Heat Flow Curves Research Articles

Study on evolution characteristics of thermal contribution functional groups in low temperature oxidation process of bituminous coal

In order to study the key functional groups that play a major role in heat during the low-temperature oxidation of bituminous coal, this paper takes three kinds of bituminous coals with different degrees of metamorphism as the research objects, analyzes the variation law of thermal effect parameters during low-temperature oxidation of coal by C80 microcalorimetry experiment, and calculates and analyzes the activation energy of different oxidation stages by oxidation kinetics method. The change rule of functional groups during low temperature oxidation was analyzed by Fourier transform infrared experiment. The correlation between heat and functional groups was comparatively analyzed by using the grey correlation degree and frequency ratio model method, and the key functional groups of bituminous coal that play a major role in heat at different oxidation stages were identified. The results showed that with the deepening of metamorphism degree, the heat flow curve and exotherm gradually decrease, while the characteristic temperature and activation energy gradually increase. Among them, from Caojiatan's long-flame coal to Dongtan's gas coal, the exotherm decreased by 35.5 %, and the maximum endothermic rate temperature and heat balance temperature increased by 3.43 °C and 13.32 °C respectively. With the deepening of metamorphism degree, the content of hydroxyl groups keeps increasing, the content of aliphatic hydrocarbons, aromatic functional groups and fatty ethers first decreases and then increases, and the content of carboxyl groups first increases and then decreases. The correlation analysis between the gray correlation method and the frequency ratio model method shows that as the degree of metamorphic deepened, the key functional groups that play a major role in heat change from COOH to OH in the rapid endothermic stage, from-CH2- to COC in the slow endothermic stage, and from OH to COOH in the rapid exothermic stage. The key functional groups of bituminous coal, which play a major role in heat, follow the evolution of OH → CH2, COC → COOH as the oxidation stage advances.

Hydration mechanism and phase assemblage of ternary blended cements based on sewage sludge ash and limestone: Modified by Na2SO4

This study developed an eco-friendly ternary system based on sewage sludge ash and brought in Na2SO4 as a modifier to increase the mechanical property. The changes of the hydration kinetics, microstructure and phase assemblage of the ternary system with the addition of Na2SO4 were investigated. The results show that Na2SO4 accelerates the hydration kinetics of the ternary system and promotes the reaction of both clinker and sewage sludge ash, leading to the increase in compressive strength. The formation of the aluminate phases is promoted by the addition of Na2SO4, which contributes to porosity refinement and strength development. Moreover, Na2SO4 modifies the composition and microstructure of the aluminate phases in the ternary system. The addition of Na2SO4 promotes the formation of carboaluminate phases due to the increase in the alkalinity of the pore solution, which reflects on the appearance of the third hydration peak in the heat flow curves. However, with the increase in the addition level of Na2SO4, the formation of carboaluminate phases is inhibited due to the fact that the aluminates react preferentially with sulfate rather than limestone.

Impact of thermal radiation on turbine blades with film cooling structures

With increasing temperature in front of turbine blades, the impact of thermal radiation on the cooling of turbine blades continues to increase. Therefore, to ensure the long-term reliable operation of blades, the impact must be accurately evaluated. In this study, a method for separating the contribution of radiative heat flow from the total heat flow is proposed for turbine blades with different complex film-cooling structures. Here, the effect of the film hole angle on the proportion of the radiative heat flow was studied using parallel experiments and nonlinear regression methods, which allowed the radiative heat flow to be estimated with a high degree of accuracy. The method was verified by computational fluid dynamics (CFD) simulation data results showing that after considering the change in the physical properties of the boundary layer, the convective heat flow curve inclines upward along the wall temperature, and is no longer a linear function of the wall temperature. The radiative heat flow is driven by the 4th power of temperature, and the curve instead inclines downward. When the mainstream gas temperature was 1600 K, the radiant heat flow of the film cooling plate with a 30° hole angle accounted for up to 40 % of the total heat flow, with the 45° and 60° hole angles both accounting for more than 20 %. Future research must focus on the effect of thermal radiation in the heat transfer design of the blade.

