Reduction Of Void Fraction Research Articles

Nanofluid application in post SB-LOCA transient in VVER-1000 NPP

This paper is the third in a series of four papers that the application of nanofluid as a coolant to improve heat transfer in a VVER-1000 nuclear reactor is investigated. In the first and second papers, neutronics and thermo-hydraulic behavior of nano-particles in normal operation mode of the reactor were reported. In this study, the effects of nanofluids in Small Break Loss of Coolant Accident (SB-LOCA) to complement the existing safety systems is investigated. During SB-LOCA transient, due to reduced mass flow rate and Reynolds number, flow boiling along with vapor formation around the fuel rods occur. The fuel assembly coolant channel of a VVER-1000 core is modeled using a CFD code and heat transfer coefficients, pressure drop and volume fraction distribution of phases are computed for water/Al2O3 nanofluid. We observe that with the escalation of heat transfer enhancement, due to reduction in void fraction, pressure drop along the channel is reduced. This new phenomenon of lower pressure drop along with heat transfer enhancement, would be a significant factor for the use of nanofluids during reactor SB-LOCA transients.

Mechanical properties of bamboo-epoxy composites a structural application

In this study, the physical and mechanical properties of bamboo fiber reinforced epoxy composites were studied. Composites were fabricated using short bamboo fiber at four different fiber loading (0 wt%, 15 wt%, 30 wt% and 45 wt%). It has been observed that few properties increases significantly with respect to fiber loading, however properties like void fraction increases from 1.71% to 5.69% with the increase in fiber loading. Hence, in order to reduce the void fraction, improve hardness and other mechanical properties silicon carbide (SiC) filler is added in bamboo fiber reinforced epoxy composites at four different weight percentages (0 wt%, 5 wt%, 10 wt% and 15 wt%) by keeping fiber loading constant (45 wt%). The significant improvement of hardness (from 46 to 57 Hv) at 15 wt%SiC, tensile strength (from 10.48 to 13.44 MPa) at 10 wt% SiC, flexural strength (from 19.93 to 29.53 MPa) at 5 wt%SiC and reduction of void fraction (from 5.69 to 3.91%) at 5 wt%SiC is observed. The results of this study indicate that using particulate filled bamboo fiber reinforced epoxy composites could successfully develop a composite material in terms of high strength and rigidity for light weight applications compared to conventional bamboo composites. Finally, SEM studies were carried out to evaluate fibre/matrix interactions.

A numerical study of pellets having both catalytic- and capture properties for SE-SMR process: Kinetic- and product layer diffusion controlled regimes

Sorption enhanced steam methane reforming (SE-SMR) is a promising process for the production of H2. The in situ removal of CO2 in the steam methane reforming (SMR) process shifts the reaction equilibrium according to the Le Chatelier's principle towards higher H2 production. Most of the earlier studies worked on the SE-SMR process using two different pellets (2-P): one for the reforming catalyst and the other one for the CO2 adsorbent. In this study, we propose a novel pellet design where both the catalyst and sorbent are put together in a support (1-P). In the 1-P system, the produced CO2 molecules due to the reforming reactions will be captured by the sorbent inside the pellet. Whereas in the 2-P system, the CO2 molecules have to transported in between the pellets to be captured by the sorbent. Hence, 1-P design has less resistance for mass transfer between the catalyst and sorbent active sites. In this study, the internal mass transport limitations of the 1-P design for the SE-SMR process have been investigated.The dusty gas multicomponent mass diffusion pellet model formulated for the SE-SMR process describes the evolution of species mole fractions, pressure, total concentration, temperature, fluxes, and convection within the voids of the porous pellet. The internal effectiveness factors of the SE-SMR process have been calculated for the SE-SMR process. The effective diffusivities are described according to the parallel pore- and random pore models. The carbonation reaction in the SE-SMR process is a gas–solid reaction system, which generally follows one out of two controlled regimes, namely the kinetic- and product layer diffusion (PLD) controlled regimes. A mathematical model based on the grain model was applied modelling the carbonation reaction. The change in pellet void-fraction due to the formation of the solid CaCO3, the diffusion of gaseous phase through the product layer, the structural changes of the spherical grains by the inclusion of variable diffusion coefficient and multiple carbonation/calcination cycles were considered in the pellet model. The effect of pellet diameter on the carbonation reaction rate is investigated to observe the complex mechanism of gas–solid reaction system.The 1-P performance is promising compared to the conventional 2-P design. Further, the reduction in void-fraction with the CaO conversion, product layer diffusion, pellet diameter and multiple cycles are all important effects in the SE-SMR process because these mechanisms influence on the reaction rate, capture capacity, and the effectiveness factors.

