Lubos Hes

Effect of Mutual Bonding of Textile Layers on Thermal Insulation and Thermal Contact Properties of Fabric Assemblies

This paper deals with thermal insulation and thermal contact properties of woven fabric assemblies used to produce mens jackets. An important part of the paper is the description of the new computer-controlled instrument, the Alambeta, to measure insulation and thermal contact properties of fabrics. The heat flow passing between the textile sample and measuring head during thermal contact is measured directly by a special thin sensor, whose thermal inertia is similar to that of human skin. Thus, the instruments warm-cool feeling sensitivity aproximates human skin. Spotbonding the outer fabric interlining and lining together reduces the total thermal resistance of the assembly and simultaneously increases (makes cooler) thermal absorptivity, a new parameter used to describe the warm-cool feeling of fabric. The meaning of this pa rameter, which mainly reflects the surface properties of the fabrics and whose level does not depend on experimental conditions, is explained in detail. The effect of temperature drop on the thermal properties of the fabric assemblies is also investigated. The increased resulting thermal conductivity with the average temperature of the assembly is a consequence of the increased portion of the heat transferred through the system by radiation.

Heat transfer and rheology of stirred yoghurt during cooling in plate heat exchangers

In the present work an experimental investigation was conducted to obtain a correlation for the determination of convective heat transfer coefficients of stirred yoghurt in a plate heat exchanger. A rheological study was carried out in order to characterise the stirred yoghurt flow behaviour, evaluating its dependency both on shear rate and temperature. A shift in the temperature dependency was evidenced at 25 °C. It is also shown that the material shows a complex flow behaviour, changing from a Bingham fluid to a power-law fluid at shear stresses in excess of approximately 6.7 Pa. As regards the heat transfer behaviour of the non-Newtonian stirred yoghurt a correlation for the convective heat transfer coefficient was obtained that reveals the large effects of the thermal entry length due to the high Prandtl numbers and to the short length of the plate heat exchanger.

Simulation of the Effect of Air Gaps between the Skin and a Wet Fabric on Resulting Cooling Flow

As the moisture content of a fabric increases, the relative water vapor permeability (the relative heat flow responsible for the cooling of the body) also increases and the fabric temperature drops due to the evaporation of the water from the surface of the fabric. In this work on the experimental study of water vapor permeability of wet fabrics, the effect of air layers between the skin of the wearer and the fabric on the total relative cooling heat flow (cooling effect) experienced by the skin of the garment wearer is investigated. It was found out that when layers of 2- and 4-mm thickness were introduced between the skin and the fabric, the relative water vapor permeability or relative cooling heat flow was smaller than when the fabric was in direct contact with the skin, and in this case it did not depend significantly on the fabric moisture content.

Aerogel based nanoporous fibrous materials for thermal insulation

In this research work, the thermo physiological properties of polyester/polyethylene nonwoven composite wraps of varying thicknesses impregnated with aerogel were studied and compared. The SEM images were also taken to compare the physical configuaration of the aerogel based fibrous composites. Specific thermal properties like thermal conductivity, thermal resistance, thermal diffusivity and thermal absorptivity were measured using alambeta instrument. The air permeability of the thermal wraps was measured in air permeability tester. The relative water vapor permeability and absolute water vapor permeability was measured in Permetest. These tests were conducted to understand thermal properties, air and water vapor permeability of flexible aerogel based composites with nanoporous structure. The results of the experiments were statistically analyzed and found to be within confidence intervals.

Thermal Comfort of Bedsheets Under Real Conditions of Use

Thermal comfort properties of various fabrics in dry state were systematically studied by many authors, but papers on thermal comfort properties under real conditions of use are sporadic in the professional literature. In this paper, thermal conductivity, thermal resistance, water vapor permeability, and thermal absorbtivity in dry and wet state of commercial cotton, cotton/PU, and cotton/PES bedsheets are presented. The samples were wetted by means of the so-called sweating impulse. It was found that the warmer (dryer) contact feeling and highest thermal insulation in wet state exhibited knitted cotton bedsheets containg some percentage of PU or PES, and also 100% cotton woven fabrics with hairy surface.

