Heat Flux Direction Research Articles

Effects of processing conditions on solidification characteristics and mechanical properties of in situ magnesium metal matrix composites derived from polysilazane precursor

Polymer-derived in situ magnesium metal matrix composites (P-MMMCs) were fabricated by injecting a liquid or cross-linked polysilazane precursor into molten magnesium by a stir-casting method at two different melt temperatures of 700 and 800℃. Microstructural analysis reveals that the composites fabricated at 700℃ exhibit uni-modal grain size distribution having more or less columnar-shaped grain morphology. On the contrary, bi-modal grain size distribution with predominantly dendritic grain morphology occurs in the Mg matrix composites fabricated at 800℃. Such difference in grain morphology can be associated with variation in the availability of heterogeneous nucleation sites, and direction of heat flux during solidification. All of the fabricated composites were investigated for their solidification characteristics, microstructural evolution, micro/nano-hardness and compression properties. This article discusses the correlation between the processing parameters, microstructural evolution and mechanical properties of the as-cast in situ composites fabricated by liquid metallurgical route. Polymer-injection followed by in situ pyrolysis holds the potential of revolutionary processing technologies for producing castings of metal matrix nanocomposites, for example by bubbling the organic liquid with a carrier gas, e.g. nitrogen, into the molten metal by a Bessemer-like process.

Modelling cascaded caloric refrigeration systems that are based on thermal diodes or switches

Solid state cooling based on caloric materials has a large potential, because there is no need for fluid refrigerants. The state of the art system approach relies on a heat transfer fluid, which is pumped back and forth through a regenerator of caloric material. This system approach has fundamental limitations regarding the cycle frequency. Different system approaches are considered. One is the utilization of fast thermal diodes or switches. The aim of this work is to provide a universal model to describe a cooling system based on caloric materials using thermal diodes or switches. The heat capacitance of the caloric material is defined to be constant, allowing a lumped elements representation. An analytical model based on an analogy to an electrical low pass filter is introduced. The analytical model is compared with good agreement to lumped elements simulations and measurement data from the literature. It is shown, that the newly introduced cut-off frequency as merit for the potential cycle frequency is proportional to the heat transfer coefficient of the diodes/switches in flux direction and inverse proportional to the length scale of the caloric material in heat flux direction. Different heat transfer mechanisms are discussed as basis for such thermal diodes and switches in combination with different caloric effects. To increase the cycle frequency compared to state of the art systems, latent heat transfer is a promising candidate for all caloric effects, whereas for example a dry press contact for a magnetocaloric system presents many challenges.

Transverse thermoelectric multilayer generator with bismuth-substituted calcium cobaltite: Design optimization through variation of tilt angle

Sintering of Ca2.7Bi0.3Co4O9 pellets and multilayer laminates at 920 °C results in a ceramic microstructure with low density with a pronounced anisotropy. The electrical conductivity of multilayers is 56 S/cm at 400 K (perpendicular to pressing direction). The Seebeck coefficient is positive, and the power factor increases from 60 μW/(K²m) at 400 K to 200 μW/(K²m) at 900 K. The thermal conductivity (parallel to pressing direction) is 0.65 W/(mK). Transverse multilayer thermoelectric generators (TMLTEG) were fabricated by stacking layers of Ca2.7Bi0.3Co4O9 green tapes, screen-printing of AgPd stripes at various tilt angle φ relative to the heat flux direction (20°, 45°, and 65°), and co-firing at 920 °C. For φ = 65° the power output is 8 mW at ΔT = 200 K with room temperature at the cold side. FEM modelling as well as analytical calculations agree well with measurements, and the optimum tilt angle is found to be φ = 58°.

Abnormal data rejection range in the Bowen ratio and inverse analysis methods for estimating evapotranspiration

First, this paper proposes a definition of the abnormal data that occur as the Bowen ratio (B0) approaches −1. The data are defined as α times of the heat flux supply (Rn-G), thus indicating the rejection range for the B0. The method is applied to observed data. Second, we compare the results of the B0 method with those of the inverse analysis of Bowen ratio method (IABM). The IABM can estimate the sensible and latent heat flux using temperature and humidity observed at a single height above the ground by optimization. Third, the estimation of the monthly evapotranspiration required for water resources and irrigation planning is adequate for α = 1˜3 in the B0 method and IABM, and the initial humidity for IABM is adequate for 80% of the observed humidity plus 20%. Fourth, the features of the analysis of both methods regarding the direction of latent heat flux (lE) and sensible heat flux (H) associated with temperature difference (ΔT) and vapor pressure (Δe) clarified. And we found that the number of the rejected data in hourly and monthly is remarkably large in B0 method than IABM. Based on above results, the IABM has almost the same performance compared with the B0 method excluding initial value determination in the optimization process.

