Two-phase Thermal Conductivity Research Articles

Influence of Scattering Phase Function on Estimated Thermal Properties of Al2O3 Ceramic Foams

Open ceramic foams are usually constituted of three-dimensional networks with randomly interconnected solid struts and fluid within pores. The heat transport within this material can be understood as the coupling of conduction, convection as well as radiation. The cooperative control of different heat transfer mechanisms is critical to successful design and optimization of components working at high temperatures. An answer to this problem usually requires good knowledge and understanding of the corresponding thermal properties in a wide range of temperatures. In the present paper, an inverse identification method was developed to determine coupled thermal properties from transient thermal measurements at temperatures up to 900 K for full description of conduction/radiation heat transports of foam media with absorbing, emitting, and anisotropic scattering effects. The influence of postulated phase function on the identified equivalent extinction coefficient, scattering albedo, anisotropic scattering factor, and two-phase thermal conductivity was discussed for better understanding of thermal behavior within ceramic foams. The estimated thermal properties under each postulated phase function of the sample at transient temperature profiles were used to calculate equivalent thermal conductivities, which were then compared with the measured results at more than 1000 K. The accordance between them indicated that linear anisotropic scattering phase function demonstrates superiority in description of radiation behavior within Al2O3 ceramic foam.

Influence of Gas-Liquid Interface on Temperature Wave of Pulsating Heat Pipe

The influence of the interface on the amplitude and phase of the temperature wave and the relationship between the attenuation of the temperature wave and the gas-liquid two-phase physical parameters are studied during the operation of the pulsating heat pipe. The numerical simulation shows that the existence of the phase interface changes the direction of the temperature gradient during the propagation of the temperature wave, which increases the additional “thermal resistance.” The relative size of the gas-liquid two-phase thermal conductivity affects the propagation direction of heat flow at phase interface directly. The blockage of the gas plug causes hysteresis in the phase of the temperature wave, the relative size of the gas-liquid two-phase temperature coefficient will gradually increase the phase of the temperature wave, and the time when the heat flow reaches the peak value is also advanced. The attenuation of the temperature wave is almost irrelevant to the absolute value of the density, heat capacity, and thermal conductivity of the gas-liquid two phases, and the ratio of the thermal conductivity of the gas-liquid two phases is related. When the temperature of the heat pipe was changed, the difference of heat storage ability between gas and liquid will lead to the phenomenon of heat reflux and becomes more pronounced with the increases of the temperature wave.

Radial two-phase thermal conductivity of ceramic sponges up to high temperatures – experimental results and correlation

The experimental determination of the radial two-phase thermal conductivity with forced convection (often also called effective thermal conductivity) at high temperatures up to 1100 K for different ceramic sponges (variation of material, porosity and cell size) is presented. The experimental results are shown and dependencies are clarified. The sponges investigated were made of alumina, mullite, oxidic-bonded silicon carbide (OBSiC) and silicon infiltrated silicon carbide (SiSiC). They have porosities between 75% and 88% and cell densities in the range of 10 ppi to 30 ppi. Due to the high temperature domain, the thermal radiation is significant and cannot be neglected. The radial two-phase thermal conductivity under forced convection conditions is highly dependent on the superficial air velocity, the temperature, the porosity and the cell size of the sponge sample. Furthermore, for future prediction of the radial two-phase thermal conductivity of sponges, a correlation based on an approach summing up the different underlying heat transfer mechanisms is developed.

High temperature two-phase thermal conductivity of ceramic sponges with stagnant fluid – Experimental results and correlation including thermal radiation

Experimental results for the two-phase thermal conductivity (often called “effective thermal conductivity”) with stagnant fluid at temperatures up to 1200 K for different ceramic sponges (variation of material, porosity and cell density) are presented in this publication. A two-plate test facility was used for the experiments. Samples investigated have porosities higher than 75% and cell densities in the range of 10–45 ppi (pores per linear inch). They are made of alumina, mullite and oxidic-bonded silicon carbide. The two-phase thermal conductivity is strongly dependent on temperature, porosity and cell sizes of the sponge sample. A model based on the superposition of the two heat transfer mechanisms, thermal conduction and thermal radiation is used to predict the two-phase thermal conductivity of ceramic sponges. A model based on combination of thermal resistances is suggested for predicting the thermal conductivity. The so-called Rosseland equation is used as an initial model for predicting the part of thermal radiation on the two-phase thermal conductivity. Measurements of material properties are included in this work as model implementation requires an exact knowledge of sponge data.

Radial two-phase thermal conductivity and wall heat transfer coefficient of ceramic sponges – Experimental results and correlation

Abstract In this publication, experimental results for the radial two-phase thermal conductivity and the wall heat transfer coefficient of different ceramic sponges (variation of material, porosity and pore size) are presented, revealing a strong influence of the superficial air velocity and of the porosity of the ceramic sponges on both the radial two-phase thermal conductivity and the wall heat transfer coefficient. The experimental data are correlated in dimensionless form to offer a “universal” correlation for estimating the radial two-phase thermal conductivity and the wall heat transfer coefficient of sponges in the future.

Axial two-phase thermal conductivity of ceramic sponges – Experimental results and correlation

In this publication experimental results for the axial two-phase thermal conductivity of different ceramic sponges (variation of material, porosity and pore size) are presented, revealing a strong influence of the superficial air velocity and of the porosity of the ceramic sponges on the axial two-phase thermal conductivity. The experimental data are correlated in dimensionless form to offer a “universal” equation for calculating the axial two-phase thermal conductivity of sponges in the future.

Determination of the thermal properties of ceramic sponges

The knowledge of thermal properties of technical components or internals in chemical reactors is often a key characteristic for planning and designing chemical engineering processes. As an alternative to packed beds or packings, sponges turned out to be used in new application fields in chemical and process engineering. Therefore an experimental study was performed to investigate the two-phase thermal conductivity of solid ceramic sponges made of alumina, mullite and oxidic-bonded silicon carbide (OBSiC) at moderate temperatures. A two-dimensional model is used for analysing the measured temperature profiles and for calculating the thermal conductivity. It can be observed, that the thermal conductivity increases with decreasing porosity and is nearly constant when the pore size (ppi number) is varied. The thermal conductivity data are modelled by an approach similar to the well known Krischer model. Compared to a packed bed of spherical particles, the values of the thermal conductivity of sponges turn out to be about five times higher.

Modelling of two-phase thermal conductivity

The prediction of the conductivity of a random distribution of spheres in a continuum of different material is addressed in this work. The approach of Ohm's law based on the unit cell is the starting point. Firstly, it is shown that the approach is compatible with the Fourier law approach which until now was considered irreconcilable. It is then demonstrated how one may use this result to model non-Maxwell interactions and thereby arrive at an equation for the conductivity of disperse media which predicts the property simply and in a physically clear fashion. Comparisons are made with existing models. The new equation is easy to use and can accommodate extensions to account for secondary effects.