Search Results (2,913)

This research analyzes the wavy, axisymmetric flow of a Ree–Eyring non-Newtonian nanofluid, infused with motile microorganisms, within a porous, tapered cylindrical channel under a transverse magnetic field. This investigation presents a theoretical framework that may inform the improvement of energy efficiency and thermal management in biomedical engineering applications, such as drug delivery systems and microfluidic biosensors. The work provides an extended insight by a contribution to the evaluation of entropy generation, explicitly considering the influence of motile microorganisms, thereby bridging a gap in the existing literature. The comprehensive physical model further incorporates the combined effects of Joule heating, viscous dissipation, nonlinear thermal radiation, and chemical reactions. Methodologically, the governing nonlinear equations of the system were rendered tractable under long-wavelength and low-Reynolds-number assumptions and subsequently solved using the numerical Runge–Kutta–Fehlberg technique. The key conclusion is that, based on the present numerical model, careful selection of magnetic field strength and microorganism motility parameters may reduce irreversible energy losses, potentially improving the net usable work in advanced nanofluid transport systems for biomedical applications, subject to experimental validation. The most significant finding reveals that the magnetic field serves as a dual-purpose control parameter: increasing its strength boosts total entropy generation by 20–30% while simultaneously raising the Bejan number, confirming heat transfer as the dominant irreversibility mechanism in the system. Additionally, nanoparticle concentration diminishes substantially with elevated chemical reaction rates and Schmidt numbers, while microorganism density is highly sensitive to the Péclet number, which causes flow disruptions. Full article

A solar air heater generates heated air using solar energy. This system has a relatively simple design, which reduces the initial cost and facilitates maintenance compared with other solar systems. However, its thermal conversion efficiency is limited by the poor thermal conductivity of air. Previous studies have improved thermal efficiency by enhancing either the heat transfer area or the heat transfer coefficient, but most have applied only one of these approaches. In this work, a novel solar air heater with longitudinal fins and blocks, designed to simultaneously enhance the heat transfer area and heat transfer coefficient, is investigated for various block shapes (rectangular, forward-chamfered, backward-chamfered, and triangular blocks) utilizing computational fluid dynamics. Compared to the smooth fin channel, heat transfer is enhanced by a maximum of 1.61 times with the backward-chamfered block, while the corresponding enhancement factors for the rectangular, forward-chamfered, and triangular blocks are 1.52, 1.46, and 1.54, respectively. The thermo-hydraulic performance parameter, which simultaneously evaluates heat transfer augmentation and frictional penalty, further indicates that the backward-chamfered block is most effective at Reynolds numbers below 6000, while the rectangular block performs best above 9000. Full article

Dixit et al. proposed an asymptotic drag scaling method for zero-pressure-gradient flat-plate turbulent boundary layers based on the approximation $M\sim U_{\tau }^2\delta$, where M is the kinematic momentum rate through the boundary layer, $U_{\tau }$ is the friction velocity, and $\delta$ is the boundary-layer thickness. In the present paper, an explicit Reynolds-number-dependent correction to this approximation is derived from the logarithmic mean-velocity profile. Integration of the log law across the layer yields $M\sim U_{\tau }^2\delta f\left(R{e}_{\tau }\right)$, where $R{e}_{\tau }=\delta U_{\tau }/\nu$ is the friction Reynolds number and $f\left(R{e}_{\tau }\right)$ is given analytically. Application of the correction to the dataset compiled by Dixit et al. shows that the corrected scaling gives an exponent consistent with the asymptotic value $-1/2$ within bootstrap confidence intervals, whereas the uncorrected formulation does not. The correction should be viewed as a leading-order amendment, since the derivation uses the logarithmic law outside its strict range of validity. Full article

Accurate three-dimensional characterization of turbulent flows in natural waterways is essential for the effective design of tidal farms and other critical infrastructure situated along or across rivers. High-fidelity predictions based on the large-eddy simulation (LES) method capture the necessary physics but incur computational costs that hinder rapid scenario testing. Statistically, a relatively long history of instantaneous flow fields is required to generate reliable turbulence statistics, e.g., mean velocity and Reynolds stresses, of river flow. Such a requirement often incurs high simulation runtime and data storage costs. This study seeks to develop a neural network surrogate model that learns from a limited number of instantaneous flow realizations and approximates the outputs of the corresponding time-averaged fields with LES-level accuracy. Such a surrogate would eliminate the need to accumulate extensive ensembles, enabling faster hydrodynamic assessment and making LES-informed analyses more accessible for practical engineering decisions. Full article

