Latent Heat Of Phase Transformation Research Articles

An Experimental Study on the Influence Solution Concentration and Nano-Additives on Cold Storage Performance of Tetrabutylammonium Bromide

Tetrabutylammonium bromide (TBAB) is considered a promising alternative cold energy storage material. Due to the high dissociation heat of phase transition at an atmospheric pressure of 278–293 K, which reaches 200–500 kJ/kg, this substance is considered an effective cold energy storage medium for air conditioning systems. In this paper, the cold storage crystallization process of TBAB solution with different concentrations was tested by conducting experiments and the phase transition’s temperature and latent heat were measured. Finally, the growth characteristics of TBAB hydrate crystals with different concentrations (10%, 20%, 30% and 40%) were analyzed. Considering the cold storage temperature, phase transformation temperature and latent heat, the cold storage effect is the best when 40% TBAB solution is used. Although single substance phase change materials have a long service life, they have problems with low thermal conductivity and high undercooling. Therefore, researchers usually improve the performance of phase change materials by adding other auxiliary materials, thereby enhancing their application prospects. Among these auxiliary materials, adding nano additives to phase change materials can significantly improve latent heat, thermal conductivity and nucleation ability, while also reducing undercooling. Therefore, we studied the influence of different nano-additives (Al2O3, SiC, TiO2 and ZnO) on phase change materials. The composites with excellent properties were screened by cooling step cooling curve and differential scanning calorimeter (DSC). Compared with pure TBAB solution, the phase transition latent heat of the composite phase change materials (PCMs) prepared by adding nanoparticles were significantly increased. The results show that adding nano-SiC into 40% TBAB solution can obtain better performance. This work not only provides reference for the further research, but also a sight to design the phase change materials for the application of new phase change cold storage materials.

Thermal simulation of microelectrode machining process during block electrical discharge grinding

Electrical discharge machining is widely used in aerospace industries, semiconductor applications, micro-electromechanical systems, and other fields of the machining of micro-holes, micro-shafts, and micro-structures due to non-contact stress during the electrical discharge machining process. In the electrical discharge machining process, the machining precision of electrical discharge machining depends on the dimensional precision of the tool electrode, therefore, the machining process of microelectrode has attracted growing concern. In this study, a thermal model of the microelectrode machining process was proposed to simulate the formation process of microelectrode based on the temperature distribution during block electrical discharge grinding of the microelectrode process. According to the microelectrode machining process, the thermal model considers the main influential factors such as random distribution of discharge points, time-dependent discharge channel radius, Gauss heat source, phase transformation latent heat, etc. Through the thermal simulation analysis, the temperature distribution and the dynamic spark-erosion process of the microelectrode based on random repetitive spark discharges were obtained during block electrical discharge grinding of the microelectrode. Besides, it is found that the maximum temperature value and discharge crater volume of the electrode surface fluctuate and gradually decrease with the increase of discharge times. In addition, the effect of pulse width on the machining process of microelectrode was investigated by numerical simulation and block electrical discharge grinding of microelectrode experiment. The results indicated that the crater volume and the maximum temperature value on the surface of the microelectrode increased with the increase of pulse width. The formation process of microelectrode was considered a significant indicator to determine the machining precision and machining efficiency of the electrical discharge machining process.

Thermal behavior and morphology evolution of polyamide 12 in laser powder bed fusion process: Experimental characterization and numerical simulation

A particle scale numerical model of laser powder bed fusion (L-PBF) for semi-crystalline polymer PA12 is presented in this work. The model is developed by the finite element method under the framework of the level-set method. The asymmetrical and kinetical thermal behavior of semi-crystalline polymer is implemented in the thermal model, enabling better considering the latent heats during different phase transformation processes. Particle consolidation under laser irradiation is simulated by hydrodynamics. The physical properties of PA12 powder are identified by experiments regarding the inputs of the numerical model. Before performing L-PBF processing simulations, the numerical powder bed model, the implementation of crystallization kinetics, and the related hydrodynamic parameters are verified by comparing simple simulations with the experiments. The influence of phase transformation latent heat on the temperature calculation and the consequent melt pool profile is discussed in the single-track simulation: the peak temperature on the top surface of the substrate is significantly lower when properly considering the re-melting fusion latent heat with the modified crystallization kinetics. As a result, a shallower melt pool is obtained. The effect of processing parameters on the temperature field and shape of the melt pool is also modeled. This parametric study indicates that decreasing the scanning velocity can eliminate the inter-layer lack of fusion phenomenon. Additionally, the thermal interaction between adjacent scanning tracks can improve the fusion continuity in the gap area of neighbor tracks. Finally, optical observations are conducted to validate the simulated morphology of the melt pool. A good agreement between numerical results and experimental observations is obtained, indicating the ability of the numerical model to simulate the PA12 L-PBF process.

