Increase In Heat Capacity Research Articles

Controlling Thermodynamic Properties of Ferromagnetic Group-IV Graphene-Like Nanosheets by Dilute Charged Impurity

Using the Kane-Mele Hamiltonian, Dirac theory and self-consistent Born approximation, we investigate the effect of dilute charged impurity on the electronic heat capacity and magnetic susceptibility of two-dimensional ferromagnetic honeycomb structure of group-IV elements including silicene, germanene and stanene within the Green’s function approach. We also find these quantities in the presence of applied external electric field. Our results show that the silicene (stanene) has the maximum (minimum) heat capacity and magnetic susceptibility at uniform electric fields. From the behavior of theses quantities, the band gap has been changed with impurity concentration, impurity scattering strength and electric field. The analysis on the impurity-dependent magnetic susceptibility curves shows a phase transition from ferromagnetic to paramagnetic and antiferromagnetic phases. Interestingly, electronic heat capacity increases (decreases) with impurity concentration in silicene (germanene and stanene) structure.

Estudio del régimen de combustión sin llama ante la variación de la carga térmica

El presente estudio evaluó numérica y experimentalmente en un horno de 20 kW (a gas natural y con un factor de aireación de 1.2) la estabilidad del régimen de combustión sin llama ante la variación de la carga térmicausando diferentes flujos de aire (43 scfm, 63 scfm y 83 scfm) y una mezcla de Aire-Helio (Aire 82.02 scfm y Helio 7.06 scfm); esta última se utilizó con el fin de evaluar el comportamiento del sistema ante un incremento en el calor especifico del fluido de carga. El estudio se dividió en dos par-tes, una parte numérica y otra experimental. A partir de la simulación numérica se calculó el perfil de temperatura, el perfil de CO, el factor de recirculación Kv y la masa de productos de combustión recirculados. En la fase experimental los perfiles de temperatura mostraron una disminución en la temperatura de la cámara de combustión a medida que el flujo de aire de carga incrementaba, sin embargo, esta disminución no afectó la uniformidad térmica dentro del horno. Además la eficiencia útil mostró un incremento al pasar de un flujo de aire de carga de 43 scfm a 63 scfm,seguido de una disminución al incrementar la carga térmica.

Thermodynamic scaling of relaxation: insights from anharmonic elasticity

Using molecular dynamics simulations of a molecular liquid, we investigate the thermodynamic scaling (TS) of the structural relaxation time in terms of the quantity , where T and ρ are the temperature and density, respectively. The liquid does not exhibit strong virial–energy correlations. We propose a method for evaluating both the characteristic exponent and the TS master curve that uses experimentally accessible quantities that characterise the anharmonic elasticity and does not use details about the microscopic interactions. In particular, we express the TS characteristic exponent in terms of the lattice Grüneisen parameter and the isochoric anharmonicity . An analytic expression of the TS master curve of with as the key adjustable parameter is found. The comparison with the experimental TS master curves and the isochoric fragilities of 34 glassformers is satisfying. In a few cases, where thermodynamic data are available, we test (i) the predicted characteristic exponent and (ii) the isochoric anharmonicity , as drawn by the best fit of the TS of the structural relaxation, against the available thermodynamic data. A linear relation between the isochoric fragility and the isochoric anharmonicity is found and compared favourably with the results of experiments with no adjustable parameters. A relation between the increase of the isochoric vibrational heat capacity due to anharmonicity and the isochoric fragility is derived.

