ASHRAE ST-16-002

Abstract

Scope: The primary purpose of this study is to develop engineering methods to assess the impact of increased makeup air velocity in atria. The current restriction defined by NFPA 92 (NFPA2015) states:“Themakeupair velocity shall not exceed 200 ft/min (1.02 m/sec) where the makeup air could come into contact with the plume unless a higher makeup air velocity is supported by engineering analysis.” This limitation not only limits creative and aesthetic atria designs but may also represent a significant cost. This study analyzes the effect of makeup air injected by a variety of vent sizes at elevations at or below the limiting elevation of the flame through numerical simulations. This study focuses on identifying worst-case scenarios for the interaction of makeup air with an axisymmetric plume by applying computer modeling to simulate multiple configurations, observe the results, and adapt further simulations to elicit the most extreme cases. A mass flow rate diagnostic is used to assess the increase in entrainment (i.e., smoke production.) This mass flow diagnostic is developed to provide a comparative analysis, assessing the increase in the rate of smoke production with a specified makeup air velocity with that produced with no mechanical makeup air. The proportional increase in entrainment is defined asanalpha factor.The most significant smoke production increase and smoke layer stabilization descent is associated witha1MW(950 Btu/s) fire, with lesser increases observed for 2.5 and 5 MW (2370 and 4740 Btu/s) fires. As the makeup air is introduced further from the edge of the flame, the apparent effect of the airflow velocity is reduced.