat transfer and critical heat flux enhancement—Research gap to engineering application

As a novel strategy to improve heat transfer characteristics of fluids by the addition of solid particles with diameters below 100 nm, nanofluids exhibit unprecedented heat transfer properties and are being considered as potential working fluids to be used in high heat flux systems such as electronic cooling systems, solar collectors, heat pipes, and nuclear reactors. The present paper reviews the state-of-the-art nanofluid studies on such topics as thermo-physical properties, convective heat transfer performance, boiling heat transfer performance, and critical heat flux (CHF) enhancement. It is indicated that the current experimental data of nanofluids thermal properties are neither sufficient nor reliable for engineering applications. Some inconsistent or contradictory results related to thermo-physical properties, convective heat transfer performance, boiling heat transfer performance, and CHF enhancement of nanofluids are found in data published in the literature. No comprehensive theory explains the energy transfer processes in nanofluids. To bridge the research gaps for nanofluids' engineering application, the urgent work are suggested as follows. (1) Nanofluid stability under both quiescent and flow conditions should be evaluated carefully; (2) A nanofluid database of thermo-physical properties, including detailed characterization of nanoparticle sizes, distribution, and additives or stabilizers (if used), should be established, in a worldwide cooperation of researchers; (3) More experimental and numerical studies on the interaction of suspended nanoparticles and boundary layers should be performed to uncover the mechanism behind convective heat transfer enhancement by nanofluids; (4) Bubble dynamics of boiling nanofluids should be investigated experimentally and numerically, together with surface tension effects, by considering the influences of nanoparticles and additives if used, to identify the exact contributions of solid surface modifications and suspended nanoparticles to CHF enhancement in boiling heat transfer. Once we acquire such details about the above key issues, we will gain more confidence in conducting application studies of nanofluids in different areas with more efficiency.     

过冷流动沸腾中汽泡脱离直径的理论研究

汽泡脱离直径模型是壁面沸腾计算中的一个重要子模型。为了正确预测过冷流动沸腾中的壁面传热情况,研究结合新改进的汽泡生长模型,采用力平衡方法对过冷流动沸腾中的汽泡脱离直径进行了模拟。汽泡生长模型同时考虑了微液层、过热层和汽泡顶部过冷液体层对汽泡生长所做的贡献,并采用饱和沸腾与过冷沸腾2个实验对其进行了验证,结果表明预测曲线与实验值吻合良好。另外,选取了3个过冷流动沸腾实验来验证汽泡脱离直径模型,模拟结果均在可接受的误差范围内。     

Thermal-hydraulic characteristics of a single-phase natural circulation loop with water and Al2O3 nanofluids

A natural circulation system operates on the basis of natural laws like gravity and buoyancy. Although natural circulation is a benign gift of nature for applications to several heat removal systems due to their simplicity in design, elimination of hazards related to pumps, better flow distribution, cost reduction, etc. however, the potential threat of flow instabilities still eludes for its wide applications. Although addition of local losses (orificing) may suppress instabilities, however, it is accompanied by significant flow reduction which is detrimental to the natural circulation heat removal capability. In this paper, we have demonstrated experimentally, with nanofluids, not only the flow instabilities are suppressed but also the natural circulation flow rate is enhanced. The increase in steady natural circulation flow rate due to addition of nanoparticles is found to be a function of its concentration in water. The flow instabilities are found to occur with water alone only during a sudden power addition from cold condition, step increase in power and step decrease in power (step back conditions). With a small concentration of Al₂O₃ nanofluids, these instabilities are found to be suppressed significantly.     

