An enhanced thermal conduction model for the prediction of convection dominated solid–liquid phase change

An enhanced thermal conduction model for predicting convection dominated solid–liquid phase change is presented. The main feature of the model is to predict (1) the overall thermal behavior of the system and (2) the phase front position without recurring to the full solution of the Navier–Stokes equations. The model rests entirely on the conduction equation for both the solid and liquid phases. The effect of convection in the melt is mimicked via an enhanced thermal conductivity that depends on the dimensionless numbers and the geometry of the flow. The model is tested and confronted to full CFD solutions for a freezing duct flow problem and for buoyancy driven melting in an enclosure. In both cases, the predictions of the enhanced thermal conduction model show excellent agreement with that of the CFD model. Not only is the enhanced thermal conduction model simpler to implement but its simulations run at least ten times as fast as those of the CFD model. Consequently, the enhanced thermal conduction model is well suited for controlling real-time solid–liquid phase change processes that occur in industrial applications as well as in latent heat thermal energy storage systems.     

Control-oriented thermal management of solid oxide fuel cells based on a modified Takagi–Sugeno fuzzy model

Thermal management for a solid oxide fuel cell (SOFC) is actually temperature control, due to the importance of cell temperature for the performance of an SOFC. An SOFC stack is a nonlinear and multi-variable system which is difficult to model by traditional methods. A modified Takagi–Sugeno (T–S) fuzzy model that is suitable for nonlinear systems is built to model the SOFC stack. The model parameters are initialized by the fuzzy c-means clustering method, and learned using an off-line back-propagation algorithm. In order to obtain the training data to identify the modified T–S model, a SOFC physical model via MATLAB is established. The temperature model is the center of the physical model and is developed by enthalpy-balance equations. It is shown that the modified T–S fuzzy model is sufficiently accurate to follow the temperature response of the stack, and can be conveniently utilized to design temperature control strategies.     

Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10⁵–10⁸) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart.     

A new target-oriented methodology of decreasing the regeneration temperature of solid–gas thermochemical sorption refrigeration system driven by low-grade thermal energy

In this paper, an innovative target-oriented desorption methodology for decreasing the regeneration temperature of solid–gas thermochemical sorption refrigeration system is presented. This method uses a two-stage desorption thermodynamic cycle with two different reactive salts to decrease the overall driving heat source temperature. The working principle of the proposed desorption methodology is based on the different thermochemical equilibrium characteristics of reactive salts. Experimental verification showed that the proposed two-stage desorption methodology is feasible and effective in lowering the regeneration temperature of solid–gas thermochemical sorption refrigeration system. Moreover, the extent to which the regeneration temperature is lowered can be regulated by choosing the appropriate secondary reactive salt according to the available driving heat source temperature. The presented target-oriented desorption methodology can contribute to the widening of the scope of application of thermochemical sorption refrigeration technology utilizing low-grade thermal energy or renewable energy.     

Retrieval of thermal properties in a transient conduction–radiation problem with variable thermal conductivity

This article reports an inverse analysis of a transient conduction–radiation problem with variable thermal conductivity. Simultaneous retrieval of parameters is accomplished by minimizing the objective function represented by the square of the difference between the measured and the assumed temperature fields. The measured temperature field is calculated from the direct method involving the lattice Boltzmann method (LBM) and the finite volume method (FVM). In the direct method, the FVM is used to obtain the radiative information and the LBM is used to solve the energy equation. With perturbations imposed on the measured temperature data, minimization of the objective function is achieved with the help of the genetic algorithm (GA). The accuracies of the retrieved parameters have been studied for the effects of the genetic parameters such as the crossover and the mutation rates, the population size, the number of generations and the effect of noise on the measured temperature data. A good estimation of parameters has been obtained.     

Solar–gas solid sorption heat pump

Solid sorption short cycle heat pump (⩽10 kW) which uses physical adsorption and is of interest to the space and domestic application is designed and tested. This heat pump has a very short (12 min), nonintermittent, two adsorber heat recovery cycles with an active carbon fiber as a sorbent bed and ammonia as a working fluid. It has two energy sources: solar and gas flame. The system management consists only in actuating the special type valves to change the direction of the heating circuit and water valves to change the water cooling circuit.     

