NSHEV Free Flow Area Simulations using CFD

The free flow area of NSHEV (Natural smoke and heat exhaust ventilation) systems can be calculated using a validated NSHEV simulation setup via CFD. The purpose of NSHEV systems is to remove heat and smoke in case of fire.

NSHEV Simulation: Pakhus 54

At Aerotak, we have developed a validated CFD model able to capture the experimental results specified in EN 12101-2. The CFD model be used to determine the total free flow area of a NSHEV system accourding to “Bygningsreglementets vejledning til kapitel 5 – Brand”. To demonstrate the process, consider the NSHEV simulation at Pakhus 54, as shown in Figure 1a and 1b.

As seen in Figure 1b, the windows are not necessarily identical (different types) and they may be open or closed. These circumstances are taken into account in the CFD model.

CFD Simulation Setup

The CFD model consists of a 1:1 scale window module (shown in Figure 2a) fitted in a virtual wind tunnel. At the wind inlet a velocity of 10 m/s is applied. At the pressure inlet, a pressure of 8.5 Pa is applied. This is shown in Figure 2b.

As the wind may come from different directions, a range of different wind directions is tested by rotating the window module accordingly in the wind field. Here, five different directions were simulated as shown in Figure 3:

The mass flow rate of ventilated air (fume) changes with wind direction. Based on the simulated mass flow rate, a discharge coefficient is then calculated as specified by EN 12101-2:

Results

The results consist of a discharge coefficient calculation for each wind direction. This is shown in Figure 4.

As shown in Figure 4, the discharge coefficient is negative for low wind angles. This means that air is pushed into the building from the NSHEV system instead of discharging air. This is a worrying situation, causing potentially dangerous situations. However using the CFD tools, different compensation methods can be analyzed, in order to resolve the problem. In this case, a wind direction measurement device can be used to shut the windows to eliminate the negative ventilation scenarios. Given other circumstances, a similar benefit could have been achieved by changing the design of the windows (shape and size) or by sheltering the windows from low wind angles.

In order to examine the flow, visualizations of the flow fields can be employed. This is shown in Figure 5 left picture. The wind is coming from right to left and is pushed into the building. In contrast, air is sucked out through the window shown in the right hand picture, because the open window creates a low pressure zone just at the window opening.

Conclusion

For this project, the NSHEV free flow area could be approved if the windows having negative discharge coefficient were automatically closed and the remaining windows opened. This can be achieved using a wind direction measurement device.

Results of other NSHEV cases have shown a sufficient free flow area with the application of CFD, even though the more simplified approach in EN 12101-2 resulted in an insufficient free flow area.

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References

Chen, Q. and Srebric, J. 2002. “A procedure for verification, validation, and reporting of indoor environment CFD analysis”, HVAC&R Research, 8(2), 201-216.