Textile heat fluxmeter

Thus, this chapter describes an innovative smart textile a heat fluxmeter with a textile auxiliary wall, also called a textile heat fluxmeter (THF), which can detect, analyze, and monitor the heat and mass transfers with minimum dismrbance due to their porosity. It is a yam-based sensor that can be defined as the yarn itself as a sensing element and thus it is easier to be used by conventional knitting and weaving processes [15]. Moreover, it is desirable to use flexible electronics and this is especially tme when they need to be in contact with the human body, in which case the flexibility and nonirritability requirements are of utmost importance (Fig. 19.2) [16]. 

This textile heat fluxmeter consists of a network of thermocouples (assembly of two dissimilar conductor or semiconductor), also called a thermopile, assembled into a textile auxiliary wall. Thus, heat and mass transfer properties of textile substrate used as auxiliary wall will be smdied in the first part of this work. Afterward, the principle and the production technology of the conventional heat fluxmeters and the textile heat fluxmeter will be defined. 

Figure 19.2 Photographs of heat fluxmeters (a) textile heat fluxmeter, and (b) conventional heat fluxmeter (Captec Entreprise, France).

Since a textile substrate is used as an auxiliary wall to create the textile heat flux-meter, die performance of this heat fluxmeter is influenced by textile substrate properties. The first part of this chapter is especially concerned with the complex phenomena of the heat and mass transfers. The following section presents the principles and the production processes of the conventional and textile heat fluxmeters. 

In order to eliminate inconveniences of conventional heat fluxmeters such as impermeability, rigidity, a heat fluxmeter with a textile auxiliary wall, also called a textile heat fluxmeter (THF) has been developed. 

Figure 19.8 Equivalent circuit model of a textile heat fluxmeter (under steady state). Where T is the temperature (K), / (h is the thermal resistance (m K/W), is the heat flux (W), and AT is the temperature gradient (K).

Figure 19.12 Schema of textile heat fluxmeter structure biconductor wire passes on the five warp yams, and there is one textile warp yam between two BC wires (Ex. Twill 5Z).

Initially, the sensitivity of textile heat fluxmeters was calibrated in order to compare their performance amongst them and also against a commercial heat fluxmeter (Captec Entreprise, France). Afterward, the heat fluxmeters were characterized with the Skin Model with regard to a physiological application. Finally, the characterization of the coupling between the heat and mass transfers was carried out. 

In order to determine sensitivity of the textile heat fluxmeters, the conductive heat flux, which is constant at a steady state through aU of the elements of the unidirectional (z) thermodynamic system, was used (Eq. [19.9]). It was considered that the loss of the energy to the axes x and y is insignificant. 

Sensitivity was calculated due to the output voltage delivered from terminals of the textile heat fluxmeter and the heat flux density created by heating resistance according toEq. [19.11]. 

The textile heat fluxmeter was soaked in distilled water in order to have a maximum retention rate. After soaking, it was immediately placed on a heating resistance. The whole system was fixed to the insulation material and placed on a scale. Simultaneously, the output voltage supplied by the THE and the weight of the measuring unit were recorded. 

The heat flux density measured by textile heat fluxmeter changes depending on the humidity factor. 

Three textile heat fluxmeters, ie, PES/CO2, PES3, and PES/C03 which were produced with subtractive method or additive method, were compared to a commercial heat flux-meter for heat and mass transfer properties. 

The sensitivity of the larger-sized heat fluxmeters, either textile or reference, is higher than the sensitivity of the smaUer-sized heat fluxmeters due to an increase in number of thermocouples (cf. Eq. [19.4]). Despite the low number of thermocouples, the sensitivities of the THFs are in the same range as the reference ones. The highest sensitivity for THF is observed for satin structure with PES/CO material. This can be attributed to (1) the thermal resistance of the whole textile heat fluxmeter, which is more important than the others and (2) the better contact with the measuring unit due to its smooth surface. 

Table 19.4 Sensitivity comparison between textile heat fluxmeters and reference heat fluxmeters for subtractive method... 

Table 19.5 Sensitivity comparison between subtractive method and additive method for small-sized textile heat fluxmeters (same number of thermocouples)...

Instead of using sensitivity given by the manufacturer, also called nominal sensitivity, heat flux performance of the commercial reference heat fluxmeter was characterized under the same conditions as textile heat fluxmeters with the Skin Model simulation tool. 

If the first characterization method (sensitivity calibration with a heating resistance) is compared with the second one (performance for physiological applications with Skin Model), the results have the same trend and the textile heat fluxmeter PES/CO satin gives the higher sensitivity among all the THFs. 

Large-sized textile heat fluxmeters (5x5 cm) were characterized in order to analyze the impact of humidity on heat transfer properties. Three different heat flux densities (273 W/m, 369 W/m, and 464 W/m ) were used to supply heating resistance. 

The coupling of the heat and mass transfers in a textile heat fluxmeter includes several phenomena. Different steps of this coupling are presented in Fig. 19.18 for the textile heat fluxmeter PES/CO3 and the supplied heat flux density 464 W/m depending on the heat flux density measured by THF and the retention rate. 

The drying time increases when the heat flux density supplied from heating resistance decreases from 464 W/m to 273 W/m. Textile heat fluxmeter measures a heat flux density lower than the supplied heat flux density at steady state, which can be caused by the heat loss and the thermal resistance difference between difference interfaces. 

Figure 19.19 The impact of the heat flux density supplied hy the heating resistance on drying time for the textile heat fluxmeter PES/CO3.

The most appropriate auxiliary wall for textile heat fluxmeter development was considered in the first part of this chapter. PES/CO fabric with twill stmcmre was chosen due to its better thermal insulation property, which was observed using the Skin Model. The satin stmcmre was preferred for both PES and PES/CO materials because of the smooth fabric pattern and higher weft density. 

The second part focused on the textile heat fluxmeter development and characterization. Different methods showed that the heat flux performances of the THFs are in the same range as the reference one. The textile heat fluxmeter PES/CO with a satin stmc-ture gives slightly higher sensitivity values than the fluxmeters PES3 and PES/CO2. Therefore, PES/CO2 can be preferred as a heat fluxmeta- for future physiological applications. 

The characterization of the couphng between heat and mass transfers showed that textile heat fluxmeters take into account the evaporation phenomenon. This coupling is mainly influenced by the THE material. 

Einally, this textile heat fluxmeter will be integrated into the garment for physiological apphcations, ie, personal protective clothing for firefighters, in order to detect and quantify the heat and mass transfers and inform the user. 

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