Adding surfactant can improve the stability of hybrid nanofluids. However, positive or negative effects of the surfactant on the thermophysical properties of hybrid nanofluids should be further ...investigated. In the present study, effects of different surfactants, including SDS, PVP and CTAB, with various mass concentrations and temperatures on the stability of Al2O3-CuO/Water (W) and Al2O3-TiO2/Water (W) hybrid nanofluids are investigated. Results show that increment of surfactant concentration improves the stability and viscosity. Then obtained results from UV–Vis Spectrophotometer, TEM and sedimentation methods are compared, and PVP surfactant has the highest degree of stability, with the minimal sedimentation up to 25 days. At the optimum PVP surfactant concentrations of 0.005 wt% and 0.01 wt%, the maximum thermal conductivity enhancement is 12% and 14% at 60 °C for Al2O3-CuO/W and Al2O3-TiO2/W hybrid nanofluids. Therefore, adding even little amount of surfactant can improve the stability and thermal conductivity of hybrid nanofluids.
Synergistic mechanism of enhanced thermal conductivity and the corresponding economic analysis are analyzed to select a reasonable nanofluid. To this end, effects of nanoparticles (NP) and base ...fluids (BF) mixture ratios on thermal conductivity enhancement (Al2O3-TiO2/EG-W, Al2O3-CuO/EG-W and Al2O3-Cu/EG-W) are carried out at 1.0 vol% and temperature of 20 °C to 60 °C. Results show that the maximum thermal conductivity for Al2O3-TiO2/EG-W nanofluid increases from 0.482 (20 °C) to 0.543 W/(m.K) (60 °C). Moreover, synergistic mechanism of thermal conductivity enhancement is caused by dispersing different sizes of nanoparticles to form order arrangement of liquid molecules around nanoparticles, results in more compact solid-liquid interface and reasonable heat transfer network. Then, sensitivity analysis indicates that NP mixture ratio is a more important factor to determine the alignment of nanoparticles. Finally, Al2O3-CuO/EG-W nanofluid is an efficient fluid in laminar flow, while Al2O3-EG/W single nanofluid is the best choice for turbulent flow from economic analysis.
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•Effect of nanoparticles and base fluid on thermal conductivity is investigated.•Synergistic mechanism is analyzed and guideline is provided to select nanofluids.•Expand range of PPF to estimate economy in the convective heat transfer.•Maximum thermal conductivity of hybrid nanofluids occurs for the ratio of 20:80.
•The behavior of cladding melting and melt migration in lead-cooled coolant was studied.•The viscosity function was used to evaluate the cladding viscosity at different enthalpies.•Surface tension ...promoted the molten cladding to adhere on fuel rod surface.•The “center-peaked distribution” case was easy to cause re-solidified metal in the upper part of the fuel rod.•Flowing coolant accelerated the cooling of melt and softened cladding.
The behavior of cladding melting and melt migration is an important phenomenon during the early stage of core disruptive accidents (CDA) in lead-cooled fast reactors (LFRs), which can affect the accident evolution trend and severity. In this paper, the Moving Particle Semi-implicit (MPS) method with the advantage of phase-interface capture was employed to further reveal this key phenomenon. In the simulation, the cladding was regarded as a high-viscosity fluid, and the viscosity function related to solid fraction was used. Meanwhile, the modified algorithm with the viscosity term being placed after the pressure term was adopted to ensure the cladding was stationary before the temperature reached the melting point. The computational model was verified by the experiments of tin cladding tube melting and melt migration in seven-pin bundle. After that, the effects of surface tension coefficient, fuel initial temperature and coolant flow speed on cladding melting and melt migration in lead-bismuth coolant were investigated with the improved MPS method. The results indicated that surface tension promoted the melt to adhere on fuel rod surface and the melt-coolant contact area decreased when the surface tension was introduced. The fuel initial temperature with “center-peaked distribution” caused a temperature gradient in the flow channel and led to re-solidified metal at the upper part of the fuel rod. Compared with the “uniform distribution” case, the size of molten droplet in “center-peaked distribution” case was smaller and more fragmented droplets could be observed. When the lead-bismuth coolant was flowing, the enhanced heat transfer accelerated the cooling of melt as well as softened cladding, and the solidified melt blocked the flow channel at the midstream and downstream regions.
