Isothermal Heat Research Articles

Transition of the thermal boundary layer and plume over an isothermal section-triangular roof: an experimental study

The development of thermal boundary layers and plume near a section-triangular roof under different isothermal heating conditions has been the focus of numerous numerical studies. However, flow transition in this type of flow has never been observed experimentally. Here, phase-shifting interferometry and thermistor measurements are employed to experimentally observe and quantify the flow transitions in a buoyancy-driven flow over an isothermal section-triangular roof. Visualisation of temperature contours is conducted across a wide range of Rayleigh numbers from laminar at 103 to chaotic state at 4 × 106. Power spectral density of the temperature measurements reveals the type of bifurcations developing as the Rayleigh number is increased. This flow transition is characterised as a complex bifurcation route with the presence of two fundamental frequencies, a low and a high frequency. We found that the thermal stratification in the environment plays a significant role in the flow transition. The spatial development of flow is also quantitatively and qualitatively described. In addition to clarifying flow transition in experiments, the work demonstrates the implementation of phase-shifting interferometry and punctual temperature measurements for characterisation of near-field flow over a heated surface.

The Role of Heater Size and Location in Modulating Natural Convection Behavior in Cu-Water Nanofluid-Loaded Square Enclosures

Enhancing the energy efficiency of thermal systems reduces their consumption, lowers costs, and reduces undesired environmental impact, thus making these systems more sustainable. The current work introduces a passive method for heat transfer enhancement that is carried out using natural convection by nanofluid. This work introduces a computational study of the process of natural convection within a square cavity containing Cu/H2O nanofluid. The cavity wall on the left side undergoes partial isothermal heating, while the opposing side is fully cooled isothermally, with all other boundaries maintained adiabatic. A mathematical model formulated based on a 2-D model was used to provide the solution for the system of governing equations of mass, momentum, and energy conservation, employing the finite element technique. A commercial CFD package is utilized to perform the computational simulation. The present investigation delves into the impact of the Rayleigh number, nanoparticle concentration, heater length, and heater location on the flow field and heat transfer characteristics. The model outcomes were displayed for a wide range of the pertinent parameters as 103 ≤ Ra ≤ 106, 0.25 ≤ lh ≤ 1.0, 0.125 ≤ hc ≤ 0.875, and 0.02 ≤ ϕ ≤ 0.10. Also, correlation equations relating the average Nusselt number to these crucial parameters are derived. These equations are simple and can be applied in practice easily in many fields, such as electric and electronic equipment cooling and thermal management of heat sources. Also, these equations gather all the parameters that affect the heat transfer process. They are shedding light on the intricate interplay between these parameters in the natural convection heat transfer process.

Experimental evidence of phase transition of silica polymorphs in basaltic eucrites: implications for thermal history of protoplanetary crust

Silica polymorphs occur under various pressures and temperature conditions, and their characteristics can be used to better understand the complex metamorphic history of planetary materials. Here, we conducted isothermal heating experiments of silica polymorphs in basaltic eucrites to assess their formation and stability. We revealed that each silica polymorph exhibits different metamorphic responses: (1) Quartz recrystallizes into cristobalite when heated at ≥ 1040 °C. (2) Monoclinic (MC) tridymite recrystallizes into no other polymorphs when heated at ≤ 1070 °C. (3) Silica glass recrystallizes into quartz when heated at 900–1010 °C, and recrystallize into cristobalite when heated at ≥ 1040 °C. These results suggest that MC tridymite in eucrites does not recrystallize into other polymorphs during the reheating events, nor does it recrystallize from other silica phases below the solidus temperature of eucrite (~ 1060 °C). Additionally, we found that pseudo-orthorhombic (PO) tridymite crystallizes from quenched melts in the samples heated at ≥ 1070 °C. Previously, cristobalite has been considered as the initial silica phase, which crystallizes from eucritic magma. Our findings suggest that the first crystallizing silica minerals may not always be cristobalite. These require a reconsideration of the formation process of silica minerals in eucrites.

