Improved sensitivity of micro thermal sensor for underwater wall shear stress measurement

Abstract

Knowledge of such wall shear stress is essential for understanding the dynamics of the fluid flow, and its measurement holds great importance for investigating and controlling wall-bound turbulence and flow separation (Bellhouse and Schultz 1966;Goldstein 1996;Kimura et al. 1999;lin et al. 2005;Wang et al. 2007;Sturm et al. 2012). Presently, there are many measurement techniques for shear stress. however, most of them will greatly interfere with fluid flow during measurement (Zhang et al. 2008). Miniaturized shear stress sensors fabricated using MeMS technology offer superior spatial resolution, fast time response and minimized interference with fluid flow (liu and Yuan 2007;liu and Zhu 2009;Beutel et al. 2013a). Therefore, MeMS sensors will play an important role for studying gaseous or liquid fluid flows experimentally.Micro thermal shear stress sensors are based on measurement of heat transfer from a heated thin-film element to the fluid flow (Osorio and Silin 2011). constant current (cc) mode can be used to drive them. There are many reported thermal shear-stress sensors working in cc mode. The shear-stress sensitivity of a micromachined thermal shear stress sensor reported by chang liu is 15 mV/Pa at a power consumption of 12 mW (I = 2 ma) when the shear stress input was about 0.2 Pa (liu et al. 1994). Mark Sheplak has presented a characterization of a silicon-micromachined thermal shear-stress sensor (Sheplak et al. 2002). The sensitivity of this sensor is 11 mV/Pa at a power consumption of 9 mW (I = 10 ma) when the shear stress input was about 0.2 Pa.In this paper, we studied how to improve sensitivity of thermal shear stress sensor working underwater with allowable drive currents. They were obtained by analyzing I-V characteristic and output voltage-shear stress relationship. and they were be allied to drive the sensor in two different shear stress input ranges to explore the advancement of sensitivity.Abstract Drive current is an important parameter of thermal shear stress sensor. Increasing drive current is helpful for enhancing its sensitivity. however, there must be an allowable drive current for the sake of safe working temperature of the sensor. how to make full use of drive current to increase the sensor's sensitivity working underwater was studied. If the allowable drive current in still water and the current in stream water are used to drive the sensor in lower and higher shear stress input ranges, respectively, sensitivity of the sensor will be enhanced with the sensor working under a safe temperature. The both currents were separately determined by analyzing I-V characteristic and output voltage-shear stress relationship. We can improve the sensor's sensitivity from 11.8 to 27.5 mV/Pa when shear stress input was 0.4 Pa.

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