Stability Analysis of Unsteady Hybrid Nanofluid Flow Past a Permeable Stretching/Shrinking Cylinder

The study of boundary layer flow has gained considerable interest owing to its extensive engineering applications. Thus, this numerical study aims to investigate the stability analysis of unsteady flow in the hybrid Al2O3-Cu/H2O nanofluid past a shrinking permeable cylinder. The impacts of suction and unsteadiness parameters are considered in this study. The partial differential equations are converted into a system of nonlinear ordinary differential equations by selecting suitable similarity transformation and solved using the bvp4c code in the MATLAB program. The findings revealed that the existence of dual solutions is visible. The skin friction coefficient and the local Nusselt numbers of Al2O3-Cu/H2O increase with the inclusion of the suction parameter. The presence of the unsteadiness parameter actively promotes heat transfer degradation on the shrinking cylinder. Stability analysis indicates that a stable and physically realizable solution appeared in the first solution, whereas the second solution is unstable.

Burnout Investigation of Small Diameter Tubes Immersed in Nanofluids

This paper deals with research into pool boiling critical heat flux (CHF) of water–Al2O3, water–TiO2 and water–Cu nanofluids on horizontal stainless steel tubes. The experiments were conducted under atmospheric pressure. Nanoparticles were tested at concentrations of 0.001%, 0.01%, 0.1% and 1% by weight. Ultrasonic vibration was used in order to stabilize the dispersion of the nanoparticles. Although dispersants were not used to stabilize the suspension, the solutions tested showed satisfactory stability. Experimental measurements were performed with stainless steel tubes of three outside diameters: 1.6, 3 and 5 mm. Enhancement of CHF was observed to be independent of the concentration and material of the nanoparticles and tube diameter, with simultaneous heat transfer degradation. Built up during the boiling process, nanolayers improve substantially the heating surface wettability. A correlation is suggested for the CHF prediction during pool boiling of nanofluids.

Simulation of Porous Magnetite Deposits on Steam Generator Tubes in Circulating Water at 270 °C

In the secondary side of pressurized water reactors (PWRs), the main corrosion product accumulated on the steam generator (SG) tubes is magnetite, which has a porous structure. The purpose of this work is to simulate the porous magnetite deposited to the SG tubes using a loop system. We newly developed a circulating loop system for a porous magnetite deposition test. A test section was designed as a single hydraulic flow channel, and a cartridge heater was fabricated and mounted into a commercial SG tube to provide an equal heating source for the primary water. After the deposition test, the simulated magnetite deposits were characterized for comparison to real SG tube deposits collected from an operating PWR plant. The magnetite deposits produced using the loop system were appropriate for simulating the real SG tube deposits because the particle characteristics, phase, and porous morphology are closely similar to those of real deposit samples. Using the loop system, the chemical impurities such as Na and Cl can be easily concentrated within the pores of the simulated magnetite deposits. These simulated magnetite samples are expected to be widely utilized in various research fields such as the heat transfer degradation and magnetite accelerated corrosion of SG tubes.

Experimental Heat Transfer From Heating Source Subjected to Rigorous Natural Convection Inside Enclosure and Cooled by Forced Nanofluid Flow

Mixed convection heat transfer characteristics from heat source located symmetrically inside square enclosure and cooled by Al2O3/water-based nanofluid flow was experimentally investigated. The configuration was subjected to high levels of natural convection and low rates of nanofluid flow. The nanofluid thermophysical properties were characterized using the available correlations in the literatures except the viscosity which was measured and correlated in terms of the nanoparticles loading ratios. Comparative analysis indicated that the application of nanofluid could not guarantee heat transfer enhancement in configurations dominated by natural convection. Exception heat transfer enhancement was only found when very low nanoparticles loading ratio was applied. Instead, heat transfer degradation was found especially in the cases of highest nanoparticles loading ratios. Alternatively, heat transfer enhancement was observed when the forced convection effect was substantial at the highest nanofluid flow rate. The present conclusions were justified and correlated to the findings reported in the literature.