Directly measured electrocaloric heat in multilayer capacitors of lead scandium tantalate

Isothermal electrocaloric (EC) heat is a key—but rarely reported—parameter for cooling applications. Here, we employ calorimetry with large-area Peltiers (10 × 10 mm2) to study a multilayer capacitor (MLC) of well-ordered PbSc0.5Ta0.5O3 (PST) with a first-order ferroelectric phase transition. Our direct measurements of isothermal EC heat are consistent with complementary EC measurements of similar MLCs that display large EC effects, i.e., indirect EC measurements and direct measurements of adiabatic EC temperature change [Nair et al., Nature 575, 468 (2019)]. Using a field of 7.9 V μm−1, the EC heat associated with the electrically active PST peaks at 11 MJ m−3 near 300 K. At higher temperatures, as one approaches the critical point, the transition is seen to become less first order. Separately, we find that the inactive PST compromises quasi-direct EC measurements. In the future, isothermal EC heat should be directly measured as a matter of routine.

Thermodynamic study of semi-closed rankine cycle based on direct combustion of hydrogen fuel

Electrolysis of water for hydrogen-production has enormous potential. This study proposes a semi-closed Rankine cycle for efficiently utilizing direct hydrogen combustion via thermal power conversion. Unlike traditional closed Rankine cycles, combustion occurs inside the combustion chamber with hydrogen and pure oxygen, followed by a mixed heat exchange with the mainstream working fluid, water. The heat absorption process of the semi-closed Rankine cycle occurs within the working fluid and exhibits characteristics of both open and closed cycles. Thus, this cycle can combine the advantages of desirable parameters and isothermal heat rejection of the Rankine cycle. Applying the semi-closed Rankine cycle to hydrogen fuel utilization yields a higher energy efficiency, especially when coupled with a supercritical compression regeneration process. The main steam parameters were 620 °C/30 MPa, the cycle's average heat absorption temperature reached 561 °C, and its energy efficiency was 59.35%. When the main steam parameters were increased to 1200 °C/30 MPa, the average heat absorption temperature rose to 1021 °C and its energy efficiency was 68.27%, demonstrating a significant efficiency advantage. The newly proposed semi-closed Rankine cycle based on direct combustion of hydrogen fuel provides a novel and feasible path for exploring the efficient utilization of hydrogen.

Shrinkage and deformation of material extrusion 3D printed parts during sintering: Numerical simulation and experimental validation

Material extrusion 3D printing (ME3DP) combined with sintering is a low-cost additive manufacturing technique for fabricating components of difficult-to-print metals, such as copper, aluminum, and ceramics. However, the sintering process includes complex material science, such as volumetric shrinkage and free structural bending, to identify the relative density and deformation of a 3D printed sample. The prediction of the relative density and deformations during the sintering process provides information to the design engineer to optimize the design of the CAD model before sintering. In this study, a phenomenological model based on constitutive equations was developed to predict the density and structural deformation during the sintering process of pure copper components fabricated by ME3DP using metal injection molding feedstock. The densification rate was determined using shrinkage estimation with an isothermal stairway heating cycle in a vertical dilatometer. Furthermore, different sets of experiments were performed with a load on the probe with long isothermal heating cycles at 850, 900, 950, 1000, and 1050 °C in a vertical dilatometer to estimate the axial viscosity of the copper. The constitutive equations were solved using the solid mechanics module with user-defined creep in COMSOL Multiphysics by considering isotropic assumptions. Two types of geometries, cube and overhanging I section, were used to predict shrinkage and deformation during the sintering process. The developed model successfully predicted the relative density based on shrinkage and structural deformation owing to gravity during the sintering process.

Response of Foldable Protein Conformations to Non-Physiological Perturbations: Interplay of Thermal Factors and Confinement.

Technological advances frequently interface biomolecules with nanomaterials at non-physiological conditions, necessitating response characterization of key processes. Similar encounters are expected in cellular contexts. We report in silico investigations of the response of diverse protein conformational states to lowering of temperature and imposition of spatial constraints. Conformational states are represented by folded form of the Albumin binding domain (ABD) protein, its compact denatured form, and structurally disordered nascent folding elements. Data from extensive simulations are evaluated to elicit structural, thermodynamic and dynamic responses of the states and their associated environment. Analyses reveal alterations to folding propensity with reduced thermal energy and confinement, with signatures of trend reversal in highly disordered states. Across temperatures, confinement has restrictive effects on volume and energetic fluctuations, leading to narrowing of differences in isothermal compressibility (κ) and heat capacities (Cp). While excess (over ideal gas) entropy of the hydration layer marks dependence on the conformational state at bulk, confinement triggers erasure of differences. These observations are largely consistent with timescales of protein-water hydrogen bonding dynamics. The results implicate multi-factorial associations within a simple bio-nano complex. We expect the current study to motivate investigations of more biologically relevant interfaces towards mechanistic understanding and potential applications.

