3D Magneto-buoyancy-thermocapillary convection of CNT-water nanofluid in the presence of a magnetic field

Anumerical study is performed to investigate the effects of adding Carbon Nano Tube (CNT) and applying a magnetic field in two directions (vertical and horizontal) on the 3D-thermo-capillary natural convection. The cavity is differentially heated with a free upper surface. Governing equations are solved using the finite volume method. Results are presented in term of flow structure, temperature field and rate of heat transfer. In fact, results revealed that the flow structure and heat transfer rate are considerably affected by the magnitude and the direction of the magnetic field, the presence of thermocapillary forces and by increasing nanoparticles volume fraction. In opposition, the increase of the magnetic field magnitude leads to the control the flow causing flow stabilization by merging vortexes and reducing heat transfer rate. © 2020 by the authors

NUMERICAL ANALYSIS OF NATURAL CONVECTION IN A PRISMATIC ENCLOSURE

Natural convection heat transfer and fluid flow have been examined numerically using the control-volume finite-element method in an isosceles prismatic cavity, submitted to a uniform heat flux from below when inclined sides are maintained isothermal and vertical walls are assumed to be perfect thermal insulators, without symmetry assumptions for the flow structure. The aim of the study is to examine a pitchfork bifurcation occurrence. Governing parameters on heat transfer and flow fields are the Rayleigh number and the aspect ratio of the enclosure. It has been found that the heated wall is not isothermal and the flow structure is sensitive to the aspect ratio. It is also found that heat transfer increases with increasing of Rayleigh number and decreases with increasing aspect ratio. The effects of aspect ratio become significant especially for higher values of Rayleigh number. Eventually the obtained results show that a pitchfork bifurcation occurs at a critical Rayleigh number, above which the symmetric solutions becomes unstable and asymmetric solutions are instead obtained

Impacts of double rotating cylinders on the forced convection of hybrid nanofluid in a bifurcating channel with partly porous layers

Impacts of using double rotating cylinders and partly porous layers in the bifurcating channels on the hydro-thermal performance were numerically assessed. Hybrid nanoparticles were used in water and finite element method was selected as the solver. Effects of Reynolds number, rotational speeds of the cylinders and their locations in the bifurcating channels, porous layer sizes and nanoparticle solid volume fractions on the hydro-thermal performance features were explored. The contribution of different hot wall parts was changed with varying Reynolds number and rotational velocity of the cylinders. Depending upon the rotational direction of the cylinders, the vortex occurrence and size at the bifurcations change significantly. Heat transfer considering all hot walls rise with higher rotational speeds in both directions. The amount of improvement in the heat transfer rate becomes 25% and 19% with varying speeds of the cylinders as compared to motionless cylinders. The pressure coefficient reduces with increasing the second cylinder speed in clockwise direction and this is favorable for thermal performance since the heat transfer also increases. The overall impact of the varying horizontal locations of the cylinders on the heat transfer rate is slight. The separated zones at the branching depends on the porous layer sizes. The overall heat transfer behavior becomes opposite when varying the sizes of the porous layers in the horizontal and vertical channels. By using nanoparticles in the base fluid, 35.75% improvement in the heat transfer rate is achieved for vertical wall at Re = 350 while pressure drop coefficient rises by about 8.5%. The overall improvement in the heat transfer rate by using nanofluid is 26%. Owing to diverse use of bifurcating channels in thermal engineering from fuel cells to electronic cooling, the proposed methods of heat transfer enhancement techniques can be considered simultaneously for effective control the thermal performance of those systems

Experimental Analysis of the Thermal Performance Enhancement of a Vertical Helical Coil Heat Exchanger Using Copper Oxide-Graphene (80-20%) Hybrid Nanofluid