A standardized method for genus Colletotrichum characterization by isothermal microcalorimetry using thermokinetic parameters

A new standardized method, using isothermal microcalorimetry (IMC), was established to determine thermokinetic parameters from heat flow curves and to demonstrate the reproducibility and repeatability of the parameters of five Colletotrichum species on different days. Measurements on IMC were made at different periods and by two operators. Repeatability and reproducibility (R&R) measurement system analysis was performed on the technique used to measure the heat flow of Colletotrichum strains. The results showed that the %GageR&R was found to be within the acceptable ranges of a measurement system. Also, the parameters obtained from the curves were subjected to a combination of Principal Component Analysis (PCA) and Clustering, the data showed that the total heat (Ht) and maximum growth rate (μmax) are probably the most specific distinguishing characteristic of the strains evaluated in this study. This study demonstrates, for the first time, the usefulness of IMC in obtaining heat flow curves and thermokinetic parameters, providing repeatable and reproducible measurements over a period and under controlled conditions, for future identifications of phytopathogenic fungi.

Hydration mechanism and phase assemblage of blended cement with iron-rich sewage sludge ash

The hydration of Portland cement blended with iron-rich sewage sludge ash (IRSSA) was investigated. The addition of IRSSA results in a retarding effect on cement clinker hydration during early ages, especially C3S, while the reaction degree of cement clinker increases at later ages. The second peak observed in the deceleration period of heat flow curves in blended systems is due to the reaction between aluminum/iron phases in IRSSA and portlandite, generating additional AFt and AFm. Iron can also be incorporated and absorbed in C-S-H, leading to the increase in the Fe/Ca atomic ratios. Special areas with disorder distribution of bright spots can be observed in BSE images, which is due to the formation of C-S-H induced by IRSSA. The low mobility of Fe3+ in the pore solution leads to a higher Fe/Ca atomic ratios in the C-S-H induced by IRSSA than that in the C-S-H induced by cement clinker.

Quest for optimal nanoconfinement for hydrate-based CO2 capture

CO2 capture is a viable solution for mitigating the global CO2 emission in the atmosphere. Hydrate-based CO2 capture is a promising technology. In this study, the CO2 hydrate formation thermodynamics and kinetics, along with the factors that influence CO2 capture in nanoporous silica gels (named S9, S26, and S77 with an average pore size of 9, 26 and 77 nm, respectively), with different degrees of nanoconfinement were investigated. The CO2 hydrate phase equilibrium and dissociation heat were determined by heat flow analysis. The heat flow curves revealed the formation of both confined- and bulk-phase hydrates in S9 and only bulk-phase hydrate in S26 and S77. The phase equilibrium temperature of the confined-phase hydrate was about 10 K lower than that of the bulk-phase hydrate and the dissociation heat of confined phase hydrate is 285.81 J/g in S9. According to the pressure curves, the CO2 hydrate formation time in S9 was 2–3.5 times longer than that in S26 and S77. Compared with dry silica gel, the supersaturated one was more conducive to CO2 capture. Under the same water saturation condition, the CO2 capture ability of S26 and S77 was stronger than that of S9, and the effect of water saturation was dominant over that of pressure. Thus, nanoconfinement in S9 has an adverse effect on hydrate-based CO2 capture whereas that in S26 and S77 is advantageous to it. This study provides an important fundamental basis for enhancing the hydrate-based CO2 capture ability via nanoconfinement in silica gel.

