High temperature gradient calorimetric wall shear stress micro-sensor for flow separation detection

• Published in: Sensors and actuators. A. Physical. 2017-10, Vol.266, p.232-241
• Main Authors: Ghouila-Houri, Cécile, Gallas, Quentin, Garnier, Eric, Merlen, Alain, Viard, Romain, Talbi, Abdelkrim, Pernod, Philippe
• Format: Article
• Language: English
• Subjects: Aerodynamics; Convective heat transfer; Electric bridges; Electric wire; Electrical insulation; Engineering Sciences; Flow control; Flow separation; Flow separation detection; Fluid dynamics; Fluids mechanics; Heat measurement; Heat transfer; High aspect ratio; High temperature; Instrumentation and Detectors; Mechanics; MEMS sensors; Physics; Platinum; Separation; Shear stress; Silica; Silicon dioxide; Studies; Substrates; Temperature distribution; Temperature gradients; Thermal insulation; Thermistors; Turbulence; Turbulent boundary layer; Wall shear-stress sensor; Wind tunnel testing; Wind tunnels
• ISSN: 0924-4247; 1873-3069
• DOI: 10.1016/j.sna.2017.09.030

•A MEMS calorimetric sensor for bi-directional wall shear stress measurement is presented.•Thermal and electrical characterizations are performed.•The calibration of the sensor in a wind tunnel is performed.•The sensor is able to detect flow separations in a turbulent flows. The paper describes and discusses the design and testing of an efficient and high-sensitivity calorimetric thermal sensor developed for bi-directional wall shear stress measurements in aerodynamic flows. The main technical application targeted is flow separation detection. The measurement principle is based on the forced convective heat transfer from a heater element. The sensor structure is composed of three parallel substrate-free wires presenting a high aspect ratio and supported by periodic perpendicular SiO2 micro-bridges. This hybrid structure takes advantages from both conventional hot-films and hot-wires, ensuring near-wall and non-intrusive measurement, mechanical toughness and thermal insulation to the bulk substrate, and it allowed to add the calorimetric sensor functionality to detect simultaneously the wall shear stress amplitude and direction. The central wire is made of a multilayer structure composed of a heater element (Au/Ti) and a thermistor (Ni/Pt/Ni/Pt/Ni) enabling measurement of the heater temperature and a layer of SiO2 between them for electrical insulation. The upstream and downstream wires are thermistors enabling operation in the calorimetric mode. This design provides a high temperature gradient and a homogeneous temperature distribution along the wires. The sensor operates in both constant current and constant temperature modes, with a feedback on current enabled by uncoupling heating and measurement. Welded on a flexible printed circuit, the sensor was flush mounted on the wall of a turbulent boundary layer wind tunnel. The experiments, conducted in both attached and separated flow configurations, quantify the sensor response to a bi-directional wall shear stress up to 2.4Pa and demonstrate the sensor ability to detect flow separation.