Keywords

1 Introduction

Urban heat island and extreme heat have been occurring more frequently in cities across the world [1], increasing the discomfort of people and leading to a rise in the rate of heat-related mortality [2]. Passive and active cooling are the two main methods for adjusting the human thermal environment [3]. Passive cooling technologies, such as retro-reflective materials and green vegetation, are less effective for rapid cooling of specific areas. In contrast, the active cooling technology adopting artificial equipment allows for fast and stable cooling of the environment [4].

The mist spraying is an active cooling technology based on the principle of evaporative cooling [5], which can effectively affect micro-climate [6]. Ulpiani [7, 8] and Giuseppe [9] conducted experiments in Ancona and Roma cities in Italy and found that the spraying could reduce air temperature by up to 7.9℃ in hot-dry climates. Sureshkumar [10] carried out experiments in India. Yamada [11] and Oh [12] executed experiments in Japan. Zhang [13] and Huang [14] completed their experimental studies in Qingdao and Shanghai cities in China. Many studies showed that spray was more effective in hot-dry regions, while some papers had also demonstrated the positive effect of spray in hot-humid areas. Mist spray also affects the thermal perception of the human body. Ulpiani [7] and Vanos [15] believed that people showed high satisfaction with the wetness of the mist area in hot-dry climates. While in hot-humid climates, Desert [16] and Wong [17] found that severe discomfort was increased due to increased moisture levels of the skin. The effect of increased humidity caused by mist spraying on thermal perception has not been consistently observed in related studies. The benefits of mist spray for enhancing physiological comfort could vary for people in different regions and climates [18]. As Ulpiani [5] and Meng [19] concluded, the cooling potential was affected by climatic factors and weather conditions as well as the adaptation of local population. On the question of determining the suitable conditions for the application of spray, Huang [20], Yamada [11] and Zheng [21] claimed different conclusion. It is crucial to conduct targeted experimental research.

There is no clear conclusion as to when spray should be used and its cooling potential in hot-humid regions yet. Consequently, this study aimed to investigate the relationship among the weather indicator, spray cooling effects, and improvements in human thermal perception. Considering the potential damage to the spray equipment's lifespan and the increase in water consumption, an intermittent spraying aligned with practical application scenarios was adopted to avoid excessive accumulation of water vapor in the air. This study established a periodic mist spray system to analyze the dynamic effect on the microclimate when the spray was on, running, and off within all the experimental periods. The research explored how the ambient thermal environment impacts the efficacy of misting. Additionally, participants entered the mist at different periods and assessed their thermal perception levels every minute. The dynamic influences of the spraying environment on the thermal sensation and thermal comfort of local residents were evaluated in different ambient thermal environments. The findings are expected to provide guidance for the application, control, and adjustment of the mist spray system in outdoor engineering scenes.

2 Methods

2.1 The Mist Spray System

The mist spray system was set up before the experiment. After the water was filtered, it entered the system through the inlet pipe. Following the start-up and debugging of the host machine, the water was pressurized and passed through the high-pressure outlet pipe. Finally, the mist was ejected from high-pressure nozzles (see Fig. 1).

The experiment period was set to 30 min. The mist system ran non-stop for the first 25 min of each period and stopped running in the last 5 min. A lightweight square shed was constructed with a side length of about 2.20 m. Four nozzles were installed near the center of the shed, 2.30 m above the ground. The height adopted based on the reference to the work of Su [22]. Experiments were conducted in and around this shed (in Dongguan City, China, during June 27–29, 2022), including environmental measurement and questionnaire survey.

Fig. 1.
figure 1

Schematic illustration of the mist spray system.

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2.2 Environmental Measurement

The instrument used in the environmental experiment to measure air temperature and relative humidity was a HOBO logger, which recorded data at intervals of 30 s. The probe of the instrument was positioned 1.10 m above the ground. A radiation box with three blades providing shelter was placed outside the HOBO instrument. Two groups of HOBO instruments were utilized in the experiment. The first group was positioned in the center of the spray area to measure the temperature & humidity changes within the mist. The second group was placed at the four corners on the outer edge of the spray area to measure ambient temperature & humidity changes surrounding the mist (see Fig. 1). By comparing the measurement results of these two groups, the cooling & humidification effects of the mist could be analyzed. A weather station (Watch Dog, WD for short) was also included in the study, recording solar radiation, ambient temperature, wind speed, and other background environmental data at intervals of 10 min. It was located in an open area 25 m away from the mist spray area, with no obstruction from buildings within a 45-m radius.

