Thermal Power Density Research Articles

Fabrication, characterization, and performance of YbDSB ternary compounds for IT‐SOFC applications

Abstract(Yb2O3)x(Dy2O3)y(Bi2O3)1−x−y (0.04≤x+y≤0.20) powders (xYbyDSB) were synthesized by modified sol‐gel Pecchini method. The powders were characterized for structural, surface morphological, thermal, and electrical properties and power density measurements by means of X‐ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis/thermal gravimetry (DTA/TG), and impedance spectroscopy, respectively. Lattice parameters and crystalline size of δ‐phase of Yb2O3‐ and Dy2O3‐doped Bi2O3 samples were calculated from the X‐ray diffraction data. Surface and grain properties of the related phases were determined by SEM analysis. In the investigated system, the maximum electrical conductivity was observed as σ=0.954 S cm−1 for 6% mol Yb2O3 and 6% mol Dy2O3 at 800°C among all δ‐YbDSB systems. Cathode supported electrochemical cell was fabricated and 6Yb6DSB was used as the electrolyte. Maximum power density of single cell with an active area of 1.5 cm2 is 72.50 mW/cm2 at 700°C.

Development of fuel temperature calculation code for HTGRs

In order to ensure the thermal integrity of fuel in High Temperature Gas-cooled Reactors (HTGRs), it is necessary that the maximum fuel temperature in normal operation is to be lower than a thermal design target. In the core thermal-hydraulic design of block-type HTGRs, the maximum fuel temperature should be evaluated considering data such as thermal power, core geometry, power density and neutron fluence distributions, and core coolant flow distribution. The fuel temperature calculation code used in the design stage of the High Temperature engineering Test Reactor (HTTR) presupposes to run on UNIX systems, and its operation and execution procedure are complicated and are not user-friendly. Therefore, a new fuel temperature calculation code named FTCC which has a user-friendly system such as a simple and easy operation and execution procedure, was developed. This paper describes calculation objects and models, basic equations, improvement points from the HTTR design code in FTCC, and the result of a validation calculation with FTCC. The calculation result obtained by FTCC provides good agreement with that of the HTTR design code. In addition, the effect of cooling forms on fuel is investigated by using FTCC.

Entropy analysis of thermoelectric heat pumps including multi-channel heat exchangers: design considerations

This work aims to propose routes for thermoelectric heat pump design based on Entropy Generation Minimization (EGM). The system considered is composed of a thermoelectric module sandwiched between two parallel multi-channels heat exchangers. Co-optimization of the heat exchanger and the thermoelectric module designs are lead in order to increase the system performance, depending on realistic manufacturing constraints. The optimized variables considered in this study are: the thermoelectric leg number, their length and section; and the number of channels and their length and diameter in both heat exchangers. The following dissipative contributions are identified: thermal conduction and Joule effect in the thermoelectric module, and heat transfer and viscous dissipation in both heat exchangers. On the one hand, the thermoelectric module design has to respect optimal design ratios derived analytically to meet a given thermal power demand with maximum COP. On the other hand, dissipative contributions competition in the heat exchangers results in two distinct optimal designs, depending on the thermal power density. Taking into account manufacturing constraints, realistic system design is derived and discussed. Although the entropy generation in the heat exchangers is low compared to that in the thermoelectric module (dominated by Joule effect), the heat exchanger design highly impacts the global system performance.

Laser power dependence of mechanoluminescence in metals.

Mechanoluminescence (ML) glow is produced on the back side when the front of a metal sample is irradiated with infrared Nd:YAG laser pulses. An incident laser beam with a power density below the plasma-flare onset threshold causes a rise in temperature in the studied metal. As the incident laser power density increases, the intensity of the ML glow signal also increases. On the basis of the laser power density-induced temperature, an expression is derived for the temperature-induced thermal stress. An expression is derived for the correlation between thermal stress and laser power density, which indicates that the temperature-induced thermal stress is directly related to the incident laser power density. In the region of plastic deformation, temperature-induced thermal stress is related to the strain and, consequently, to the emitted ML intensity. Finally, an expression is derived for the laser power dependence of the ML intensity, and good agreement is found between the theoretical and experimental results. Copyright © 2016 John Wiley & Sons, Ltd.

