Heat Diffusion Coefficient Research Articles

Heat diffusion coefficient study of polymers based on interpretable machine learning

Abstract. Polymers hold significant application value across various fields of modern society, with different application scenarios requiring specific thermal diffusivity coefficients. Finding polymer materials with targeted thermal diffusivities is crucial. However, due to the vast variety and complex structures of polymers, constructing a unified structured dataset for machine learning modeling is challenging. Although machine learning has shown great potential in materials science, it has rarely been applied to predict the heat diffusion coefficient of polymers. This paper constructs a dataset for predicting the thermal diffusion coefficient of polymers using a publicly available dataset by transforming the SMILES code of polymers into eight features with practical physical and chemical meanings. Using the Random Forest algorithm, training with 400 of these data and randomly selecting 200 of them for cross-validation, the accuracy of the test set reached 0.9. Additionally, through interpretability analysis, we found that the molecular weight of the polymer monomers, the TPSA (the polar surface area of the molecule), and the NRB (the number of rotatable bonds) are the main features affecting the polymer thermal diffusion coefficient. An increase in the TPSA and the NRB positively contributes to the thermal diffusivity, while an increase in molecular weight negatively contributes. Our study provides a new method for the prediction of polymer thermal diffusivity and creates a new paradigm for the study of polymer thermal diffusivity, promoting further development in this field.

Durable and reliable thermal management system with superior temperature uniformity for sidewall coring tool in extreme thermal environments

The sidewall coring tool (SCT) plays a crucial role in exploring the distribution of oil and gas resources, which needs to operate in a challenging high temperature (205 °C) and vibrating downhole environment for 21 h. However, the internal electronics are unable to withstand such extreme temperatures. Previous investigations on thermal management of downhole electronics have failed to ensure long operational duration and reliability. To solve this issue, a durable and reliable thermal management system (TMS) with superior temperature uniformity was proposed for the SCT in this paper. First, the proposed system adopted heat storage modules (HSMs) with high heat storage density and heat diffusion coefficient to enhance the heat storage rate and extend the operational duration. Second, HSMs were utilized for storing heat from nearby heat sources to improve temperature uniformity. Meanwhile, the reliability of HSMs was verified by high-temperature vibration experimental tests. Finally, through iterative optimization, the system utilized a well-designed configuration of thermal insulators and HSMs to further improve temperature uniformity, thereby extending the operational duration of the system. The performance of the total system was evaluated by thermal simulations. The maximum temperature of the heat sources remained below 150 °C after 21 h, with a maximum temperature difference between any two sources not exceeding 12 °C. Furthermore, the experiment was conducted to validate the simulated results, with the relative error within 5 %. The proposed TMS demonstrates a significantly extended operational duration and improved reliability compared to previous works, which has been successfully applied to downhole operations.

Validation of SOLPS-ITER simulations against the TCV-X21 reference case

This paper presents a quantitative validation of Scrape-Off Layer Plasma Simulation-ITER (SOLPS-ITER) simulations against the TCV-X21 reference case and provides insights into the neutral dynamics and ionization source distribution in this scenario. TCV-X21 is a well-diagnosed diverted L-mode sheath-limited plasma scenario in both toroidal field directions, designed specifically for the validation of turbulence codes (Oliveira, Body et al 2022 Nucl. Fusion 62 096001). Five new, neutrals-related observables are added here to the extensive, publicly available TCV-X21 dataset. These are three deuterium Balmer lines in the divertor and neutral pressure measurements in the common and private flux regions. The quantitative agreement metric used in the validation is combined with the conjugate gradient method to approach the SOLPS-ITER input parameters that return the best overall agreement with the experiment. A proof-of-principle test of this method results in a modest improvement in the level-of-agreement; the shortcomings impacting the result and how to improve the methodology are discussed. Alternatively, a scan of the particle and heat diffusion coefficients shows an improvement of 10.4% in the level-of-agreement, approximately twice as high as that achieved by the gradient method. This result is found for an increased transport coefficient compared to what is usually used for TCV L-mode plasmas. The simulations further indicate that ∼65% of the total ionization occurs in the SOL, from which ∼70% in the divertor regions, despite being a sheath-limited regime, motivating the inclusion of self-consistent neutral models in future turbulence simulations on the path towards improved agreement with the experiment.