2.3 Questionnaire Survey

Dozens of participants were invited to the experimental site. Each participant was given one questionnaire form with three parts: 1. Personal information including gender, age, and height; 2. Thermal perception questions before participants entered the mist area; 3. Thermal perception questions after participants entered the mist area. Thermal perception aspects included thermal sensation, thermal comfort, and preferences for temperature & humidity. Thermal sensation evaluation adopted 7-point scales from Very hot (3) to Cold (–3). Thermal comfort also employed 7-point scales from Very comfortable (3) to Very uncomfortable (–3). In addition, the 5-point scale from Substantially increase (2) to Substantially decrease (–2) was used to rate the preferences for temperature and humidity.

After filling in their personal information, participants completed their first assessment outside the spray area, then entered the mist and performed the second assessment, and at one-minute intervals thereafter. The mist spraying kept running continuously for each participant during the questionnaire assessment.

3 Results

3.1 Experimental Results and Analysis

The experiment lasted from 13:00 to 16:00 on June 27, 8:30 to 16:00 on June 28, and 8:30 to 13:00 on June 29, including 30 experimental periods. During the experiment, the maximum solar radiation was 967 kW/m2, with the highest ambient temperature of 36.9℃, while the average wind speed was 0.18 m/s. This result suggests a typical summer heat climate in Dongguan City. Since there was little wind on the days of the experiment, the mist did not travel beyond the spray area. The measurement results from the weather stations were compared with the mean results from the second group of HOBO instruments located at the four corners of the outer edge of the spray area. The maximum differences in temperature and humidity were found to be 0.53℃ and 6.46%, respectively, while the average differences were 0.53% and 3.21%, respectively. Measurement results from HOBO instruments showed consistency to the weather stations, indicating that they were minimally affected by the mist. Therefore, they can be considered as representative of the ambient temperature and humidity outside the mist spray area for comparison purposes.

Figure 2 compares the temperature and humidity inside and outside the spraying area. The variations are essentially the same, with a decrease in air temperature and an increase in relative humidity. Additionally, the periodic changes in temperature and humidity align with the time periods of the mist spray system. At the start of each period, the air temperature drops, and the relative humidity rises. When the system stops running after 25 min of each period, the air temperature increases, and the relative humidity decreases, approaching the values observed outside the spray area at the end of the period. Throughout the experiment, the spray reduces the air temperature by approximately 0.12℃ to 5.68℃, with an average decrease of 2.42℃. Moreover, the relative humidity in the mist area increases by around 0.04% to 22.21%, with an average humidification of 9.02%.

Fig. 2.
figure 2

Comparison of the temperature & humidity inside and outside the mist area during the experiment: (a) Air temperature on June 28th; (b) Relative humidity on June 28th.

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Figure 3 analyzes the relationship between ambient temperature and the magnitude of cooling & humidification. The significance (p-value) is reported using a T-test, as are the graphs below in this article. The data indicate a strong correlation, which aligns with the findings of Desert [16]. The analysis demonstrates that as the ambient temperature increases, the cooling and humidification effect of the mist spray becomes more pronounced. For every 1 ℃ increase in ambient temperature beyond 31 ℃, the temperature reduction caused by spraying increases by approximately 0.5 ℃. At an ambient temperature of around 31.28 ℃, the mist spray can achieve a temperature reduction of about 1.0 ℃. While at an ambient temperature of about 35.48℃, the mist spray can lower the temperature by approximately 3.0 ℃.

Fig. 3.
figure 3

Relationship between ambient temperature and magnitude of cooling & humidification.

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3.2 Results and Analysis of the Questionnaire Survey

A total of 57 participants (29 males and 28 females) were invited to participate in the experiment, with 86% aged between 19 and 29 (8 persons aged under 20, 41 persons aged 20–29, 5 persons aged 30–39, and 3 persons aged over 40), entering the mist area randomly under different ambient environmental conditions.

