Hot wire anemometers

The resistive type of anemometer (hot wire or hot film) described earlier relies on the temperature dependence of the hot sensing element to monitor the convective heat loss. The resistance is maintained at a constant value by adjusting the Joule heating to maintain a constant temperature. That power input is used as a measure of the convective or other heat losses. This type of anemometer is in widespread use today. However, the energy loss of the resistive anemometer depends on the temperature of the fluid ambient in which it is immersed. Changes in the fluid ambient temperature are reflected in the power required to maintain the anemometer s resistance and this is the reason why temperature compensation is needed for this type of sensor. 

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. 

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). 

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). 

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. 

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). 

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 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). 

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. 

Manufacturers of thermal anemometers provide small rigs for their calibration. They typically consist of a nozzle, an air supply unit, and a regulating valve. The probe is placed into the nozzle jet. The reference velocity is calculated from the nozzle upstream pressure and nozzle characteristics. Due to its small size, this type of rig can be used only for hot-wire or other thermal anemometers. ... 

Z. Yue, T. G. Malmstrora. A simple method fot low-speed hot-wire anemometer calibration. Measurement Science and Technology, 9, 1998, pp. 1506-1510. 

Hot film anemometer An instrument for the measurement of fluid velocity similar to the hot wire anemometer, but more robust as it consists of a thin quartz rod covered with a film of platinum rather than a wire. 

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. 

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. 

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. 

If a heated wire is immersed in a fluid, the rate of loss of heat will be a function of the flowrate. In the hot-wire anemometer a fine wire whose electrical resistance has a high temperature coefficient is heated electrically. Under equilibrium conditions the rate of loss of heat is then proportional to /2f2, where Q. is the resistance of the wire and / is the current flowing. 

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. 

Measurement based on heat flux effects This approach uses local probing devices such as hot-wire anemometers and microthermocouples. The hot-wire anemometer can be either a constant-temperature system or a constant-heat-flux system. Because of the difference in heat transfer between the exposed fluid (liquid or gas)... 

Thus they were able to calculate the velocity intensity from the mass-transfer intensity and the spectral distribution function of mass-transfer fluctuations. By measuring and correlating mass-transfer fluctuations at strip electrodes in longitudinal and circumferential arrays, information was obtained about the structure of turbulent flow very close to the wall, where hot wire anemometer techniques become unreliable. A concise review of this work has been given by Hanratty (H2). 

Figure 28 (Y an, Yao and Liu, 1982) presents the results of turbulence measurement above a distributor plate by means of a hot-wire anemometer, indicating that turbulence intensity increases sharply near the distributor, especially for the perforated plates and the cap plate, and then diminishes beyond a certain distance (40 mm in the present case) to some nearly constant low values.

The hot wire technique appears to be the most useful one among the various electrical methods. However, the hot wire anemometer can operate only at relatively low droplet velocities. 

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