Numerical investigation of boiling regime transition mechanism by a Lattice–Boltzmann model

A numerical study has been performed to investigate the hydrodynamic aspects of the pool boilingon horizontal-, vertical- and downward-facing surfaces. The FlowLab code, which is based on a Lattice–Boltzmann (LB) model of two-phase flows, is employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), are implemented through the fluid–fluid (G_σ) and fluid–solid (G_t) interaction potentials. The model is found to express a linear relation between the macroscopic properties (σ, β) and microscopic parameters (G_σ, G_t). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of two-phase flow regime transition mechanism are investigated for different surface–coolant configurations. Results of the LB simulation clearly demonstrate that not only the bubble nucleation site density (related, e.g. to the heater surface condition and heat fluxes), but also the surface position have a profound effect on the flow regime (pool boiling) characteristics. The results of the LB simulation of hydrodynamics of two-phase flow on the horizontal surface provide the pictures quite similar to the experimental observation for saturated pool boiling. Two mechanisms of flow (boiling) regime transition on the vertical surface are predicted for the local bubble coalescence at bubble generation site and the downstream bubble coalescence. On the downward-facing surfaces, friction between bubbles and the surface wall is found to significantly enlarge the bubble size prior the bubble slip upwards. This behavior is responsible for the earlier bubble coalescence, and therefore, lowers the maximum heat removal rate, in a similar regime of nucleate boiling on a downward-facing surface.     

Recovery of neutron-irradiation-induced defects of Al2O3, Y2O3, and yttrium-aluminum garnet

SiC fiber-reinforced SiC matrix composites (SiC_f/SiC) are considered as one of the candidates for blanket materials in future fusion reactors and as an advanced fuel cladding material for next-generation fission reactors. Generally, the densification of SiC needs sintering additives and oxides such as Al₂O₃, Y₂O₃, and yttrium-aluminum garnet (YAG, Y₃Al₅O₁₂), which are frequently added to SiC. However, the effects of neutron irradiation on sintering additives are still unclear. In this study, we performed the neutron irradiation of Al₂O₃, Y₂O₃, and YAG at fluences up to 2.0–2.5 × 10²⁴ n/m² (E > 0.1 MeV) at 60–90 °C. The isochronal recovery of the macroscopic volume of Al₂O₃ against annealing temperature showed smooth and continuous shrinkage at a temperature of up to 1200 °C, and the volume slightly increased above that temperature. In contrast, the volume of Y₂O₃ showed quick shrinkage at the low temperature range, and slower and smooth recovery was observed up to ～1100 °C. In the case of YAG, the recovery of volume occurred in a step-wise manner at 600–750 °C, and continuous shrinkage occurred at temperatures lower and higher than that temperature range. The activation energies for the macroscopic volume recoveries of three oxides were obtained from the Arrhenius plots of the rate coefficients. Two-stage recovery was observed for Al₂O₃, whereas more complicated recovery processes were suggested for Y₂O₃ and YAG.     

CHF enhancement by honeycomb porous plate in saturated pool boiling of nanofluid

One strategy for severe accidents is in-vessel retention (IVR) of corium debris. In order to enhance the capability of IVR in the case of a severe accident involving a light-water reactor, methods to increase the critical heat flux (CHF) should be considered. Approaches for increasing the IVR capability must be simple and installable at low cost. Moreover, cooling techniques for IVR should be applicable to a large heated surface. Therefore, as a suitable cooling technology for required conditions, we proposed cooling approaches using a honeycomb porous plate for the CHF enhancement of a large heated surface in a saturated pool boiling of pure water. In this paper, CHF enhancement by the attachment of a honeycomb-structured porous plate to a heated surface in saturated pool boiling of a TiO₂-water nanofluid was investigated experimentally under atmospheric pressure. As a result, the CHF with a honeycomb porous plate increases as the nanoparticle concentration increases. The CHF is enhanced significantly up to 3.2 MW/m² at maximum upon the attachment of a honeycomb porous plate with 0.1 vol.% nanofluid. To the best of the author's knowledge, under atmospheric pressure, a CHF of 3.2 MW/m² is the highest value for a relatively large heated surface having a diameter exceeding 30 mm.     