Numerical simulation of the thermal behaviors for two typical damping alloys during selective laser melting

In this work, the three-dimensional transient temperature field model of two typical damping alloys (i.e., AZ31 Mg alloy and Mn-Cu alloy) were developed during selective laser melting (SLM) using ANSYS finite element analysis software, considering thermophysical properties and phase transformation latent heat of the materials. The simulated molten pool width values of the Mn-Cu alloy show a 6 % error with the experimental SLMed Mn-Cu sample, verifying that the developed model agrees well with the experimental results. Based on the developed numerical model, the temperature distribution, temperature gradients, and cooling rates of the two alloys were investigated during horizontal deposition and vertical deposition of SLM at different scanning speeds (300–700 mm/s). The results showed that the simulated peak temperatures of the AZ31 Mg alloy during the SLM process show a small fluctuation and are well below its boiling point (1107 °C) at the scanning speed of 500–700 mm/s. The temperature gradient increases from 8.5 × 106 °C/m to 20.2 × 106 °C/m with the laser scanning speed. The cooling rate increases from 1.7 × 106 °C/s to 4.1 × 106 °C/s, then reaches stable at 500–600 mm/s. As for Mn-Cu alloy, the cooling rate fluctuates between 7.5 and 8.8 × 106 °C/s during the SLM process at a scanning speed of 300–700 mm/s. By comparing the thermal behavior of the two alloys, the peak temperatures, temperature gradients, and cooling rate of both alloys are closely associated with their thermophysical properties and the laser energy input.

Effect of Different Melt Treatment Methods on Microstructure and Thermal Stability of Al-Si Alloy

The effects of modification, electromagnetic stirring and compound treatment (modification and electromagnetic stirring) on the microstructure and thermal stability of Al-13% Si alloy are experimentally investigated. The results show that the three melt treatments can change the microstructure of the alloy. The modification has obvious refining effect on primary silicon and eutectic silicon grains, electromagnetic stirring has refining effect on primary silicon grains, and eutectic silicon grains appear coarsening phenomenon. The latent heat is obviously decreased by electromagnetic stirring, while the latent heat is increased by modification. Refining eutectic silicon grains will increase the latent heat of phase transformation of the alloy. When 560 °C remains unchanged, the melting temperature of the alloy generally decreases with the increase of holding time, but fluctuates, with a decrease of 7 °C. The latent heat decreases with the increase of constant temperature time, and the decrease amplitude reaches 8.4%.

Formulation and phase change mechanism of Capric acid/Octadecanol binary composite phase change materials

Fatty acids and fatty alcohols have the advantages of high latent heat of phase change, good thermal stability, no corrosion, no supercooling and phase separation. They can be used as phase change energy storage materials for passive temperature control. However, their popularization and application are limited because of their high phase transition temperature and narrow phase transition range. This study develops a novel binary composite phase change materials (PCMs) of Capric acid (CA) and Octadecanol (OD) by a melt blending method. The theoretical calculation and hot melt-step cooling were carried out to generate an optimal molar ratio, followed by DSC thermal characterization. ATR-FTIR and XRD were performed to determine the phase transformation and chemical and structure changes. The results showed the binary CA-OD binary composite PCMs has a high latent heat of fusion, a melting temperature Tm = 26.48 °C and △H = 181.06 J/g at optimal mass ratio of 85.15:14.86 (CA:OD), which is higher than the theoretically predicted latent heat of phase transition, indicating a good synergistic effect beneficial to energy storage. Solid CA exists in the form of dimer and –OH in solid OD exists in form of association, and intermolecular hydrogen bonds weakens in liquid. There are hydrogen bonds in the CA-OD binary composite PCMs, and the molecular structure changes before and after the phase transformation were similar to that of a single component CA or OD. The crystal structures of the two compounds also change and the latent heat of phase transformation is improved. Finally, through TG and high and low temperature cycle test, CA-OD binary PCMs demonstrates good thermal stability and practicability in the field of building energy conservation.