Low thermal conductivity for Sr1−xLaxTiO3

Abstract

N type thermoelectric materials Sr_1?xLa_xTiO₃ (x?=?0·02, 0·06 and 0·10) have been obtained by hydrothermal method and sintering at 900 and 1100°C. The thermoelectric properties have been measured, and all of the samples in the measured temperature range have been found to be insulating. Very low thermal conductivity κ?=?0·75 W m^?1 K^?1 is found at 500 K for Sr_0·9La_0·1TiO₃ after sintering at 900°C. With increasing doping level x, the thermal conductivity κ decreases. With increasing sintering temperature, the grain size of samples increases, and the thermal conductivity increases. This phenomenon is ascribed to the strong suppression of the electron and phonon conductivities due to scattering by the numerous grain boundary scattering.     

Electrochemical and thermal properties of SmBa0.5Sr0.5Co2O5+δ cathode impregnated with Ce0.8Sm0.2O1.9 nanoparticles for intermediate-temperature solid oxide fuel cells

SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC55) impregnated with nano-sized Ce_0.8Sm_0.2O_1.9 (SDC) powder has been investigated as a candidate cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The cathode chemical compatibility with electrolyte, thermal expansion behavior, and electrochemical performance are investigated. For compatibility, a good chemical compatibility between SBSC55 and SDC electrolyte is still kept at 1100 °C in air. For thermal dilation curve, it could be divided into two regions, one is the low temperature region (100–265 °C); the other is the high temperature region (265–850 °C). In the low temperature region (100–265 °C), a TEC value is about 17.0 × 10^?6 K^?1 and an increase in slope in the higher temperatures region (265–800 °C), in which a TEC value is around 21.1 × 10^?6 K^?1. There is an inflection region ranged from 225 to 330 °C in the curve of d(δL/L)/dT vs. temperature. The peak inflection point located about 265 °C is associated to the initial temperature for the loss of lattice oxygen and the formation of oxygen vacancies. For electrochemical properties, the polarization resistances (R_p) significantly reduced from 4.17 Ω cm² of pure SBSC55 to 1.28 Ω cm² of 0.65 mg cm^?2 of SDC-impregnated SBSC55 at 600 °C. The single cell performance of SBSC55∣SDC∣Ni-SDC loaded with 0.65 mg cm^?2 SDC exhibited the optimum power density of 823 mW cm^?2 at operating temperature of 800 °C. Based on above-mentioned properties, SBSC55 impregnated with an appropriate SDC is a potential cathode for IT-SOFCs.     

Miniature heat-pipe thermal performance prediction tool – software development

The software for flat miniature heat-pipe parameters (Q_max, R_hp, temperature field along the pipe surface, heat transfer coefficients in the evaporator and condenser zones h_e, h_c, etc.) prediction and numerical modeling was developed. The experimental data received for the flat miniature heat pipe (2.5–4 mm thickness, 50–250 mm length, 8–11 mm width) with a copper sintered powder wick saturated with water were compared with the data of numerical analysis and results showed that experimental verification testifies the validity of the software application.     

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

In this paper, a novel process for the production of pure hydrogen from natural gas based on the integration of solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs) is presented. In this configuration, the SOFC is fed by natural gas and provides electricity and heat to the SOEC, which carries out the separation of steam into hydrogen and oxygen. Depending on the system layout considered, the oxygen available at the SOEC anode outlet can be either mixed with the SOFC cathode stream in order to improve the SOFC performance or regarded as a co-product. Two configurations of the cell stack are studied. The first consists of a stack with the same number of SOFCs and SOECs working at the same current density. In this case, since in typical operating conditions the voltage delivered by the SOFC is lower than the one required by the SOEC, the required additional power is supplied by means of an electric grid connection. In the second case, the electricity balance is compensated by providing additional SOFCs to the stack, which are fed by a supplementary natural gas feed. Simulations carried out with Aspen Plus show that pure hydrogen can be produced with a natural gas to hydrogen LHV-efficiency that is about twice the value of a typical water electrolyzer and comparable to that of medium-scale reformers.     

Performance evaluation of photovoltaic/thermal–HDH desalination system

The efficiency of photovoltaic (PV) panel drops with increase in cell temperature. The temperature of the PV panel can be controlled with various cooling techniques. In the proposed work the PV panel is cooled by circulating water and the recovered heat energy is used to run a humidification and dehumidification desalination to produce distilled water from sea water (or) brackish water. This work deals with a detailed analysis of performance of combined power and desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system. A mathematical model of PV/thermal–humidification dehumidification plant was developed and simulations were carried out in MATLAB environment. The performance of photovoltaic/ thermal desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system was investigated under various solar radiation levels (800–1000 W/m²). For each solar radiation level the effect of mass flow rate of coolant water (30–110 kg/h) on water outlet temperature, PV efficiency, PVT thermal efficiency, distilled water production, and plant efficiency was studied. Results show that under each solar radiation level increasing coolant flow rate increases efficiency of PV panel and reduces the plant efficiency. The highest PV efficiency (16.598%) was reached under 800 W/m² at mass flow rate of 110 kg/h and the highest plant efficiency (43.15%) was reached under 800 W/m² at a mass flow rate of 30 kg/h. The maximum amount of distilled water production rate (0.82 L/h) was reached under 1000 W/m² at water mass flow rate of 30 kg/h.     

