Decrease In Specific Heat Research Articles

Transcritical pressure Organic Rankine Cycle (ORC) analysis based on the integrated-average temperature difference in evaporators

Integrated-average temperature difference (ΔTave) was proposed to connect with exergy destruction (Ieva) in heat exchangers. Theoretical expressions were developed for ΔTave and Ieva. Based on transcritical pressure ORCs, evaporators were theoretically studied regarding ΔTave. An exact linear relationship between ΔTave and Ieva was identified. The increased specific heats versus temperatures for organic fluid protruded its T–Q curve to decrease ΔTave. Meanwhile, the decreased specific heats concaved its T–Q curve to raise ΔTave. Organic fluid in the evaporator undergoes a protruded T–Q curve and a concaved T–Q curve, interfaced at the pseudo-critical temperature point. Elongating the specific heat increment section and shortening the specific heat decrease section improved the cycle performance. Thus, the system thermal and exergy efficiencies were increased by increasing critical temperatures for 25 organic fluids. Wet fluids had larger thermal and exergy efficiencies than dry fluids, due to the fact that wet fluids shortened the superheated vapor flow section in condensers.

Finite-time thermodynamic modeling and analysis of an irreversible Miller cycle working on a four-stroke engine

The performance of an irreversible air standard Miller cycle in a four-stroke free-piston engine is analyzed using finite-time thermodynamic. In the model, the relation between the internal irreversibility described by using the compression and expansion efficiencies, the specific heat of the working substance depending on its temperature, the heat transfer loss as a percentage of fuel's energy and the friction loss computed according to the mean velocity of the piston is considered. Moreover, the influences of the excess air coefficient, initial temperature, compression ratio and another compression ratio corresponding to expansion level on the Miller cycle are analyzed by detailed numerical examples. The results show that the efficiency increases with the decrease of specific heat of working substance. The heat transfer loss and friction loss have negative effects on the performance. Comparison of the Miller and Otto cycles shows that the Miller cycle has a higher efficiency through extra expansion work. The conclusions of this investigation are of importance to provide guidelines for the performance evaluation and improvement of practical Miller engines.

First-principles calculations on mechanical and elastic properties of 2H- and 3R-WS2 under pressure

The structure, mechanical stability and elastic properties of 2H- and 3R-WS2 under pressure have been investigated using first-principles calculations based on density functional theory (DFT). The equilibrium lattice parameters of 2H- and 3R-WS2 at 0GPa are consistent with experimental and other theoretical values. 2H-WS2 is more stable than 3R-WS2 when pressure is less than 5.8GPa whereas 3R-WS2 is more stable than 2H-WS2 when pressure is higher than 5.8GPa. According to the mechanical stability criteria, both 2H- and 3R-WS2 exhibit mechanical stability under the pressure range from 0 to 20GPa. With the increasing pressure, the elastic moduli (E, B, G), sound velocities (vs, vp, vm) and Debye temperatures of 2H- and 3R-WS2 increase monotonously whereas volume and specific heat decrease. Large elastic anisotropies in compressibility and in shear were demonstrated for 2H- and 3R-WS2 at 0GPa. As the pressure increases, anisotropies in compressibility and in shear become weak for 2H- and 3R-WS2. Moreover, 2H- and 3R-WS2 under pressure have higher hardness and better ductility than those at 0GPa.

Specific Heat of the 1D Spin-1/2 Ising Model with Added Dzyaloshinskii–Moriya Interaction

The 1D spin-1/2 Ising model in a uniform magnetic field with added the Dzyaloshinskii–Moriya (DM) interaction is considered. The energy spectrum is obtained using the fermionization approach and defining some mean field order parameters. Then the specific heat is determined in an infinite chain system. Results show that there is only one peak in the specific heat as a function of the temperature. In the Neel phase, the position of the peak is found almost field-independent, but the maximum value of the specific heat decreases by increasing the magnetic field. Also, in the region of the saturated ferromagnetic phase, by increasing h, the peak becomes wider and goes to higher temperatures, which is a signal for the saturated ferromagnetic phase. In addition, the specific heat at low temperature shows two peaks structure on both sides of the quantum critical point.

Surfactant-free ionic liquid-based nanofluids with remarkable thermal conductivity enhancement at very low loading of graphene

We report for the first time the preparation of highly stable graphene (GE)-based nanofluids with ionic liquid as base fluids (ionic liquid-based nanofluids (Ionanofluids)) without any surfactant and the subsequent investigations on their thermal conductivity, specific heat, and viscosity. The microstructure of the GE and MWCNTs are observed by transmission electron microscope. Thermal conductivity (TC), specific heat, and viscosity of these Ionanofluids were measured for different weight fractions and at varying temperatures, demonstrating that the Ionanofluids exhibit considerably higher TC and lower viscosity than that of their base fluids without significant specific heat decrease. An enhancement in TC by about 15.5% and 18.6% has been achieved at 25 °C and 65 °C respectively for the GE-based nanofluid at mass fraction of as low as 0.06%, which is larger than that of the MWCNT-dispersed nanofluid at the same loading. When the temperature rises, the TC and specific heat of the Ionanofluid increase clearly, while the viscosity decreases sharply. Moreover, the viscosity of the prepared Ionanofluids is lower than that of the base fluid. All these advantages of this new kind of Ionanofluid make it an ideal fluid for heat transfer and thermal storage.

