@article{MTMT:36416899,
	title = {Identifying correlations between metal foam thermal characteristics and non-Fourier behavior},
	url = {https://m2.mtmt.hu/api/publication/36416899},
	author = {Fehér, Anna Éva and Cseh, Dániel and Kovács, Róbert Sándor},
	doi = {10.1016/j.icheatmasstransfer.2025.110008},
	journal-iso = {INT COMMUN HEAT MASS},
	journal = {INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER},
	volume = {170},
	unique-id = {36416899},
	issn = {0735-1933},
	abstract = {Due to their low density and large specific surface area, metal foams are increasingly used as cellular materials that combine excellent structural and thermal properties. Their cellular structure makes them particularly suitable for use in heat exchangers, insulation, and fire protection layers. The heat transport that takes place within them is a complex phenomenon characterized by the simultaneous presence of heat conduction, heat transfer, and heat radiation, making their modeling a significant challenge. The aim of the research is to develop a one-dimensional, time-dependent, discrete numerical model capable of describing the effective thermal behavior of metal foams. The model takes into account heat conduction through the solid phase, conductive heat transfer in the closed cavities, thermal radiation between the pore walls, and by-passing heat conduction around the cavity. The results highlight that geometric features such as cavity size and arrangement have a significant impact on temperature distribution and confirm that classical Fourier-based models are not accurately applicable to porous materials. We found that the proposed one-dimensional approach is eligible to reproduce the experimentally observed non-Fourier effects for which modeling the Guyer–Krumhansl equations is a proper candidate. Identifying correlations between the thermal diffusivity and metal foam parameters, we showed that the emerging effective non-Fourier behavior is not purely a material property but depends on the geometrical structure as well.},
	year = {2026},
	eissn = {1879-0178},
	orcid-numbers = {Fehér, Anna Éva/0000-0002-2366-6388; Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:36161803,
	title = {The piston effect in supercritical fluids investigated via a reversible–irreversible vector field splitting-based explicit time integration scheme},
	url = {https://m2.mtmt.hu/api/publication/36161803},
	author = {Takács, Donát M. and Fülöp, Tamás and Kovács, Róbert Sándor and Szücs, Mátyás},
	doi = {10.1063/5.0271333},
	journal-iso = {PHYS FLUIDS},
	journal = {PHYSICS OF FLUIDS},
	volume = {37},
	unique-id = {36161803},
	issn = {1070-6631},
	abstract = {In the vicinity of the liquid–vapor critical point, supercritical fluids behave strongly compressible and, in parallel, thermophysical properties have strong state dependence. These lead to various peculiar phenomena, one of which is the piston effect where a sudden heating induces a mechanical pulse. The coupling between thermal and mechanical processes, in the linear approximation, yields a non-trivially rich thermoacoustics. The numerous applications of supercritical fluids raise the need for a reliable yet fast and efficient numerical solution for thermoacoustic time and space dependence in this sensitive domain. Here, we present a second-order accurate, fully explicit staggered space–time grid finite difference method for such coupled linear thermoacoustic problems. Time integration is based on the splitting of the state space vector field representing the interactions that affect the dynamics into reversible and irreversible parts, which splitting procedure leads to decoupled wave and heat equations. The former is a hyperbolic partial differential equation, while the latter is a parabolic one; therefore, different time integration algorithms must be amalgamated to obtain a reliable, dispersion error-free, and dissipation error-free numerical solution. Finally, the thermoacoustic approximation of the supercritical piston effect is investigated via the developed method.},
	year = {2025},
	eissn = {1089-7666},
	orcid-numbers = {Takács, Donát M./0000-0002-8463-745X; Fülöp, Tamás/0000-0003-2708-7065; Kovács, Róbert Sándor/0000-0001-5822-6035; Szücs, Mátyás/0000-0002-2492-0392}
}

@article{MTMT:36300608,
	title = {Comparative analysis of lumped parameter and one-dimensional continuum models for droplet evaporation at elevated pressures and temperatures},
	url = {https://m2.mtmt.hu/api/publication/36300608},
	author = {Csemány, Dávid and Kovács, Róbert Sándor},
