Anemometers

Gup and Vane Anemometers. A number of flow meter designs use a rotating element kept in motion by the kinetic energy of the flowing stream such that the speed is a measure of fluid velocity. In general, these meters, if used to measure wind velocity, are called anemometers if used for open-channel Hquids, current meters and if used for closed pipes, turbine flow meters. 

Cup anemometers have shaped cups mounted on the spokes of a wheel. The cups, under the action of the fluid forces, spin in a horizontal plane about a vertical shaft mounted in bearings. Vane or propeller types use a multibladed rotor, the axis of which is parallel to the flow direction as the rotating member. Both designs are commonly used for wind speed measurement or similar appHcations such as the velocity in ventilation ducts. Because of inertia, anemometers are most accurate under steady conditions. Velocity fluctuations cause readings that are too high. 

Current Meters. Various vane designs have been adapted for open-channel flow measurement. The rotating element is partially immersed and rotates rather like a water wheel. Operation is similar to that of vane anemometers. 

Measurement by Thermal Effects. When a fine wire heated electrically is exposed to a flowing gas, it is cooled and its resistance is changed. The hot-wire anemometer makes use of this principle to measure both the average velocity and the turbulent fluctuations in the flowing stream. The fluid velocity, L, is related to the current, /, and the resistances, R, of the wire at wire, and gas, g, temperatures via... 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Nonintrusive Instrumentation. Essential to quantitatively enlarging fundamental descriptions of flow patterns and flow regimes are localized nonintmsive measurements. Early investigators used time-averaged pressure traverses for holdups, and pilot tubes for velocity measurements. In the 1990s investigators use laser-Doppler and hot film anemometers, conductivity probes, and optical fibers to capture time-averaged turbulent fluctuations (39). 

AH closed loop control systems must measure the amount of air needed under all conditions of engine demand. Air measurement is most often done using a hot wire anemometer, usually referred to as a mass air meter (99,100). 

Anemometers An anemometer may be any instrument for measurement of gas velocity, e.g., a pitot tube, but usually the term refers to one of the following types. 

The bot-wire anemometer consists essentially of an electrically heated fine wire (generally platinum) exposed to the gas stream whose velocity is being measured. An increase in fluid velocity, other things being equal, increases the rate of heat flow from the wire to the gas, thereby tending to cool the wire and alter its electrical resistance. In a constant-current anemometer, gas velocity is determined by measuring the resulting wire resistance in the constant-resistance type, gas velocity is determined from the current required to maintain the wire temperature, and thus the resistance, constant. The difference in the two types is primarily in the electric circmts and instruments employed. 

The hot-wire anemometer can be modified for hquid measurements, although difficulties are encountered because of bubbles and dirt adhering to the wire. See Stevens, Borden, and Strausser, David Taylor Model Basin Rep. 953, December 1956 Middlebrook and Piret, Ind. Eng. Chem., 42, 1511-1513 (1950) and Piret et al., Ind. Eng. Chem., 39, 1098-1103 (1947). 

The heated-thermocouple anemometer measures gas velocity from the cooling effect of the gas stream flowing across the hot junctions of a thermopile supplied with constant electrical power input. Alternate junctions are maintained at ambient temperature, thus compensatiug for the effect of ambient temperature. For details see Bunker, Proc. Instrum. Soc. Am., 9, pap. 54-43-2 (1954). 

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

Once these traverse points have been determined, velocity measurements are made to determine gas flow. The stack-gas velocity is usually determined by means of a pitot tube and differential-pressure gauge. When velocities are very low (less than 3 m/s [10 ft/s]) and when great accuracy is not required, an anemometer may be used. For gases moving in small pipes at relatively high velocities or pressures, orifice-disk meters or venturi meters may be used. These are valuable as continuous or permanent measuring devices. 

In their experimental measurement Schwartz and Smith used circular-shape, hot-wire anemometers at several radii. These had to be placed somewhat above the top (discharge) end of the bed in up-flow operation, to dampen out large variations in velocity in the position of the anemometer. Right over a pellet the velocity was low and between the pellets it was high. These localized differences disappeared and a uniform velocity resulted somewhat above the bed. 

Fig. 19-7. Microvane and three-cup anemometer. Source Photo courtesy of R- M-Yourg Co.

The elevation angle, and through appropriate data processing a, can be measured with a bivane (a vane pivoted so as to move in the vertical as well as the horizontal). Bivanes require frequent maintenance and caUbra-tion and are affected by precipitation and formation of dew. A bivane is therefore more a research instrument than an operational one. Vertical fluctuations may be measured by sensing vertical velocity w and calculating o- , from the output of a propeller anemometer mounted on a vertical shaft. 

Rather than using separate systems for horizontal and vertical wind measurements, a u-v-w anemometer system Fig. 19-9) sensing wind along three orthogonal axes, with proper processing to give average wind direction and (Tj, from the combination of the u and v components and w and o-, from the lO component may be used. 

Gas velocihes can also be measured with anemometers (rotating vane, hot wire, etc.), from visual observations such as the velocity of smoke puffs, or from mass balance data (knowing the fuel consumption rate, air/ fuel ratio, and stack diameter). 

Variable Anemometer Height Option - The anemometer height is used in adjusting the wind speed to stack height wind speed for cavity calculations based on the following power law function ... 

UOTEN = initial wind speed value set to 20 m/s UlTEN = initial wind speed value set to 1 m/s HS- stack height ZREF - anemometer height... 

UOTEN is adjusted downward in speed and UlTEN is adjusted upward in speed in an iterative process until the minimum wind speed, UC, that will entrain the plume into a building s cavity is found. The critical wind speed is then adjusted to the anemometer height, using the reverse of the power law above, as follows ... 

Anemotherm - The Anemotherm, a hot wire anemometer, has been extensively applied to flare gas measurement and has been successfully used after modifications to reduce probe fouling. A technique used to reduce fouling is the installation of a steam coil around the tip of the probe, to prevent gases from condensing on the probe. 

The sensor is the element of an instrument directly influenced by the measured quantity. In temperature measurement the thermal mass (capacity) of the sensor usually determines the meter s dynamics. The same applies to thermal anemometers. In IR analyzers used for concentration measurement, the volume of the flow cell and the sample flow rate are the critical factors. Some instruments, like sound-level meters, respond very fast, and follow the pressure changes up to several kHz. 

The hot-wire anemometer sensor is a very fine wire with a diameter of few micrometers and length of few millimeters. This wire is connected to a measurement bridge and an electrical current is fed through the wire. The wire is heated to a temperature above the air temperature and the air velocity is determined by the cooling effect of the wire. The voltage over the wire, U, is a function not only of the velocity but also of the excess temperature and the fluid properties in the following way ... 

Modern hot-wire anemometers are normally used in the constant temperature (CT) mode, where the wire resistance and w ire temperature are kept virtually constant. In the CT-mode the wire is one part of a Wheatstone bridge circuit, which has a feedback from the bridge offset voltage to the top of the bridge (see Fig. 12.18). 

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