The properties of water (W)/ethylene glycol (EG) mixtures vary significantly with the proportion of EG and temperature, so it is suitable to use such fluids as exchange heat mediums in a waste heat ...system with temperature fluctuations. The experiments were conducted with 1.0 wt% Cu/Al2O3- EG/W hybrid nanofluids at temperatures ranging from 20 to 50 °C, where the base fluid (EG/W) mixture ratio was varied from 20:80 to 80:20. To search individuals which contain optimal weights and thresholds, a genetic algorithm (GA) and a mind evolutionary algorithm (MEA) coupled with a back-propagation neural network (GA-BPNN and MEA-BPNN, respectively) were used to improve the accuracy in the predicted thermal conductivity. The results show that the thermal conductivity increases nonlinearly with the ratio of water to ethylene glycol and temperature, due to the higher thermal conductivity of water and stronger collision frequency between molecular and nanoparticles. Binary Polynomial Regression (BPR) was fit with (coefficient of determination) R2 = 0.9984 as functions of temperature and mixture ratio. Comparisons of the prediction performance and capability of BPR, the performance of R2 increases by 0.11% and 0.13% for GA-BPNN and MEA-BPNN. It indicates that the combined BPNNs both predicate more accurately, particularly MEA-BPNN has the highest prediction accuracy.
A compound method is established by adding Al2O3-TiO2/Water (W) nanofluids and porous media in a microchannel to reduce pressure drop and enhance heat transfer in the limited heat dissipated area. ...Effects of porosity and hybrid nanofluids are numerically investigated in the laminar region. Pressure drop, Nusselt number, velocity and temperature fields are used as indicators to evaluate the characteristics of flow and heat transfer. Furthermore, the analytical methods including thermodynamic evaluation and nanoparticle migration were performed to reveal the essential mechanism of heat transfer enhancement. Compared with non-porous microchannels, the maximum reduction of porous microchannels is 9.73% (ε = 0.5), 8.3% (ε = 0.6) and 6.8% (ε = 0.7) for pressure drop, while the maximum agument is 23.53% (ε = 0.5), 20.59% (ε = 0.6) and 16.67% (ε = 0.7) for Nusselt number. The permeability of fluid reduces pressure drop and enhances heat transfer in the porous region. The maximum heat transfer irreversibility reduces up to 64%, and exergy efficiency reaches 88% by adding nanofluids and porous media. Nanoparticle migration originating from thermophoresis affects the concentration distribution near the wall, and cold-hot fluids fully mix in the porous region, thereby increasing heat transfer. The effect of thermophoretic diffusion of dilute nanofluids in micro-scale convective heat transfer is higher than that in the macro-scale one.
Based on previous works, a performance evaluation plot (PEP) is presented to theoretically evaluate the techniques used in the determination of various thermophysical properties of fluids in energy ...saving. From it, PEP is divided into “three lines and four zones”. Zone-1 indicates enhanced heat transfer without energy-saving, and enhanced heat transfer are in Zone-2, Zone-3 and Zone-4 with energy-saving for a fixed pumping power, pressure drop and flow rate, respectively. Then, microchannel heat sinks (MCHS) with cavities and different ribs arrangements were designed and simulated by using water-Al2O3 nanofluids. Based on the estimation indicators of comprehensive thermal performance (PEP and the thermal enhancement factor η), all working points located in the Zone-3 and 4 indicate that the increase in the heat transfer ratio is higher than that in the friction factor. The maximum enhancement of η is 2.2517 and it is measured in an odd-symmetric ribs structure (Channel D) with Reynold number equal to 582. The reasons are that the arrangement of odd-symmetric ribs continuously interrupts the development of the flow and thermal boundary layers. Moreover, cavities facilitate the full mixing of cold and hot fluids and reduce pressure drop.
