The idea of applying bio-mimetic leaf vein structures to fins is inspired by the excellent nutrient transport capabilities of leaves in nature, which is the result of natural selection, and this coincides with the principles used in heat exchangers. The study established a three-dimensional, steady-state mathematical model for the air-side fluid of a leaf vein fractal fin. Through mathematical simulations, this study focused on the impact of the fractal branch angle, fin spacing, and Reynolds number on air-side heat transfer coefficient and pressure drop with the Reynolds number ranging from 500 to 2500. Comprehensively considering the uniformity of the flow field, the air-side heat transfer coefficient, and the resistance factor, the leaf vein fractal fin with a branching angle of 30°, a first-level vein width of 1mm, a second-level vein width of 0.8mm, and a third-level vein width of 0.4mm exhibits the optimal air-side heat transfer performance. At a Reynolds number Re = 575, the heat transfer coefficient is improved by 51.6%.

In the present work, a direct oil cooling strategy using a multi-nozzle configuration is proposed for the thermal management of high-power density electric machines. The stator and winding temperatures, heat transfer coefficient, injection pressure, and power consumption are investigated for different nozzle types, nozzle numbers, heights of nozzle combinations, and oil flow rates. In addition, an artificial neural network (ANN) model based on two algorithms is developed for predicting thermal performance under various operating conditions. The flat jet nozzle shows the lowest maximum winding temperature of 120.3 °C and a superior heat transfer coefficient of 3028.6 W/m2-K compared to both full cone nozzles. The power consumption for the flat jet nozzle is higher at 123.9 W compared to other nozzle types. The combination of four flat jet nozzles shows improved oil spray distribution and enhanced cooling compared to combinations of two and six flat jet nozzles. Further, the thermal performance of oil spray cooling with four flat jet nozzles improves when height and oil flow rate are increased. Oil spray cooling with the best configuration shows a winding temperature, heat transfer coefficient, and injection pressure of 98.9 °C, 3408.6 W/m2-K and 4.86 bar, respectively, at a flow rate of 20 LPM. The proposed neural network model with a Levenberg–Marquardt (LM) training variant and logarithmic–sigmoidal (Log) transfer function shows the lowest prediction error within ±2%.

The heat transfer and flow characteristics of TiO2/R123 nano-organic working fluid are investigated and compared with that of R123. A three-dimensional numerical model of the smooth circular tube with a diameter of 10 mm and a length of 1 m is established, and the thermodynamic properties of the nano-organic working fluids are rectified with the volume of fluid model. The grid independence validation is conducted, and the simulation results from three models (the k-ε model, the realizable k-ε model, and the Reynolds Stress Model) are evaluated against experimental data. When using the TiO2/R123 nano-organic working fluid, the error between the simulation and experimental results is 6.1%. The flow field distribution is examined, and the effect of mass flux on heat transfer coefficient and pressure drop is discussed. Results demonstrated that the inclusion of TiO2 nanoparticles significantly enhances heat transfer performance. At a 0.1 wt% nanoparticle concentration, the heat transfer coefficient increases by 23.2%, reaching a range of 1430.11 to 2647.25 W/(m2·K), compared to pure R123. However, this improvement in heat transfer performance is accompanied by an increase in flow resistance, with the flow resistance coefficient rising from 0.0353 to 0.0571. Additionally, pressure drops increase by up to 18.7%.

Abstract - Heat exchangers play a crucial role in various industries, including power generation, chemical processing especially ventilation and air conditioning. Performance and efficiency of heat exchangers is observed to have dependent on various fluid flow parameters like Nusselt and Reynold numbers. In this research work, an attempt is made to investigate the performance of dimpled tube heat exchangers under different flow conditions, characterized by the Reynolds number and Nusselt Number. Results show that dimpled tubes outperform smooth tubes, with significant improvements in heat transfer rates and moderate increases in pressure drop. From tabular values and plots it was found that using spherical dimples leads to a significant increase in the heat transfer rate as compared to that of a normal tube without dimples. Also, it was seen that the change of dimple arrangement from inline to staggered arrangement enhances the heat transfer characteristics to a noticeable amount as compared to others but may further be studied for higher scale implementation with some corresponding moderations. This research has important implications for industries that rely on heat exchangers, offering a potential solution to improve their efficiency and reduce energy consumption which can aid in developing more efficient and sustainable heat transfer systems. Key Words: heat transfer coefficient (h), dimpled tube, nusselt Number (Nu), reynolds Number (Re), smooth tubes