Role of thermal expansion anisotropy on the elastocaloric effect of shape memory alloys with slim-hysteresis superelasticity

We reveal the mixed-mode mechanism of elastocaloric effect in the course of slim-hysteresis superelasticity in a severely deformed NiTi shape memory alloy with tailored (negative/zero/positive) thermal expansion anisotropy. It is shown that in addition to the latent heat of phase transformation, the reversible heat from elastic deformation (known as the thermoelastic effect) plays a significant role in the overall elastocaloric response. The magnitude of the adiabatic temperature change associated with the thermoelastic effect $|\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}|$ can reach 3.1 K, which is comparable to that from the phase transformation latent heat ($|\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{PT}}|=2.9$ K). The sign and magnitude of the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}$ scales with the sign and magnitude of the coefficient of thermal expansion (CTE) along the stressing direction. For the directions with negative CTEs, the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}<0$ while for those with positive CTEs the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}>0$ upon rapid release of tensile stresses. As such, the cooling performance is strongest when the material is tensioned along the direction of the strongest negative CTE $(--14.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ owing to the synergistic interplay of $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{PT}}$ and $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}$. The results reveal the importance of thermal expansion property on the elastocaloric effect.

Analysis of scale and frame interface effects on the solidification properties of solar salt in mesopores

The intermittency and instability of solar irradiation can be effectively overcome by using medium- and high-temperature composite phase change materials (PCMs) for latent heat storage. Studies using metals and organics as phase change materials show that the phase change behavior of mesoporous composite phase change materials is affected by the constraint effect, and its thermophysical parameters (such as phase change point and phase change latent heat) are significantly different from those of macro volume. However, at present, research on molten salt phase change materials, which are widely used in the field of medium- and high-temperature research, is insufficient. Molecular dynamics simulations combined with experimental methods were used to analyze the changes in the freezing point, supercooling degree and enthalpy of solidification of solar salt in mesoporous composite phase change materials, which were based on the self-diffusion coefficient and the potential energy-temperature curve. The results show that a larger mesoporous size leads to a higher freezing point of the solar salt. At the same scale, with the increase in Al2O3 content in the framework, the freezing point of the solar salt first decreases and then increases. The supercooling degree of solar salt decreases with increasing scale. When the mass content of Al2O3 in the framework is in the range of 25 wt%-40 wt%, the supercooling degree of solar salt increases gradually; in contrast, it decreases in the range of 40 wt%-50 wt%, and finally, it increases again in the range of 50 wt%-75 wt%. The latent heat of phase transition increases with increasing scale. With the increase in Al2O3 content in the framework, the latent heat of phase transformation first decreases, then increases, and finally stabilizes. Al2O3 and SiO2 in the interface of the frame can promote heterogeneous nucleation and provide a substrate for the nucleation of molten solar salt. They also reduce the degree of supercooling. It is found that the effect of Al2O3 on nucleation is more significant.

Calculation of phase transformation latent heat based on cooling curves in end-quench test and its application in nickel-based superalloy

To obtain the phase transformation latent heat corresponding to different cooling rates with low test workload and cost, the Newton thermal analysis method and the improved Newtonian thermal analysis method were discussed based on the cooling curve obtained in the end-quench test. The validity of two methods was given by the latent heat calculation of 45# steel. The results show that the relative error of latent heat is 5.20% through the improved Newtonian thermal analysis method, which is more accurate than the Newtonian thermal analysis method. Furthermore, the latent heat release of phase transformation of the self-designed CSU-A1 powder metallurgy nickel-based superalloy increases from 4.3 to 12.29 J/g when the cooling rate decreases from 50.15 to 33.40 °C/min, because there is more sufficient time for the alloy microstructure to complete the phase transformation process when the cooling rate is smaller.

Cyclic degeneration of elastocaloric effect for NiTi shape memory alloy: Experimental observation and constitutive model

