Volume 50, Issue 4

Enclosure design is essential for thermal engineering technology and applications, including electronics, heat transfer equipment, power reactors, cooling mechanisms, solar energy systems, and nuclear power plants. This study aims to analyze the numerical and Response Surface Methodology (RSM) optimization for natural convection and entropy generation in a wavy triangular cavity with Casson fluid under inclined magnetohydrodynamic and radiation influences. The finite element approach (FEM) is utilized to compute the numerical solution for the simulation framework, while RSM is applied to determine the optimal heat transfer rate among four different parameters. The study presents streamlines, velocity profiles, isothermal lines, total entropy generation, and average Nusselt number in graphically and tabularly. The results show that an increase in the number of undulations and the Casson parameter leads to an increase in the thermal transfer rate and total entropy generation, whereas the Hartmann number has a decreasing effect on both. The Nusselt number rises with the rising number of undulations and the radiation parameter. The peak stream function is observed at an inclination angle of 60°. The significant R 2 value of 0.9967 shows a good agreement between the expected and actual values.

Accurately predicting turbulent water flow in duct systems remains a challenging problem, particularly when anisotropic turbulence effects are significant. Bridging the gap between industrial applications and academic research requires a deeper understanding of such complex flows. This study investigates a less commonly analyzed configuration involving a horizontal aluminum duct transitioning into a converging wavy duct. The wavy section consists of 2.5 full sinusoidal periods, ending in a reduced outlet diameter. In addition, the effect of incorporating four minor/secondary inlets, arranged as branches at different angles, was examined and presented herein. Aluminum was selected for its low density and corrosion resistance, which are beneficial in experimental and industrial setups. Initially, the duct was analyzed in an unbranched configuration. The study then progressed to include the four secondary/minor branch inlets at various angles. The simulation results were validated by comparison with a solution for a simple flow in a 70 mm duct. Additional verification was provided by employing other CFD codes, along with grid convergence index and mesh sensitivity analyses, improving the confidence in the simulation results. Branch angles influences turbulence intensity depending on flow conditions and angle magnitude. Sharper branch angles are particularly effective, inducing greater turbulence at the converged outlet. Higher inlet temperatures and velocities lead to increased Reynolds stress due to enhanced energy transfer and elevated turbulent kinetic energy. Specifically, an increase in inlet velocity at a 45 ° branch angle further augments turbulent momentum transfer, resulting in more controlled mixing along the duct.

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) inherently produce waste heat, leading to component degradation, increased cooling demands, and reduced efficiency and longevity. To mitigate these challenges, this study introduces isopropanol-acetone-hydrogen chemical heat pumps (IAH-CHPs), selected for their proven ability to efficiently upgrade and store the waste heat from HT-PEMFCs in a high-value form. Grounded in thermodynamic and electrochemical principles, a comprehensive mathematical model, incorporating key irreversible losses, is developed to evaluate the potential. Numerical calculations predict a 29 % increase in the hybrid system’s maximum power density compared to a standalone HT-PEMFC operating at 443 K, along with corresponding enhancements of 14.17 % in energy efficiency and 14.16 % in exergy efficiency. Preliminary predictions confirm the feasibility of this approach, and the optimal operating ranges for maximizing power density are identified. Additionally, exhaustive parametric studies reveal the impacts of various structural and operational parameters – such as leakage current density, phosphoric acid doping, relative humidity, operating temperatures, and critical factors within the heat pump cycle – on the system’s thermodynamic performance and key current density indicators. Local sensitivity analyses highlight effective performance regulation strategies. These results provide essential insights for mitigating waste heat challenges, enhancing system efficiency, and extending the operational lifespan for HT-PEMFCs.

We propose a new low-dissipation model for heat engines with two cold reservoirs and one hot reservoir. This model is useful for studying systems with two energy rejection channels toward two cold sources of very different temperatures, where the energy fluxes per channel are of very different magnitudes. We show that it is necessary to add a new parameter to the model: the reversible entropy partitioning ratio, which ensures the model’s correctness in the reversible limit. We study the properties of some heat engines when operating at maximum power, demonstrating that the new model allows the system’s working properties to be optimized almost as easily as other thermal engine models in the low-dissipation approximation. In the final part of the work, we numerically study some properties of the machines when operating at maximum power.

The current trend of reducing the size of electronic devices in the industry has extensively increased the demand for effective heat dissipation, thereby intensifying the need for high-performance heat-dissipating devices. A promising approach to solve this challenge is the use of single-phase (SP), two-phase (TP), and supercritical fluids in micro-channels (MCs). Two-phase cooling is applicable only to those devices in which the tip temperature is high enough to allow the cooling fluid to convert into a two-phase state. In all other cases, only single-phase cooling can be utilized. In this work, numerical and experimental investigations on MC have been performed using water as the working fluid to predict TP behavior and heat dissipation from electronic devices using SP and TP flow. A numerical model of flow boiling heat transfer was developed based on conservation equations, which is solved to identify the existence of single and two-phase regions in the MC and to study the variation of pressure along its length at different heating powers. Further, experiments were performed in both SP and TP conditions to observe the nature of flow regimes and the impact of various parameters on effective heat dissipation through MCs well as temperature distribution. Numerical results were validated with experimental results, which showed good agreement. Several experiments were also carried out to develop an empirical correlation between mass flow rate and heat power to maintain the electronic device temperature below 40 °C. The developed correlation is experimentally validated at three different heat powers 6 W, 8 W and 10 W.

