New Flexible Flow Sensor Enhances Fluid Machinery

The greenhouse gas emissions caused by the energy and transportation industries due to inefficient fluid machinery contribute to global warming. To enhance efficiency, defining and decreasing flow separation on curved surfaces is essential.

Scientists from Japan have created a malleable, thin-film microelectromechanical system-based airflow sensor that can be employed to quantify complex, three-dimensional (3D) flow separation in curved walls for high-speed airflows.

The energy and transportation industries regularly utilize various types of fluid machinery, including turbines, pumps, and aircraft engines, all of which involve a high carbon footprint. This result is primarily from inefficiencies in the fluid machinery triggered by flow separation around curved surfaces, which are usually complex in nature.

Therefore, to enhance the effectiveness of fluid machinery, the near-wall flow on the curved surface must be defined to subdue this flow separation. The challenge to achieve this is multifaceted. 

First, traditional flow sensors are not flexible enough to fit the curved walls of fluid machinery. Second, currently available flexible sensors appropriate for curved surfaces cannot detect the fluid angle (flow direction). Moreover, these sensors are restricted to only sensing flow separation at speeds below 30 m/second.

In a new paper, Prof. Masahiro Motosuke from the Tokyo University of Science (TUS) in Japan and his contemporaries, Mr. Daiki Shiraishi, Mr. Koichi Murakami, and Dr. Yoshiyasu Ichikawa from TUS, in partnership with Mitsubishi Heavy Industries, Japan, and Iwate University, Japan, accepted this challenge.

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.

Professor Masahiro Motosuke, Department of Mechanical Engineering, Graduate School of Engineering, Tokyo University of Science

The details of their study were reported in Volume 13 Issue 8 of Micromachines on August 12^th, 2022.

The researchers built a polyimide thin film-based malleable flow sensor that can be effortlessly fixed on curved surfaces without unsettling the surrounding airflow, a core requisite for competent measurement.

The sensor was established on microelectromechanical system (MEMS) technology to support this. Furthermore, the unique design permitted numerous sensors to be combined for concurrent measurement of the flow angle and wall shear stress on the wall's surface.

To quantify the shear stress on the walls, the sensor assessed the heat loss from a micro-heater. At the same time, the flow angle was predicted using a collection of six temperature sensors around the heater that enabled multidirectional measurement.

The team conducted numerical simulations of the airflow to enhance the geometry of the sensor arrays and heaters. Using a high-speed airflow tunnel as the test setting, the researchers accomplished operational flow measurements with wide-ranging airflow speeds from 30 to 170 m/second. The new sensor showed high flexibility as well as scalability.

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, minimizing the effect on the surrounding flow.

Professor Masahiro Motosuke, Department of Mechanical Engineering, Graduate School of Engineering, Tokyo University of Science

The researchers estimated the heater output to vary as the one-third power of the wall shear stress. The sensor output comparing the temperature variance between two oppositely positioned sensors showed an odd sinusoidal oscillation as the flow angle was altered.

The new sensor possesses the potential for a broad range of applications in industrial-scale fluid machinery that repeatedly involve difficult flow separation around 3D surfaces. Furthermore, the working standard used to create this sensor can be extended beyond high-speed subsonic airflows.

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.

Professor Masahiro Motosuke, Department of Mechanical Engineering, Graduate School of Engineering, Tokyo University of Science

If combined, the innovative MEMS sensor could be revolutionary in the progress of efficient fluid machinery with decreased harmful effects on the environment.

Journal Reference

Murakami, K., et al. (2022) Development of a Flexible MEMS Sensor for Subsonic Flow. Micromachines. doi.org/10.3390/mi13081299.