Figure 4 shows thermal perception rating results at different times throughout the questionnaire survey. Before participants entered the mist, 89% of the participants felt their thermal sensation above 2 (Hot), and 77% reported 3 (Very hot), with an average rating result of 2.65. In terms of thermal comfort, 79% of the subjects selected -2 (Uncomfortable), with 53% selecting -3 (Very uncomfortable). The average rating result was -2.23. A total of 98% of the participants believed that the air temperature should be reduced. 86% of the participants were adaptive to the ambient humidity, while 14% were looking forward to a slight increase in humidity.

Among the participants who had just entered the misting area, only 2% reported their thermal sensation 3 (Very hot), while 39% selected 1 (Slightly hot). The average rating was 1.19. During the subsequent 5 min, the percentage of participants rating 0 and 1 increased to 35% and 54%, respectively. The average thermal sensation rating gradually decreased to 0.65. No participants reported 3 after 1 min in the mist. Regarding thermal comfort, after entering the misting area, 30% of the participants reported below -1 (Slightly uncomfortable), 35% selected 0 (Neutral), and another 35% selected higher than 1 (Slightly comfortable). The average rating was 0.05. Over the next 5 min, the percentage of participants rating 0 and -1 decreased to 5% and 7%, respectively. No participants reported -2 (Uncomfortable) after 2 min in the mist. As the participants continued to stay in the mist, the percentage of participants reporting 2 (Comfortable) gradually increased, reaching 37% towards the end. The average rating for thermal comfort eventually reached 1.42. It seemed that there was a larger range of change in thermal comfort (−2.23 to 1.42) compared to thermal sensation (2.65 to 0.65), indicating that the mist spraying provided a greater improvement in thermal comfort than in thermal sensation.

Fig. 4.
figure 4

Questionnaire results at different moments: (a) Thermal sensation; (b) Thermal comfort; (c) Temperature preference; (d) Humidity preference.

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When entering the mist area, 68% of the participants believed that the air temperature should be reduced, while 32% voted to remain constant. However, the desire to lower the air temperature steadily decreased over the next 5 min. Despite 61% of participants feeling “Slightly hot” or “Hot”, only 35% believed that the air temperature should be “Slightly decreased”. On the contrary, the percentage of participants who believed that the temperature could remain constant increased to 65% after staying in the mist for 5 min, indicating that they were gradually adapting to the temperature in the mist environment. Regarding humidity preference, most participants were comfortable with the ambient humidity outside the misting area. When entering the spraying, the percentage of participants who felt the humidity should remain unchanged dropped to 53%. Over the next 5 min, the percentage of participants who desired a slight decrease in humidity increased to 49%. Participants were able to feel the humid environment. As they spent more time in the mist, their demand for reduced humidity increased. However, participants tend to have a high tolerance for high humidity. By the 5-min mark, with 61% of participants were requesting lower humidity, and only 7% feeling “Slightly uncomfortable”.

3.3 Comparative Analysis of Experimental and Questionnaire Results

The relationship between thermal perception evaluation results and ambient temperatures was analyzed. Figure 5 illustrates that there is a correlation between them prior to participants entering the mist area. When the ambient temperature exceeds 31.3 ℃, most participants feel “Slightly hot”. When it exceeds 32.6 ℃, most participants feel “Hot”. Additionally, when the ambient temperature surpasses 32.4 ℃ and 34.4 ℃, most participants feel “Slightly uncomfortable” and “Uncomfortable”, respectively. Once the subjects entered the mist, the correlation weakened. When the temperature is above 31.7 ℃, most participants feel “Slightly hot”. People are more likely to identify “Hot” outside the mist area at the same temperature, with a sudden increase in the tolerance for hot environment after entering the mist area.

Fig. 5.
figure 5

The relationship between evaluation result and ambient temperature outside the mist.