In-Situ Polymer Framework Strategy Enabling Printable and Efficient Perovskite Solar Cells by Mitigating "Coffee Ring" Effect.

Organic-inorganic hybrid perovskites are considered ideal candidates for future photovoltaic applications due to their excellent photovoltaic properties. Although solution-printed manufacturing has shown inherent potential for the low-cost, high-throughput production of thin-film semiconductor electronics, the high-quality and high-reproducibility deposition of large-area perovskite remains a bottleneck that restricts their commercialization due to the droplet coffee-ring effect (CRE). In this study, these issues are addressed by introducing an in situ polymer framework. The 3D framework formed by spontaneous cross-linking improves the precursor viscosity and homogenizes its heat diffusion coefficient, counteracting the lateral capillary flow of the colloidal particles and anchoring their flocculent movement. Thus, the Marangoni convection intensity is properly controlled to ensure high-quality perovskite films, which significantly enhances reproducibility in printing efficient photovoltaics by mitigating the CRE. Subsequently, the perovskite solar cells and modules achieve power conversion efficiencies of 23.94 and 17.53%, and exhibit positive environmental stability, retaining over 90 and 78% efficiency after storage for 2500 and 1600h, respectively. This work may serves as a foundation for exploring precursor rheology to match the homogeneous deposition requirements of perovskite photovoltaics and facilitating the advancement of their printing manufacturing and commercialization transition.

Isotope mass scaling and transport comparison between JET Deuterium and Tritium L-mode plasmas

The dimensionless isotope mass scaling experiment between pure Deuterium and pure Tritium plasmas with matched ρ∗ , ν∗ , β n , q and Te/Ti has been achieved in JET L-mode with dominant electron heating (NBI+ohmic) conditions. 28% higher scaled energy confinement time BtτE,th/A is found in favour of the Tritium plasma. This can be cast in the form of the dimensionless energy confinement scaling law as ΩiτE,th∼A0.48±0.16 . This significant isotope mass scaling is consequently seen in the scaled one-fluid heat diffusion coefficient Aχeff/Bt which is around 50% lower in the Tritium plasma throughout the whole plasma radius. The isotope mass dependence in the particle transport channel is negligible, supported also by the perturbative particle transport analysis with gas puff modulation. The comparison of the edge particle fuelling or ionisation profiles from the EDGE2D-EIRENE simulations show that the absolute density differences that are necessary for the dimensionless match in the confined plasma dominate over any isotope mass dependencies of particle fuelling and ionization profiles at the plasma edge. Local GENE simulation results indicate a mild anti-gyroBohm effect at ρtor = 0.6 and thereby a small isotope mass dependence in favour of Tritium on heat transport and a negligible effect on particle transport. A significant fraction of the isotope scaling and reduced heat transport observed in the Tritium plasma is not captured in the GENE and ASTRA-TGLF-SAT2 simulations by simply changing the isotope mass for the same input profiles.

An analytical model of three-hot-wire acoustic particle velocity sensors

A thermal-based acoustic vector sensor consisting of three heated wires can detect acoustic particle velocity instead of pressure. This paper proposes an analytical model for calculating the temperature distribution and acoustic-caused temperature perturbation of the sensor with a three-hot-wire configuration. In this model, the nonlinear effect caused by non-uniform thermal conductivity is considered. The nonlinear effects will not only affect the temperature of the three hot wires, but also the frequency response of the sensor. Based on the stationary temperature distribution calculated by the nonlinear model, we correct the heat diffusion coefficient in the acoustic perturbation model. The nonlinear stationary model and the corrected perturbation model are found to be in good agreement with the numerical results. Furthermore, we design a chip to measure the temperature distribution of the three heated wires and compare the sensitivity of the sensor with the corrected model. The experimental results can verify the proposed model. This study provides a theoretical basis for sensor performance optimization.