Periodic amplification of radiative heat transfer

We demonstrate that the direction and values of the radiative heat flux exchanged between a non-phase-change material and a phase-change one excited with a temperature difference modulated in time can efficiently be tuned by means of their common steady-state temperature. This heat-flux modulation occurs in both the far- and near-field regimes as a result of the strong temperature dependence of the emissivity and permittivity of the phase-change material, respectively. It is shown that the heat pumping into or out of the phase-change material can not only be amplified but also canceled out for temperatures around its critical temperature. This nullification of the radiative heat flux can be used as a mechanism to rectify heat currents and to insulate the two bodies from each other, even when their temporal temperature difference is different than zero. The obtained results thus open a new pathway for the heat-flux control of nonequilibrium radiating systems.

Detection of heat flux at single-atom scale in a liquid-solid interfacial region based on classical molecular dynamics

In this study, we examine heat flux at the single-atom scale in a liquid-solid interfacial region by calculating local quantities based on classical molecular dynamics. The heat flux was calculated over a subatomic area defined on the liquid-solid interfacial region, and a two-dimensional map of the local heat flux at the liquid-solid interface was obtained. The results clearly showed directional heat flux at the single-atom scale between the liquid and solid phases; the spatial heat conduction was not uniform along a temperature gradient in the immediate vicinity of the solid surface, which suggests that the interfacial thermal resistance can be interpreted more precisely based on the local quantity of the heat flux. The methodology and results given in this study should prove useful to more precisely interpret and control heat transfer and thermal resistance at interfaces.

Design Rules for Additive Manufacturing – Understanding the Fundamental Thermal Phenomena to Reduce Scrap

Abstract The goal of this work is to predict the effect of part geometry and process parameters on the direction and magnitude of heat flow ‒ heat flux ‒ in parts made using metal additive manufacturing (AM) processes. As a step towards this goal, the objective of this paper is to develop and apply the mathematical concept of heat diffusion over graphs to approximate the heat flux in metal AM parts as a function of their geometry. This objective is consequential to overcome the poor process consistency and part quality in AM. Currently, part build failure rates in metal AM often exceed 20%, the causal reason for this poor part yield in metal AM processes is ascribed to the nature of the heat flux in the part. For instance, constrained heat flux causes defects such as warping, thermal stress-induced cracking, etc. Hence, to alleviate these challenges in metal AM processes, there is a need for computational thermal models to estimate the heat flux, and thereby guide part design and selection of process parameters. Compared to moving heat source finite element analysis techniques, the proposed graph theoretic approach facilitates layer-by-layer simulation of the heat flux within a few minutes on a desktop computer, instead of several hours on a supercomputer.

Mechanistic Approach of Goss Abnormal Grain Growth in Electrical Steel: Theory and Argument

The first Si-Fe electrical steel was produced in 1905, and the grain-oriented steel was discovered in 1930 after Goss demonstrated how optimal combinations of heat treatment and cold rolling could produce a texture giving Si-Fe strip good magnetic properties when magnetised along its rolling direction. This technology has reduced the power loss in transformers greatly and remains the basis of the manufacturing process today. Since then many postulations reported on the mechanism on abnormal grain growth (AGG) which is the key for Si-Fe superior magnetic properties, however, none have provided a concrete understanding of this phenomenon. Here, we established and demonstrated a new theory that underlines the fundamental mechanistic approach of abnormal grain growth in 3% Si-Fe steel. It is demonstrated, that the external heat flux direction applied during annealing and Si atom positions in the solid solution disordered α-Fe cube unit cell that cause lattice distortions and BCC symmetry reduction are the most influential factors in the early stage of Goss AGG than what was previously thought to be dislocation related stored energy, grain boundary characteristics and grain size/orientation advantages.