Thermoelectric properties of copper selenide with ordered selenium layer and disordered copper layer

Thermoelectric figure-of-merit (ZT) of ∼1.6 at 700°C is achieved in β-phase copper selenide (Cu2Se) made by ball milling and hot pressing. The β-phase of such material possesses a natural superlattice-like structure that combines ordered selenium (Se) and disordered copper (Cu) layers in its unit cell, resulting in a low lattice thermal conductivity of 0.4–0.5Wm−1K−1. A λ-shaped specific heat peak indicates a phase transition from cubic β-phase to tetragonal α-phase at around 140°C upon cooling and vice versa. An abnormal decrease in specific heat with increasing temperature was also observed due to the increased random motion of disordered Cu atoms. These features indicate that β-phase Cu2Se could be potentially an interesting thermoelectric material, competing with other conventional thermoelectric materials.

Specific heat of graphene nanoribbons

We studied the specific heat of graphene nanoribbons (GNRs) using an extended force constant model. We found that at low temperature, the specific heat decreases, and its variation with temperature increases with increasing GNR width. However, the specific heat increases with increasing GNR width after crossing a chaotic region. Free boundary conditions, –CHOH-terminated and armchair-edge-induced phonon nondegeneracy, shift and distortion and localized vibrational modes significantly influence GNR specific heat compared with periodic boundary conditions and bare and zigzag edges in GNRs. Finally, we found a uniform expression for specific heat vs. width at every temperature except for the chaotic region.

Butterflylike specific heat, magnetocaloric effect, and itinerant metamagnetism in(Er,Y)Co2compounds

The field-induced first-order phase transition in $({\text{Er}}_{1\ensuremath{-}x}{\text{Y}}_{x}){\text{Co}}_{2}$ with the yttrium concentration $x=0.45$ is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient $\ensuremath{\gamma}$ of the $T$-linear specific heat decreases by $\ensuremath{\sim}48%$ under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of $\text{Co}\text{ }3d$ electrons. The isothermal magnetic entropy change $\ensuremath{\Delta}{S}_{m}$ in ${\text{Er}}_{0.55}{\text{Y}}_{0.45}{\text{Co}}_{2}$ includes a large contribution associated with spin fluctuations induced by the $f\text{\ensuremath{-}}d$ exchange interaction in the hybridized $3d\text{\ensuremath{-}}5d$-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the $R{\text{Co}}_{2}$ type compounds. The maximal $\ensuremath{\Delta}{S}_{m}$ value observed for ${\text{ErCo}}_{2}$ just above the Curie temperature is ascribed to the closeness of the ${T}_{\text{C}}$ value to the spin-fluctuation temperature ${T}_{\text{sf}}$ of itinerant $\text{Co}\text{ }3d$ electrons. The nonmonotonous change in $\ensuremath{\Delta}{S}_{m}$ with the Curie temperature of ${R}_{1\ensuremath{-}x}{R}_{x}^{\ensuremath{'}}{\text{Co}}_{2}$ compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.

Thermophysical property of undercooled liquid binary alloy composed of metallic and semiconductor elements

The thermophysical properties of the liquid Ni–Si binary alloy system were investigated by the molecular dynamics method. The properties investigated include density, excessive volume, enthalpy, mixing enthalpy and specific heat at both superheated and undercooled states. It is found that the density decreases with an increase in the Si content, and so do the temperature coefficients. If the Si content is smaller than 30%, the density changes linearly with the temperature. If it is larger than 30%, the density is a quadratic function of the temperature. The simulated enthalpies of different composition alloys increase linearly with a rise in temperature. This indicates that the specific heats of Ni–Si alloys change little with temperature. The specific heat versus composition first decreases to a minimum value at 50% Si, then experiences a rise to a maximum value at 90% Si and finally falls again. According to the excessive volume and mixing enthalpy, it can be deduced that the Ni–Si alloy system seriously deviates from the ideal solution. Moreover, a comparison was also performed between the present results and the approximated values by the Neumann–Kopp rule. It reveals that this work provides reasonable data in a broad temperature range, especially for the metastable undercooled liquid state.

