Al-Qadisiyah Journal for Engineering Sciences

Abstract The integration of nanofluid effects and channel shape effects in a heat sink, which exhibits both variable and constant cross-section, has gained significant traction as an efficient cooling method for thermal devices, particularly microelectronic devices. This research presents an experimental and numerical analysis to compare the performance of microchannel heat sink designs (straight, zigzag, wavy, and circular cavities). In addition, the study dealt with the use of pure water and nanofluid (CuO-H2O) with volumetric concentrations of (0.01, 0.02, and 0.03) as coolants. COMSOL Multiphysics was used for numerical analysis to simulate and solve the problem of fluid and heat flow in 3D. The bottom wall of the four microchannels is subjected to a steady heat flux of 170 kW/m². The simulations were only performed within the laminar domain, encompassing a spectrum of Reynolds numbers ranging from 50 to 150. The influence on the microchannel's wall temperature, thermal resistance, pressure drop, and friction factor is exhibited. According to the findings, the wavy and zigzag microchannel heat sink cooled by nanofluid displays higher performance in terms of heat transmission and dissipation in comparison to the heat sink that was cooled by distilled water. as evidenced by a 12% increase in mussel number at volume concentrations of 0.03%.

Abstract The aim of this research was to increase the engine car efficiency by adding an innovative material called hybrid MWCNT/Al2O3 nanofluids. A volume ratio instance for investigations is 0.05 MWCNT mixed with 0.05 Al2O3 in distilled water (DW). The experimental temperature setup was in the range from 50 °C to 70 °C. The results demonstrate that as temperatures increase, specific heat and thermal conductivity increase significantly, while viscosity and density gradually decrease. At 70 °C, the highest thermal conductivity of 1.143 W/m.K was achieved in the presence of hybrid MWCNT/Al2O3 nanofluids. Furthermore, it was found that the correlation coefficient for thermal conductivity is 97.06% R2.

Abstract Researchers in heat transfer are paying close attention to nanofluids because of their potential as high-performance thermal transport media. In light of natural convection's enormous significance, the addition of nanoparticles significantly enhances the thermophysical properties of the nanofluids compared to the base fluid. In this study, experimental work was used to evaluate the influence of CuO nanoparticles on natural convection with magnetohydrodynamic (MHD) flow in a square cavity. The cavity’s left and right vertical walls were maintained at different temperatures, and the top and bottom walls of the cavity were insulated. This experimental study applied a horizontal magnetic field with uniform strength. Results were obtained for a variety of Hartmann numbers ranging from 0–300, Rayleigh numbers going from 2.76E+8 to 6.89E+8, and solid volume fractions ranging from 0 to 1.5%Vol. Results showed that the heat transfer coefficient and Nusselt number values decreased with the increase in the values of the Hartmann number, except for the heat transfer coefficients at Ha=100 and 150 are larger than the heat transfer coefficients at Ha= 0. The maximum heat transfer coefficient and Nusselt number enhancement were 40.8% and 28.5%, respectively, at a 1.5% volume concentration of CuO-water nanofluid, Ra= 6.7E+8 and Ha=100 compared with pure fluid (water) at Ha=0.

Abstract In this study, numerical computations of the influence of adding ribs in a rectangular channel on the forced convection heat transfer and laminar fluid flow characteristics has been carried out. The analysis was carried out by using the finite element method to solve the dimensionless governing equations for two-dimensional channel with 80 mm height and 2000 mm length at the Reynolds number of (10, 100, and 500), rib height e=8mm with different aspect ratios (AR =2.5, 3.125, 3.75, 4.375, and 5). Also, the study compared two cases of investigations with and without nanofluid (Water/ TiO2) at the volume fractions of nanoparticles of 0, 2 and 4%. The results concluded that, for a certain arrangements, the use of extended surfaces within a rectangular channel can significantly enhance the rate of heat transfer and when the aspect ratios decreases, the Nusselt number increased. However, the existence of ribs within channel in case of constant heat flux can cause a significant improvement of heat transfer compared to that in the corresponding channel under the variable heat flux.

