A micromachined flow shear-stress sensor based on thermal transfer principles

Microhot-film shear-stress sensors have been developed by using surface micromachining techniques. The sensor consists of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sensing element, made of polycrystalline silicon material, resides on top of the diaphragm. The underlying vacuum cavity greatly reduces conductive heat loss to the substrate and therefore increases the sensitivity of the sensor. Testing of the sensor has been conducted in a wind tunnel under three operation modes-constant current, constant voltage, and constant temperature. Under the constant-temperature mode, a typical shear-stress sensor exhibits a time constant of 72 μs

Overview of sensors suitable for active flow control methods

Hlavným cieľom tejto bakalárskej práce bolo vytvorenie prehľadu vyvíjaných a už aplikovaných senzorov pre účely aktívneho riadenia prúdov. Senzory musia splňovať niektoré podmienky, preto výber senzorov bol naviazaný na reálnych výsledkoch testovacích programov, popis ktorých tvorí prvú časť tejto bakalárskej práce. Opis technológie a princíp fungovania senzorov je popísaný v druhej časti tejto práce.The main purpose of this bachelor thesis was to create the overview of the sensors developed for the future active flow control applications and overview the sensors already used in the active flow control applications. The sensors have to fulfil several requirements, so selection for the overview was based on the real flight test programs results, which were described in the first part of the thesis. The sensors technology description and operation principles were included in the second part of the thesis

micromachined flow sensors in biomedical applications

Application fields of micromachined devices are growing very rapidly due to the continuous improvement of three dimensional technologies of micro-fabrication. In particular, applications of micromachined sensors to monitor gas and liquid flows hold immense potential because of their valuable characteristics (e.g., low energy consumption, relatively good accuracy, the ability to measure very small flow, and small size). Moreover, the feedback provided by integrating microflow sensors to micro mass flow controllers is essential to deliver accurately set target small flows. This paper is a review of some application areas in the biomedical field of micromachined flow sensors, such as blood flow, respiratory monitoring, and drug delivery among others. Particular attention is dedicated to the description of the measurement principles utilized in early and current research. Finally, some observations about characteristics and issues of these devices are also reported

Micro-Electro-Mechanical-Systems (MEMS) and Fluid Flows

The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electro-mechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research

A surface-micromachined shear stress imager

A new MEMS shear stress sensor imager has been developed and its capability of imaging surface shear stress distribution has been demonstrated. The imager consists of multi-rows of vacuum-insulated shear stress sensors with a 300 /spl mu/m pitch. This small spacing allows it to detect surface flow patterns that could not be directly measured before. The high frequency response (30 kHz) of the sensor under constant temperature bias mode also allows it to be used in high Reynolds number turbulent flow studies. The measurement results in a fully developed turbulent flow agree well with the numerical and experimental results previously published

Microsensors and actuators for macrofluidic control

Microsensors and actuators suitable for macrofluidic control have been designed, fabricated, tested, and optimized over the span of the last decade. MEMS-based shear stress sensor arrays using polysilicon hot filaments have been fabricated on both rigid (silicon) and flexible (parylene) substrate for application on all types of fluid dynamic and aerodynamic surfaces. In addition, MEMS bubble flap-type pneumatic actuators have been tested and used in turbulent boundary layer drag reduction in conjunction with the rigid MEMS shear stress sensor arrays acting as high-speed shear stress imagers. The flexible MEMS bubble actuator arrays have also been used with the flexible shear stress sensor arrays for generating maneuvering forces in the wind tunnel for a delta wing model and on UAV-type radio-controlled aircraft

A Smart Dual-Mode Calorimetric Flow Sensor

A smart thermal flow sensor system is presented. It makes use of a novel heater control circuit which can automatically set the operating mode to either constant power or constant temperature difference. It overcomes the limitations of single-mode thermal flow sensors, such as temperature overshoots at low flow rates at constant power mode, or excessive power consumption at high flow rates at constant temperature difference mode. The system is especially useful for temperature sensitive and portable applications, such as respiratory monitoring for medical diagnostics. In this paper, detailed description of the sensor’s design, implementation, and experimental validation are presented. The proposed dual-mode flow sensor achieves an overtemperature reduction up to 9.5% compared with thermal flow sensors operating in constant power mode alone, and a power reduction up to 13.6% compared with thermal flow sensors operating in constant temperature difference mode alone for the flow range of 0 to 50 slm while offering an improved overall sensitivity

Fabrication of an in-plane SU-8 cantilever with integrated strain gauge for wall shear stress measurements in fluid flows.

We present a cantilever fabricated from the polymer SU-8 for the measurement of wall shear stress in fluid flows. The pressure induced deflection of the cantilever, measured using a calibrated and integrated nichrome strain gauge, can be related to the wall shear stress on the surface. The initial degree of curvature of the cantilever can be controlled via the exposure dose, which allows a small positive deflection to be achieved, and so minimises the intrusion into the flow. Wind tunnel testing results show a sensitivity greater than 2.5 mV/Pa, with a shear stress of 0.38 Pa and excitation of 1 V

Heat Transfer Measurements with Surface Mounted Foil-Sensors in an Active Mode: A Comprehensive Review and a New Design

A comprehensive review of film-sensors shows that they are primarily operated in a passive mode, i.e. without being actively heated to an extent, whereby they create a heat transfer situation on their own. Only when these sensors are used for wall shear stress measurements, the detection of laminar/turbulent transition, or the measurement of certain flow velocities, they are operated in an active mode, i.e. heated by an electrical current (after an appropriate calibration). In our study we demonstrate how these R(T)-based sensors (temperature dependence of the electrical resistance R) can also be applied in an active mode for heat transfer measurements. These measurements can be made on cold, unheated bodies, provided certain requirements with respect to the flow field are fulfilled. Our new sensors are laminated nickel- and polyimide-foils manufactured with a special technology, which is also described in detail