Heat fluxmeters

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. 

Heat flux ( >) can be defined as the rate of heat energy transfer through a given surface (W), and heat flux density q>) is the heat flux per unit area (W m ). The fluxmeter, which measures this density, is called a heat fluxmeter or a heat flux sensor [14,40]. 

The principle of the gradient heat fluxmeter is to observe a heat flux due to the evaluation of the temperature gradient between the two faces of the conducting support, which has known thermal characteristics. This temperature gradient is measured by a thermocouple (assembly of two dissimilar conductors or semiconductors) or rather a number of thermocouples forming a thermopile. 

Fig. 19.4 presents a gradient heat fluxmeter with a thermal conductivity A and a thickness h. The heat flux density in a conduction phenomenon can be defined by Fourier s law (Eq. [19.2]). 

Figure 19.4 Schema of gradient heat fluxmeter with an auxiliary wall [40].

The inverse relationship between the thermal conductivity and thermal resistance is expressed with Eq. [19.1]. Therefore, the temperature gradient between two faces of the heat fluxmeter can be calculated due to the thermal resistance and the heat flux density (Eq. [19.3]). 

In order to increase the signal delivered by the heat fluxmeter, a large number of thermocouples connected electrically in series can be used to form a thermopile. 

Only a few companies commercialize gradient heat fluxmeters and all of them use the same principle described above the transfer of the heat flux generates a temperature gradient on the thermopile, which delivers an output voltage proportional to the heat flux. 

As mentioned previously, there are relatively few companies that commercialize gradient heat fluxmeters. There are three companies in the United States (VateU, Rdf, Omega) and three companies in Europe (Hukseflux, Wuntronic, Captec) (Table 19.2) [43-48]. 

These commercial heat fluxmeters listed in the Table 19.2 use electrochemical deposition process with electroplating technology, which is a deposition process using electrical current to obtain a metallic layer on a sample s surface. The sample is used as the cathode and the anode is made of the depositing material. As the sample to be... 

Table 19.2 Characteristics of different commercial heat fluxmeters [49]...

Figure 19.5 Cutaway view of a heat fluxmeter with printed circuit technology (Captec Entreprise, France) [14].

The thermocouple of constantan and copper is preferred for the production of conventional heat fluxmeters for different reasons deposition of copper can be done by chemical or electrochemical processes this thermocouple is well suited to electroplating technology because of the higher electrical conductivity of copper (59.1 x 10 S/m) as plots and lower electrical conductivity of constantan (1. 9 X 10 S/m) as a track [51]. 

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. 

Fig. 19.7(b) shows that the THF consists of a thermopile and textile substrate. Moreover, this thermopile is made up a number of thermocouples that are connected electrically in series in order to increase the output voltage delivered from the terminals of the heat fluxmeter (Fig. 19.8). 

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).

The thermocouple of constantan/copper (type t) was achieved by an electrochemical deposition of copper to a wire of constantan with electroplating technology. This thermocouple (type t) was chosen for different reasons this type is already confirmed by conventional heat fluxmeters it has good sensitivity for the temperature range we are looking at for physiological applications (0—350°C). 

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 calibration of the system was undertaken with a commercial heat fluxmeter (Captec Entreprise, France) with two different dimensions, ie, 2 x 2, 5 x 5 cm. 

Where (jsskin Model is the heat flux density measured by the Skin Model (W/mO, and l fluxmeter is the output voltage measured by heat fluxmeter (V). 

The greater the gradient of the curve, the greater the sensitivity of the heat fluxmeter. 

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. 

Textile auxiliary wall characteristics such as thermal resistance and water vapor permeability are presented in Table 19.3. Additionally, reference heat fluxmeter characteristics, ie, weight, thickness, and thermal resistance, are compared with the textile auxiliary walls. However, the impermeability and rigidity of the reference heat fluxmeter limit measuring the porosity, air permeability, and water vapor permeability index. 

According to Eq. [19.4], the observed output voltage AV depends on the thermal resistance of the heat fluxmeter under steady state. It is observed that fabrics with twill stmctures provide better thermal insulation properties since they have higher thermal resistance than plain or satin stmctures. Moreover, the porosity and water vapor permeability values are slightly higher for PES/CO fabrics than pure PES fabrics. Therefore, PES/CO fabric with a twill stmcmre (PES/CO2) was selected as a textile auxiliary wall. 

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. 

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