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Hotwire lab stab F09

Hotwire lab stab F09 - HOT-WIRE AND HOT-FILM ANEMOMETRY...

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1 HOT-WIRE AND HOT-FILM ANEMOMETRY INTRODUCTION The hot-wire anemometer has been used extensively for many years as a research tool in fluid mechanics. In this paper hot-wire anemometry will refer to the use of a small, electrically heated element exposed to a fluid medium for the purpose of measuring a property of that medium. Normally, the property being measured is the velocity. Since these elements are sensitive to heat transfer between the element and its environment, temperature and composition changes can also be sensed. Figure 1 shows a hot-wire anemometer probe. Typical dimensions of the wire sensor are 0.00015 to 0.0002 inches (0.0038 to 0.005 mm) in diameter and 0.040 to 0.080 inches (1.0 to 2.0 mm) long. This is the type of hot wire that has been used for such measurements as turbulence levels in wind tunnels, flow patterns around models and blade wakes in radial compressors. The film type of sensor is shown in Figure 2. The hot film is used in regions where a hot wire probe would quickly break such as in water flow measurements. More detailed descriptions of film sensors and a comparison between hot wires and films will be presented below. You will be using a Constant Temperature Anemometer (CTA). It works based on the fact that the probe’s resistance will be proportional to the temperature of the hot wire. The bridge circuit shown in Figure 3 below is set up by setting the adjustable resistor to the resistance you wish the probe and its leads to have during operation. (The other two legs of the bridge have identical resistance.) The servo amplifier tries to keep the error voltage zero (meaning the resistances of the two lower legs of the bridge match). It will adjust the bridge voltage such that the current through the probe heats it to the temperature which gives the selected resistance. When we put the probe in a flow, the air (or water) flowing over it will try to cool it. In order to maintain the temperature (resistance) constant, the bridge voltage will have to be increased. Thus, the faster the flow, the higher the voltage. A very fine hot wire by itself cannot respond to changes in fluid velocity at frequencies above about 500 Hz. By compensating for frequency lag with a non-linear amplifier this response can be increased to values of 300 to 500 kHz.
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2 PROBE TYPES 1. Hot-Wire Sensors A hot-wire type sensor must have two characteristics to make it a useful device: A high temperature coefficient of resistance An electrical resistance such that it can be easily heated with an electrical current at practical voltage and current levels. The most common wire materials are tungsten, platinum and a platinum-iridium alloy. Tungsten wires are strong and have a high temperature coefficient of resistance, (0.004/ o C). However, they cannot be used at high temperatures in many gases because of poor oxidation resistance. Platinum has good oxidation resistance, has a good temperature coefficient (0.003/ o C), but is very weak, particularly at high temperatures. The platinum-iridium wire is a
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