To enhance the thermal management of lithium-ion batteries in new-energy vehicles, various bio-inspired liquid-cooled plate channel designs were investigated to improve hotspot dissipation within the laminar flow regime. A series of three-dimensional numerical simulations were conducted to compare leaf vein-, tree branch-, honeycomb-, and spider web-inspired channels, followed by further optimization to improve thermohydraulic performance. The selected optimized bio-inspired channels were subsequently evaluated against conventional structures. Simulation results indicate that the honeycomb-inspired liquid-cooled plate channel achieved the best performance, followed by the tree branch- and spider web-inspired channels, which exhibited comparable thermohydraulic performance. The leaf vein-inspired channel demonstrated the lowest performance. The key design element for enhanced heat dissipation is the inclusion of longitudinal branch channels, which minimize flow zones with near-zero velocity and effectively mitigate local hotspots. Furthermore, the combination of longitudinal and inclined branch channels can redirect flow direction and enhance fluid mixing. Compared with the conventional channel widely adopted in existing studies, within the Reynolds number range of 260 to 920, the optimized honeycomb-inspired liquid-cooled plate channel achieves a 44.0–49.3% increase in Nusselt number and an 81% enhancement in comprehensive performance metric. Concurrently, thermal resistance is diminished by 2.6–9.2%, and pumping power is reduced by 50.0–56.8%. Full article

Fractured coal in the residual-strength stage is a primary medium for gas migration and drainage in deep mining areas. To investigate the hydro–mechanical seepage response of post-peak fractured coal under constant-pressure-difference conditions, triaxial CO₂ seepage tests were conducted on coal specimens collected from the Xinyuan Coal Mine. A Weibull-based damage constitutive model was established to characterize the confining-pressure-induced hysteresis in the damage-evolution path. The flow-rate evolution and Reynolds number analysis indicated that gas flow remained within the linear Darcy regime. A controlled-variable analysis was used to examine the competing effects governing permeability evolution. Mechanical compaction induced an exponential decrease in permeability, whereas the decrease in permeability with increasing pore pressure was interpreted, within the proposed model framework, as the combined effect of possible adsorption-induced matrix swelling and weakened gas slippage. To address the limitations of conventional constant-slip-factor models, a pressure-dependent slip modulation coefficient was introduced into a composite permeability equation incorporating effective stress, adsorption-related deformation, and dynamic gas slippage. Global nonlinear fitting yielded R² = 0.97 and an RMSE of 0.1909, with the residuals generally distributed around zero, supporting the fitting reliability of the model within the investigated stress–pressure range. Response-surface analysis identified mechanical compaction as the dominant controlling mechanism, while adsorption-related deformation and gas slippage acted as secondary correction mechanisms. The proposed framework provides a quantitative basis for distinguishing the mechanical and fluid-related effects governing permeability evolution in post-peak fractured coal. Full article

The drainage panel is one of the defining components of green roofs. Its hydraulic behavior is often assessed using simplified flow equations. However, the geometric complexity of the panel can lead to inaccuracies in estimating head losses. Numerical analysis of an individual panel element reveals that, depending on the Reynolds number, complex and highly unsteady spatial and temporal flow structures may develop, potentially affecting an accurate representation of the overall flow dynamics. Full article

This study presents a combined experimental and numerical investigation of xanthan gum solutions at 100 and 300 ppm in turbulent smooth pipe flow under temperatures of 30–50 °C and Reynolds numbers of 8000–12,000. Water was used as the Newtonian reference fluid, while xanthan gum was modelled using temperature- and concentration-dependent shear-thinning properties. Experimental pressure-drop data were used to evaluate drag-reduction behaviour, whereas numerical simulations were employed to analyse the associated flow and heat-transfer responses. The results show that XG 100 ppm produced a relatively stable drag-reduction response of approximately 31–39%, while XG 300 ppm showed a wider and more condition-dependent range of about 25–45%. Water exhibited higher Nusselt numbers of approximately 68–106. In contrast, XG 100 ppm produced Nusselt numbers of approximately 45–69, while XG 300 ppm showed lower values of about 35–61. The corresponding heat-transfer reduction ranged from approximately 26–48% for XG 100 ppm and 23–46% for XG 300 ppm. These findings confirm a clear hydraulic–thermal trade-off, indicating that the xanthan gum concentration should be optimised according to both pressure-loss reduction and heat-transfer requirements. Full article