Stability threshold for 2D shear flows of the Boussinesq system near Couette

In this paper, we consider the nonlinear stability for the shear flows of the Boussinesq system in a domain T×R. We prove the nonlinear stability of the shear flow (US,ΘS)=((eνt∂yyU(y),0)⊤,αy) with U(y) close to y and α ≥ 0 in Sobolev spaces for the following two cases: (i) α ≥ 0 is small scaling with the viscosity coefficients and initial perturbation ≲min{ν,μ}1/2 and (ii) α > 0 is not small, the heat diffusion coefficient μ is fixed, and initial perturbation ≲ν1/2.

Performance analysis of heat exchanger- evacuated tube assisted drying system (HE-ETADS) under unload condition

A prototype heat exchanger-evacuated tube assisted drying system (HE-ETADS) has been fabricated and tested under unload conditions in active mode at different water flow rates (10, 20, and 30Ltr/hr). Experimental work was conducted in three different categories. Category-I: drying system in stagnation condition, Category-II: all fans are in operating condition and Category-III: ventilation window is open. Coefficient of performance, heat consumption factor, convective heat transfer coefficient (CHTC) and diffusion coefficient have been calculated in thermal performance analysis of advanced system. Highest coefficient of diffusivity (0.19) was on third day of experimentation under Category-III at 30Ltr/hr water flow rate. Maximum heat consumption factor (0.60) was calculated during the second day for Category-I at 20Ltr/hr and for Category-II it was 0.775 during third day of experimentation at 30Ltr/hr, whereas in Category-III it was 0.782 at 10Ltr/hr water flow rate during first day of experiment. Higher coefficient of performance was 0.902 on third day of experiment for Category-I at 30Ltr/hr water flow rate whereas 0.940 was on first day of experimentation for Category-II at 10Ltr/hr flow rate. Category-III achieved 0.97 at 10Ltr/hr water flow rate during first day of experiment. CHTC of north wall insulated greenhouse dryer (NWIGHD) for ground to drying cabin air was 46.622 W/m2℃, while for advanced HE-ETADS it was found 47.542 W/m2℃, which is 1.98% greater than NWIGHD. Hence, HE-ETADS has potential to enhance drying rate and reduce payback period.

Bridgman growth and characterization of Dy:YCa4O(BO3)3 crystal

A 1-inch Dy3+:YCa4O(BO3)3 single crystal has been grown by the Bridgman method. The segregation coefficient of Dy3+ is 0.784. The structures, thermal and optical properties of Dy3+:YCOB crystal was studied. The results show that the Dy3+:YCOB crystal has high transmittance, specific heat and thermal diffusion coefficient. The Y-axis-oriented Dy3+:YCOB crystal has the largest absorption cross-section σabs(0.85×10−21cm2@444nm) and emission cross-section σem(2.54×10–21cm2@585nm). In addition, the J-O intensity parameters Ωt(t=2,4,6) of the Dy3+:YCOB crystal were calculated, where Ω2, Ω4, Ω6 are 5.01×10−20cm2, 0.33×10−20cm2, 0.52×10−20cm2 respectively. The fluorescence branching ratio β is 78.7%, and the radiative lifetime τrad is 1.588 ms. The fluorescence lifetime τf is 384.47 μs at 585 nm. The above data confirms that the Y-axis-oriented Dy3+:YCOB crystal is a good yellow laser gain material, which has potential application value in the field of yellow laser operation.