Crystallization Behavior and Structure of CaO–SiO2–Al2O3 Melts Representing the Oxide Inclusions in Si‐Mn Deoxidized Steel

For further optimization of the inclusion plasticization control process of Si‐Mn deoxidized steel, the crystallization behavior and structure of CaO–SiO2–Al2O3 system inclusions are investigated. The crystallization behavior of four typical low melting point CaO–SiO2–Al2O3 inclusions at 1000 to 1250 °C is studied by steel crucible isothermal heating experiment. The obtained results reveal that the glassy stability of the inclusions is high when the composition of low melting point inclusion is located in the eutectic region of tridymite, pseudowollastonite, and anorthite in the ternary phase diagram. However, when the composition of the inclusion is located in the eutectic region of melilite, pseudowollastonite, and anorthite, it is easy to undergo glass‐crystallization transformation and precipitate wollastonite and anorthite crystalline phases when isothermal heating at 1100 to 1200 °C. In addition, the crystallization mode of these inclusions is mainly surface crystallization, and the crystallization degree of inclusions increases with the increase of heat treatment temperature and time. The activation energy of crystallization and the content of nonbridging oxygen in the silicate structure NBO/T can be used to predict the crystallization ability of inclusions. The greater the activation energy of crystallization and the smaller the NBO/T value, the higher the glassy stability of the inclusions.

Strength-ductility synergy of Mg-7.5 wt.% Sn binary alloys promoted by semisolid isothermal treatment coupled with hot extrusion

In this paper, we propose to adopt a new process for as-extruded Mg-7.5wt.%Sn binary alloy, i.e., semisolid isothermal heat treatment coupled with hot extrusion. Semi-solid isothermal treatment achieved nanosizing of micron-layer lamellar eutectic in the Mg-7.5wt.% Sn binary alloy, while subsequent hot extrusion crushed this nanoeutectic to nanoparticles, resulting in high strength (303 ± 2.8MPa for UTS and 246 ± 0.7MPa for YS,) and high elongation (11.5 ± 0.1% for EL) of the Mg-7.5wt.% Sn binary alloy. Grain refinement, multi-scale particles (broken nanoscale and submicron particles, and dynamically precipitated nanoscale phases), and high-density dislocations are the main contributors to the high strength. Meanwhile, the good ductility of the alloy is promoted by the softening effect due to dynamic recrystallisation as well as the weakening and random distribution of the texture.

Radiative recombination model for BiSeI microcrystals: unveiling deep defects through photoluminescence

Abstract Pnictogen chalcohalides are semiconductors that have emerged as promising materials for energy conversion due to their exceptional optoelectronic properties. Their electronic configuration (ns2), particularly for Bi- and Sb-based compounds, can be a key factor in efficient carrier transport and defect tolerance, similarly, to Pb-perovskites. In the present study, the Bi-containing chalcohalide, bismuth selenoiodide (BiSeI) was synthesized via isothermal heat treatment of binary precursors in evacuated quartz ampoules. The synthesized BiSeI microcrystals exhibited a characteristic needle-like morphology and a near-stoichiometric composition. Both indirect and direct band gap energies of BiSeI were determined by ultraviolet–visible–near-infrared diffuse reflectance spectroscopy, with room temperature values of 1.17 eV and 1.29 eV, respectively. This study presents the first experimental investigation of the photoluminescence properties of BiSeI microcrystals resulting in a recombination model involving multiple defect states. This work provides valuable insights into the defect structure and recombination mechanisms within BiSeI, paving the way for further exploration of its potential in optoelectronic devices.