Flow and Heat Transfer Study of an Impinging Piezoelectric Fan Over a Vertical Surface

Piezoelectric fans are low-form-factor cooling devices, which have gained recent attention for electronics cooling. These devices feature a vibrating blade, which sheds vortices from its tip during its motion. The performance of a piezoelectric fan is based on its location, orientation, and operating condition. Thus, we investigated the heat transfer and flow field of an impinging flow produced by a piezoelectric fan. The heat transfer tests are conducted using a vertical, 2.54 cm × 2.54 cm copper heater, which is configured with the piezoelectric fan positioned along its centerline. The fan is operated at its fundamental frequency of 60 Hz, where it achieves maximum heat transfer and fan deflection. There is significant heat transfer degradation with increasing heater-to-fan spacing and off-resonance operating conditions. To better understand this thermal performance, we require information about the flow field produced by this pulsating flow. Hence, we performed particle image velocimetry (PIV) measurements of the flow field for free and impinging cases with different heater-to-fan spacing. We used instantaneous and time-averaged PIV to depict the response in a region within approximately two times the fan oscillation amplitude. In this region, there was a stagnation flow close to the heater, which would result in significant heat transfer. However, this flow also featured high-magnitude velocity vectors towards the sides of the heater rather than towards its center, which would likely result in non-uniform heat transfer.

Investigation of MHD RANS Modeling Base on DNS Database Under the Advanced Blanket Design Conditions Utilized Molten Salt

In this study, validations of Reynolds Averaged Navier-Stokes Simulation (RANS) based on Kenjeres & Hanjalic MHD turbulence model (Int. J. Heat & Fluid Flow, 21, 2000) coupled with the low-Reynolds number k-epsilon model have been conducted with the usage of Direct Numerical Simulation (DNS) database. DNS database of turbulent channel flow imposed wall-normal magnetic field on, are established in condition of bulk Reynolds number 40000, Hartmann number 24, and Prandtl number 5. As the results, the Nagano & Shimada model (Trans. JSME series B. 59, 1993) coupled with Kenjeres & Hanjalic MHD turbulence model has the better availability compared with Myong & Kasagi model (Int. Fluid Eng, 109, 1990) in estimation of the heat transfer degradation in MHD turbulent heat transfer.

Investigation of MHD RANS Modeling Base on DNS Database Under the Advanced Blanket Design Conditions Utilized Molten Salt

In this study, validations of Reynolds Averaged Navier-Stokes Simulation (RANS) based on Kenjeres & Hanjalic MHD turbulence model (Int. J. Heat & Fluid Flow, 21, 2000) coupled with the low-Reynolds number k-epsilon model have been conducted with the usage of Direct Numerical Simulation (DNS) database. DNS database of turbulent channel flow imposed wall-normal magnetic field on, are established in condition of bulk Reynolds number 40000, Hartmann number 24, and Prandtl number 5. As the results, the Nagano & Shimada model (Trans. JSME series B. 59, 1993) coupled with Kenjeres & Hanjalic MHD turbulence model has the better availability compared with Myong & Kasagi model (Int. Fluid Eng, 109, 1990) in estimation of the heat transfer degradation in MHD turbulent heat transfer.

Effect of Diamond Nanolubricant on R134a Pool Boiling Heat Transfer

This paper quantifies the influence of diamond nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a roughened, horizontal, and flat surface. Nanofluids are liquids that contain dispersed nanosize particles. A lubricant based nanofluid (nanolubricant) was made by suspending 10 nm diameter diamond particles in a synthetic ester to roughly a 2.6% volume fraction. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) up to 129%. A similar enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 91% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in boiling heat transfer degradation of approximately 19% for the best performing tests. It was speculated that the poor quality of the nanolubricant suspension caused the performance of the (99.5/0.5), and the (98/2) nanolubricant mixtures to decay over time to, on average, 36% and 76% of the of pure R134a/polyolester performance, respectively. Thermal conductivity and viscosity measurements and a refrigerant\lubricant mixture pool-boiling model were used to suggest that increases in thermal conductivity and lubricant viscosity are mainly responsible for the heat transfer enhancement due to nanoparticles. Particle size measurements were used to suggest that particle agglomeration induced a lack of performance repeatability for the (99.5/0.5) and the (98/2) mixtures. From the results of the present study, it is speculated that if a good dispersion of nanoparticles in the lubricant is not obtained, then the agglomerated nanoparticles will not provide interaction with bubbles, which is favorable for heat transfer. Further research with nanolubricants and refrigerants are required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.