Structure and Dynamics of Premixed Swirl Flames at Elevated Power Density

The Turbomeca gas turbine model combustor was studied to understand the effect of increased thermal power density on the structure and dynamics of premixed swirl flames. The burner was operated under two flame conditions: one stable, and one with self-excited thermoacoustic oscillations. Simultaneous measurements of scalar and three-component velocity fields were acquired at 3 and 6 kHz frequencies. Nonreacting flow measurements were used to approximate the spatial and temporal resolution with respect to the scales of the incoming flow. Time-resolved velocimetry revealed the presence of periodic fluctuations in both flames, occurring at shifted frequencies that did not correspond to a resonant acoustic mode or any corresponding harmonics. Proper orthogonal decomposition analysis revealed stably forced inner shear-layer oscillations that periodically formed and ejected symmetric vortex pairs under stable flame operation. The unstable flame was found to exhibit a single spatiotemporally evolving flow structure consistent with a helical precessing vortex core. A reconstructed time series elucidated the interactions of the reactant jets, the precessing vortex core, and the central recirculation bubble over a thermoacoustic cycle. This coupled interaction, and the resultant modulation of transport within the inner shear layer, were identified as a mechanism by which the precessing vortex core was linked to elevated power density flame dynamics.

Role of ion bombardment, film thickness and temperature of annealing on PEC activity of very-thin film hematite photoanodes deposited by advanced magnetron sputtering

A High Power Impulse Magnetron Sputtering (HiPIMS) system was used for deposition of hematite thin films. Ion flux density on substrate was studied for different pulse discharge current densities corresponding to different pulsing frequencies of cathode voltage. Total thermal power flux density on substrate was investigated for the same plasma conditions as ion flux density was measured. Our findings revealed the pulse ion flux density on substrate linearly increases with pulse discharge current density whilst the total thermal power density decreases on about 50% when discharge pulsing frequency falls from 50 kHz to 100 Hz. Our previous investigation proved that the change in the discharge pulsing frequency has significant influence on microstructure of deposited hematite thin film. Results of our study imply that magnitude of the ion flux density on substrate leads to highly textured hematite thin film oriented along the desired (110) plane. Nevertheless the ion flux density on substrate is not substantial contribution to the total energy balance on substrate. Other elementary processes taking place on substrate probably can play a significant role in hematite thin film formation. The highest photocurrents of 0.86 mA cm−2 at 0.55 V and 0.99 mA cm−2 at 0.7 V vs. Ag/AgCl was revealed for the HiPIMS deposited films having the thickness of 25 nm and after the thermal treatment at 750 °C for 30 min. The photocurrents of 0.7 mA cm−2 at 0.55 V and 0.82 mA cm−2 at 0.7 V vs. Ag/AgCl of hematite films annealed at 650 °C for 30 min were reached when the voltage bias was applied to the FTO substrate during the deposition.

Study of Natural Gas Engine using Non-Premixed Combustion Model

Direct injection natural gas engines are used in many heavy duty vehicles. Similar to diesel engines, high thermal efficiency and power density is maintained in such direct injection natural gas engines. In such engines, natural gas is injected directly into the combustion chamber. Then the gas mixes with the high pressure air in the combustion chamber and combustion occurs. The main objective is to simulate a cycle, from the end of the compression stroke to 50° after TDC. Simulation would be done keeping methane as fuel. A 30 degree sector of a 4-stroke engine which corresponds to one fuel injector hole is modeled. Since combustion simulation is to be studied, the simulation starts at 20 degree crank angle (CA) before the start of injection (SOI) and ends at 50 degree CA after top dead center (TDC). A simplified model of the engine with no valves is modeled since during the entire combustion process, both the valves remain closed.

A Thermo-hydraulic Modeling for the Formation Process of the Discharge Crater in EDM

In EDM, the extremely high thermal power density results in melting and evaporating followed by removal electrode material, forming a discharge crater on electrode surfaces. Thus, during the formation process of discharge crater, there are the three physical forms (including solid, liquid and vapor) of electrode material and the different phase transformation between them simultaneously. In order to clarify the processes of discharge crater formation and material removal in electrical discharge machining (EDM), in this paper, the thermo-hydraulic coupling numerical model of discharge crater formation was established and analyzed with finite element method (FEM). The model is based the theory of Multi-physics coupling related to the electrode material removal process, and the equations of heat conduction and fluid flow were coupled. In addition, the level set method was used to track the variation of the discharge crater surface. The simulation of the processes of discharge crater formation and material removal in EDM were realized. The temperature distribution, the velocity distribution and the acceleration distribution of heated zone were analyzed. It can be found that after discharge is ignited, material removal began to occur. It was also found that the metal removal efficiency was 0.04, leaving most of the molten zone re-solidified. It can be considered that the flow flied formed in the heated zone is one of mechanisms of the material removal and the forming of the discharge crater.