SIMULATION HEATING PROCESS FOR WEAKLY REACTIVE COAL IN THE HORIZONTAL REACTOR

In this study results of the active medium parameters influence formed by hot air on the thermal heating process of weakly reactive coal from the formation «Maikubensk 3B» for the extraction of additional heat are represented. In the course of the research the regularities of the hygroscopic moisture and volatile combustible substances release for the obtaining additional heat have been determined. Optimal temperature ranges of the stage heating taking into account thermal destruction of coal in the temperature range from 20 C to 600 C, the main steps of the insulated heating process, the moisture content gradient propagation lines and dependence of the heat transfer and mass transfer coefficients on the temperature and the humidity during the thermal heating process in the horizontal coal heating reactor. The results show that it is possible to release volatile combustible gases with a gradual increase of temperatures in the obtained mode. The heating mode comprises a pre-heating stage with hydroscopic moisture release, in the temperature range from 20 C to 105 C, the stage of isothermal heating in the temperature range from 105 C to 400 C and the phase of flame-free combustion in the temperature range from 400 C to 600 C with the temperature rise above the set value. Process simulation was carried out in the Comsol Multiphisics software environment for the weakly reactive coal real-world heating conditions. Keywords: coal heating, weakly reactive coal, isothermal heating, step-by-step heating, Lagrange method, non-liner equation, non-stationary process.

Thermodynamic prediction and experimental verification of phase transformation kinetics in 3Mn steel with Ti and V microadditions

AbstractIn this work, the phase transformation kinetics and precipitation processes were studied using thermodynamic calculations and a dilatometric method to determine the optimal chemical composition of medium-Mn steel and its initial parameters of heat treatment. The investigated steel is intended for forgings with the microstructure composed of bainitic matrix and retained austenite (RA). An influence of Al and Si on the Ms temperature was characterized to obtain the best chemical composition in terms of RA stabilization. Theoretical continuous cooling transformation (CCT) and time–temperature transformation (TTT) diagrams were determined and compared with dilatometric results. Microstructural observations were compared with the dilatometric results. The hardenability of steel was high due to increased Mn and Mo additions. A very small fraction of cementite is expected in the microstructure due to Al and Si additions. The shortest time to start and finish the bainitic transformation was noted for the isothermal heat treatment at 420 °C. The completion of the bainitic transformation took about 25 min and is acceptable from the industrial point of view. The obtained results constitute a good basis for designing thermomechanical processing routes of bainitic steels with RA.

Microstructure analysis of cement-biochar composites

The use of biochar as a concrete constituent has been proposed to reduce the massive carbon footprint of concrete. Due to the low density and complex porosity of biochar, microstructural analysis of Portland cement-biochar composites is challenging. This causes challenges to the improvement of the micro-scale understanding of biochar composite behavior. This work advances the microstructural understanding of Portland cement composites with 0, 5, and 25 volume percent (vol%) of cement replaced with wood biochar by applying common characterization techniques of mercury intrusion porosimetry (MIP), gas sorption, scanning electron microscopy, and isothermal heat flow calorimetry (HFC) in conjunction with 1H nuclear magnetic resonance (NMR) and micro-X-ray computed tomography (XCT) analysis techniques. The combination of these techniques allows a multi-scale investigation of the effect of biochar on the microstructure of cement paste. NMR and XCT techniques allow the observation and quantification of the pore space. HFC and MIP confirmed that biochar absorbs moisture and reduces the effective water-cement ratio. Gas sorption, MIP, and NMR shows that 5 vol% replacement does not significantly affect the gel and capillary pore structures. Results from XCT (supported by MIP and NMR) show that biochar can reduce the formation of larger pores. Importantly, XCT results suggest that biochar can act as a flaw in the microstructure which could explain reductions in the mechanical properties. Overall, the mechanical properties already analyzed in the literature are consistent with the microstructural changes observed, and these results highlight the need to carefully tailor the volume fraction of biochar to control its effect on the paste microstructure.