Investigation of dilution effect with N2/CO2 on laminar burning velocity of premixed methane/oxygen mixtures using freely expanding spherical flames

The main objective of this work was to measure the dilution effect of nitrogen and carbon dioxide on the unstretched laminar burning velocities and differential diffusional stability of premixed methane-oxygen mixtures at ambient conditions and different equivalence ratios. Freely expanding spherical flame method was used. The combustible mixture was prepared by following (100-Z)%CH4+2O2ϕ+Z%(N2/CO2) where Z corresponds to the percentage of diluent in the mixture by volume and ϕ refers to equivalence ratio. The percentage of N2/CO2 in the mixture was varied between 50–70%/30–50%. The equivalence ratio was varied from 0.6 to 1.4. Necessary stretch correction to the measured data was done following weak stretch theory. PREMIX with GRIMech3.0 was used to simulate all the experimental operating conditions. The addition of diluents has reduced the burning velocity of the mixture considerably due to the decrease in energy content of the mixture, increase in specific heat capacity and the corresponding reduction in the flame temperature and decrease in thermal diffusivity. At 50%N2/CO2 in the premixed methane-oxygen mixtures, the burning velocity measured with CO2 was 3.6 (average value estimated for all studied equivalence ratios) times smaller than the mixtures diluted with N2. Burning velocity was reduced by 88% due to the addition of 50%CO2 as compared to 0%CO2 in the combustible mixture and out of which reduction due to thermal effect was 81.5% and chemical effect (participation of added CO2 in the chemical reactions) was 6.5% (average value estimated for all studied equivalence ratios). For all the studied dilution cases, the burned gas Markstein length remains positive indicating that the mixtures were stable towards the differential diffusion effects. Agreement of burning velocities obtained between PREMIX with GRIMech3.0 and measurements was not good at lower levels of diluents. Based on the literature, a new species, and few elementary reactions were added to original GRIMech3.0. By doing sensitivity analysis, kinetic parameters of some elementary reactions were also modified. Simulations with the modified GRIMech3.0 showed good agreement with most of the experimental data.

Simulation of hydrogen diffusion in TiH x structures

Quantum-mechanical calculations of the energies of the formation of Frenkel pairs and barriers for hydrogen migration via different mechanisms in the titanium hydride δ-TiH x and in the α phase of titanium have been carried out. Using the potential of interaction developed for the molecular-dynamic simulation, diffusion coefficients of hydrogen in fcc and hcp lattices of TiH x were calculated depending on the temperature. The opportunity to approximate the coefficients of hydrogen self-diffusion has been analyzed in terms of the model of non-interacting point defects. For δ-TiH x , the range of concentrations and temperatures was separated where the self-diffusion of hydrogen becomes liquid-like (ceases be dependent on the hydrogen concentration), upon the transition into which there takes place a sharp increase in the isochoric heat capacity. The effect of defects in the Ti sublattice on the coefficient of self-diffusion of H has been investigated.

Thermal properties of multiferroic Bi1−xEuxFeO3 (х = 0–0.40) ceramics

A study of thermal diffusion, heat capacity and thermal conductivity of multiferroic Bi1−xEuxFeO3 (x = 0–0.4) within the range of 130–1200 K is reported. Modifying by admixture of Eu is found to change substantially the thermal anomalies of diffusion and thermal conductivity of the antiferromagnetic phase transition, to increase heat capacity over a wide range of temperatures and to shift the antiferromagnetic transition temperature. The excess heat capacity is shown being related to Schottky effect of three-level states. The mechanisms dominating thermal transfer of phonons at the phase transition and dependence of the mean free path of phonons on the temperature are determined. The results are discussed with account of structural data.

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

ABSTRACT We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations ( ) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions ( ) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C16H ) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C16H ) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H2 formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H2 formation at least for small hydrogenated PAHs like pyrene.

From ultrafast to slow: Heating rate dependence of the glass transition temperature in metallic systems

When a solid crosses the glass transition, viscous flow sets in and the glass transforms into a supercooled liquid. The glass transition temperature exhibits a heating rate dependence which was previously observed to follow a Vogel–Fulcher–Tammann (VFT) equation. Here we report on a change in the heating rate dependence of the glass transition, determined by the distinct increase in the specific heat capacity, of a metallic glass. We combine standard techniques such as differential scanning calorimetry with recently developed ohmic heating techniques in order to cover seven orders of magnitude in heating rate. Contrary to previous reports, our results show that the change of the glass transition temperature cannot be described by a single VFT equation. Instead we find two distinct regimes which independently follow a VFT expression.

