Thermal Shear Stress Sensors

Constant current (CC) mode Here, the current through the sensor is kept constant, and the change in input current needed to maintain its constant value is related to the flow signal, that is, shear stress. 

The thermal sensors for shear stress measurements can have two principal modes of variation (1) elevated hot-wire probe and (2) surface-mounted hot-wire probe. The principles of operation of these two types of thermal shear stress sensors are discussed in the following sections. 

In elevated hot-wire approach, the hot wire is mounted at small distance away from the wall. The velocity increases linearly with the wall distance in the near-wall region of both laminar and turbulent flow. For turbulent flow, this assumption is valid for the instantaneous velocity profile till the wall normal location j 5. The linear relationship between the shear 

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Lin Q, Jiang F, Wang X-Q, Han Z, Tai Y-C, Lew J, Ho C-M (2000) MEMS Thermal Shear-Stress Sensors Experiments, Theory and Modehng, Technical Digest, Solid State Sensors and Actuators Workshop, Hilton Head, SC, 4—8 June 2000, pp 304-307 Lin TY, Yang CY (2007) An experimental investigation of forced convection heat transfer performance in micro-tubes by the method of hquid crystal thermography. Int. J. Heat Mass Transfer 50 4736-4742... 

Sheplak M et al (2002) Characterization of a silicon-micromachined thermal shear-stress sensor. AIAA J 40(6) 1099-1104... 

A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633... 

Shear Stress Sensors, Fig. 1 (a) Schematic of a hot wire sensor connected to a Wheatstone bridge circuit and (b) schematic of a flush-mounted thermal shear stress sensor... 

In experimental aerodynamics, the surface hot wire probe has proved to be the most successful standard measurement technique to determine the laminar-to-turbulent flow transition, local separation, and shear stress fluctuations. The flush-mounted thermal shear stress sensor is one of the most successful techniques for shear stress measurement and is available in various forms, i.e., sensor skin, etc. [4], due to the rapid development of MEMS manufacturing processes. 

Stainback PC, Nagabushana KA (1996) Review of Hot-Wire Anemometry Techniques and the Range of their Applicability for Various Hows. Electron J Huids Eng, Transaction of the ASME King LV (1914) Phil. Trans R Soc, London 4. Liu CJ, Huang J, Zhu Z, Jiang F, Tung S, Tai YC, Ho CM (1999) A Micromachined How Shear-stress Sensor Based on Thermal Transfer Principles. J MEMS 8(l) 90-99 Sheplak M, et al (2002) Characterization of a silicon-micro-machined thermal shear-stress sensor. AIAA J 40(6) 1099-1104 Hsiai TC, SK Wong P, Ing M, Salazar A, Hama S, Navab M, Demer L, Ho CM (2003) Monocyte Recruitment to Endothelial Cells in Response to Oscillatory Shear Stress. EASED J 17 1648-1657... 

Padmanabhan A, et al (1996) A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes. J Microelectromech Syst 5(4) 307-315 Jiang F, Tai Y-C, Huang J-B, Ho C-M (1995) Polysilicon structures for shear stress sensors. In TENCON 95. IEEE Region 10 International Conference on Microelectronics and VLSI Qiao Lin YX, Tai Y-C, Ho C-M (2005) A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633 Rouhanizadeh M, et al (2006) MEMS sensors to resolve spatial variations in shear stress in a 3D blood vessel bifurcation model. IEEE Sens J6(l) 78-88... 

Xu Y, et al (2005) Micromachined thermal shear-stress sensor fo" underwater apphcations. J Microelectromech Syst 14(5) 1023-1030... 

Shear Stress Sensors, Figure 9 Static calibration curve (a) the bridge output ( ) versus shear stress (zw) and (b) the input power versus (shear stress) of a thermal shear stress sensor... 

Figure 12.12 The flow and heat transfer mechanism of a flush-mounted thermal shear stress sensor...

Figure 12.12 shows the heat transfer mechanism from a surface-mounted thermal sensor. The total heat transfer to the fluid from the thermal sensor (Qohmic) has two components, that is, the heat transfer to the fluid (<2fluid) and the heat lost to the substrate (Gsubstrate)- Th heat transfer to the fluid has two parts, that is, direct heat transfer from the sensor element (Qfi) and indirect heat transfer from the substrate heated by the conduction of heat from the sensor to the substrate ( 2f2)- The heat transferred to the fluid via the substrate effects the temperature distribution near the sensor. This affects the net heat transfer rate from the sensor element and limits the performance of thermal shear stress measurement. The effective length of the thermal sensor is higher than the size of the sensor element, thus limiting the spatial resolution of shear stress measurement. Therefore, effective thermal isolation between the sensor element and substrate is an important issue for optimum performance, fabrication, and packaging of thermal shear stress sensors. For thermal isolation, the resistor of the sensor sits on the top of a diaphragm above a vacuum cavity (see Figure 12.12). The presence of vacuum cavity and thin diaphragm reduces the convective and conductive heat transfer to the substrate. Better insulation improves the thermal sensitivity of the sensor, that is, higher temperature rise T - Tq) of the thermal sensor is achieved for a particular power input (F).

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