Manuela Campanale

Analytical and Experimental Investigations on the Heat Transfer Properties of Light Concrete

It is well known that the thermal performance of some insulating and building materials is related to the actual operating conditions because the thermal conductivity of such materials is highly dependent on the moisture content. Since the thermal conductivity of liquid water is about 25 times greater than that of air, it is quite easy to understand how even small variations of the moisture content can have a significant impact on thermal performance. For this reason it is important to find a correlation between the moisture content in a specimen and its thermal conductivity. The purpose of this paper is to investigate both experimentally and theoretically the moisture contribution during the measurement of the heat transfer properties in light concrete slabs (autoclaved concrete and concrete lighted with polystyrene pearls) and to correlate its thermal transmissivity with the moisture content.

Effect of Moisture Movement on Tested Thermal Conductivity of Moist Aerated Autoclaved Concrete

The purpose of this work was to study both theoretically and experimentally the process of moisture redistribution and heat transfer due to phase changes during the tests of thermal conductivity in aerated autoclaved concrete (AAC) moist specimens. The different moisture contents of the test samples were obtained in climatic chamber at equilibrium conditions reached with constant air temperature and variable relative humidity. The moist specimens were sealed inside highly impermeable polyethylene bag, as required by UNI 10051, and placed in a heat flow meter apparatus. During the experimental thermal conductivity measurements, the temperature and heat flow rate were measured under transient and steady state conditions. A theoretical analysis of the heat and mass transfer process was performed and then a suitable numerical model was used to predict the moisture redistribution and heat transfer due to the phase changes. The theoretical model has been compared against the experimental data. Substantial agreement between numerical results and experimental data was found. Then several numerical simulations have been performed to study the influence of the errors due to phase changes and non-uniform moisture distribution during the test of thermal conductivity of moist AAC specimens.

Thermal Conductivity of Moist Autoclaved Aerated Concrete: Experimental Comparison Between Heat Flow Method (HFM) and Transient Plane Source Technique (TPS)

In this work, measurements of the thermal properties of moist autoclaved aerated concrete specimens have been performed using both the heat flow meter apparatus (HFM) and the transient plane source technique Hot Disk (TPS). When testing moist materials, the steady-state condition can take a long time to be reached; furthermore, an additional difficulty occurs, because the temperature gradients inside the material cause a moisture transport with a moisture redistribution and a latent heat due to phase change. Therefore, for a correct determination of the thermal conductivity, it is necessary to separate these effects from the measurements. The transient plane source technique Hot Disk is a transient technique which carries out measurements in few seconds to make negligible the moisture redistribution; on the contrary, using the heat flow meter apparatus the steady-state condition is reached only when there is a total redistribution of the moisture contained in the specimen. Measurement data obtained from these two different methods (HFM and TPS) have been analyzed and compared; only in this way it was possible to perform an accurate computation of the moisture conversion coefficient

Simplified Procedure for the Determination of Thermal Resistance of Thick Specimens Enclosing Air Only

f_\mathrm{u}

Influence of Surface Emissivity and of Low Emissivity Shields on the Thermal Properties of Low Density Insulating Materials

fu of the thermal conductivity according to the EN 10456.

Thermal Imbalance in Hot Box Apparatus and In-Situ Measurements

Some standardized experimental procedures for the characterization of most common homogeneous insulating materials, in particular, low-density high- thickness mineral wool products, are based on some analytical models (two-flux model) which take into account combined heat transfer by conduction and radiation in homogeneous media. Interlaboratory comparisons and experimental validation of the models during some years has now covered most commercial products and proved that agreement is far better than testing accuracy. However, the above proce dures can no longer be applied when a density gradient occurs along the thickness of the specimen and hence a gradient of the radiative extinction coefficients is orig inated. The gradient occurs due to the weight of the upper layers of the product on the lower layers during binder polymerization. The system of differential equations which described the above models was therefore improved to take this effect into ac count. The solution was only possible by splitting the insulation into three layers, two facing the bounding surfaces of the product and a third forming a core. The solution is then used as the interpolating function of measured data in a procedure to characterize mineral wool products with density gradients, exceeding the maximum specimen thickness for the apparatus to be used, and such that the homogeneity assumption of standard models does not supply acceptable accuracy levels.

Low GWP refrigerants condensation inside a 2.4 mm ID microfin tube

Procedures are presented in this paper for finding the thermal resis tance of thick products when the thermal resistance can not be measured directly because the specimen thickness exceeds the apparatus capabilities, typically 10-15 cm for guarded hot plate and heat flow meter apparatus, and when it can not simply be calculated as the sum of the thermal resistances of slices cut from the product because of the so-called thickness effect. The proposed method is applicable to air filled in sulating materials, i.e., only air in the cells or among the fibres. It consists of using interpolating equations, one measurement, and a set of material parameters that are known for the family products. For some insulating materials, diagrams are also sup plied which correlate the specimen transfer factor (called thermal conductivity) with specimen thickness and material thermal transmissivity (the measured thermal con ductivity at thicknesses such that the thickness effect may be neglected).

A procedure for the determination of thermal resistance of insulating materials with density gradients

Heat and moisture transfer in wood wool slabs (shredded wood with Portland cement binder) has been investigated both experimentally and theoretically. The purpose of this study is the analysis of the process of moisture redistribution taking place inside wood wool slabs conditioned to moisture equilibrium (e.g., in a climatic chamber) and then mounted in a guarded hot plate or heat flux meter apparatus to measure the specimen thermal resistance. A theoretical analysis of the heat and mass transfer process for such a material was performed and then a suitable numerical model was used to predict and analyse the moisture redistribution. The porous media may be modelled as a multiphase system constituted by a solid matrix and a network of interconnected pores, partly filled with liquid water and partly with moist air. Three equations have been derived: conservation of the mass of the dry air, the conservation of the mass of water species (liquid and vapour) and the energy balance. These equations have been supplemented with the thermodynamic relations and the constitutive equations needed for the closure of the model. The experimental work was performed on a slab cut into several slices. During the experimental tests, the temperature and heat flow rate data was collected both during the steady state conditions to get the thermal resistance and during the initial transient period, when the heat flow rate is continuously varying. At the end of the test, the slices were weighed to get information on the moisture redistribution. The moisture data versus heat flow rate and temperature on the two faces of the specimen, and information about the final moisture distribution were compared with those obtained by numerical simulation. Substantial agreement between numerical result and experimental data was found.