	doi = {10.1016/j.icheatmasstransfer.2025.109547},
	journal-iso = {INT COMMUN HEAT MASS},
	journal = {INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER},
	volume = {169},
	unique-id = {36300608},
	issn = {0735-1933},
	abstract = {Droplet evaporation plays a critical role in a wide range of industrial and technological applications, from combustion engines to spray coating and refrigeration. While Eulerian-Lagrangian models are widely used for predicting droplet behavior, they often assume a uniform internal temperature, which may not be valid under high-temperature and high-pressure conditions. This study compares the traditional lumped parameter model (LPM) with a novel one-dimensional model employing a staggered grid (1D-SG) approach. Both models are applied to water droplets in air over a broad range of initial diameters, relative velocities, and ambient pressures and temperatures. The analysis evaluates deviations in droplet lifetime and evaporation rate between the two approaches, emphasizing the influence of Biot number, which is more sensitive to pressure than to temperature. Results show that LPM remains accurate for small droplets and low convection conditions, with deviations under 1 %. However, for larger droplets and enhanced convection, discrepancies in evaporation rate and lifetime reach up to 6 % and 8 %, respectively. The findings demonstrate that while LPM is computationally efficient, its applicability depends on droplet size and flow conditions. The proposed 1D-SG model offers a more physically consistent alternative when higher accuracy is required, providing guidance for model selection in spray-related simulations.},
	year = {2025},
	eissn = {1879-0178},
	orcid-numbers = {Csemány, Dávid/0000-0002-5103-5540; Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34225898,
	title = {Nonlinear thermal analysis of two-dimensional materials with memory},
	url = {https://m2.mtmt.hu/api/publication/34225898},
	author = {Munafò, C.F. and Rogolino, P. and Kovács, Róbert Sándor},
	doi = {10.1016/j.ijheatmasstransfer.2023.124847},
	journal-iso = {INT J HEAT MASS TRANS},
	journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER},
	volume = {219},
	unique-id = {34225898},
	issn = {0017-9310},
	abstract = {A nonlinear hyperbolic heat transport equation has been proposed based on the Cattaneo model without mechanical effects. We analyze the two-dimensional Maxwell-Cattaneo-Vernotte heat equation in a medium subjected to homogeneous and non-homogeneous boundary conditions and with thermal conductivity and relaxation time linearly dependent on temperature. Since these nonlinearities are essential from an experimental point of view, it is necessary to establish an effective and reliable way to solve the system of partial differential equations and study the behavior of temperature evolution. A numerical scheme of finite differences for the solution of the two-dimensional non-Fourier heat transfer equation is introduced and studied. We also investigate the attributes of the numerical method from the aspects of stability, dissipation and dispersive errors.},
	keywords = {THERMODYNAMICS; Engineering, Mechanical; extended irreversible thermodynamics; second sound; Nonlinear heat transport; GENERALIZED HEAT-CONDUCTION; 2ND SOUND},
	year = {2024},
	eissn = {1879-2189},
	orcid-numbers = {Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34297744,
	title = {Heat equations beyond Fourier: From heat waves to thermal metamaterials},
	url = {https://m2.mtmt.hu/api/publication/34297744},
	author = {Kovács, Róbert Sándor},
	doi = {10.1016/j.physrep.2023.11.001},
	journal-iso = {PHYS REP},
	journal = {PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS},
	volume = {1048},
	unique-id = {34297744},
	issn = {0370-1573},
	year = {2024},
	eissn = {1873-6270},
	pages = {1-75},
	orcid-numbers = {Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34691294,
	title = {Challenges in the thermal modeling of highly porous carbon foams},
	url = {https://m2.mtmt.hu/api/publication/34691294},
	author = {Fehér, Anna Éva and Kovács, Róbert Sándor and Sudár, Ákos and Barnaföldi, Gergely Gábor},
	doi = {10.1007/s10973-024-12927-w},
	journal-iso = {J THERM ANAL CALORIM},
	journal = {JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY},
	volume = {149},
	unique-id = {34691294},
	issn = {1388-6150},