A new approach to time-resolved electrochemical impedance spectroscopy using the Impedance Camera to track fast hydration processes in cement-based materials

In this paper, usefulness and accuracy of the new Impedance Camera working in the time-resolved electrochemical impedance spectroscopy (TR-EIS) mode was demonstrated. Flash setting monophase hydraulic binder material Ca7ZrAl6O18 (C7A3Z), and traditional both Ordinary Portland Cement (OPC) and calcium aluminate cement (CAC)-based alumina spinel castables (CMAS castables) were selected and evaluated. The TR-EIS measurements were conducted in the frequency range from 100 Hz to 1 MHz with 17 point per decade. TR-EIS results were correlated with the hydration heat release determined by calorimetry, and thermal behavior (DSC-TG-EGA(MS)) and phase composition (XRD) of the C7A3Z cement paste. In this work, a new approach to divide the curve of speed impedance changes into few characteristic periods of hydration, different than presented so far as the periods on the heat flow curve obtained by isothermal calorimetry was implemented.

Improved impurity determination of pure organic substances by differential scanning calorimetry with a dynamic method.

Differential scanning calorimetry can be used to measure the impurity contents of pure organic substances on the principle of freezing-point depression. Impurity determination by differential scanning calorimetry with a dynamic method, which has the advantages of speediness and convenience, remains to be explored. Here, a series of acetanilide and dibenzothiophene samples with various purities was prepared through zone melting, and the samples were then analyzed by gas chromatography-mass spectrometry. A modified dynamic method, including encapsulating the analyte in a volatile pan through cold welding, remelting the analyte with a low heating rate, calculating the melted fraction considering the area of the tailing under the heat-flow curve, and reducing the error from solid-solution formation, is proposed. Encapsulating with a volatile pan using a proper torque gave an accurate result. Remelting gave a lower impurity content and a more narrow and sooth peak of heat-flow compared with the first melting. The impurity-content results calculated by the modified method were usually higher than those calculated by the ASTM standard method. For acetanilide and dibenzothiophene with impurity contents of less than 0.30%, the modified dynamic method showed good accuracy. The proposed method is applicable to determination of reference materials of organic substances with high purity owing to its accuracy and convenience.

Effect of tartaric acid on the hydration of oil well cement at elevated temperatures between 60 °C and 89 °C

In order to study the abnormal viscosity evolution behavior of oil well cement slurries containing tartaric acid, the influence of tartaric acid on the early-age hydration of oil well cement at high temperatures was investigated by isothermal calorimetry (IC), thermogravimetric analysis (TGA) and quantitative XRD (QXRD) analysis in the temperature range of 60 °C to 89 °C. Results show that, a high dosage of tartaric acid can fundamentally change the typical shape of the cement hydration heat flow curve - two additional exothermic peaks associated with aluminate phase hydration (C3A and C4AF) appear before the main hydration peak. The unsynchronized hydration of the aluminate phases and the silicate phases was found to be the cause of premature setting. Additionally, long-term curing study (up to 28 d) showed that increasing the dosage of tartaric acid decreased the ultimate hydration extent of C2S while it increased the ultimate hydration extent of C4AF.

Influence of Hydroxypropyl Methylcellulose Dosage on the Mechanical Properties of 3D Printable Mortars with and without Fiber Reinforcement

Hydroxypropyl Methylcellulose (HPMC) is one of the most frequently used viscosity modifying admixtures in 3D printable cement-based materials. In this study, the effects of HPMC dosage on the mechanical properties of 3D printable cement-based mortars were investigated. For this purpose, mortar mixtures with and without micro steel fibers containing three different HPMC dosages (0%, 0.15%, and 0.30% by weight of cement) were produced. Reliant on the HPMC dosage, heat flow and cumulative heat curves were obtained. At the end of 7 and 28 days of standard curing, flexural, compressive, and shear bond strengths, as well as flexural toughness, were measured. Additionally, porosity values were obtained on molded, single-layer, and three-layer printed specimens. The results showed that the increase in HPMC dosage prolonged the setting times and decreased the heat release. Moreover, the porosity values increased with an increase in the HPMC dosage and the number of printed layers. All mechanical properties were drastically decreased with the use of HPMC. The decrements were more significant at the first 0.15% HPMC dosage and the shear bond strengths. Prolonging the curing period from 7 to 28 days did not lead to meaningful recovery in the mechanical properties. The negative effects of HPMC on flexural and shear bond performances were more pronounced in fiber-reinforced mortars.