Thermal resistance models of selected fabrics in wet state and their experimental verification

This study investigated the structural model of textile fabrics affected by moisture. The model has been verified through five fleece fabrics made up of various textile materials and subjected to several mechanical surface treatment levels. Thermal resistance is one of the key parameters of thermal comfort along with water vapor permeability. In recent times, a keen interest has been focused on the mathematical modelling of this parameter and its experimental verification. However, most of these efforts are made to describe dryness in thermal resistance of fabrics, ignoring the wet condition found in protective and outdoor clothing. To determine the thermal resistance value of the studied fabrics, the ALAMBETA semiautomatic nondestructive thermal tester was used. The findings show that the proposed model displays substantial harmony with the experimental data.

Determination of heat transfer by radiation in textile fabrics by means of method with known emissivity of plates

High thermal resistance of thermal insulating fabrics is a major factor in the decision to use them in protective apparel applications, sleeping bags and other functional textiles. New standards and new applications of fibre layers in industrial textiles require more accurate methods of testing of their thermal resistance and conductivity, which minimise the dependence of the measured parameters on the testing conditions, such as the temperature drop between the plates of testing instrument and infrared radiation properties of these plates. In this article, the use of a testing method based on two different temperature drops between the plates of the ALAMBETA testing instrument revealed that the portion of heat transferred by radiation through the tested woven fabrics reached 12–17%. From the second series of measurements it follows that decrease in surface emissivity of the plates of the mentioned testing instrument influences the level of heat transfer by radiation between the plates of the ALAMBETA testing instrument and may cause the changes in the experimentally determined thermal conductivity and thermal resistance of certain porous fabrics. These changes presented up to 8% for the studied non-woven fabrics with low density.

The Effective Thermal Resistance of Fibrous Layers in Sleeping Bags

This paper describes a simple method for evaluating the thermal insulation capacity of fibrous layers in sleeping bags. Our measurements have kept the thickness layer at 75per cent of the thickness when the sleeping bag is in free state. Thus, the expansion (volumetric)characteristics of the tested fibrous layers are respected, resulting in a more objective evaluation and comparison of the tested layers. The results were recalculated to the same square mass of layers. It was found that various layers differ substantively from one another, and that these first results also differ from those achieved under common testing procedures, where the tested layers are subjected to constant pressure between two plates of different temperature.

Using a Thermal Simulator to Determine the Amount of Time that Humans are Thermally Protected by Fabrics Containing Phase Change Materials

In this paper, a simple original method and instrument for testing the thermal efficiency of garments containing PCM particles is described. The efficiency is characterized by the length of time that the garment can offer protection, when the effective thermal resistance is at least two times higher then the thermal resistance of the same garment without any PCM function. The related instrument can simulate any boundary conditions typical for the wearing of PCM garments. The good functionality of the instrument was verified by the tests. The experiments revealed that the length of time that the tested commercial garments could offer protection is quite short.

P-TEST - computerized instrument for testing of the water vapour and thermal resistance of fabrics

The water vapour permeability of fabrics is presented, along with thermal resistance, the most important characteristic of clothing comfort. The patented measuring method and original facility are briefly introduced. The paper describes a prototype of stand-alone instrument that employs and automates this method. An embedded microcontroller together with appropriate sensors replaces the number of facility devices and greatly simplifies the instrument operation. The concept of analog signal conditioning and conversion is discussed. Firmware is completely written in C language including interrupt handlers. The function of core subroutine that handles A/D converter with channel multiplexing and makes temperature control, averaging, conversion and linearization of integer data to the SI values is explained. Both the measurement process and the formulas are used to compute the results. PC application that supports built-in diagnostics in the instrument through serial link is mentioned. Finally, the experience with the prototype electronics and its calibration is released and future improvements are outlined.