	abstract = {The heat pulse (flash) experiment is a well-known, widely used method to determine thermal diffusivity. However, for heterogeneous, highly porous materials, neither the measurement nor the evaluation methodologies are straightforward. In the present paper, we focus on two open-cell carbon foam types, differing in their porosity but having the same sample size. Recent experiments showed that a non-Fourier behavior, called ’over-diffusive’ propagation, can be present for such a complex structure. The (continuum) Guyer–Krumhansl equation stands as a promising candidate to model such transient thermal behavior. In order to obtain a reliable evaluation and thus reliable thermal parameters, we utilize a novel, state-of-the-art evaluation procedure developed recently using an analytical solution of the Guyer–Krumhansl equation. Based on our observations, it turned out that the presence of high porosity alone is necessary but not satisfactory for non-Fourier behavior. Additionally, the mentioned non-Fourier effects are porosity-dependent; however, porous samples can also follow the Fourier law on a particular time scale. These data serve as a basis to properly identify the characteristic heat transfer mechanisms and their corresponding time scales, which altogether result in the present non-Fourier behavior. Based on these, we determined the validity region of Fourier’s law in respect of time scales.},
	year = {2024},
	eissn = {1588-2926},
	pages = {3605-3616},
	orcid-numbers = {Fehér, Anna Éva/0000-0002-2366-6388; Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34733253,
	title = {On the dynamic thermal conductivity and diffusivity observed in heat pulse experiments},
	url = {https://m2.mtmt.hu/api/publication/34733253},
	author = {Fehér, Anna Éva and Kovács, Róbert Sándor},
	doi = {10.1515/jnet-2023-0119},
	journal-iso = {J NON-EQUIL THERMODY},
	journal = {JOURNAL OF NON-EQUILIBRIUM THERMODYNAMICS},
	volume = {49},
	unique-id = {34733253},
	issn = {0340-0204},
	abstract = {Determining the thermal properties of materials with complex structures is still a major engineering challenge today. The well-known heat pulse experiment can be used to determine the thermal diffusivity by measuring the temperature history as a thermal response for a fast excitation. However, the evaluation of the measurements can be challenging, especially when dealing with non-homogeneous samples. The thermal behavior of such heterogeneous materials may exhibit a response including two-time scales. Therefore, the Fourier equation is not necessarily applicable. The simplest possible alternatives are the 2-temperature models the Guyer–Krumhansl and Jeffreys heat equations. In the present paper, we focus on the interpretation of the Jeffreys heat equation; studying its analytical solution, we present a fitting method for determining the unknown parameters. We also discuss its relation with the other two heat equations, and we offer an interpretation of how to characterize the transient response of heterogeneous materials.},
	year = {2024},
	eissn = {1437-4358},
	pages = {161-170},
	orcid-numbers = {Fehér, Anna Éva/0000-0002-2366-6388; Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34746761,
	title = {Sustainable printed circuit board substrates based on flame-retarded PLA/flax composites to reduce environmental load of electronics: Quality, reliability, degradation and application tests},
	url = {https://m2.mtmt.hu/api/publication/34746761},
	author = {Farkas, Csaba and Gál, László and Csiszár, András and Grennerat, Vincent and Jeannin, Pierre-Olivier and Xavier, Pascal and Rigler, Dániel and Krammer, Olivér and Plachy, Zbynek and Dusek, Karel and Kovács, Róbert Sándor and Fehér, Anna Éva and Géczy, Attila},
	doi = {10.1016/j.susmat.2024.e00902},
	journal-iso = {SUSTAINABLE MATERIALS TECHNOLOGIES},
	journal = {SUSTAINABLE MATERIALS AND TECHNOLOGIES},
	volume = {40},
	unique-id = {34746761},
	issn = {2214-9929},
	abstract = {The present paper introduces a novel, sustainable approach to produce an eco-friendly Printed Circuit Board (PCB) substrate; a substitute for traditional substrates, to significantly reduce e-waste. We present the prepreg technology, the road to actual circuit assembly with application studies, life cycle analysis (LCA), and sustainability analysis. The flame-retarded prepregs and resulting PCB assemblies were based on polylactic acid (PLA), the structure is reinforced with flax textiles. After copper lamination, subtractive PCB production was performed, and thermal and mechanical reliability was investigated in the case of both laminated and bare substrates. Steps of surface roughness, peel and thermal analysis followed. After a new set of assemblies, the post-assembly analysis was extended with further shear strength analysis