Evaluation of factors affecting mean radiant temperature in a public apace (Case study: Shanbedi passage way in Bushehr)

Rapid urban growth and expansion cause reduction in the level of thermal comfort in urban open spaces. Reducing the use of air-conditioning systems in buildings is one of the positive interventions for protecting the environment in the cities in order to create suitable thermal conditions in the interior space. There is a direct relationship between thermal parameters of interior spaces and the outside ambient temperature. Creation of fair exterior condition for buildings and public spaces in city is the first and foremost action in order to control and optimize the thermal behavior of buildings. In a warm climate, creation of enclosure in urban open space leads to create more shadow and decrease radiant temperature which they result in improvement of comfort condition and declining in building energy use for cooling and saving energy consumption. Today, Outdoor design should be put in our urban planning for improvement of environmental conditions.
Using appropriate Enclosure in urban open spaces especially in hot and humid cities, one can improve the microclimate condition. Todays, morphological changes, plants and landscape reduction, air pollution and inappropriate construction lead to disturb the environment and increase temperature. Historic fabric is a good source for harmonizing urban planning with the climate. The meteorological parameters such as temperature, humidity, wind speed and radiant temperature have significant role in human and thermal comfort.
Thermal comfort is a necessary factor that should be considered in every stages of urban design process. One of the main issues in the assessment of the human comfort in outdoor spaces (especially in sunny outdoor conditions) is the amount of mean radiant temperature which is the sum total of short-wave and long-wave of absorbed radiation flows by the human body. The mean radiant temperature assessment is more sensitive because radiation is one of the meteorological factors which has a huge effect on human thermal comfort. The Mean Radiant Temperature is used as a benchmark more than the air temperature or the temperature for analysis of the impact of weather on the comfort of people. In the past, mean radiant temperature of historic fabric was balanced with creation of shades and it can be seen in several design and construction strategies like high walls, narrow passage lanes, planting trees and awnings on façade and its glaring area. The mean radiant temperature is influenced by climatic parameters such as temperature, humidity, radiant temperature, wind, orientation and space enclosure.
The aim of this study is to evaluate the effect of urban fabric and physical design on the mean radiant temperature to control level of thermal comfort for inhabitants. Focus on the mean radiant temperature is essential for pedestrian and urban open spaces design. Bushehr is a coastal city in the south-west of Iran (Latitude: 28°55 N, Longitude 50°55 E) which has a hot and humid climate. In cities, promoting use of streets and open spaces by pedestrians has physical, environmental, economic and social benefits. Thus, ensuring people's comfort in open spaces is essential for improving the quality of urban life. In a complex urban environment, the radiation is considerably different in open spaces because of buildings and vegetation shading and due to various surface materials.
Shadow creation and conducting wind flow into open spaces are very important for Bushehr to improve comfort. Shanbadi district is in the eastern part of historic fabric of Bushehr. In historic fabric of Bushehr using enclosure, orientation of passages and buildings and use of appropriate materials, thermal stress in city was reduced through ventilation and shading. Also high density of physical environment has influenced the thermal comfort. Street enclosure has been created by varying the width of street and height of the buildings. Considered passage way in Shanbadi has north-west south-east direction. Shanbadi is one of the 4 historic districts in Bushehr and have lots of climatic design strategies which create harmony of urban fabric and local climate. Some passages leading to this area are input and output of the wind in different hours.
2. Materials & Methods
In this study meteorological data are obtained through field study in three seasons (summer, fall and winter) and in 4 period of time. In this research summer study is analyzed. The site is in shanbedi district, one of the four old district in Bushehr. 7 sections with different enclosure were chosen in study area and climate data is surveyed during the day separately in each of these sections. RAY man 1.2 software was used for calculating the mean radiant temperature. RAY man 1.2 software is a tool for calculating the mean radiant temperature and heat index such as PET, PMV and SET in urban studies. Mean radiant temperature calculation in RAY man software requires some information such as geographic location (latitude, altitude and time difference with Greenwich), the time of study data (date and time of study), characteristics of meteorological data (air temperature, humidity , cloud cover, wind speed and vapor pressure), surrounding surface reflection, scattered radiation ratio, the Bowen ratio and the global radiation. Air temperature and relative humidity for calculation of the mean radiant temperature were determined by WBGT device every 30 seconds and the amount of air flow by ANOMETR 3880 every 5 minutes. Both devices were set at a height of 1.7 meters above the ground.
3. Results and Conclusions
The result indicates that in the area surveyed in the range period of 10 to 12 o'clock enclosure has the most effect on the mean radiant temperature. In many hours of the days wind has the most influence on the mean radiant temperature. There is a direct relationship between wind speed and enclosure. At 8 to 10 and 10 to 12 o'clock there is a relatively strong relationship between wind speed and enclosure, but between the period of 12 to 14 o'clock and 14 to 16 o'clock the impact of the wind speed on enclosure is reduced until it loose its effect. In this section one of the influential factor increasing wind speed is multiple passages lead to this section that guide the wind to this area. In different hours of a day we can see change in mean radiant temperature due to change of wind speed changes. Another influential factor, especially in the middle of the days, is the solar orientation when it is vertical and other factors have less influence than enclosure on the mean radiant temperature. For this time of the days awning and canopy in street is advised to decrease the amount of mean radiant temperature. Generally, in the most hours of the days wind has the greatest impact on improvement in the mean radiant temperature in this area.
To improve thermal comfort in the new urban context and decrease mean radiant temperature, especially in the middle of the day, shade should be created in the pedestrian path areas, sidewalks and public spaces. It can be made through artificial roofs, locating trees which provide shadow or building projections. Smart shading devices can be used to respond to need of radiation and shading in different time of the day and different seasons. These shading devices are capable of opening and closing automatically. Other green design parameters such as photovoltaic cells and water facilities to store water can also added to this devices. It can be a good design alternative for maintaining of thermal comfort in urban open public spaces in Bushehr.
In addition, creation of space with enclosure degree (ratio of height to width) of 1.73 make it possible to create a favorable space. Enclosure can be created through use of the large tree canopy with radius more than 2m in streets which the width is more than its height. Awing projection should be at least half of the height of the openings to create thermal comfort and reduce mean radiant temperature.