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...numerically, the solutions of which are known as direct numerical simulations (DNSs), is apparently the only way to obtain complete description of a turbulent flow. However, the current computer technology, even with the fastest supercomputers, is still insufficient to simulate turbulent flow in a large space such as a room using an efficient DNS code (Graham 2003; Gharib 1996). The computing power limitation of direct simulations is usually addressed by the use of simplified mathematical flow models combined with experimental validation. To obtain reliable and precise information on actual airflow patterns and velocity distributions, experimental measurements are still a must at present.
The great need for experimental data has led to the development of many flow measurement systems for flow velocity and the velocity distributions. A flow velocimetry technique can be regarded as point-wise or global-wise. Point-wise measurement techniques, such as Pitot tubes, thermal anemometers, and laser Doppler velocimetry, can be used to obtain the velocity information at the point of the probe. In order to measure the global velocity distribution in a whole flow field, arrays of these point-wise measurement probes have to be used, which may either disturb the flow field, such as with intrusive methods (e.g., thermal anemometers) or excessively increase the complexity of equipment settings and the measurement cost for the nonintrusive or partially intrusive methods, such as laser Doppler velocimetry. Particle image velocimetry (PIV), particle tracking velocimetry (PTV), and other global-wise techniques, which have been intensively studied in recent years, can measure fluid velocities over the flow field or, in other words, a global domain. The global domain can be either a plane (e.g., PIV technique) or a volumetric space (e.g., holographic PIV and volumetric PTV).
The room airflow is normally characterized by relatively low velocities, by high turbulence levels, and, in most cases, by unsteady behavior in a relatively large enclosure. The commonly used methods at present and the potential techniques for the near future for indoor airflow measurements include rotating vane anemometers, thermal anemometers, ultrasonic anemometers, Laser-Doppler velocimetry, visualization techniques, particle image velocimetry, particle tracking velocimetry, and molecular tagging velocimetry. These techniques are reviewed in the remainder of this paper.
ROTATING VANE ANEMOMETERS
Rotating vane anemometers (Figure 1) sense the air speed from the pressure differentials. Airflow causes the anemometer rotor to rotate with an angular speed that is directly proportional to the airflow speed. The typical measuring range of a modern vane anemometer for ventilation application is from 0.25 to 30 m/s (50 to 6000 ft/min) with a resolution of 0.01 m/s (2 ft/min), and the accuracy of [+ or -]0.05~0.1 m/s ([+ or -]10~20 ft/min) is in the low measurable velocity range. For low-speed airflows under 0.25 m/s (50 ft/min), the measurement accuracy of vane anemometers is very low and cannot be used. The anemometers are simple, but their responsiveness, typically longer than 0.5 seconds, is not good enough to measure a turbulent flow in a common fluid field.
[FIGURE 1 OMITTED]
Rotating vane anemometers are point-wise and intrusive. The sensing head has a large size. The probe intrusiveness will unavoidably disturb the flow field. When used in an array, the distance between two probes should be larger than two diameters of the vane anemometer. Because of the requirement for alignment with the airflow direction and their poor performance for low-speed airflows, rotating vane anemometers typically have been applied in in-situ measurements of face velocities at the room air inlets or outlets where the airflow has a relatively high speed and a steady direction. The price of a rotating vane anemometer ranges from less than $100 to $1000 at present.
THERMAL ANEMOMETERS
Thermal anemometers measure the local flow velocity through its relationship to the convective cooling of electrically heated sensing elements (Webster 1999). Thermal anemometers can be further classified as constant-current anemometers (CCAs), constant-temperature anemometers (CTAs), and constant-temperature-difference anemometers (CTDAs). The operating principle of a typical commercial thermal anemometer is shown in Figure 2. The heat transfer from the sensing elements is a function of the air velocity and the temperatures of the surface and the air. The dimensions of thermal anemometer sensing parts are very small (e.g., the hot-wire diameter is only several micrometers), and current manufacturing technology is incapable of maintaining sufficiently small tolerances to ensure sensor reproducibility. Therefore, each thermal anemometer must be calibrated individually. One of the most significant error sources of thermal anemometers is the difference between measurement and calibration conditions. In low-velocity flow fields, the free convection, or thermal buoyancy, generated by the heat from the sensing elements becomes significant around the sensor itself, and the resulting output is consequently prone to considerable inaccuracy. The shape of the sensors could be wires (hot-wire anemometry), films (hot-film anemometry), or spheres (thermisters).