Principal component analysis on properties of binary and ternary hydrides and a comparison of metal versus metal hydride properties

Principal component analysis (PCA) is used to investigate interrelationship among material properties of hydrides. Property data which consist of ∼200 compounds (binary and ternary metal hydrides) were analyzed. A comparative study was carried out among the metal properties with that of their hydrides. The observed decrease or increase of entropy, molar volume and specific heat of hydrides from that of the metals can be attributed to hydrogen bond formation, charge transfer and corresponding change in crystal structure.

Surface and quantum-size effects on the specific heat of layered and nonlayered nanotubes

A detailed analysis of room temperature specific heat of unrelaxed layered and nonlayered nanotubes is presented. Our results exhibit a close interplay between surface and quantization contributions of specific heat, i.e. Csurface and Cquantum. The calculated specific heat of single-layer and multi-layer layered nanotubes increases with diameter regardless of the species. However, slowly increasing Cquantum and rapidly decreasing Csurface with diameter result in the decrease of specific heat of nonlayered nanotubes. Moreover, our theoretical results are compared with experimental observation and physical mechanisms are discussed. These results imply that the existence of surface in nonlayered nanotubes has an important influence on specific heat.

Low temperature specific heat of heterocyclic polymer networks: Effect of network density

Measurements of low frequency Raman scattering and low temperature specific heat between 2 and 20 K have been performed in two glassy, heterocyclic polymer networks. The effective network densities of heterocyclic polymer networks are varied by changing the ratio of bi- and mono-functional isocyanate monomers, leaving the overall chemical structure essentially unchanged. A boson peak at ∼ 20 cm−1 characterizes the Raman spectra at room temperature of both samples. Below 10 K, the specific heats deviate from a cubic temperature dependence, as predicted by the Debye theory, and reveal an excess specific heat, having the shape of a well-defined peak in a C p/T 3 plot with a maximum at ∼ 5 K. The increase in effective network density is accompanied by a slight decrease in excess specific heat. These observations have been explained in terms of additional low-energy vibrations associated with the monomers building the polymer networks.

Increase of the energy efficiency in paper industry

In this work, phenomena of heat and mass transfer in process of paper drying are given, certain technology units are analyzed, while possibility for decrease of specific heat and electric energy consumption by modernization of technology is examined. Some of the solutions applied on paper machines worldwide in order to improve energy efficiency are shown. Theoretic and practical discoveries in this area are applied in Board factory UMKA, and these results are shown in the work as well.

Application of aluminum oxide nanofluids in diesel electric generator as jacket water coolant

This paper presents a study on nanofluids applications, such as a coolant in a diesel electric generator (DEG). Specific heat measurements of aluminum oxide nanofluid with various particle concentrations have been presented demonstrating a reduction in their values with an increase in the particle concentration and an increase with temperature. Experiments were performed on the DEG to assess the effect of nanofluids on cogeneration efficiency. The investigation showed that applying nanofluids resulted in a reduction of cogeneration efficiency. This is due to the decrease in specific heat, which influences the waste heat recovery from the engine. However, it was found that the efficiency of waste heat recovery heat exchanger increased for nanofluid, due to its superior convective heat transfer coefficient.

Heavy-mass fermi liquid near a ferromagnetic instability in layered ruthenates.

Low temperature magnetic, thermal, and transport measurements in Ca2-xSrxRuO4 clarify the appearance of a cluster glass phase, after the evolution of a nearly ferromagnetic heavy-mass Fermi liquid from the spin-triplet superconductor Sr2RuO4. As the Mott transition is approached across a 2nd-order structural transition, both the magnetization and specific heat decrease considerably while the transport scattering rate diverges. A metamagnetic transition to a highly spin polarized state, with a local moment S=1/2, is observed. We argue that an orbital rearrangement with Ca substitution changes itinerant ferromagnetism to antiferromagnetism of localized moments.

繊維媒体を用いた蓄冷式熱交換器の低温性能評価

In this study, the cryogenic performance of a regenerator that utilizes fibrous media is estimated. The decrease in the specific heat of the matrix causes a loss of thermal efficiency in cryogenic regenerators. Water vapor condenses in regenerators as frost, which is one of the serious problems in cryogenic air separation plants. A single-blow experiment and numerical analysis considering the effect of temperature dependence on thermal properties as well as the effect of phase change of water vapor were performed. Good agreement of the temperature time variation in the regenerator was obtained for experimental and numerical results. The decrease in specific heat of the matrix causes a loss in efficiency, especially when the switching period is long. Furthermore, the evaporation and condensation of water vapor in a regenerator is simulated under the condition of continuous operation. It was calculated that frost deposition occurs where the temperature gradient is large, and the amount of frost increases linearly with operating time. The drying process is needed once a day for the present condition. The interval increases for lower inlet temperature and higher gas pressure.

