D'Ambrosio Alfano Francesca Romana, Palella Boris Igor, Riccio Giuseppe
DIIN-Dipartimento di Ingegneria Industriale, Università di Salerno, Via Ponte Don Melillo, 84084 Fisciano, Salerno, Italy.
Ann Occup Hyg. 2012 Nov;56(9):1063-79. doi: 10.1093/annhyg/mes036. Epub 2012 Jul 17.
This paper deals with the indirect evaluation of the natural wet bulb temperature, t (nw), one of the two quantities forming the basis of the well-known wet bulb globe temperature (WBGT) index, considered worldwide to be a suitable and user-friendly tool for the preliminary assessment of hot thermal environments. This quantity can be measured by a wet bulb thermometer (a temperature sensor covered with a wetted wick naturally ventilated) or, if this is not available, calculated from other microclimatic parameters (i.e. the air temperature, the globe temperature, the air velocity, and the humidity) using a quite trivial energy balance equation. Because of the strong non-linear structure of such an equation, the risk of a multiplicity of steady state solutions could result in the failure to obtain a reliable index evaluation. To dispel all doubts, this work carries out an in-depth analysis of the heat balance equation to be solved for the indirect evaluation of the natural wet bulb temperature. A preliminary investigation of each heat flow term involved in the heat balance on the sensor has been carried out; in a second phase a special continuation method has been implemented, highlighting the effect of microclimatic parameters on the multiplicity of solutions. Results show that under free convection the evaluation produces a single solution only under uniform conditions, whereas in the presence of even slight differences between the air temperature and the mean radiant temperature, there can be as many as three solutions. This phenomenon, if confirmed by a further experimental investigation, could become a difficult matter since a sensor, in principle, has to read a unique value of the quantity measured. In any case, from a numerical point of view, the presence of many values of tnw greatly reduces the possibility of an indirect WBGT calculation from the other involved physical quantities; as a consequence, the indirect evaluation of WBGT should be clearly avoided based on ISO 7243 Standard.
本文探讨自然湿球温度t(nw)的间接评估方法。自然湿球温度是著名的湿球黑球温度(WBGT)指数的两个基本量之一，被全球公认为是一种适用于热环境初步评估的、用户友好型工具。该温度可通过湿球温度计（一种覆盖有自然通风的湿棉芯的温度传感器）测量，若没有该温度计，也可利用一个非常简单的能量平衡方程，根据其他微气候参数（即气温、黑球温度、风速和湿度）来计算。由于该方程具有很强的非线性结构，存在多个稳态解的风险可能导致无法获得可靠的指数评估结果。为消除所有疑虑，本文对用于间接评估自然湿球温度的热平衡方程进行了深入分析。首先对传感器热平衡中涉及的每个热流项进行了初步研究；在第二阶段，实施了一种特殊的延拓方法，突出了微气候参数对解的多重性的影响。结果表明，在自由对流条件下，只有在均匀条件下评估才会产生单一解，而当气温和平均辐射温度之间即使存在微小差异时，可能会有多达三个解。如果进一步的实验研究证实了这一现象，可能会成为一个难题，因为原则上传感器必须读取所测物理量的唯一值。无论如何，从数值角度来看，t(nw)存在多个值会大大降低根据其他相关物理量间接计算WBGT的可能性；因此，根据ISO 7243标准，应明确避免对WBGT进行间接评估。
