hotwireusage

hotwireusage - AAE 520 Notes, 9-Jan-04 Page 1 Background...

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AAE 520 Notes, 9-Jan-04 Page 1 Background Information for Use of Pitot Tube, Manometer, Hot Wires, and Hot Films 1 Background The following is adapted from the handout in AAE333L . 1.1.1 Specific Applications: Hot wires are the most common instrument for measuring unsteady fluid flows. Most real fluid flows are turbulent and unsteady. Pitot tubes are the simplest standard flow-measurement technique. DO NOT FORCE THE HOT FILM OR PITOT TUBE INTO THE WALL OR STOPS! BE GENTLE WITH THE TRANSLATION STAGE! 1.1.2 The Pitot-Static Tube For inviscid incompressible (i.e., ignoring the effects of viscosity and assuming density is constant) flow the principle of continuity and the Bernoulli equation become useful in determining average flow properties along a streamline. Time-average velocity measurements can be made with a pitot-static tube. The operation of a pitot-static tube is based upon the Bernoulli equation which for a steady incompressible flow takes the form: P s + 1 2 ρ U 2 = P t where: U = average flow velocity ρ = fluid density (assumed constant in Bernoulli equation) P s = static pressure P t = total pressure 1 2 ρ U 2 = dynamic pressure (by definition) The pitot-static tube is a combination of a pitot or total head tube for measuring total pressure and a static tube for measuring static pressure in the flow, thereby allowing the velocity to be determined at the point of measurement (see Figure 1).
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AAE 520 Notes, 9-Jan-04 Page 2 Figure 1 The flow entering the pitot-static tube at the open end is brought to rest (U=0) at the stagnation point. This tube provides the total pressure. The static pressure must be measured perpendicular to the flow, thus ignoring the effects of velocity. This is accomplished by using carefully positioned holes in the side of the pitot-static tube. A differential manometer can read directly the dynamic pressure q = 1/2 ρ U 2 = P. Therefore the velocity of the fluid is ( p = P t - P s ): U = 2( ) PP ts ρ 1.1.3 Hot Wire and Hot Film Sensors The principle on which the operation of a hot wire or a hot film sensor is based is very simple and can be explained as follows: If a piece of electrically heated fine wire is placed normal or at some angle to a flow stream, the wire will be cooled by the flowing fluid due to the heat transferred from the wire to the fluid provided the sensor is at a temperature above the fluid temperature. The amount of heat transferred is related to the magnitude of the velocity and should increase with increasing velocity. THE PHYSICAL QUANTITY WHICH IS MEASURED BY THE HOT FILM ANEMOMETER IS THE HEAT TRANSFER RATE FROM THE WIRE TO THE FLUID. Figure 2 Sensor
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AAE 520 Notes, 9-Jan-04 Page 3 1.1.4 The Sensor Two kinds of sensors have been developed and they are presently in use, the hot wire and the hot film. Both are attached to fine needles that point forward into the flow (Fig. 2). Both are usually placed perpendicular to the primary flow direction. The hot wire is typically made of tungsten or platinum-rhodium, and is a fine wire that is typically 0.00005 to 0.0002 inches in diameter and 0.010 to 0.040 inches long.
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hotwireusage - AAE 520 Notes, 9-Jan-04 Page 1 Background...

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