Lactose permease, a secondary active transporter from Escherichia coli , facilitates the co-transport of protons and lactose across the cytoplasmic membrane. Unlike passive diffusion mechanisms, lactose permease operates via conformational switching that alternately exposes the binding pocket to either membrane side. In this study, we present a theoretical treatment combining irreversible thermodynamic principles and kinetic modeling to quantify its operation. Onsager’s reciprocity is applied to analyze proton-lactose coupling, and a bisubstrate kinetic scheme is employed to simulate system behavior under various proton gradients and lactose concentrations. The complete catalytic cycle is characterized by associated rate constants and energetic transitions, highlighting that lactose permease exhibits a dissipative nature as a hallmark of secondary active transport. Altogether, this study provides a novel thermodynamic perspective on lactose permease, aiming to bridge molecular transport kinetics with the formalism of irreversible processes. This work is the first to integrate Onsager relations with the Michaelis-Menten kinetic model to quantify the energetic efficiency of lactose permease.

Based on an irreversible vacuum thermionic generator model, this paper performs single-, two-, three-, and four-objective optimizations for it by utilizing NSGA-II algorithm, finite-time thermodynamics and three decision-making techniques. Output-power, thermal-efficiency, ecological-function and efficient-power are taken as optimization-objectives, and collector work-function and output-voltage are taken as optimization-variables. Total number of optimizations completed consists of one four-objective optimization, four three-objective optimizations, six two-objective optimizations, and four single-objective optimizations. Deviation indexes under various combinations of optimization objectives are compared by using three decision-making techniques: Shannon Entropy, LINMAP, and TOPSIS. Based on these comparisons, optimal design scheme is determined. Findings indicate that multi-objective optimization can take into account multiple performance indicators at the same time and have a better coordination ability. The deviation index is 0.1761 when selecting LINMAP decision mode to optimize the combination of output-power, efficient-power, thermal-efficiency and ecological-function which is the best scheme for four-objective optimization, and optimal ranges of collector work function and output voltage are 1.295–1.355 and 0.22–0.44, respectively. The important contribution herein is introducing NSGA-II algorithm and three decision-making techniques for performance optimization of vacuum thermionic generator and performing totally 15 optimization tasks for in the first time in open literature.

Extended irreversible thermodynamics is a theory that expands the classical framework of nonequilibrium thermodynamics by going beyond the local-equilibrium assumption. A notable example of this is the Maxwell–Cattaneo heat flux model, which introduces a time lag in the heat flux response to temperature gradients. In this paper, we develop a variational formulation of the equations of extended irreversible thermodynamics by introducing an action principle for a nonequilibrium Lagrangian that treats thermodynamic fluxes as independent variables. A key feature of this approach is that it naturally extends both Hamilton’s principle of reversible continuum mechanics and the earlier variational formulation of classical irreversible thermodynamics. The variational principle is initially formulated in the material (Lagrangian) description, from which the Eulerian form is derived using material covariance (or relabeling symmetries). The tensorial structure of the thermodynamic fluxes dictates the choice of objective rate in the Eulerian description, and plays a central role in the emergence of nonequilibrium stresses – arising from both viscous and thermal effects – that are essential to ensure thermodynamic consistency. This framework naturally results in the Cattaneo–Christov model for heat flux. We also investigate the extension of the approach to accommodate higher-order fluxes and the general form of entropy fluxes. The variational framework presented in this paper has promising applications in the development of structure-preserving and thermodynamically consistent numerical methods. It is particularly relevant for modeling systems where entropy production is a delicate issue that requires careful treatment to ensure consistency with the laws of thermodynamics.

Despite advancements in cooling solutions for electronic devices, heat dissipation remains the primary challenge in optimizing heat sink performance in a competitive industry. The current study numerically investigates the performance of plate-fin heat sink (PHS) and pin-fin heat sink (PnHS) using a hybrid nanofluid (GnP-MWCNT/Water) as the working fluid. Key performance parameters, including pressure drop, thermal resistance, effectivity, and turbulent kinetic energy, are analyzed across different Re and nanofluid concentrations. Findings indicate that while the PnHS exhibits higher convective heat transfer due to increased flow disturbances, it also suffers from greater thermal resistance and pressure drop. In contrast, the PHS offers an adequate balance between heat dissipation and flow efficiency, leading to higher overall effectiveness. At Re = 5,334, the pressure drop for 0.20 % nanofluid is 88.5 Pa in the PnHS and 73 Pa in the PHS. Additionally, at Re = 1,333, the effectiveness values for PHS are 0.333 (water) and 0.354 (nanofluid), while for PnHS, they are lower at 0.186 and 0.195, respectively. The current study highlights the interplay between enhanced heat transfer and increased flow resistance, emphasizing the importance of optimizing fin design and nanofluid concentration for efficient thermal management.

We studied planar compressible Poiseuille flows of an ideal gas, both in steady and unsteady states, to identify the minimal number of state parameters required to describe changes in internal energy. In previous work (Phys. Rev. E 104, 055107 (2021)), five parameters were needed for steady flows. Here, using global non-equilibrium thermodynamics, we reduce this number to three: non-equilibrium entropy S * , volume V , and number of particles N . The internal energy U ( S * , V , N ) of such systems in stationary and non-stationary states is the function of non-equilibrium entropy S * , volume V and number of particles N in the system irrespective of any processes, number of boundary conditions or imposed constraints. We tested this by placing a cylinder inside the channel, finding that U depends on the cylinder’s location y c only via the state parameters S * ( y c ) and N ( y c ) for V  = const. Moreover, in cases where the flow becomes unstable and parameters such as velocity and pressure oscillate, U depends on time t only through S * ( t ) and N ( t ) for V  = const. These results demonstrate that this formulation of internal energy remains robust and consistent, even in unsteady flows with varying boundary conditions.