Experimental Determination of Polycrystalline Salt Rock Thermal Conductivity, Diffusivity and Specific Heat From 20 to 240°C

Salt rock is recognized as one of the most suitable parent rocks for geological disposal of high level radioactive waste due to its low permeability, good ductility, good thermal conductivity and damage self-healing properties. The thermal conductivity of salt rock directly affects the temperature of disposal storage and surrounding rock, high temperature will lead to a series of problems such as nuclear waste storage tank rupture, mechanical and permeability reduction of surrounding rock. In this paper, the thermal conductivity, specific heat and thermal diffusion coefficient of NaCl single crystal made by crystallization method in the laboratory and polycrystalline salt rock from Khewra salt mine were measured in the range of 22–240°C by the transient plane source method. The results showed that the thermal conductivity and thermal diffusivity of salt rock decrease gradually with the increase of temperature, while the specific heat capacity increases with the increase of temperature. The thermal conductivity of salt rock is slightly lower than that of single-crystal NaCl. The reason for this phenomenon may be that there are a few pores in salt rock. Based on the experimental data, the models of thermal conductivity, thermal diffusivity and specific heat of salt rock with temperature were established. The results can provide a reference for the construction of underground salt rock high-level radioactive waste disposal repository. Based on the thermal conductivity model of polycrystalline salt rock established in this paper, the temperature field evolution during the operation of the underground salt rock high-level nuclear waste repository in 1,000 years was studied. It was found that the temperature of the glass solidified of high-level radioactive waste reached the highest (177.6°C) and then dropped rapidly. The decay heat radiation influence radius of the nuclear waste reaches its maximum in about 50 years of operation of the repository and then gradually decreases.

Study of Electron Anomalous Heat Diffusion Affected by Mode Number of Perturbed Magnetic Island in Tokamak Plasmas

Anomalous electron heat diffusion across high mode magnetic islands and the nonlocal high mode stochastic magnetic field are researched and compared with earlier low mode research work. The work in this paper uses two typical magnetic islands and a high mode stochastic magnetic field that are aroused by five incompact high mode perturbed magnetic islands. It is found that the mode number of the perturbed magnetic island is another key factor of anomalous electron heat diffusion across the minor radius in tokamak plasmas, besides the ratio of the parallel heat diffusion coefficient to the perpendicular coefficient and the width of diffusive layers.

Global nature of magnetic reconnection during sawtooth crash in ASDEX Upgrade

This paper discusses the toroidal localisation of magnetic reconnection during sawtooth crashes. Numerical analysis with realistic heat diffusion coefficients shows that heat distributes itself helically along the torus faster than the temporal resolution of any existing ECE diagnostics. It makes local and global (helically axisymmetric) magnetic reconnection indistinguishable for an observer, while a local crash where the heat stays confined within a finite helical region could be distinguished. Statistical analysis of sawtooth crashes with the ECEI diagnostic is conducted in ASDEX Upgrade. The displacement of the heat within a finite helical region has not been observed. The statistical data supports global magnetic reconnection.

Eddy heat exchange at the boundary under white noise turbulence.

We prove the existence of an eddy heat diffusion coefficient coming from an idealized model of turbulent fluid. A difficulty lies in the presence of a boundary, with also turbulent mixing and the eddy diffusion coefficient going to zero at the boundary. Nevertheless, enhanced diffusion takes place. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.

Molecular insight into the diffusion/flow potential properties initiated by methane adsorption in coal matrix: taking the factor of moisture contents into account.