常压下ATF锆合金包壳Cr涂层表面饱和池式沸腾气泡行为实验研究

铬（Cr）涂层锆合金包壳是最有前途的耐事故燃料（ATF）的新型包覆材料之一,对其表面的气泡动力学进行研究有助于评估是否具有更好的传热性能。在常压下的Cr涂层锆合金包壳池式沸腾实验装置中对不同工艺方法下制备的Cr涂层锆合金包壳进行实验,研究了粗糙度等表面状态对气泡产生、长大以及脱离等气泡行为的影响。结果表明,气泡接触角与Cr涂层表面粗糙度有关,粗糙度越大,表面气泡接触角越小;不同涂层工艺下制备的4种Cr涂层锆合金包壳样件表面的气泡脱离直径范围为1.256～1.446 mm,气泡脱离频率范围为29.99～50.97 Hz;气泡脱离直径与粗糙度呈负相关,脱离频率与粗糙度呈正相关;气泡脱离直径预测模型与实验数据之间的偏差为±6%,脱离频率预测模型与实验数据之间的偏差为±3%。     

Pressure effect on CHF enhancement in pool boiling of nanofluids

This paper investigates critical heat flux (CHF) in saturated pool boiling for water and TiO₂ nanofluid on a 7-mm-diameter vertical copper surface at pressures of 0.1–0.8 MPa. The nanofluid was prepared by dispersing 0.002 wt% TiO₂ nanoparticles in deionized water. The CHF of the nanofluid was enhanced about two times over that of water boiling at atmospheric pressure. With the increasing pressure, however, the CHF enhancement with the nanofluid decreases, and almost disappears at 0.8 MPa.     

水基磁性流体池沸腾传热强化的实验研究

通过水和水基磁性流体池沸腾传热的对比实验,确定了水基磁性流体强化沸腾传热的效果,并进行了机理分析。实验结果显示,热流密度相同时,水基磁性流体的沸腾换热系数比水至少增强2倍,施加磁场可进一步强化沸腾传热,增强倍数可超过5倍。通过分析磁场对磁性流体中沸腾汽泡的影响,认为施加磁场有使汽泡脱离直径减小,生长速度加快和脱离频率增加的作用。     

Volume change of Al2O3 and MgAl2O4 induced by 14-MeV neutron irradiation

Volume change of Al₂O₃ and MgAl₂O₄ induced by irradiation with 14-MeV neutrons at 50°C has been measured. It is shown that the volume change of A1₂O₃ is anisotropic and is larger than that of MgAl₂O₄ about a factor of five. The result for MgAl₂O₄ is compared with that of fission neutron irradiation.     

Effect of carbide on Al2O3/B4C burnable poison sintered in Ar

Al₂O₃–14 wt%B₄C composites with 0–1 wt% C addition were sintered in Ar at 1550–1650 °C. The influence of the C additive on the B loss, densification behavior, and microstructure of the Al₂O₃–B₄C composites were investigated. The results show that there are B₂O₃, H₃BO₃ and Al₁₈B₄O₃₃ exist between Al₂O₃ and B₄C interface, which result in B loss because of B₂O₃'s high vapor tension at above 1500 °C. The presence of Al₁₈B₄O₃₃ grains formed by chemical reaction of Al₂O₃ with surface oxides on B₄C inhibit the densification of pellets by reducing the specific free surface energy of the Al₂O₃. However, the added C eliminates those oxides to reduce B loss because of its higher activity than B₄C, and it also coarsens Al₂O₃ grain although the density of pellets is decreased by gas products.     

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.     

纳米流体饱和池沸腾传热及CHF模型研究

纳米流体传热是一种新兴的换热方式,目前研究多集中在单相研究领域,而纳米流体沸腾传热特性的相关研究较少。本文采用热通量拆分方法,将壁面传热分为4种模型(微液层蒸发、气泡脱离前的瞬态导热、气泡脱离后的瞬态导热以及微对流换热),对这4种模型的传热量分别进行计算,结合壁面核化中心密度等参数,计算了壁面平均传热系数和CHF。结果表明,本文计算结果与国际上已发表的实验数据符合较好,充分验证了所建立模型的合理性。