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Scientists formulate a heat wave survival guide

August 06, 2003

Scientists from Imperial College London have found a simple solution to city dwellers' despair as temperatures soar during summer heat waves. In a bid to cool down they have to learn how to ventilate and cool rooms by using windows and the building properly.

Dr Gary Hunt of the Department of Civil and Environmental Engineering who is leading research at Imperial on the fluid mechanics of natural ventilation explains:

"Many of us have forgotten how to correctly use the sash windows so carefully installed by the Edwardians and Victorians to maximise airflow.

"If used correctly it is possible to significantly improve comfort in the office or at home without using air conditioning units that place high demands on energy and increase carbon-dioxide emissions."

Using laboratory modelling techniques that accurately replicate the flow of air in small-scale physical models, Dr Hunt and his team are able to measure and clearly visualise how air circulates within rooms and buildings.

"The trick to getting the maximum flow of air through the window is to slide the sashes so the window is open equally at the top and bottom.

"By separating the in-flow and out-flow, cooler incoming air efficiently flushes the warm air out of the room. The warm air inside the room tumbles out of the top opening and the cooler air form the outside comes in through the lower opening," explains Dr Hunt.

By applying mathematical analysis and using small-scale laboratory models, the researchers were also able to calculate the size and placement of windows to maximise ventilation.

"The laboratory modelling technique relies on examining the movement of water through a model typically one twentieth to one hundredth of the building size. At this scale water moves through the model room in the same way as air moves through a real room," said Dr Hunt.

Results show if the windows of a room are too small or their location is not appropriately chosen, hot air, which collects at the ceiling may extend down to the occupied region making the environment unpleasantly hot and muggy.

"Offices typically experience the largest heat gains during the occupied daylight hours, when the need for ventilation is greatest. Minimal gains occur at night when the space is unoccupied.

"Our research shows a good strategy is to leave sash windows in the mid-position overnight - providing it's safe to do so. The cool external air flushes the warm air out of the room and also cools the walls, floor and ceiling. The cool walls then absorb heat the following day and prevent the internal temperatures from rising as high," he added.

Work is now underway at Imperial on a range of problems concerned with the fluid mechanics of airflows in buildings. Projects include modelling airflow in large multi-story buildings, which allows researchers to calculate how to naturally ventilate them effectively.

"The aim of our research is to gain an improved understanding of the physics of airflow in buildings through the use of laboratory and mathematical modelling techniques and to develop simple design guidelines that will help architects create energy efficient buildings of the future," said Dr Hunt.

Imperial College, University of London