Optimal local skin temperatures for mean skin temperature estimation and thermal comfort prediction of seated person in thermally stratified environments

Thermally stratified environments are universal in "real world" buildings. However, the studies on the machine learning model and mean skin temperature (MST), which was based on the analysis of Local Skin Temperatures (LSTs), were insufficient in thermally stratified environments. To create thermally stratified environments in this study, the air temperatures at the lower body parts in a climatic box were controlled independently from the upper body parts exposed in climate chamber, with 12 air temperature combinations of 22, 25, 28, and 31°C. Sixteen human subjects were recruited to collect thermal perceptions and measure their LSTs. The variations of LSTs and the optimal LSTs to estimate MST and predict thermal state were analyzed. Based on the classifications of LSTs and area of local skin, a new method using chest (0.42), forearm (0.21), thigh (0.30), and foot (0.07) was proposed to estimate MST. Its errors decreased by at least 22.8% as compared to the existing methods. Then, the model based on Random Forest was used to filter the optimal LSTs for the predictions of Thermal Sensation Vote (TSV) and Local Thermal Comfort (LTC). Results showed at least three LSTs were needed to reach a robust model prediction accuracy and generalization ability. The optimal LSTs for the predictions of TSV and LTC were (Forearm, upper arm, foot) and (Forearm, chest, thigh), respectively. This study contributes to provide the basic information of optimal LSTs to improve the accuracies of the thermal comfort predictions and MST estimation in the thermally stratified environments.

Thermoregulatory and cardiovascular responses in the elderly towards a broad range of gradual air temperature changes

This study aimed to determine age-related differences in thermoregulatory and cardiovascular responses to a wide range of gradual ambient temperature (T_a) changes. Morphologically matched normotensive elderly and young males participated. The participants wearing only shorts rested during the 3-h experiment. After 30 min of baseline at 28 °C, T_a increased linearly to 43 °C in 30 min (warming) and then gradually decreased to 13 °C in 60 min (cooling). T_a was rewarmed to 28 °C in 30 min (rewarming), and that temperature was maintained for an additional 30 min (second baseline). During the warming phase, there were no age-related differences in blood pressure (BP) and rectal temperature (T_re), despite a significantly lower cutaneous vascular conductance and heart rate in the elderly (P < 0.05). At the end of the cooling phase, systolic blood pressure (SBP) in the elderly was significantly higher than the young (155.8 ± 16.1 and 125.0 ± 12.5 mmHg, P < 0.01). There was a consistent age group difference in SBP during rewarming. Mean skin temperature was significantly lower in the elderly during rewarming (P < 0.05). T_re decreased more in the elderly and was significantly lower at the end of the experiment than the younger participants (36.78 ± 0.34 and 37.01 ± 0.15 °C, P < 0.05). However, there were no age group differences in thermal sensation. In conclusion, even normotensive elderly participants have a greater and more persistent BP response to cold than younger adults, suggesting that the elderly might be at a higher risk of cardiac events during cooling and subsequent rewarming.

Thermal comfort in environments with different vertical air temperature gradients

The use of displacement ventilation for cooling environments is limited by the vertical temperature gradient. Current standards recommend a temperature difference of up to 3 K/m between the head and the feet. This paper reviews the scientific literature on the effect of vertical temperature gradients on thermal comfort and compares this to the results of our own experiments. Early experiments have demonstrated a high sensitivity of dissatisfied test subjects to changes in the temperature gradient between head and foot level. Recent studies have indicated that temperature gradients of 4-5 K/m are likely to be acceptable, and the mean room temperature may have a greater sensitivity on the percentage of dissatisfied (PD). In new experiments, test subjects have evaluated the thermal comfort of different vertical air temperature gradients in a modular test chamber, the Aachen comfort cube (ACCu), where they have assessed vertical temperature gradients of ΔT/Δy = 1, 4.5, 6, 8, and 12 K/m at a constant mean room temperature of 23°C. The results of the different temperature gradients are in contrast to ANSI/ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy, Atlanta GA, American Society of Heating, Refrigerating and Air Conditioning Engineers, 2013) as the PD increases almost constantly with higher vertical air temperature gradients. The PD for the overall sensation increases by approximately 7% between gradients of 1 and 8 K/m. The evaluation of our own tests has revealed that vertical temperature gradients of up to 8 K/m or higher are likely to be acceptable for test subjects.