[FIGURE 2 OMITTED]
Hot-Wire Anemometers
The sensing element of a hot-wire anemometer is a tungsten or platinum-alloy wire 0.8~1.5 mm (0.03~0.06 in.) long and 2.5-7.5 [micro]m in diameter. The wire is mounted at its two ends on thin metallic prongs, usually tapered and having diameters of less than 1 mm. The wire is heated by electric current and cooled by the local airflow. This thermal effect is recorded and converted together with the current information to find the local flow velocity. The small sensing elements
allow hot-wire anemometers to have fast time responses; thus, they are capable of measuring velocity fluctuation frequencies up to 10,000 Hz and have a millimeter range spatial resolution. Although it is possible to measure airflow as low as 0.01 m/s (2 ft/min) with a single-component hot-wire probe, the accuracy is subject to the meter's design capability. The inaccuracy is on the order of 10%-25% at 0.05-0.5 m/s (10-100 ft/min) for a single-component hot-wire anemometer. The hot-wire sensors are fragile and thus suitable only for clean airflows.
More than one component of velocity can be measured if multiple wires are mounted. For two-or three-dimensional airflow studies, probes with, respectively, two or three perpendicular wires in an "X" pattern are used. Usually, a thermal anemometer also includes a sensor for the fluid temperature and a bridge circuit to compensate for temperature variations. Air velocity components cannot be measured simultaneously with multicomponent hot-wire probes if the air velocity is below 0.2 m/s (40 ft/min) (Zhang 1991) due to the thermal buoyancy generated by the heat from the sensing elements.
The price range for a single-component hot-wire anemometer is $1000 to $15,000. For a three-component hot-wire anemometer system, the price ranges from $15,000 to $250,000.
Hot-Film Anemometers
The sensors of hot-film anemometers are fabricated by depositing a thin film of conducting metal (usually platinum or alumina) over the surface of a firm nonconducting substrate (usually quartz). Cylindrical film sensors are typically 50 [micro]m (0.002 in.) in diameter. Compared to hot-wire sensors, these larger diameter film sensors generally have a smaller signal-to-noise ratio and slower frequency response. The response of hot-film sensors is about 10 to 20 Hz, which is still sufficient for turbulence measurements in many indoor airflow applications. The primary advantages of hot-film sensors over hot-wire sensors are that film sensors are sturdier and more stable in retaining their calibrations than hot-wire sensors. Therefore, they may be used in dirty airflows with particulates. Film sensors generally are more expensive than wire sensors.
Hot-Sphere Anemometers
Hot-sphere anemometers were specially designed for indoor airflow applications. The sensors are of a spherical shape with a diameter of 1 to 3 mm (0.04 to 0.125 in.) and operate at a lower temperature. Unlike hot-wire or hot-film sensors, hot-sphere sensors are omnidirectional, i.e., they are insensitive to the ambient airflow direction and therefore essentially measure the total speed. The typical measuring range is from 0.1 to 5 m/s (20 to 1000 ft/min) with a resolution of about 0.01 m/s (2 ft/min). Due to the relatively large size of the sensing elements, this type of sensor has a time constant of typically about two seconds. Therefore, they are mainly for measuring mean velocity and not suitable for measuring velocity fluctuations. However, in one study (Li 1995), a spherical thermister used as a sensing unit was able to detect room airflow fluctuations up to 20 Hz. The inaccuracy of hot-sphere anemometers is on the order of 10%-20% at 0.1~0.5 m/s (20~100 ft/min), and there is a need for regular calibration. The sensors may not be completely omnidirectional. Directional sensitivity exists due to the probe support, the sensor guard, and the orientation.
Hot-sphere anemometers cost less...
NOTE: All illustrations and photos
have been removed from this article.
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