Temperature-dependent Physical Properties Research Articles

Investigation of thermo-mechanical coupled behavior within irradiated biological tissues with temperature-dependent properties

The thermo-mechanical coupled behavior of biological tissues excited by a transient heating has been studied in this paper, in which the temperature-dependent physical properties are considered. A non-linear bio-thermo-mechanical coupled model with variable physical parameters has been proposed firstly in the context of the generalized theory with the dual-phase relaxation mechanism. An effective procedure constructed on an explicit finite difference scheme is then implemented to solve this non-linear model numerically. On this basis the thermo-mechanical coupled response and relative thermal damage within a skin tissue irradiated by a pulse laser has been explored. The temporal and spatial distributions of each physical field resulting from various cases have been obtained and illustrated. The results stated that the thermo-mechanical interaction induced by a laser heating would be inhibited by temperature-dependent physical parameters and this inhibition is more significant on the tissue displacement and normal stresses than tissue temperature. The thermal damage is also inhibited when the temperature dependence is considered and the time that healthy tissues can be tolerated would be a larger extension for its accumulative effect with tissue temperature.

A Two-Dimensional Thermoelastic Analysis of a Cylindrical Shell Made of FGM with Temperature-Dependent Physical Properties

A Two-Dimensional Thermoelastic Analysis of a Cylindrical Shell Made of FGM with Temperature-Dependent Physical Properties

Temperature-dependent properties of the antiferroelectric model PbZrO3 : An effective Hamiltonian study

A novel atomistic effective Hamiltonian scheme, incorporating an original and simple bilinear energetic coupling, is developed and used to investigate the temperature-dependent physical properties of the prototype antiferroelectric ${\mathrm{PbZrO}}_{3}$ (PZO) system. This scheme reproduces very well the known experimental hallmarks of the complex Pbam orthorhombic phase at low temperatures and the cubic paraelectric state of $\mathrm{Pm}\overline{3}\mathrm{m}$ symmetry at high temperatures. Unexpectedly, it further predicts a novel intermediate state also of Pbam symmetry, but in which antiphase oxygen octahedral tiltings have vanished with respect to the Pbam ground state. Interestingly, such a new state exhibits a large dielectric response and thermal expansion that remarkably agrees with previous experimental observations and the x-ray experiments we performed. We also conducted direct first-principles calculations at 0 K, which further support such a low-energy phase. Within this fresh framework, a reexamination of the properties of PZO is thus called for.

Characterization of Cubic Zirconia as a Lens Material Suitable for Autonomous Driving

As the development of autonomous driving technology is now in full swing, the demand for miniaturized optical modules mounted on various sensors has increased. Particularly, the optical lens used for such autonomous driving must demonstrate stable performance and durability despite rapid changes in the external environment. In this regard, cubic zirconia (CZ) can be used as an optical lens due to its high refractive index, which is above 2.1 in visible and near-infrared wavelengths, along with its chemical and mechanical durability. Thus, in this paper, we investigated the temperature-dependent physical properties of CZ fabricated by the skull melting method. The temperature coefficient of the refractive index (dn/dT) of the fabricated CZ plate in the temperature range of 25–100 °C decreased from 9.76 × 10−5/K to 7.00 × 10−5/K as the wavelength increased from 447.0 nm to 785.0 nm. The estimated Abbe number decreased from 33.98 at 25 °C to 33.12 at 100 °C, while the measured coefficient of thermal expansion (CTE) was 9.91 × 10−6/K, which revealed that the dn/dT value of the CZ plate was more affected by the dispersion than by the CTE. In addition, the CZ samples with a high refractive index, coated with a dielectric multilayer showed a high average transmittance of 98.2% at the investigated wavelengths, making it suitable for miniaturization or wide-angle optical lens modules. To secure the durability required for automobile lenses, the variation in the surface profile of the CZ before and after the external impact was also analyzed, revealing much better performance than TAF glass. Therefore, the observed results demonstrate that the CZ material is suitable for use as an optical lens for autonomous vehicles.

Vertical inhomogeneous magnetic order in FeRh film

FeRh films, known as antiferromagnetic materials at low temperatures, exhibit unexpected ferromagnetic (FM) characteristics, unlike the bulk. The detailed temperature-dependent physical properties of FeRh films were examined to elucidate the causes. Temperature-dependent magnetic and electrical measurements showed that the coexistence of antiferromagnetic and residual FM states induces thermomagnetic irreversibility (e.g., spin-glass-like behavior) and negative magnetoresistance. Polarized neutron reflectometry (PNR) revealed the presence of non-uniform FM states at the interfaces. Additional temperature-dependent PNR profile and room temperature structural and spectroscopic analyses confirmed that the bottom FM region is correlated with structural distortion. In contrast, the top FM region is temperature-dependent and originates from non-stoichiometry.

