The steadily growing interest in applying granular media in various novel and advanced technologies, particularly in the energy sector, entails the need to gain in-depth knowledge of their thermal and flow behaviour and develop simulation predictive tools for systems’ design and optimisation. The focus of the present study is on the numerical modelling of the thermal decomposition of solid fuel grains in a packed bed while considering a non-classical description of heat transfer in such a medium. The work aims to assess the influence of the relaxation time and thermo-physical properties of the medium on the nature of the solution and highlight the factors that are the source of local non-equilibrium affecting thermal wave speed propagation. The analysis of the predicted temperature distribution was carried out based on the developed transient one-dimensional thermal and flow model, taking into account the moisture evaporation and the devolatilization of fuel particles. Obtained simulation results showed a significant increase in the temperature gradients with increased relaxation times for the case of wet granular bed. They also demonstrated the variable dynamics of thermal wave propagation due to the change in the packed bed structure with the process progress. For a relaxation time of 100 s, a several-fold increase in the temperature signal propagation speed during the fuel bed thermal decomposition was predicted.

The paper presents a modified in-house model for calculating heat transfer coefficients during flow condensation, which can be applied to a variety of working fluids, but natural refrigerants in particular, at full range thermodynamic parameters with a particular focus on increased saturation pressure. The modified model is based on a strong physical basis, namely the hypothesis of analogy between the heat transfer coefficient and pressure drop in two-phase flow. The model verification is based on a consolidated database that consists of 1286 data points for 7 natural refrigerants and covers the reduced pressure range (the ratio of critical pressure and saturation pressure) from 0.1 to 0.8 for different mass velocities and diameters. The new version of the in-house model, developed earlier by Mikielewicz, was compared with 4 other mathe-matical models widely recommended for engineering calculations and obtained the best consistency results. The value of the mean absolute percentage error was 28.13% for the modified model, the best result among the scrutinised methods.

The article presents a comprehensive computational fluid dynamics analysis of the adsorption and desorption cycles in adsorption refrigeration systems, focusing on the impact of the adsorbent bed geometry. The entire adsorption/desorption cycle has been modeled, allowing for the observation of events during the transitional period between processes and how these influence their progression. This approach is a novelty in the field. The developed numerical model was verified against experimental data available in the literature, demonstrating excellent convergence with the experiment, with a de-viation not exceeding 2%. The study illustrates how the geometrical parameters such as height and length of the bed affect the efficiency of the adsorption and desorption processes, emphasizing the importance of bed geometry in the adsorption of heat and mass exchangers in energy and adsorbate transfer. The research findings provide valuable insights for designing more efficient cooling devices using adsorption technology, highlighting the role of bed geometry in optimizing these systems. Modeling the entire adsorption/desorption cycle is a novelty and allows for the observation of what happens during the transitional period between processes and how this influences their progression.

This study provides a simple and effective decision-making method to choose the best phase-change material for different energy storage applications. Three case studies are provided to demonstrate the proposed decision-making method. The first case study addresses the problem of best phase-change material selection for a domestic water heating latent heat storage system by considering 15 different phase-change materials and 8 selection attributes; the second case study addresses the problem of selecting the best phase-change material for a triple tube heat exchanger unit by considering 12 different phase-change materials and 5 selection attributes; the third case study addresses the problem of best phase-change material selection for latent heat thermal energy storage within the walls of Trombe to enhance performance considering 11 phase-change materials and 4 selection attributes. The results of the proposed decision-making method are compared with those of other well-known multi-attribute decision-making methods. The proposed method is shown to be simple to implement, providing a logical way for allocating weights to the selection attributes and adaptable to phase-change material selection problems in different energy storage contexts.

This article presents the results of experimental measurements of the physical properties of new environmentally friendly bio-based composite building materials containing hemp shives bonded with a magnesium binder. Some of the tested materials contained an admixture of phase change material (PCM) of variable proportions in the binder to increase the heat capacity of building elements (walls), which can positively affect room temperature regulation. Densities and porosities are key parameters describing building materials, directly affecting mechanical, acoustic, and, most importantly, hygro-thermal properties, including thermal conductivity, water vapor permeability, water absorptivity, and sorption curves. The experiment was carried out for ten different samples of bio-based building composites, differing in the bulk density ob-tained during the manufacturing process and in the PCM proportion. As part of the experiment, true density tests were conducted on a helium pycnometer. Then, the geometric densities of the tested materials (which may differ from the bulk density obtained during production) were measured using the Archimedes method, making it possible to obtain the total, closed, and open porosity values. Tests were also carried out for selected traditional building materials, such as red brick and autoclaved aerated concrete, to compare the results obtained.

