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This research paper focuses on CFD analysis for comparative evaluation of different hybrid nanofluids (NFs): MWCNT-Al 2 O 3 /H 2 O, MWCNT-MgO/H 2 O, Al 2 O 3 -CuO/H 2 O performance as working fluid (WF) inside parabolic trough solar collector (PTSC). The performance of the collector is evaluated using different performance indicators, including Nusselt number (Nu), friction factor (f), and Performance Evaluation Criterion (PEC) for different Reynolds number (Re) (10000-50000). Among the investigated hybrid nanoparticles (NPs), MWCNT-Al 2 O 3 emerges as the most promising choice for significantly enhancing the system's thermal performance with the highest PEC value of 1.9. This outcome signifies the superior efficiency of MWCNT-Al₂O₃/H₂O hybrid NF in enhancing convective heat transfer, thus underlining the paramount significance of adopting them in thermal systems operating within the specified Re range. [ABSTRACT FROM AUTHOR]

Abstract In the process of drilling, the tubing-casing annulus are easily to be eccentric. Heat transport phenomenon occurring in such tubes and annulus can be complex, and this paper investigates forced convection in a vertical eccentric annulus with different radius ratios and eccentricity (normalized by the radius difference). Physical experiments and numerical simulation have been conducted with constant inner tube inlet temperature and flow rate of hot water. Results show that with the increase of radius ratios, the convection heat transport coefficient increases. For a radius ratio of 1.875, the heat transport coefficient increases as the eccentricity increases when the Re > 2000 and it decreases when the Re < 2000. For a radius ratio of 2.25 and Re < 1500, the influence of eccentricity is trivial, but for Re > 1500, the increase of eccentricity strengthens the heat convection. For a radius ratio of 3.06, the heat transport coefficient increases as the eccentricity increases. An empirical correlation for Nusselt number is derived as a function of Reynolds number, Prandtl number and eccentricity. Highlights • For D o / D i = 1.875, the effect of eccentricity is identified into two zones. • For D o / D i = 3.06, the increase of eccentricity enhances the convective heat transport. • The increase of radius ratio strengthens the heat transport of annular channels. • With low eccentricity and low Reynolds number, the increase of radius ratio can hardly improve the heat exchange efficiency. • Three kind of empirical correlations are regressed through parametric analysis. [ABSTRACT FROM AUTHOR]

• Thermal and hydrodynamic characteristics of staggered drop-shaped tubes were studied. • The effect of the number of tube rows N R on the performance was investigated. • The effect of increasing Reynolds number Re on the performance was analyzed. • The investigation covered 1.78 × 103 ≤ Re ≤ 18.72 × 103 and 1 ≤ N R ≤ 20. • New correlations for N u ¯ , f , and ε considering N R were developed. In this paper, for the first time, air-side correlations were developed and introduced to describe the average Nusselt number N u ¯ , friction factor f , and thermal − hydraulic performance ε of compact heat exchanger employing drop-shaped tubes considering the number of tube rows (N R). Drop-shaped tubes were arranged in a staggered formation and placed in 1 to 20 consecutive rows in the flow direction, each consisting of 7 tubes in the transverse direction. The Reynolds number (Re) varied from 1.78 × 103 to 18.72 × 103. The calculations were carried out using Ansys Fluent software package. Tube row and Reynolds number effects on the thermal and hydrodynamic characteristics of the studied heat exchangers were analyzed. The simulation results were in a good agreement with the available literature. The results showed that N u ¯ and ε increase, while f decreases as Re increases. Moreover, an increase in N R leads to an increase of f and a decrease of ε. The maximum values of N u ¯ were obtained for N R = 2, which were higher by 3.95–6.69% and 19.91–24.62% than those for N R = 1, and N R = 20, respectively. Heat transfer was stabilized only from the 19-row onwards. In addition, a 1-row heat exchanger had better thermal − hydraulic performance compared to that with a larger N R (ε for N R = 1 was about 22.32–27.24 times higher than those for N R = 20). The maximum deviation between the numerical results and the obtained correlations developed from them was ± 8.87%, which provides a useful reference for future studies and designs of heat exchangers with different row numbers of drop-shaped tubes. [ABSTRACT FROM AUTHOR]