XVI Международная конференция "Термоэлектрики и их применения --- 2018" (ISCTA 2018), Санкт-Петербург, 8-12 октября 2018 г. Новое направление применения термоэлектрических преобразователей энергии

AbstractIn southern countries, including European states, solar collectors for additional water-heating systems are widely used. However, the disadvantage of such systems is that a considerable proportion of solar energy cannot be used with increasing water temperature and dissipates into the environment. It is proposed to use waste heat at a high temperature, which is supplied to a thermoelectric generator (TEG) operating on the basis of a temperature difference between the hot water in the solar collector and cold water supplied to a radiator on the other TEG side. This is a new application of thermoelectric converters, in which the converter can act not only as a source of electrical energy but also as a source of low-potential heat coming from the radiator. The total conversion efficiency in such devices can reach 90%. It is shown that the use of p -type legs in a TEG with the cleavage planes of the legs parallel to the heat-flux direction instead of the conventional parallel orientation results in an increase in the thermoelectric efficiency by 25% on average in the temperature range of 100–300°C.

A transient regime for transforming thermal convection: Cloaking, concentrating, and rotating creeping flow and heat flux

By treating a set of equations governing transient heat and mass transfer simultaneously, here we develop the transformation theory for thermal convection with unsteady creeping flow in porous media, whose steady counterpart has been previously studied. We find that the transformation theory can still be valid when the temperature, density, and velocity of fluids vary with time. As applications, we design thermal cloaks, concentrators, and rotators at transient states examined by finite-element simulations, which can be used to control the magnitude or direction of heat flux in convection. Also, we discuss both the effects of natural or mixed convection and the differences between steady and unsteady states. This work develops a theory for dynamically controlling the flow of heat associated with thermal convection.

Effects of process parameters on microstructures and tensile properties of laser melting deposited CrMnFeCoNi high entropy alloys

In this paper, a laser melting deposition (LMD) technique has been applied to fabricate CrMnFeCoNi high entropy alloys (HEAs). The microstructures and tensile properties of CrMnFeCoNi HEAs prepared under different laser power and scanning strategies have been investigated. It has been observed that the laser power and scanning strategy have significant effects on the columnar to equiaxed transitions (CET) of the microstructure of LMD CrMnFeCoNi HEAs because of their effects on heat flux direction and the temperature gradient. Due to small molten size and rapid cooling rate in LMD process which results in a significant solute-trapping effect and thus avoids component segregation, the elements distribution of LMD samples are more homogeneous than as-cast samples. Besides, tensile properties of the LMD samples can be adjusted by changing laser power and scanning strategy, which be corresponding to the changes of microstructure.

The evolution of bainite and mechanical properties of direct laser deposition 12CrNi2 alloy steel at different laser power

In this paper, 12CrNi2 alloy steel samples were successfully fabricated by direct laser deposition (DLD) technology. The evolution of bainite in the DLD 12CrNi2 alloy steel process at different laser power of 1800 W, 2000 W and 2200 W and mechanical properties of as-deposited samples were investigated. The results showed that the middle-upper microstructure of the samples fabricated at different laser power transformed from lath bainite (LB) to granular bainite (GB) with increasing laser power. In addition, granular bainite was divided into blocky granular bainite (GB1) and lath-like granular bainite (GB2) according to different formation mechanisms, and the amount of GB1 increased and GB2 decreased in the range of laser power from 2000 W to 2200 W. No preferred texture was observed in the EBSD maps due to the complex heat flux direction which was resulted in the reciprocating scanning strategy. With the increase of laser power, the proportion of high-angle grain boundaries increased from 33.1% to 46.4% and that of low-angle grain boundaries decreased from 66.9% to 53.6%. The sample fabricated at 2000 W had the highest mean microhardness (331 HV0.2) and the best combination of ultimate tensile strength (757 MPa) and elongation (9.1%). However, the sample fabricated at 2200 W had the best impact toughness (aku = 100.0 J/cm2) because it contained a large amount of GB1 with dispersive and spherical-like island structures. This study provides the theoretical and experimental basis for the design of laser power and controllability of microstructure and properties in the DLD alloy steel process.