This paper presents a mathematical model of a vapour vacuum system, which is a crucial component of steam power plants of critical importance for energy efficiency. This system consists of three stages, with each stage containing a steam ejector and a gas phase separator in the form of an interstage heat exchanger. The primary purpose of this system is to remove inert gases and maintain the appropriate level of vacuum in the power plant condenser. The presented mathematical model can be used to analyse the operation of the vacuum system in a steady state. Preliminary pressure calculations in various components of the vacuum system show the influence of additional measurement orifice resistance on the vacuum drop in the condenser, which can reduce the efficiency of the entire energy system. It is worth noting that the presented model can be used as a tool for analysing elements of the vacuum system in energy systems.

Graphene oxide nanoparticles with higher thermal conductivity aid in enhancing the flow and heat transport in magnetohy-drodynamic devices such as magnetohydrodynamic pumps. Modelling such devices with promising applications inherently necessitates entropy studies to ensure efficient models. This investigation theoretically studies the entropy generation in magnetohydrodynamic flow of graphene oxide in an inclined channel. Buongiorno nanofluid model is used including the impacts of nanoparticle attributes, namely thermophoretic and Brownian diffusion along with viscous dissipation effects. The spectral quasi-linearization method with Chebyshev’s polynomials is adapted to solve the differential equations under slip conditions. On studying the effects of implanted parameters, it is concluded that the conductive heat transfer enhancement by the Hartmann number is remarked. The Bejan number is found to be greater than 0.9 and hence, heat transfer primarily causes the entropy generation. A good agreement is found between the results for special cases and the results from the literature. Furthermore, investigations conclude that entropy is contributed primarily by heat transfer.

This paper presents an analysis of the heat flow in a plate heat exchanger located at a building heat exchange station. The plate heat exchanger is the main source of heat for the building system based on microsubstations in the building apartments. The co-operation of the heat exchange station with the substations in the apartments is also described. Such microstations are intended for both domestic hot water preparation and apartment heating. The method of calculating the product of the heat transfer coefficient k and the heat exchange surface area A is presented. In order to verify the correctness of the measured values of the temperatures of hot and cold water at the heat exchange station inlet and outlet, they were compared to the values calculated using the -NTU method. Good agreement was found between the results of the calculations and the meas-urements. Recommendations were made for the temperature of return water to the heating station. The cost of operating the district heating network could be reduced by increasing the surface area of central heating radiators in apartments, so that the temperature of return water to the heating station could be lowered.

Impinging jets are one of the most effective techniques of heat transfer intensification, therefore they are continuously applied in various engineering areas. On the other hand, a numerical modelling of complex phenomena contributing to an overall heat transfer effect (and the Nusselt number value) is still not sufficient and suffers from lack of generalization. The extensive studies have been conducted to unify approach to the impinging jet modelling and construct the model (in Ansys Fluent software), which allows mirroring of the results. Presented work discusses differences in representation of impinging jet between various turbulence models based on the turbulence kinetic energy, momentum and energy budgets. It allows deep understanding of influence of geometrical and flow parameters on fluid mechanics phenomena interaction and final effect. The most significant results are connected with linking of Nusselt number distribution with analyzed budgets’ terms. Each term contributes to the distribution and cannot be omitted. Drawn conclusions explain the origin of reported in litera-ture differences and includes suggestions, how to evaluate the Nusselt number distribution results coming from various turbulence models. At this stage of research to have a complete image of relation between the particular quantities budgets and heat transfer effect it is suggested to consider also the turbulence kinetic energy dissipation budget, which will fil opened by this research gap.

There is no doubt that the miniaturization of various electronic devices, including processors, servers, micro-electromechan-ical system devices, etc. has resulted in increased overall performance. However, there is a major problem with thermal management in these devices, as well as in many others. One of the most promising solutions is liquid cooled microchannel heat sink. In the current work, different cases of open micro pin-fin configurations of heat sink were considered. The con-figurations considered were a uniform height micro pin-fin heat sink, three-stepped unidirectional micro pin-fin heat sink and three-stepped bi-directional micro pin-fin heat sink. These configurations were also oriented in two dissimilar fashions, i.e. inline and staggered, so the total of six heat sink configurations are compared and analysed. Using single phase water as a coolant and copper as a substrate, these configurations were simulated numerically for different Reynolds numbers (10−160) under heat flux of 500 kW/m². It can be concluded that at low Reynolds numbers, steepness does not contribute much in both inline and staggered arrangements, while at higher Reynolds numbers, 3 stepped staggered configurations has revealed the best performance due to boosted fluid mixing and more projecting secondary flow. Furthermore, bi-direction-ality in steepness shows augmented performance only in inline arrangement.

