Re-entrainment of building exhausts may lead to poor indoor air quality, potential health hazards, worker complaints, and lower productivity. To minimize re-entrainment, the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommends minimum dilution models D-r and D-s to estimate worst-case dilutions at fresh-air intakes. The D-r and D-s models predict plume center-line (worst-case) dilution at roof level, assuming that the plume has a Gaussian concentration profile in both the vertical and lateral directions. The D-r model considers the effect of plume rise; however, the D-s model assumes negligible plume rise and is primarily recommended for wall vents and capped stacks. This paper evaluates the ASHRAE (2003, 2007) dilution models using data from wind tunnel and field experiments carried out with typical low-rise and high-rise buildings. Some comparisons with the 2011 dilution models are also presented. The effectiveness of the dilution models in modeling the downwash effect of a rooftop structure (RTS) on plume dispersion is also evaluated. Comparisons between estimated and measured concentration data obtained from present and previous studies indicate that the ASHRAE model estimates are sensitive to building geometry, stack height, exhaust speed, sampler location, and the downwash effect of RTS. Depending on the interpretation and user experience, the models may significantly underpredict or overpredict the dilution level at fresh-air intakes. Fora typical low-rise building, the ASHRAE 2003 D-r model overpredicted dilutions by a factor of 10 when the upwind RTS was within 15 m (49.2 ft) of the stack and by a factor of 2 with no RTS present. On the other hand, the ASHRAE 2007 D-r model was conservative for nearly all cases evaluated with dilution estimates 10 to 100 times lower than the measured values. The ASHRAE 2003 and 2007 D-s models underpredicted dilutions by a factor of 10 to 100 irrespective of building height and whether or not an RTS was present. The 2011 dilution models are generally more conservative than the 2007 model. For both D-r and D-s models, the predictions improved with increase in distance from the stack. Some basic knowledge of building aerodynamics is helpful in the application of the ASHRAE dilution models, which otherwise should be used with caution.

This paper investigates the downwash effect of a rooftop structure (RTS) representing a typical RTS on plume dispersion. The effect of wind direction, exhaust speed, stack location, stack height, and RTS crosswind width on the severity of the downwash effect on the plume is assessed. Wind tunnel experiments were conducted to obtain plume centerline concentrations on the roof of typical low-rise and high-rise buildings. Measurements were obtained downwind of an RTS with height h = 4 m, along-wind length I = 8 m for 3 crosswind widths w = 10 m, 20 m and 30 m. Flow visualization was also conducted to obtain a qualitative assessment of the flow downwind of the RTS. The downwash produced by the RTS caused a significant increase in roof level concentration depending on building height, stack location, stack height, exhaust speed, wind direction and RTS crosswind width. An attempt is made to provide design guidance for determining stack height required to avoid the downwash effect for an exhaust placed downwind of the RTS. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.

A wind-tunnel study has been carried out to assess wind pressures acting on parapets, including their top surfaces. Local and area-averaged pressure coefficients were measured on parapets of flat-roof models with a length to height ratio (L/H) of 1:1, 2:1, and 3:1. The results were obtained for full-scale equivalent parapet heights of 1 and 2 m and for wind directions ranging from zero to 315°. The local wind load on the parapet was found to be approximately 30% larger at the windward corner of the building than at the midspan location. Maximum parapet loads for the low building model were approximately 30% larger than those for the cubical model. Parapet height did not significantly affect the peak local load on the parapet except in the corner region, where the inward load (toward the roof) for the 1 m parapet was 25% higher than that for the 2 m parapet.