Thermal sensation and percentage of dissatisfied in thermal environments with positive and negative vertical air temperature differences

Considering the percentage of dissatisfied due to local thermal sensation (PD_LTSV), a vertical air temperature difference (ΔT_d) threshold of about 3 °C was recommended in standards. However, some novel air distribution methods might create large positive (which means the head warmer than the feet, vice versa) or negative ΔT_d, with no suitable proved criteria to be used. In this study, sixteen subjects were seated in a climatic box placed in a climate chamber to evaluate thermal sensation and percentage of dissatisfied with negative and positive ΔT_d in different whole-body thermal conditions. Air temperatures were controlled independently at the upper and lower body parts, with 13 different air temperature sets combined with 22 °C, 25 °C, 28 °C, and 31 °C (i.e. -9 °C ≤ ΔT_d ≤ 9 °C). Results showed that subjects were more thermally sensitive at the upper body and with positive ΔT_d than at the lower body or with negative ΔT_d. The 80% acceptable ΔT_d range is about -8 to 7 °C in overall neutral (TSV = 0), -7 °C to 6 °C in slightly cool (TSV = -0.5) conditions, which is wider than -3 to 3 °C in slightly warm conditions (TSV = +0.5). By considering the factors of both TSV and ΔT_d, a new overall percentage of dissatisfied index (OPD_P) was proposed. Case studies show that the new OPD_P index is more precise and suitable for the evaluations of different air distributions to predict overall percentage of dissatisfied in thermal environments with vertical air temperature difference.