New non-iterative method for computation of tubular cross-flow heat exchangers

A novel semi-analytical method was proposed for the performance and design computation of the tubular heat exchangers with cross and mixed flows of fluids. The heat exchanger is segmented into small elements (finite volumes). Closed analytical relationships express the fluid temperatures at the outlets of the finite volumes. Calculations of two heat exchangers, a steam superheater and a car cooler, are presented to illustrate the effectiveness of the developed method. Exact formulas for calculating the temperature of steam and flue gas in the superheater and of water and air in the automotive radiator are derived to evaluate the accuracy of the proposed method. The results obtained by the proposed method were found to be in perfect agreement with the results obtained by exact formulas and experimental results. Temperature-dependent physical properties of fluids can also be easily considered. The method presented in this paper is also competitive in terms of computation time due to the elimination of iterations when determining the fluid temperatures. When the one-pass of the superheater under analysis is divided into 20 control volumes, the computing time on a personal computer for temperature-dependent heat capacities of the steam and flue gas is less than 0.5 s.

Temperature-controlled spectrophotometry: a simultaneous analysis of phase transition, thermal degradation and optical properties of semi-transparent composites from 20 °C to 450 °C.

So far, optical and effective radiative properties of polymer matrix based composites were investigated at temperatures well below their degradation temperature. At the same time, polymers exhibit temperature dependent physical properties and may undergo structural changes as their temperature raises. In this work, we employ the "Temperature-Controlled Spectrophotometry", a new method enabling to identify simultaneously phase transitions, thermal degradation and radiative properties of semi-transparent composites over a large temperature range. The method consists of measuring simultaneously the normal-normal and the normal-hemispherical transmittances and reflectances of the sample subjected to a laser irradiation with tuneable wavelength while the temperature is rised from room temperature up to 450 °C by means of a CO2 laser. Physical changes of the sample are identified from the temperature variation of normal-normal transmittance and specular reflectance measurements. Most of the results here are presented at a specific wavelength of 1070 nm but the proposed method is suitable over the semi-transparency spectral domain of the material by changing the wavelength of the probe laser. An inverse method for parameter identification based on normal-hemispherical measurements is employed to determine the transport effective radiative properties of the sample, namely the transport extinction coefficient and the transport scattering albedo from room temperature to 325 °C.

Comparison of Two Numerical Models for Friction Stir Welding

In this paper, two numerical models of friction stir welding of two aluminum plates are considered. The coupled mechanical-temperature problem is solved with explicit integration over the time. The two models are based on the finite elements method (FEM) and have different complexity and number of participating parameters. The aim of the development is to highlight the weaknesses and strengths of the models of different complexity. The first model is simpler and it is based on the free penetration of the tool into the parts. The second model is based on the arbitrary Lagrange-Euler method and take in to the account the instrument penetration that allows to take into account the movement of the material in the process area. Convective and radiative heat transfers are accounted in the two models. The results for the temperature, stress and strain in the welded joint are studied and compared. The calculation time is given for each model. A parametric analysis was performed for the influence of the rotational speed of the pin, welding velocity and the pressing force. The influence of the temperature dependent physical properties of the welded plates is determined and compared for the both finite element models.

A two-phase model for studying the complex interplay between natural convection and magnetic field in aluminum-oxide/water nanofluid

While previous experimental studies revealed the existence of optimal heat transfer performance in natural convection of magnetohydrodynamic (MHD) nanofluid flows, it is believed that this phenomenon is not unique in different levels of magnetic flux density. In order to prove this speculation, we numerically investigate the influence of the magnetic field on steady natural convection of aluminum-oxide (Al2O3) nanofluids inside an enclosure. The MHD flow model without the consideration of nanofluids is well validated against theoretical, numerical, and experimental results. The nanofluid in the present investigation has temperature-dependent physical properties and is modeled by a two-phase mixture considering thermophoresis and Brownian diffusion. While varying the volume fraction of nanoparticles from 0 to 6%, we methodically examine the effect of the Hartmann number on the heat transfer performance through the thermal behavior map in terms of the Hartmann–Rayleigh plane. Depending on the combination of magnetic intensity and buoyancy-induced flow, the nanofluid heat transfer with the addition of various concentrations of nanoparticles can be categorized into two patterns separated by a critical Hartmann number: the presence of a maximum Nusselt number (Type A) and continuously increased heat transfer rates (Type B) over a wide range of the Rayleigh number from 103 to 106 and the Hartmann number from 0 to 300. Described by a piecewise-defined function, the critical Hartmann number is exponentially proportional to the Rayleigh number from 104 to 106 and rises linearly in a logarithmic scale with elevating the Rayleigh number from 103 to 104.

