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STUDY OF THE MAGNETIC TAYLOR-COUETTE FLOW WITH THE AXIAL FLOW IN ROTATING MACHINERY UNDER QUADRUPOLE MAGNETIC FIELD

In this study, the effect of applying a magnetic field resulting from quadrupole magnets on the hydrodynamics and heat transfer in magnetic Taylor-Couette flow with axial flow was investigated numerically for the first time. The studied geometry consisted of two vertical concentric cylinders. The inner cylinder rotated at a specific angular velocity to create the Taylor-Couette flow, while the outer cylinder was stationary. The outer cylinder was also considered to be under constant heat flux boundary conditions. The magnetic nanofluid was a water-based nanofluid consisting of iron oxide nanoparticles with diameters of 8 nm and the magnetic field was produced by quadrupole magnets. The modeling of the problem was implemented by using the two-phase mixture model and control volume technique. The obtained results indicated that in the absence of a magnetic field and using our considered range of Taylor numbers, the flow consisted of many vortices, which is called the Taylor vortex flow. By applying the magnetic field, the ferrofluid flow was pushed toward the outer cylinder wall, which also considerably increased the stability of the flow. Moreover, by imposing the magnetic field, the local heat transfer coefficient increased and the heat transfer coefficient decreased as the Taylor number of the flow increased.

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EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS FOR VERTICAL DOWNFLOW USING TRADITIONAL AND 3D-PRINTED MINI TUBES

In this study, an investigation on the influences of different manufacturing techniques on the heat transfer and pressure drop in the developing and fully developed regions of mini-tube under different flow regimes is introduced. The purpose of this research is to experimentally investigate the heat transfer and pressure drop characteristics using 3D-printed tubes and traditional stainless steel tubes in the vertical direction under isothermal and non-isothermal boundary conditions. Experiments are conducted using distilled water (Prandtl numbers varying from 4 and 7) at Reynolds numbers of 800-10000 with heat fluxes between 30 and 500 kW·m<sup>-2</sup>. Test tubes with inside diameters of 2 mm are used, and the average surface roughness is 1.6 μm and 15.3 μm, respectively. The results are compared with previous studies. It is verified that the heat transfer characteristics are almost the same for the traditional tube and the 3D-printed tube in the laminar region. The average deviation between these two tubes is 7.7%. However, for the turbulent region, the Nusselt numbers of 3D-printed tube in the turbulent region increases by an average of 45% as compared with a traditional tube. The friction factors under heating conditions also increased by an average of 209%. In addition, the 3D-printed tube enters the transition region earlier. The results show that the average critical Reynolds number of a traditional tube and 3D-printed tube is around 2300 and 2000, respectively. Correlations in the turbulent region are developed to predict the friction factors and heat transfer coefficients with good accuracy.

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RECENT PROGRESS ON HEAT TRANSFER PERFORMANCE AND INFLUENCING FACTORS OF DIFFERENT MICROCHANNEL HEAT SINKS

The microchannel heat sink (MCHS) is an efficient thermal management technology widely used in various fields, including electronic equipment, automobiles, and aerospace. In this paper, the recent advances in cross-sectional shape, coolant type, flow channel shape, flow pattern, and application scenarios of the MCHS are systematically reviewed. The liquid film thickness in circular microchannels is the smallest, followed by rectangle, trapezoid, and triangle sections. Conversely, the pressure drop experienced exhibits an inverse relationship with the liquid film thickness. Comparatively, the heat transfer performance of the liquid phase surpasses that of the gas phase, and the two-phase coolant consistently outperforms the single-phase coolant. The study also investigates the impact of flow direction and shape on heat transfer performance. It is found that the implementation of wavy, fractal, and cavity structures enhances heat transfer performance at the expense of increased fluid motion variability, resulting in a loss of pressure drop. Additionally, this paper discusses the occurrence of laminar and turbulent flow phenomena within MCHSs and summarizes their respective influences on heat dissipation performance. On the basis of the aforementioned findings, four key applications of MCHSs are emphasized, accompanied by recommendations for their present utilization and future development. Future research endeavors will concentrate on striking a balance between altering the shape and material characteristics of MCHSs to optimize heat transfer performance while developing novel theoretical models continuously.