Heat exchangers are widely recognized as eco-friendly devices that transfer heat between two or more fluids without mixing. Double Pipe Heat Exchangers (DPHE) are used in many industrial applications such as power generation, chemical processing, HVAC, and renewable energy systems. Traditional DPHEs are simple and reliable, however, they often face limitations in heat transfer. Improving the thermal performance of DPHE can significantly enhance the operational efficiency of thermal energy systems. This study presents a novel fin arrangement to the traditional DPHE using different diamond-shaped fins to improve its thermal performance. The thermal and hydraulic properties of DPHE with different diamond-shaped fin configurations are investigated using CFD analysis. The optimization process is carried out using the Response Surface Method (RSM) for optimal diamond-shaped fin design. The results indicate that novel diamond-shaped fins improve thermal performance, particularly at high mass flow rates. The thermal enhancement factor (TEF), overall heat transfer coefficient, and pressure drop are used to evaluate the thermal performance of DPHE. The diamond-shaped fins exhibit a 55% increase in overall heat transfer coefficient compared to conventional DPHE. The TEF for diamond-shaped fin configurations is higher than 1 with a maximum value of 1.63 for DPHE-HF45 depicting a 63% increase in thermal enhancement. The optimization results show that the optimal fin design achieves a desirability of 81.3%, with a pressure drop of 870.726 Pa and an overall heat transfer coefficient of 2199.85 W/m 2 K at a mass flow rate of 2.711 lit/min.

ABSTRACT Graphene nanofluid synthesis is the main aim of this study by using two surfactants, Tween 80 and polyvinyl pyrrolidone (PVP), for electronic chip cooling application. The mechanical exfoliation method was used to synthesize the graphene nanofluid from graphite with the aid of a kitchen blender. Graphite, Tween 80 concentration, and mixing time were optimized by the Taguchi method. Graphite concentration proved to be the most prominent parameter leading to graphene exfoliation as per signal-to-noise (S/N) ratio analysis. The optimized parameters were 10 g of graphite, 3 mg/ml of Tween 80, and 60 min of blending. Furthermore, the addition of 3 mg/ml of PVP in kitchen-blended graphene nanofluid using Tween 80 shows the increase in the stability of graphene nanofluid. The graphene nanofluid obtained using PVP and Tween 80 has few layers. The heat transfer coefficient (HTC) of graphene nanofluid obtained using PVP and Tween 80 is 72.4% higher than base fluid deionized water and 64.3% more than the graphene nanofluid containing Tween 80 at a flow rate of 1.5 L/min. Additionally, pumping power required is lower for graphene nanofluid obtained using PVP and Tween 80 at a flow rate of 1.5 L/min as compared to base fluid.

The Centrifugal Hypergravity Interdisciplinary Experiment Facility (CHIEF) at Zhejiang University represents a significant advancement in centrifugal modeling, set to be the world’s fastest and largest hypergravity centrifuge to date. Large hypergravity centrifuges operate with enclosed, high-Reynolds-number rotating flows where aerodynamic power and air-side heat removal both depend on pressure. Lowering chamber pressure is a common strategy to curb windage losses, but it may also diminish the convective heat-transfer coefficient (CHTC) of air. Here we perform facility-scale, plate-based CHTC measurements in a concentric annular chamber representative of CHIEF, spanning 10–101 kPa and rotation up to 1000 g. We recast the correlation in a non-dimensional form, and validate it against measurements. Reducing pressure from 101 to 10 kPa lowers the air-side CHTC by ≈74% and the aerodynamic power by ≈81.5% at 1000 g, revealing a trade-off between power savings and heat-removal capacity. The proposed correlation reproduces the measured CHTC within ±10% across the tested pressure–rotation space, while straight-duct baselines (Dittus-Boelter/Gnielinski) systematically under-predict the rotating annulus. A 95% coverage analysis for CHTC and the cooling load ( Q air ) confirms the robustness of these trends. The wall-air temperature difference decreases non-linearly with pressure, consistent with the correlation. These results quantify how vacuum pumping reshapes internal heat dissipation and provide operational guidance for selecting pressure-rotation settings that balance heat production and air-side heat removal in large centrifuges.

