This study investigates the thermal performance of nanoPCM emulsions (nanoPCMEs) as working fluids in parallel plate thermal energy storage system, comparing two configurations: solid aluminum walls and aluminum profile walls filled with PCM + metal foam. Three distinct nanoPCM emulsions were evaluated against pure water as a base fluid. The experimental results demonstrated superior convection heat transfer performance for all nanoPCMEs compared to pure water. In the solid aluminum wall configuration, maximum thermal energy storage reached approximately 900 kJ, with 2.0 wt% nanoPCME achieving this level twice as fast as pure water. The PCM + metal foam configuration exhibited enhanced performance, storing up to 2250 kJ of thermal energy due to PCM's high latent heat, reaching maximum storage 1.6 times faster than pure water. Notable improvements in heat transfer were observed, with nanoPCME achieving the highest average Nusselt numbers across all configurations. Maximum enhancements in Nusselt number were 34.5 % and 41.9 % for solid wall and PCM + metal foam configurations, respectively, using 2.0 wt% nanoPCME. These findings demonstrate the significant potential of nanoPCMEs for improving thermal energy storage systems and thermal control applications.

The thermal management of next-generation hybrid electric regional aircrafts poses critical challenges due to extreme heat loads, which could reach more than 2 MW and must be dissipated. This rejected heat can be used in a passive system such as Thermoelectric Generators (TEGs), which can directly convert thermal energy into electrical energy. This work is carried out in the framework of the EU Clean Aviation-funded project TheMa4HERA and it numerically explores the possibility of integrating thermoelectric (TE) technology in the next generation of regional aircrafts. Two case studies are considered: energy recovery from the outflow valve originally used to control the pressure of the cabin and the integration of TEG modules in skin heat exchangers used to partially dissipate heat coming from the fuel cells and/or from the power electronics. The results will permit us to understand the feasibility of implementing TEG technology into these specific conditions in terms of overall power generation. The findings indicate that while TEG integration in the outflow valve offers limited power density, the skin heat exchanger shows significantly higher potential for effective energy recovery.

The low thermal conductivity of Phase Change Materials (PCMs) limits their use in thermal energy storage systems. Embedding PCMs within metal foams is an effective method to improve their thermal conductivity and energy storage capacity by creating a fast energy-absorbing zone with low thermal resistance. This study presents a pore-scale numerical analysis of the melting behavior of PCM embedded in Kelvin cell-structured aluminum foams. Three cubic domains of different sizes (small: 25.4 mm, medium: 50.8 mm, and large: 101.6 mm) were analyzed, along with four porosity levels (s = 0.875, 0.914, 0.930, and 0.956), at a constant Cell Per Length (CPL = 6). The enthalpy-porosity method was used to simulate the phase change process. Results show that lower porosity improves heat conduction and leads to faster melting. For instance, at s = 0.875, the melting time was almost 50 % shorter compared to s = 0.956. As porosity increases, the complete melting time becomes approximately 2.24 times longer. When the domain size increases from small to large, the complete melting duration increases by about 10 times. It was also observed that as domain size increases, heat transfer shifts from being conduction-dominated to convection-dominated. Lower porosity promotes a more uniform temperature distribution and faster phase change, while higher porosity increases the influence of natural convection and creates non-uniform melting fronts.

