Gradient heat fluxmeter

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].

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]. 

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]). 

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

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