•A numerical model is proposed and validated to predict ground temperature profiles.•Three typical meteorological year databases are compared to provide the weather parameters for the numerical ...model.•Two-harmonic analytical correlation (THAC) with high accuracy is introduced to obtain ground temperature rapidly.•THACs of main cities in two climate regions in China are provided on the standard condition.•THACs of all types of soil can be derived according to the developed universal correction formulas and the standard THACs.
The lacking of ground temperature measurement data has been preventing the development of underground engineering in China. Therefore, this paper proposed and validated a numerical prediction model that comprehensively considered the effects of short-wave solar radiation, long-wave radiation, latent evaporation energy, heat convection, heat conduction, soil freezing and thawing latent heat on ground temperature. Three frequently-used typical meteorological year databases were used to provide the weather parameters for the numerical model. Comparison of three city cases showed that the model using the “Chinese typical year weather” performed best in accuracy. Moreover, due to the much calculation time required for the numerical model, the two-harmonic analytical correlation (THAC) with higher accuracy than Kusada correlation and Baggs correlation was introduced to obtain ground temperature rapidly. Considering the variety of soil types, the universal correction formulas about the relationship between the THAC coefficients and the soil thermal diffusivity were developed through a series of typical city cases. Simultaneously, THACs of main cities in two climate regions corresponding to soil thermal diffusivity of 8.13*10-2 m2/day were provided as the standard THACs. The THACs of other types of soil were proved to be derived according to the universal correction formulas and the standard THACs.
The application range of existing real scale mobile thermal storage units with phase change materials (PCM) is restricted by the low phase change temperature of 58 ∘ C for sodium acetate trihydrate, ...which is a commonly used storage material. Therefore, only low temperature heat sinks like swimming pools or greenhouses can be supplied. With increasing phase change temperatures, more applications like domestic heating or industrial process heat could be operated. The aim of this study is to find alternative PCM with phase change temperatures between 90 and 150 ∘ C . Temperature dependent thermophysical properties like phase change temperatures and enthalpies, densities and thermal diffusivities are measured for the technical grade purity materials xylitol (C 5 H 12 O 5 ), erythritol (C 4 H 10 O 4 ) and magnesiumchloride hexahydrate (MCHH, MgCl 2 · 6H 2 O). The sugar alcohols xylitol and erythritol indicate a large supercooling and different melting regimes. The salt hydrate MgCl 2 · 6H 2 O seems to be a suitable candidate for practical applications. It has a melting temperature of 115.1 ± 0.1 ∘ C and a phase change enthalpy of 166.9 ± 1.2 J / g with only 2.8 K supercooling at sample sizes of 100 g . The PCM is stable over 500 repeated melting and solidification cycles at differential scanning calorimeter (DSC) scale with only small changes of the melting enthalpy and temperature.
In this paper, the effects of diluted gas (N2, CO2) on the pressure and time parameters of hydrogen explosion are tested by experiments. The combustion heat loss and thermodynamic state parameters ...during combustion were also calculated. The results show that as the diluent gas content increases, the maximum hydrogen explosion pressure decreases, and the rapid hydrogen deflagration time increases. The effect of dilution gas on hydrogen explosion pressure parameters and explosion time parameters in the rich-combustion state is more significant than that in the lean-combustion state. When φ = 1.0 and 20% of N2 and CO2 are added, the heat loss per unit area increases approximately 3.23 times and 4.97 times, respectively. The thermal diffusivity did not change as the N2 content increased under different equivalence ratios but decreased linearly as the CO2 content increased. When the CO2 content increases from 5% to 30% at φ = 0.6, α decreases by 14.1%. N2 has no significant effect on the adiabatic flame temperature under the lean-combustion state. At different equivalence ratios, the adiabatic flame temperature decreases linearly as the CO2 content increases, and the inhibition effect of CO2 on the adiabatic flame temperature is significantly higher than that of N2.
•Diluent gas has a significant suppression effect on hydrogen explosion under rich burn conditions.•Dilution gas increases the rapid deflagration time and continuous combustion time.•The effect of CO2 on thermal diffusivity is more significant.•The heat loss per unit area increases with the increase of the volume fraction of diluted gas.
An important proposed mechanism in nanothermites reactions - reactive sintering - plays a significant role on the combustion performance of nanothermites by rapidly melting and coalescing aggregated ...metal nanoparticles, which increases the initial size of the reacting composite powders before burning. Here, we demonstrate a high-speed microscopy/thermometry capability that enables ~ µs time and ~ µm spatial resolution as applied to highly exothermic reaction propagation to directly observe reactive sintering and the reaction front at high spatial and temporal resolution. Experiments on the Al+CuO nanocomposite system reveal a reaction front thickness of ~30 μm and temperatures in excess of 3000 K, resulting in a thermal gradient in excess of 10
K m
. The local microscopic reactive sintering velocity is found to be an order of magnitude higher than macroscale flame velocity. In this observed mechanism, propagation is very similar to the general concept of laminar gas reaction theory in which reaction front velocity ~ (thermal diffusivity x reaction rate)
.
In the present paper, we consider the two-dimensional Euler-Boussinesq equations with temperature-dependent thermal diffusivity. More precisely, we prove the global-in-time existence and uniqueness ...for this system under Yudovich-type initial data. In addition, the global propagation of striated regularity is also considered.