Based on the cyclic tension-unloading tests with various peak strains (4%-12%), the effect of loading level on the cyclic deformation and evolution of elastocaloric effect (eCE) for NiTi shape memory alloy (SMA) wires was investigated. Experimental results show that the transformation ratchetting and degeneration of eCE simultaneously occur during the cyclic deformation, and these two phenomena aggravate with the increase of loading level. Although the refrigeration capacity (maximum temperature drop in the reverse phase transformation process) exhibits a positive relativity with loading level in the 1st loading cycle, it changes non-monotonically with the variation of loading level in the steady-state cycle, which can be attributed to the interaction between the phase transformation and plasticity deformation. Moreover, it is found that the phase transformation between austenite (A) and martensite (M) phases occurs as a two-step process involving an intermediate phase, rhombohedral (R) phase. Then, a thermo-mechanically coupled cyclic constitutive model is constructed based on the fundamental laws of irreversible thermodynamics. Besides the thermo-elastic deformation, four different inelastic deformation mechanisms, i.e., R phase transformation (RT), martensite transformation (MT), plasticity in A and R phases (ARP), transformation-induced plasticity (TRIP) and their interactions are considered in the proposed model. The volume fractions of R phase, M phase, martensite affecting zone (MAZ) and the free zone (FZ) of mixed A + R phases are introduced. The thermodynamic driving forces for the four considered inelastic deformation mechanisms, i.e., RT, MT, ARP and TRIP are derived by the energy dissipation inequality and a new proposed Helmholtz free energy. Internal heat generation caused by the mechanical dissipation and the latent heat of phase transformation, and the heat balance equation are obtained by applying the conservation law of energy. The forward and reverse inheritances of dislocation (i.e., plastic deformation) caused by the moving martensite interfaces during MT are addressed. Adopting a first-order approximation of the constitutive equations and lumped analysis of heat transfer, the proposed constitutive model for a material point is extended to describe the overall responses of the whole wire. Finally, the predicted results for the cyclic deformation of NiTi SMA and correspondent evolution of eCE are compared against the experimental ones to validate the capability of the proposed model.

The effect of cooling on machining and phase transformation responses of Ni-rich NiTiHf high-temperature shape memory alloy

A Ni-rich Ni50.3Ti29.7Hf20 (at. %) high-temperature shape memory alloy was machined under different cooling conditions, specifically cryogenic cooling and flood cooling, and results were compared with those from dry machining. The effects of cooling on machinability measures such as tool wear, cutting forces and surface quality were evaluated for various cutting speeds. Microhardness, phase transformation temperatures and latent heat of transformation corresponding to the machined specimens were determined using hardness and differential scanning calorimetry tests. Cryogenic cooling was found to notably improve the machinability of the Ni-rich NiTiHf alloy by reducing tool wear, cutting force and surface roughness. This study provides clear indication that the machining process alters the phase transformation temperature of the Ni-rich NiTiHf alloy. After the machining process, austenite finish temperature of specimens increased by 5–10% depending on the machining conditions. The smallest amount of latent heat required for transformation (ΔH) was obtained for each of the two specimens: 2.33 J/g (exothermic) for the cryogenically machined specimen and 3.38 J/g (exothermic) for the specimen machined under flood cooling.

Influence of mesoporous size and structure on heat transport characteristics of mixed nitrate

The effects of mesoporous size and structure on the solidification characteristics of solar salt are simulated by molecular dynamics (MD). The mixed nitrate model with different scales and two structures is established by using Material Studio software, and the model is applied to the Lammps software package for simulation calculation. The changes of freezing point, supercooling, and phase transformation latent heat are summarized. The micro mechanism of solidification characteristics of nano solar salt is analyzed by radial distribution function, potential energy temperature curve and Gibbs free energy theory. The results show that the freezing point of solar salt first increases and then decreases with the increase of nanopore scale. The nanowire structure will also increase the phase transition temperature on the same scale, and the phase transition points of the two eventually tend to be stable with the increase of scale. The supercooling of solar salt decreases with the increase of mesoporous scale, but there is an abnormal increase. Under the two different structures, the solidification enthalpy gradually decreases with the increase of scale, and the phase transition latent heat of nanowire solar salt is 30%–37% higher than that of nanoparticle structure on the same scale.

Effects of Mesoporous Size and Structure on Solidification Characteristics of Solar Salt

The effects of mesoporous size and structure on the solidification characteristics of mixed nitrate were simulated by molecular dynamics (MD). The solar salt models of different scales are established by using material Studio software, and the models are exported to the Lammps software package for simulation calculation. The changes of parameters such as radial distribution function, potential energy temperature curve, freezing point, and phase transformation latent heat are summarized, and the micro mechanism of solar salt solidification characteristics at the nanoscale is analyzed. The results show that the freezing point of solar salt increases first and then decreases with the increase of nanopore size, and finally tends to be stable with the increase of nanopore size. The enthalpy of solidification decreases with the increase of scale.

