Flow measurements anemometer

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. 

Pulsed hot wire anemometer A device used for gas flow measurement, similar to the hot grid anemometer, in which measurement, are made by pulses of hot air at a downstream sensor. 

The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically heated, fine platinum wire which is immersed into the flow. As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect on the wire electrode occurs, causing its electrical resistance to change. In a constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance. In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, the resistance constant. 

In this work, measurements of the mean velocity and the turbulent fluctuations in the flows generated by the two impellers were confined to the center line of the radial jet since this is the zone where the flow is most strongly influenced by the impeller style. Turbulent flow parameters were determined using a DISA Electronics Laser Anemometer System. As the name implies, this is an optical device that measures the instantaneous velocity at a point. The technique is linear which allows accurate flow measurements at very high turbulence levels such as found in mixing vessels. Since there is no probe, there are no flow disturbances and the measurement is independent of fluid properties. The anemometer used in this work was sensitive to the direction of the instantaneous velocity. 

Air flow measurement technique at different hood surfaces, volumetric flow rate, transport and capture velocity, hood static pressure, pitot tube, thermal anemometer, and magnehelic gauge. 

Yu Dun, Hsieh H Y and Zemel J N 1993 MicroChannel pyroelectric anemometers for gas flow measurements Sensors Actuators 39 29-35 Pfahler J, Harley J C, Bau H H and Zemel J N 1990 Liquid transport in micron and submicron channels Sensors Actuators A 21-23 431 Wilding P, Pfahler J, Bau H H, Zemel J N and Kricka L J 1994 Manipulation and flow of biological fluids in straight channels micromachined in silicon Clin. Chem. 40 43-7... 

The chapter begins with the fundamental measurements of resistance, capacitance, charge, and particle force. We proceed with flow measurements with various probes followed by a listing of some commercial electrostatic instruments. Nonelectrosatic measurements in multiphase flow such as the laser-Doppler anemometer, radioactive tracers, and stroboscopic techniques (Polaskowski, et. al, 1995 Soo, 1982) have not been discussed unless in relation to an electrostatic effect. 

Anemometry is a general term to represent the measurement of wind speed— anemos is the Greek word for wind. The earliest anemometer for meteorology is credited to Alberti in 1450. Hooke reinvented the device, which relied on cups or disks mounted on a pole that would rotate by the force of wind. Modern instruments to measure wind speed rely on laser Doppler shift, ultrasonic waves, propellers, and hot wire anemometers. The hot wire anemometer is commonly used for fluid flow measurements and in particular for research applications that require a detailed analysis of velocity in localized areas or for conditions... 

Nezu I Rodi W. 1986. Open-channel Flow Measurements with a Laser Doppler Anemometer. Journal... 

Horiuchi T, Hidaka H, Fukui T, Kubo Y, Horio M, Suzuki K, Mori T (1998) Effect of added basic metal oxides on CO adsorption on alumina at elevated temperatures. Appl Catal A Gen 167 195-202 Houser EJ, Mlsna TE, Nguyen VK, Chung R, Mowery EL, McGill RA (2001) Rational materials design of sorbent coatings for explosives applications with chemical sensors. Talanta 54 469 85 Hsieh HY, Spetz A, Zemel JN (1991) Wide range pyroelectric anemometers for gas flow measurements. In Digest of technical papers of TRANSDUCERS 91. International conference on solid-state sensors and actuators, 24-27 June 1991, San Francisco, CA, pp 38-40... 

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. 

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... 

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). 

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 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. 

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. 

A hot-wire anemometer, working in the CT mode, is capable of measuring rapid velocity fluctuations. This is an advantage in the measurement of flow turbulence and is also the main area of application for the hot-wire anemometer. It is an instrument mainly for scientific purposes. 

