Effect of two sweating simulation methods on clothing evaporative resistance in a so-called isothermal condition

Abstract

The effect of sweating simulation methods on clothing evaporative resistance was investigated in a so-called isothermal condition (T _manikin  = T _a  = T _r ). Two sweating simulation methods, namely, the pre-wetted fabric “skin” (PW) and the water supplied sweating (WS), were applied to determine clothing evaporative resistance on a “Newton” thermal manikin. Results indicated that the clothing evaporative resistance determined by the WS method was significantly lower than that measured by the PW method. In addition, the evaporative resistances measured by the two methods were correlated and exhibited a linear relationship. Validation experiments demonstrated that the empirical regression equation showed highly acceptable estimations. The study contributes to improving the accuracy of measurements of clothing evaporative resistance by means of a sweating manikin.

Annex A: Correction of the clothing evaporative resistance determined in the so-called isothermal condition (T manikin  = T a  = T r )

For the “Newton” sweating manikin, a fabric “skin” (the skin must be thin and tightly fitted to the manikin body; i.e., there should not be any air gap in between) should dress up the manikin when performing wet tests. During the wet test, this piece of fabric skin should be kept fully wetted either by buffering enough amount of water or by continuously supplying water. Evaporation occurring at the surface of this diffusive fabric skin will absorb heat from the skin. Unfortunately, the surface temperature of the fabric skin is not controlled. Therefore, there is a temperature difference between the controlled manikin surface and the wetted fabric skin surface. Thus, the skin will draw heat from the manikin surface through conduction, and in the so-called isothermal condition where T _air  = T _manikin  = T _r , the fabric skin will also probably draw heat from the ambient environment through both convection and radiation due to the negative temperature difference (i.e., T _sk,f < T _air). The surface temperature at the fabric skin T _sk,f may be calculated by Eq. (A-1)

$$ {T}_{\mathrm{sk},\mathrm{f}}={T}_{\mathrm{manikin}}-{I}_{\mathrm{sk},\mathrm{wet}}\times {H}_{\mathrm{manikin}} $$

(A-1)

where T _sk,f is the surface temperature of the wetted fabric skin, °C; H _manikin is the observed heat loss from the manikin, W/m²; and I _sk,wet is the wet conductive thermal resistance of the wetted fabric skin, m² K/W.

Wang et al. (2010) have developed a universal equation to estimate the wet fabric skin surface temperature. This equation has been further validated by Ueno and Sawada (2012). The equation reads

$$ {T}_{\mathrm{sk},\mathrm{f}}=34.0-0.0132\times {H}_{\mathrm{manikin}} $$

(A-2)

It should be noted that the value of 0.0132 m² K/W for I _sk,wet is dependent on the fabric skins construction and thickness; different values (e.g., 0.0092 m² K/W reported by Ueno and Sawada (2012) were reported in other laboratories for their specific fabric skins.

The clothing standard evaporative resistance R _et,standard should be calculated as

$$ {R}_{\mathrm{e}\mathrm{t},\mathrm{standard}}=\frac{p_{\mathrm{sat}}\left({T}_{\mathrm{sk},\mathrm{f}}\right)-{p}_{\mathrm{a}}}{H_{\mathrm{e}}} $$

(A-3)

It should be noted that the prevailing way uses the evaporative heat loss observed on the manikin surface H _manikin to calculate clothing real evaporative resistance. The clothing evaporative resistance (determined in the so-called isothermal condition) calculated by the prevailing heat loss method may be called as the standard evaporative resistance. However, in the so-called isothermal condition, the wet fabric skin-clothing system may gain heat from the environment. Thus, the heat loss observed on manikin H _manikin is not equal to the actual energy H _e used for water evaporation of the wet fabric skin-clothing system. The actual thermal energy used for water evaporation H _e may be calculated by

$$ {H}_{\mathrm{e}}={H}_{\mathrm{manikin}}+\frac{T_{\mathrm{air}}-{T}_{\mathrm{sk},\mathrm{f}}}{I_{\mathrm{t},\mathrm{wet}}} $$

(A-4)

where I _t,wet is the wet thermal insulation of the tested clothing ensemble, m² K/W.

The clothing thermal insulation decreases as it gets wet. Based on the meta-analysis (Wang et al. 2015a), the relationship between the increase of total clothing weight w _t (in grams) and the reduction in the clothing thermal insulation ΔI _t was further explored. The equation used for calculating the decrease of clothing thermal insulation reads

$$ \varDelta {I}_{\mathrm{t}}\left(\%\right)=0.0000001\times {w}_{\mathrm{t}}^3-0.00016\times {w}_{\mathrm{t}}^2+0.1004\times {w}_{\mathrm{t}}\;\left(0 < {w}_{\mathrm{t}} < 1000\right) $$

(A-5)

Thus, the clothing wet thermal insulation I _{t, wet} should be calculated as

$$ {I}_{\mathrm{t},\mathrm{wet}}={I}_{\mathrm{t}}\times \left(1-\varDelta {I}_{\mathrm{t}}/100\right) $$

(A-6)

Combining Eqs. (A-2)–(A-6) with the Eq. (A-3), we can finally use the Eq. (A-7) to calculate the clothing real evaporative resistance (the air temperature in Eq. (A-7) was T _air = 34.0 °C)

$$ {R}_{\mathrm{et},\mathrm{real}}=\frac{ \exp \left(18.956-\frac{4030.18}{269-0.0132\times {H}_{\mathrm{manikin}}}\right)\times 100- \exp \left(18.956-\frac{4030.18}{269}\right)\times {\mathrm{RH}}_{\mathrm{a}}}{H_{\mathrm{manikin}}\times \left[1+\frac{0.0132}{I_t\times \left(1-1\times {10}^{-9}\times {w}_t^3+1.6\times {10}^{-6}\times {w}_t^2-1.004\times {10}^{-3}\times {w}_t\right)}\right]} $$

(A-7)