Thermal diffusivity measurement of glass at high temperature by using flash method

A measurement of the thermal diffusivity of a semi-transparent material (glass) by means of the "Flash Method" is investigated in the present work. By taking into account the heat losses on the two faces of the sample, and using a new experimental technique design, an improvement of the determination of the thermal diffusivity of the semi-transparent material (glass) at high temperature is realized. The experimental design presented here is an original technical concept that enables a significant reduction in heat loss during the experiments. A very simple model based on the quadrupole method is used to theoretically determine the thermal diffusivity of the semi-transparent material by taking into account both conduction and radiation. Theoretical results clarify the effect of the absorption coefficient and the thickness of the sample on the heat transfer in the semi-transparent medium.     

Solar heating of a fluid through a semi-transparent plate: Theory and experiment

A theoretical and experimental study of solar radiation passing through a thin semi-transparent slab to heat a fluid is presented. The system of differential equations describing the temperature of the slab and the fluid as a function of time is derived and solved; the theoretical curves generated by the solution for the fluid temperature are compared with experimental readings obtained using water as the fluid and acrylic plastic (methyl---methacrylate) as the semi-transparent material. The theoretical solution assumes that the solid absorbs radiation according to Beer's law and that the fluid completely absorbs the transmitted radiation. The transient theoretical fluid temperature curves for heating oil through plastic and through an opaque (copper) plate are compared; a general criterion for steady-state has been derived, showing that, for the same parameters of solar intensity, convection coefficients, ambient temperature and container dimensions, the fluid temperatures attained by using semi-transparent materials are considerably higher than those obtained with opaque plates.     

Transient temperature and thermal stress profiles in semi-transparent particles under high-flux irradiation

Transient temperature and thermal stress profiles in semi-transparent spherical particles heated by concentrated solar radiation are studied by means of a theoretical model. The analysis of radiative–conductive interaction is based on the spectral radiation transfer model in a refracting and absorbing particle. The stress–strain state of the particle is described by the thermoelastic approach. An analytical self-similar solution for the particle temperature profiles and thermal stresses during the quasi-steady period of the particle heating is derived. It is shown that the circumferential tensile stress near the particle surface is a non-monotonic function of the particle radius. The range of physical parameters corresponding to the maximal tensile stress near the particle surface is determined. The model is applied to ZnO and CaCO₃ particles, which are used as reactants in industrially-relevant high-temperature processes. It is shown that tensile stresses in the selected types of particles exposed to concentrated solar radiation cannot lead to their mechanical destruction. At the same time, the considerable temperature difference and thermal stresses in non-isothermal particles can be an interesting issue in a detailed analysis of the thermal decomposition of semi-transparent particles.     

Equivalent circuit model parameters of a high-power Li-ion battery: Thermal and state of charge effects

Equivalent circuit model (EMC) of a high-power Li-ion battery that accounts for both temperature and state of charge (SOC) effects known to influence battery performance is presented. Electrochemical impedance measurements of a commercial high power Li-ion battery obtained in the temperature range 20 to 50 °C at various SOC values was used to develop a simple EMC which was used in combination with a non-linear least squares fitting procedure that used thirteen parameters for the analysis of the Li-ion cell. The experimental results show that the solution and charge transfer resistances decreased with increase in cell operating temperature and decreasing SOC. On the other hand, the Warburg admittance increased with increasing temperature and decreasing SOC. The developed model correlations that are capable of being used in process control algorithms are presented for the observed impedance behavior with respect to temperature and SOC effects. The predicted model parameters for the impedance elements R_s, R_ct and Y₀₁₃ show low variance of 5% when compared to the experimental data and therefore indicates a good statistical agreement of correlation model to the actual experimental values.     

Viscosity of nanofluids based on an artificial intelligence model

By using an FCM-based Adaptive neuro-fuzzy inference system (FCM-ANFIS) and a set of experimental data, models were developed to predict the effective viscosity of nanofluids. The effective viscosity was selected as the target parameter, and the volume concentration, temperature and size of the nanoparticles were considered as the input (design) parameters. To model the viscosity, experimental data from literature were divided into two sets: a train and a test data set. The model was instructed by the train set and the results were compared with the experimental data set. The predicted viscosities were compared with experimental data for four nanofluids, which were Al₂O₃, CuO, TiO₂ and SiO₂, and with water as base fluid. The viscosities were also compared with several of the most cited correlations in literature. The results, which were obtained by the proposed FCM-ANFIS model, in general compared very well with the experimental measurement.     

