Thermal Flow Sensors

Fig. 19. Resistor temperatures as function of the flow with constant power dissipation in the heater resistor. The insert shows the layout of the thermal flow sensor with three resistive elements in the center of the flow channel...

Since thermal transfer accompanies the mass transfer of a fluid flow, microflow sensors based on thermal effects are the most common. An example of a MEMS thermal flow sensor is shown in Fig. 2. 

Hot wire Hot-wire anemometer Thermal flow sensors... 

Pyroelectric flow sensors are a kind of thermal flow sensor. They can be classified according to the thermal transfer mechanism, i.e.,... 

Thermal flow sensors based on convective heat transfer are usually used to measure the flow rate, since a heater can easily be realized by applying electric power to an electric resistor. For instance, HWA, one of the most widely used velocimetries, is a thermal flow sensor. 

All thermal flow sensors are based on the convective heat transfer between a flowing fluid and a sensing solid element. Figure 1 shows a simplified case of convective heat transfer which explains the thermal flow sensing principle. A heater heats a solid sensor, which faces the flowing fluid. When power is supplied to the heater, the sensor temperature increases to T. As the fluid flows through the solid sensor, there will be a convective heat transfer from the sensor to the fluid. If Q is the heat transfer rate, Aj, is the interface area perpendicular to the direction of heat flow between the sensor and the fluid. and Tf are the temperature of the sensor surface and fluid, respectively. The heat transfer can then be described as... 

A thermal flow sensor typically consists of a heater and one or more temperature sensors. The heater can operate either at constant heating power or at a constant heating temperature. In thermal microflow sensors, the flow rate is evaluated by measuring the heating power, the temperature of the heater or fluid, and the time of flight of a heat pulse [7]. 

There are three types of thermal flow sensors that combine the evaluation methods and heater modes hot wire (hot film), calorimetric, and time of flight. 

Electrical heater Temperature sensors Thermal flow sensors Thermoelements... 

Due to their compactness and standard fabrication technology, the temperature in thermal flow sensors is often measured by thermocouples, which rely on the thermoelectric effect. The thermoelectric effect describes the coupling between the electrical and thermal currents, especially the occurrence of an electrical voltage due to a temperature difference between two material contacts, known as the Seebeck effect. In reverse, an electrical current can produce a heat flux or a cooling of a material contact, known as the Peltier effect. A third effect, the Thomson effect, is also connected with thermoelectricity, where an electric current flowing in a temperature gradient can absorb or release heat from or to the ambient [10, 11]. The relation between the first two effects can be described by methods of irreversible thermodynamics and the linear transport theory of Onsager in vector form. 

Ashauer M, Glosch H, Hedrich F, Hey N, Sandmaier H, Lang W (1999) Thermal flow sensors for liquids and gases based on combinations of two principles. Sens Actuators A 73 7-13... 

Buchner R, Sosna C, Maiwald M, Benecke W, Lang W (2006) A high-temperature thermopile fabrication process for thermal flow sensors. Sens Actuators A 130 262-266... 

Kuo JTW, Yu L, Meng E (2012) Micromachined thermal flow sensors - a review. Micromachines 3 550-573... 

A thermal flow sensor using porous Si thermal isolation as the one described above was also used in a Si thermal accelerometer without solid proof mass (Goustouridis et al. 2007) and in a Si thermoelectric generator (Hourdakis and Nassiopoulou 2013). 

Bolometer 7 Gas flow sensor 7 Micro-hotplates 6 Molecular dynamics approach 4 Monte Carlo simulation 5 Photonic device 7 Room temperature 2-3 Temperature dependence 2 Thermal conductivity 1, 4 Thermal flow sensor 7 Thermally isolating substrate 5... 

Pyroelectric sensors Pyroelectric Thermal flow sensors Definition... 

Jacobs, T., Kutzner, C., Kropp, M., Brokmann, G., Lang, W., Steinke, A., Kienle, A., and Hauptmann, P. (2009) Novel impedimetric and perforated thermal flow sensor for inline chemical process analysis in micro residence time reactors. Sensors, 2009 IEEE, pp. 719-722. 

It is important to know the thermal conductivity of snow when designing an artificial skiing facility. In this subsection, the measured thermal conductivity of the snow layer will be introduced. As shown in Fig. 4, a thermal flow sensors and thermocouples are embedded in the snow layer to measure the heat flow and temperature distribution within the snow layer. Similarly, thermocouples are also installed on the snow surface to measure the temperature distribution at the snow surface as well as the representative room temperature Im above the snow surface. Figure 5 shows the temperature distribution and other values within the snow layer and the room. From these measurements, the thermal conductivity of the snow as measured to be 0.26 W/mK for powdered snow and the ice layer beneath the powdered snow was 1.23 W/mK. 

---