Numerical analysis of mixed convection flow in an open channel cavity was performed by placing PCM and using ferro nanofluids (water based Fe3O4 nanofluid) with different volume fractions (1.0%, 3.0% and 5.0%) as working fluids. The effect of different Richardson numbers under laminar flow conditions with constant Reynolds number was analyzed to reveal how the nanofluid volume fraction and PCM position in the cavity affect the flow and heat transfer characteristics. The main reason for considering the use of nanofluid as a working fluid with the PCM layer is the thermal control of the cavity. It has been determined that the use of ferro nanofluids has a positive effect on the thermal control of the flow by increasing the thermal energy storage capacity of the PCM. As the Ri number increased, the importance of mixed convection effects increased, and for the highest Ri number, natural convection played a dominant role, causing a 65.5% decrease in the heated wall dimensionless temperature. Since the heat flux applied for pure water at highest Ri number is one third of that applied for 5.0% nanofluid, the specified temperature increase is quite lower by comparing heat flux thanks to the nanofluid high convection capability. Different than pure water, in all PCM cases the rate of melting is higher for 5.0% ferro nanofluid due to the higher convective heat transfer. In the case of the PCM on the bottom wall, the highest thermal energy storage capacity was obtained for each working fluid and 50% more energy was stored compared to the right heated wall at the same Ri number. Moreover, the maximum average Nu number was determined for the bottom heated case and for the same Ri number at 5.0% ferro nanofluid which was obtained approximately twice as much as pure water when flow reached steady state.

Estimating gas transport through a hierarchical micro/nanoporous system is challenging due to non-equilibrium gas dynamics. The primary difficulty lies in determining the rarefaction level, because identifying a representative flow dimension in a complex porous system with multiple pore scales is not straightforward. Our study performed a pore-level analysis for gas permeability in dual-scale porous media with varying porosity, throat size, and secondary pore size under different rarefaction conditions. We found that secondary porosity negatively affects permeability due to increased friction forces, with this influence growing as the secondary pore size and porosity increase until the secondary pore becomes comparable to the throat. However, rarefaction reduces the effects of secondary pores due to boundary slip. Traditional Knudsen number (Kn) calculations based on Darcy-defined height failed to accurately describe the rarefaction effects on gas permeability. Instead, we introduced an equivalent diameter to calculate the Kn, which provided an accurate normalization of apparent gas permeability independent of pore geometry. The extended Kozeny–Carman–Klinkenberg model developed in our previous study successfully yielded a macroscopic model for apparent gas permeability in hierarchical micro/nanoporous systems as a function of the traditional Darcy height and porosity.

A numerical study on thermal energy storage systems with parallel plates to collect sensible heat is conducted with porous and direct model approaches. The simulations in a two-dimensional domain are performed with COMSOL Multiphysics commercial software. For the equivalent porous medium, the permeability and effective thermal conductivity as well as the specific area, and interfacial convective coefficient are numerically evaluated, considering a thermally and hydrodynamically fully developed flow. A stack of parallel plates is the system with assigned length and height, and the external heat losses effect is considered. The analysis allows to evaluate an optimized configuration as Channels Per Length (CPL) by means of a balance in the channels between pressure drop and heat transfer. Moreover, the effect of CPL values and heat loss from the parallel plate system is estimated in terms of charging time and heating capacity. The results exhibit that as the CPL increases, the time required for the charging process decreases while heat accumulation inside the system increases significantly. In fact, at the highest CPL, charging time is 2.7 times faster and the amount of heat accumulation is approximately 20% higher in adiabatic case. It is illustrated that the amount of heat accumulation inside the system varies considerably for different heat loss values. Ultimately, this study shows that porous model is more practical and accurate to be used for higher CPL cases.

The present numerical study analyzes the melting process of phase change material (PCM) embedded in a metallic foam structure at pore scale level. The computational domain consists of two different sizes of 3D cubic boxes. The analyzed domain is filled with Kelvin cell-structures with different Cell Per Length (CPL) and constant porosity of 0.956. A constant temperature, higher than the melting temperature of PCM, is assigned to one external surface of the enclosure, while the other surfaces are adiabatic. The conjugate problem for the heat transfer between the PCM and the solid structure with Kelvin cells is developed. Enthalpy-porosity method is used to describe the PCM melting process. The finite volume method is used to solve the conjugate heat transfer problem at pore scale level by Ansys-Fluent code. A comparison of different CPL values is reported in terms of liquid fraction, average temperature of the PCM, and energy storage. The comparison is also considered between the two different volumes of the cubic boxes. The presence of the metallic structured Kelvin cells increases the overall heat transfer rate and decreases the melting time. Results for smaller cavity indicates that as the CPL number increases, the time required for the PCM melting process decreases. Furthermore, the total heat accumulation process takes a shorter time to reach the maximum value. The melting time and the duration of heat accumulation are worsened for the large cubic box ( L = 4 inch) at CLP > 6. This is due to the dominant viscous effect, which decreases the velocity induced by the buoyancy forces because of higher contact surface area. In these cases, heat transfer between liquid and solid phases of the PCM decreases substantially. & COPY; 2023 Elsevier Ltd. All rights reserved.