Entropy features of the PeTa effect during phase transformations of water

The article discusses a hypothesis put forward by V. A. Tatarchenko and M. E. Perelman. According to it, the first order phase transition during vapour condensation or melt crystallisation (PeTa effect) is accompanied by the appearance of nonthermal radiation of the media. The generally accepted point of view is that the latent heat of phase transformation can only be released in the form of heat. When the authors of the hypothesis tried to prove the existence of the effect of nonthermal radiation and considered the facts confirming it, they did not take into account the peculiarities of the initial and final states of the medium (i.e. their entropy). To clarify the physics of the process of liquid crystallisation and to consider the possibility of nonthermal radiation, we studied the peculiarities of water crystallisation and the formation of ice. This isthe process the authors referred to in order to prove their hypothesis. It was shown that in various experiments, it is necessary to consider both the state (structure) of the initial water samples and the formed ice, which can consist of various crystalline modifications with chaotic packing. These features of initial and final states, i.e. the entropy of water and ice samples in real experiments and under observed natural phenomena, make it more difficult to assess the characteristics of a possible radiation. The entropy of the initial and final states was determined by the procedure of the system preparation and the peculiarities of the phase transition dynamics. Its values depend on macroscopic parameters, as well a s on themicrostructure of the media, the determination of which is a very challenging task in each specific case. In addition, in many cases, we have to deal with metastable media, for which it is necessary to take into account the influence of fluctuations on the process of the phase transition. Therefore, the concepts of equilibrium thermodynamics are not applicable to them. However, these are the media where non-heat radiations may occur in accordance with the laws of self-organisation in nonlinear weakly nonequilibrium objects. This work shows a method for preparing low-entropy medium with its subsequent phase transformation into ice. To do so we conducted an experiment which involved freezing concentrated alcohol in order to obtain deeply supercooled water. It appears that to find the characteristics of the PeTa radiation it is necessary to takeinto account the entropy constraints for each specific case, which will allow assessing the spectrum of possible non-heated radiations and their characteristics.

Study on the preparation and thermal properties of binary mixed chloride salts

Nitrate phase change materials (PCMs) are the most widely used PCMs in solar thermal power generation technology. The maximum service temperature of Nitrate phase change materials is only 600°C. Therefore, to find a phase change material with large heat capacity, wide temperature range, low heat loss and low price is the focus of current research. According to different mass ratios, nine binary molten salt mixtures were prepared by mixing lithium chloride and sodium chloride. The phase change temperature and latent heat of phase transition of them were studied by differential scanning calorimeter (DSC). The experiment results showed that since the melting point of sodium chloride was high, when the content of sodium chloride in the binary mixture of lithium chloride and sodium chloride was large, a small amount of lithium chloride could not reduce the melting point of the mixture below 600°C, the mixture could not be melted. Meanwhile, when sodium chloride and lithium chloride were melted, the phase transition temperature of lithium chloride and sodium chloride remained at about 540°C and floated at ±15°C. The melting temperature and crystallization temperature of the binary mixture of 90% lithium chloride and 10% sodium chloride were quite different, and the latent heat of phase transformation was relatively high. Therefore, the binary mixed molten salt can be used in the heat transfer and storage technology of solar power generation.

A new finite element model for Mn-Si-Cr bainitic/martensitic product quenching process: Simulation and experimental validation

Microstructure difference between the center and the surface caused by temperature gradient during quenching process may lead to inhomogeneous properties of large-size products. A new finite element model (FEM) is proposed in the present work to precisely simulate temperature distribution of a bainite/martensite multiphase material during water-air alternating surface quenching process. Parameters like nonlinear variation of thermo-physical parameters, nonlinear variation of heat transfer coefficient, phase transformation latent heat, and transformation kinetics were considered when developing the FEM. In particular, the experimental investigation of bainite and martensite transformation kinetics provides the basis for the accurate calculation of microstructure. The model helps identify the temperature distribution, cooling rate profile, microstructure and hardness characteristic of Mn-Si-Cr bainitic/martensitic large-size products. A quarter of axle block experiments were conducted to validate the proposed FEM, which shows high precision. The error of simulated temperature was lower than 5 % and that of hardness was within 2 %. Then, the model is used to simulate the discrepancy between surface and center location under as-employed quenching. Both experiment and simulation results confirmed that the different cooling rate leads to the temperature gradient. Different temperature distribution resulted in inhomogeneous properties of large-size product. Effects of the intensity of water spraying on large-size product are compared thoroughly in the simulation analysis. It is reflected that the improvement of microstructure and corresponding diminution of non-uniformity in hardness can be realized through optimizations of key processing parameters. Therefore, it is helpful for quenching process design of large-size products.

