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Thermo, Volume 2, Issue 3 (September 2022) – 7 articles

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Article
Group Combustion of Dispersed Spherical Core–Shell Nanothermite Particles
Thermo 2022, 2(3), 209-231; https://doi.org/10.3390/thermo2030016 - 08 Aug 2022
Viewed by 202
Abstract
The group combustion characteristics of core–shell nanothermite particles differ from other dispersed solid or liquid fuels. In a core–shell structure, each discrete nanothermite particle can undergo an exothermic reaction as the oxygen atoms in the metal oxide shell undergo a solid state diffusion [...] Read more.
The group combustion characteristics of core–shell nanothermite particles differ from other dispersed solid or liquid fuels. In a core–shell structure, each discrete nanothermite particle can undergo an exothermic reaction as the oxygen atoms in the metal oxide shell undergo a solid state diffusion to oxidize the metal core. This feature allows the spherical core–shell nanothermites to react in the absence of gaseous oxygen, thus modifying their group combustion characteristics compared to char or liquid fuels. Using a number of simplifying assumptions, a theoretical framework was established—based on existing group combustion theory—to examine the characteristics of mass and heat diffusion in nanothermite combustion. First, a model for the quasi-steady state single-particle combustion, in quiescent air, was established. The isolated particle combustion theory serves as the basis for the combustion interaction and mass transfer in a spherical cloud of dispersed nanothermite particles. The type of group combustion is strongly dependent on the diffusion of vapour products, i.e., the interaction is more pronounced when the diffusion of vapour products is higher. The group combustion regimes in dispersed nanothermites were identified and delineated. Full article
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Article
Separation of Alcohols from n-Tetradecane Using 1-Ethyl-3-methylimidazolium Hydrogensulfate
Thermo 2022, 2(3), 200-208; https://doi.org/10.3390/thermo2030015 - 20 Jul 2022
Viewed by 272
Abstract
Extraction of alcohols from n-tetradecane using various extraction solvents has been investigated at a range of temperatures from 295 to 393 K under ambient pressure. On the basis of the experimental liquid–liquid equilibrium data, the distribution ratio and selectivity were calculated for [...] Read more.
Extraction of alcohols from n-tetradecane using various extraction solvents has been investigated at a range of temperatures from 295 to 393 K under ambient pressure. On the basis of the experimental liquid–liquid equilibrium data, the distribution ratio and selectivity were calculated for the extraction of 1-octanol, 1-decanol, and 1-dodecanol (C8–C12) in 1-ethyl-3-methylimidazolium hydrogensulfate [C2mim][HSO4] and sulfolane. Results showed that moderate selectivities were obtained in sulfolane with very low distribution coefficients. In contrast, [C2mim][HSO4] showed similar selectivity values with higher distribution coefficients. A study of a number of different 1-alcohols (C4–C12) showed that the decrease in hydrogen bonding compared to the increased van der Waals interactions between n-tetradecane and the higher-chain alcohols decreased the extraction selectivity in [C2mim][HSO4]. Increasing the temperature of the ionic liquid extraction medium resulted in increased chemical extraction for 1-butanol and 1-hexanol due to the formation of the corresponding alkylsulfate ionic liquid. In contrast, the selectivity decreased for 1-octanol, 1-decanol and 1-dodecanol due to the partial dissolution of the corresponding alkylsulfate ionic liquid into the n-tetradecane phase. Full article
(This article belongs to the Special Issue Vapor–Liquid Equilibrium and Chemical Thermodynamics)
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Article
Semi-Analytical Method for 3D Transient Heat Transfer in Thermally Interacting Fields of Ground Heat Exchangers
Thermo 2022, 2(3), 171-199; https://doi.org/10.3390/thermo2030014 - 08 Jul 2022
Viewed by 286
Abstract
The study of heat transfer in ground heat exchangers (GHEs) considering the fluid advection inside the pipes; the heat transfer between the fluid and the ground through the grout material; and the thermal interaction between GHEs is a challenging task. The present paper [...] Read more.
The study of heat transfer in ground heat exchangers (GHEs) considering the fluid advection inside the pipes; the heat transfer between the fluid and the ground through the grout material; and the thermal interaction between GHEs is a challenging task. The present paper presents a new semi-analytical method that takes into account the aforementioned effects to consider both the short- to long-term effects of GHEs. The heat transfer between the fluid and grout was studied by a transient multipole expansion considering time-dependent fluid temperatures and an advection model for the pipes obtained from an energy balance on the heat carrier fluid. Thermal interactions were analyzed using an equivalent borehole method while penalizing the transient multipole expansion to include thermal interaction effects. Validation of the short-term predictions was performed by comparing the proposed model to experimental data found in the literature and to an FEA model. The proposed model was then compared with a FEA model in long-term simulations of a geothermal field comprised of 24 GHEs for multi-annual simulation. The method resulted in substantial reduction in computational time while preserving good accuracy when compared with the FEA model. Full article
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Article
Single-Parameter Aging in the Weakly Nonlinear Limit
Thermo 2022, 2(3), 160-170; https://doi.org/10.3390/thermo2030013 - 06 Jul 2022
Viewed by 198
Abstract
Physical aging deals with slow property changes over time caused by molecular rearrangements. This is relevant for non-crystalline materials such as polymers and inorganic glasses, both in production and during subsequent use. The Narayanaswamy theory from 1971 describes physical aging—an inherently nonlinear phenomenon—in [...] Read more.
