This study investigates a hybrid tab-cooled heat sink combining sintered-wick copper heat pipes embedded in a high-conductivity aluminum block with computer numerical control machined serpentine liquid cooling channels for a 15-cell lithium-ion battery pack (capacity: 2.5 Ah per cell, parallel connection). Performance was evaluated under constant-current discharge rates (1, 3, and 4 C, where C denotes nominal capacity rating) and automotive drive cycles (Urban Dynamometer Driving Schedule (UDDS), Short Dynamometer Driving Schedule (SDDS), and Worldwide Harmonized Light Vehicles Test Procedure) at coolant flow rates from 0 to 0.5 liters per minute. At 3 C discharge, peak temperature decreased from 81.6 degrees C to 62.9 degrees C with active cooling, extending safe operation from 50 to 93% depth-of-discharge before reaching the 60 degrees C threshold. Temperature variation among cells was reduced by 40%. Dynamic drive cycles showed peak temperature reductions of 19 degrees C (UDDS) and 22 degrees C (SDDS). Thermal resistance analysis revealed maximum reductions of 66% (1 C) and 39% (3 C) versus baseline. At low discharge rates, the system is convection-limited, making liquid cooling highly effective. However, at high rates (3-4 C), heat pipe operational limits become the primary constraint, indicating applications requiring sustained operation above 3 C would benefit more from enhanced heat pipe capacity than from increased liquid cooling flow rates.
This study experimentally investigates the pool boiling heat transfer characteristics of R134a on the outer surfaces of three double-sided enhanced tubes with different surface structures at a saturation temperature of 6 degrees C (corresponding to a saturation pressure of 361.98 kPa) and heat fluxes ranging from 25 to 60 kW/m2. The internal helical grooves enhance boundary-layer disturbance and turbulent mixing, while the external fin arrays increase the available surface for bubble nucleation and the density of active nucleation sites, thereby promoting bubble generation and departure. The experimental results show that the refrigerant-side heat transfer coefficients of the E1, E2, and E3 tubes are 5.78-6.79, 5.33-6.71, and 4.31-6.02 times higher than those of the smooth tube, respectively. When the heat flux exceeds 35 kW/m2, the E1 tube facilitates bubble growth and timely detachment, reduces thermal resistance, and exhibits superior heat transfer performance. Four existing correlations were evaluated, and a new correlation was developed based on the present experimental data. The proposed correlation predicts 93% of the measured data within +/- 15% error.
The present study investigates the entropy-generation and irreversibility characteristics of air flow across a fin-and-tube heat exchanger fitted with rectangular winglet pairs (RWPs). A three-dimensional computational fluid dynamics analysis was carried out for Reynolds numbers between 2000 and 4000 and angles of attack of 15 degrees-60 degrees, considering RWP locations at upstream, adjacent, and downstream positions relative to the tube centerline. The total, thermal, and viscous entropy-generation rates were evaluated using second-law formulations, and exergy-based parameters such as the entropy-generation ratio (EGR), irreversibility distribution ratio, Bejan number, and thermal efficiency index were employed to assess overall performance. Results show that an upstream RWP at 30 degrees angle of attack and Reynolds number of 3000 provides the best thermodynamic performance, reducing the total entropy-generation rate by approximately 17% (EGR = 0.83) and increasing the heat-transfer improvement number by about 18%. The Bejan number remains above 0.95 for all cases, confirming that heat-transfer-related irreversibility dominates over viscous effects. Exergy analysis further indicates that the total irreversibility decreases while the thermal improvement factor increases with optimized RWP placement. The findings offer practical guidance for the design of energy-efficient fin-tube heat exchangers using vortex generators.