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The change in participants' thermal perception upon entering the spray area is related to the immediate temperature difference between inside and outside the area, as shown in Fig. 6. The increase in thermal comfort outweighs the decrease in thermal sensation. For every 1 ℃ increase in temperature difference, the thermal sensation decreases by 0.53, while thermal comfort increases by 0.64. The thermal sensation rating decreases for 95% of participants, leading to a more comfortable feeling for all individuals. Even though the air temperature inside the mist remains relatively high after cooling, participants still feel more comfortable.

Fig. 6.
figure 6

The relationship between the magnitude of variation in thermal perception and the instantaneous temperature difference.

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By combining the results of Figs. 3, 5 and 6, it could infer suitable ambient thermal environmental conditions for the mist spray, as well as the potential for cooling and improvement in thermal perception for the local population.

Previous research [11, 20] has suggested that temperature above 30 ℃ is favorable for spray application. However, this study shows that when the ambient temperature is 30 ℃, the average level of thermal sensation is −0.29, and the average level of thermal comfort is 0.44. This indicates that local residents, who are accustomed to hot weather, can tolerate such outdoor thermal conditions. Although the mist can lower the temperature by approaching 0.40 ℃, its necessity appears to be relatively low.

When the ambient temperature reaches 32.5 ℃, the average level of thermal sensation is 1.88 (approaching Hot), with the average level of thermal comfort −1.03 (around Slightly uncomfortable). At this point, it is advisable to consider utilizing the mist spray as it can lower the temperature by 1.58 ℃. After participants enter the mist area, the average level of thermal sensation decreases to 1.02 (Slightly hot), while the average level of thermal comfort increases to 0.54 (between Neutral and Slightly comfortable). 32.5 ℃ may be considered as an ambient temperature threshold.

At an ambient temperature of 34 ℃, the average level of thermal sensation is 2.64, while the average level of thermal comfort is −1.80. When participants enter the mist, which provides a cooling effect of 2.30 ℃, the average level of thermal sensation decreases to 1.39, and the average level of thermal comfort increases to 0.23. Although most of the participants still feel slightly hot, they do not feel uncomfortable. This indicates that the mist system is effective. Spraying could help local residents maintain a physiological state close to slightly hot and neutral comfort.

4 Discussions

Since the amount of spray is assumed to be constant and completely evaporates, the amount of cooling and humidification should not change with the ambient temperature changes. However, in the experiment, some of the mist might be attached to the subjects' hair, skin or surrounding objects, such as the surface of the radiation box and the ground surface, without evaporating. In other words, the amount of spray is not equal to the amount of evaporation. Due to the changing of the participants and environment, the actual amount of mist evaporation was changed, that may be the reason that the amount of cooling and humidification varied with the ambient air temperature, which is consistent with Zhang's research [13].

It was observed that participants were able to perceive the humid environment, and this sensation did not diminish over time. However, the presence of moisture in the air did not compromise the effectiveness of the mist spray. Individuals living in hot-humid climates generally have a higher tolerance for humidity, and they provided positive feedback when evaluating their thermal comfort. This supports the findings from other studies in the literature. Consequently, humidity is not considered a key indicator for triggering the activation of the spray in this study.

The experiment showed that the temperature variation and changing rate caused by the on-off operation of the system were influenced by the ambient temperature. The running and stopping time of the spray may also lead to different experimental results. In order to fully explore the potential of spraying, the number of subjects should be increased to enhance the credibility of the research conclusions, and the participants’ age, height, and weight can be more diverse in future study.

5 Conclusions

This paper studied the effects of the periodic mist spray on the outdoor thermal environment and the thermal perception of local residents. Main conclusions are enumerated as follows: The spray could reduce the temperature by an average of 2.42 ℃, with a maximum cooling value of 5.68 ℃. In hot weather, the participants felt a greater improvement in thermal comfort than in thermal sensation due to the mist spray, and the pleasure brought by the cooling continued to increase as they stayed longer in the mist. Higher ambient temperatures led to a more substantial cooling and humidification effect. The mist spray system should be activated if the ambient temperature reached 32.5 ℃, and the mist could help local residents maintain a physiological state close to slightly hot and neutral comfort.

In conclusion, the mist spray is expected to improve the outdoor thermal environment more efficiently and provide people more comfortable experience. Further research will be conducted to facilitate the application, control, and adjustment of the mist spray system in actual outdoor scenes.