Physical aging deals with slow property changes over time caused by molecular rearrangements. This is relevant for non-crystalline materials such as polymers and inorganic glasses, both in production and during subsequent use. The Narayanaswamy theory from 1971 describes physical aging—an inherently nonlinear phenomenon—in terms of a linear convolution integral over the so-called material time ξ. The resulting “Tool–Narayanaswamy (TN) formalism” is generally recognized to provide an excellent description of physical aging for small, but still highly nonlinear, temperature variations. The simplest version of the TN formalism is single-parameter aging according to which the clock rate dξ/dt is an exponential function of the property monitored. For temperature jumps starting from thermal equilibrium, this leads to a first-order differential equation for property monitored, involving a system-specific function. The present paper shows analytically that the solution to this equation to first order in the temperature variation has a universal expression in terms of the zeroth-order solution, R0(t). Numerical data for a binary Lennard–Jones glass former probing the potential energy confirm that, in the weakly nonlinear limit, the theory predicts aging correctly from R0(t) (which by the fluctuation–dissipation theorem is the normalized equilibrium potential-energy time-autocorrelation function). Full article
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Article
Simulation of Single Vapor Bubble Condensation with Sharp Interface Mass Transfer Model
Thermo 2022, 2(3), 149-159; https://doi.org/10.3390/thermo2030012 - 30 Jun 2022
Viewed by 286
Abstract
Pure numerical simulation of phase-change phenomena such as boiling and condensation is challenging, as there is no universal model to calculate the transferred mass in all configurations. Among the existing models, the sharp interface model (Fourier model) seems to be a promising solution. [...] Read more.
Pure numerical simulation of phase-change phenomena such as boiling and condensation is challenging, as there is no universal model to calculate the transferred mass in all configurations. Among the existing models, the sharp interface model (Fourier model) seems to be a promising solution. In this study, we investigate the limitation of this model via a comparison of the numerical results with the analytical solution and experimental data. Our study confirms the great importance of the initial thermal boundary layer prescription for a simulation of single bubble condensation. Additionally, we derive a semi-analytical correlation based on energy conservation to estimate the condensing bubble lifetime. This correlation declares that the initial diameter, subcooled temperature, and vapor thermophysical properties determine how long a bubble lasts. The simulations are carried out within the OpenFOAM framework using the VoF method to capture the interface between phases. Our investigation demonstrates that calculation of the curvature of interface with the Contour-Based Reconstruction (CBR) method can suppress the parasitic current up to one order. Full article
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Article
Multi-Scale Modelling of the Bound Metal Deposition Manufacturing of Ti6Al4V
Thermo 2022, 2(3), 116-148; https://doi.org/10.3390/thermo2030011 - 23 Jun 2022
Viewed by 291
Abstract
Nonlinear shrinkage of the metal part during manufacturing by bound metal deposition, both on the ground and under microgravity, is considered. A multi-scale physics-based approach is developed to address the problem. It spans timescales from atomistic dynamics on the order of nanoseconds to [...] Read more.
Nonlinear shrinkage of the metal part during manufacturing by bound metal deposition, both on the ground and under microgravity, is considered. A multi-scale physics-based approach is developed to address the problem. It spans timescales from atomistic dynamics on the order of nanoseconds to full-part shrinkage on the order of hours. This approach enables estimation of the key parameters of the problem, including the widths of grain boundaries, the coefficient of surface diffusion, the initial redistribution of particles during the debinding stage, the evolution of the microstructure from round particles to densely-packed grains, the corresponding changes in the total and chemical free energies, and the sintering stress. The method has been used to predict shrinkage at the levels of two particles, of the filament cross-section, of the sub-model, and of the whole green, brown, and metal parts. Full article
(This article belongs to the Special Issue Feature Papers of Thermo in 2022)
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Article
Thermal Properties of Li2BeF4 near Melting Point
Thermo 2022, 2(3), 107-115; https://doi.org/10.3390/thermo2030010 - 22 Jun 2022
Viewed by 274
Abstract
The LiF–BeF2 system is used as a heat transfer medium in molten salt reactors (MSRs). The thermal diffusivity of Li2BeF4 was studied using the laser flash analysis (LFA) method in solid and transition states. While the thermal diffusivity is [...] Read more.
The LiF–BeF2 system is used as a heat transfer medium in molten salt reactors (MSRs). The thermal diffusivity of Li2BeF4 was studied using the laser flash analysis (LFA) method in solid and transition states. While the thermal diffusivity is shown to decrease slightly in solid-state Li2BeF4, it drops significantly at temperatures close to phase transition. The heat capacity of Li2BeF4 was measured by differential scanning calorimetry (DSC). Some differences were observed between the results obtained in cooling and heating modes. Thermal conductivity was calculated using thermal diffusivity-, density-, and heat-capacity data. The good thermal conductivity of the Li2BeF4 compound in solid and liquid states justifies its use as a heat transfer medium for molten salt reactors. Full article
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