The effects of heated wall locations on mixed convection flow in a 2-D rectangular open cavity are investigated numerically using ANSYS-Fluent software. The flow is supposed to be a two-dimensional laminar and is regulated by parameters such as Reynolds, Re (100 and 200), and Richardson numbers, Ri (1, 5, 10, 15 and 20). One of the walls is heated uniformly, while the others are adiabatic. The heated wall might be vertical on the inflow and outflow sides or horizontal on the bottom of the cavity. The flow is stable at all Re and Ri values except at Ri = 20 and Re = 100 for bottom heated wall. It is found that using phase change material (PCM) reduces flow variations in the cavity. Further, the mixed convection effects are developed with increased Richardson number. This pushes the recirculating zone upstream and creates an unstable flow behavior. According to the results, the condition that minimized the fluctuations in the flow direction was the application of PCM to the bottom heated wall case. Therefore, thermal control of the flow is most efficient in this condition. The PCM melting rate is quite high for the Re = 200 condition as there is higher convective heat transfer in all cases than the Re = 100. Dimensionless wall temperature values with PCM case are lower than without PCM case for all conditions. The decrease of this parameter in bottom heated case is about 18.8% while 43.1% and 56.3% for left and right heated case, respectively. Additionally, the highest average Nusselt number is found for the bottom heated case for the same Re, with and without PCM cases.

Energy storage technologies are strategic and necessary components for the efficient utilization of renewable energy sources and energy conservation. An analysis on sensible and latent heat thermal energy storage (TES) in honeycomb systems is accomplished. The analysis is focused on the possibility to simulate the honeycomb system as a porous medium both for the sensible and latent heat TES systems. To obtain these models, it is necessary to evaluate the corresponding main parameters of the equivalent porous medium simulating the honeycomb system. After an introduction, a presentation of the main literature on sensible and latent heat TES systems is provided. The governing equations for direct model obtained considering the single channel related to the honeycomb structures and the porous medium model for both local thermal equilibrium and local thermal non-equilibrium are presented. Some results are given in terms of average temperature and stored energy for sensible and latent heat TES.

Porous metallic foams are a key material in numerous thermal and hydraulic applications. Gas flows in such micro/nanoporous systems deviate from classical continuum descriptions due to nonequilibrium in gas dynamics, and the resulted heat and mass transport show variation by rarefaction. This study performed a wide range of pore-level analysis of convective gas flows in a Kelvin cell model at different porosities and working conditions. Rarefaction effects onto permeability and heat transfer coefficients were calculated through Darcy to Forchheimer flow regimes. Permeability increased up to 60% by increasing rarefaction while this enhancement decreased by increasing porosity. At the same time, rarefaction lessened inertial effects such that Forchheimer coefficients decreased substantially. At high flow velocities, the increase in rarefaction considerably decreased the effect of drag forces. Hence, hydrodynamic enhancement due to rarefaction was found to increase by increasing Reynolds number. On the other hand, positive influence of boundary slip and negative influence of temperature jump developing between gas and solid almost canceled each other for the studied low heat flux region of highly conductive metal foam structures. Hence, Nusselt numbers were found mostly related to Reynolds number independent from rarefaction. We described Nusselt value based on power law model as a function of Reynolds and porosity. Results and the proposed model are important to accurately predict the thermal and hydrodynamic performance of metal foams in the 80 PPI range.