Comparing microstructure and mechanical properties of AlSi10 alloy produced by powder bed fusion-laser beam and high pressure die casting

PurposePowder bed fusion-laser beam (PBF-LB) is a rapidly growing manufacturing technology for producing Al-Si alloys. This technology can be used to produce high-pressure die-casting (HPDC) prototypes. The purpose of this paper is to understand the similarities and differences in the microstructures and properties of PBF-LB and HPDC alloys.Design/methodology/approachPBF-LB AlSi10Mg and HPDC AlSi10Mn plates with different thicknesses were manufactured. Iso-thermal heat treatment was conducted on PBF-LB bending plates. A detailed meso-micro-nanostructure analysis was performed. Tensile, bending and microhardness tests were conducted on both alloys.FindingsThe PBF-LB skin was highly textured and softer than its core, opposite to what is observed in the HPDC alloy. Increasing sample thickness increased the bulk strength for the PBF-LB alloy, contrasting with the decrease for the HPDC alloy. In addition, the tolerance to fracture initiation during bending deformation is greater for the HPDC material, probably due to its stronger skin region.Practical implicationsThis knowledge is crucial to understand how geometry of parts may affect the properties of PBF-LB components. In particular, understanding the role of geometry is important when using PBF-LB as a HPDC prototype.Originality/valueThis is the first comprehensive meso-micro-nanostructure comparison of both PBF-LB and HPDC alloys from the millimetre to nanometre scale reported to date that also considers variations in the skin versus core microstructure and mechanical properties.

Corrosion of aluminium alloy AA2024-Τ3 specimens subjected to different artificial ageing heat treatments

The effect of artificial-ageing on corrosion behaviour of aluminium alloy (AA)2024-T3 was investigated. The natural ageing which takes place during the aircraft's lifespan was simulated with isothermal heat-treatments at 190 °C. Electrochemical impedance spectroscopy measurements were performed in different heat-treated specimens to examine the prevailing corrosion mechanisms. Additionally, pre-corroded tensile specimens from different heat-treatment conditions were mechanically tested to assess the corrosion-induced degradation. Different forms of corrosion were revealed in the investigated ageing tempers; intense localized attack was noticed in the initial (T3) and under-aged (UA) tempers. UA condition exhibited the highest susceptibility to corrosion propagation followed by T3, according to the charge transfer resistance RCT and degradation rate of tensile elongation at fracture Af ( ≈ 0.118) and ( ≈ 0.103), respectively. Corrosion-induced degradation rates for the peak-aged (PA) and over-aged (OA) tempers ( ≈ 0.042 and 0.054, respectively) were almost one third of UA, attributed to volume fraction and size of the precipitated second-phase particles.

Effect of Low-Temperature Tempering Temperature on the Mechanical Properties of NM500 Wear-Resistant Steel

Abstract In order to explore the effect of low temperature tempering heat treatment temperatures on Brinell hardness (HB), impact absorption energy (KV2) and tensile mechanical properties of NM500 wear-resistant steel, isothermal tempering heat treatment of NM500 wear-resistant steel sheets after quenching were carried out at 200~260 °C. The microstructures, Brinell hardness, -20 °C impact energies and tensile mechanical properties of NM500 wear-resistant steel after tempered were analyzed by optical microscope, Brinell hardness tester, pendulum impact testing machine and tensile testing machine respectively. The results show that when the tempering temperature increases from 200 °C to 260 °C, the Brinell hardness of NM500 wear-resistant steel decreases from 490 HBW to 460 HBW, the offset yield strength (Rp0.2) decreases from 1396.3MPa to 1351.0MPa, and the tensile strength (Rm) decreases from 1708.5 MPa to 1615.0 MPa. However, the low temperature tempering temperature has no significant effect on the microstructure and elongation after fracture. According to GB/T 24186-2022, 200~220 °C is the suitable low temperature tempering temperature range for NM500 wear-resistant steel.

Kinetic analysis of dehydration/dehydroxylation from carbonaceous chondrites by in situ heating experiments under an infrared microscope

