Journal of Heat and Mass Transfer Research
Volume:8 Issue: 2, Summer-Autumn 2021

This paper presents a series of numerical simulations of nanofluid natural convection inside an F-shaped enclosure equipped by heat source. A hybrid nanofluid consisting of Ag and MgO nanoparticles and water as base fluid was used. Lattice Boltzmann method (LBM) was applied and the effects of Raleigh number (103 ≤ Ra ≤ 106), solid volume fraction of nanoparticle (0 ≤ ϕ ≤ 0.02), and heat source location (0 ≤ S ≤ 0.9) on the flow field, distribution of temperature and heat transfer performance were analyzed according to streamlines, isotherms, and profiles of average Nusselt numbers. The results indicated that the average Nusselt number enhances by increasing the ϕ, although the addition of nanoparticles cannot change the flow pattern and the thermal field significantly. At low Ra, the effect of Ra on average Nu is weak. However, for high Ra, the heat transfer increases significantly by increasing the Ra. The position of heat source also affects the average Nu. The S = 0.6 is the best position of the hot obstacle for enhancing the heat transfer and S = 0.9 is the worst choice. This trend cannot be affected by Ra and ϕ.

This study simulated the flow and temperature field inside hollow bricks using well-known geometries to investigate the transient thermal behavior in both solid and fluid regions. To this end, a set of governing equations was solved simultaneously for an absorbing-emitting, isotropically scattering gas and solid region. To discretize the equations, the finite volume method was applied and the radiative transfer equation was calculated using the discrete ordinate method. Furthermore, the block off method is used to distinguish between solid and fluid media in the computational domain. The obtained results show that the rate of heat transfer is minimized in geometries having vertical rectangle sub cavities and their wall emissivity tends to zero. Moreover, the time required to reach steady state condition is an increasing function of total heat flux.

Tubular-Fin heat exchangers are a type of compact heat exchangers with prominent features like high levels of exchanged heat and less space occupancy. These heat exchangers are commonly used for exchanging heat between gas and liquid. In this study, for a tubular-fin heat exchanger, the heat transfer and pressure drop for circular and serrated fins with the triangular arrangement are numerically calculated and compared with the existing experimental data. A three-dimensional numerical study with the Reynolds mean-averaged Navier-Stokes (k-ε) model for turbulence is conducted. In the Reynolds range of 6000 to 25000, the performance of four types of fin geometry (serrated, semi-serrated, circular and semi-circular) are compared. The results show that the circular fin has the highest heat transfer rate, while the serrated fin has the highest reduction in the gas temperature. It is also found that the semi-circular fin has the highest thermal enhancement factor and the semi-serrated fin has the highest heat transfer coefficient. The results of the present study can be beneficial in the selection of optimal fins in a heat exchanger from both practical and economic aspects.

The main purpose of the current work is the analysis of the pulsating effect on the flow and heat transfer from impinging jet on a concave surface. In this way, the heat flux of 2300 W/m2 has been applied constantly on the surface with the radius of 120mm. The intermittent jet has been created for the frequency range of 1-100Hz by the pulsed-jet generator. Nusselt number distribution and flow field have been investigated for dimensionless distance of nozzle to surface (H/d) 2 to 5 and Reynolds number from 7000 to13000. The comparison of the experimental data with Numerical simulation shows that the k-ε RNG turbulence model is appropriately capable of predicting the Nusselt number on the concave surface under the pulsed jet impinging. Results of the present research indicate that, pulsating the jet is more effective on the concave surface in comparison with a flat surface. Also as compared to steady jet, when pulsating applies to the inlet jet with low frequency, reduction in Nusselt number is acquired. Furthermore, at each Re number and H/d, a threshold Strouhal number is found above which the Nusselt number of the pulsed jet is greater than that of the steady jet. Moreover, for the low nozzle to surface distance, Nu of stagnation point at low and high frequency is varied with Sr0.05 and Sr 0.15, respectively. At Re=10000, pulsating the impinging jet with f =100Hz causes an increase in the time and area-averaged of Nusselt number by 22% and 20% in comparison to steady jet at H/d=5 and 2, respectively.

In the numerical present study, MHD natural convection heat transfer of non-Newtonian power-law fluid in a two-dimensional enclosure with variable aspect ratio in the presence of heat absorption/generation is investigated by using the lattice Boltzmann method (LBM). The magnetic field is applied to the enclosure in uniform and periodic forms. The vertical wall and curved walls of the enclosure are at constant hot and cold temperature, respectively. The present work is validated with previous studies and the accuracy of the results is ensured. The effect of the Hartmann number, non-Newtonian power-law index, heat absorption/generation coefficient, aspect ratio of the enclosure and the type of magnetic field applied on the nature of flow and heat transfer are studied. The results show that increasing the non-Newtonian power-law index, Hartmann number and the heat absorption/generation coefficient reduce the Nusselt number. By increasing the heat generation/absorption coefficient, aspect ratio and decreasing the non-Newtonian power-law index, the effect of the magnetic field increases. Applying a magnetic field periodically compared to a uniform form leads to an increase of in Nusselt number and flow strength that this effect is greatest for shear thinning fluid and negligible for shear thickening fluid. Increase of Hartmann number and heat absorption/generation coefficient simultaneous leads to further decrease of average Nusselt number. This research can be helpful in the optimal design of heat transfer equipment.

