Comparative Analysis Between Air Movement, Air Temperature and Comfort Case study: Hot and Dry region of Iran

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Abstract

Sustainable and energy efficient design issues are key to the way good buildings are designed in the 21st century. No building designer today should be without the basic skills offered by the unique multidisciplinary studies of architectural design, based on energy use in buildings. However, this paper is focusing of effect of air movement on human comfort, as a key element of building design. Two variables namely; air movement between spaces and shading of spaces are important in urban design, particularly in hot and dry condition. They are also equally important for human thermal comfort, when people are walking or working or relaxing in the urban spaces. Therefore, as related to urban design, the main concern is how to provide the potential for cooling by suitable air movement and enough shading spaces. Under harsh condition of hot-dry cities, it is difficult to provide good outdoor open spaces such as squares, streets and so on, especially when the ambient wind speed is under high ambient temperature. Regarding the daytime conditions of cities in this region, it is commonly assumed that urban temperatures are higher than rural ones, due to lack of vegetation and heat generated in the towns. Because of such phenomena, air motion sometimes is horrible and shade of urban sepses is not easy to get. Hot-dry regions commonly have strong winds during daytime, where outdoor temperatures exceed about 32°C, so urban open spaces are not desirable. As mentioned above, the main objectives related to the street layout are to provide maximum shade in summer for pedestrians and minimum solar exposure of the buildings along the streets. In line with shading, the effect of air movement is essential and depends on environmental temperature and humidity, as well as on the clothing and metabolic rate of people. When air temperature is above the skin temperature, the effect of air movement will be the same as other climatic factors and the increase of air movement will raise the skin temperature. As the temperature changes, the level of clothing, the air movement (which can cool the body by convection and/or evaporation of sweat) and the moisture of the skin will change. We should know, air movement is more noticeable when the air is cool and the difference between skin and air temperature is large. Conversely, if the air is only slightly below skin temperature, very large increases in air speed are needed to achieve an increase in convective cooling. However, in what temperature this change will happen? This is the main question of the present paper. ASHRAE standard - 55 sets an upper limit of around 0.2 m/s (assuming typical turbulence around 40%) for air velocities within the basic comfort zone to reduce the risk of discomfort from drafts. In this study air movement reduced discomfort from heat at temperature of 32.5?C; below this temperature there were few subjects indicating heat discomfort. A theoretical analysis of the data suggests that where the air velocity is above 0.1 m/s, this allowance can be equivalent to rising the comfort temperature. However, air velocity is one of the important variables for achieving comfort condition without using electrical or mechanical energies.

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