Thermo, Volume 2, Issue 3 (September 2022) – 13 articles

This review explores the use of microwave heating and microwave-generated plasma for biosecurity applications. Microwave heating has been shown to rapidly heat and kill a wide range of pests and pathogens. Examples of microwave thermal disinfestation of soils, grains, hay, and timber are presented and discussed. Microwave energy can also ionize various gasses, including air, to create plasma. Plasmas are described by many characteristics, such as temperature, degree of ionization, and density. In the “after glow” (cold plasma) of a plasma discharge, there are sufficient charged particles and excited atoms to generate elevated UV levels and ionize the surfaces of objects. Examples of cold plasma and plasma-activated water disinfestation of grains and other commodities are also presented and discussed. Brief comments on the scale-up of this technology have also been presented. Full article

The results obtained from a study of the thermal transformations of polymorphic long-chain normal paraffins (n-C₃₂H₆₆ and n-C₃₆H₇₄) are presented here. The research was performed using a power-compensated Differential Scanning Calorimeter (DSC). Both heating and cooling experiments were performed in dynamic nitrogen atmosphere. Thermodynamic data for both polymorphic transitions, as well as the fusion endotherms, were determined from the DSC thermal curves. Using the heats of transition (∆H), in Joules/gram, obtained from the data in the DSC thermal curves, molar heats of transition (∆H), in kJ/mol, were calculated and compared to previously published values. The molar entropy of transition (∆S) was then calculated for each of the observed thermal events. Additional information is given by the author on obtaining the best results from the use of DSC for the thermal behavior of n-paraffins. This manuscript opens with a review of most of the early work and the results it provided dealing with polymorphism of n-paraffin solids. Some of this referenced work was performed prior to the advent of computerized analytical instrumentation. Full article

It is shown that the inert properties of a stationary random process can be expressed in terms of the ratio of its correlation interval ${\tau }_x$ to the doubled variance D_x. When using a fixed value of Planck’s constant h as a proportionality factor, the ratio $h{\tau }_x/2D_x$ has the dimension of a kilogram and can be used as an equivalent of a mass standard. It is proposed to use thermal (i.e., Johnson–Nyquist) noise as a reference Gaussian stationary random process. The theoretical substantiation of the project for the creation of “thermoelectric semiconductor ampere-balances” for balancing the inert mass of a quasi-ideal silicon-28 ball is also given. Combining these two projects can provide the basis for a stable and reproducible mass standard. Full article

Thermodynamics, as a scientific tool, advises on the control of variables involved in processes of different nature and is particularly useful in the design of equipment, or to obtain previous simulations. However, to generate more accurate models, an exact science is required. Thus, the thermodynamic–mathematical binomial is able to relate the fundamental variables of a system using the potential functions directing the process, although these relationships are not always completely satisfactory, as it is necessary to complete the modelling with a set of parameters, which depend on the experimentation. To ensure a better description of the behavior of a system, in this work a multi-objective optimization procedure (MOP) is applied to the NRTL model, comparing the results with other conventional procedures used to characterize the real properties of the binary methyl methanoate + pentane. The results obtained with the MOP confirmed a better representation of the experimental information with NRTL, analyzing its impact on the simulation/design processes. The set of optimal parametrizations obtained allow several options to be process engineered to select the most appropriate one depending on the specific problem to be designed. Full article

Qualitatively, the non-covalent interactions are well-known and help to explain many phenomena in chemistry and biochemistry. Quantitatively, determination of strength this force is a challenging task. The vaporization enthalpy is a reliable measure not only for the intermolecular interactions in the liquid phase, but also as the measure of intermolecular non-covalent interactions in the gas phase for the specific group of compounds, e.g., for the triglycerides. The vaporisation thermodynamics of four triglycerides were studied by using transpiration method, quartz crystal microbalance, and thermogravimetric analysis. Vapour pressure–temperature dependences were used to derive the enthalpies of vaporisation of these very low volatile liquids. Vaporisation enthalpies of the triglycerides available in the literature were collected and uniformly adjusted to the reference temperature 298.15 K and validated using structure–property relationships (chain-length dependence, correlation with retention indices, and correlation with normal boiling points). The consistent sets of evaluated vaporisation enthalpies for the linear and branched triglycerides were used to develop the “centerpiece” based group-additivity method for predicting enthalpies of vaporisation of triglycerides. It has turned out that the family of triglycerides do not obey the group-additivity rules. The reason for that is that the evaporated in the gas phase triglycerides exhibit intensive non-covalent attractive dispersion interactions strongly dependent on the alkyl-chain length. For the first time the intensity of the dispersion interactions was quantified for the family of aliphatic linear triglycerides with the chain length from 3 to 18 carbon atoms. The influence of the branching and unsaturation of the alkyl chains to the strength of the non-covalent interactions was also discussed. Full article

Cooling vests containing phase change materials (PCMs) are used to reduce heat stress in hot environments and maintain the body core temperature within a safe range. The performance of such cooling vests depends in a complicated way on the PCM material and mass, the insulation value of the clothing layers and heat loss to the environment. Conventionally, these performance parameters are evaluated experimentally or using a numerical model, both of which do need a certain amount of evaluation time. The objective of this paper is to develop a transient heat transfer model which includes metabolic heat production in the human body, as well as clothing and PCM layers and radiation to the environment but which is presented as a series of closed-form equations that can be evaluated without the need of a numerical solver. We present solutions for the body and PCM temperature as well as for the heat flux, cooling power and cooling duration. The model equations are validated by comparing them with experiments of ice PCM packs on a hotplate, as well as with published experimental and numerical data for the core temperature, heat flux and percentage of environmental heat loss using a Glauber salt type of PCM. Both the hotplate experiments and the model predictions show that the cooling power during PCM melting drops from about 70 to 32 W for increasing insulation layer thicknesses. In addition, the model is seen to compare well with experimental and simulation data in the literature. In a parametric study, we show how the equations can be used to evaluate the effects of PCM melting temperature and PCM thickness on cooling performance. The paper, therefore, can be considered as a practical means to help select the best cooling vest configuration for workers in a hot and humid environment. Full article

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

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

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

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 $\xi$. 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 $d\xi /dt$ 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, $R_0\left(t\right)$. 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 $R_0\left(t\right)$ (which by the fluctuation–dissipation theorem is the normalized equilibrium potential-energy time-autocorrelation function). Full article

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

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

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