Structural investigation of nC-Si/SiOx:H thin films from He diluted (SiH4 + CO2) plasma at low temperature

Detail structural characterizations of the nC-Si/SiOx:H films prepared from low temperature (300°C) SiH4 plasma, have been performed using various spectroscopic and microscopic probes, e.g., IR spectroscopy, ellipsometry, scanning electron microscopy and atomic force microscopy. The growth structure has been probed by Bruggeman effective medium approximation fitting to the ellipsometry data, considering a three-layer growth model, which has been identified by FESEM studies. It has been observed that with the reduction in pressure (p) the overall crystallinity improves along with the lowering in the incubation layer thickness, and the reduction of void fraction in the bulk as well as in the growth zone and surface layer. The maximum crystallinity in the bulk has been identified at p=0.6Torr, corresponding to the lowest roughness on the surface. Oxygen incorporation has been found to be favored at lower gas pressure in the plasma, along with simultaneous dehydrogenation of the silicon network which remains the key criteria for attaining enhanced nanocrystallinity. Plausible formation mechanism of the nC-Si/SiOx:H structure, activated by chemical reactions occurring in the He diluted (SiH4+CO2) plasma has been investigated.

A numerical study of a two property catalyst/sorbent pellet design for the sorption-enhanced steam–methane reforming process: Modeling complexity and parameter sensitivity study

In this study the performance of a combined catalyst/sorbent pellet design for the sorption-enhanced steam–methane reforming process has been investigated. Different mathematical model complexities have been studied and parameter sensitivity analyses have been performed. The mathematical pellet model formulated for the process describes the evolution of species mole fractions, pressure, total concentration, temperature, fluxes, and convection within the voids of the porous pellet. The effective diffusivites are described according to the parallel pore and random pore models, and the mass diffusion fluxes are described according to the Maxwell–Stefan and dusty gas models. Moreover, models proposed in the literature for void fraction changes, product layer diffusion resistance, and degeneration due to multiple carbonation/calcination cycles are employed. The simulated pellet effectiveness factor is a convenient parameter frequently used in modeling and simulations of chemical reactors indicating the relative importance of diffusion and reaction limitations. Thus, in this study, the effectiveness factor behavior due to different mathematical modeling assumptions and model parameter values is elucidated. The combined pellet performance is promising compared to the conventional two-pellet design. For further improved modeling and simulations of the pellet, the characterizing of the pore size distribution is important because of the Knudsen diffusion mechanism. Moreover, the reduction in void fraction with the CaO conversion, product layer growth, and degeneration due to multiple cycles are all important effects in the SE-SMR process because these mechanisms influence on the cycle life time, reaction rate, and capture capacity. For industrial applications, the pellet is only of interest in the capture kinetic controlling step, i.e. before the process becomes controlled by the product layer diffusion resistance.

