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Heat transfer systems have been employed in numerous industrial and residential settings to convert and recover heat. One effective approach for enhancing heat transfer rates involves incorporating artificial roughness in the form of regularly spaced ribs on the heated surface. In this particular study, the researchers conducted numerical simulations to analyze the turbulent heat and flow properties within a rectangular duct of a solar air heater (SAH). The duct featured three different types of transverse rib shapes: equilateral‐triangular, trapezoidal, and square sectional ribs. The governing equations have been solved numerically using a finite‐volume technique via ANSYS FLUENT. For solving the transport equations of turbulent kinetic energy and its dissipation rate, the RNG k–ε $\varepsilon $ turbulence model was utilized. At a broad range of Reynolds numbers (6000 ≤ Re ≤ 18,000), relative roughness height (0.021≤e/Dh≤0.043 $0.021\le e/{D}_{h}\le 0.043$), and a fixed relative roughness pitch (P/e=10 $P/e=10$), the Nusselt number (Nu ${Nu}$), friction factor (f $f$), and thermal performance factor (TPF) have been examined. The maximal Nu was obtained for equilateral‐triangular ribs, while the minimum friction factor was seen for square sectional ribs. The highest TPF of 1.591 was achieved with equilateral‐triangular ribs at Re=6000 ${Re}\,=\,6000$ and e/Dh = 0.032. Contours of kinetic energy, turbulent intensity, velocity, and pressure produced significant findings for comprehending the SAH's physical phenomena. [ABSTRACT FROM AUTHOR]

The function of energy is becoming increasingly vital in meeting the requirements of modern societies and sustaining rapid economic and industrial growth globally. Heat transfer equipment has been employed for heat recovery and conversion in various domestic and industrial applications. Therefore, due to the recent global energy crisis, boosting the thermal efficiency of energy systems has become an essential requirement, which would reduce both their size and rates of energy demand. There are active and passive methods for boosting heat transfer rates. As they have no moving components, passive methods are more affordable and dependable than active ones. Applying two or more passive techniques concurrently will result in a higher heat transmission rate than any approach working independently. The current article comprehensively reviews experimental and computational investigations of passive compound forced convection heat transfer augmentation techniques at laminar, transition, and turbulent flow regimes. This article focuses on compound techniques arrangement in the form of turbulators, typical twisted tapes (TTs), surface roughness, vortex generators, and so on. The pioneering research suggests that using a lower twist ratio, lower pitch, and smaller winglet angles in TTs can result in higher heat transfer rates, albeit with a slightly increased friction factor. The combination of alternate‐axes and wings in TTs leads to more effective heat transfer enhancement within the tube. [ABSTRACT FROM AUTHOR]

In this paper, the performance characteristics of an ejector-expansion refrigeration cycle using six low GWP alternative refrigerants for R134a are presented through the first and second laws of thermodynamics. The ejector is modelled by assuming a constant-mixing pressure. The investigated refrigerants are compared based on the optimum ejector area ratio, discharge temperature, volumetric cooling capacity (VCC), coefficient of performance (COP) and exergy efficiency (ηex). The drop-in analysis is considered based on the same operating/designing parameters, and the VCC, COP and ηex improvements over the conventional refrigeration cycle are presented. The improvements in COP and VCC are between 7.914%–18.46% and 5.712%–22.82%, respectively, for the investigated range of evaporating and condensing temperatures. These values are higher than that of R134a by about 21% and 16%, respectively. The VCC using R1234yf is very close to R134a with a maximum reduction of 6.5%. Therefore, R1234yf refrigerant appears to be the best alternative to R134a in an ejector-expansion system with an appreciable COP and ηex devaluation. R152a outperforms R134a in terms of COP and ηex, thus it is a suitable alternative to R134a except for safety. [ABSTRACT FROM AUTHOR]

The convective heat transfer coefficient in solar air heaters suffers from a reduction due to the formation of a laminar sub-layer between the absorber plate and the air in motion. A viable and efficient strategy to augment the heat transfer coefficient involves introducing artificial roughness in the form of ribs. This study employs numerical simulations to examine the turbulent heat and flow characteristics within a rectangular duct of a solar air heater (SAH) featuring trapezoidal ribs as roughness elements. The governing equations were numerically solved using the finite-volume method and ANSYS FLUENT. The transport equations of turbulent kinetic energy and its dissipation rate were addressed by the RNG k-ε turbulence model. The Nusselt number (Nu), friction factor (f), and thermal performance factor (TPF) were computed across a broad range of conditions, including Reynolds number (6,000≤Re≤18,000), relative roughness height (0.021≤e/Dnd relative roughness pitch (5≤P/e≤20) under constant heat flux conditions. It was determined that the maximum enhancement ratio of Nu was 2.124 times that of a smooth duct when e/Dh=0.043, P/e=20, and Re=6,000. Additionally, the TPF for the SAH featuring trapezoidal ribs attained a peak value of 1.64. [ABSTRACT FROM AUTHOR]