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Novel sensor enables high-speed airfl ows on curved surfaces
The multidirectional sensor developed by researchers from Japan can help improve the efficiencies of industrial-scale fluid machinery
I
nefficient fluid machinery used in the energy and transportation sector are responsible for greenhouse gas emissions and the
resulting global warming. To improve efficiency, it is necessary to characterise and reduce flow separation on curved surfaces. To this end, researchers from Japan have developed a flexible, thin- film MEMS airflow sensor that can measure complex, three-dimensional flow separation in curved walls for high-speed airflows. The energy and transportation sector often make use of different kinds of fluid machinery, including pumps, turbines and aircraft engines, all of which have a high carbon footprint. The challenge in characterising
near-wall flow on the curved surface to suppress flow separation is manifold. First, conventional flow sensors are not flexible enough to fit into the curved walls of fluid machinery. Second, existing flexible sensors suitable for curved surfaces can’t detect the fluid angle (direction of flow). Moreover, these sensors are limited to only detecting flow separation at speeds below 30m/s. In a new study, a team from the Tokyo University of Science (TUS) in Japan led by Professor Masahiro Motosuke and in collaboration with Mitsubishi Heavy Industries and Iwate University in Japan addressed the problem. “Sensing the shear stress and its
direction on curved surfaces, where flow separation easily occurs, has been difficult to achieve in particular without using a novel technique,” said Motosuke.
The team developed a polyimide thin- film-based flexible flow sensor that can easily be installed on curved surfaces without disturbing the surrounding airflow, a key requirement for efficient measurement. To enable this, the sensor was based on microelectromechanical system (MEMS) technology. Moreover, the novel design allowed
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multiple sensors to be integrated for simultaneous measurement of the wall shear stress and flow angle on the surface of the wall. To measure the shear stress on the walls, the sensor measured the heat loss from a micro-heater, while the flow angle was estimated using an array of six temperature sensors around the heater that facilitated multi- directional measurement. The team conducted numerical simulations of the air flow to optimise the geometry of the heaters and sensor arrays. Using a high-speed airflow tunnel as the testing environment, the team achieved effective flow measurements with wide range of airflow speeds from 30-170m/s. The developed sensor demonstrated both high flexibility and scaleability. “The circuits around the sensor can be pulled out using a flexible
printed circuit board and installed in a different location, so that only a thin sheet is attached to the measurement target, minimising the effect on the surrounding flow,” said Motosuke. The team estimated the heater output to vary as the one-third power of the wall shear stress, whilst the sensor output comparing the temperature difference between two oppositely- placed sensors demonstrated a peculiar sinusoidal oscillation as the flow angle was changed.
The developed sensor has the potential for a wide range of applications in industrial-scale fluid machinery that often involve complex flow separation around three- dimensional surfaces. Moreover, the working principle used to develop this sensor can be extended beyond high- speed subsonic airflows.
Thin and flexible sensor developed by the Japanese team of researchers
“Although this sensor is designed for fast airflows, we are currently developing sensors that measure liquid flow and can be attached to humans based on the same principle. Such thin and flexible flow sensors can open up many possibilities,” said Motosuke. Taken together, the novel MEMS sensor could be a game-changer in the development of efficient fluid machineries with reduced detrimental effects on our environment.
Automation | November 2022 35
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