THERMAL CONDUCTlVlTY MEASUREMENTS ON WET PAPER SAMPLES AT HIGH TEMPERATURES

The effective thermal conductivity of a number of paper sheets was measured at high temperatures and various water contents. This was done by enclosing a pile of sheets between brass plates which were subjected to a sinussoidally varying temperature. The temperature variations at two positions in the pile were measured using thermocouples. From amplitude ratio and phase shift the conductivity was calculated. At low water contents, conductivity was proportional to the density of the paper and independent on temperature. At intermediate water contents, the conductivity increased markedly with temperature, attaining values higher than for pure water.     

THERMAL CONDUCTlVlTY MEASUREMENTS ON WET PAPER SAMPLES AT HIGH TEMPERATURES

ABSTRACT

The effective thermal conductivity of a number of paper sheets was measured at high temperatures and various water contents. This was done by enclosing a pile of sheets between brass plates which were subjected to a sinussoidally varying temperature. The temperature variations at two positions in the pile were measured using thermocouples. From amplitude ratio and phase shift the conductivity was calculated. At low water contents, conductivity was proportional to the density of the paper and independent on temperature. At intermediate water contents, the conductivity increased markedly with temperature, attaining values higher than for pure water.     

Lumped-parameter model for the retorting of a large block of oil shale with an internal temperature gradient

A lumped-parameter model which combines the internal and external heat transfer resistances was proposed for the retorting of a large block of oil shale. In this model the temperature of oil-shale block is obtained by solving the ordinary differential equation of energy balance using an overall heat transfer coefficient U, where U is equal to h/(1 + Bi/5). The results of this model were compared with those of the uniform-temperature model which uses the external heat transfer coefficient h, instead of U, in the energy balance equation and of the rigorous model which calculates the block temperature profile more rigorously by a partial differential equation. The comparison showed that the lumped-parameter model is much better than the uniform-temperature model and is a useful alternative to the rigorous model for modelling the retorting of a large block of oil shale with an internal temperature gradient.     

真空玻璃隔热原理分析及实验

利用辐射-边部传导耦合模式分析真空玻璃的隔热原理,通过传热通量和传热系数的计算,建立真空玻璃的传热方程,计算出真空玻璃结构沿厚度方向的温度分布和热流通量,对计算值和实验值进行了对比分析。结果表明,真空玻璃传热系数与真空压强正相关,真空度决定真空玻璃隔热性能;真空玻璃双面镀低辐射膜隔绝了较多的辐射热交换,其效果比单面镀膜好。     

Identification of local heat flux to membrane water-walls in steam boilers

A heat flux entering steam generating tubes in power station boilers may be a critical factor in considering the safety of the tubes. The knowledge of the distribution and magnitude of this flux during the operation of the power boiler is very important. The design of a modern boiler furnace requires the computation of furnace wall metal temperatures for the proper selection of the tube thickness and material. These temperatures are functions of the heat fluxes and the internal heat transfer coefficients. In this study, a measuring device (flux-tube) and a numerical method for determining the heat flux in boiler furnaces, based on experimentally acquired interior tube temperatures, are presented. An inverse method was developed, which can estimate the following parameters: the absorbed heat flux, the heat transfer coefficient on the inner tube surface and the temperature of water-steam mixture from temperature measurements at several interior locations of the flux-tube. The least squares method was used to minimize the differences between the calculated and measured temperatures. The unknown parameters are found using the Levenberg-Marquardt method. The number of temperature sensors (thermocouples) is greater than three because the additional information can help in more accurate estimations of the unknown parameters. The temperature dependent thermal conductivity of the flux-tube material was assumed. The developed flux-tube can work for a long time in the destructive high temperature atmosphere of a coal-fired boiler.     

