Influences of street aspect ratios and realistic solar heating on convective heat transfer and ventilation in full-scale 2D street canyons

Abstract

Considering the influence of natural solar heating, this paper numerically evaluated the sustainability of various urban street layouts from three different views: the pedestrian-level ventilation capacity assessed by the normalized net escape velocity NEVped; the transport and removal processes of heat and vehicular pollutant across the street top surface; the convective heat transfer capacity (CHTC, convective heat transfer coefficient) at each wall surface and from the entire street unit to the outer atmosphere. The airflow and heat transfer for the full scale 2D street canyons (aspect ratio AR= H/W= 1,3,6, W= 24 m) were simulated with the natural solar heating at local solar time (LST) of LST09 (9 a.m.), LST12 (12 a.m.) and LST15 (3 p.m.).Results show that: 1) For pedestrian-level ventilation (NEVped), the isothermal single vortex exists when AR = 1, 3, producing one order greater NEVped than AR= 6 in which two main vortices appear. Thus, high-rise deep street design (e.g., AR= 6) should be avoided due to poor ventilation. Solar heating leads to more complicated multi-vortex structure and basically enlarges NEVped as AR= 6. 2) Turbulent diffusions always dominate pollutant transfer across street top surface, but for heat removal, the dominant factor varies between mean flows and turbulent diffusions, which depends on ARs and solar angles. Pollutant and heat transfer should be evaluated together when assessing the sustainability of urban layout. 3) CHTC distributes ununiformly at different building surfaces and varies with aspect ratios and solar angles. The total convective heat transfer capacity over the unit lot area or for the entire street unit can be enhanced in deeper canyons.