Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications

Abstract

MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices.

Mems thermal film sensors for unsteady flow measurement

“…Third, the present fabrication process is much more simplified than those designed for shear stress measurement. For the latter, to minimize the heat loss to the substrate, a design with a cavity beneath the sensing element [13,18] was recommended. However, such a design would limit the choices of the substrate material, such as the silicon-based substrate.…”

“…However, such a design would limit the choices of the substrate material, such as the silicon-based substrate. Each sensing element was patterned on the silicon substrate, called a silicon island [13,18,20], which was rigid. In order to be applied to a contoured surface, these silicon islands needed to be patterned on a flexible skin [20,21].…”

“…flow control [9][10][11][12][13]. Nevertheless, the measurement of shear stress using thermal film sensors was considered an indirect method [14,15] since, in contrast to the direct method using a floating element or a fence, a calibration procedure is required.…”

“…Unlike most of the MEMS thermal film sensors reported in the literature for wall shear stress measurement, the present interest is focused on exploiting the characteristics of the high frequency response of the self-made sensors. Therefore, the concerns associated with shear stress measurement, such as the calibration required prior to measurement and the drift issue due to heat conduction between the substrate and the film [7,8,13,16], are deemed unnecessary. The present thermal film sensors are characterized by a frequency response of up to 30 kHz when operating in constant temperature mode.…”

Integrated flow sensing for focal biochemical stimulation

“…Furthermore, this focal chemical delivery platform is the first to feature integrated thermal flow sensors for monitoring flow rate of each microchannel and is thus capable of feedback control of focal delivery for each pore. Arrayed resistive sensors have previously been demonstrated to be simple to incorporate into MEMS fabrication processes, allow multiple modes of operation (hot-film, calorimetric, and time-of-flight), and can also be used for the determination of thermal properties of the working fluid [7][8][9]. The thermal sensors are capable of nL/min range flow rate measurements.…”