A new parameter (uniformity coefficient of nanoparticles distribution (UCND)) was introduced for quantitatively estimating the uniformity of nanoparticles distribution in nanofluids. Comparisons ...between UCND and measurements of the size distribution of nanoparticles show that the average diameters of nanoparticles in nanofluids (46:54, 50:50, 52:48, and 54:46) are 43, 54, 52, and 39 nm and the UCND are 0.9251, 0.9102, 0.9275, and 0.9513. They indicate that a mixture ratio of 54:46 is the most uniform with the lowest average diameter of nanoparticles and highest UCND. Moreover, nanofluids with the greatest uniformity exhibit the highest thermal conductivity with 0.426 W/(m.K) and relatively low viscosity with 3.761 mPa.s at 54:46, which is highly desirable for engineering applications. Finally, ANN models describing the thermal conductivity and viscosity were developed. The data predicted with the ANN models was agree with the experimental data, with (coefficient of determination) R2 = 0.9846 and R2 = 0.9755 for thermal conductivity and viscosity.
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•A parameter, UCND, was derived for quantitative estimating uniformity of nanofluids.•The effect of uniformity on thermophysical properties was investigated.•ANN models for predicting thermophysical properties were presented.
The melt migration and solidification behavior on the surface of metal structure and in the debris voids is a key phenomenon of debris bed melting, which has an important impact on the assessment of ...reactor pressure vessel damage and melt leakage. In this study, the moving particle semi-implicit (MPS) method with high-order discretization scheme was coupled with a surface tension model that was established based on interface reconstruction. The migration and solidification behavior of UO2–ZrO2 and Fe–Zr melts on the metal structure surface and in the debris voids were investigated with the improved MPS method. The effects of temperature, velocity, contact angle and droplet diameter on the melt migration and solidification were analyzed. The results showed that the molten UO2–ZrO2 droplet on the metal structure surface presented three stages with solidification rate decreasing, while the solidification rate of molten Fe–Zr droplet had little change. The maximum spread factor and solidification rate increased with the increase of droplet falling velocity, but the spread factor was independent of the melt type at the same velocity. The high-temperature molten droplet penetrated the voids formed by the debris with diameter of 5 mm more easily compared to the voids formed by the debris with diameter of 3 mm. The influence of contact angle on the migration of molten droplet with initial velocity was small, and the maximum difference in droplet mass fraction was about 6%. Three groups of molten droplets with different diameters penetrated the voids, and the average increments of penetration mass fraction were 2.3%, 2.7% and 5.8%, respectively, with the increase of velocity. Near the bottom of the debris bed, the molten droplet without initial velocity solidified and blocked the flow channel, but the molten droplet with initial velocity of 0.5 m/s penetrated through the debris voids. The droplet with initial velocity of 0.5 m/s had a faster solidification rate in the voids compared to the droplet without initial velocity, and the droplet solidification mass fraction was 96.27% when the initial melt temperature was 1410 K.
Hydraulic fragmentation of melt jet is an important phenomenon during fuel-coolant interaction (FCI) of nuclear reactor severe accident. If the melt jet has not broken up completely before contacting ...the lower head, it may ablate the vessel wall and a compact debris bed may form. The jet breakup length and melt-coolant contact area are important parameters describing melt fragmentation. In this study, therefore, a numerical model for analyzing melt jet hydraulic fragmentation was developed and validated against the experiment. Through the analysis of UO2 melt jet fragmentation in coolant under various conditions, the relationship between the morphological structures and axial velocities of melt jet was clarified, and an accurate criterion for judging jet breakup length was proposed. The effects of initial melt jet velocity and diameter on the dimensionless breakup length were investigated, and an empirical equation for calculating breakup length was established with a maximum deviation about 10%. The dimensionless contact area of melt and coolant increased as the melt was injected into the coolant, but it finally reached a stable value for each case. It increased as the increase of initial injecting velocity, but was not affected by the initial melt jet diameter. The empirical equation for calculating the dimensionless contact area was established as well, and the prediction deviation was less than 10%.
•Hydraulic fragmentation of melt jet in coolant was investigated under different Weber numbers.•The relationship between morphological structures and axial velocities of the melt jet was clarified.•Melt jet breakup length was at the point where the wave peak value of dimensionless velocity less than 1.0.•An empirical equation for calculating breakup length was established with the maximum deviation about 10%.•An empirical equation for calculating the dimensionless contact area was established with the prediction deviation less than 10%.
