NSHEV Free Flow Area Simulations using CFD

Challenge

Replace physical testing of the Natural Smoke and Heat Eaxhaust Ventialition (NSHEV) area using CFD to efficiently quantify the free flow area.

Approach

The CFD model consists of a 1:1 scale window module (shown in the figure below) 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.

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 the figure below.

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:

Result

The results consist of a discharge coefficient calculation for

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. The wind is coming from right to left and is pushed into the building. In contrast, air is sucked out through the window in cases with higher wind angles, as the open window creates a low pressure zone just at the window opening.