Films with nanoengineered surfaces hold promise for enhancing phase-change cooling and offering a solution to the bottleneck of efficient thermal management in high-power electronics and energy systems. Interfacial thermal resistance between substrates and films, however, hinders further enhancement of heat dissipation during the phase-change process. Furthermore, rapid phase-change bubble detachment at high heat fluxes generally causes peeling off of films and performance deterioration. Here, we developed a uniform carbon nanofiber film (CNFF) on a copper substrate via a facile and low-cost bubble-induced self-assembly strategy and medium-temperature annealing process for efficient phase-change cooling by synergistic surface-interface engineering. The CNFF with micro/nanoporous morphology showed superwetting to electronic fluorinated liquid and ultralow under-liquid bubble adhesion force. Additionally, C-O-Cu covalent bonding can be generated at the copper/CNFF interface due to the interfacial interaction between copper and carboxyl-functionalized carbon nanofiber during annealing of CNFF. This reliable bonding not only decreased interfacial thermal resistance between copper substrate and CNFF but also enabled robustness of CNFF against bubble detachment and liquid jetting. Therefore, the CNFF achieved the simultaneous enhancement of the critical heat flux and heat transfer coefficient up to 47.1 W/cm2 and 46.6 kW/(m2·K) with enhanced ratios of 121.1 and 380.4% over the pristine copper, respectively. Based on surface-interface engineering on CNFF, high-performance and long-term phase-change thermal management for high-power chips was finally accomplished. It is expected that the advancement in phase-change heat transfer by combining nanoengineered surfaces with interfacial covalent bonding in this work can provide an energy-efficient path for next-generation electronics cooling.

Optimization for Accurate Acquisition of Interface Heat Transfer Coefficient in Directional Solidification of Nickel-Based Superalloy

A new DCMD module design that introduces an insulation barrier of negligible thickness to divide the open duct of the hot-saline feed into two subchannels for dual-flow operation was investigated. This configuration enables one subchannel to operate in a cocurrent-flow mode and the other in a countercurrent-flow recycling mode, thereby significantly enhancing the permeate flux. Theoretical and experimental investigations were conducted to develop modeling equations capable of predicting the permeate flux in DCMD modules. These studies demonstrated the technical feasibility of minimizing temperature polarization effects while improving flow characteristics to boost permeate flux. Results indicated that increasing both convective heat-transfer coefficients and residence time generally improved device performance. The dual-flow operation increased fluid velocity and extended residence time, leading to reduced heat-transfer resistance and enhanced heat-transfer efficiency. Theoretical predictions and experimental results consistently showed that the absorption flux improved by up to 40.77% under the double-flow operation with internal recycling configuration compared to a single-pass device of identical dimensions. The effects of inserting the insulation barrier on permeate flux enhancement, power consumption, and overall economic feasibility were also discussed.

Porosity formation due to solidification shrinkage and inadequate liquid metal feeding during the casting of Sn-0.3Ag-0.7Cu (SAC0307) is a critical issue that impairs quality and subsequent processing. However, the opacity of the casting process often obscures the quantitative relationships between process parameters and defect formation, creating a significant barrier to science-based optimization. To address this, the present study utilizes finite element method (FEM) analysis to systematically investigate the influence of pouring temperature (PCT, 290–390 °C) and interfacial heat transfer coefficient (HTC, 900–5000 W/(m2·K)) on this phenomenon. The results reveal that PCT exerts a non-monotonic effect on porosity by modulating the solidification mode, which governs the accumulation of dispersed microporosity. In contrast, HTC plays a critical role in determining porosity morphology by controlling both the solidification rate and mode. Consequently, an optimal processing window was identified at 350 °C PCT and 3000 W/(m2·K) HTC, which significantly enhances interdendritic feeding and improves the ingot’s internal soundness. The efficacy of these optimized parameters was experimentally validated through macro- and microstructural characterization. This work not only elucidates the governing mechanisms of solidification quality but also demonstrates the value of numerical simulation for process optimization, offering a reliable scientific basis for the industrial production of high-quality SAC0307 alloys.

An artificial neural network (ANN) model has been developed to investigate the flow and thermal characteristics of aqueous humor (AH) within the anterior chamber (AC) of the human eye. The impact of convective heat transfer coefficients (CHTC) and thermal conductivity (TC) on intraocular fluid velocity and temperature distributions, plays a critical role in regulating ocular function and preserving overall eye health. An ANN is trained on data derived from modified Navier–Stokes equations formulated using a lubrication theory, incorporating convective and no-slip boundary conditions at the corneal surface. The model produced mathematical formulations for profile of temperature, velocity, and stream function, providing a close relation with analytical expectations. Graphical analysis highlights the ANN's ability to capture variations in AH velocity and temperature distribution with changing TC and CHTC. To ensure the reliability of our results, we validated the ANN model predictions against established experimental data and numerical simulations. Our findings align well with previous simulations, enhancing the understanding of ocular fluid mechanics and health implications.

