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A two-dimensional numerical investigation of turbulent convective heat transfer due to a confined slot jet impinging on an isothermal moving surface is accomplished. The confined geometry has an upper adiabatic surface parallel to the heated moving plate and the slot jet is in the middle of the confining adiabatic wall. The working fluids are pure water or a nanofluid, which in this case was a mixture of water and Al2O3 nanoparticles. The governing equations are written adopting the k-ε turbulence model with enhanced wall treatment and the single-phase model approach for the nanofluids. The numerical model is solved using the finite volume method with the Ansys Fluent code. Two geometric configurations regarding two values of the jet distance from the target surface are considered in the simulations. The concentration of nanoparticles ranges from 0% to 6%, with a single diameter equal to 30 nm, Reynolds numbers ranging from 5000 to 20000, and a moving surface-jet velocity ratio between 0 and 2 are examined in the investigation. The aim is to study the system behaviors by means of local and average Nusselt numbers, local and average friction factor/skin friction factor, stream function, and temperature fields. Results show that the presence of nanoparticles determines an increase in the dimensionless heat transfer but, as expected, does not affect the friction factor. The local and average increase in Nusselt numbers is also due to a combined effect of the moving plate and nanofluids.

Direct numerical simulations have been performed for heat and momentum transfer in internally heated turbulent shear flow with constant bulk mean velocity and temperature, $u_{b}$ and $\\theta _{b}$, between parallel, isothermal, no-slip and permeable walls. The wall-normal transpiration velocity on the walls $y=\\pm h$ is assumed to be proportional to the local pressure fluctuations, i.e. $v=\\pm \\beta p/\\rho$ (Jiménez et al., J. Fluid Mech., vol. 442, 2001, pp. 89–117). The temperature is supposed to be a passive scalar, and the Prandtl number is set to unity. Turbulent heat and momentum transfer in permeable-channel flow for the dimensionless permeability parameter $\\beta u_b=0.5$ has been found to exhibit distinct states depending on the Reynolds number $Re_b=2h u_b/\\nu$. At $Re_{b}\\lesssim 10^4$, the classical Blasius law of the friction coefficient and its similarity to the Stanton number, $St\\approx c_{f}\\sim Re_{b}^{-1/4}$, are observed, whereas at $Re_{b}\\gtrsim 10^4$, the so-called ultimate scaling, $St\\sim Re_b^0$ and $c_{f}\\sim Re_b^0$, is found. The ultimate state is attributed to the appearance of large-scale intense spanwise rolls with the length scale of $O(h)$ arising from the Kelvin–Helmholtz type of shear-layer instability over the permeable walls. The large-scale rolls can induce large-amplitude velocity fluctuations of $O(u_b)$ as in free shear layers, so that the Taylor dissipation law $\\epsilon \\sim u_{b}^{3}/h$ (or equivalently $c_{f}\\sim Re_b^0$) holds. In spite of strong turbulence promotion there is no flow separation, and thus large-amplitude temperature fluctuations of $O(\\theta _b)$ can also be induced similarly. As a consequence, the ultimate heat transfer is achieved, i.e. a wall heat flux scales with $u_{b}\\theta _{b}$ (or equivalently $St\\sim Re_b^0$) independent of thermal diffusivity, although the heat transfer on the walls is dominated by thermal conduction.

Passive heat transfer enhancement by spanwise rollers associated with the Kelvin–Helmholtz instability was studied through direct numerical simulations of high-aspect-ratio longitudinal ribs at the friction Reynolds number $300$. The temperature was treated as a passive scalar with Prandtl number unity to discuss the similarity between the heat and momentum transfer. The results reveal that the high-aspect-ratio longitudinal ribs lead to a favourable breakdown of the Reynolds analogy, that is, the enhancement of the heat transfer rate surpasses that of the frictional resistance. The favourable breakdown of the Reynolds analogy can be attributed to the enhanced turbulent heat flux compared with the Reynolds shear stress, whereas the rib-induced secondary flow plays a role in reducing the favourable breakdown of the Reynolds analogy. The conditional average statistics reveal that the high-pressure region accompanied by the spanwise rollers suppresses the spanwise roller-induced sweep and ejection motions, leading to smaller Reynolds shear stress than for the turbulent heat flux.