The fabrication of ceramic composites with low thermal expansion and excellent mechanical properties is essential for thermal barrier coating (TBC) applications. For this purpose, using the sol-gel ...process, novel ceramic composites were developed by combining varying weight percentages of β-spodumene (0–20 wt%) with zirconia toughened alumina (ZTA). The influence of β-spodumene addition on the densification, phase composition, microstructure, and mechanical, thermal, and fracture behavior of the studied composites was investigated experimentally and numerically. Increasing the β-spodumene content reduces the sintering temperature of the formed composites by 75 °C. Introducing 5 wt% β-spodumene to ZTA composite improves the densification process by serving as a mineralizer. X-ray diffraction (XRD) revealed that the t-m ZrO2 transformation was enhanced by increasing the β-spodumene content. The results demonstrated that composites with β-spodumene content of 5 wt% exhibited the best mechanical performance. The numerical prediction of fracture toughness agreed well with the experimental results for different composites.
Using quantum gas microscopy, we study the late-time effective hydrodynamics of an isolated cold-atom Fermi-Hubbard system subject to an external linear potential (a “tilt”). The tilt is along one of ...the principal directions of the two-dimensional square lattice and couples mass transport to local heating through energy conservation. Because of this coupling, the system quickly heats up to near infinite temperature in the lowest band of the lattice. We study the high-temperature transport and thermalization in our system by observing the decay of prepared initial density waves as a function of wavelengthλand tilt strength and find that the associated decay timeτcrosses over as the tilt strength is increased from characteristically diffusive to subdiffusive withτ∝λ4. In order to explain the underlying physics and emphasize its universal nature, we develop a hydrodynamic model that exhibits this crossover. For strong tilts, the subdiffusive transport rate is set by a thermal diffusivity, which we are thus able to measure as a function of tilt in this regime. We further support our understanding by probing the local inverse temperature of the system at strong tilts, finding good agreement with our theoretical predictions. Finally, we discuss the relation of the strongly tilted limit of our system to recently studied 1D models that may exhibit nonergodic dynamics.
The synthesis and characterization of epoxy‐based composites with few‐layer graphene fillers, which are capable of dual‐functional applications, are reported. It is found that composites with certain ...types of few‐layer graphene fillers reveal an efficient total electromagnetic interference shielding, SEtot ≈ 45 dB, in the important X‐band frequency range, f = 8.2 −12.4 GHz, while simultaneously providing high thermal conductivity, K ≈ 8 W m−1 K−1, which is a factor of ×35 larger than that of the base matrix material. The efficiency of the dual‐functional application depends on the filler characteristics: thickness, lateral dimensions, aspect ratio, and concentration. Graphene loading fractions above the electrical and thermal “percolation thresholds” allow for strong enhancement of both the electromagnetic interference shielding and heat conduction properties. Interestingly, graphene composites can block the electromagnetic energy even below the electrical percolation threshold, remaining electrically insulating, which is an important feature for some types of thermal interface materials. The dual functionality of the graphene composites can substantially improve the electromagnetic shielding and thermal management of airborne systems while simultaneously reducing their weight and cost.
Graphene‐enhanced composites with dual functionality are reported. Graphene composites reveal an efficient electromagnetic interference shielding in the important X‐band frequency range while simultaneously providing the high thermal conductivity for heat removal. The composites can block the electromagnetic energy even below the electrical percolation threshold, remaining electrically insulating, which is required for certain thermal management applications.
In this study, thermoelectric chalcostibite (CuSbS 2 ) compounds were fabricated using mechanical alloying (MA) and hot pressing (HP), and phase identification, microstructural observation, and ...thermal analysis were conducted. The thermal properties were then measured and compared with those of other Cu- Sb-S ternary compounds synthesized by the same solid-state process, namely, skinnerite (Cu 3 SbS 3 ), famatinite (Cu 3 SbS 4 ), and tetrahedrite (Cu 12 Sb 4 S 13 ). Both the MA powder and HP-sintered samples contained a single-phase chalcostibite with an orthorhombic structure, and relative densities of 94.6-99.7% were obtained based on HP temperature. The full width at half maximum of the X-ray diffraction peak was significantly reduced for the HP specimens compared to that of the MA powder due to stress relaxation and grain growth during HP at elevated temperatures. However, practically no changes were observed in the lattice constants based on HP temperature. Differential scanning calorimetric analysis revealed that one endothermic reaction occurred at 814-815 K for the MA powder and at 818-821 K for the HP specimen, which were interpreted as the melting points of chalcostibite. Densely sintered compacts with densities close to the theoretical density were obtained using HP at temperatures of 623 K or higher. The constituent elements of the chalcostibites were uniformly distributed. As the HP temperature increased, thermal diffusivity and conductivity increased, but they decreased significantly as the measurement temperature increased. For the chalcostibite specimen hot-pressed at 623 K, the thermal diffusivity and conductivity were (0.75-0.36) × 10 -2 cm 2 s -1 and 1.47-0.72 W m -1 K -1 at 323-623 K, respectively. Compared with other Cu-Sb-S ternary compounds, the thermal diffusivity was higher at low temperatures but similar at high temperatures, and the thermal conductivity above 500 K was lower than 1 W m -1 K -1 .
(Received 7 February, 2024; Accepted 19 March, 2024)
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