The formation of frost has substantial impacts on various industries such as aviation, refrigeration, and air conditioning. This process is highly intricate, involving transient heat and mass transfer. Recently, significant attention has been given to the periods of frost growth through physical, mathematical, and experimental approaches. The lattice Boltzmann method has recently gained recognition as an effective tool for simulating complex phenomena. Despite the widespread use of circular cylinders in the refrigeration industry under natural convection conditions, research on frost growth on these surfaces has been limited. Therefore, this study aims to explore the characteristics of frost growth on circular cylinders using the lattice Boltzmann method. The findings indicate that although the computational time for the lattice Boltzmann method is less than twice that of the finite volume method, its implementation is significantly simpler and requires less coding time. Also, in problems where the system has phase changes and mass transfer, in addition to simplification and less coding time, the computational time of the lattice Boltzmann method is also much less than that of the finite volume methods. In conventional Computational fluid dynamics approaches such as the finite-volume method, modeling phase-change phenomena typically requires additional closure relations, interface-tracking schemes, or empirical correlations, which introduce uncertainties and complexity. In contrast, the lattice Boltzmann method inherently captures microscale transport mechanisms without relying on such auxiliary models, resulting in a simpler formulation and more accurate prediction of frost evolution on the cylindrical surface. Finally, the results demonstrate that the relative error between experimental outcomes and the numerical model ranges from 5 to 20%, with the finite volume method exhibiting a similar order of error.
Selecting the optimal working fluid is essential for enhancing the performance of medium-to-low temperature geothermal organic Rankine cycle systems, which must achieve a balance between energy efficiency, economic viability, and environmental sustainability. Current multi-criteria evaluation methods often assign weights subjectively, lacking a quantitative basis for trading off different performance objectives. This study establishes a thermodynamic model to analyze five organic working fluids. System performance is evaluated using indicators such as net output power, thermal efficiency, exergy efficiency, total heat transfer area, and annual carbon dioxide emission reduction. A comprehensive evaluation framework is developed by integrating the analytic hierarchy process with the entropy weight method, which combines expert judgment with objective data to derive balanced weighting criteria. Results show that the fluid designated R1234ze (E) exhibits the best overall performance, effectively harmonizing energy, economic, and environmental benefits. The proposed hybrid method offers a more reliable tool for working fluid selection, and the findings provide practical guidance for designing efficient and sustainable geothermal power generation systems.
Conventional solar collectors are impeded by low convective heat transfer rates, leading to low conversion efficiencies. Mathematical models are formulated in the present study to simulate the thermal response of a double-pass solar air heater (DPSAH) for evaluating the influence of absorber plate configuration and associated geometric features. Initially, flat plate absorber DPSAH is simulated at a specific mass flow rate for its performance with different aperture heights and number of glass layers. Maximum thermal performance is achieved with double-glazed flat plate DPSAH of 0.05 m aperture height at the flow rate at 0.025 kg/s. With these characteristics, thermal and thermo-hydraulic performance simulations were carried out further on DPSAH with different absorbers. Outlet air temperatures were obtained as 338.11, 346.23, 346.67, and 348.79 K, for DPSAH with flat plate, V-corrugated, longitudinal-finned, and porous mesh absorbers, respectively. The maximum thermal efficiencies are 69.72%, 87.03%, 88.43%, and 94.17%, while thermo-hydraulic efficiencies are 54.4%, 69.5%, 70.7%, and 75.7% for flat plate, V-corrugated, longitudinal-finned and porous-mesh DPSAHs, respectively. Porous mesh DPSAH achieves the highest exit air temperature, thermal efficiency, and thermo-hydraulic efficiency of 348.79 K, 94.17%, and 75.73%, respectively. The predicted outcomes of thermal and overall performance are successfully validated with literature.
This paper presents a numerical investigation of the Nusselt number for compressible gas flow in a microtube under constant wall heat flux values, both positive and negative, ranging from -20000 to 20000 W/m2. The Nusselt number is defined based on the difference between the surface temperature during heat transfer and the adiabatic wall temperature, which represents the temperature in the absence of heat transfer. In high-speed microchannel gas flow, the gas temperature decreases as thermal energy is converted into kinetic energy due to significant gas expansion and flow acceleration near the outlet. This approach prevents negative Nusselt numbers or heat transfer coefficients when the gas temperature falls below the wall temperature, particularly in cases involving negative wall heat flux. The numerical methodology is based on the arbitrary Lagrangian-Eulerian method. The Nusselt number is determined by solving the governing equations incorporating density variation and viscous dissipation for compressible flow. The results reveal that Nusselt numbers calculated using adiabatic wall temperatures differ from those obtained using bulk temperatures and are lower than those observed in incompressible flow. A correlation is proposed for predicting the heat transfer coefficient in gaseous flow through microtubes.