The thermal performance enhancement of a vertical helical coil heat exchanger using distilled water-based copper oxide-graphene hybrid nanofluid has been analyzed experimentally. Accordingly, the focus of this study is the preparation of CuO-Gp (80-20%) hybrid nanoparticles-based suspensions with various mass fractions (0% ≤ wt ≤ 1%). The volume flow rate is ranged from 0.5 L·min−1 to 1.5 L·min−1 to keep the laminar flow regime (768 ≤ Re ≤ 1843) and the supplied hot fluid’s temperature was chosen to equal 50 °C. To ensure the dispersion and avoid agglomeration an ultrasound sonicator is used and the thermal conductivity is evaluated via KD2 Pro Thermal Properties Analyzer. It has been found that the increment in nanoparticles mass fraction enhances considerably the thermal conductivity and the thermal energy exchange rate. In fact, an enhancement of 23.65% in the heat transfer coefficient is obtained with wt = 0.2%, while it is as high as 79.68% for wt = 1%. Moreover, increasing Reynolds number results in a considerable augmentation of the heat transfer coefficient

Experimental Analysis of the Thermal Performance Enhancement of a Vertical Helical Coil Heat Exchanger Using Copper Oxide-Graphene (80-20%) Hybrid Nanofluid

The thermal performance enhancement of a vertical helical coil heat exchanger using distilled water-based copper oxide-graphene hybrid nanofluid has been analyzed experimentally. Accordingly, the focus of this study is the preparation of CuO-Gp (80-20%) hybrid nanoparticles-based suspensions with various mass fractions (0% ≤ wt ≤ 1%). The volume flow rate is ranged from 0.5 L·min−1 to 1.5 L·min−1 to keep the laminar flow regime (768 ≤ Re ≤ 1843) and the supplied hot fluid’s temperature was chosen to equal 50 °C. To ensure the dispersion and avoid agglomeration an ultrasound sonicator is used and the thermal conductivity is evaluated via KD2 Pro Thermal Properties Analyzer. It has been found that the increment in nanoparticles mass fraction enhances considerably the thermal conductivity and the thermal energy exchange rate. In fact, an enhancement of 23.65% in the heat transfer coefficient is obtained with wt = 0.2%, while it is as high as 79.68% for wt = 1%. Moreover, increasing Reynolds number results in a considerable augmentation of the heat transfer coefficient

Thermal and Phase Change Process in a Locally Curved Open Channel Equipped with PCM-PB and Heater during Nanofluid Convection under Magnetic Field

Thermal performance and phase-change dynamics in a channel having a cavity equipped with a heater and phase-change material (PCM)-packed bed (PB) region are analyzed during nanoliquid convection under an inclined magnetic field. Curvature of the upper wall above the PCM zone is also considered by using the finite element method. Impacts of curvature of the upper wall (between 0.01H and 0.6H, H-channel height), strength of magnetic field (MGF) (Hartmann number between 0 and 40), height (between 0.1H and 0.4H) and number (between 5 and 17) of heaters on the thermal performance and phase-change dynamics are studied. In the interior and wall near regions of the PCM-PB, the curvature effects become opposite, while phase completion time (tF) rises by about 42% at the highest radius of the curvature. Imposing MGF and increasing its strength has positive impacts on the phase change and thermal performance. There is a reduction in tF by about 45.2% and 41.8% when MGF is imposed at Ha = 40 for pure fluids and nanofluids. When thermal performance for all different cases is compared, using MGF+nanofluid+PCM provides the most favorable case. When the reference case (only pure fluid without MGF and PCM) is used, including nanoparticles results in an improvement of 33.7%m while it is further increased to 71.1% when PCM-PB is also installed. The most favorable case by using MGF, nanofluid and PCM-PB results in thermal performance improvement of about 373.9% as compared to the reference configuration

Thermal and Phase Change Process in a Locally Curved Open Channel Equipped with PCM-PB and Heater during Nanofluid Convection under Magnetic Field

Thermal performance and phase-change dynamics in a channel having a cavity equipped with a heater and phase-change material (PCM)-packed bed (PB) region are analyzed during nanoliquid convection under an inclined magnetic field. Curvature of the upper wall above the PCM zone is also considered by using the finite element method. Impacts of curvature of the upper wall (between 0.01H and 0.6H, H-channel height), strength of magnetic field (MGF) (Hartmann number between 0 and 40), height (between 0.1H and 0.4H) and number (between 5 and 17) of heaters on the thermal performance and phase-change dynamics are studied. In the interior and wall near regions of the PCM-PB, the curvature effects become opposite, while phase completion time (tF) rises by about 42% at the highest radius of the curvature. Imposing MGF and increasing its strength has positive impacts on the phase change and thermal performance. There is a reduction in tF by about 45.2% and 41.8% when MGF is imposed at Ha = 40 for pure fluids and nanofluids. When thermal performance for all different cases is compared, using MGF+nanofluid+PCM provides the most favorable case. When the reference case (only pure fluid without MGF and PCM) is used, including nanoparticles results in an improvement of 33.7%m while it is further increased to 71.1% when PCM-PB is also installed. The most favorable case by using MGF, nanofluid and PCM-PB results in thermal performance improvement of about 373.9% as compared to the reference configuration