Insights of quantitative imaging and instantaneous heat transfer coefficient measurement in spouted bed column

Correlation for convective heat transfer coefficient of a solar still glass cover with mixed convection consideration

The relentless pursuit of higher power density and miniaturization of modern electronics demand have exposed the limitations of conventional passive cooling systems. This study presents an innovative quasi-honeycomb architecture composed of vertically aligned and interconnected graphene nanosheet arrays (VIG) synthesized via plasma-enhanced chemical vapor deposition (PECVD) on copper substrates, achieving dual-mode heat dissipation through synergistic radiative and convective enhancement. The engineered graphene-copper hybrid interface demonstrates exceptional thermal performance, achieving an enhanced heat transfer coefficient of 35.6 W m-2 K-1 through synergistic optimization of infrared emissivity and specific surface area. Systematic evaluations reveal a 21.6% improvement in cooling efficiency compared to pristine copper substrates. Practical implementation as a conformal passive heat sink effectively suppresses temperature rise in high-power LED arrays (ΔT reduction: 28.1 °C at 2.7 W) and lithium-ion battery modules (thermal mitigation: 7.0 °C under 3C discharge). Notably, the ultrathin (≈2.5 μm) and ultralight (≈0.073 mg cm-2) structure enables spontaneous self-assembly on sub-100 μm metallic foils, providing geometrically adaptive heat dissipation for irregular surfaces. This work establishes a universal paradigm for developing conformal thermal management solutions compatible with geometrically complex surfaces in next-generation compact electronics.

Physical meaning of advection velocity estimated from phase delay of heat transfer coefficients

Comparative study of flow boiling heat transfer coefficient of R134a, R1234yf, and their mixture (44/56 R134a/R1234yf) in a 3 mm round tube

New generalized expression of convective heat transfer coefficients for flat steel box girders

Charging and discharging electric vehicle (EV) batteries generate heat, which impacts their longevity and operational efficiency over time. This study evaluates the thermal performance of various cooling fluids in a battery thermal management system (BTMS). N-Octadecane, hybrid nanofluid, ternary hybrid nanofluid, synthetic ester oil, and water are compared across heat transfer coefficients of 5, 10, and 20 W/m2K. N-Octadecane demonstrates superior thermal regulation, maintaining a low maximum cell temperature of 304.987 K with a minimal temperature difference of 0.684°C. Hybrid and ternary hybrid nanofluids exhibit enhanced heat transfer efficiency, while water shows effective heat distribution due to its high specific heat capacity and thermal conductivity. Synthetic ester oil, despite its strong electrical insulation, lags in thermal performance. N-Octadecane outperforms other fluids by 35–40%, surpassing synthetic ester oil by 66.5% under non-flow conditions. Moreover, varying the heat transfer coefficient has minimal influence on n-octadecane's liquid fraction (60–62% of total volume). This research emphasizes the significance of thermal properties in BTMS, advocating for the integration of PCMs and nanofluids to optimize thermal control in batteries.

Current research on single-box multi-compartment concrete box girders ignores the effect of the wind field on the temperature distribution of the box girder. Incoming wind has a significant effect on the convective thermal transfer coefficient on the surface of the box girder structure, and affects the temperature field analysis accuracy. In this paper, a single-box three-cell concrete box girder is taken as the research object; the temperature field model of the world’s largest single-box three-cell concrete box girder is established, and a dot matrix temperature field modeling test is conducted. Based on the wind field distribution characteristics of the single-box three-cell concrete box girder, more accurate convective thermal transfer boundary conditions were determined. The model’s temperature field and gradient distributions were validated and compared with traditional methods that neglect wind effects. The results showed a 71% reduction in root mean square error and a 69% reduction in mean absolute error, with the vertical temperature gradient in the web closer to actual conditions. To solve the difficult problem of determining the thermal boundary conditions at different cross sections, a wind speed reduction coefficient model of a concrete box girder with variable cross section and a single-box and three rooms is established, which is linearly related to the aspect ratio of the box girder. The influence of incoming wind speed and box girder geometry on the wind speed reduction coefficient at different cross sections of the bridge is quantitatively evaluated, and the empirical formula for the convective heat transfer coefficient used in previous research is improved.