Experimental characterization and mechanistic modeling of carbon monoxide fueled solid oxide fuel cell

The paper presents an elementary reaction based solid oxide fuel cell (SOFC) model coupled with anodic elementary heterogeneous reactions and electrochemical charge transfer reactions for CO/CO₂ fuel based on an anode supported button cell. The model is calibrated and validated using experimental data obtained for various CO/CO₂ fuel compositions at 750, 800 and 850 °C. The comparison shows that the modeling results agree well with the experimental data. The effects of operating conditions on the cell performance and the detailed species concentration distribution are predicted. Then, the carbon deposition on the SOFC anode with CO/CO₂ fuel is experimentally measured and simulated using the elementary reaction model. The results indicate that lower temperature and lower operation voltage are helpful to reduce the possibilities of carbon deposition on Ni particle surfaces.     

Combined wall-to-fluidized bed heat transfer. Bubbles and emulsion contributions at high temperature

A theoretical and experimental investigation of combined wall-to-fluidized bed heat transfer is presented. Bubble and emulsion contributions are modelled assuming both phases to be semi-transparent grey media. For a radiant bubble phase component the general situation of absorbing gases including particulate is described. Tests were performed with a two-dimensional fluidized bed column in the temperature range 500–900°C. Experimental and theoretical results emphasized that (i) a non-linear relation exists between h and T_b if the conduction-to-radiation parameter (N) is smaller than 5, indicating that radiative and conductive contributions are not independent, (ii) the dense phase radiant contribution becomes significant at temperatures higher than 700°C, (iii) at 900°C the bubble phase radiative component cannot be neglected.     

Investigation of thermal aspects of hydrogen storage in a LaNi5‐H2 reactor

In this work, hydrogen absorption in a LaNi₅‐H₂ reactor is investigated experimentally and numerically. Experimental measurements were carried out on a cylindrical metal‐hydride reactor filled with LaNi₅ alloy. During the experiments hydrogen was charged at a constant pressure. The performance of the reactor during hydriding process was obtained at different fluid temperatures and hydriding process was identified from measured temperature histories. The temperature changes in the reactor were measured at several locations and recorded in a computer. The numerical simulation of the reactor was also performed. A two‐dimensional mathematical model has been established and solved numerically by the method of finite volume for the simulation. The numerical results are compared with the measured data to validate the mathematical model. The predicted results are in good agreement with the experimental measurements. Copyright © 2005 John Wiley & Sons, Ltd.     

Modeling and simulating the transcritical CO2 heat pump system

In this paper, a mathematical model for steady-state simulation of transcritical CO₂ water-to-water heat pump system with an expander has been developed. It is used to simulate the performance of transcritical CO₂ system with CO₂ expander prototype. Simulated results are compared with experimental data to verify the accuracy of the simulation model. The comparison results show the average deviation of about 15% for COP_c(cooling coefficient of performance) and COP_h(heating coefficient of performance), about 17% for cooling and heating capacity at experimental high pressure ranges. With this model, which has been validated in a limited high pressure range, the influence of water mass flow rate and water inlet temperature of both evaporator and gas cooler on the performance of transcritical CO₂ expander system is analyzed. The results show that decreasing inlet temperature and increasing mass flow rate of cooling water cannot only increase the system performance but also reduce the optimal heat rejection pressure, at which the maximum COP (coefficient of performance) can be obtained. For chilling water, increasing its inlet temperature and mass flow rate is favorable for increasing the system performance, while the optimal heat rejection pressure does not vary very much.     

Supercritical water gasification of oil-containing wastewater with a homogeneous catalyst: Detailed reaction kinetic study

Supercritical water gasification (SCWG) is a promising technology for oil-containing wastewater treatment. This paper aims to establish a reaction kinetic model to provide better guidance for optimal industrial reactor design. The model is developed based on the experimental results obtained from K₂CO₃-catalyzed SCWG of diesel (the model compound of oil containment in wastewater) at various conditions of 500–650 °C and 15.23–64.45 s. Then the model validation by using the experimental data from other conditions. The validation results showed that the kinetic model can predict the gas concentration with an acceptable deviation. Afterward, the indicators of carbon gasification efficiency and gas yield versus residence time are predicted. The results show that the required residence time for the complete gasification is varied from 214.2 to 2150.8 s when the temperature changes from 500 to 650 °C. Moreover, the reaction rate analysis result indicates that the two reactions contributed most to the hydrogen production are the forward water-gas shift and the reverse CO methanation reaction. Additionally, the sensitivity analysis result reveals that the hydrolysis reaction of diesel has a significant influence at the initial stage, while the CO and CO₂ methane reactions play a vital role at the late stage for gas production.     