ВЛИЯНИЕ ТЕПЛОВЫДЕЛЕНИЯ И ФОРМЫ КАМЕРЫ СГОРАНИЯ НА КОНЦЕНТРАЦИЮ НЕСГОРЕВШИХ УГЛЕВОДОРОДОВ В ОТРАБОТАВШИХ ГАЗАХ

The paper presents the results of experimental studies of the influence of heat release and the combustion chamber shape on the concentration of unburned hydrocarbons in the exhaust gases of reciprocating electric power plants with spark ignition. The authors determined that the increase in the combustion chamber area causes the increase in the unburned hydrocarbons concentration due to the increase in the area of near-wall layers formed on the combustion chamber walls. Moreover, it was noted that, except the combustion chamber shape, the process of fuel combustion in the final phase of combustion, the intensity of which was estimated by the heat release value, played the important role in the mechanism of unburned hydrocarbons formation. The analysis of experimental data showed the tendency of reduction of the unburned hydrocarbons concentration with the heat release increase. The authors also revealed the reasonability of application of complex influence of fuel-air mixture composition, the combustion chamber design, and the processes in it on the unburned hydrocarbons dynamics. The authors proposed using the K parameter complex considering the influence of carbon, hydrogen and oxygen mass fractions in the fuel mixture, the combustion chamber area, and the heat release during combustion final phase on the unburned hydrocarbon emission. It is expected that the measures aimed to increase thermal power density and to reduce carbon concentration in the fuel-air mixture will reduce K , and the unburned hydrocarbons concentration as well. The authors received the linear relationship between the proposed K parameter complex and the unburned hydrocarbons concentration. Thus, the use of K complex during the stages of design and operational development of power plants will allow evaluating the effectiveness of design solutions aimed to improve environmental performance.

Comprehensive parametric study of using carbon foam structures saturated with PCMs in thermal management of electronic systems

The focus of the present work is to perform detailed parametric studies for electronic thermal management systems using different carbon foam structures of different porosities and skeleton thermal conductivities saturated with different phase change materials (PCMs) of different fusion temperatures, heat of fusions and thermal conductivities. Different thicknesses of thermal management modules and power densities levels are also included in the parametric study. The analysis was carried out using a validated finite element numerical model based on volume averaging technique and single-domain energy equation. The results show that decreasing CF and PCM thermal conductivities, increasing carbon foam porosity and increasing module height increase the module temperature and delay the approaching steady state temperature. The transient module temperature decreased and the time of approaching steady state temperature is delayed with increasing PCM heat of fusion. However the module fusion temperature does not show any strong effect on module temperature. Design guidelines for selecting the combinations of CF/PCM thermo-physical properties are presented for different operating conditions and power levels.

Improvement of Thermal Reliability and Power Density of SOFCs by Preparing Nano LSC Particles between GDC Electrolyte and LSCF Cathode

For enhancement of thermal reliability and reduction of operating temperature of SOFCs, it is effective to increase contact area and to extend triple phase-boundary area (TPB). In this work, we inserted approximately 50 nm LSC particles between the electrolyte and cathode by pulsed laser deposition (PLD). Thermal cycle testing of unit cells with or without LSC were carried out from 200 to 600°C with heating and cooling rates of 400°C h‒ 1. Cathode thin film in the unit cell without LSC delaminated from the electrolyte after only one thermal cycle, whereas cathode thin film in the unit cell with LSC survived 50 cycles. Unit cells with LSC exhibited a maximum power density of 0.53 W cm‒ 2 at 600°C; unit cells without LSC exhibited 0.29 W cm‒ 2 at 600°C. We succeeded in enhancing the thermal reliability and reducing the operating temperature of SOFCs simultaneously by inserting the cathode nanoparticles.

Improvement of Thermal Reliability and Power Density of SOFCs By Preparing Nano LSC Particles Between GDC Electrolyte and Lscf Cathode