Experimental validation of residual stress thermomechanical simulation in as-quenched superalloy discs by using diffraction and incremental hole-drilling methods

Mechanical properties of commercial Ni-based superalloys at elevated temperature rely strongly on the cooling rate during solution heat treatment. Quenching-induced residual stress can be crucial to machining accuracy and fatigue strength of industrial superalloy components.In this study, an existing finite-element-based methodology is improved by inverse heat transfer calculation and phase transformation latent heat input, with a maximum temperature deviation of ∼44℃. The simulated thermal stress and strain rate evolution demonstrate the origin of the quenching stress field. Robust non-destructive methods, i.e., neutron diffraction and X-ray diffraction, are employed to validate the simulated surface and internal residual stresses in both air-cooled and water-quenched discs. The simulated in-depth normal residual stress profiles are proven to be consistent with the results of non-destructive measurement, with a maximum average deviation of ∼34 MPa. After a rapid quenching process, the near-surface stress gradient measured by incremental hole-drilling method at a depth of 0.45 mm is ∼150 % lower than simulation result.

Development of Sheet Metal Forming Analysis Incorporating Phase-transformation Model for Hot Stamping

To predict the spring back after hot stamping of sheet metals, a user material routine incorporated with phase transformation effects has been developed and implemented in general-purpose commercial finite element software. The mechanical response of the metallographic structure incorporates variation of transformation plastic strain and transformation volume expansion. Additionally, the thermal response incorporates the dependencies of thermal properties and latent heat of phase transformation. The start timing of diffusive transformation was determined using Scheil’s law, and the Kolmogorov–Johnson–Mehl–Avrami equation was adopted for the progression of diffusive transformation. The Koistinen and Marburger equation was used for the progression of martensite transformation. Using the developed model, we considered the effects of phase transformation on shape fixability. We conducted experiments that focused on measuring the shape fixability of U-bend hot formed parts. In this study, we show that the spring back of the U-bend parts increases under conditions in which phase transformation occurs before formation has been completed. Furthermore, the spring back could accurately predict by using the developed model.

Effect of laser scanning speed on the thermal-mechanical coupling field of laser remelting of valve seat

A numerical simulation model for the thermal-mechanical coupling of laser remelting of valve seat was established. The changes of thermophysical properties of the material with temperature, latent heat of phase transformation, and the absorption rate of laser energy on the surface of the material were considered in the model. The results showed that as the laser scanning speed increased, the maximum temperature and the area of the molten pool decreased, and the cooling rate of the molten pool increased. In the laser irradiation area, the temperature gradient in the molten pool decreased and the temperature gradient in the phase transformation region near the molten pool increased. When the laser scanning speed was increased from 1 mm/s to 4 mm/s, the circumferential residual stress was increased from 132 MPa to 183 MPa, and the radial residual stress was increased from 20 MPa to 51 MPa. In the laser remelting process, there was a competitive relationship between the increase in the deformation trend caused by the increase in the temperature amplitude and the decrease in the deformation constraint due to the decrease in the temperature gradient, resulting in a reduction in the residual stress accompanying the decrease in the laser scanning speed. The change trend of the simulation results was basically consistent with the experimental results, which showed the effectiveness of the simulation model.

Temperature characteristic of crushed coal under liquid coolant injection: a comparative investigation between CO2 and N2

For large-scale underground coal fires (LUCF), how to achieve significant cooling was a key to prevent and extinguish spontaneous combustion of coal in mining-out area. The liquid CO2 or liquid N2 injection, which has stronger cooling, wider coverage, and inerting effect, was a reliable technology that can improve the efficiency of LUCF control in the coal industry. However, before application, it is of fundamental importance for understanding of the heat transfer mechanisms and temperature variations of crushed coal during and after liquid CO2/N2 injection. In this paper, a series of laboratory experiments was designed and used to research the process about the phase change and convective diffusion of liquid CO2 and liquid N2 in coal under controlled conditions. Liquid CO2/N2 was separately injected into a cylindrical container which is filled with crushed coal, and the transient variation of coal temperature was measured on specified sites in the system by using temperature sensors. A comparative analysis between the liquid CO2 and liquid N2 injection for temperature characteristic of coal was conducted. In addition, the temporal-spatial characteristics of the cooling energy caused by latent heat of phase transformation of liquid CO2 and liquid N2 were revealed, and its correlation with the transient coal temperature field was identified.