Topology optimization for a water-cooled heat sink in micro-electronics based on Pareto frontier

• An efficient Pareto frontier based design strategy for heat sinks is proposed. • A fitting formula of the Pareto frontiers is introduced to accelerate the design. • The 2D topology design results are validated by 3D CFD simulations. • Effects of temperature dependent properties on the results are significant. Topology optimization has been increasingly adopted as a non-intuitive methodology for heat sink design in the thermal management of microelectronics. In this work, a bi-objective topology optimization method for heat sink design in micro-electronics is suggested by considering the trade-off between two contradicting objectives, i.e., the heat transfer and hydraulic performances. The method in this work focused on the laminar water flow with Reynolds number less than 100 based on the channel width for a wide temperature range that covers different types of micro-electric devices. A series of Pareto frontiers were acquired for heat sink designs with different topologies, which can be used to decide the optimum design in respect of heat transfer and pressure drop. Particularly, an analytical formula in terms of inlet Reynolds numbers was obtained for the fast generation of the Pareto frontiers under different working conditions which help to determine the weight parameters of two contradicting objectives readily. The new strategy for weight parameter determination significantly reduces the workload of trial-and-error during the weight parameter determination in the traditional bi-objective topology optimization and thus accelerates the design process. Besides, the temperature-dependent physical properties (viscosity and thermal conductivity) in the design model were considered and were found to affect the design results significantly. Finally, the 3D heat sink model was reconstructed from the 2D topology optimization, and the thermal-fluid analyses were conducted. The results indicate that the 3D reconstructed model can well satisfy the design objective specified in the topology optimization, thereby confirming the validity and robustness of our design method.

Thermodynamic stability of microheterogenic states in Fe – Mn – C melts

Possibility of existence of microheterogeneous states in Fe – Mn – C melts was analyzed carried out according to the concepts of chemical thermodynamics. Microheterogeneous state of a chemically inhomogeneous Fe – Mn – C melt was understood as presence of dispersed Fe – C particles in it, which are suspended in Mn – C environment and separated from it by interface. Hypothesis of microheterogeneous state of Fe – Mn – C melts is supported by numerous experimental data on their thermodynamic and physical properties. Identification of anomalies in temperature dependences of physical properties of Fe – Mn – C melts made it possible to determine temperature values above which the melt superheating treatment (MST) leads to destruction of microheterogeneity, i.e., the liquid – liquid structure transition (LLT) in the melt. LLT is understood by authors as a structural transition “microheterogeneous melt – homogeneous solution” and this is expressed in destruction of microheterogeneous state when the melt is heated to a temperature determined for each composition (MST). This paper describes a method for theoretical determination of temperature range where microheterogeneous state of the Fe – Mn – C melt is thermodynamically stable. Thermodynamic stability of dispersed Fe – C particles in the Mn – C medium was estimated according to the equations proposed by Kaptay for a regular solution. It was assumed that interface between the melt of dispersed Fe – C particles with sizes from 2 to 34 nm, distributed in the Mn – C dispersion medium and separated from it by an interface with increased carbon content. This result of the assessment is consistent with the data on size of the structural units of a viscous flow obtained earlier within framework of the theory of absolute reaction rates.

