Obstructed Plume

This page provides an overview of the development and results of the obstructed plume methodology as outlined in EPRI 3002005578 / NUREG-2178

Chapter 5: Correlation of Obstructed Plume Models with Heskestad Plume Predictions

The Heskestad plume correlation is typically used for unobstructed plume simulations and modified to account for changes in plume temperature due to an obstruction. This modification is estimated through a comparison of the results of fire model simulations against the existing Heskestad plume correlation. The software used to simulate and estimate fire plume temperatures subject to obstructions in this study is FDS Version 6.0.1.

EPRI 3002005578 / NUREG-2178 presents methodology to correct for the ZOI due to the obstructed plume effect encountered in electrical enclosures.

Plume flows subject to different obstruction geometries. From left to right: flat plate, arch, three-wall, unobstructed

In order to investigate the effect of the enclosure metal top on the plume, a review of past studies and plume theory was conducted. From the review, a series of FDS simulations were conducted. The FDS simulations were performed used the following parameters:

• Room size ~ 16½ ft. square, 20 ft. height
• Cabinet size = 1, 2, 3, 4 ft. square, 7½ ft. height
• FDS grid size ~ 1½ in. cube

Using FDS, 156 simulations were performed

• 39 unobstructed
• 117 with obstructions

The fire scenario configurations were also varied to study the range of effects. The parameters varied include:

• HRR
• Fire diameter
• Fire base height, within the cabinet
• Number of cabinet walls

Unobstructed and Obstructed Temperature Predictions

Chapter 6: Correction to Plume Zone of Influence due to Obstructed Plume Effects

Chapter 6 discusses the impact on the fire plume modeling. The results determined that the unobstructed model overpredicts the vertical zone of influence. To correct this, an obstructed plume bias, B = 0.62, which is equivalent to a reduction of 38% for estimating the plume temperature rise is recommended for use.

Heskestad versus Obstructed Plume Temperature - Bias and Uncertainty

In other words, the ZOI credit for obstructed plume offers a reduction in vertical ZOI of 24%.

Recommended ZOI for an Electrical Enclosure Fire. Left: Without top obstruction, Right: with top obstruction credit

The thermal radiation (horizontal) ZOI extends farther than the plume will be shifted by the top obstruction. This means that horizontal ZOI is unaffected by the obstructed plume.

Obstructed Plume Shift

The obstructed plume correction does not apply to:

• Electrical enclosures where more than 5% of the total top surface area is open (sealed penetrations do NOT count as openings)
• Fires with a base elevation located below the half height of the enclosure
• Configurations that include burning of secondary combustible materials
• The analysis that determines the hot gas layer (HGL) damage
• The evaluation of the horizontal thermal flame radiation ZOI

This is ONLY applicable for electrical cabinet ignition sources.