Abstract
Thermal environment in an urban street canyon is primarily affected by prevailing air conditions, wind flow, solar radiation as well as thermal properties of the surrounding urban structures and pavement surfaces that affect the reflection, absorption and re-emission of solar radiation. Experiments were conducted in a 1:5 scale test setup consisting of North-South oriented street canyon (height to width ratio 1.7) located in Singapore. Test cases covering two levels solar reflectance of walls (0.35 and 0.57) and road (0.12 and 0.55) were conducted in a three-month period. Environmental parameters including direct beam and diffuse solar radiation, net radiation (incoming and outgoing shortwave and longwave radiation) and wind speed were continuously measured at the top of the canyon. Thermal comfort parameters including air temperature, relative humidity, air velocity and globe temperature were also monitored continuously inside the street canyon. When the solar reflectance of canyon surfaces increases, mean radiant temperature (MRT) reduces by up to 1.2°C during daytime and 2.5°C during the night. Such reduction leads to reduced occurrence of heat stress by 34% and 42% during the day and night times, respectively, as measured by the universal thermal comfort index (UTCI). This paper further discusses the effect of longwave radiation on MRT in the street canyon due to changes in canyon solar reflectance.
Highlights
The thermal environment of an urban area depends on the heat transfer mechanism between different types of built-up surfaces as well as heat emission sources in the surrounding area
Experiments were conducted in a North-South oriented street canyon set up in Singapore to study the impact of wall and road solar reflectance on thermal comfort in the street canyon
When the solar reflectance of the walls increases from 0.35 to 0.57, mean mean radiant temperature (MRT) in the street canyon decreases by 1.2°C during daytime and by 2.5°C during night time
Summary
The thermal environment of an urban area depends on the heat transfer mechanism between different types of built-up surfaces as well as heat emission sources in the surrounding area. The increased air temperatures could be estimated by knowing the amount of absorbed solar irradiation and the re-emitted infrared radiation from built-up surfaces. Low solar reflectance of built-up surfaces can elevate outdoor temperatures by means of convective and radiative heat exchanges. Simulation studies carried out by Pearl mutter et al [3] show that pedestrian energy balance is predominantly affected by radiative exchange with urban fabrics rather than with air temperature. In this context, the aspect ratio of urban street canyons contributes significantly to surface and air temperatures and the resultant energy fluxes. The impact of outdoor thermal comfort parameter due to changes in solar reflectance of the wall and road surfaces in a street canyon setup will be investigated through an experimental setup
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