A Study on Temperature Distribution within HVDC Bushing Influenced by Accelerator Content during the Curing Process

Power transmission technology plays an important role in energy sustainability. Bushing is an indispensable type of equipment in power transmission. In production, the accelerator changes the temperature distribution during the curing process, influencing the formation of defects and thus the safety output of renewable energy. In this study, uncured epoxy resin samples with different accelerator contents were prepared and measured by differential scanning calorimetry (DSC). The obtained heat flow curves were analyzed for curing kinetics. Then, the curing process of large length–diameter ratio bushings was simulated by using the finite element method combined with a curing kinetics model, transient Fourier heat transfer model, and stress–strain model. The study reveals that the curing system can be established by the Sestak–Berggren autocatalytic model with different accelerator contents. The overall curing degree and the maximum radial temperature difference of the capacitor core tend to increase and then decrease with the accelerator content. This is mainly attributable to the rapid exotherm excluding the participation of some molecular chains in the reaction, resulting in permanent under-curing. As the accelerator content increases, the strain peak decreases and then increases. This paper provides guidance for the comprehensive evaluation and manufacturing of the low-defect capacitor cores of large-size high voltage direct current (HVDC) bushings.

Reaction kinetics of supplementary cementitious materials in reactivity tests

This work characterizes the reaction kinetics of supplementary cementitious materials (SCMs) with calcium hydroxide in the modified R3 test. The heat flow curves of 58 SCMs of varying reactivities were studied. Based on the heat flow curves, the SCMs were classified as more reactive, less reactive, and inert. Most of the heat flow curves in the modified R3 test exhibit, after the peak of heat flow, an initial slow decaying power-law regime that transitions into a longer and faster decaying power-law regime. The pre-exponent of the first regime depends on the initial SCM reactivity and correlates well with the 24-hour heat release in the modified R3 test, thus making it a useful metric for rapid classification of SCMs.

Coupled Transport Effects in Solid Oxide Fuel Cell Modeling.

With its outstanding performance characteristics, the SOFC represents a promising technology for integration into the current energy supply system. For cell development and optimization, a reliable quantitative description of the transport mechanisms and the resulting losses are relevant. The local transport processes are calculated by a 1D model based on the non-equilibrium thermodynamics (NET). The focus of this study is the mass transport in the gas diffusion layers (GDL), which was described as simplified by Fick’s law in a previously developed model. This is first replaced by the Dusty-Gas model (DGM) and then by the thermal diffusion (Soret effect) approach. The validation of the model was performed by measuring -characteristics resulting in a maximum deviation of experimental to simulated cell voltage to up to 0.93%. It is shown that, under the prevailing temperature, gradients the Soret effect can be neglected, but the extension to the DGM has to be considered. The temperature and heat flow curves illustrate the relevance of the Peltier effects. At K and A/m2, 64.44% of the total losses occur in the electrolyte. The exergetic efficiency for this operating point is 0.42. Since lower entropy production rates can be assumed in the GDL, the primary need is to investigate alternative electrolyte materials.