Three-Dimensional Numerical Analysis of Mini-Grooved Flat Heat Pipe Filled with Different Working Fluids with Experimental Validation

Mini-grooved flat heat pipe (FHP) is an efficient approach to cool the electronics within small space. As it was hard to measure the flow and thermal characteristics inside the FHP, a novel three-dimensional model was set up to couple velocity, pressure, temperature, and mass transport inside the FHP, anticipating understanding the working mechanisms of FHP filled with different working fluids under transient and steady states. Temperature-dependent physical properties of working fluid and simulated thermal conductivity of wick were employed. The numerical results were validated by experiment with the maximum deviations in thermal resistance and maximum temperature of 17.9% and 5.0%, respectively. It was found that working fluid types, properties and input heat affected the FHP performance. The deionized water filled FHP shows more excellent thermal characteristics. Under input heat of 10 W, the thermal resistance, maximum temperature, and temperature nonuniformity of deionized water filled FHP were numerically lower than those of anhydrous ethanol filled FHP by 48.1%, 11.5% and 56.4%, and lower than those of hexane filled FHP by 54.4%, 14.3% and 62.6%, respectively. However, the starting performance of deionized water filled FHP is comparatively poorer. FHP filled with anhydrous ethanol or hexane is more appealing for electronics cooling.

Numerical investigation of thermal residual stress distribution for the sustainability of laser welded joints of dissimilar materials

In this work, a three-dimensional (3D) time dependent finite element method is used to simulate the thermo-mechanical behavior of laser welded joints. Laser welding of two dissimilar materials (AISI 316L and Ti6Al4V) of 2 mm thickness has been considered while modeling in COMSOL multiphysics with a coupling of heat transfer and solid mechanics module. A total of six temperature dependent physical properties, i.e. density, specific heat, thermal conductivity, coefficient of thermal expansion, Young’s modulus, and Poisson’s ratio have been considered for making the numerical model more realistic. The experimental results show that AISI 316L experiences more than 2 times more maximum stresses than Ti6Al4V near the weld bead region during the process. High value of Von mishes stress in the welded zone can be observed due to high temperature gradients of welds. The simulated value of residual after one entire thermal cycle is near to 80 MPa on a point over weld bead.

Convential exergy aspect of heat transfer and fluid flow through spiral passage subjected to consant wall temperature

Exergy destruction of heat transfer and turbulent convective fluid flow through spiral passage subjected to constant wall temperature is analyzed. Constant and temperature dependent thermo physical properties models of process fluid have been adopted. Heat transfer characteristics and both thermal as well viscous fluid friction exergy destruction are investigated. The local variation of total (thermal and viscous dissipation) exergy destruction was studied along the spiral passage. Some of second law of thermodynamic dimensionless parameters, such as Bejan (Be) and entropy generation (Ngen) numbers were considered. It is found that De (Dean Number) with its magnitude is a measure of the secondary flow, and has opposite influence on these two numbers along the passage. The study concluded that total exergy destruction was dominated by thermal effects due to temperature difference. But the influence of irreversibility due to pressure drop was found less significant. This effect is of a particular interest in the heat transfer and fluid flow in spiral passages where secondary flow phenomenon plays an important role in thermal mixing and conversion of viscous dissipation into thermal energy through narrowing passage.

Mesoscopic evolution of molten pool during selective laser melting of superalloy Inconel 738 at elevating preheating temperature

Due to the poor weldability of Inconel 738 alloy, defects (cracks, pores) always exist in the selective laser melting(SLM) process. In this paper, the molten pool evolutionof the single track, including the morphology, porosityand crack formation, wassystematically investigated by numerical simulation, andthe effects of different preheating temperature and process parameters were verified by experimental analysis. The detailed temperature-dependent thermal physical properties were applied into the Volume of Fluid (VOF) methodto simulate a powder bed computational domain which was established by Discrete Element Model (DEM).The simulation results show that the change of molten pool width by different laser power and preheating was not as significant as that of depth. The results also demonstrate that higher energy density and preheating temperature induced the generation of pores at the bottom of molten pool, which can be explained by the variation in cooling rates . Moreover, the change of temperature gradients in three-dimensional directions with the increase of preheating temperature was also investigated, which explained the reason for crack forming and extending through the center line. Finally, the evolution of molten pool at a higher preheating temperature1000℃ was simulated to further underpin the positive effect of preheating on preventing cracks formation.

Analytical-numerical method for calculating cross-flow tube heat exchangers considering temperature-dependent fluid heat capacities

The paper presents a mathematical model of a steam reheater in a steam boiler installed in a power unit of 1000 MW. A new numerical-analytical method was used to determine the temperature distribution of steam, flue gas, and the reheater’s tube walls. The reheater is a two-pass cross-current heat exchanger. There are 44 panels of tubes on the width of the boiler. The steam flows parallel through 20 pipes in each pass. An analytical model of the superheater was developed with the assumption of the constant physical properties of steam and flue gas. The results of steam and flue gas temperature calculations obtained by the proposed method were compared with the exact method’s results. Very high accuracy of estimates was attained even if the whole superheater is divided into a few control volumes. The results of the first and second pass steam temperature calculations were compared using the P-NTU (Effectiveness - Number of Transfer Units) method. A perfect match between the steam temperature and the average temperature of the flue gas at the outlet of the reheater was found. The advantage of the developed method of superheaters modeling is the possibility to model heat exchangers with complex flow systems. The calculations were also carried out for temperature-dependent physical properties of steam and flue gas.