We scrutinize the possibility of boosting the functionality of an isothermally heated horizontal capsule filled with the phase change material (PCM) as the thermal energy storage (TES) system. There is the conjugate heat transfer at the wall. The constrained inward melting of the water/ice (Pr=6.2) at Ra=105 in this system should be improved since the thermal conductivity of the base PCM is low. The thermal performance of the PCM may be manipulated by adding the magnesia (MgO) and hybrid Ag/MgO nanoparticles and by using the multilobed capsules. The iterative explicit lattice Boltzmann method (LBM) is implemented to investigate the effects of the nanoparticle loading, aspect ratio (AR) and circumference of the cross-section on the full melting time. The use of the 2-lobe capsule with the highest AR and increased circumference reduces the full melting time by 37% in contrast to the pure PCM melting in the cylindrical tube. Using the MgO nanoparticles with a lower loading (0.01) within the 2-lobe capsule diminishes the complete melting time for the pure PCM by 55%. It is the best nanofluid-based case when we consider the price of nanoparticles and the capacity of the TES system. The hybrid nanoparticles/PCM composites with (50:50) weight proportions are not prescribed as the increment of the viscosity of the PCM is further than that of the thermal conductivity of the PCM. To decrease the thermal conduction resistance at the bottom section of the horizontal cylindrical capsule, it is suggested to use the multilobed capsule for the pure PCM melting instead of the expensive single nanoparticles.

Over time, many studies have proven the advantages of using chilled ceiling systems as an assistant device for covering the barriers of the stratified air distribution systems. However, previous investigations are still insufficient, especially in analyzing indoor air quality. With the aid of the computational fluid dynamics techniques and Airpak software, we attempted to determine the possible effects of the chilled ceiling on the performance of displacement ventilation by evaluating contaminant removal, ventilation effectiveness, the freshness of air, and air change efficiency. In addition, we tried to find a solution to maintain the stratified form of the contaminant profile, and reduce the impact of the negative buoyant flow caused by the chilled surface. In order to investigate, the study was analyzed with variable exhaust vent location. The results indicated that the indoor air quality indexes had the best operation compared to the other cases when the exhaust vent was placed near the occupants. Actually, by local exhaust vent strategy, the inversion phenomenon caused by chilled surface was minimized and the contaminant concentration was safer at the inhaled zone compared to the other cases. In addition, the chilled ceiling had entirely adverse effects on the mean age of air, contaminant concentration, air change efficiency and ventilation effectiveness. Nevertheless, the disturbance effect was lower when the exhaust vent placed in the vicinity of heat/contaminant sources.

Recently, water scarcity has been intensified in arid areas because of depletion of freshwater resources, reduction of rainfall, population, and urbanization growth. Therefore, the need to use desalination systems has increased in these areas. On the other hand, the increase of building energy consumption for achieving enhanced thermal comfort has become a global crisis due to the depletion of fossil fuel resources and related environmental problems. In this study, a small-scale solar polygeneration system using photovoltaic-thermal solar collectors and hybrid humidification-dehumidification and reverse osmosis desalination units is proposed to supply the electricity, domestic hot water, space heating, and freshwater demands of a one-story house. The dynamic simulation of the system performance in the Hot-Dry climate zone is done using the TRNSYS-MATLAB co-simulator. The results indicate that using the thermal and electrical energy generated by the proposed system, the building annual energy consumption for providing domestic hot water, and space heating demands reduce 100% and 27.2%. The increase of the annual solar fraction of domestic hot water and space heating, because of using the electrical energy generated by the system, is 11.3% and 15.6%, respectively. The electricity and freshwater demand of the building is completely supplied by the proposed system and the excess electricity is sold to the grid. Economic analysis indicated that fuel saving cost of 29479 $ and water saving cost of 23779 $ are obtained during the life cycle of the system and the payback period is 3.75 years. The results show that the considerable energy savings are achieved using the proposed solar polygeneration system for providing the required electricity, heating, and fresh water demands of the residential buildings.