Coal seam permeability is one of the key parameters affecting coalbed methane (CBM), and plays an important role in resource evaluation and regional selection. To fully explore the diffusion/flow potential properties initiated by methane adsorption beneath diverse moisture contents (1-5%) in coal molecules. The pore size distribution and methane adsorption capacities were discussed based on Monte Carlo (MC) and molecular dynamics (MD) methods. The potential properties of diffusion/flow induced by methane adsorption were investigated using the maximum absolute adsorption capacities as benchmark. The variation patterns of the pore structure were analyzed using SEM scanning experiment to verify the results of simulation analysis. It is found that the free pores facilitate methane molecular adsorption and increase adsorption spaces; the skeleton pores restrict the flow and transport of water molecules. Reduction values in surface free energies increase at different temperatures, and released heat diffusion coefficients and permeabilities for methane molecules drop as moisture contents increase. Interestingly, however, enhancements in temperatures increase the methane molecular diffusion coefficients. The lower the activation energies, the easier they are to diffuse. Sufficiently, the optimum conditions for gas drainage of coal seam are at temperature of 293K and moisture content of 5%, indicating greater contributions to gas pressure relief for coal seam. By comparing the results of molecular simulation and SEM scanning, trend of change is basically the same. Moreover, it is explored that hydraulic measure was the most significant to the CBM stimulation technology through field engineering application. This research is expected to provide guidance for facilitating the effectiveness of gas extraction for coal seam.

Exploration of hydration and durability properties of ferroaluminate cement with compare to Portland cement

This study investigated the hydration process and durability properties of ferroaluminate cement (FAC). Hydration heat, hydration products, compressive strength, gas permeability and chloride diffusion coefficient were investigated experimentally for FAC mortar samples with different water-to-cement ratio (0.5, 0.45 and 0.4) and were compared with OPC mortar samples with the same mixing proportion. The mechanism of the difference in the durability of two cements was revealed from the perspective of pore size distribution and hydration products. The results show that FAC has a relatively fast hydration rate and the compressive strength at early age is larger than OPC. The gas permeability and chloride diffusion coefficient of FAC mortar samples are approximately 3–4 times lower than that of OPC mortar samples. The permeability and diffusivity of both FAC and OPC decrease significantly when the w/c decreases from 0.5 to 0.4. The main hydration products of FAC are ettringite, ferric hydroxide and aluminum hydroxide gel, large amounts of ettringite make FAC to form a dense structure, the proportion of the pores with pore size of 10–100 nm in FAC sample decreases greatly with compare to the OPC sample, which endows FAC superior impermeability properties.

Modeling of glass transition process and elastic properties of Zr-Nb amorphous alloys

The glass transition of supercooled Zr−Nb melt was studied by molecular dynamics modeling. Glass transition temperature was determined using by different criteria such as change of the heat capacity, diffusion coefficient and viscosity, the obtained data are in good agreement with each other. It is shown, that elastic modulus rise with the decrease of the free volume in structure, that agrees with the supposed model of the amorphous alloys’ deformation.

Simulations of COMPASS vertical displacement events with a self-consistent model for halo currents including neutrals and sheath boundary conditions

The understanding of the halo current properties during disruptions is key to design and operate large scale tokamaks in view of the large thermal and electromagnetic loads that they entail. For the first time, we present a fully self-consistent model for halo current simulations including neutral particles and sheath boundary conditions. The model is used to simulate vertical displacement events (VDEs) occurring in the COMPASS tokamak. Recent COMPASS experiments have shown that the parallel halo current density at the plasma-wall interface is limited by the ion saturation current during VDE-induced disruptions. We show that usual magneto-hydrodynamic boundary conditions can lead to the violation of this physical limit and we implement this current density limitation through a boundary condition for the electrostatic potential. Sheath boundary conditions for the density, the heat flux, the parallel velocity and a realistic parameter choice (e.g. Spitzer’s resistivity and Spitzer–Härm parallel thermal conductivity) extend present VDE simulations beyond the state of the art. Experimental measurements of the current density, temperature and heat flux profiles at the COMPASS divertor are compared with the results obtained from axisymmetric simulations. Since the ion saturation current density () is shown to be essential to determine the halo current profile, parametric scans are performed to study its dependence on different quantities such as the plasma resistivity and the particle and heat diffusion coefficients. In this respect, the plasma resistivity in the halo region broadens significantly the profile, increasing the halo width at a similar total halo current.

