Frequency and Duty Cycle Effects of Intermittent Mist-Cooled Airflows on Outdoor Comfort Compared with Steady Misting

Abstract

Climate change, driven by urbanization, is increasing outdoor heat stress, diminishing thermal comfort, and affecting public health. Mist cooling, a widely adopted evaporative cooling strategy, enhances outdoor thermal comfort by lowering air temperature and elevating relative humidity through droplet evaporation. This study proposes intermittent mist-cooled airflows as a method to improve outdoor thermal comfort in hot climates, achieving similar comfort to continuous misting with reduced water use, while also enabling higher comfort levels under certain environmental conditions. The method builds on temporal alliesthesia. Intermittent cooling pulses generate fluctuating temperature gradients that induce cooler thermal sensations and higher comfort, similar to the refreshing effect of natural wind. Wind tunnel experiments under constant misting were used to validate a CFD model developed in ANSYS Fluent. After validation, parametric simulations examined the influence of mist injection frequencies of 0.3-1 Hz and duty cycles of 40-80% on temperature, relative humidity, turbulence, and evaporation under a constant airflow of 2 m/s. The instantaneous outputs of air temperature and relative humidity provided dynamic boundary conditions for a multi-segment human thermoregulation model for prediction of local thermal sensation and comfort. Compared with constant misting at the same water consumption, intermittent misting improved thermal sensation from “slightly warm” to “slightly cool” and comfort from “just uncomfortable” to “just comfortable” at an optimal frequency of 0.5 Hz and 60% duty cycle. Fluctuations amplified alliesthesia, improving average comfort by 0.59 points on Zhang’s -4 to +4 scale, while maintaining the same water consumption as continuous misting.