The erratic natural convection and mass transit flow of a viscous incompressible as well as electrically conducting fluid over an impetuously started immeasurable vertical flat plate immersed in the porous medium may occur in many engineering applications. By virtue of these various applications, in the present article, we have considered Soret and Dufour effects on erratic Magnetohydrodynamic natural convection fluid flow. With aid of similarity transformation, the partial differential equations of the flow, which are non - linear, are converted into a group of ordinary differential equations. Later on, those equations are linearized and solved by making use of numerical technique called implicit finite difference method. The derived results of velocity, temperature and concentration profiles are demonstrated through graphs for various values of the physical parameters, which influences the fluid flow. The temperature distribution of the fluid increases with increase in Dufour number, whereas the concentration profiles decreases with the increase in Dufour number or decrease in Soret number.Lastly, the impact of various parameters on local-skin friction, local-Nusselt number, and local-Sherwood number are also presented in the tabular form.

Using nanoparticles can lead to an increase in mass transfer rate in liquid–liquid extraction systems. Increasing the concentration of nanoparticles in liquids results in the deposition of nanoparticles and thus limits its use in extraction systems. In this paper, the effect of adding a surfactant to nanoparticle in liquid-liquid extraction systems on extraction efficiency is investigated. The effect of surfactant concentration on the extraction efficiency has been investigated both separately and in the presence of nanoparticles. In this research, the effect of continuous and dispersed phase velocity, and pulsation intensity on the hydrodynamic characteristics of the system has been investigated for the first time with the simultaneous use of silica nanoparticles and SDS surfactant in the vertical pulsed packed column.Using hydrodynamic system and in the presence of nanoparticles and surfactant, this research article provides optimum conditions to obtain maximum efficiency with minimum additives and pulsation intensity. ANOVA analysis (three-level Box–Behnken experimental design) has been used to investigate the effective parameters and sensitivity analysis. The results showed that pulsation intensity is the most effective factor on response. With increasing pulsation intensity from 1 to 2.5, the droplet size decreases and hold-up is increased from 0.02 to 0.05 (at Qd=Qc= 2 lit/s) in the system. Also, the effects of adding SiO2 nanoparticles and Sodium dodecyl sulfate (SDS) surfactant into a chemical system on the hydrodynamic characteristics were studied. The results showed that by adding nanoparticles the droplet size decreases while hold-up increases. Finally, a semi-empirical correlation has been proposed to predict the droplet size in terms of operational parameters, the system chemical properties and the nanoparticle volume fraction. It was found that when pulsation intensity, nanoparticle concentration and surfactant concentration were 1.75, 0.1, and 0.05 respectively, extraction efficiency increased to 0.98.

This study investigates numerically the hydro-thermal behavior of nanofluids in double-pipe heat exchangers with two different cross-sections using four different nanofluids. Two types of different circular and cam-shaped cross-sections, four types of different nanofluid of various concentrations are assessed. The results show that cam-shaped cross-section has reduced the heat transfer rate and the pressure drop compared to the circular cross-section. The heat transfer rate has been increased in both types of heat exchanger by increasing the concentration of nanoparticles, but the pressure drop and friction coefficient has been increased compared to pure water. The results of investigating energy ratio show that the highest performance evaluation criterion (PEC) is related to the silver nanoparticles and the energy ratio (PR) increases by increasing the percentage of nanoparticle concentration. But the energy ratio has been decreased for other nanoparticles by increasing the volume percentage of nanoparticles. The results of thermal and hydraulic studies show that the highest PR value is related to water/TiO2 nanofluid and cam pipes have a higher energy loss. Maximum heat transfer improvement for circular pipes is 26.74% related to Ag nanoparticles while this value is 21.15% for cam-shaped pipes.

Particle manipulation using Dielectrophoretic (DEP) force is widely used in microfluidic systems. Because this force is highly sensitive to the electrical properties of particles and the medium as well as the frequency of the electric field. Therefore, regarding the electrical properties of particles and the medium, the attractive and repulsive DEP forces can be created by adjusting the electric field frequency. In this numerical study, two electric fields with different frequencies are employed for simultaneous separating/trapping of particles and dual trapping of particles by taking advantage of the DEP force. At first, by proposing a single-chamber microchannel, the effects of frequency and voltage are investigated for trapping the 5µm Polystyrene particles within the microchannel chamber and for separating and ejecting the 2µm polystyrene particles from the microchannel. At this stage, the optimum voltages are obtained for trapping the 5µm particles and ejecting the 2µm particles according to the obtained performance diagrams. Then, another chamber is added to the microchannel for dual trapping of polystyrene particles. By utilizing the optimum voltages, the particles with different sizes are trapped in different chambers of microchannel. In this section, the performance cartography of the proposed system is also evaluated for the first time to select the optimum values and smart separation. In all numerical simulations, two electric fields with different frequencies are used, one electric field absorbs the particles and the other field repels the particles.