Purpose A computational fluid dynamics (CFD) analysis has been carried out on the aerodynamic and thermal behavior of an incompressible Newtonian fluid having a constant property and flowing turbulently through a two-dimensional horizontal high-performance heat transfer channel with a rectangular cross section. The top surface of the channel was kept at a constant temperature, while it was made sure to maintain the adiabatic condition of the bottom surface. Two obstacles, with different shapes, i.e. flat rectangular and V-shaped, were inserted into the channel; they were fixed to the top and bottom surfaces of the channel in a periodically staggered manner to force vortices to improve the mixing and consequently the heat transfer. The first fin-type obstacle is placed on the heated top channel surface, and the second baffle-type one is placed on the insulated bottom surface. Five different obstacle situations were considered in this study, which are referred as cases FF (flat fin and flat baffle), FVD (flat fin and V-downstream baffle), FVU (flat fin and V-upstream baffle), VVD (V-downstream fin and V-downstream baffle) and VVU (V-Upstream fin and V-upstream baffle). Design/methodology/approach The flow model is governed by Reynolds-averaged Navier–Stokes equations with the k-epsilon turbulence model and the energy equation. These governing equations are discretized by the finite volume method, in two dimensions, using the commercial CFD software FLUENT software with the Semi Implicit Method for Pressure Linked Equations (SIMPLE) algorithm for handling the pressure-velocity coupling. Air is the test fluid with the flow rate in terms of Reynolds numbers ranging from 12,000 to 32,000. Findings Important deformations and large recirculation regions were observed in the flow field. A vortex causes a rotary motion inside the flow field, which enhances the mixing by bringing the packets of fluid from the near-wall region of the channel to the bulk and the other way around. The largest value of the axial variations of the Nusselt number and skin friction coefficient is found in the region facing the baffle, while the smallest value is in the region near the fin, for all cases. The thermal enhancement factor (TEF) was also introduced and discussed to assess the performance of the channel for various obstacle situations. It is found that the TEF values are 1.273-1.368, 1.377-1.573, 1.444-1.833, 1.398-1.565 and 1.348-1.592 for FF, FVD, FVU, VVD and VVU respectively, depending on the Re values. In all cases, the TEF was found to be much larger than unity; its maximum value was around 1.833 for FVU at the highest Reynolds number. Therefore, the FVU may be considered as the best geometrical configuration when using the obstacles to improve the heat transfer efficiency inside the channel. Originality/value This study can be a real application in the field of shell-and-tube heat exchangers and flat plate solar air collectors.

This study investigates the performance of a double-tube heat exchanger employing perforated twisted tape and aluminum oxide nanoparticles. Turbulent flow conditions, considered by Reynolds numbers fluctuating from 4800 to 7800, are examined. The perforated steel tape utilized measures 1000 mm in length, and 16 mm in width, with a twist ratio of 6.25. The impact of incorporating aluminum oxide nanofluid at volume concentrations of 0.1%, 0.2%, and 0.3% on heat exchanger efficiency is analyzed. The numerical model’s validity is established through a comparison of the numerical results with experimental data, demonstrating significant agreement. Findings show that both Nusselt number (Nu) and exergy efficiency enhance with the insertion of perforated twisted tape and utilization of nanofluid. The Nusselt number ratio ranges from 127 to 146% when employing only perforated twisted tape inserts. When combined with nanofluid volume concentrations of 0.1%, 0.2%, and 0.3%, the ratio increases to 146–162%, 155–168%, and 158–175%, respectively. The pressure drop ratios range from 176 to 186% for the case with only perforated twisted tape inserts. The ratio rises to 196–201% when paired with 0.3% nanofluid volume concentrations. The perforated twisted tape increases pressure drop via added surface area and flow obstruction, while Al 2 O 3 /water nanofluid raises viscosity, exacerbating friction forces. Numerical simulations reveal uniform temperature and pressure distribution in the plain tube, with increased turbulence and swirl flow using perforated twisted tape and nanofluids.

This study presents an in-depth computational investigation of the thermohydraulic performance of water-based hybrid nanofluids containing graphene and aluminum oxide (Al₂O₃) nanoparticles. Using computational fluid dynamics (CFD) software, the research focused on understanding the behavior of these nanofluids under turbulent flow conditions in a circular tube. The analysis included examining how various parameters, including the Nusselt number, friction factor, and performance evaluation criteria, were influenced by the addition of nanoparticles. Five different nanoparticle volume concentrations, ranging from 0.1 to 1%, were analyzed. The simulation was conducted for turbulent flow regimes using Reynolds numbers between 20,000 and 80,000. A constant surface heat flux boundary condition was applied to the tube. The k-ε RNG (Renormalization Group) turbulence model was used as it is suitable for capturing turbulence effects in such flows. The thermophysical properties of the hybrid nanofluids were calculated using correlations available in the literature. The addition of graphene and Al₂O₃ nanoparticles significantly increased the Nusselt number, demonstrating enhanced heat transfer performance. The improvement in the Nusselt number was observed across all volume concentrations and Reynolds numbers. The maximum enhancement (28%) was recorded compared to pure water at 1% volume concentration. The friction factor increased with the addition of nanoparticles due to the higher viscosity of the hybrid nanofluids. The highest increase in the friction factor was 7.2% at the maximum volume concentration of 1%. The increase in viscosity contributed to an increased pressure drop in the system. However, the magnitude of this increase was relatively modest compared to the heat transfer benefits. The PEC (Performance Evaluation Criteria) value was found to be greater than 1 in most cases. The highest PEC value of 1.26 was achieved at the lowest Reynolds number (20,000) and the highest nanoparticle concentration (1%). This indicates that the use of the hybrid nanofluid is thermohydraulically advantageous under these conditions. The study concludes that water-based hybrid nanofluids containing graphene and Al₂O₃ nanoparticles enhance heat transfer performance significantly, making them suitable for applications requiring efficient thermal management. The slight increase in flow resistance was outweighed by the substantial heat transfer improvements, particularly at higher volume concentrations and lower Reynolds numbers, as reflected by the favorable PEC values.