Temperature-explicit formulation of energy equation for thermal–hydraulic analyses

A temperature equation which is derived from an enthalpy transport equation by using an assumption of a constant specific heat is very attractive for analyses of heat and fluid flows. It can be used for an analysis of a solid–fluid conjugate heat transfer, and it does not need a numerical method to obtain temperature from a temperature–enthalpy relation. But its application is limited because of the assumption. A new method is derived in this study, which is a temperature-explicit formulation of the energy equation. The enthalpy form of the energy equation is used in the method. But the final discrete form of the equation is expressed with temperature. The discretized equation from the temperature-explicit formulation can be used for a heat transfer analysis in a solid–fluid coupled region without any special treatment at the solid–fluid interface. And it can be applied for multiphase flows with a real gas effect. It is found by numerical tests in this study that the proposed method is very efficient and as accurate as the standard enthalpy formulation.     

Forced oscillation in diffusion flames near diffusive–thermal resonance

In this work, we carry out a systematic analysis of forced oscillation in planar diffusion flames under weak external forcing. The external forcing is introduced by independently imposing a flow field with small amplitude fluctuations. Employing the asymptotic theory of Cheatham and Matalon, the linear response is first examined. It is shown that when the Damköhler number Da is close to the critical value Da¹ corresponding to the marginal state of diffusive–thermal pulsating instability, the imposed velocity fluctuation may induce very large amplitude of flame oscillation as the frequency of velocity fluctuation c approaches c₀, the flame oscillation frequency at the onset of instability. This is a resonance phenomenon between the imposed flow oscillations and the intrinsic flame oscillations that are driven by the diffusive–thermal instability, and hence we refer to this as the diffusive–thermal resonance. The nonlinear near-resonant response is then examined with the Damköhler number Da chosen to be very close to the critical Damköhler number Da¹, and we derive an evolution equation for the amplitude of forced oscillation. Examination of the evolution equation reveals that in most situations, flames with larger Lewis number of fuel, smaller initial mixture strength, and smaller temperature difference between the oxidant and fuel stream are more responsive to the external forcing.     

Numerical study of a dense gas–solid jet flow

A new approach using both Eulerian and Lagrangian coordinates and taking account of inter-particle interactions has been developed for the study of gas–solid flows. A numerical algorithm is presented. Comparison with experimental results shows reasonably good agreement.     

Study of chemical looping co-gasification (CLCG) of coal and rice husk with an iron-based oxygen carrier via solid–solid reactions

The chemical looping gasification (CLG) is a promising gasification technology for syngas production. It reduces the demand for pure oxygen and heat from outside by the cycle of oxygen carriers. The lattice oxygen is transferred by oxygen carrier like Fe₂O₃ in CLG. Considering the synergy between lignite and rice husk, the chemical looping co-gasification (CLCG) of lignite and rice husk with Fe₂O₃ as oxygen carrier was studied in this work. The mass loss of lignite increased by about 3% with the help of rice husk. Due to the synergetic effect, rice husk developed the pyrolysis of coal in the co-gasification. It is found that the most contributing reaction at around 800 °C–1000 °C in CLG is the gasification of char with Fe₂O₃via solid-solid reactions. The kinetic fitting was used to explore the reaction mechanism of CLCG. The modified random pore model (MRPM) fitted the experimental data well, which confirmed the solid-solid reactions between char and Fe₂O₃, and the synergy between lignite and rice husk in CLCG. Finally, the gas analysis was conducted in a fixed bed system with gas analyzers. It is found that Fe₂O₃ enhanced the concentration of CO and CO₂ in CLG process.     