Investigation of transport processes in a commercial hot wall CVD reactor with multi-substrates for high-quality pyrocarbon deposition

A 3D CFD model is developed to illustrate the complex reacting flow in the commercial hot-wall CVD reactor using detailed gas and surface chemistry. This helps to understand multi-species transport and their interplay with the gas and surface reactions for pyrocarbon (PyC) deposition. The detailed transport phenomena models with temperature-dependent physical properties are incorporated into the model to improve accuracy. Valuable information such as microscopic local variations in gas velocity, temperature, and species distributions is obtained, which otherwise difficult to acquire experimentally. A sensitivity analysis is also performed to realize how the operating conditions affect the distributions of intermediate species and, ultimately, the growth rate of PyC. The understanding gained in this study is further used to optimize the CVD reactor to obtain high-quality PyC. The response surface methodology is employed to examine the interactions among various operating parameters that influence film performance.

Numerical simulation of hardfacing remanufacturing for large-scale damaged grinding roller

The present study is directed to the temperature and residual stress fields in the hardfacing remanufacturing for a large-scale grinding roller with damage. For this purpose, a numerical procedure for the hardfacing process based on the thermal cycle curve method was established, and the effectiveness of the proposed procedure was verified by the measurements of a multi-pass welding experiment with V-groove butt joint. A local model of bead-on-plate welding was used to check the heat source and extract the thermal cycle curve. The resulting thermal cycle curve was applied as a thermal load to the corresponding welding layer one by one. Based on the calculated material properties, the data files of the base metal (Grade:KmTBCr26) and the filling layers (Grade:ARCFCW9024) were re-developed to define the temperature-dependent physical and mechanical properties of the materials used in the hardfacing remanufacturing. The numerical simulation results reveal the changing trend and distribution of the temperature and residual stress fields during the hardfacing remanufacturing for the damaged grinding roller. The distribution of temperature and residual stress is very complicated due to the numerous welding layers in the hardfacing process. It is pointed out that with the increase of welding layers, the heat-affected zone of the grinding roller expands gradually, and the maximum tensile stress always appears at the position near the weld toe, which is prone to fatigue failure or interface peeling due to stress concentration.

Modeling and numerical simulation of a parabolic trough collector using an HTF with temperature dependent physical properties

This paper describes the mathematical modeling of a parabolic trough collector where the heat transfer fluid has temperature dependent physical properties for which we have developed our own mathematical expressions. For a specific location and date, the direct solar radiation and other sun–earth parameters are computed on a Julian day basis. Also, we have used certain known and recent correlations for the heat transfer coefficients appearing in this model. The mathematical model is governed by three nonlinear partial differential equations of parabolic type with initial/boundary value conditions. We describe a numerical algorithm in order to obtain the approximate solution of this model. Then, we carry out some numerical simulations which yield valuable information for the performance and efficiency of a solar PTC plant at the specific location.

Thermally Tunable Acoustic Beam Splitter Based on Poly(vinyl alcohol) Poly(N-isopropylacrylamide) Hydrogel.

In this study, we demonstrated a thermally tunable acoustic beam splitter using a poly(vinyl alcohol) poly(N-isopropylacrylamide) hydrogel (PVA-pNIPAM). The nature of PVA-pNIPAM hydrogel offers exceptional temperature-dependent physical properties due to its phase transition around its lower critical solution temperature. The acoustic impedance of the hydrogel can be tuned below, above, or matched to that of water by changing the environmental temperature. An acoustic wave propagating in water can be split into transmitted and reflected components by the PVA-pNIPAM hydrogel slab on varying its angle of incidence. The intensity ratio between the reflected and the transmitted componence can be adjusted by tuning the temperature of the medium. The acoustic beam can be entirely reflected at a temperature corresponding to the matched impedance between hydrogel and water. The beam-splitting behavior was observed for acoustic waves from both a monochromatic wave and broadband pulse source. In addition, the phase of beam split pulses can be reversed by selecting the hydrogel’s operating temperature.