Computational fluid dynamic study on the effect of cooling tower plume in an alley of a Mega City

Abstract

In recent years, cooling towers have become more popular in mega cities mainly because of their superior energy efficiency compared with air-cooled chillers. However, under specific environmental conditions, e.g. the humid air in Hong Kong or the calm wind inside compact built structure, exhausting plume often appears as saturated air (moisture) over cooling towers that adversely affects the visual appearance of the premises. Moreover, visible moist plume arouses public concern due to the possible transport of contaminants or even harmful bacteria such as Legionnella. In this paper, a computational fluid dynamic (CFD) study was performed to examine the mechanism of plume development over cooling towers, the subsequent moisture transport and the resultant impact on the buildings nearby in a hypothetical dense urban environment. The velocity of the cooling tower exhaust is kept constant under different prevalent wind speed. This configuration examines the velocity ratio (v r) at 0.33, 0.83, 1.53 and 2.17 for atmospheric condition of light air, light breeze, gentle breeze and moderate breeze respectively. When the v r is 0.83 and 2.17 (light breeze), the short-circuiting can occur, in which a representative moisture content can reach up to 17.5 and 17.73 gM/kgDA respectively at different distance ratio. This indicates that the discharge air re-enters the cooling tower as some is trapped within the alley. Whilst, as v r increases to 1.53 (gentle breeze), short-circuiting of cooling tower is reduced to approximately by an average of 0.5 gM/kgDA for the specified distance ratio. This is due to the stronger upward draught that pushes the exhaust air further up yet still lingers within the alley. The CFD analysis shows that further increase in the prevalent wind speed suppresses the crosswind mixing of cooling tower exhaust, thus further reduces the chance of short-circuiting. This indicates the environment condition affects the severity of plume. More attention should be pay on the potential risk of public health.