Thermophysical properties and conduction mechanisms in AsxSe1−x chalcogenide glasses ranging from x = 0.2 to 0.5

The arsenic (As) to selenium (Se) ratio in AsxSe1-x glasses ranging from x = 0.2 to 0.5 was varied in order to examine the effect of chemical and topological ordering on the glass' thermal transport behavior. The fundamental thermal properties of glass transition temperature (Tg), thermal conductivity (k), and heat capacity (cp) were experimentally measured using differential scanning calorimetry, transient plane source method, and ultrasonic testing. Based on topological constraint theory, inflections in Tg and k were found at the structural coordination number ⟨r⟩ of 2.4, whereas a slight increase in heat capacity (cp) with increasing ⟨r⟩ was observed. A maximum in total thermal conductivity of 0.232 W/m·K was measured for the composition with x = 0.4, which corresponds to the stoichiometric As2Se3. Gas kinetic theory was used to derive an expression for the photon (kp) portion of thermal conductivity, which was calculated by measurements of the glass' absorption coefficient (α) and refractive index (n). Models based on Debye theory were used to derive expressions for specific heat (cv) and the lattice (kl) portion of thermal conductivity. The maximum value for kp was 0.173 W/m·K for the composition with x = 0.2, and a minimum value of 0.144 W/m·K was measured for the composition with x = 0.4. Photonic conduction was found to be the dominant carrier mechanisms in all compositions, comprising 60% to 95% of the measured total thermal conductivity.

Tailor-making thermocouple junction for flame temperature measurement via dynamic transient method

Traditional dynamic method for temperature measurement is generally based on the assumption that the thermal inertia coefficient of a thermocouple junction is invariable (time constant) within the whole measuring process. Furthermore, the effect of heat conduction and heat radiation on the temperature response of the thermocouple is simply neglected under certain conditions. In this study, a new concept is proposed for improvement of the traditional dynamic method. The key point of the newly-proposed method is to tailor-make a thermocouple junction diameter to realize accurate compensation for radiation heat loss and specific heat capacity rising with the heat conduction from wires to the junction, which eventually makes the temperature response curve of the thermocouple still comply with the first-order response equation. The proposed method can be used for temperature measurement of high temperature flame, fine particle-laden flame as well as lab-scale small flame. Heat sink at the thermocouple junction due to heat conduction from neighboring wires to the junction is observed via numerical calculation, which can compensate the temperature rise lag of the junction caused by the radiation heat loss and the increase of specific heat capacity. Thus, there is no need to neglect the effect of heat conduction, heat radiation and thermal inertia coefficient variation on temperature measurement as in the traditional dynamic method. A thermocouple with the junction diameter of 0.7mm can retain relatively long time (ca. 2s) of steady apparent thermal inertia coefficient. The tailor-made thermocouple is applied to measure the temperature of TiO2 aerosol flame and sooting flame. The temperature measurement results agree well with both simulation results and benchmark experimental results, which indicates that the improved dynamic method and the tailor-made thermocouple can be employed to measure the temperature of fine particle-laden flame in a relatively straightforward manner.

Impact of wellbore fluid properties on trapped annular pressure in deepwater wells

To reduce the threat of trapped annular pressure on deepwater wells safe production, this study established a model of calculating trapped annular pressure, examined the influence of wellbore fluid properties on trapped annular pressure, and analyzed the sensitivities and engineering feasibilities of controllable factors. To realize the calculation of trapped annular pressure under multiple annuli with liquid, a volume balance equation set was built according to compatibility principle and a wellbore temperature computing model was built based on wellbore-formation coupled heat transfer. Annular pressure decreases as the expansion-compression ratio of annular fluid reduces. Decreasing annular saturation can eliminate annular pressure radically and then a formula was proposed to give extreme annular saturation. The increase of production fluid specific heat capacity and flow rate leads to enhancement of annular pressure. Annular pressure keeps a linear relation to production fluid hole bottom temperature and the wellhead temperature can reflect the value of annular pressure. The water ratio increase of production fluid causes dynamic increase of annular pressure. The sensitivity of annular saturation is much higher than other factors. Decreasing annular liquid thermal conductivity has relatively higher engineering feasibility. The annular pressure can be controlled effectively by developing subsea wellheads with the ability to release annular fluid, highly compressible materials and downhole thermal-insulated fluids.