This study experimentally investigates the mixed convection heat transfer performance of CuZnFe₂O₄–water-based magnetic nanofluids in a cylindrical minichannel under the influence of external magnetic fields. Nanofluids with three different volumetric concentrations (0.25%, 0.50%, and 0.75%) were synthesized and characterized in terms of thermophysical properties. The experiments were conducted within the Richardson number range of 0.1–10 to ensure mixed convection conditions, while magnetic field intensities of 220 G, 300 G, and 380 G were applied using custom-built electromagnets. Results show that suspending CuZnFe₂O₄ nanoparticles significantly enhances the heat transfer rate compared to pure water, mainly due to increased thermal conductivity and particle–fluid interactions. The application of a magnetic field further augments the Nusselt number by disturbing the thermal boundary layer and intensifying particle motion, leading to up to 64.4% improvement compared with pure water at similar Reynolds numbers. In addition, Analysis of Variance (ANOVA) and Response Surface Methodology (RSM) were employed to determine the most influential parameters on heat transfer performance and to develop a predictive correlation for the Nusselt number as a function of Reynolds number, nanoparticle concentration, and magnetic field intensity. The findings highlight the combined effects of nanoparticle suspension and magnetic field application as a promising approach for enhancing heat transfer in low-flow mixed convection regimes, offering valuable insights for thermal management in miniaturized cooling systems. Full article

A computational fluid dynamics (CFD) analysis is conducted to systematically investigate heat transfer enhancement in tubes fitted with grooved twisted tapes and to identify the groove geometry that provides the best thermo-hydraulic performance. Three grooved twisted tape configurations—circular-grooved twisted tapes (CGTT), rectangular-grooved twisted tapes (RGTT), and triangular-grooved twisted tapes (TGTT)—are evaluated and compared with a smooth tube and a conventional twisted tape over a Reynolds number range of 5000–20,000 under isothermal wall conditions. The grooved twisted tapes enhance heat transfer through the combined effects of swirl-induced secondary flows and groove-generated flow disturbances, which intensify turbulent mixing and reduce the thickness of the thermal boundary layer. Compared with the plain tube, the grooved configurations increase the Nusselt number by 1.472–1.98 times while increasing the friction factor by 3.21–3.58 times. Relative to the conventional twisted tape, the grooved designs provide an additional 8.0–12.1% enhancement in heat transfer with only a marginal increase of 0.2–1.5% in friction factor. The thermodynamic analysis indicates that the CGTT configuration exhibits the lowest entropy generation rate and exergy loss throughout the investigated Reynolds number range. In particular, the CGTT achieves a Bejan number of 0.999841 at Re = 5000, demonstrating an excellent balance between heat transfer enhancement and frictional losses. Furthermore, the CGTT attains the highest thermal performance factor (TPF) of 1.294 at Re = 5000 and maintains TPF > 1.0 over the entire Reynolds number range. The overall performance ranking is consistently established as CGTT > TGTT > RGTT based on comprehensive analyses of velocity fields, streamline patterns, turbulent kinetic energy distributions, temperature contours, and thermodynamic characteristics. Although the present study identifies the circular-groove configuration as the optimal design for a twist ratio (y/W) of 3.0, further parametric investigations involving variations in twist ratio, groove dimensions, and groove pitch are required to develop generalized design guidelines. Full article

Gas transport in rough nanochannels under rarefied conditions is of considerable interest in microscale and nanoscale flow applications. However, the influence of surface roughness on flow resistance in the transitional regime remains insufficiently understood. In this study, Event-Driven Molecular Dynamics (EDMD) simulations are used to investigate the effects of surface roughness height (k) and periodicity (Λ) on friction-factor behavior for Knudsen numbers between 0.25 and 0.35 and reported Reynolds numbers up to approximately 10². Here, Re is calculated from molecularly averaged density and mean velocity, the effective channel height, and the reduced MD-unit dynamic viscosity used in post-processing. Friction factors were evaluated from the equivalent pressure drop associated with the imposed periodic driving parameter after statistically steady conditions were reached. The results reveal variations in flow resistance with roughness geometry, enabling the development of empirical relations between the normalized friction factor and relative roughness. The resulting correlations describe the observed simulation trends within the parameter range investigated. In addition, velocity-profile and density-field analyses provide physical insight into the mechanisms governing the observed behavior. The findings suggest that classical continuum-based correlations may not fully capture roughness effects under the conditions investigated. The proposed formulation may serve as a practical tool for estimating friction-factor behavior within the investigated transitional rarefied-flow regime. Full article

Nanocolloid research has undergone a complete transformation, renouncing the empirical estimation of properties and relying on real case scenarios. The main objective of this paper is to compare a large number of samples that were experimentally studied in terms of thermophysical properties in order to be able to draw a conclusion in terms of the heat transfer efficiency of a certain surfactant addition to a 2 wt.% TiO₂ nanoparticle-enhanced fluid. The analysis discusses both the advantages and drawbacks in terms of surfactant type and concentration influence over the Prandtl number, thermal diffusivity, and Nusselt number, as well as the heat transfer coefficient for different Reynolds numbers in laminar flow. The investigation also includes a different figure of merits and performance evaluation criteria that are extensively employed in the literature in order to have a complete overview of the efficiency of surfactants in improving nanocolloids. In conclusion, even if surfactants are considered for improving nanocolloid stability, their drawbacks have not been debated in depth in the open literature. The main conclusion that arises from this study outlines that among all tested samples, F127 at a concentration of 0.25 wt.% consistently demonstrates the best overall performance, achieving an optimal balance between enhanced thermal properties and acceptable pumping requirements. Full article