Reentrant strain glass transition in Ti-Ni-Cu shape memory alloy

Reentrant glass transition, referring to a reversible transition from a high-temperature ordered phase to a low-temperature glassy phase, is unusual and difficult to be understood, because at low temperature the glassy phase is usually less thermodynamically stable than the ordered phase. In this work, we report a systematic study of a reentrant strain glass (RSTG) transition between a strain-ordered martensitic phase and a strain glass in a Ti50Ni34Cu16 shape memory alloy. This transition is characterized by a temperature invariance of average B19 martensitic structure, a deviation in heat flow and resistivity curves, a frequency-dependent behavior of storage modulus and internal friction curves, and a peak in a zero-field cooling curve. In-situ microstructural observations show that the B19 martensitic domain pattern keeps unchanged upon cooling while nanodomains with local 4H symmetry gradually emerge and grow in large martensitic domains. Moreover, the RSTG transition exhibits low modulus (∼24 GPa) and high damping (tan δ > 0.075) over a large temperature range. Based on experimental results, we established a new Ti50Ni50-xCux phase diagram with the RSTG state included. This phase diagram helps understand the abnormal formation of RSTG: since Cu dopants stabilize the B19 phase instead of B19’ phase, with sufficient Cu dopants (x = 16) the B19-B19’ transition is thermodynamically suppressed and the RSTG transition appears, which resembles the conventional strain glass formation in a Ti50-xNi50+x phase diagram. The microstructural evolution of RSTG with nanodomains embedded in large domains explains novel properties and puzzles in Ti-Ni-Cu alloys. We further predict more unusual properties could be found in RSTG materials.

Thermal hazard and mechanism study of 5-(4-Pyridyl)tetrazolate (H4-PTZ)

Thermal decomposition experiment of 5-(4-Pyridyl)tetrazolate (H4-PTZ) was carried out. The heat flow curve and reaction rate data under different heating rates were obtained. The characteristic parameters were obtained. The apparent activation energy for each individual reaction was calculated by applying different methods. On this basis, Malek method was used to predict the most probable mechanism function of thermal decomposition reaction of H4-PTZ. The thermal safety parameters, including self-accelerating decomposition temperature, hot spot fire temperature and thermal explosion critical temperature were predicted also. The activation enthalpy, activation entropy, and activation Gibbs free energy of H4-PTZ are calculated. Gaussian16 program was used to optimize the molecular structure, search the transition state and calculate the intrinsic reaction coordinates of H4-PTZ. The most probable decomposition path of H4-PTZ was found, and the activation energy calculated by experiment was compared with that calculated by theory.

Numerical Simulation of the Effects of Wedge Subduction on the Lithospheric Thermal Structure and the Seismogenic Zone South of Chile Triple Junction

In contrast to common subduction, the young and thin part of the Antarctic Plate subducts first to the south of the Chile Triple Junction (CTJ), followed by the old and thick part, corresponding to wedge subduction. A finite element model was used to simulate the wedge subduction of the Antarctic Plate and to compare it with the slab subduction of the Nazca Plate. The results show that the CTJ is not only a wedge subduction boundary but also an important factor controlling the lithospheric thermal structure of the overriding plate. The computed heat flow curves are consistent with the data observed near the trench of the two selected profiles. The different slab dips to the north and south of the CTJ are considered to be caused by wedge subduction. When the slabs are young and at the same age, the deep dip of the Antarctic slab is 22° smaller than the Nazca slab. Southward from the CTJ, the slab age of the wedge subduction increases, which leads to a larger slab dip, a colder slab, and a wider seismogenic zone. The effect of the slab age of wedge subduction on the focal depth is smaller than that of the convergence rate. A 4.8-cm/year difference in convergence rate of the wedge subduction results in an 11-km difference in the width of the seismogenic zone and a 10-km difference in the depth of the downdip limit. Among these controlling factors, the convergence rate plays a major role in the different focal depths south and north of the CTJ.

On the optimization of BOF slag hydration kinetics

In this study, the early age hydration kinetics of BOF slag is investigated. Isothermal calorimetry and in-situ X-ray diffraction (XRD) are used to examine the effects of K3-citrate concentration and nanosilica addition on slag reactivity. The results reveal that brownmillerite is the most reactive slag phase during the first 24 h of hydration. The kinetics of brownmillerite hydration is controlled by K3-citrate dosage, and the reaction is strongly accelerated with highly concentrated solutions (0.6 M). A similar phenomenon is observed for amorphous C2S at the early hydration stages. Among hydration products, crystalline siliceous hydrogarnet is identified. Precipitation of hydrogarnet (as observed with the XRD technique) coincidences well with the occurrence of a shoulder in the heat flow curves after the heat flow maximum due to brownmillerite dissolution. It is shown that nanosilica slightly delays the hydration of brownmillerite while enhancing the precipitation of C-S-H gel. In consequence, the early age mechanical properties of BOF slag mortars with nanosilica addition are improved.