Modeling the effective thermal conductivity of nanofluids using full factorial design analysis

In this paper, a full factorial design analysis is proposed for predicting nanofluid thermal conductivity ratio (TCR) as well as determining the effects of critical factors and their interactions. A statistical design of experiment approach with three variables (volume fraction, temperature, and nanoparticle diameter) at two levels is carried out. Three types of oxide‐water nanofluids (Al₂O₃‐water, CuO‐water, and TiO₂‐water) are used to evaluate the effectiveness of the proposed mathematical model. The significance and adequacy of the regression model were evaluated by the analysis of variance. The predicted model has a root mean square error equals to 0.0074, R² = 0.99, and P < .0013, thus showing good results compared to a set of experimental data as well as other mathematical model results. The results illustrate that the TCR of metallic oxide nano?uids increases with temperature and nanoparticles volume fraction but decreases when nanoparticle size intensi?es. Furthermore, it is found that the nanoparticles volume fraction has a great impact on the nano?uids thermophysical properties. Finally, the obtained results confirm that the proposed model is considerably accurate and capable of predicting nano?uids thermal conductivity and that it can be used with ease as an alternative to many other models.     

Increase of COP for heat transformer in water purification systems. Part I – Increasing heat source temperature

The integration of a water purification system in a heat transformer allows a fraction of heat obtained by the heat transformer to be recycled, increasing the heat source temperature. Consequently, the evaporator and generator temperatures are also increased. For any operating conditions, keeping the condenser and absorber temperatures and also the heat load to the evaporator and generator, a higher value of COP is obtained when only the evaporator and generator temperatures are increased. Simulation with proven software compares the performance of the modeling of an absorption heat transformer for water purification (AHTWP) operating with water/lithium bromide, as the working fluid–absorbent pair. Plots of enthalpy-based coefficients of performance (COP_ET) and the increase in the coefficient of performance (COP) are shown against absorber temperature for several thermodynamic operating conditions. The results showed that proposed (AHTWP) system is capable of increasing the original value of COP_ET more than 120%, by recycling part of the energy from a water purification system. The proposed system allows to increase COP values from any experimental data for water purification or any other distillation system integrated to a heat transformer, regardless of the actual COP value and any working fluid–absorbent pair.     

Thermal radiation effects in phase-change energy-storage systems

A numerical model was developed to determine the transient temperature distribution, solid/liquid interface location, and energy-storage capacity of a semi-transparent phase-change medium. The medium is bounded between two concentric cylinders and internal energy transfer occurs simultaneously by conduction and thermal radiation. The radiation transport equation was coupled with the energy equation; both enthalpy and temperature were employed as dependent variables. The spherical harmonic approximation (P-N approximation) was used to obtain solutions for the radiative heat flux. The coupled conservation of energy and moment differential equations were solved by using iterative numerical finite-difference schemes with appropriate thermal and radiant boundary and interface conditions. The numerical model was used to study the effects of radiation on solidification (melting), transient temperature distribution and energy-storage capacity of an absorbing, emitting, and isotropically scattering, semi-transparent, gray medium contained in a cylindrical annulus. The results increase our understanding of internal energy transfer and show the effects of optical properties, conduction/radiation parameter, and geometric dimensions and should lead to better designs and optimization of phase-change energy-storage systems.     

Thermal dispersion for water or air flow through a bed of glass beads

The one-temperature model for thermal dispersion in a porous medium is based on the notion of an average ‘enthalpic’ temperature, solution of an energy equation of the convection–diffusion type. It requires the determination of thermal dispersion coefficients. The functional form of the correlations that relate them to dimensionless groups is established as well as the limits of this model. An experimental bench has been built to measure these coefficients for water or air flow through a bed of glass beads. They are estimated through a Bayesian inversion of several local temperature measurements, with uncertainty on their location, and with the use of the analytical solution of a corresponding model. Results are presented in terms of variation of these two coefficients with the Reynolds or Péclet numbers and with the nature of the fluid and corresponding correlations are given.     

Lifted methane-air jet flames in a vitiated coflow

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H₂/air flame. The fuel stream consists of CH₄ mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.     