Transient Coupled Radiation and Conduction Heat Transfer in a Scattering Layer of ARLEN

ARLEN is a heat-resistant, modified polyamide with a high melting point and a rigidity level comparable to super engineering plastics. It possesses strong dimensional stability and is widely used in engineering and high-temperature applications. At elevated temperatures, radiative transfer becomes important, and transient temperature responses, including radiation, can be significantly different from those by conduction alone. This work considers transient radiative and conductive heat transfer in a plane, absorbing, emitting and isotropically scattering layer of ARLEN. The solution includes the radiative two-flux equation, which is coupled with the transient energy equation, and both equations are solved simultaneously. The two-flux method is used as a simplification for obtaining the radiative heat source term in the energy equation. The external boundaries of the layer are exposed to a radiative environment and can be convectively heated or cooled.     

Surface temperature measurements on burning wood specimens in the Cone Calorimeter and the effect of grain orientation

Surface temperatures were measured on dry Douglas fir sapwood specimens during Cone Calorimeter tests using thermocouples and an infra-red pyrometer. Good agreement between the thermocouples and the pyrometer was obtained when (1) the emissivity was assumed to be 1.0 and (2) the thermocouples were in good contact with the surface and were not located in the proximity of a fissure. The major fissures were normal to the grain of the wood and the volatiles vented through the fissures. Char oxidation in the region between the vertical fissures resulted in higher surface temperatures.     

Determination of Heat Conductivity and Thermal Diffusivity of Waste Glass Melter Feed: Extension to High Temperatures

The heat conductivity (λ) and the thermal diffusivity (a) of reacting glass batch, or melter feed, control the heat flux into and within the cold cap, a layer of reacting material floating on the pool of molten glass in an all‐electric continuous waste glass melter. After previously estimating λ of melter feed at temperatures up to 680°C, we focus in this work on the λ(T) function at T > 680°C, at which the feed material becomes foamy. We used a customized experimental setup consisting of a large cylindrical crucible with an assembly of thermocouples, which monitored the evolution of the temperature field while the crucible with feed was heated at a constant rate from room temperature up to 1100°C. Approximating measured temperature profiles by polynomial functions, we used the energy equation to estimate the λ(T) approximation function, which we subsequently optimized using the finite‐volume method combined with least‐squares analysis. The heat conductivity increased as the temperature increased until the feed began to expand into foam, at which point the conductivity dropped. It began to increase again as the foam turned into a bubble‐free glassmelt. We discuss the implications of this behavior for the mathematical modeling of the cold cap.     

Evaluation of surface energy of solid polymers using different models

In the present work, contact angles formed by drops of diethylene glycol, ethylene glycol, formamide, diiodomethane, water, and mercury on a film of polypropylene (PP), on plates of polystyrene (PS), and on plates of a liquid crystalline polymer (LCP) were measured at 20°C. Then the surface energies of those polymers were evaluated using the following three different methods: harmonic mean equation and geometric mean equation, using the values of the different pairs of contact angles obtained here; and Neumann's equation, using the different values of contact angles obtained here. It was shown that the values of surface energy generated by these three methods depend on the choice of liquids used for contact angle measurements, except when a pair of any liquid with diiodomethane was used. Most likely, this is due to the difference of polarity between diiodomethane and the other liquids at the temperature of 20°C. The critical surface tensions of those polymers were also evaluated at room temperature according to the methods of Zisman and Saito using the values of contact angles obtained here. The values of critical surface tension for each polymer obtained according to the method of Zisman and Saito corroborated the results of surface energy found using the geometric mean and Neumann's equations. The values of surface energy of polystyrene obtained at 20°C were also used to evaluate the surface tension of the same material at higher temperatures and compared to the experimental values obtained with a pendant drop apparatus. The calculated values of surface tension corroborated the experimental ones only if the pair of liquids used to evaluate the surface energy of the polymers at room temperature contained diiodomethane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1831–1845, 2000     

Thermal conductivity of neutron-irradiated reactor graphites

The thermal conductivity of neutron-irradiated reactor-grade graphites made from needle coke (H-327), Gilsocoke (H-328), and isotropic petroleum coke (H-451) was measured between 22 and 800°C. Irradiation temperatures were between 600 and 1600°C, and fast neutron fluences ranged up to 10²² n/cm² (E>0·18 MeV). Irradiation reduces the conductivity toward a saturation level which increases with irradiation temperature. For given irradiation conditions, the reduction in conductivity is greater for the needle-coke graphites than for the isotropic graphites. The dependence of conductivity on measurement temperature is reduced by irradiation. The temperature dependence of the irradiation-induced increase in thermal resistivity is in fair agreement with that predicted from lattice dynamics theory for those vacancy loops which are small compared with phonon wave-lengths. The temperature dependence is not consistent with calculations which assume a temperature-independent phonon mean free path for irradiation-induced defects.     