Abstract In this paper, an experimental electrical distribution transformer was studied and a new technique was proposed to improve the performance of a new mixed cooling consisting of pure transformer oil, paraffin wax and nanoparticles. The experiment was carried out on a small transformer that was done by taking a model with dimensions (15 * 10 * 10) cm to facilitate calculations. Paraffin wax absorbs the heat generated in the transformer due to the smelting process that can be used to cool electrical appliances. Nanoparticles have good thermal properties and lead to increased oil insulation to thermal improvements in transformer oil with dispersal of solid nanoparticles and their effects on transformer cooling. Three types of solid nanoparticles were used in this experiment (Al2O3, TiO2, and Sic) with a different volume concentration (1%, 3%, and 5%) and 4% paraffin wax as a certified added percentage for each process. The obtained results showed that when mixing paraffin wax and solid nanoparticles with transformer oil, the transformer cooling performance is improved by reducing the temperature. The best selected nanoparticles were found to be Sic and the reason for this is that Sic has a higher thermal conductivity compared to (Al2O3 and TiO2). The proposed hybrid oil reduces the temperature by 10 ° C (in the case of PCM and Sic) and it is possible to improve the cooling performance of electrical transformers.

Abstract The present study, experimentally investigated the mixed convection in a square enclosure partitioned in two layers. The experiments were performed with Al₂O₃–water nanofluid (upper layer) and superposed porous medium (lower layer) with an adiabatic rotating cylinder at the center of the cavity. The boundary conditions of the experimental study were; the upper and lower walls were assumed adiabatic, the right wall was heated, and the left wall was cooled. Experimentally, 15 K-type thermocouples and thermal imaging camera were employed to measure the temperatures distribution inside the cavity when the concentration of nanoparticles (ɸ = 0.06), the temperature difference (∆T) between the cold and hot walls was (6, 8, and 10) °C, and angular rotational velocity (-50, -25, 0, 25, and 50) rpm. The results of experimental data showed that in general, the distribution of temperatures was very well along the upper half of the enclosure, while in the lower half the temperature distribution was confined near the hot wall region. When the circular cylinder rotates in counter-clockwise, it noted that the effect of speed is evident in the downside of the cylinder, while the temperature distribution in the left upper part of the enclosure decreasing. When the circular cylinder rotates in the clockwise direction, the results showed that the effect of cylinder rotation was around cylinder only. Moreover, the results demonstrated that the increasing temperature difference leads to a noticeable increment in the intensity of the flow.

Abstract The characteristics of the conjugate natural convection of (Al₂O₃-water) nanofluid inside differentially heated enclosure is numerically analyzed using COMSOL Multiphysics (5.3a). The enclosure consists of two vertical walls, the left wall has a thickness and maintain at a uniform hot temperature, while the opposite wall at cold temperature and the horizontal walls are isolated. A high thermal conductivity thin baffle has been added on the insulated bottom wall at a different inclination angles. The effect of the volume fractions of nanoparticles (f), Rayleigh number (Ra), solid wall thermal conductivity ratio (Kr), baffle incline angles (Ø) and the thickness of solid wall (D) on the isothermal lines, fluid flow patterns and the average Nusselt number (Nu)  has been investigated. At low Rayleigh number (Ra=10³ to 10⁴) the Isothermal lines are parallel with the vertical wall which is characteristic of conduction heat transfer. on the other hand, when Rayleigh number increase to (Ra=10⁶),  the isotherms lines distribution in the inner fluid become parallel curves with the adiabatic horizontal walls of the enclosure and smooth in this case convection heat transfer becomes dominant. As the Rayleigh number further increases, the average Nusselt number enhance because of buoyancy force become stronger. In addition, the fluid flow within the space is affected by the presence of a fin attached to the lower wall that causes blockage and obstruction of flow near the hot wall, hence the recirculation cores become weak and effect on the buoyant force. The maximum value of the stream function can be noticed in case of nanofluid at (Ø=60), whereas they decrease when (Ø > 60), where the baffle obstruction causing decreases in flow movement. So that the left region temperature increases which cause reduction of the convective heat transfer by the inner fluid temperatures. This is an indication of enhancing of insulation. When the inclination angle increases (Ø >90), the baffle obstruction on flow and fluid resistance becomes smaller and the buoyancy strength increase, as a result, the heat transfer is increasing in this case. As a result of increasing the thermal conductivity from 1 to 10, an increase in the amount of heat transferred through the solid wall to the internal fluid have been noticed. This change can be seen in the isothermal lines, also, there was growth and an increase in the temperature gradient. The increasing of wall thickness from (D=0.1 to 0.4) leads to reduce the intensive heating through the solid wall as well as small heat transferred to the inner fluid. Therefore, it can be noticed that when the wall thickness increases the stream function decrease.

