Thermal Flow Sensors. Anemometers. Calorimeters. Ensemble of both

Transcription

1 Thermal Flow Sensors Anemometers Calorimeters Ensemble of both IST Presentation V

2 Constant Temperature Anemometer At the heart of a thermal anemometer are two sensors: an air velocity sensor and a temperature compensation sensor. The velocity sensor is heated to an elevated temperature (relative to the surrounding air) by means of control electronics. The temperature compensation sensor senses the ambient, or surrounding, air temperature and forces the velocity sensor to stay at a constant "overheat" above the ambient. The sensors form two opposite legs of a Wheatstone bridge (Figure 1). The circuit forces the voltage at points A and B to be equal by means of an operational amplifier. Air flowing past the velocity sensor tends to cool the sensor, thus driving down its resistance. The operational amplifier responds by immediately delivering more power to the top of the bridge to maintain voltage equilibrium at points A and B. As more air flows past the sensor, more power is required to maintain a balanced bridge. Thus, the power going into the top of the bridge is related to the velocity of the air flowing past the sensor. This is the basic principal of operation for constant temperature thermal anemometers. Figure 1: Wheatstone Bridge Diagram Types of thermal CTA -Hot wire anemometer -Hot film anemometer (IST AG) IST Presentation V

3 U_out vs velocity (Example) volts point of free convection, forced air velocity = 0) Features of Constant Temperature Anemometry Measures velocities from a few cm/s to supersonic High temporal resolution: fluctuations up to several hundred khz High spatial resolution: eddies down to 1 mm or less Measures all three velocity components simultaneously Provides instantaneous velocity information m/s Measurement principles of CTA Introduction Constant Temperature Anemometry (CTA) is used to measure fine structures in turbulent gas and liquid flows. The working principle is based on the cooling effect of a flow on a heated body. The CTA measures velocity at a point and provides continuous velocity time series, which can be processed into amplitude and time-domain statistics. Examples are mean velocity, turbulence intensity, higher order moments, auto-correlations and power spectra. Heat transfer from cylinders Convective heat transfer Q from a wire is a function of the velocity U, the wire over-temperature Tw -T0 and the physical properties (k,r,m) of the fluid. The basic relation between Q and U for a wire placed normal to the flow was suggested by L.V. King (1914). In its simplest form it reads: IST Presentation V

4 A graphic picture how an anemometer works FS1, FS5 series are anemometric flow sensors. They only use the heat transfer to medium due to reducing heater temperature Thermography of flow monitor with no flow. The color graduations around the sensor tip show how the fluid is heated up by the heat source within the flow monitor. Advantages -Very simple to use, because no individual sensor housing (channel requiered) -measure in turbulent and laminar flow profiles -Heater is also the sensor. It measures by itselfs -Reference sensor is on the same chip -High flow rates possible Thermography with fluid flowing. The heat generated by the flow monitor is conducted away from the sensor, essentially reducing the sensor temperature. Disadvantages -Flow signal depends on sensor alignment -There is a thermal coupling between heater and sensor -Chip contamination possible, no flow direction detectable -Relatively high heating power necessary IST Presentation V

5 Calorimeter Flow sensors FS2T, Mikroflowsens sensor 1 heater sensor 2 sensor 1 heater sensor 2 FS2 T They use the temperature distribution over a heating element with two or more temperature sensors left and right For accurate measurings a laminar flow is required to keep up the temperature distribution gradient Strömungsgeschwindikeit [m/s] Brückenspannung [V Durchfluss [ml/min] IST Presentation V

6 Calorimeter Flow sensors FS2T, Mikroflowsens S/H1 S/H2 S/H3 S/H4 S/H1 S/H2 S/H3 S/H4 Mikroflowsens They use the temperature distribution over 4 heating/ sensor elements with serial down / up- stream arrangement For accurate measurings a laminar flow is required to keep up the temperature distribution gradient AC=VCC Flow velocity [m/s] R A R C 40 AD=V1 V BC=V2 Vout [mv] Flow rate [ml/min] R D R B -40 BD=GND IST Presentation V