Mohammad Mastiani

Entropy generation and natural convection of nanoparticle-water mixture (nanofluid) near water density inversion in an enclosure with various patterns of vertical wavy walls

Entropy generation due to laminar natural convection of Cu-water nanofluid near the density maximum of water in a two-dimensional enclosure with various patterns of vertical wavy walls is investigated numerically. In order to study the nature of irreversibility in terms of entropy generation in the presence of nanoparticle near water density inversion, the second law of thermodynamics is applied. The governing equations are formulated using both the Boussinesq and non-Boussinesq homogenous models and solved on a non-uniform mesh using a pressure-based finite volume method. The calculations are performed for Ra number of 10^5, 10^6 and a range of nanoparticle volume fraction from 0 and 0.05. The results are presented and discussed in terms of streamlines, isotherms patterns, contours of local entropy generation, average Nusselt number and average entropy generation. It was found that both the density inversion and the presence of nanoparticles play a significant role in the flow field structure, heat transfer characteristics and entropy generation. It was concluded that the Boussinesq approximation gave rise to the higher average heat transfer rate and entropy generation as compared to non-Boussinesq approximation. In addition, the average Nusselt number and entropy generation were found to decrease as the volume fraction of nanoparticle increased. Finally, the formation of bi-cellular flow structure substantiates the effective role of density inversion of water in the free convection characteristics.

NUMERICAL STUDY OF SOLIDIFICATION OF A NANO-ENHANCED PHASE CHANGE MATERIAL (NEPCM) IN A THERMAL STORAGE SYSTEM

The effects of nanoparticle dispersion on solidification of a Cu-n-hexadecane nanofluid inside a vertical enclosure are investigated numerically for different temperatures of the left vertical wall. An enthalpy porosity technique is used to trace the solid-liquid interface. The resulting nanoparticle-enhanced phase change materials (NEPCMs) exhibit enhanced thermal conductivity in comparison to the base material. The effect of the wall temperature and nanoparticle volume fraction are studied in terms of the solid fraction and the shape of the solid-liquid phase front. It has been found that a lower wall temperature and a higher nanoparticle volume fraction result in a larger solid fraction. The increase in the heat release rate of the NEPCM shows its great potential for diverse thermal energy storage applications.

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

Aqueous microdroplet generation involving high inertial air flow inside a T-junction microchannel was studied numerically. The volume of fluid method was employed to track the interface between two immiscible fluids: water and air. The effects of high inertial air flow on the water droplet generation were investigated. At various Re and Ca numbers, unique flow regime mapping including squeezing, dripping, jetting, unstable dripping, and unstable jetting and their transitions were determined. Unstable dripping and unstable jetting flow regimes are new regimes which have not been previously reported in the liquid–liquid system. The flow structure in these two flow regimes is affected by the high inertial nature of the continuous phase which is negligible in the conventional liquid–liquid system. It was found that the stable aqueous droplets are generated in the squeezing and dripping flow regimes. On the other hand, the unstable dripping flow regime is unable to sustain spherical droplets as they travel downstream. In the unstable jetting flow regime, a stream of water is fragmented into multi-satellite droplets and threads of different sizes as it moves downstream. The behavior of the unstable jetting flow regime cannot be characterized due to the effect of high inertial air flow on the water stream. The results show that droplet size increases as Ca and Re numbers increase and decrease, respectively. As both Ca and Re numbers increase, droplet generation frequency increases, reaching its maximum at 223 Hz. Finally, the effect of different contact angles at 120–180° on droplet size, detachment time, and droplet generation frequency was investigated. The results of this research provide valuable insight into the understanding of high throughput oil-free aqueous droplet generation within a gas flow.

Microbubbles Loaded with Nickel Nanoparticles: A Perspective for Carbon Sequestration

This work reports a microfluidic study investigating the feasibility of accelerating gaseous carbon dioxide (CO2) dissolution into a continuous aqueous phase with the use of metallic nickel (Ni) nanoparticles (NPs) under conditions specific to carbon sequestration in saline aquifers. The dissolution of CO2 bubbles at different pH levels and salinities was studied to understand the effects that the intrinsic characteristics of brine in real reservoir conditions would have on CO2 solubility. Results showed that an increased shrinkage of CO2 bubbles occurred with higher basicity, while an increased expansion of CO2 bubbles was observed with a proportional increase in salinity. To achieve acceleration of CO2 dissolution in acidic brine containing high salinity content, the catalytic effect of Ni NPs was investigated by monitoring change in CO2 bubble size at various Ni NPs concentrations. The optimal concentration for the Ni NPs suspension was determined to be 30 mg L-1; increasing the concentration up to 30 mg L-1 showed a significant increase in the dissolution of CO2 bubbles, but increasing from 30 to 50 mg L-1 displayed a decrease in catalytic potential, due to the decreased translational diffusion coefficient that occurs at higher concentrations. The optimal additive concentration of Ni NPs was tested with variations of solution at acidic and basic conditions and different levels of salinity to reveal how effectively the Ni NPs behave under real reservoir conditions. At the acidic level, Ni NPs proved to be more effective in catalyzing CO2 dissolution and can sufficiently alleviate the negative impact of salinity in brine.