Influence of carbon nanotubes and polycarboxylate superplasticiser on the Portland cement hydration process

The aim of this study was to investigate the changes in the heat of hydration of Portland cement in case of its modification by a complex admixture (CA) based on multi-walled carbon nanotubes/carboxymethyl cellulose (MWCNT/CMC) suspension and polycarboxylate ether (PCE) plasticiser. The application of the CA leads to the prolongation of the duration of the induction, acceleration and later appearance of the peak at deceleration periods of Portland cement hydration by 4 h 55 min, 15 h 45 min and 21 h 30 min, respectively. The CA decreases the heat flow of cement by 35% in the acceleration period and total hydration heat emission by 26% after 48 h. The CA decreases the formation of portlandite and ettringite while the stability of gypsum increases. The identified tendency of changes in formed hydrates explains the retardation of C3S hydration. The evaluation of the cement hydration rate during the acceleration period by the angle of the linear function of the cement hydration heat flow curve was proposed. The cement modified by the CA can be useful in construction practice for the concreting of massive structures where low heat emission is a requirement, for the development of the mixture for new concrete technologies like massive concrete structures, hot weather concreting etc.

Dual Nakamura model for primary and secondary crystallization applied to nonisothermal crystallization of poly(ether ether ketone)

AbstractNon‐isothermal crystallization kinetics has been traditionally predicted using the Nakamura crystal growth model, which is an integral form of the Avrami equation under the assumption that the non‐isothermal kinetics can be explained using a series of infinitesimal isothermal steps. However, the Nakamura model often overpredicts crystal fraction near the end of crystallization because of ignoring slower secondary crystallization kinetics. In this study, the dual Nakamura model is proposed to capture both primary and secondary crystallization kinetics of poly(ether ether ketone) (PEEK). First, non‐isothermal crystallization kinetics of PEEK is monitored at five constant cooling rates of 10, 30, 50, 70, and 90 K/min via differential scanning calorimetry. The dual Nakamura model, which contains a smaller secondary Avrami exponent () to account for the secondary crystallization, is fitted to the exothermal heat flow curves. The dual Nakamura model successfully describes the final stage of crystallization at all cooling rates by reducing the crystal growth dimension from = 3 for primary crystallization to .

Experimental and theoretical study on dissociation thermodynamics and kinetics of hydrogen-propane hydrate

Hydrate-based hydrogen storage is a promising technology in the clean energy field. In this study, propane was selected as the accelerator guest molecule for hydrogen storage in hydrates. Hydrogen–propane (0.67/0.33, mole ratio) hydrate dissociation properties are investigated by experiment and molecular dynamics simulation. The hydrogen–propane hydrate phase equilibrium is measured by different methods and the average dissociation heat is 77.03 J/g under 0.98–1.72 MPa. While hydrate dissociation heat flow curves show the characteristic of two-step dissociation, which is further confirmed via Raman spectroscopy. Simultaneously, hydrogen–propane hydrate dissociation driven by depressurization and heating are also investigated by Raman spectroscopy. During dissociation, the Raman peak area of hydrogen in hydrate continues to decrease, whereas it is relatively stable for propane before the hydrate dissociation complete. In addition, hydrogen–propane hydrate reformation occurs more intensely during the heating process, especially when the temperature exceeds 273 K than with depressurization. This indicates that depressurization is more feasible for continuous hydrogen releasing, a concept of recycling hydrate clathrate structure was proposed to shorten hydrate formation time and accelerates hydrogen storage rate. In addition, the molecular dynamics simulation results confirmed that the hydrogen release rate from hydrate is higher than that of propane from the microscopic view point. The results of this study provide a potential pathway of hydrogen storage using hydrates.