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EXPERIMENTAL AND NUMERICAL CONVECTIVE HEAT TRANSFER INVESTIGATION IN LAMINAR RECTANGULAR-CHANNEL FLOW ACROSS V-SHAPED GROOVES

The influence of staggered V-grooves on the hydrothermal performance of a rectangular-channel flow is systematically investigated through a combination of numerical and experimental approaches. The 3-D numerical simulation is developed adopting computational fluid dynamics (CFD) (ANSYS FLUENT) for a range of Reynolds numbers (Re) from 100 to 1000. The experiments are conducted on straight and V-grooved channels (with pitch-to-height and height-to-hydraulic diameter ratios of 2 and 0.75) for distilled water under constant wall heat flux conditions to validate the computational model. Additionally, the impact of V-shaped groove arrangements, forward V-grooved channel (FVGCH) and backward V-grooved channel (B-VGCH), on the flow and heat fields, as well as the effect of groove depths (<i>d</i> = 1.5, 2.5, and 3.5 mm), are also studied. In both experimental and numerical results, the performance evaluation criterion (PEC) grows with rising Reynolds numbers. The highest PEC values of the numerical and experimental findings for the F-VGCH are 2.18 and 2.29, respectively, at Re = 1000. Whereas the highest PEC values of the numerical and experimental results for the B-VGCH are 1.81 and 1.96, respectively, at the same Re (Re = 1000). In addition, the values of PEC for F-VGCH are greater than the PEC values of B-VGCH for all examined groove depths over the entire range of Reynolds numbers. Thus, the F-VGCH offers the best performance evaluation criterion in comparison with B-VGCH.

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ENHANCED NEAR-FIELD RADIATION TRANSPORT AND NANO-ENTITY DETECTION IN AQUEOUS SOLUTION WITH A SI3N4-BASED INTEGRATED WGM/SPR MICROSENSOR

In this emerging proof-of-concept simulation study, we demonstrated the enhancement of near-field radiation transport in a whispering-gallery mode (WGM) ring resonator via integration with surface plasmon resonance (SPR). The integrated sensor is made of a Si<sub>3</sub>N<sub>4</sub> micro-ring with the internal core coated with a thin metal film of silver or gold. It is used for nano-entity detection in an aqueous solution environment. The radiation enhancement F-factor is adopted to quantify the performance of the integrated sensor. It was found that the sensitivity of the integrated sensor was enhanced about 2 to 4.8 times compared to a pure Si<sub>3</sub>N<sub>4</sub> WGM ring sensor without SPR. The integrated WGM/SPR microsensor may be combined with the reverse transcription-polymerase chain reaction technology to extend the limit of detection. The Q-factor of the proposed Si<sub>3</sub>N<sub>4</sub>-based integrated sensor is one to two orders of magnitude higher than that of a similar silica-based integrated sensor; thus, the new sensor may effectively detect nano-entities in aqueous solutions and has outstanding advantages in terms of small size, rapid detection with fewer samples, and high accuracy.

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ANALYSIS OF PLATE-FIN HEAT SINK INFUSED WITH PHASE CHANGE MATERIALS FOR INTERMITTENT SPACE MISSIONS

In this work, we numerically investigated the heat transfer effectiveness of different phase change materials (PCMs) when infused in a plate-fin heat sink with a fixed volume fraction of thermal conductivity enhancer. The PCM's ability to absorb and release large amounts of thermal energy at constant temperature is a desired feature in transient electronics cooling applications. In this study, we focused on examining the effect of the number of fins, type of PCM, heat flux, PCM volume fraction, and heat sink bottom wall thickness. The results showed that increasing the number of fins improved the performance of the PCM-infused heat sink. When a heat flux of 4000 W/m<sup>2</sup> was applied for 30 minutes on a plate-fin heat sink infused with paraffin wax, the maximum temperature did not exceed 70°C in the four-fin design, while it exceeded 80°C in the two-fin design. A salt hydrate PCM outperformed paraffin wax and RT35. The bottom wall of the heat sink acted as a thermal spreader and a nonlinear relationship existed between the bottom wall thickness and the maximum electronics temperature. Compared to the two- and four-fin heat sink models, the zero-fin model required the longest time to fully melt the entire PCM due to the additional amount of PCM present in the heat sink gaps.

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