Numerical study of mixed convection within porous square cavities using Bejan’s heatlines: Effects of thermal aspect ratio and thermal boundary conditions

The present numerical study deals with mixed convection flows within square enclosures filled with porous media. The influence of various thermal boundary conditions on bottom and side walls based on thermal aspect ratio (A) is investigated for a wide range of parameters (1 ? Re ? 100, 0.015 ? Pr ? 7.2, 10^?5 ? Da ? 10^?3 and 10³ ? Gr ? 10⁵). A penalty finite element method with bi-quadratic elements has been used to investigate the results in terms of streamlines, isotherms and heatlines and average Nusselt numbers. Lid driven effect is dominant at low Darcy number (Da = 10^?5), whereas buoyancy driven effect is dominant at high Darcy numbers (Da = 10^?4 and Da = 10^?3) for Re = 1. Asymmetric pattern is observed in isotherms and heatlines for Re = 100. It is found that thermal gradient is high at the center of the bottom wall for A = 0.1 due to large dense heatlines at that zone and that is low for A = 0.9 irrespective of Re, Pr and Gr. Overall heat transfer rates are higher for A = 0.1 compared to other thermal aspect ratios (A = 0.5, A = 0.9) irrespective of Darcy number, Prandtl number and Reynolds number.     

KNO3/NaNO3 – Graphite materials for thermal energy storage at high temperature: Part I. – Elaboration methods and thermal properties

Composites graphite/salt for thermal energy storage at high temperature (～200 °C) have been developed and tested. As at low temperature in the past, graphite has been used to enhance the thermal conductivity of the eutectic system KNO₃/NaNO₃. A new elaboration method has been proposed as an alternative to graphite foams infiltration. It consists of cold-compression of a physical mixing of expanded natural graphite particles and salt powder. Two different compression routes have been investigated: uni-axial compression and isostatic compression. The first part of the paper has been devoted to the analysis of the thermal properties of these new graphite/salt composites. It is proven that cold-compression is a simple and efficient technique for improving the salt thermal conductivity. For instance, graphite amounts between 15 and 20%wt lead to apparent thermal conductivities close to 20 W/m/K (20 times greater than the thermal conductivity of the salt). Furthermore, some advantages in terms of cost and safety are expected because materials elaboration is carried out at room temperature. The second part of the paper is focused on the analyses of the phase transition properties of these graphite/salt composites materials.     

Interface thermal characteristics of flip chip packages – A numerical study

Flip chip ball grid array (FC-BGA) packages are commonly used for high inputs/outputs (I/O) ICs; they have been proven to provide good solutions for a variety of applications to maximize thermal and electrical performance. A fundamental limitation to such devices is the thermal resistance at the top of the package, which is characterized θ_JC parameter. The die-to-lid interface thermal resistance is identified as a critical issue for the thermal management of electronic packages. This paper focuses on the effect of the interface material property changes on the interface thermal resistance. The effect of package’s junction to case (Theta-JC or θ_JC) thermal performance is investigated for bare die, flat lid and cup lid packages using a validated thermal model. Thermal performance of a cup or flat lid attached and bare die packages were investigated for different interface materials. Improved Theta-JC performance was observed for the large die as compared to the smaller die. Several parametric studies were carried out to understand the effects of interface bond line thickness (BLT), different die sizes, the average void size during assembly and thermal conductivity of interface materials on package thermal resistance.     

Pressurized thermal shock screening criteria re-evaluation effort — US industry activities

A joint project is under way between the US nuclear power industry and the US Nuclear Regulatory Commission (NRC) to re-evaluate the pressurized thermal shock (PTS) screening criteria as presently defined in title 10 of the Code of Federal Regulations, 10 CFR 50.61. Advances in probabilistic risk assessment (PRA), probabilistic fracture mechanics (PFM), thermal hydraulics (TH), and overall plant risk assessment are being incorporated into a comprehensive program to establish a technical basis for revising the present screening criteria. US industry activities are being coordinated through the Reactor Pressure Vessel Integrity Issue Task Group (RPV Integrity ITG) of the EPRI Materials Reliability Project (MRP). The EPRI MRP was formed in 1998 to identify and address issues that could affect operability of major components in pressurized water reactor (PWR) plants. Major activities are coordinated with the nuclear steam supply system (NSSS) vendors, the vendor owner's groups, The Nuclear Energy Institute (NEI), and the NRC. The MRP provides for a unified industry approach to resolution of technical and regulatory issues related to PWR materials degradation. A major task under the Reactor Pressure Vessel Integrity Issue Task Group (RPV Integrity ITG) is to support industry activities associated with the pressurized PTS re-evaluation effort. This paper provides a brief overview of the EPRI MRP program, the RPV Integrity ITG, and industry activities associated with the PTS re-evaluation effort.