1.Introduction Solid oxide fuel cells (SOFCs) have received much attention as promising electrochemical power generation devices because of their high electrical efficiency and environment adaptability. Development of SOFCs is now focused on lowering the operating temperature from the conventional range of 750-900°C to an intermediate range of 500-700ºC, allowing a wider range of materials for SOFC components and improvement of thermal reliability. Preparing the Yttlia Stabilized Zirconia (YSZ) and Gadolinia-doped ceria (GDC)) bilayer electrolytes are effective for reducing operating temperature [1]. In the present work, NiO-YSZ anode supported cells with YSZ/GDC bilayer electrolytes and (La,Sr) (Co,Fe) O3 (LSCF) thin film cathodes were fabricated by pulsed laser deposition (PLD) and screen printing. However, one thermal cycle test of a cell between 200°C and 600°C, with heating and cooling rates of 400°C /h, resulted in detachment of the LSCF thin film and thus reduced durability. To improve the adhesion between the LSCF and GDC, (La,Sr)CoO3(LSC), nano particles were deposited on the GDC thin films by PLD. This step was expected to increase contact area between electrolyte and cathode. We investigated the influence of the LSC nano particle on thermal reliability and I-V characteristic of SOFCs. 2.Experimental Details NiO-YSZ cermet (NiO:YSZ＝60:40 wt%, 500 μm thick) was used as the anode substrate, YSZ (8 mol% Y2O3) and GDC (10 mol% GdO2) as electrolyte materials, and La0.6Sr0.4Co0.2Fe0.8 (LSCF) and La0.6Sr0.4Co (LSC) as cathode materials. YSZ (8 μm thick) and LSCF (30 μm thick) films were prepared by screen printing and GDC (3 μm thick) was deposited by PLD. Two sets of unit cells were prepared: one set was with LSC nano particles deposited between GDC and LSCF thin films by PLD (NiO-YSZ / YSZ / GDC / LSC particle / LSCF) and the other set was without LSC nano particles (NiO-YSZ / YSZ / GDC / LSCF).Thermal cycle test between 200 and 600°C with heating and cooling rates of 400° C /h were performed. Power densities and I-V characteristics were measured. 3.Results (a) Deposition of LSC nano particles by PLD on the GDC thin film. Figure 1 shows a scanning electron microscopy image of the surface of a GDC thin film. Many LSC nano particles (50-nm diameter) were deposited on the dense GDC thin film by PLD, which increased the area of contact between the GDC and LSCF thin films. (b) Thermal cycle test of NiO-YSZ anode supported cells with or without LSC nano particles. Two types of unit cells, (1) NiO-YSZ / YSZ / GDC / LSC particle / LSCF and (2) NiO-YSZ / YSZ / GDC / LSCF were prepared by screen printing and PLD. Photographs of unit cells after one thermal cycle are shown in Fig. 2. It is evident that adding the LSC nano particles succeeded in suppressing detachment of the LSCF thin film. Spike effect resulted in high adhesion between LSCF and GDC. The unit cell with LSC nano particles survived 50 thermal cycles and yielded higher thermal reliability. (c) I-V characteristics of NiO-YSZ anode supported cells with or without LSC nano particles. Figure 3 shows the I-V characteristics of unit cells with and without LSC nano particles. The maximum power densities of unit cells with or without LSC nano particles were 0.5 and 0.3 W·cm-2, respectively. Cell performances were found to be better when LSC nano particles were deposited, because of increase in reactive sites.[1] T. Mukai et al., Journal of Fuel Cell Science and Technology , 10(2013) 061006-1-6.

Cross-Linked Carbon Nanotube Heat Spreader

ABSTRACTAmong the exceptional properties of isolated individual carbon nanotubes (CNTs), exceptional thermal conductivity along their axis has been demonstrated, However they have also shown poor thermal transfer between adjacent CNTs. Thick bundles of aligned CNTs have been used as heat pipes, but the thermal input and output power densities are the same, providing no heat spreading effect. We demonstrate the use of energetic argon ion beams to join overlapping CNTs in a thin film to form an interpenetrating network with an isotropic thermal conductivity of 2150 W/m K. Such thin films may be used as heat spreaders to enlarge the thermal footprint of laser diodes and CPU chips, for example, for enhanced cooling. At higher ion energies and fluence, the CNTs appear to collapse and reform, aligned parallel to the ion beam axis, and form dense high aspect ratio tapered structures. The high surface area of these structures lends themselves to applications in energy storage, for example. We consider the mechanisms of energetic ion interaction with CNTs and junction formation of two overlapping CNTs during the subsequent self-healing process, as well as the formation of high aspect ratio structures under more extreme conditions

Investigation on the effects of internal reflector size on the neutron flux of PBMR reactors by means of MCNP code

In this paper, the effects of internal reflector size on the neutron flux of PBMR reactors by means of MCNP code has been investigated as a part of establishing Monte Carlo computation system for PBMR core analysis. At first, high temperature library in this reactor is built by NJOY code then, cross sections in the library are used by MCNP code. Because of complex fuel structure, several simplifications are assumed in order to limit the need for any further approximations when defining code models. In neutronic simulation, at first, all the important neutronic parameters such as thermal and fast flux distribution and power density of the reactor core are compared with the results of other codes and the accuracy of the simulation has been validated. Then the best size of internal reflector has been chosen by comparing the neutron flux in some different sizes and obtaining the best form factor.

