15-01-2013, 11:01 AM
Pitot Tubes
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When a fluid stream of uniform velocity flows onto a blunt body, the stream lines looks as follows:
Some of the fluid moves above the body and some of the fluid moves below the body. But the fluid in the center hits the object and stops. The velocity of the fluid at point 2 is zero, and this point is referred to as the stagnation point. 1
A Pitot tube is an instrument for measuring the velocity of a fluid. It was invented by Henri Pitot (1695–1771), and it consists of a tube with a short, right-angled bend, which is placed vertically in a moving fluid with the mouth of the bent part directed upstream. The pressure is measured with a pressure-measuring device. Pitot tubes are used to measure airspeed aboard aircraft in flight and they are also used to measure the flow of liquids. 2 A Pitot tube measures a fluid’s velocity by converting the kinetic energy of the flow into potential energy. This conversion occurs at the stagnation point.
Holes are drilled into the sides of the stack at particular intervals and the Pitot tube is placed into the stack with the angled bend against the flow of air. The flow is measured many times across the entire diameter of the stack and the values are averaged. If the Pitot tube measured flow only at the center of the stack it would overestimate the volumetric flow since flow is greatest at the center. Conversely, if the flow were measured only near the walls of the stack, it would underestimate the volumetric airflow since flow is less near the walls due to friction.
Tall stacks, such as those associated with power plants and large industrial facilities have platforms that are used as staging areas for stack tests. These platforms can be several hundred feet above ground level. If on a cold windy day you see several people huddled on such a platform, they are probably conducting a stack test.