A novel type of pipe insert was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Pipe inserts with different dimensionless spacer length (S/D = 3.33, S/D = 5, S/D = 6.67, S/D = 8.33, S/D = 10, S/D = 11.67) were investigated at the Reynolds number range of 100–1750. Liquid water was used as the working fluid. The results showed that the maximal Nusselt number was enhanced by 3.4–10.3 times as that of the smooth tube. The friction factor resulted in an increase of 5.6–13.5 times. Performance Evaluation Criterion (PEC) values were approximately 1.91–4.33. The Nusselt number increased with the decrease in dimensionless spacer length. A small spacer length resulted in a high heat transfer coefficient. However, it also brought a high flow resistance and eventually deteriorated the heat transfer performance. Therefore, a suitable dimensionless spacer length S/D = 6.67 was recommended in this paper. The effect of pipe shape (R/D = 3.33, R/D = 5, R/D = 6.67) on the thermal characteristic was also investigated in this study. It indicated that the pipe shape had a little impact on the heat transfer performance in the laminar regime. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their structure, because their special structure make it possible for the fluid to flow from the central region to the wall region, which reconstructs the velocity profile and temperature profile in the tube and results in a high heat transfer performance. [ABSTRACT FROM AUTHOR]

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers ( Re ), rib spacing ratio ( P / e ) and rib angles ( α ) on steam and air convective heat transfer were obtained and compared. The blockage ratio ( e / D h ) of the tested channel is 0.078 and the aspect ratio ( W / H ) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio ( P / e = 10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P / e = 8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. [ABSTRACT FROM AUTHOR]

In this paper the flow of an incompressible Newtonian fluid past a stationary unconfined spheroid under the mixed convective heat transfer regime is studied numerically. The spheroid is assumed to be maintained at a constant wall temperature. The effect of Reynolds number (Re), Prandtl number (Pr), aspect ratio ( E ) and Richardson number (Ri) on the average drag coefficient and Nusselt number is investigated. The range of parameters considered in this study is: 10 ≤ Re ≤ 100; 0 < Ri < 2 and 0.5 ≤ E ≤ 1.5 and for two Prandtl numbers of 1 and 5. The drag coefficient increases monotonically with the increase of surface temperature for all Reynolds number. Prolate spheroids have higher drag coefficient and Nusselt numbers compared to oblate spheroids. The variation of Nusselt number with Ri at a fixed Re is non monotonic at large Re and this effect is stronger for prolate spheroids when compared to oblates. For aspect ratios greater than unity and for Re > 60, Nusselt numbers for small Ri (∼0.5) are lower compared to Nusselt numbers for Ri = 0. This difference in Nusselt numbers becomes appreciable for larger Prandtl numbers. A correlation for average Nusselt number was developed using least squares regression analysis. [ABSTRACT FROM AUTHOR]

• Nusselt number measurements in a wide range of Reynolds and Rayleigh numbers resulting in a Richardson number between 0.1 to 100. • Particle image velocimetry (PIV) and Liquid crystal thermography (LCT) measurements using a refractive index matched porous medium. • Higher Nusselt number for the cavity with porous medium as compared to the pure-fluid cavity at high Reynolds numbers. • Three flow and heat transfer regimes depending on Richardson number. • New correlation for Nusselt number as a function of Reynolds and Rayleigh numbers. This paper reports on an experimental study of mixed convection flow and heat transfer in a vented, differentially side-heated cubical cavity filled with a porous medium consisting of relatively large solid low-conductivity spheres. Rayleigh numbers and Reynolds numbers are varied over the ranges 6 × 106 < Ra < 7 × 107 and 240 < Re < 4250, respectively, for a fixed Prandtl number of Pr = 6.75, thus covering more than three decades in Richardson numbers Ri = Ra/(Re2 Pr). Heat transfer measurements were combined with measurements of the velocity field (using particle image velocimetry) and the temperature field (using liquid crystal thermography) to better understand the dependence of the Nusselt number, Nu, on the Richardson number. We observed three different flow and heat transfer regimes depending on the Richardson number. For Ri < 10, the flow structure and the Nusselt number scaling are similar to those for the pure forced convection, i.e., the Nusselt number scales as Nu ~ Re0.61 independent of Rayleigh number. For Ri > 40, natural convection dominates the flow in the vicinity of the heating wall. The Nusselt number becomes less sensitive to the Reynolds number and is mainly determined by the Rayleigh number. In the intermediate regime for 10 < Ri < 40, the upward directed natural convection flow at the heating wall competes with the downward directed forced flow leading to a minimum effective Nusselt number. A Nusselt number correlation is derived that is valid in the range 0.1 < Ri < 100 covering all three regimes. [ABSTRACT FROM AUTHOR]