on the soldered components and mass analysis regarding thermal processes. The evaluation showed that PLA/Flax substrates provide reliable structural performance up to 200 °C in the reflow soldering process; this allows limited but stabilized application possibilities with specific eco-friendly lead-free solders. A basic blinker circuit and a field programmable gate array (FPGA)–based design was produced and tested; the latter has the general complexity of a commercial circuit. A vol% and wt% analysis extended our discussion with a reduction of harmful components in waste in the range of 90%, which is a disruptive and significant result. Life cycle analysis (LCA) quantified the ecological impact of the assembly, highlighting a significant ease on environmental load (∼10%) for the total assembly. Finally, a qualitative degradation study was introduced to the prepared samples to investigate short-term stability with mechanical-, colour-, mass- and scanning electron microscopy (structure) analysis. Early results show that the boards can withstand the harsh environment of a composting bin for a few days, but in the time of a few weeks, degradation starts, pointing to eventual decomposition. The work directly connects with multiple sustainability development goals. © 2024 The Authors},
	keywords = {life cycle; scanning electron microscopy; recycling; Substrates; Surface roughness; Thermoanalysis; environmental protection; Electric network analysis; Sustainable development; Soldering; Electronic equipment; Field programmable gate arrays (FPGA); Waste reduction; Lead-free solders; Environmental loads; Electronic waste; PCB; Printed circuit boards; Eco-friendly; Timing circuits; E-wastes; sustainable electronics; Flame-retarded; Biodegradable PCB; Environmentally-friendly electronics; Biodegradable printed circuit board; E-waste reduction; E-waste reduction; Environmentally-friendly electronic; Printed circuit board substrate; Printed circuit board substrate},
	year = {2024},
	eissn = {2214-9937},
	orcid-numbers = {Kovács, Róbert Sándor/0000-0001-5822-6035; Fehér, Anna Éva/0000-0002-2366-6388}
}

@article{MTMT:34755651,
	title = {Thermal and mechanical properties of AlSi7Mg matrix syntactic foams reinforced by Al2O3 or SiC particles in matrix},
	url = {https://m2.mtmt.hu/api/publication/34755651},
	author = {Fehér, Anna Éva and Maróti, János Endre and Takács, Donát M. and Orbulov, Imre Norbert and Kovács, Róbert Sándor},
	doi = {10.1016/j.ijheatmasstransfer.2024.125446},
	journal-iso = {INT J HEAT MASS TRANS},
	journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER},
	volume = {226},
	unique-id = {34755651},
	issn = {0017-9310},
	year = {2024},
	eissn = {1879-2189},
	orcid-numbers = {Fehér, Anna Éva/0000-0002-2366-6388; Takács, Donát M./0000-0002-8463-745X; Orbulov, Imre Norbert/0000-0002-5358-0630; Kovács, Róbert Sándor/0000-0001-5822-6035}
}

@article{MTMT:34757577,
	title = {Two-field mixed hp-finite elements for time-dependent problems in the refined theories of thermodynamics},
	url = {https://m2.mtmt.hu/api/publication/34757577},
	author = {Tóth, Balázs and Molnár, Zsombor and Kovács, Róbert Sándor},
	doi = {10.1007/s00161-024-01300-9},
	journal-iso = {CONTINUUM MECH THERM},
	journal = {CONTINUUM MECHANICS AND THERMODYNAMICS},
	volume = {36},
	unique-id = {34757577},
	issn = {0935-1175},
	abstract = {Modern manufacturing technologies allow heterogeneous materials with complex inner structures (e.g., foams) to be easily produced. However, their utilization is not straightforward, as the classical constitutive laws are not necessarily valid. According to various experimental observations, the Guyer–Krumhansl equation is a promising candidate for modeling such complex structures. However, practical applications need a reliable and efficient algorithm capable of handling both complex geometries and advanced heat equations. In the present paper, we derive new two-field variational formulations which treat the temperature and the heat flux as independent field variables, and we develop new, advanced hp -type mixed finite element methods, which can be reliably applied. We investigate their convergence properties for various situations, challenging in relation to stability and the treatment of fast propagation speeds. That algorithm is also proved to be outstandingly efficient, providing solutions four magnitudes faster than commercial algorithms.},
	year = {2024},
	eissn = {1432-0959},
	pages = {825-838},
	orcid-numbers = {Tóth, Balázs/0000-0002-5419-2234; Kovács, Róbert Sándor/0000-0001-5822-6035}
}