Exploring heating performance of gas engine heat pump with heat recovery

In order to evaluate the heating performance of gas engine heat pump (GEHP) for air-conditioning and hot water supply, a test facility was developed and experiments were performed over a wide range of engine speed (1400-2600 r/min), ambient air temperature (2.4-17.8 °C) and condenser water inlet temperature (30-50 °C). The results show that as engine speed increases from 1400 r/min to 2600 r/min, the total heating capacity and energy consumption increase by about 30% and 89%, respectively; while the heat pump coefficient of performance (COP) and system primary energy ratio (PER) decrease by 44% and 31%, respectively. With the increase of ambient air temperature from 2.4 °C to 17.8 °C, the heat pump COP and system PER increase by 32% and 19%, respectively. Moreover, the heat pump COP and system PER decrease by 27% and 15%, respectively, when the condenser water inlet temperature changes from 30 °C to 50 °C. So, it is obvious that the effect of engine speed on the performance is more significant than the effects of ambient air temperature and condenser water inlet temperature.

Relating phase transition heat capacity to thermal conductivity and effusivity in Cu2Se

Accurate measurement of thermal conductivity is essential to determine the thermoelectric figure‐of‐merit, zT. Near the phase transition of Cu2Se at 410 K, the transport properties change rapidly with temperature, and there is a concurrent peak in measured heat capacity from differential scanning calorimetry (DSC). Interpreting the origin as a broad increase in heat capacity or as a transient resulted in a three‐fold difference in the reported zT in two recent publications. To resolve this discrepancy, thermal effusivity was deduced from thermal conductivity and diffusivity measurements via the transient plane source (TPS) method and compared with that calculated from thermal diffusivity and the two interpretations of the DSC data for heat capacity. The comparison shows that the DSC measurement gave the heat capacity relevant for calculation of the thermal conductivity of Cu2Se. The thermal conductivity calculated this way follows the electronic contribution to thermal conductivity closely, and hence the main cause of the zT peak is concluded to be the enhanced Seebeck coefficient. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Urban warming in the 2013 summer heat wave in eastern China

The impact of urban warming during the 2013 July–August extreme heat wave event across the Yangtze River Delta (YRD) in China was assessed. Using a newly developed high-resolution, land-use dataset, urban stations were identified from a total of 101 stations in the YRD. The difference between urban and non-urban/rural stations indicates that urban warming reached 1.22 °C in the 2013 summer heat wave. The new land-use dataset was then input to the Weather Research and Forecasting model to further understand the dynamical/physical processes of the urban warming during the heat wave. The model-simulated urban warming is ~1.5 °C. Impacts of urbanization on near-surface temperature had strong diurnal variation, reaching a peak at 19:00 LST, around sunset. In the daytime, urban warming was mainly caused by enhanced sensible heat fluxes and longwave radiation from the surface. Because of reduced latent heat flux and increased heat capacity, urban ground stored much more heat than rural ground during the daytime, which is later released as sensible heat flux from the surface at night, leading to the nocturnal urban warming. The simulation results also suggest a positive feedback between urban warming and heat wave intensity, which makes the heat wave more intense in urban than rural areas and the urban warming during the extreme heat wave stronger than its climatological mean.