VELOCITY AND TEMPERATURE DISTRIBUTIONS BETWEEN PARALLEL POROUS PLATES WITH HALL EFFECT AND VARIABLE PROPERTIES

Unsteady hydromagnetic flow and heat transfer between two parallel porous plates is studied with the Hall effect and temperature-dependent properties. The fluid is acted upon by a constant pressure gradient and an external uniform magnetic field, and uniform suction and injection are applied perpendicular to the parallel plates. A numerical solution for the governing nonlinear equations of motion and the energy equation is obtained. The effects of the Hall term and the temperature-dependent viscosity and thermal conductivity on both the velocity and temperature distributions are examined.     

VELOCITY AND TEMPERATURE DISTRIBUTIONS BETWEEN PARALLEL POROUS PLATES WITH HALL EFFECT AND VARIABLE PROPERTIES

Unsteady hydromagnetic flow and heat transfer between two parallel porous plates is studied with the Hall effect and temperature-dependent properties. The fluid is acted upon by a constant pressure gradient and an external uniform magnetic field, and uniform suction and injection are applied perpendicular to the parallel plates. A numerical solution for the governing nonlinear equations of motion and the energy equation is obtained. The effects of the Hall term and the temperature-dependent viscosity and thermal conductivity on both the velocity and temperature distributions are examined.     

Thermo-physical properties of coal under transient conditions

Coal samples were obtained along the vertical axis in an eight-inch diameter experimental moving bed coal gasifier. This system had a throughput of half a tonne of sized (6 × 20 mm) coal per day. The specific heat and thermal conductivity were measured using the line heat-source method with a transient sample temperature. The coal sample was heated at a rate of 4 K/min during the property measurement. The heat capacity and thermal conductivity were obtained from an iterative fit of the experimental data to the heat conduction equation taking due account of the probe size and contact resistance. The apparatus was tested with samples of known properties such as alumina powder and glass micro-beads. The measurements reported here cover the range from 300–700 K. Effects of surface moisture and pyrolysis reactions can be identified. The results are compared with data in the literature measured at constant sample temperatures. The line source technique is well suited to transient property measurements.     

Characterization of epoxy resins by photochemical hole burning (PHB) spectroscopy

The phonon frequencies, E_s, measured as the energy differences between zerophonon hole peak and pseudophonon side hole peak in photochemical hole burning (PHB) spectra, were studied for tetraphenylporphin (TPP) in epoxy resin films under various conditions of sample preparation and annealing. The values of phonon frequency, E_s, reflecting low energy excitation modes of the matrix epoxy resins were about 13 to 16 cm^?1. They decreased when the epoxy resin was cured at relatively low temperatures and vice versa. At the same cure temperatures the sample quenched to liquid nitrogen showed a lower value than that of the annealed sample. These results are in agreement with the results of qualitative free volume measurements. The thermal stability of holes burned at 20 K was also tested, and the results were compared with the extent of cure and glass transition temperature of the resins.     

REMOTE SPECTRAL SENSING OF TEMPERATURE PROFILES IN FLAMES

Knowledge of the temperature profile of the gases produced by the combustion of fossil fuels with air is a desirable step in the thermal protection systems. These gases are generally associated with high temperatures and steep temperature gradients. The measurement of temperature profiles of combustion products are often hard to make using thermocouples and other immersed pyrometers because such probes may have to be immersed in inaccessible and possibly destructive media. Determination of gas temperature profiles from multi-frequency radiative measurements seems an attractive alternative. This could be accomplished by radiative heat transfer probing and the application of the principle of inversion, where inversion is the process of inferring the internal parameters of a system of outside measurements, without the interruption of the combustion system itself.

