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

Abstract

The paper describes and discusses the design and testing of an efficient and high-sensitive calorimetric thermal sensor to measure simultaneously the magnitude and the direction of wall shear stress in aerodynamic flows. The main technical application targeted is the back flow and the flow separation detection for active flow control. The measurement principle is based on the flow-induced forced heat convection transfer on a heater element. The sensor is micro-structured with three parallel substrate-free wires presenting a high aspect ratio and supported by periodic perpendicular SiO2 micro-bridges ensuring a mechanical toughness and a thermal insulation relatively to the bulk substrate with high thermal inertia. The central wire is made of a multilayer structure (Au/TiSiO2/Ni/Pt/Ni/Pt/Ni/SiO2) and is composed of a heater element (Au/Ti) and a thermistor (Ni/Pt/Ni/Pt/Ni) enabling to measure the heater temperature. The upstream and downstream wires are thermistors enabling to operate 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 mode and constant temperature mode, with a feedback on current enabled by uncoupling heating and measure. 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, demonstrate the sensor sensitivity to the wall shear stress up to 2.4 Pa and the ability of the sensor to perform flow direction sensing for back-flow detection in a separated flow configuration.

Design and Evaluation of a Multidirectional Thermal Flow Sensor on Flexible Substrate

“…Determination of flow velocity and direction in harsh environments is a major concern in various applications, such as gas flow measurement in pipelines [1], meteorology [2], agricultural production [3], avionics [4] and active flow control in aviation [5], intelligent air-conditioning systems [6,7], and wind turbine installations [8]. Flow sensors also find critical applications in biomedical fields, for example, installation in external ventricular drains [9], tracheal intubation flow sensing [10], sleep apnea monitoring [11], and blood flow sensing [12].…”

Unsteady flows measurements using a calorimetric wall shear stress micro-sensor

“…Moreover, thermal losses due to conduction into the kapton foil of the hot-film probe are part of the hot-film sensitivity and are not separable from the heat transfer by forced convection with the fluid. On the contrary, the suspended micro-structure of the HTGC microsensor avoid this issue: the heat losses from the micro-bridges are negligible due to the fact that they are suspended, as it was shown using thermal imaging in previous work (Ghouila-Houri 2017 [8]). Therefore, the assumption that the whole sensitivity of the HTGC micro-sensor is entirely due to forced convection with the fluid is valid.…”

“…The thermal coefficient of resistance (TCR) of the HTGC micro-sensor was measured close to 2400 ppm/˚C. The temperature rise coefficient was measured at 9˚C/mW for the central element and 5˚C/mW for the lateral elements (Ghouila-Houri 2017 [8]). Therefore only 7 to 10 mW are required for the HTGC micro-sensor to work efficiently compared to 100 et 300 mW for conventional hot-film sensors.…”

“…The HTGC micro-sensor is flush-mounted in the wind tunnel wall on a plexiglas plug, as shown in figure 2 (b). Electrical contacts are welded on a flexible band using gold-wire micro-bonding technique that ensured that the sensing part was perfectly flush-mounted, unlike in previous work (Ghouila-Houri 2017 [8]) where the micro-sensor was slightly under the wall, in a small cavity due to front side welding. The wire bond is 15 µm high.…”

“…This paper is divided in three parts. The first one shortly describes the design and fabrication that have been more precisely presented before (Ghouila-Houri 2016 [7], Ghouila-Houri 2017 [8]). The second part presents the testing in wind tunnel with the measurement of the mean wall shear stress, the detection of the flow direction and the measurement of the turbulent spectrum.…”