Influence of connectivity on the rigidity of the covalently bonded (GeTe4)100−x(As2Se3)x glasses

Bulk (GeTe4)100−x(As2Se3)x(0≤x≤60) glasses in the tie-line GeTe4-As2Se3prepared by melt quenching method are characterized by differential scanning calorimetry (DSC), X-ray diffraction and Raman spectroscopy. In covalent network glasses, the structural network transform from a floppy to a rigid structure at an average coordination number (Zav) 2.40. (GeTe4)100−x(As2Se3)x glasses in the range 5≤x≤15 exhibit two stage crystallization whereas glasses with x>15 exhibit single stage crystallization. Annealing of the glasses results into hexagonal GeTe and Te structures for 0≤x≤10 whereas glasses in the range 15≤x≤40 show only Te crystallites. For higher As2Se3 mol. % (x≥50) As2Te3 phase along with Te has been observed. Raman studies indicate the presence of GeSe2, GeTe2, As2Te3, and -Te-Te-structural units. The average coordination number of the glasses prepared in this tie-line is 2.4. Though the overall bond energy of the system increases with the increase of As2Se3, the density (ρ), glass transition (Tg) and specific heat capacity (∆Cp) show a decreasing trend. The rate of decrease of ρ, Tg and ∆Cp is slow for 0≤x<25 and rapid for 25≤x≤40. These results indicate that the rigidity of the structural network decreases with an increase of As2Se3 though the average coordination number remains constant for the entire range of composition.

METROLOGICAL PERFORMANCES OF BOMB CALORIMETERS AT REAL CONDITIONS

The high-usage measurement equipment for heat of combustion of organic fuels are bomb isoperibol calorimeters with a water thermostat. The stability of work of calorimeters at real conditions is important for maintenance of reliability of measurement results. The article purpose – the analysis of stability for parameters of calorimeters to environment changes. In this work influence room temperature (Тк) and heat exchange conditions on metrological characteristics of two models of calorimeters is considered with different degree of thermal protection: V-08МА and BIC 100. For calorimeters V-08МА the increase in a effective heat capacity (W) on 0,1 % by growth of Tк on everyone 5 °С is established. To use value W in all interval laboratory temperatures Tк = 14–28 °С it is necessary to correct W on 2,8 J/°C on everyone 1 °С changes of Tк. Updating W is required, if the correction exceeds error in determination W. For calorimeter BIC 100 it is not revealed dependences W from Tк. BIC 100 have constant-temperature cap, high stability a temperature in thermostat and stabilized heat exchange. It is established that an standard deviation of cooling constant for all calorimeters in direct proportional to standard deviation W.

Effects of nitrogen dilution on the NOx and CO emission of H2/CO/CH4 syngases in a partially-premixed gas turbine model combustor

In this study, the nitrogen dilution effect in H2/CO/CH4 syngas flames on the characteristics of NOx and CO emissions were investigated in a partially premixed gas turbine model combustor. Nitrogen was diluted into the fuel side and combustor liner temperature, and the flame structures were observed in order to find information that was critical for analyzing the results of the NOx and CO emissions. The addition of nonflammable N2 was effective in decreasing the combustion temperature, which was attributed to the reduction in the reaction rate of syngas oxidation as well as the increase in the overall heat capacity of the air-fuel mixture at a constant heat input. NOx suppression was successfully achieved up to the one digit ppmv level for all heat input and all fuel composition by the 60%-fuel side dilution of nitrogen. However, increases in nitrogen dilution increased the possibility of CO emission; therefore, extremely high CO was emitted in a 100%-CO flame at a low load with 60%-nitrogen dilution reaching 10,000 ppmv. The dilution of nitrogen in syngas considerably changed the flame structure, such as flame length, thickness, angle, location, and intensity. These flame structures provided valuable clues for the analysis of the NOx and CO emission results in relation to the flame temperature results. Based on the integration of the test results, a cause-and-effect diagram of NOx and CO emissions was plotted, and the root causes of NOx suppression were determined to be the decreases in both temperature and residence time in the hot combustion zone, and the critical path of these root causes was clarified. Although the nitrogen dilution technique has been investigated in detail, and the effects are fully understood, the careful monitoring of CO and flame detachment from the fuel nozzle is recommended when applying the nitrogen dilution technique to reduce NOx emission, especially in cases of low load operation. A practical dataset was collected from the combustion test at gas turbine relevant conditions for a wide range of fuel compositions. These results and the knowledge gained from this study could be usefully applied for the environmentally friendly and safe operation of integrated gasification combined cycle and synthetic natural gas firing gas turbine power plants.