AbstractCI, CM, and CR carbonaceous chondrites contain hydrous minerals, indicating that their parent bodies underwent aqueous alteration at low temperatures. Some of these chondrites, such as heated CM, CI, and CY chondrites, experienced thermal dehydration by impacts or solar radiation after aqueous alteration. This study conducted heating experiments on carbonaceous chondrites and evaluated their dehydration/dehydroxylation kinetics in an effort to explain the thermal history of the parent asteroids of heated carbonaceous chondrites using their degrees of dehydration/dehydroxylation of hydrous minerals. Murchison (CM2.5) and Ivuna (CI1), relatively primitive (having not undergone thermal alteration) carbonaceous chondrites, were used as starting materials. Weakening in the OH band at ~3680 cm−1 (2.72 μm) with isothermal heating at 350–500°C (Murchison) and 450–525°C (Ivuna) were observed under in situ infrared spectroscopy (FT‐IR) equipped with a heating stage. To determine the rate constants, the decrease in the OH band was fitted using kinetic models such as first‐order reactions, two‐dimensional diffusion, and three‐dimensional diffusion. The apparent activation energies and frequency factors were determined using the Arrhenius equation. Time–temperature transformation diagrams were drawn to represent the decrease in the OH‐band intensity as a function of temperature and heating duration. Such kinetic approaches can provide constraints on the temperature and time of the dehydration/dehydroxylation processes and enable us to estimate long‐term effects from experiments in the laboratory within a short time.

Recrystallization behavior of a high γ′-fraction Ni-based single crystal superalloy induced by residual strain

The static recrystallization (SRX) of the designed high γ′-fraction Ni-based single crystal superalloy (SX) AlloyX was investigated. The three-dimensional SRX threshold of AlloyX involves plastic strain – deformation temperature – heat treatment temperature established by isothermal deformation and heat treatment tests. The influence of different dislocation configurations obtained at different deformation temperatures on critical SRX temperature was analyzed. The high-density entangled dislocations were observed to gradually transition into low-angle grain boundaries at heat treatment temperature within the γ-channel, potentially serving as the primary driving force for the SRX of AlloyX. Compared with the lower γ′-fraction SX DD32, it is further determined that the preferential dissolution of the small-sized γ′ in the interdendritic region (IDR) caused by high Al is the important factor that induces the recrystallization to occur preferentially in the IDR of AlloyX rather than the dendrite region (DR).

Entropy generation in mixed convection rotating flow over a heated surface

This work aims to study entropy generation in mixed convection flow of Newtonian viscous fluid. Water and kerosene oil we can take as counter examples in this case. This fluid flows past a vertical plate. This fluid near the wall is heated with isothermal heating. The plate moves with constant acceleration and the fluid flows in a rotating fame. Mixed convection is considered here due to buoyancy force and constant plate linear translation. This problem is modeled in terms of partial differential equations (PDEs) and then using suitable variables, the problem is transformed into dimensionless form. Exact solutions of the problem are attained using the Laplace transform. The equations for Bejan number and entropy generation are also developed in dimensionless form, and then by via the exact solutions of velocity and energy, the results of entropy generation and Bejan number are figured. Graphical results are provided to discuss about the behavior of embedded parameters. Results of Nusselt number and skin friction are also figured in tabular form and discussed.

Laser-directed energy deposition of low-carbon, low-temperature ultra-fine bainitic multi-physical modeling, microstructure and performance studies

An innovative iron-based alloy powder characterized by reduced carbon content and elevated Mn content was developed, leading to the fabrication of iron-based ultrafine bainitic steel with exceptional mechanical properties utilizing laser-directed energy deposition (L-DED). The layers' morphology, microstructure, phase composition, and elemental distribution were analyzed, indicating that the melting depth of the layer was deep and ultra-fine dendrite formed in the top region. A multi-physical powder and melt pool simulation was conducted to analyze the melt flow and pool solidification characteristics, showing that the strong downward flow inside the molten pool caused by the powder impingement increased the melting depth. The cooling rate G × R, where G was the temperature gradient and R was the solidification rate, was higher at the top pool boundary, facilitating the ultra-fine dendrite formation at this location. Following subsequent low-temperature isothermal heat treatment, a refined bainitic microstructure emerged within the deposited layer. The bainitic plates in the 573 K isothermally transformed bainite were interspersed with film-retained austenite (RA). Conversely, the bainitic plates formed at a lower temperature (523 K) exhibited a coalesced morphology stemming from the diminished stability of super-cooled austenite. The bainite plates underwent growth and coalescence, ultimately giving rise to the bainitic microstructure. The 523 K transformed coatings demonstrated superior hardness (509.2 HV), strength (1435 MPa), and elongation (11.5 %) compared to the 573 K transformed coatings (456.5 HV, 1241 MPa, 10.1 %). This research holds significant implications for advancing low-carbon bainitic materials with remarkable comprehensive mechanical properties in additive manufacturing applications.