Computational and experimental study of solar thermal energy store for low-temperature application

Abstract This paper evaluates solar thermal energy storage (STES) systems for a low-temperature application. The STES used in this study was an annular cavity storage unit filled with heat storage material. Two latent heat storage materials (benzoic acid and stearic acid), and one sensible heat storage material (palm olein) were selected for this study. Transient thermal analysis was carried out on the STES models using ANSYS 16.0. The performances of the STES systems were evaluated by installing them in solar cookers to heat water. The heating power, sensible heat efficiency (SHE), the amount of energy and exergy stored were estimated. The temperature distribution, total heat flux and directional heat flux of the STES systems were observed to be affected by the type of heat storage materials used. The heating power, the energy stored and exergy stored reduced, while the SHE increased with an increase in the amount of water heated. In the absence of solar radiation, the STES systems sustained the temperature of 1 and 1.5 kg of water above 60 and 50 °C respectively for 200 min; this shows the suitability of the annular cavity storage unit for low-temperature applications.

Effect of material anisotropy on the transverse thermoelectricity of layered composites

Thermoelectric devices can achieve conversion between thermal and electrical energies. In contrast to longitudinal thermoelectrics, transverse thermoelectrics can decouple the directions of heat flux and electric current, which in turn offers more flexibility in device design. While many studies have focused on composite structures constructed using materials with isotropic properties, this work investigated the effect of material anisotropy on the transverse thermoelectricity of layered composite materials. A simplified analytical model was derived based on the equivalent circuit principle to correlate the effective thermoelectric properties of the composite structure and the properties of the constituting phases. The effects of the geometrical design parameters of the composite structure and the material anisotropy were investigated. The transverse thermoelectric figure of merit increases with the increase in material anisotropy. The analytical model was then compared with the finite element analysis model with respect to the cooling capacity of transverse thermoelectric devices. The good agreement achieved between the two approaches indicated the effectiveness of the simplified analytical model in predicting the transverse thermoelectric performance of layered structures with anisotropic material properties.

Effects of tool path in remanufacturing cylindrical components by laser metal deposition

Laser metal deposition as an additive manufacturing process has been widely utilized for component repair. In this study, in order to investigate the influence of tool path on characteristics of coatings for cylindrical part repair, multi-layer cobalt-based alloy was coated on cylindrical tool steel substrates using the blown powder laser metal deposition process following the helix (H), circle-line-circle (CLC), and line-arc-line (LAL) routes. A series of analysis was performed on the coatings including shape’s profile, powder-catch efficiency, microstructure, EDS, XRD, and Vickers hardness. The result shows coatings fabricated using the H and CLC routes have consistent thickness while more material was deposited near the middle for the LAL route. Powder-catch efficiency for the CLC and LAL paths was up to 28% while it was only 14% for the H route. The microstructure near the coating’s starting point was columnar dendrites growing parallel to the heat flux direction. Cooling speed reduced after several layers’ coating and equiaxed-like morphology appeared. Much gas porosity was discovered near the interface for LAL-coated samples. EDS and XRD analysis show Fe was diffused into the coatings. Microhardness measurement reveals that the hardness of LAL fabricated samples was slightly higher than the hardness of H and CLC samples. The result shows that the CLC path is more suitable for repairing/coating cylindrical components due to a relatively consistent shape, a high powder-catch efficiency, and defect-free deposits.

Microstructures and mechanical properties of CrMnFeCoNi high entropy alloys fabricated using laser metal deposition technique

In this paper, a laser metal deposition (LMD) process has been applied to the fabrications of CrMnFeCoNi high entropy alloys. The microstructures and mechanical properties of the CrMnFeCoNi alloys prepared using casting technique and LMD technique have been investigated. It has been found that the CrMnFeCoNi samples prepared using casting show a coarse dendritic structure. The microstructures of LMD samples contain both columnar grains and/or equiaxed grains, and the proportion of columnar grains and equiaxed grains can be adjusted by changing the laser power. Besides, the mechanical properties of LMD samples can be adjusted by changing laser power, due to columnar to equiaxed transitions (CET) that are effected by solidification mode and the heat flux direction of LMD process. It is worth to mention that the mechanical properties of the 1400 W samples produced using LMD are better than those produced by casting, which is rarely observed. Furthermore, CrMnFeCoNi alloys prepared using LMD show excellent mechanical properties at cryogenic temperatures.