On the specific heat capacity enhancement in nanofluids.

Molten salts are used as heat transfer fluids and for short-term heat energy storage in solar power plants. Experiments show that the specific heat capacity of the base salt may be significantly enhanced by adding small amounts of certain nanoparticles. This effect, which is technically interesting and economically important, is not yet understood. This paper presents a critical discussion of the existing attendant experimental literature and the phenomenological models put forward thus far. A common assumption, the existence of nanolayers surrounding the nanoparticles, which are thought to be the source of, in some cases, the large increase of a nanofluid’s specific heat capacity is criticized and a different model is proposed. The model assumes that the influence of the nanoparticles in the surrounding liquid is of long range. The attendant long-range interfacial layers may interact with each other upon increase of nanoparticle concentration. This can explain the specific heat maximum observed by different groups, for which no other theoretical explanation appears to exist.

Development of pavement temperature predictive models using thermophysical properties to assess urban climates in the built environment

This study developed pavement temperature predictive models based on the characterized thermophysical properties of different pavements to assess urban climates in the built environment. A database comprising of six pavement types including conventional and modified asphalt, and cement concrete mixtures was available with their thermophysical properties: specific heat capacity, thermal conductivity and material density. Models were developed to predict temperature at the surface, and at 40mm depth using the measured thermophysical properties, and recorded climatological parameters: air temperature, wind speed, and relative humidity. The two predictive models were robust and rational depicted by low bias and high precision. An increase in heat capacity increased pavement surface temperature indicating that higher energy is required to raise the pavement temperature, and also be able to release as much energy as stored, which would be best suitable at different times of the day to counter urban heat island (UHI) effects. An increase in thermal conductivity decreased pavement temperature illustrating that the pavement would store more heat within the system for a longer duration, and may release this heat at a particular timeframe changing the urban climate at that moment. An increase in wind speed by about 1m/s increased pavement temperature by 1°C, and this may increase UHI if there is already higher temperature in the environment. Overall, based on rational correlations between model predictions and actual field measurements it is recommended that the pavement temperatures of the systems be comfortably predicted for pavements using the developed models.

Experimental and theoretical analysis of effects of equivalence ratio on mixture properties, combustion, thermal efficiency and exhaust emissions of a pilot-ignited NG engine at low loads

This paper presents the effects of equivalence ratio (Φ) on mixture properties, combustion, thermal efficiency and exhaust emissions of a pilot-ignited natural gas (NG) engine using diesel as an ignition source at low loads. Φ was changed in a wide range (0.4–0.9) by changing throttle opening with other combustion boundaries being unchanged with different NG percentage energy substitutions (PES) at 1335rpm and 25% load. The experiments were arranged based on an electronically controlled heavy-duty, turbocharged, intercooler, 6-cylinder and four-stroke pilot-ignited NG engine. Thermodynamic properties of the NG–air mixture with decreasing air were analyzed theoretically using thermodynamic relations and ideal gas equations. In addition, the time-sequenced characteristic and the heat release rate (HRR)-imbalanced characteristic were found in the engine combustion process, with their quantitative indicators being defined. The results show that as Φ increases, the density, the specific heat ratio and the heat capacity of the NG–air mixture decrease, but the gas constant and the specific heat capacity increase. CA10 moves far away from top dead centre (TDC), CA50 moves near TDC first and then far away with increasing Φ. Three kinds of profiles of HRR curves with different time-sequenced and HRR-imbalanced characteristics appear as Φ increases. The engine thermal efficiency increases first and then decreases with increasing Φ. In addition, as Φ increases, total hydrocarbons (THC) emissions decrease significantly (decrease by 97% as Φ increases from 0.45 to 0.91 with 90% PES) and nitrogen oxides (NOx) emissions increase first and then decrease.