The present study explores the characteristics of reacting flow in a scramjet combustor with struts, focusing particularly on implementing different injection strategies. A three-dimensional DLR scramjet combustor is utilised to assess the impact on the system, incorporating multiple injections and varying injection angles on the triangular wedge. The analysis considers three injectors with parallel, upward and downward injections at angles of 15° and 30°. The numerical investigation is con-ducted under a constant total pressure of 7.82 bar, a temperature of 340 K, and an airspeed of Mach 2 at the inlet. The results highlight the significance of injector location and shape in promoting flame stabilization. Furthermore, injection angles play a crucial role in mitigating shockwave intensity. The numerical analysis involves a steady-state Reynolds-averaged Navier-Stokes equation with the shear stress transport k–ω turbulence model. The obtained results were analyzed by examining the critical variables such as Mach number, static pressure and combustion efficiency across the combustor. Based on the com-putational results, injecting fuel upward not only increases the overall pressure loss but also enhances the subsonic regime downstream of the strut, which leads to better mixing and combustion efficiencies. This is primarily due to shockwave generation from the edges of the strut and the interactions with the fuel stream shear layers.

The aim of the present work is to discuss the effect of varying thermal conductivity in a semiconducting medium under photothermal theory. An infinite elastic half-space is overlying the infinite semiconducting medium, and a constant me-chanical force is applied along the interface. The normal mode analysis method is applied to find the analytic components of displacement, stress, carrier density and temperature distribution. It was found that all physical quantities are affected by variable thermal conductivity. The novelty of the paper lies in the fact that no such a problem of variable thermal conductivity has been discussed by any researcher so far.

In order to improve the output performance of direct methanol fuel cell, the finite-time thermodynamic model of direct methanol fuel cell is developed in this paper. Then, mathematical expressions for energy efficiency, power density, exergy efficiency and exergy coefficient of performance are derived. In addition, the effects of operating temperature, inlet pres-sure and membrane thickness on the performance of direct methanol fuel cells are considered. The results show that the exergetic performance coefficient not only considers the exergy loss rate to minimize the loss, but also the power density of the direct methanol fuel cell to maximize its power density and improve its efficiency. Therefore, the exergetic perfor-mance coefficient is a better performance criterion than conventional power and efficiency. In addition, increasing the inlet pressure and decreasing the membrane thickness can significantly improve the exergetic performance coefficient and en-ergy efficiency.

The investigation of the couple stress fluid flow behaviour between two parallel plates under sudden stoppage of the pressure gradient is considered. Initially, a flow of couple stress fluid is developed between the two parallel plates under a constant pressure gradient. Suddenly, the applied pressure gradient is stopped, and the resulting unsteady flow is studied. This type of flow is known as run-up flow in the literature. Now the flow is expected to come to rest in a long time. Usually, these types of problems are solved by using the Laplace transform technique. There are difficulties in obtaining the inverse Laplace transform; hence, many researchers adopt numerical inversions of Laplace transforms. In this paper, the problem is solved by using the separation of variables method. This method is easier than the transform method. The velocity field is analyti-cally obtained by applying the usual no-slip condition and hyper-stick conditions on the plates, and hence the volumetric flow rate is derived at subsequent times. The steady state solution before the withdrawal of the pressure gradient is matched with the initial condition on time. The rest time, i.e. the time taken by the fluid to come to rest after the pressure gradient is withdrawn is calculated. The graphs for the velocity field at different times and different couple stress parameters are drawn. In the special case when a couple stress parameter approaches infinity, couple stress fluid becomes a viscous fluid. Our results are in good agreement with this special case.

In engineering phase-change phenomena are found in a multitude of applications, ranging from refrigeration and air con-ditioning to steam turbines and petroleum refining. This study investigates the flow of moist air in a circular duct where water vapour condenses in contact with the cold wall of the duct. The investigation delves into the relationship between the condensation mass transfer rate, the heat transfer between the bulk flow and the wall, and the temperature of the wall. The volume of fluid model coupled with the Lee evaporation-condensation model was employed. Five simulations were carried out, involving different wall temperatures while maintaining the same inlet conditions. Condensation was more pronounced at lower wall temperatures, which aligns with the expectations. The heat transfer between the bulk flow and the wall decreased with the decreasing temperature difference. Interestingly, the findings revealed that the surface heat transfer coefficient increased as the wall temperature approached the temperature of the bulk flow. The success of the study suggests potential applications in optimising thermal management systems, with implications for industries where accurate predictions of moisture behaviour and heat transfer are crucial.

