Natural History Museum - Fume Dispersal

Introduction

Phase 2 of the Darwin Centre at the Natural History Museum will link directly onto Waterhouse's historic Cromwell Road Building. It features a 'cocoon' situated within a 78m wide by 32m high atrium, which will house entomological specimens.

Laboratory fume cupboards are required in the Phase 2 building which require careful extraction. This will be achieved with an arrangement of exhaust stacks situated on the roof of the building.

Clearly there is some concern as to the environmental impact of the exhaust, and in particular, if there is a risk that it will be drawn back into the atrium. Flow Analysis have been commissioned to analyse the exhaust flows and help in the design of the exhaust stack arrangement. Some preliminary work is presented here.

Methodology

The path taken by the exhaust fumes is dependent on the local air flow in the vicinity of the Phase 2 building, which in turn is highly influenced by the surrounding buildings. All buildings which may be of importance must be included in the model.

The CFD model has been built using the architect's drawings for the Phase 2 building itself, the Phase 1 building, the adjacent waterhouse building, Jerwood Gallery and Whale Hall. Other buildings have been included by reference to maps, aerial photographs, and on-site photographs.

The exhaust stacks are arranged in groups of four. Each one is 300mm in diameter and emits fumes at 10 m/s. In due course, several exhaust stack arrangements and various climatic conditions will need to be considered.

So far, a worst case 'negative buoyancy' situation has been examined, i.e. the exhaust is at a lower temperature than the surrounding air.

Results

Several prevailing south-westerly wind cases have been analysed by Flow Analysis. For a wind speed of 5 m/s (about average), some complex re-circulations are predicted (see the vector plot above), especially downwind of the Phase 2 building. This air movement tends to deflect the main air stream slightly upwards, causing the exhaust to be blown harmlessly away.

At a lower wind speed of only 0.1 m/s (still south-westerly), the exhaust plume has significant momentum compared to the wind, and the effects of the negative buoyancy of the exhaust is clearly visible (see the animation to the left).

When the wind is from a southerly direction, the wind hits most of the buildings more obliquely and the air flow is less disturbed downwind of the Phase 2 building. Consequently the plumes do not rise as high (see the streamlines to the right).

Conclusions

For the limited number of cases examined to date, the southerly wind is likely to be more problematic than the south-westerly wind. Further work is required to examine more climatic conditions and exhaust stack configurations and to quantify the effects of the predicted exhaust concentrations.

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