Influence Of Mist Spraying Design Parameters On Pedestrian Thermal Comfort In Outdoor Open Space In China’s Cold Region

Global warming and urban heat island（UHI）caused by rapid urbanization deteriorated the urban thermal environment and reduced the thermal comfort of urban outdoor open spaces.One small–scale thermal mitigation approach is to install mist spraying systems that are flexible and cost-effective.It has been widely used around the world.Nowadays,to relieve outdoor heat stress and support energy conservation and emission reduction strategies,optimizing the design of mist spraying systems is needed.However,few studies have focused on the influence of design parameters of mist spraying systems on outdoor thermal comfort.Therefore,nozzle density and height were selected for this study to investigate the effect of nozzle density and height of mist spraying systems on pedestrian thermal comfort.The experiment was conducted in an open square（Sky View Factor:0.598-0.699）on a campus in Xi’an,China.Different misted environments were created by changing nozzle densities（4,6and 8 nozzles/4.5m²）and heights（2.3m,2.7m and 3.1m）.Thermal perceptions were investigated using longitudinal questionnaire surveys and physiological parameters paired with meteorological measurements.Universal Thermal Climate Index（UTCI）and skin temperature（ST）at five body parts（forehead,upper arm,chest,low back and upper leg）were selected to evaluate thermal perception and physiology.Results demonstrated that:（1）The effect of mist spraying on increasing relative humidity（RH）and reducing globe temperature（T_g）and mean radiant temperature(T_mrt)increased significantly as the nozzle densities increased,and the effect of humidification declined significantly as the nozzle height increased;（2）After entering the misted environment for 10minutes,Subjects’thermal sensation vote（TSV）decreased as the nozzle densities increased and increased with nozzle height increased;Subjects’wet sensation vote（WSV）decreased as the nozzle number increased;（3）After entering the misted environment for 10 minutes,ST declinations at each body part were:upper arm（0.7℃）>forehead（0.69℃）>chest（0.59℃）>upper leg（0.38℃）>lower back（0.09℃）;When the nozzle height were 2.3m and 2.7m,the reduction in skin temperature of the upper arm and the reduction in Mean Skin Temperature（MST）of the subjects increased significantly as the nozzle densities increased;（4）Meteorological factors were the most important factors affecting subjective perception.For thermal perception,psychological factors had a greater influence than individual factors,and for wet perception,individual factors had a greater influence than psychological factors.（5）The UTCI reduction resulting from adding a nozzle was equivalent to lowering nozzle height by 0.82m.A model of the nozzle density and height was built to predict when respondents would perceive neutral UTCI in a misted environment.Our results can provide a theoretical basis and practical guidelines for effectively mitigating heat stress in urban open spaces when designing mist spraying systems.