Factors affecting apparent plume-rise entrainment

Field measurements for 466 cases of plume rise from moderately sized sources were statistically analyzed for the ability of source and plume parameters to explain variations in the apparent entrainment coefficient used in the two-thirds law. The average value of the entrainment coefficient was 0.83, which was significantly greater than the 0.6 value characterizing entrainment caused by vertical rise of a bent-over plume and conventionally used by the USEPA. Correlation analyses demonstrated that increased coefficient values were most strongly associated with plumes that could not escape the stack wake, which increased entrainment and exerted a downward body force on the plume. Using an entrainment coefficient equal to 0.6 causes plume rise to be overestimated and ground-level concentration to be underestimated.