Abstract In this paper a microchannel heat sink with expanded microchannels and nanofluids is numerically investigated. The object of this paper is to study and improve the cooling performance of microchannel heat sink. Both the geometrical parameters and working fluids were studied and a comparison was made between them. Expanded microchannels (sudden expanded and diverging) were used instead of straight microchannels, also micro pin fins with square and triangular shapes were used for heat transfer enhancement. Sudden expanded microchannels were studied with different expansion ratios and expansion lengths. Three types of nanofluids (Cu-water, Al2O3-water and Diamond-water) with volume concentration (1 – 5) % were studied as working fluids and their effects on overall performance of heat sink were compared with pure water. The results obtained shows that the overall performance of microchannel heat sink increased with increasing the expansion ratio or decreasing the expansion length. For the same expansion ratio the sudden expanded microchannels gives higher modification compared with diverging microchannels. Also using of nanofluids lead to enhance the heat transfer and the improvement got by geometric parameters such as using of expanded microchannels or fins is much larger than that obtained by using nanofluids for the same heat sink.

Abstract In this work, a numerical simulation of natural convection in an oblique enclosure filled with silver-water nanofluid is obtained for different values of Rayleigh numbers, volume fraction and inclination angle of sloping walls. The considered oblique enclosure with left and right side walls are maintained at constant cold temperature (Tc). The horizontal top wall of enclosure is kept insulated, but the bottom is maintained at constant hot temperature (Th). The present work is utilized to obtain results in the range of Rayleigh number (103-106) , volume fraction of nanofluid varied from (0-0.2) and inclination angle of side walls are (-60o, -30o, 0o, 30o, 60o). The Prandtle number is 6.0. The governing equations in the two-dimensional are solved numerically by using finite-difference technique. Comparisons with other works are performed and the results are found to be in good agreement. The obtained results are shown in the form of stream function, isothermal lines and average Nusselt numbers. It observed from results that acute shaping wall and Ag-nanoparticles with high concentration are effective to enhance the rate of heat transfer, also; the average Nusselt number for all range of inclination angle increases with increase in the Rayleigh number and the solid volume fraction of the Nanofluid.

Abstract A new implementation for the volume of fluid model with finite volume method is applied to simulate two-phase flow involving nano-particles. The model is used to investigate the laminar flow utilizing nanofluids in a lid-driven cavity. The fluid in the cavity is a water-based nanofluid containing Cu, CuO or Al2O3 nano-particles. The effects of adding extra phase (solid phase) and solid volume fraction for different nanofluids on hydrodynamic characteristics are investigated. The effective density and viscosity of nano-particles are calculated by Chon and Brinkman models, respectively. The CFD model is validated for laminar flow and the results showed good agreement with available numerical data. Then the model is tested for liquid and nano-particles where the results indicate that the effects of solid volume fraction approximately depend strongly on the effective density sequentially for Al2O3, CuO and Cu. In addition the effective viscosity has less effect on the flow field