ACOUSTIC PROPERTIES OF MgB2 SUPERCONDUCTOR UNDER PRESSURE

The pressure dependence of sound velocity of longitudinal and shear waves were measured up to 0.5 GPa using a pulse echo overlap method on polycrystalline single-phase MgB 2 superconductor which was high pressure synthesized and with a T c of 39 K. The pressure dependence of Debye temperature and specific heat were calculated based on sound velocity data. The results indicate that Debye temperature increases with the applied pressures while specific heat decreases. The Grüneisen constant was calculated as well. From the sound velocities, the pressure coefficient of the averaged phonon frequency was obtained, which is consistent with the value estimated by Vogt et al. This indicates that the importance of phonon hardening in the variation of T c under pressure. Upon changing the Debye temperature, unit cell volume and transition temperature with the pressure, volume dependence of electron-phonon coupling constant are also calculated.

A study of the influence of exhaust gas recirculation and stoichiometry on the heat release in the end-gas prior to knock using rotational coherent anti-Stokes-Raman spectroscopy thermometry

Heat release in the end-gas prior to autoignition was investigated using different experimental methods including transducers for heat flux and pressure as well as rotational coherent anti-Stokes-Raman spectroscopy, which is a laser-based method for non-intrusive instantaneous thermometry of the gas. The time history was examined in the cases of mixtures of various stoichio-metries, where some were diluted with exhaust gas recir-culation (EGR). The measured temperature history was compared with the isentropic temperature calculated from the cylinder pressure trace. This comparison revealed a difference in heat release from low-temperature reactions in the end-gas for the various mixtures tested at a constant indicated mean effective pressure and a fixed position of 50 per cent burnt charge. It is shown that lean mixtures tend to exhibit the highest knock intensity, mainly due to a decrease in specific heat, as compared to the richer mixtures, which result in an earlier knock onset and as a consequence higher knock intensity. Furthermore, the comparison of temperatures indicates that the rich mixtures have a high heat release from low-temperature chemistry, which to some extent negates the higher specific heat of the charge. As a consequence, a slight enrichment of the charge can lead to higher knock intensity in comparison with a stoichiometric mixture. In spite of the lower specific heat of the charge when a stoichiometric charge was diluted with cooled EGR, these mixtures showed a very low tendency to knock.

Magnetization, internal energy andspecific heat in a three-sublattice ferrimagnet or ferromagnet with|Jab| = |Jbc|≠Jca

The temperature dependences of the magnetization, internal energyand specific heat in a three-sublattice ferrimagnet or ferromagnetwith |Jab| = |Jbc|≠Jca are calculated withinthe framework of the linear spin-wave approximation, by employingretarded Green's functions. For both the ferromagnet and theferrimagnet, the internal energy and the specific heat decrease withincreasing J'/J and/or the value of the spins. For fixed valuesof Sa, Sb, Sc and J'/J, the internal energy and thespecific heat increase, whereas the sublattice magnetizationdecreases with increasing temperature θ. Thethree-sublattice ferrimagnet has some particular characteristicswhich are not shown by the systems with two sublattices. Forferrimagnets, the antiferromagnetism of the system becomes weakerwith increasing J'/J. The sublattice magnetization at lowtemperatures (also the magnetization M0 at 0 K) of aferrimagnet increases with increasing J'/J for fixed values ofSa, Sb and Sc. The effects of the spins Sa (Sc)and Sb on the magnetizations of other sublattices differ. Thecharacteristics of the a-sublattice are the same as those of thec-sublattice, due to their similarity as well as the symmetry ofthe system. The behaviours of the b-sublattice are different fromthose of the a- and c-sublattices, due to the asymmetry of thethree-sublattice system. The spin-value dependences of the spindeviation Δm per spin (and also the energy for thezero-point quantum fluctuation) of the system are different fordifferent sublattices. These differences are ascribed to theasymmetry of the three-sublattice systems, which leads to the newintrinsic properties of the systems.

A comparison of some thermodynamic parameters between superhard fullerite, some metals and some covalent elements

Specific heat and sound velocities measurements were carried out on samples of C60 fullerene treated at high (up to 13 GPa) pressure and temperature (up to 1200 K). With an increase in specific gravity from 2.35 to 3.15 g/cm 3 the specific heat decreases, but remains 7–8% higher than the specific heat of diamond. For the new materials obtained, the Debye temperature, the Δ C= C p− C v parameter, and the work of heat expansion were calculated in an ideal approach. Similar calculations were performed for graphite, diamond, silicon, germanium and some refractory metals for comparison. The results obtained made it possible to draw qualitative conclusions about the structural stability of the new materials and to trace general tendencies for different carbon substances. A C60 sample with specific gravity 3.15 g/cm 3 had a hardness comparable with the hardness of diamond.