Hydrophobic surfaces are well recognized to eliminate the waiting period between bubble departure and subsequent nucleation. Present work numerically investigates the growth process of an isolated bubble in nucleate pool boiling (NPB) from curved hydrophobic surfaces, specifically plane, concave, and convex surfaces. The commercial software ANSYS Fluent 2021 R1 has been employed, utilizing its integrated volume of fluid method. Water is chosen as the working fluid and the phase change process at the liquid-vapor interface has been modeled using the "saturated-interface-volume" phase change model. This was implemented using a user-defined function. The influence of surface concavity on NPB has been analyzed through extensive study of bubble growth parameters, such as bubble morphology, bubble growth period, bubble radius and bubble base radius. A comparative analysis has been performed to demonstrate the role of wettability on bubble growth and associated heat transfer with change in curvature of the surface. Temporal and spatial variations of surface heat flux have also been examined for different values of surface curvature. Moreover, the effect of wall superheat on bubble growth and heat transfer has been analyzed for different curved hydrophobic surfaces. It is observed that the growth period of the bubble decreases with change in surface curvature from concave to convex. In the early stage of bubble growth, the heat flux on convex surface is found to be lower than that on the concave surface.
This study presents a comparative numerical investigation of a novel cross-flow heat exchanger with slab fins for dual-fluid operation integrated with Phase Change Material (PCM) in single-pass (SP) and multi-pass (MP) configurations, aimed at improving thermal comfort in electric vehicle cabins during engine idle periods, such as traffic light stops, by introducing a novel SP geometry. While most previous work has focused on MP heat exchangers, the research gap lies in developing a simple, easily manufacturable single-pass heat exchanger without curved bends that can promote more uniform heating, melt a larger fraction of PCM, and enhance passive heating performance. To address this gap, this study introduces and evaluates an SP PCM-to-air and liquid heat exchanger using three-dimensional transient computational fluid dynamics simulation. The analysis compares PCM melting behavior during charging and its effect on thermal output during discharging. Results show that for PCMs with high melting point, the SP design achieves a more uniform distribution of heat transfer fluid with higher temperature and improved melting efficiency, whereas the MP configuration performs better for low melting point PCMs. These differences significantly influence discharge performance: the SP design maintains air outlet temperatures above 25 degrees C for around 60 s longer and increases total heat absorption by the air by up to 30 kJ. Using N-Octadecane, the SP system delivered the highest cumulative heat release of about 290 kJ. The findings highlight the importance of PCM selection and the effect of geometry on maximizing latent heat utilization and enhancing vehicle heating energy efficiency.
The present study investigates fluid flow and heat transfer characteristics past rectangular cylinders of various aspect ratios with active (suction or blowing applied at filleted sharp corners) and passive (sharp corners with fillet) flow control techniques for low Reynolds numbers. The equations governing fluid flow and heat transfer are solved using an in-house solver based on the Streamline Upwind/Petrov-Galerkin finite element algorithm in Cartesian coordinates. The results are validated against literature data, and parametric studies were conducted for fluid flow past rectangular cylinders of different aspect ratios (1, 3, and 5) at a Reynolds number of 150, with non-dimensional fillet radius varying from 0.0 to 0.5. Suction or blowing velocities of 5%, 12%, and 20% of the inlet velocity are applied at fillets. Results demonstrate that integrating active and passive flow control reduces drag by 19.5% for rectangular cylinders with an aspect ratio of 3 and 35% for rectangular cylinders with an aspect ratio of 5, which may be attributed to the varying pressure distribution across the cylinder surface. The temperature contour indicates an even temperature distribution, thereby mitigating the formation of thermal hot spots. Thermal performance factor analysis reveals that the rectangular cylinder with an aspect ratio of 5 and a non-dimensional fillet radius of 0.5 exhibits significantly enhanced heat transfer performance when flow control strategies are implemented, outperforming the baseline configuration while incurring acceptable pressure penalties.
