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Over the past half century, air curtains have found their way into shops and factories the world over, yet the science behind them is less well understood. By working with one air curtain manufacturer, the Department of Applied Mathematics and Theoretical Physics has developed experiments and theory showing how air curtains could be made more energy efficient.

Air curtains – those downward-facing fans which greet you with a blast of hot air as you cross shop doorways – have been in widespread use for decades. As well as retail, they are used for industrial applications from aircraft hangers to refrigeration stores, helping increase thermal comfort and air quality as well as reducing energy losses.

Yet despite their ubiquity, air curtains are poorly understood when it comes to the science of fluid dynamics, a gap in understanding that Professor Paul Linden of the Department of Applied Mathematics and Theoretical Physics discovered almost by chance. “One of my PhD students started looking at the flow of air curtains and, much to our surprise, when we began investigating the issue we found that very little was known about them – even though they are very common,” he recalls.

When the pair published their research, they received a phone call from Biddle BV, a climate control company in the Netherlands that manufactures air curtains. Biddle told Linden they were interested in his paper and were keen to understand how to optimise air curtains in order to make them more energy efficient.

After visiting Biddle’s factory and discussing how they could work together, Linden returned to Cambridge and applied for an EPSRC Partnership Development Award to fund the collaboration. The award allowed him to employ a 12-month postdoc. “We worked on a number of problems that the company was interested in, one of which was the impact people have on air curtains when they walk through them,” he says. 

At the Centre for Mathematical Sciences, they developed small-scale experiments using tanks filled with fresh water and salt solutions. Using changes in water density to mimic changes in air temperature, they added coloured dyes and small spherical beads to the tanks, enabling them to video the system and measure flow rates with precision.

“Compared with the full-scale tests done by other research groups, our small-scale experiments allow us to visualise flows very easily,” Linden explains. “Adding particles as well as dyes to the water, and using a cylinder to represent the motion of a person, we can measure flow rates extremely accurately. And because we know mathematically how to make the scaling work, we can directly extrapolate our results to the large scale.”

By varying the system dynamically, they were also able to understand the impact of changing the temperature of an air curtain, with the result that much more is now understood about how to optimise air curtain systems. “We learned a lot. I was really surprised to discover just how much a person disrupts an air curtain. I thought this would make a small difference, but a person fills an average-sized doorway so they have a major impact,” says Linden. “And I can see other ramifications that it would be interesting to explore.”

The collaboration with Biddle has now ended but it is hoped that the results will help the industry make air curtains more energy efficient. The project provided training for a postdoc who has now secured a fellowship at an Israeli university, and it enabled Linden to sponsor two interns who, among other things, have examined whether an upward flow of bubbles in water acts in a similar way.

This research will yield another peer-reviewed paper and the funding has been highly effective in generating new and interesting research questions. “Bubbles are often used to aerate water in lakes and reservoirs, and we’ve discovered new things about bubbles,” Linden concludes. “So in a way, we’ve had a long-term benefit that far exceeds the EPSRC award.”