Transient thermal model of nanosecond pulsed laser ablation: Effect of heat accumulation during processing of semi-transparent ceramics

Nanosecond ablation of semi-transparent ceramics represents a particularly interesting scenario where non-linear absorption mechanism allows absorption of laser energy for photon energy below the band gap of the material. Yttria Stabilized Zirconia (YSZ) ablated by infrared radiation represents as an example of such scenario. Ablation threshold experiments reveal a stark difference between the behaviour observed during ablation of single pulses compared to multiple pulses. During single pulses, a statistical occurrence of ablation is observed. For laser intensity below the dielectric breakdown threshold, this phenomenon has been explained on the basis of a defect-mediated thermal runaway which causes an exponential increase of the bulk material absorption coefficient as function of temperature. To investigate this phenomenon, and its effect during multi-pulse ablation, a 1D modelling framework has been developed and validated. The framework focuses on simulation of ablation of trenches (i.e. partially overlapped pulses) considered as convolution of single pulses, and account for the governing equation of heat diffusion, vaporization, plasma shielding, volumetric absorption and temperature-dependent absorption coefficient by use of a simplified defect mediated absorption model which represents the main novelty of the paper.The application of the model for various laser processing parameters reveals the importance of heat accumulation on the process. A positive feedback mechanism is established over successive pulses which leads to the material becoming progressively more absorbing, and eventually results in material removal through ablation. This phenomenon explains the sharp ablation threshold observed experimentally during ablation of trenches. The modelling framework also reveals interesting dynamics caused by the temperature dependence of the material absorption behaviour, such as a delay in the starting time for material removal, and drastic changes in the energy distribution within the sample throughout the duration of a laser pulse.Ultimately, the effect of heat conservation, which is often neglected, has been shown to represent a key dynamic during multi-pulse ablation, and should be accounted for during simulations of real-world laser processes, in particular for low diffusivity semi-transparent materials.

Nano-Fluids as a Coolant for Automotive Engine Radiators: Review Study

Low-efficiency crossflow heat exchangers which are used as radiators in the automotive sector may cause engine damage. Nano-fluids are ideal coolants due to their high heat diffusion coefficient and can be added to almost any process that requires a fast reaction to thermal performance, such as the automobile. The nanofluid's thermal conductivity is greater than that of the base fluid such as water and ethylene glycol and can achieve more than 40% and enhance the heat transfer coefficient by more than 50%. The best results for heat transfer were to use carbon nanoparticles with base fluid. This article reviews previous research investigating the effect of nano-fluid in automotive radiators in which nanoparticles such as Al2O3, CuO, TiO2, and MWCNT are used. The preparation and application of nano-fluid and measurement of thermal properties and the effect of volume fraction on efficiency are discussed. The review also is focused on the numerical and experimental studies of previous work related to nano-fluid performance in automobile radiators

Effect of Thermal Parameters on Hydration Heat Temperature and Thermal Stress of Mass Concrete

To explore the influence of concrete thermal parameters on the hydration heat temperature and thermal stress of mass concrete, four feature positions of a dam foundation were chosen to analyze the changing process of temperature and stress by varying the thermal parameters, including the thermal conductivity, specific heat, surface heat diffusion coefficient, temperature rise coefficient, solar absorption coefficient, and thermal expansion coefficient. Some conclusions were obtained as follows. Increasing the thermal conductivity and reducing the specific heat and temperature rise coefficient of concrete can effectively reduce the maximum temperature of the central concrete structure. Increasing the solar absorption coefficient, specific heat, and thermal expansion coefficient and reducing the thermal conductivity, surface heat diffusion coefficient, and temperature rise coefficient of concrete can reduce the maximum principal tensile stress in the structure to a certain extent. The maximum principal tensile stress at different positions of the structure has a linear functional relationship with the thermal conductivity, specific heat, and thermal expansion coefficient and has a quadratic function relationship with the surface heat diffusion coefficient, temperature rise coefficient, and solar absorption coefficient. Besides, this study also proposed a series of related anticracking measures. This study was expected to provide a theoretical reference for the design, construction, and cracking disease prevention of mass concrete structures.