The thermal performance is crucial in heat exchange and cooling systems, and surface structure optimization to enhance performance has become a key topic in fluid dynamics research. This study uses numerical simulations to investigate the effect of different surface patterns on the thermal behavior of a tandem cylinder system. The system consists of cylinders with identical diameters (D) and a spacing of 4D, where the upstream cylinder has grooves and the downstream cylinder is smooth. The study is conducted under laminar flow conditions (Re <= 200), analyzing three groove structures (square, triangle, and dimple) and exploring the impact of different groove frequencies (N = 2, 4, 6, 8, 10) on thermal performance. The results indicate that, at Re = 100 and N = 10, square grooves promote stable secondary vortices, thereby suppressing the continuous renewal of near-wall vorticity. In contrast, triangular and dimpled grooves enhance shear-layer oscillations and reattachment processes, resulting in a more uniform and strongly unsteady near-wall vorticity distribution. Moreover, the heat transfer efficiency increases with Re. Specifically, at Re = 200, dimpled and triangular grooves show a 2% and 2.4% improvement in heat transfer compared to square grooves. In general, higher groove frequencies reduce heat transfer efficiency, but at Re = 200 and N = 10, the heat exchange performance of dimpled and triangular grooves outperforms square grooves. The study also establishes a correlation between time-averaged Nusselt number to groove area, Re, and N.
Accurate and efficient prediction of the airflow temperature field is crucial for high-geothermal operational tunnels. This paper presented an approximate analytical framework that (i) coupled the air energy balance with radial heat conduction in surrounding rock, (ii) accounted for non-homogeneous inlet/outlet boundary conditions, and (iii) incorporated internal heat sources from vehicles and lighting via heat generation term. Dimensionless solutions for air and rock temperatures were derived using advanced mathematical methods and validated with experimental data and field measurements, showing strong agreement (laboratory rock temperature deviations <= 5%, air temperature deviations in a 3.26-km tunnel <= 1 degrees C). The model accurately quantified axial air temperature variations and key parameter influences. Parametric analyses under the computed parameters showed that: (1) tunnel length significantly impacted air temperature distribution, with a linear increase in a 500 m tunnel and a quadratic rise in a 2000 m tunnel; (2) the heat-adjusting layer radius didn't affect the maximum air temperature location, occurring at 1.14 km for a 1.5-km tunnel; (3) the insulation's critical thermal conductivity decreased with lower inlet velocity or thinner insulation (e.g., 0.08112 to 0.05866 W/m & centerdot;K). The framework enables rapid, accurate predictions for ventilation, insulation optimization, and supplemental cooling placement.
Due to its highly favorable hydrodynamic and thermal properties, liquid sodium is one of the candidates as a coolant in fast-breeder nuclear reactors. However, it is imperative that liquid sodium, employed in primary and secondary circuits, maintains nuclear-grade purity. The primary impurities of concern includes oxide, hydride, and carbide. In pool-type fast reactors like prototype fast breeder reactor, an online purification process employing a cold trap (CT) removes impurities from liquid sodium. CT works on the principle of crystallization and precipitation of impurities on a wire mesh, when the temperature of sodium is reduced below the saturation temperature. As impurities precipitate within the CT, the mesh's efficiency decreases. In order to understand and optimize the CT design, a computational fluid dynamics model has been developed to predict the impurity precipitation. Additionally, a numerical methodology has been developed to estimate the total hydride precipitation and regeneration time required. The analysis revealed a total sodium hydride impurity precipitation of 1820 kg over 4.5 years, emphasizing the effectiveness of the CT purification process.