This paper discusses the development of models to retrieve the temperature distribution in a flame system by remote spectral sensing. Mixed Gray-Gas models were developed to calculate water vapor and carbon dioxide standard emissivities. The evaluation of combustion system spectral radiant flux was simulated and a model was developed and successfully tested for the application of temperature inversion.     

Heterogeneous thermochemical decomposition of a semi-transparent particle under direct irradiation

Transient heat and mass transfer in a non-uniform emitting, absorbing and anisotropically scattering medium inside a semi-transparent, optically large and chemically reacting particle directly exposed to an external source of high-flux radiation is analyzed numerically. Thermal decomposition of calcium carbonate is selected as the model chemical reaction. The unsteady mass and energy equations are solved numerically using the finite volume technique and the explicit Euler time-integration scheme. Radiative transport is modeled using the Rosseland diffusion approximation and the Monte Carlo ray tracing method. Direct irradiation and internal radiative transfer in the particle are highly favorable for particle heating and the decomposition reaction, decreasing the total reaction time by a factor of 15, as compared to the case with external and internal radiation neglected in the analysis. In the latter case, the temperatures at the particle center and the particle surface increase monotonically to 1406 and 1417 K for the reacting particle, and 1428 and 1432 K for the non-reacting particle, respectively, after 179 s—the total reaction time of the reacting particle. With radiation included in the analysis, the surface temperatures of both reacting and non-reacting particle increase from the initial 300 to 1300 K in less than 2 s, and at the same rate until the onset of the endothermic chemical reaction at t=1.1 s. The surface temperature of the reacting particle increases further up to 2000 K after 12 s, when the whole particle is calcined. Weak dependence of the temperature, the overall reaction extent, and the total reaction time on the CaO grain size is observed in spite of strong dependence of the radiative properties of porous CaO on the CaO grain size.     

Development of empirical relation to evaluate the heat transfer coefficients and fractional energy in basin type hybrid (PV/T) active solar still

The simple empirical relation is developed to estimate the glass cover temperature for known values of water and ambient temperatures in basin type hybrid (PV/T) active solar still. The empirical relation developed is based on outdoor experimental results of water and ambient temperature in the range of 14 °C to 92 °C, and 14 °C to 36 °C respectively. The results obtained for glass cover temperature using proposed relation are validated with the experimental as well as using a numerical results (obtained by numerical solution of heat balance equation) of solar still. The proposed glass cover temperature is obtained with a maximum relative error of 1.12% compared to the value obtained through a numerical solution. The maximum relative error in evaporative mode of energy transfers from water surface is obtained as 1.2%.     

Reaction injection molding process of glass fiber reinforced polyurethane composites

Structural reaction injection molding (SRIM) was used to produce polyurethane composites containing random continuous glass fiber mats. A long rectangular mold was used to carry out the SRIM experiments. 4,4′‐diphenylmethane diisocyanate and poly(propylene oxide) triol were used to formulate a thermoset polyurethane system. Dibutyltin dilaurate was used as a catalyst. A second order Arrhenius equation described the PU polymerization kinetic data obtained from the adiabatic temperature rise measurement. A viscosity as a function of temperature and conversion was developed using rheometer data. The pressure rise at the gate was measured during filling. The flow behavior within the mold was described by Darcy's law and the Kozeny's equation. The temperature profile within the mold measured by thermocouples during filling and curing coincided fairly well with the simulation results. The thermal transient problem at the wall was solved using the overall heat transfer coefficient, and it was analyzed as a function of Biot number. The dimensional stability of the fiber reinforced PU parts was excellent compared to the pure PU parts.     

On the interpretation of contact angle hysteresis

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.     

Asymmetric temperature profiles in a single catalyst pellet

For rapid reactions the differences in local coefficients of heat and mass transfer around the surface of a catalyst particle lead to a distribution of surface temperatures and an asymmetric internal profile. The nature of these distributions and their changes with flow rate were predicted for the hydrogenation of ethylene on a single spherical pellet of nickel on silica alumina. The measured surface and internal temperature were in good agreement with the predictions.