The thermal performance of a flat plate solar collector (FPSC) is limited minimum due to higher heat loss of the collector and poor heat exchange with the heat transfer fluid. The current study investigates the effect of single-slit twisted tape (TT), double-slit TT, and double-slit straight tape passive-insert devices on enhancing the heat transfer of the FPSC at a constant water flow of 0.025 kg/s. Double-slit inserts have a higher heat transfer area and reduced hydraulic diameter than conventional single-slit TT inserts. The double-slit TT insert creates more fluid mixing and turbulence in the flow than a single-slit TT. The convective heat transfer coefficient for double-slit TT, single-slit TT, and the double-slit straight insert is 47.7%, 26.9%, and 8.7% higher than FPSC with the plain absorber. Double-slit TT inserts are observed with improved energy and exergy efficiency by 23.9% and 46.2% compared to conventional collectors without a flow insert. The average energy efficiency of FPSC without inserts is 48.2%, whereas the double-slit TT is 66.5%. The pressure drop is higher for the collector with inserts than for the collector without inserts, which leads to a little more pump power. Thus, passive inserts help to augment heat transfer in the FPSC.

Purpose This study aims to computational numerical simulations to clarify and explore the influences of periodic cellular lattice (PCL) morphological parameters – such as lattice structure topology (simple cubic, body-centered cubic, z-reinforced body-centered cubic [BCCZ], face-centered cubic and z-reinforced face-centered cubic [FCCZ] lattice structures) and porosity value ( ) – on the thermal-hydraulic characteristics of the novel trussed fin-and-elliptical tube heat exchanger (FETHX), which has led to a deeper understanding of the superior heat transfer enhancement ability of the PCL structure. Design/methodology/approach A three-dimensional computational fluid dynamics (CFD) model is proposed in this paper to provide better understanding of the fluid flow and heat transfer behavior of the PCL structures in the trussed FETHXs associated with different structure topologies and high-porosities. The flow governing equations of the trussed FETHX are solved by the CFD software ANSYS CFX® and use the Menter SST turbulence model to accurately predict flow characteristics in the fluid flow region. Findings The thermal-hydraulic performance benchmarks analysis – such as field synergy performance and performance evaluation criteria – conducted during this research successfully identified demonstrates that if the high porosity of all PCL structures decrease to 92%, the best thermal-hydraulic performance is provided. Overall, according to the obtained outcomes, the trussed FETHX with the advantages of using BCCZ lattice structure at 92% porosity presents good thermal-hydraulic performance enhancement among all the investigated PCL structures. Originality/value To the best of the authors’ knowledge, this paper is one of the first in the literature that provides thorough thermal-hydraulic characteristics of a novel trussed FETHX with high-porosity PCL structures.

Twisted tapes are widely recognized as an effective passive method to enhance heat transfer in parabolic trough solar collectors. However, a major drawback is the substantial pressure drop caused by the high friction between the fluid flow and the tape surface. This study introduces hydrophobic twisted tapes as a solution to reduce pressure drop while sustaining enhanced heat transfer. A variety of twisted tapes with differing widths and twist ratios were fabricated and tested experimentally under laminar and turbulent flow regimes, focusing on pressure drop, efficiency, and performance index. The results demonstrate that hydrophobic twisted tapes can elevate the performance index from below one to above one. At the flow rate of 50 Lph, the performance index of the twisted tape with a hydrophobic surface increased by 125%. It was observed that increasing the tape width and reducing the twist ratio led to higher pressure drop, improved efficiency, and a better performance index. Additionally, the study compared the performance of twisted tapes with fixed versus variable widths. Experimental findings revealed that twisted tapes with variable widths exhibit superior performance compared to those with fixed widths. At a flow rate of 50 Lph, the performance index of the variable width twisted tape with a hydrophobic surface increased by 143%.

Liquid-infused surfaces can reduce friction drag in both laminar and turbulent flows. However, the heat transfer properties of such multi-phase surfaces have still not been investigated to a large extent. We use numerical simulations to study conjugate heat transfer of liquid-filled grooves. It is shown that heat transfer can increase for both laminar and turbulent liquid flows due to recirculation in the surface texture. Laminar flow simulations show that for the increase to be substantial, the thermal conductivity of the solid must be similar to the thermal conductivity of the fluids, and the recirculation in the grooves must be sufficiently strong (Péclet number larger than 1). The ratio of the surface cavity to the system height is an upper limit of the direct contribution from the recirculation. While this ratio can be significant for laminar flows in microchannels, it is limited for turbulent flows, where the system scale (e.g. channel height) usually is much larger than the texture height. However, heat transfer enhancement of the order of $10\\,\\%$ is observed (with a net drag reduction) in a turbulent channel flow at a friction Reynolds number ${{Re}}_\\tau \\approx 180$. It is shown that the turbulent convection in the bulk can be enhanced indirectly from the recirculation in the grooves.