Ensuring effective thermal management of lithium-ion batteries under challenging conditions-such as high ambient temperatures and rapid discharge rates-is critical for maintaining the operational safety and reliability of electric vehicles. Therefore, this study implemented a passive battery thermal management system utilizing a composite phase change material matrix augmented with expanded graphite, positioned circumferentially around the lithium-ion cell to augment heat dissipation and maintain optimal operating temperatures. The results indicated that under an ambient temperature of 35 degrees C and a high discharge rate of 5 C, the battery cell temperature escalated to 50.57 degrees C when no active or passive thermal management system was employed. To enhance the thermal performance of the composite phase change material system, varying concentrations of expanded graphite were incorporated into RT44HC, alongside adjustments in material thickness. The composite phase change material containing 12 wt% expanded graphite with a 7.5 mm thickness was the optimum battery thermal management system condition, exhibiting a lithium-ion battery temperature of 39.34 degrees C. This study provides critical insights into enhancing the thermal management of lithium-ion batteries through the integration of composite phase change materials, thereby facilitating the design of compact and efficient battery thermal management systems.
In this work, magneto-ferro-nanofluidic thermal convection and entropy generation are investigated through the examination of heat transfer efficiencies and entropy production (both local and total) in a circle-modified butterfly-shaped cavity. The cavity features strategic protrusions at both top and bottom portions with partial heating and cooling configurations. The heated surface is maintained at a steady temperature of T-h, while the upper section is cooled to T-c. The other circular regions are kept as adiabatic boundaries. The heat flow dynamics within the cavity are characterized through analysis of isothermal contours, streamlines, heatlines, and entropy generation profiles under varying flow-determining parameters. The investigation encompasses several key parameters including Rayleigh number (10(4) <= Ra <= 10(6)), Hartmann number (0 <= Ha <= 100), and magnetic field inclination angle (0-150 degrees). The flow characteristics demonstrate strong dependence on both Ra and Ha, with distinct multi-vortex structures emerging within the cavity under various flow conditions. The findings of this work advance the understanding of geometry-optimized heat transfer systems and entropy minimization strategies for industrial applications.
Accurate prediction of plate heat exchanger (PHE) performance is essential for the design of reliable thermal management systems. However, conventional approaches-including empirical heat-transfer correlations and the epsilon-NTU method-typically assume ideal flow distribution and therefore often fail to capture the degradation present in actual PHEs. To address this limitation, this study proposes a practical epsilon-NTU method that incorporates an effectiveness efficiency, defined as the ratio of the effectiveness of an actual PHE to that of an idealized counter-current PHE with identical geometry and operating conditions. The ideal reference model, defined with the same effective heat-transfer area and flow passage area as the actual PHE, assumes perfectly uniform flow distribution and provides an upper performance bound for the given configuration. Effectiveness efficiency is correlated with the number of transfer units (NTU) and heat capacity ratio and can be determined using only a limited set of calorimetric measurements. The proposed methodology was validated through flow-visualization experiments and calorimetric tests using 4-plate and 22-plate PHEs under various flow arrangements and fin orientations. Results indicate that flow maldistribution can reduce effectiveness efficiency by up to 40%, and the proposed epsilon-NTU relationship predicts the actual thermal performance within a deviation of +/- 2.2% from the experimental data. These findings show that the method provides a practical and accurate tool for performance prediction of PHEs within the tested conditions and offers a simple framework for incorporating flow-distribution degradation into epsilon-NTU-based design.
The present study performs a comprehensive computational analysis of thermofluidic transport within a shear-driven inclined square enclosure containing a centrally positioned rotating porous cylinder, under a two-dimensional laminar mixed convection regime. The top boundary enacts a uniform translational motion, while thermal boundary impositions entail an isothermal heated base, a cooled lid, and adiabatic lateral walls. The inclination angle of the cavity is varied from 0 to 90 degrees to investigate how orientation affects the flow and the heat transfer characteristics. The interplay between the natural and the forced convection is examined by regulating the Richardson number across a moderate range of 0.5 to 5, while the permeability effects are captured through variations of the Darcy number. The influence of cylinder rotation is analyzed by applying nondimensional rotational velocity between 0 to 5, both clockwise and counter-clockwise. The results reveal a highly nonlinear interaction among shear-driven vortices, buoyant forces, and rotational motion. Entropy generation analysis shows that the thermodynamic performance is optimized when the cavity is vertical and buoyancy is weak, while the highest irreversibility occurs in the horizontal orientation with strong clockwise rotation highlighting critical configurations for thermal optimization.
