Modeling Two-way Flow in Ventus

Theory

A compartment fire drives air flow around it through buoyancy and pressure differentials. Any Zone that contains the fire and smoke path will have an extreme three-dimensional temperature gradient.

This gradient will drive flow across any large vertical openings between the fire compartment and an outside compartment. Flow characteristics will depend on the neutral plane height for both compartment one and compartment two.

Temperature differences and connections through tall vertical openings between zones can cause horizontal flow between those zones. Air, like water, is a fluid that is self-levelling; warm air will spill through openings and fill ceilings like an upside down bathtub; cold air will spill through openings in the opposite direction. At a building scale, this results in stack effect.

According to the CONTAM documentation, a doorway, or even a window, may be better characterized by a two-way flow model. Temperature differences between Zones drive vertical density gradients that create small horizontal pressure differentials across the height of openings. (Dols and Polidoro 2020)

Flow characteristics will depend on the neutral plane height; two-way flow is only possible when the neutral plane of flow is within the height of the opening. To consider these conditions and assess flow characteristics, isolate the flow as done in the CONTAM Technical Reference. The hydrostatic equation is used to calculate pressure differentials resulting from the temperature difference. (Walton 1989).

For the idealized case, in a lumped-parameter model, when no external pressures are applied:

From Figure 2, Figure 3, and Figure 4:

Additional information may be found in the ASHRAE Smoke Control Handbook, Appendix A, Fig. A1 (Handbook of Smoke Control Engineering 2024).

CONTAM Theory

Figure 5 is from the public NIST document detailing AIRNET. It is identified as Figure Four in the CONTAM technical reference (Walton 1989).

Methods

Division into Directional Flow Areas

A fire-specific multi-Zone model like CFAST divides the pressure profile into up to 10 directional flow areas, or "slabs". Visit nist.gov/cfast for more information.

CFAST splits each compartment (Zone) into one upper layer and one lower layer. The flow can be characterized through calculation of the combination of each Zone. Put into practice, a colorful visual representation of an example compartment fire (NearFldDoor.smv) emerges. See Figure 6.

The temperature difference in layers can be visualized by comparing the color of each in Figure 6. For compartment fires in the real world, a visible smoke layer forms near the fire (see this FSRI experiment: living room fire showing smoke layer).

For modeling purposes, a large building can be divided into two fields: the near-field is near the fire, and the far-field is the rest of the building (Handbook of Smoke Control Engineering 2024)

Most modern standards argue that near-fire spaces should be modeled with calculations from at least two temperature Zones (upper layer and lower layer) per compartment. (“Fire Safety in Buildings - Smoke Management Guidelines” 2018) In Ventus and ContamW, the temperature and pressure are simplified to one "lumped parameter" per compartment, making it difficult to accurately model near-field flow (Handbook of Smoke Control Engineering 2024).

One-way Flow Simulation

In this section, modify the 2-zone-contaminant.vnts model in Figure 7.

One-way Flow Results

Open Path Data and expand the filters to show all data. All flow paths are zero for all times. There is no temperature-driven circulation of flow as expected in Figure 2.

Get a quick visualization of contaminant transport through the native Ventus 2D Plot Tool as in Figure 10.

The contaminant is contained entirely in Zone 1.

Because the one-way flow path provides the only connection between Zones (and there is zero external pressure), there is no pressure differential and therefore zero flow. This is the net flow through the window – but it doesn’t account for air leaving Zone 1 and returning from Zone 2 across the vertical height of the opening. To track contaminant transport, bidirectional flow should be quantified.

Two-way Flow Simulation

Next, create a Two-way Flow simulation to track the bidirectional transport into and out of 1.

Two-way Flow Results

Open Path Data and expand the filters to show data. Now, there is temperature-driven circulation.

As in Figure 12, the sum of flow0 and flow1 add up a net flow of zero. Also, the pressure differential is 0.00136 Pa in each direction.

Get another quick visualization through the native Ventus 2D Plot Tool as in Figure 13.

This contaminant concentration data makes more sense.

When you introduce contaminants to a Ventus model with a significant horizontal temperature gradient, a two-way flow model can account for mass transport across the vertical opening.

Introduce External Pressure

Most smoke management systems will involve significant external pressures such as wind and stack effect (“Fire Safety in Buildings - Smoke Management Guidelines” 2018). These external pressures will reposition the neutral plane from mid-height of the opening and create net flow in one direction (See Figure 5).

Define an external pressure with Wind:

Results with external pressures

Open Path Data and expand the filters to show data.

As in Figure 14, the sum of flow0 and flow1 add up a net flow of 0.001546 kg/s (not zero). The external wind pressure repositions the neutral plane to create net flow in one direction, equivalent to both AREA/Area Flow Path magnitudes.

Get another quick visualization through the native Ventus 2D Plot Tool as in Figure 13.

The flow between Zones did not change enough to visually alter contaminant concentrations; Figure 13 and Figure 15 are nearly identical.