A CFD modelling on effects of ejection angle of a co-flow on the thermal characteristics for a combined wall and offset jet flow

In the present study, a CFD simulation of a flow combined an offset jet and a wall jet (noted dual-jet) with the presence of co-flow is carried out. The effect of the intensity of the co-flow CFV (co-flow velocity) as well as its ejection angle α on the heat transfer exchanged in dual-jet flow is also performed. The present simulations are carried out for a Reynolds number Re = 15000, a nozzle-to-nozzle distance equal to 4 times the thickness of the nozzle, a co-flow velocity CFV = 10% − 40 % and a co-flow ejection angle α = 0° − 40°. The results of this computational study clearly show an intensification of the heat transfer exchanged between the flow and the wall by increasing the co-flow velocity CFV as well as its ejection angle α

Jet Impingement Cooling of a Rotating Hot Circular Cylinder with Hybrid Nanofluid under Multiple Magnetic Field Effects

The cooling performance of jet impinging hybrid nanofluid on a rotating hot circular cylinder was numerically assessed under the effects of multiple magnetic fields via finite element method. The numerical study was conducted for different values of Reynolds number (100≤Re≤300), rotational Reynolds number (0≤Rew≤800), lower and upper domain magnetic field strength (0≤Ha≤20), size of the rotating cylinder (2 w ≤r≤ 6 w) and distance between the jets (6 w ≤ H ≤ 16 w). In the presence of rotation at the highest speed, the Nu value was increased by about 5% when Re was increased from Re = 100 to Re = 300. This value was 48.5% for the configuration with the motionless cylinder. However, the rotations of the cylinder resulted in significant heat transfer enhancements in the absence or presence of magnetic field effects in the upper domain. At Ha1 = 0, the average Nu rose by about 175%, and the value was 249% at Ha1 = 20 when cases with the cylinder rotating at the highest speed were compared to the motionless cylinder case. When magnetic field strengths of the upper and lower domains are reduced, the average Nu decreases. The size of the cylinder is influential on the flow dynamics and heat transfer when the cylinder is rotating. An optimum value of the distance between the jets was obtained at H = 14 w, where the Nu value was highest for the rotating cylinder case. A modal analysis of the heat transfer dynamics was performed with the POD technique. As diverse applications of energy system technologies with impinging jets are available, considering the rotations of the cooled surface under the combined effects of using magnetic field and nanoparticle loading in heat transfer fluid is a novel contribution. The outcomes of the present work will be helpful in the initial design and optimization studies in applications from electronic cooling to convective drying, solar power and many other systems

Effects of Movable-Baffle on Heat Transfer and Entropy Generation in a Cavity Saturated by CNT Suspensions: Three-Dimensional Modeling

Convective heat transfer and entropy generation in a 3D closed cavity, equipped with adiabatic-driven baffle and filled with CNT (carbon nanotube)-water nanofluid, are numerically investigated for a range of Rayleigh numbers from 103 to 105. This research is conducted for three configurations; fixed baffle (V = 0), rotating baffle clockwise (V+) and rotating baffle counterclockwise (V−) and a range of CNT concentrations from 0 to 15%. Governing equations are formulated using potential vector vorticity formulation in its three-dimensional form, then solved by the finite volume method. The effects of motion direction of the inserted driven baffle and CNT concentration on heat transfer and entropy generation are studied. It was observed that for low Rayleigh numbers, the motion of the driven baffle enhances heat transfer regardless of its direction and the CNT concentration effect is negligible. However, with an increasing Rayleigh number, adding driven baffle increases the heat transfer only when it moves in the direction of the decreasing temperature gradient; elsewhere, convective heat transfer cannot be enhanced due to flow blockage at the corners of the baffle