Indeed, nanofluids have garnered significant interest in various fields due to their numerous advantages and potential ap-plications. The appeal of SiO₂ nanofluid, in particular, lies in its low preparation cost, simple production process, controlled chemistry, environmental safety and its exceptional ability to be homogeneously suspended in the base fluid, which makes it a promising candidate for a variety of applications. In this study, we investigate the flow analysis of a water based silicon dioxide nanofluid, passing over a stretched cylinder while subjected to a continuous magnetic field, including Joule heating effects. The research involves the development of a mathematical model and the formulation of governing equations rep-resented as partial differential equations. These equations are subsequently transformed into non-linear ordinary differential equations through suitable transformations. To obtain a numerical solution, the MATLAB bvp4c solver technique is em-ployed. The study investigates the implications of dimensionless parameters on velocity and thermal distributions. It is observed that the velocity distribution f'(η) exhibits a direct relationship with the volumetric fraction ϕ and an inverse relationship with the unsteadiness parameter S, the magnetic parameter M, and the temperature distribution θ(η) shows an enhancement for the increasing ϕ and M, as well as the Eckert number. However, it declines against S and the Prandtl number. The results for local Nusselt number and skin frictions are depicted in Tables.

This article explores the phenomenon of natural convection in the rotatory flow of Cu-water nanofluid under the influence of non-uniform heat source. In order to design more effective and efficient cooling systems, this work attempts to increase our understanding of how nanofluids behave in the presence of non-uniform heat sources, convection, and rotatory force. The higher order partial differential equations governing the flow are remodelled into ordinary differential equations using similarity transformations. The remodelled equations were solved using shooting methodology and the Lobatto-III A algorithm. The impacts of various parameters such as the Richardson number (1 < Ri < 4), the Schmidt number (0.5 < Sc < 2), nanoparticle’s volume fraction (0.02 < ϕ < 0.08), etc. on velocity, concentration and temperature was ana-lysed. One of the main findings of this analysis was study of the impact of the space dependent heat source (0.2 ≤ A ≤ 1) and the temperature dependent internal heat source (0 ≤ B ≤ 0.5) on the heat regulation. Furthermore, increasing the quantity of the nano-additives and improving the fluid’s thermophysical properties intensified the acceleration of the fluid elements in the flow region. The presence of spatial and temperature-sensitive parameters facilitated quantification of the effects of a standard and variable heat source in combination of Coriolis force in the case of a Cu-water flow. The findings of the investigation will be helpful in the process of medical, architectural planning systems, oil recovery systems and so on.

The melting and solidification processes of the organic phase change material – lauric acid exposed to air were experi-mentally studied to investigate the heat exchange and its effect on the heat transfer behaviour inside a shell as well as its phase-change characteristics. Lauric acid was placed in spherical shells made of polyvinyl chloride with diameters of 44, 63, and 74 mm. This study was based on analyses of the surface temperature and vertical temperature distribution data inside the shells. We found that the phase change characteristics were strongly related to the dominant heat transfer mech-anism. In this case, melting was dominated by convection, whereas solidification was dominated by conduction. The convection intensity increased as the shell diameter increased. Further analysis revealed the melting and solidification periods. In contrast to latent heat release accompanying solidification, latent heat absorption accompanied by melting does not occur at a constant temperature, although it has a smaller temperature gradient than does sensible heat absorption. Based on the asymmetry between the melting and solidification processes, we discuss various possible strategies by which to control the charging and discharging of the phase change material by restraining the heat transfer rate to optimise its performance as a latent thermal energy storage material.

This paper investigates entropy generation rate in a temperature-dependent variable viscosity unsteady nanofluid flow past a convectively heated impulsively moving permeable cylindrical surface. The governing equations based on the modified Stokes first problem assumption are obtained and transformed using appropriate similarity variables into nonlinear ordinary differential equations. The numerical shooting method together with the Runge-Kutta Fehlberg integration scheme are employed to effectively solve the problem. The effects of related parameters on the nanofluid velocity, temperature, skin friction, Nusselt number, entropy generation rate and Bejan number are displayed graphically and quantitatively explained. It is found that an upsurge in nanoparticles volume fraction enhances the skin friction, Nusselt number, entropy production rate and the Bejan number.

In the paper, a model of a heated building using a PEM (proton exchange membrane) fuel cell is presented. This work introduces a novel and more comprehensive depiction of the thermal processes occurring within a fuel cell under transient conditions. The developed PEM fuel cell model was synergistically incorporated with a thermodynamic model of a build-ing. The resulting mathematical framework provides insights into the building's performance concerning fluctuating am-bient temperatures and the heating system powered by the PEM cell. The developed mathematical model delineates the interplay between the building's thermodynamics and the fuel cell in the context of the devised heating control system featuring an indirect heat distribution mechanism.