Usually this type of anemometer does not provide information on the flow direction. Vice versa, the. sensors are made as independent of the flow direction as possible—omnidirectional. This is an advantage for free-space ventilation measurements, as the flow direction varies constantly and a direction-sensitive anemometer would be difficult to use. Naturally, no sensor is fully omnidirectional, but satisfactory constructions are available. Due to the high sensor thermal inertia, this type of anemometer is unsuitable for high-frequency flow fluctuation measurement. They can be used to monitor low-frequency turbulence up to a given cut-off frequency, which depends on the dynamic properties of the instrument. 

The vane anemometer s physical dimensions are often quite large (compared with other local velocity measurement instruments). It does not strictly measure a local velocity at all, but rather provides a spatially integrated mean value. This is an advantage in many cases where the air volume flow rate has to be predicted using local velocities and an integration principle. 

The vane anemometer is not seriously affected by small deviations in alignment in the main flow direction. However, care is necessary since over 20° misalignment causes significant errors. With regard to providing a correction for fluid density, slightly different opinions exist. T35 Based on measurements, it is recommended- that the following density correction procedure be applied ... 

A calibration facility must produce the desired velocity range for the meter to be calibrated. The air temperature should be kept constant over the test to ensure constant density. For thermal anemometers, velocity calibration only is not sufficient. They should also be checked for temperature compensation. In the case of omnidirectional probes, sensitivity to flow direction should be tested. In the case of low-speed (thermal) anemometers, their self-convection error should be measured, and, for instruments measuring flow fluctuation (turbulence), dynamic characteristics testing should be carried out as well. ... 

The previous methods are mainly used to measure duct flow. When measuring flows on supply or exhaust terminals, different methods are used. The measurement on exhaust terminals is simple to carry out, as the velocity field near the terminal is relatively constant, with no steep gradients or swirls. In the case of a grill, traversing across the terminal surface using a suitable velocity instrument is a good alternative. A suitable instrument for most cases is the vane anemometer. 

Hot wire anemometer An instrument for the measurement of fluid velocity by measuring the resistance of a fine platinum or nichrome wire, which may or may not be shielded by a silica tube. The wire resistance is proportional to the temperature and the fluid flow rate. 

Hot wire microphone anemometer An instrument for the measurement of fluid flow. 

Optical anemometer An instrument for measuring gas flow rate using a laser, in which small frequency shifts are visualized as interference fringes. 

Rotating anemometer An instrument used to measure gas flow that depends on the rotation of vanes mounted on a spindle. 

Vortex shedding anemometer A device for measuring air velocity by placing an obstruction in a gas flow and measuring the frequency of vortex is formation downstream of the obstruction. 

Fluid Velocity. The flow of air is usually measured at or near atmospheric pressure. The typical instruments to measure fluid velocity are airborne tracer techniques, anemometers, pilot-static tubes, measuring flow in ducts. 

The static pressure within a duct is too small to he measured hy a hourdon tuhe pressure gauge, and the vertical or inclined manometer is usually employed (Figure 27.2). Also, there are electromechanical anemometers. The pressure tapping into the duct must he normal to the air flow. 

In streamline flow, E is very small and approaches zero, so that xj p determines the shear stress. In turbulent flow, E is negligible at the wall and increases very rapidly with distance from the wall. LAUFER(7), using very small hot-wire anemometers, measured the velocity fluctuations and gave a valuable account of the structure of turbulent flow. In the operations of mass, heat, and momentum transfer, the transfer has to be effected through the laminar layer near the wall, and it is here that the greatest resistance to transfer lies. 

The hot-wire anemometer is very accurate even for very low rates of flow. It is one of the most convenient instruments for the measurement of the flow of gases at low velocities accurate readings are obtained for velocities down to about 0.03 m/s. If the ammeter has a high natural frequency, pulsating flows can be measured. Platinum wire is commonly used. 

A simple quantity meter which is used for the measurement of the flow of gas in an accessible duct is the anemometer (Figure 6.29). A shaft carrying radial vanes or cups... 

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