Investigation of appropriateness of coated steel piston for aluminium alloy piston for small engines

ABSTRACT Materials substitution is an imperative research area. The aim of this research is cost reduction, improved performance on its physical, chemical or mechanical properties like improved strength, reduced weight or in terms of performances. The objective of this paper is to investigate the possibilities to use the steel piston in place of the aluminium alloy piston for small engine applications like portable generator. Aluminium alloy piston, uncoated steel piston, nickel-coated steel piston and hard chrome-coated steel piston are considered. Steel pistons plated had standard thickness and were physically experimented. The thermal analysis like thermal deformation, total and directional heat flux analysis were carried out in ANSYS software. For the outperformed piston, the CFD analyses on contoured particles were carried out with help of ANSYS Fluent software. The CFD analysis on contoured particles includes the static pressure, velocity as well as directional velocity components and path line of contoured particles.

A volume-averaged model of nitrogen–hydrogen plasma chemistry to investigate ammonia production in a plasma-surface-interaction device

Nitrogen impurity seeding is a promising technique for increasing the radiative power dissipation rate in the edge plasma of a fusion device. It will be required in future fusion devices such as ITER to reduce the directed heat flux on the divertor strike-points to within erosion limits. However, chemical reactions between nitrogen and fuel isotopes may complicate tritium control measures by increasing in-vessel retention and impacting the gas-handling plant. To gain insight into the nitrogen–hydrogen plasma chemistry a volume-averaged (global) model is developed and compared with experimental measurements in the MAGnetised Plasma Interaction Experiment plasma device. A set of 702 reactions is compiled and used to model the population dynamics of 51 relevant neutral, ionic, electron, surface and metastable excited state species. Stable equilibrium values are compared to results from an experimental investigation in which a combination of mass spectrometry, Langmuir probe analysis and optical emission spectroscopy is used to determine neutral and positive-ionic trends under the same conditions. The dominant ammonia production mechanism is found to be the Langmuir–Hinshelwood reaction between adsorbed atomic hydrogen and above 25% hydrogen concentration. For lower hydrogen proportions the Eley–Rideal reaction between free atomic hydrogen and is found to dominate. The dominant loss mechanism (for all compositions) is found to be electron impact dissociation into neutral fragments.

Residual stress and adhesion of thermal spray coatings: Microscopic view by solidification and crystallisation analysis in the epitaxial CoNiCrAlY single splat

A new approach is proposed to achieve an in-depth understanding of crystallisation, residual stress and adhesion in epitaxial splats obtained by Combustion Flame Spray. Modelling of the fundamental process mechanisms is achieved with the help of experimental observations providing details with a sub-micrometre spatial resolution. At this scope, High Angular Resolution Electron Backscatter Diffraction and Transmission Electron Microscopy analysis are employed to provide insights into crystallisation and residual stress levels, while FIB-milled microcantilever beam bending is used for fracture strength measurements in the case of single splats. A comparison to fully-developed coatings is achieved by employing the X-ray Diffraction sin2ψ technique and pull-off methods for residual stress and fracture strength, respectively. The methodology is applied to metallic CoNiCrAlY material sprayed onto a Ni-based superalloy substrate. The establishment of different crystallisation regions: epitaxial and polycrystalline, is the result of variations in the heat flux direction at the solidification front. Significant dislocation density is also reported, indicating the relevance of impact dynamics and plastic deformation mechanisms. The comparison with fully-developed coatings suggests a reduction in inter-splat bonding at splat overlapping.

Grain boundary plane manipulation in directionally solidified bicrystals and tricrystals

We report a novel crystal growth technique for manipulating the grain structure of directionally solidified bicrystals, tricrystals, and other higher order oligocrystals. Controlling both the direction of local heat flux and the thermodynamic forces acting on the grain boundary, we can continuously vary grain boundary position and orientation, a level of control not possible using traditional processing strategies. This processing method can now specify many combinations of the five crystallographic grain boundary parameters in a single specimen, meaning large portions of both the grain boundary and triple junction parameter spaces can be sampled quickly and efficiently.