Lattice-based heat sinks have attracted increasing attention as volumetric thermal management architectures for forced-convection cooling of high-power electronic systems. In contrast to conventional plate-fin, pin-fin, and straight-channel configurations, lattice geometries promote three-dimensional flow–solid interaction through interconnected ligament networks that modify boundary-layer development, wake formation, and internal heat-spreading pathways. This review synthesizes recent experimental and numerical studies to examine the thermo-fluid mechanisms governing lattice performance, with emphasis on the coupled influence of porosity, ligament dimensions, topology, orientation, and channel confinement on heat-transfer enhancement and hydraulic resistance. The analysis indicates that while lattice structures can increase average Nusselt number and improve temperature uniformity, these gains are intrinsically linked to pressure-drop penalties associated with flow tortuosity and form drag, resulting in regime-dependent thermal-hydraulic behavior. Apparent discrepancies reported across the literature are frequently attributable to differences in geometric definition, Reynolds-number normalization, and boundary-condition specification rather than to inconsistencies in physical mechanisms. By consolidating geometric scaling, performance metrics, manufacturing considerations, and system-level constraints, this review clarifies the conditions under which lattice heat sinks may provide net benefit relative to conventional cooling technologies and identifies key research directions required to support application-relevant design and evaluation. Full article

Liquid bridge formation between wet granular governs a wide range of industrial processes. In experiments aimed at observing the volume and evolution of liquid bridges, the ability to form stable and uniform liquid films on particle surfaces is an essential prerequisite. However, existing experimental setups are incapable of maintaining such uniform coating, thereby precluding a complete characterization of the bridge evolution dynamics. To address this gap, a new experimental setup is developed in this work. Uniform liquid film coating on spherical particles is achieved for the first time. The formation process is captured by high-speed imaging, and the control variable method systematically quantifies the effects of liquid film thickness, distance between two particle surfaces, and particle radius ratio on the dimensionless liquid bridge volume. Quantitatively, increasing the dimensionless liquid film thickness by 0.01 raises the maximum dimensionless liquid bridge volume by 0.2; enlarging the dimensionless initial particle spacing from 0.067 to 0.133 and 0.200 reduces the maximum dimensionless liquid bridge volume by 3.0% and 4.9%, respectively; and a radius ratio of 6:4 lowers the maximum dimensionless liquid bridge volume by 10.9% compared to 6:6. The Reynolds number exhibits no discernible effect within the viscous-dominated regime investigated. Full article

Meso-scale combustors experience major challenges associated with flame instability, excessive wall heat losses, and limited reactant residence time due to their high surface-to-volume ratios. This study numerically investigates the thermo-fluid behaviour of hydrogen-fuelled vortex flames in a frustum meso-scale combustor under stoichiometric conditions (φ = 1.0). Three outlet-diameter configurations of 6 mm, 8 mm, and 10 mm were analysed under stoichiometric hydrogen–air conditions at air mass flow rates of 40, 80, and 120 mg/s, corresponding to Reynolds numbers of approximately 624–1780, with Computational Fluid Dynamics (CFD) used to evaluate the influence of combustor geometry on thermal confinement, wall temperature distribution, and flame stabilisation characteristics. The numerical simulations were performed in ANSYS Fluent 14.0 using the RNG k–ε turbulence model coupled with the Eddy Dissipation combustion model. The results indicate that reducing outlet diameter significantly enhances thermal confinement and recirculation behaviour within the combustor core. The temperature contours showed a maximum flame temperature of approximately 2.23 × 10³ K, while the 6 mm outlet configuration produced a more compact and axially elongated high-temperature core compared with the 10 mm configuration. The 6 mm outlet enhanced thermal localisation by approximately 10.4% and increased residence time by 66.8% relative to the 10 mm outlet. The peak inner wall temperature ranged from approximately 752 K to 1085 K depending on outlet diameter and mass flow rate. The 6 mm outlet exhibited the highest average wall temperature of approximately 909 K, followed by the 8 mm outlet (879 K) and the 10 mm outlet (838 K). Compared with the 10 mm outlet, the 6 mm configuration increased the average wall temperature by approximately 8.5%, indicating improved thermal confinement and heat retention within the combustor. These results indicate that outlet diameter strongly influences the balance between thermal confinement, flame stabilisation, and flow resistance. Full article