Measurement and Analysis of Gas Bubble Evolution from an Anode in Electrolytic Cells

Measurement and analysis of oxygen gas bubbles generated from an anode in different aqueous electrolytic solutions have been studied at different applied currents (I). Bubble size distribution (BSD) was measured by using a high-speed digital camera, while corresponding voltage and current were recorded by an electrophoresis power supply. At low aqueous kinematic viscosity (νc < 0.017 cm2/s), the analyzed bubble size (b) is a strong function of νc; here, the slope (Δb/Δνc) below 0.017 cm2/s is an order of magnitude larger than that above it. In addition, at the higher νc, b is dictated predominantly by I. BSD can be described by the log-normal form of the continuous number frequency distribution. Here, the dimensionless median diameter for the distribution (β̅) and the variance (σ0) of the normalized log-normal distribution are 0.98 ± 0.029 and 0.27 ± 0.073, respectively. Further analysis indicates that ohmic resistance decreases as b increases because of the reduction in void fraction in solution. The resistivity of the solution can be described using the creeping-flow equation of motion showing that viscosity dictates the initial solution resistance.

Air–sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

Measurements of the air–sea fluxes of N 2 and O 2 were made in winds of 15–57 m s − 1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two “Lagrangian floats” measured O 2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single “EM-APEX float” profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N 2 concentrations were computed from GT and O 2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O 2 and N 2 and those measured was attributed to air–sea gas fluxes F BO and F BN. Second, the covariance flux F CO( z) = 〈 wO 2′〉( z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O 2′ is a high-pass filtered O 2 concentration and 〈〉( z) is an average over covariance pairs as a function of depth. The profile F CO( z) was extrapolated to the surface to yield the surface O 2 flux F CO(0). Third, a deficit of O 2 was found in the upper few meters of the ocean at the height of the storm. A flux F SO, moving O 2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio F BO/ F BN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O 2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of F CO( z), which peaks near 10 m instead of at the surface. At the highest winds O 2 and N 2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, F BO, F BN and F CO(0), are out of the ocean and downgradient.

Micro-Raman and ultraviolet ellipsometry studies on μc-Si:H films prepared by H2 dilution to the Ar-assisted SiH4 plasma in radio frequency glow discharge

Micro-Raman and ultraviolet ellipsometry studies have been performed on μc-Si:H films prepared by increasing the H2 dilution to the Ar-assisted SiH4 plasma in rf glow discharge. Combining the results obtained from the Bruggeman effective medium approximation fitting to the ellipsometry data and the Gaussian deconvolution of Raman spectra, it has been observed that the overall crystallinity improves along with the lowering in the incubation layer thickness, elimination of the amorphous component from the bulk, and the reduction of void fraction in the bulk as well as in the growth zone and surface layer. However, at very high H2/Ar ratio in the plasma a lowering in the crystallinity has been recorded along with an associated increase in the voids and an appearance of a small amorphous component in the bulk of the material. An increase in the voids arising at the grain boundary zone causes the hindrance to the crystallization in the network and is the result of higher H2 dilution, beyond a certain level, to the Ar-assisted SiH4 plasma, in the formation of a Si:H network. A correlation has been established between the data obtained from micro-Raman and ellipsometry in the structural characterization of a silicon–hydrogen system.

Eliipsometric Study of Rapid Thermal Annealed PECVD SiN Films

ABSTRACTA conventional parallel plate plasma enhanced chemical vapor deposition system using SiH4-NH3-N2 at 13.56MHz and 300°C was used to deposit SiN films on silicon substrates. Special care was used to assure an oxide free surface before deposition. The silane flow was varied to obtain stoichiometric Si3 N4 and silicon-rich films. Rapid thermal anneal (RTA) was performed for 30 sec at temperatures in the range 600°C — 900°C. Spectroscopic ellipsometry was used to characterize the films. Data was taken for 44 wavelengths (3200–7500Å range) at three angles of incidence on each sample. The structural and optical properties were obtained using several structural models of the film. The models assumed an effective medium approximation for the following three combinations of film constituents: Si3N4 and voids; Si3N4 and amorphous silicon (a-Si); Si3N4, voids and a-Si. In general, the three constituent model gave the best fits to the experimental data. In all films, we observed a decrease in the film thickness and void fraction as the RTA temperature increased. The a-Si percentage did not change vs. the RTA temperature. The reduction in void fraction for the silicon-rich samples was larger than for the Si3N4 film. The results are comparable to those measured by other methods on regular furnace annealed SiN films, i.e. annealing times of 30 minutes or more.