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The integro-differential equation governing the combined conduction and radiation heat transfer in a gray medium bounded by two infinite coaxial cylindrical surfaces is solved numerically to analyse the effect of radiation in transient hot-wire measurements in porous thermal insulations. The influences of the extinction coefficient, the emissivity of the wire and the heating power are studied. It is found that the hot-wire method works accurately only in cases where the extinction coefficient exceeds a minimum value which increases with the temperature. The calculated results confirm the linear relationship between measured thermal conductivities and the heating power. The problem of reference temperature for the measured thermal conductivity is also discussed in this work. [Copyright &y& Elsevier]

Open cell foams have desirable geometrical characteristics that make them a suitable choice in various heat exchanger applications. The objective of this study is to determine such volume-averaged key parameters that can characterize the complex thermal transportation process through open cell foams. These key parameters are represented by effective thermal conductivity, k e , volumetric heat transfer coefficient, h v , and dispersion conductivity, k d . In order to determine these parameters, detailed pore-scale simulations through the representative element volumes (REVs) of the actual foam structures are performed. Moreover, knowing the fact that the successful implementation of simplified foam structures as a suitable representative of the actual foam structures can simplify the complexity of the problem, it is also investigated. In the presented work, various microscopic pore-scale models are implemented for both simplified and actual foam structures to determine the key parameters. Subsequently, these key parameters are implemented into two different homogenized macroscopic models to predict the temperature fields of large-scale steady-state and transient forced convection processes. The numerical outcomes of homogenized macroscopic models are validated with the experimental data, which is available for a set of ceramic foams having different pore size (10–30 PPI) and porosity (79–87%). As a consequence of the validation process, the findings of this study reveal that the proposed methodology successfully predicts the values of the concerned key parameters. Further, it is observed that simplified foam structures cannot represent the actual foam structures, as the tortuous shape of open cell foams bound to enhance the advection and dissipation of heat due to recirculation and eddy formation. [ABSTRACT FROM AUTHOR]

Furnace walls and bottoms frequently consist of a refractory concrete, which has to be replaced from time to time due to abrasion. After relining, the furnace has to reach its operating temperature again. During this heat-up process, there is a risk of explosive spalling caused by the pore pressure build-up inside the wet concrete. These explosions may be very dangerous to the crew and the furnace-site. Those risks are reduced till today by the aid of empirically developed heat-up instructions (heat-up curves). The application of these curves doesn't really prevent explosive spalling and it is very time-consuming. To improve the first heat-up procedure, the empirically developed heat-up curves are replaced by optimized ones developed by the aid of numerical modeling. These curves are characterized by a constant maximum pressure during the heat-up drying period, and they bring a significant reduction of both, the heat-up time and the maximum pressure inside the concrete. Furthermore their dependency on wall thickness and pressure limit is investigated, as well as effects of previously not expected conditions which can occure during the heat-up of concrete under industrial conditions. Additionally, an optimized heat-up curve is used to dry a concrete brick in a laboratory experiment showing the practical applicability. Good agreement is obtained for the predicted pressure and temperature devolutions and the measured ones. [ABSTRACT FROM AUTHOR]

The first heat-up of furnaces consisting of wet refractory concrete is a challenge due to the pore pressure build-up which can exceed the strength of the concrete. To extend understanding of the complex heat and mass transfer phenomena taking place during the first heat-up, two reproducible heat-up experiments are carried out. These are performed by using a new developed pore pressure sensor. The experiments exhibit an intense change of concrete's thermal conductivity during the process. Further, even in case of one sided heating, two drying fronts are found to occur. In addition to the experimental investigation the first heat-up is simulated using a physical model, based on the coupled balances of energy and the masses of liquid water, water vapour and air as well. By the aid of numerical simulation the first heat-up is further investigated and mass flow behaviour of air, vapour and liquid water is discussed. Within the scope of a sensitivity study the concrete's thermal conductivity and it's permeability are obtained as it's most important material properties and they are used for calibration of the model aiming well representation of the experimental data. The adjusted material properties are found to be in good agreement with refractory concrete properties reported in literature. [ABSTRACT FROM AUTHOR]

To investigate thermal transportation through open cell foams there are various microscopic and macroscopic numerical models along with their limitations available in the literature. The purpose of this study is to investigate the limitations of macroscopic models and to propose some reliable ideas as conclusion that can be used to overcome the existing limitations. Therefore, a combined experimental and numerical study is presented. The experimental study comprises of steady state forced convection experiments which involve three different regimes of heat transfer. Further, in macroscopic models these three regimes of heat transfer namely conduction, thermal dispersion and interstitial convection are governed by stagnant effective thermal conductivity, k e , dispersion conductivity, k d and volumetric heat transfer coefficient, h v respectively. Moreover, the complex structure of the open cell foams is simplified into a rather realistic Kelvin cell model for the determination of k e . The influence of the geometrical parameters such as pore diameter, d and foam porosity, ε is investigated by examining 10, 20 and 30 PPI (pores per inch) alumina foams for a porosity range of 0.79–0.87. The findings of this study reveal that it is important to consider both local thermal non-equilibrium (LTNE) and thermal dispersion together for improved analysis. Further, it is revealed that although with the above consideration, it is possible to exhibit the effect of geometrical parameters on each regime of heat transfer but the accuracy of the results predicted through macroscopic models remain under question as there is no basis to validate the results. [ABSTRACT FROM AUTHOR]

Thermal transportation in case of open cell foams is a complex process that comprises of all the modes of heat transfer. In macroscopic numerical models the key parameters that govern these modes of heat transfer are stagnant effective thermal conductivity, k e , dispersion conductivity, k d , effective radiative conductivity, k r and volumetric heat transfer coefficient, h v . The purpose of this study is to determine these key parameters for the forced convection of air through open cell alumina foams. It is achieved by performing two different types of experiments on alumina foams having different PPI number (10, 20 and 30) and porosity (0.79–0.87). The first sets of experiments are based on the transient heat transfer and the aim of performing these experiments is to determine the volumetric heat transfer coefficient, h v . The second sets of experiments are based on the steady state heat transfer and they are the outcome of our previous study. The aim of implementing steady state experiments is to determine dispersion conductivity, k d by supplying the values of h v as an input parameter from the transient analysis. Further, to supply effective thermal conductivity data to both transient and steady state analyses, the Kelvin cell model is implemented. The findings of this study exhibit that the convection heat transfer in case of transient analysis and the thermal dispersion in case of steady state analysis are the dominant modes of heat transfer. Further, this study reveals that by employing the proposed combined analysis, it is possible to predict the values of key parameters rather accurately. But still, the accuracy of the results predicted through the macroscopic models largely depends on the accuracy of the experiments. [ABSTRACT FROM AUTHOR]