Experimental and numerical investigations of the effects of PBI loading and operating temperature on a high-temperature PEMFC

In this paper, experimental and numerical investigations of the effects of polybenzimidazole (PBI) loading and operating temperature on a high-temperature proton exchange membrane fuel cell (PEMFC) performance are carried out. Experiments related to a PBI-based PEMFC are performed and a two-dimensional (2-D) simulation model is developed to numerically predict the cell characteristics. Variations of 5–30 wt% in PBI amount in the catalyst layer (CL) and 160–200 °C in cell temperature are considered. On the basis of the experimental and numerical results, the negative effect of PBI content and positive effect of operating temperature on the cell performance can be precisely captured. These effects can also be shown by measurements of the impedance spectrum and predictions of O₂ concentration and current density distributions. In addition, non-uniform distributions in the O₂ concentration and the current density in the cathode compartment are also shown in the model simulation results. Cell performance curves predicted by the present model correspond well with those obtained from experimental measurements, showing the applicability of this model in a PBI-based PEMFC.     

Approximate method for computation of glass cover temperature and top heat-loss coefficient of solar collectors with single glazing

An improved equation form for computing the glass cover temperature of flat-plate solar collectors with single glazing is developed. A semi-analytical correlation for the factor f—the ratio of inner to outer heat-transfer coefficients—as a function of collector parameters and atmospheric variables is obtained by regression analysis. This relation readily provides the glass cover temperature (T_g). The results are compared with those obtained by numerical solution of heat balance equations. Computational errors in T_g and hence in the top heat loss coefficient (U_t) are reduced by a factor of five or more. With such low errors in computation of T_g and U_t, a numerical solution of heat balance equations is not required. The method is applicable over an extensive range of variables: the error in the computation of U_t is within 2% with the range of air gap spacing 8 mm to 90 mm and the range of ambient temperature 0°C to 45°C. In this extended range of variables the errors due to simplified method based on empirical relations for U_t are substantially higher.     

Some new developments on coupled radiative-conductive heat transfer in glasses—experiments and modelling

The difficulty in obtaining reliable phonic thermal conductivity of glasses at high temperature leads the authors to propose a methodology based on an experimental and numerical investigation, in order to separate the conductive and radiative part in a combined heat transfer in semi-transparent materials. The thermal conductimeter is composed of a guard plane plate and a Mach-Zehnder interferometer, supplying the total heat flux and the temperature distribution. With such a device no contacts are needed between sample and hot plate or heat sink. These measurements are treated numerically by a nodal analysis modelling of simultaneous conductive-radiative heat transfer. A monodimensional (non-diffusing) non-gray analysis including multireflections was considered. Determinations of the temperature derivative of the refractive index and the infrared spectra of materials at high temperature have been carried out. Values of the phonic conductivity of silica glass up to 900 K and borosilica glass up to 750 K for samples with different thicknesses and frontier's emissivities have been obtained by an identification process. Results are in agreement with literature data.     

Effects of high pressure,high temperature and dilution on laminar burning velocities and Markstein lengths of iso-octane/air mixtures

Spherically expanding flames are employed to measure flame velocities, from which are derived the corresponding laminar burning velocities at zero stretch rate. Iso-octane/air mixtures at initial temperatures between 323 and 473 K, and pressures between 1 and 10 bar, are studied over an extensive range of equivalence ratios, using a high-speed shadowgraph system. Effects of dilution are investigated with nitrogen and for several dilution percentages (from 5 to 25 vol% N₂). Over 270 experimental values have been obtained, providing an exhaustive data base for iso-octane/air combustion. Experimental results are in excellent agreement with recently published experimental data. An explicit correlation giving the laminar burning velocity from the initial pressure, the initial temperature, the dilution rate, and the equivalence ratio is finally proposed. Computed results using the two kinetic schemes and the Cantera code are compared to the present measurements. It is found that the mechanisms yield substantially higher values of laminar flame velocities than the present experimental results. Effects of oxygen enrichment are also investigated. A linear trend relating the percentage of oxygen in air and the unstretched laminar burning velocity is observed. Effects of high pressure, high temperature, and high dilution rate on Markstein lengths are also studied. As already done for the laminar burning velocity, an empirical correlation is proposed to describe the Markstein length for burned gases as a function of initial temperature and pressure, for equivalence ratios between 0.9 and 1.1, which has never been done before in the literature.