Thermal design and optimization of small-scale high effectiveness cross-flow heat exchangers

This paper addresses the question to what extent parallel-plate cross-flow heat exchangers can be downsized without loss of thermal-hydraulic performance. It is shown that when the characteristic length scales of the channels are reduced at a constant pressure drop, the effectiveness exhibits a maximum due to axial heat conduction in the plate material. The point of maximal effectiveness is found to correspond to a maximal thermal power density and thus to the minimal volume required for obtaining a given effectiveness. From an asymptotic analysis, closed-form expressions are derived for the optimal geometric parameters that maximize power density. The resulting expressions define optimal elemental units that allow constructing parallel-plate cross-flow heat exchangers with the lowest possible core volume for effectiveness values between 0.55 and 1.

Metal Additive Manufacturing of a High-pressure Micro-pump

For the thermal control of future space applications pumped two-phase loops are an essential part to handle the increasing thermal power densities. This study investigates the design of a reliable, leak tight, low-weight and high-pressure micro-pump for small satellite applications. The developed micro-pump uses a piezoelectric disk to create a pressure head and propel the working fluid. The micro-pumps are produced from Titanium alloy (Ti6Al4V) using Selective Laser Melting (SLM) as a metal additive manufacturing technique. Two types of micro-valve designs are explored, namely (1) no moving parts valves and (2) passive check valves. Experimental validation shows promising results especially for micro-pumps using passive check micro-valves. Finally, the assembly of the entire micro-pump has been designed for SLM production. In the end, this would enable the future use of micro- pumps for space applications. Metal additive manufacturing, as relatively new manufacturing system, showcases promising results for both research work and final production.

Structural and electrical properties of transparent conducting Al2O3-doped ZnO thin films using off-axis DC magnetron sputtering

Structural and electrical properties of off-axis DC magnetron sputtered Al2O3-doped ZnO (AZO) films were systematically investigated as a function of deposition distance from the center. With increasing distance, the AZO films showed an enhanced crystallinity and a denser microstructure with a smooth surface. The AZO film sputtered at the edge of the deposition stage (∼6cm away from the center) showed the highest mobility (∼10.1cm2/Vs) and the lowest resistivity (∼2×10−3Ωcm) due to the high plasma and thermal power density, which was suitable for transparent conducting oxide applications.

Application of the Spark Plasma Sintering Technique to Low-Temperature Copper Bonding

Planar structures, in which a power die is soldered on a substrate and wirebonds are used to connect the top of the die with the substrate, are limited in terms of thermal management and power density. 3-D packaging techniques have been proposed to overcome these limits. Here, an innovative copper-to-copper bonding solution is presented, that can be used for 3-D packaging. The bonding process is described and the effect of the bonding parameters is investigated. It is found that this technique is compatible with the requirements of power electronic packaging. A test assembly including a silicon power die and ceramic substrates is presented.

Simulation and optimization of heat flow via anisotropic material thermal conductivity

This article is focused on heat flow control within a composite material that has designed anisotropic thermal conductivity. The optimal conductive heat transfer path in the composite is specified via customized local scale properties, where the physical parameter distribution is found using an iterative procedure that couples a gradient based optimization routine with a finite element solver. A sample optimization result is presented to illustrate the procedure, and the final solution is translated into a physical embodiment having heterogeneous material properties. Numerical experiments were performed both on this synthesized material and a baseline homogeneous material with the same filler volume fraction. Heat transfer results indicate a substantial reduction in overall temperature with effective concentration of thermal power density in the designed material.

Medium wave exposure characterisation using exposure quotients

One of the aspects considered in the International Commission on Non-Ionizing Radiation Protection guidelines is that, in situations of simultaneous exposure to fields of different frequencies, exposure quotients for thermal and electrical stimulation effects should be examined. The aim of the present work was to analyse the electromagnetic radiation levels and exposure quotients for exposure to multiple-frequency sources in the vicinity of medium wave radio broadcasting antennas. The measurements were made with a spectrum analyser and a monopole antenna. Kriging interpolation was used to prepare contour maps and to estimate the levels in the towns and villages of the zone. The results showed that the exposure quotient criterion based on electrical stimulation effects to be more stringent than those based on thermal effects or power density levels. Improvement of dosimetry evaluations requires the spectral components of the radiation to be quantified, followed by application of the criteria for exposure to multiple-frequency sources.