Heat stress sensors

Torok, Z., et al. Plasma membranes as heat stress sensors from Upid-controlled molecular switches to therapeutic applications. Biochim. Biophys. Acta (BBA)-Biomembr. 1838(6), 1594-1618 (2014)... 

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... 

Additional Sensors. At this writing, other personal monitoring sensors for noise, heat stress, radiation, etc., are under development for incorporation into the Chronotox System. As previously indicated, there is no limit to the application... 

MEMS shear stress sensor supported by an underneath cavity to reduce the heat loss to the substrate as shown by the presence of a Newton s ring... 

Shear Stress Sensors, Fig. 2 The flow and heat transfer mechanism of a flush-mounted thermal shear stress sensor... 

The shear stress sensor for turbulent flow needs to accurately capture the complete turbulent fluctuation spectrum. Therefore, the shear stress sensor should possess a large bandwidth with flat and minimum frequency-phase relationship. For direct measurement, i.e., floating point sensors, the resonant frequency of the floating element and the fluidic damping determines the usable bandwidth. For the thermal sensor, the thermal inertia of the sensor element and the frequency-dependent heat conduction to the substrate influence the usable bandwidth. It is complicated to analytically predict the frequency response of the thermal sensor. Therefore, dynamic calibration is essential to characterize the frequency response of the sensor. 

Flow control systems are critical components of most of the energy systems involving fluid flow and heat transfer. These systems are essential for performance optimization of both macroscale and microscale devices. Micropumps, microvalves, microshear stress sensors, and microflow sensors are integral components of flow control systems. Capillary micropump, MHD micropump, thermocapillary micropump, and electrokinetic micropump have been presented in earlier chapters. The present chapter reports various microactuators and shear stress sensors for flow control systems. More details on microvalves and microflow sensors can be found in other references (Nguyen and Wereley, 2006). 

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).

The heat transfer rate from the flush-mounted shear stress sensor depends on the near-wall flow, that is, the magnitude of the velocity gradient. For a laminar 2-D thermal boundary layer developing over the heated sensor with an approaching linear velocity profile (Figure 12.12) and negligible free convection effect, the heat loss from the thermal element can be derived from the thermal boundary layer equation as... 

The main goal of another microhotplate design was the replacement of all CMOS-metal elements within the heated area by materials featuring a better temperature stability. This was accomplished by introducing a novel polysilicon heater layout and a Pt temperature sensor (Sect. 4.3). The Pt-elements had to be passivated for protection and electrical insulation, so that a local deposition of a silicon-nitride passivation through a mask was performed. This silicon-nitride layer also can be varied in its thickness and with regard to its stress characteristics (compressive or tensile). This hotplate allowed for reaching operation temperatures up to 500 °C and it showed a thermal resistance of 7.6 °C/mW. 

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