This paper is based on findings resulting from ASHRAE Research Project RP-1600.

The primary purpose of this study is to develop engineeringmethods to assess the impact of increased makeup airvelocity in atria. The current restriction defined by NFPA 92(NFPA2015) states: "The makeup air velocity shall not exceed200 ft/min (1.02 m/sec) where the makeup air could come intocontact with the plume unless a higher makeup air velocity issupported by engineering analysis." This limitation not onlylimits creative and aesthetic atria designs but may also representa significant cost. This study analyzes the effect of makeupair injected by a variety of vent sizes at elevations at or belowthe limiting elevation of the flame through numerical simulations.This study focuses on identifying worst-case scenariosfor the interaction of makeup air with an axisymmetric plumeby applying computer modeling to simulate multiple configurations,observe the results, and adapt further simulations toelicit the most extreme cases. A mass flow rate diagnostic isused to assess the increase in entrainment (i.e., smoke production.)

This mass flow diagnostic is developed to provide acomparative analysis, assessing the increase in the rate ofsmoke production with a specified makeup air velocity withthat produced with no mechanical makeup air. The proportionalincrease in entrainment is defined asanalpha factor.Themost significant smoke production increase and smoke layerstabilization descent is associated witha1MW(950 Btu/s) fire,with lesser increases observed for 2.5 and 5 MW (2370 and4740 Btu/s) fires. As the makeup air is introduced further fromthe edge of the flame, the apparent effect of the airflow velocityis reduced.