Purpose Fluid flow and heat transfer in a dual scale porous media is investigated to determine the interfacial convective heat transfer coefficient, numerically. The studied porous media is a periodic dual scale porous media. It consists of the square rods which are permeable in an aligned arrangement. It is aimed to observe the enhancement of heat transfer through the porous media, which is important for thermal designers, by inserting intra-pores into the square rods. A special attention is given to the roles of size and number of intra-pores on the heat transfer enhancement through the dual scale porous media. The role of intra-pores on the pressure drop of air flow through porous media is also investigated by calculation and comparison of the friction coefficient. Design/methodology/approach To calculate the interfacial convective heat transfer coefficient, the governing equations which are continuity, momentum and energy equations are solved to determine velocity, pressure and temperature fields. As the dual scale porous structure is periodic, a representative elementary volume is generated, and the governing equations are numerically solved for the selected representative volume. By using the obtained velocity, pressure and temperature fields and using volume average definition, the volume average of aforementioned parameters is calculated and upscaled. Then, the interfacial convective heat transfer coefficient and the friction coefficient is numerically determined. The interparticle porosity is changed between 0.4 and 0.75, while the intraparticle varies between 0.2 and 0.75 to explore the effect of intra-pore on heat transfer enhancement. Findings The obtained Nusselt number values are compared with corresponding mono-scale porous media, and it is found that heat transfer through a porous medium can be enhanced threefold (without the increase of pressure drop) by inserting intraparticle pores in flow direction. For the porous media with low values of interparticle porosity (i.e. = 0.4), an optimum intraparticle porosity exists for which the highest heat transfer enhancement can be achieved. This value was found around 0.3 when the interparticle porosity was 0.4. Research limitations/implications The results of the study are interesting, especially from heat transfer enhancement point of view. However, further studies are required. For instance, studies should be performed to analyze the rate of the heat transfer enhancement for different shapes and arrangements of particles and a wider range of porosity. The other important parameter influencing heat transfer enhancement is the direction of pores. In the present study, the intraparticle pores are in flow direction; hence, the enhancement rate of heat transfer for different directions of pores must also be investigated. Practical implications The application of dual scale porous media is widely faced in daily life, nature and industry. The flowing of a fluid through a fiber mat, woven fiber bundles, multifilament textile fibers, oil filters and fractured porous media are some examples for the application of the heat and fluid flow through a dual scale porous media. Heat transfer enhancement. Social implications The enhancement of heat transfer is a significant topic that gained the attention of researchers in recent years. The importance of topic increases day-by-day because of further demands for downsizing of thermal equipment and heat recovery devices. The aim of thermal designers is to enhance heat transfer rate in thermal devices and to reduce their volume (and/or weight in some applications) by using lower mechanical power for cooling. Originality/value The present study might be the first study on determination of thermal transport properties of dual scale porous media yielded interesting results such as considerable enhancement of heat transfer by using proper intraparticle channels in a porous medium.

In the present study, the fluid flow in a periodic, non-isotropic dual scale porous media consisting of permeable square rods in inline arrangement is analyzed to determine permeability, numerically. The continuity and Navier-Stokes equations are solved to obtain the velocity and pressure distributions in the unit structures of the dual scale porous media for flows within Darcy region. Based on the obtained results, the intrinsic inter and intraparticle permeabilities and the bulk permeability tensor of the dual scale porous media are obtained for different values of inter and intraparticle porosities. The study is performed for interparticle porosities between 0.4 and 0.75 and for intraparticle porosities from 0.2 to 0.8. A correlation based on Kozeny-Carman relationship in terms of inter and intraparticle porosities and permeabilities is proposed to determine the bulk permeability tensor of the dual scale porous media.