COOLING-FAN EFFICIENCY INDEX

The efficiency is the ratio of the output to the input. It can be improved by reducing input and/or improving output. In the case of fans, which are used to cool people in warm environments by increasing the air velocity around the human body, the input is the electrical energy needed to run the fan (the power requirement of a fan is almost constant, and it can be used instead of energy to make the input variable time independent), and the output is the body cooling effect.

The body cooling effect produced by a fan depends on generated air velocity and turbulence field, body area exposed to moving air, body posture, air and mean radiant temperature, air humidity, clothing insulation, metabolic rate, humidity, and skin wettedness. Sophisticated thermal manikins with full body size and a complex shape were developed and used to determine the dry-heat loss from the human body under different environmental conditions (Tanabe et al. 1994; Tsuzuki et al. 1999; Melikov et al. 2002). A manikin's body is typically divided into several segments. They can be operated to maintain constant heat flux from the body, constant body surface temperature, or to have surface temperature equal to the skin temperature of an average person in a state of thermal comfort under the particular environmental condition of the exposure. Thermal manikins can be used to measure the fan cooling effect and, thus, to determine the CFE index. Thermal manikins that can measure dry-heat loss from the human body are commonly used today, though sweating thermal manikins are under development as well (Psikuta et al. 2008). Therefore, at this stage, dry-heat loss from the human body can be used to determine the CFE. In the future, more precise or effective ways of measuring the cooling effect may be developed and used instead of thermal manikins. Clothing thermal insulation and metabolic rate (personal factors that may vary substantially in real life) can be assumed to be constant, while air humidity and skin wettedness are not taken into account. The equivalent temperature ([t.sub.eq]) is a well-known parameter that can be used to determine the CFE index. The equivalent temperature (formerly equivalent homogenous temperature) is defined as "The uniform temperature of the imaginary enclosure with air velocity equal to zero in which a person will exchange the same dry heat by radiation and convection as in the actual nonuniform environment" (SAE 1993; ISO 2004). In the definition, it is assumed that the body posture, the activity level, and the clothing design and thermal insulation are the same in both environments. The equivalent temperature is a pure physical quantity that integrates the independent effects of convection and radiation on human body heat loss in a physically sound way. The equivalent temperature [t.sub.eq] does not take into account human perception and sensation or other subjective aspects, but may correlate with them. It is important to notice that [t.sub.eq] is not a temperature that can be measured by a thermometer and that [t.sub.eq] cannot be translated to an air temperature in a complex climate (Bohm et al. 1999). The body cooling effect achieved by air movement can be quantified by the change in whole-body manikin-based equivalent temperature [t.sub.eq] from the reference condition [t.sub.eq] * (similar indoor environmental conditions but without air movement) (i.e., [DELTA][t.sub.eq] = [t.sub.eq] - [t.sub.eq] *s). The concept of [DELTA][t.sub.eq] already has been used by several authors to quantify the whole-body cooling effect of air movement (Tanabe et al. 1994; Tsuzuki et al. 1999; Melikov et al. 2002; Watanabe et al. 2005; Sun et al. 2007). Thus, the CFE is defined by Equation 1.