Laser anemometer

The result of a flow visualization experiment is a flow picture that may be recorded with a camera (for example, a photographic camera, movie camera, or video camera). The information on the state of flow is available in the recording plane x-y plane) for a specific instant of time, ii or for a number of discrete instants of time in the case of kinematic recording. This makes the information diffeient from that obtained by probe measurements (e.g., hot wire anemometer, laser-Dopplervelocimeter), where the data is measured only at one specific location (coordinate jc, y z), but as a continuous function of time t. 

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 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. 

The corresponding laser-based experimental methods are covered below, with special regard to the laser Doppler anemometer technique, which offers the greatest application use in industrial ventilation at the lowest cost. 

Laser Doppler anemometer An instrument for determining fluid velocity by measuring the difference in frequency between the incident beam and that... 

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

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

An interesting modification of this technique is the fibre-optic dynamic anemometer (FODA)143. A length of fibre-optic cable carries the laser beam to the interior of the dispersion. Back-scattered light, with its Doppler frequency shift, is returned to the detector along with reflected light and, again, the resulting beat frequency pattern is analysed. Since only a very small volume around... 

In addition to the anemometers using the laser Doppler principle, there are also laser anemometers utilizing two-focus and transmit-time principles. The laser two-focus anemometer (L2F) measures the time needed for particles to pass the known distance between two focused beams. The signal... 

If one observes the instantaneous macroscopic velocity in a turbulent-flow system, as measured with a laser anemometer or other sensitive device, significant fluctuations about the mean flow velocity are observed as indicated in Fig. 5-11, where u is designated as the mean velocity and is the fluctuation from the mean. The instantaneous velocity is therefore... 

The foregoing analysis of free-convection heat transfer on a vertical flat plate is the simplest case that may be treated mathematically, and it has served to introduce the new dimensionless variable, the Grashof number, which is important in all free-convection problems. But as in some forced-convection problems, experimental measurements must be relied upon to obtain relations for heat transfer in other circumstances. These circumstances are usually those in which it is difficult to predict temperature and velocity profiles analytically. Turbulent free convection is an important example, just as is turbulent forced convection, of a problem area in which experimental data are necessary however, the problem is more acute with free-convection flow systems than with forced-convection systems because the velocities are usually so small that they are very difficult to measure. Despite the experimental difficulties, velocity measurements have been performed using hydrogen-bubble techniques [26], hot-wire anemometry [28], and quartz-fiber anemometers. Temperature field measurements have been obtained through the use of the Zehnder-Mach interferometer. The laser anemometer [29] is particularly useful for free-convection measurements because it does not disturb the flow field. 

Bankel, J., and Olsson, E. Measurements of Particle Velocity in a Cyclone Separator Using Laser Doppler Anemometer, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 630-635. Somerset, Pennsylvania (1993). 

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. 

Measurements have been made of the combustion characteristics of an air blast kerosene spray flame and of droplet sizes within the spray boundary of isothermal sprays. Specific techniques were used to measure velocity, temperature, concentration, and droplet size. Velocities measured by laser anemometer in spray flames in some areas are 400% higher than those in isothermal sprays. Temperature profiles are similar to those of gaseous diffusion flames. Gas analyses indicate the formation of intermediate reactants, e.g., CO and Hg, in the cracking process. Rosin-Rammler mean size and size distribution of droplets in isothermal sprays are related to atomizer efficiency and subsequent secondary atomizer/vaporization effects. 

Rapid developments have taken place in the fleld of laser anemome-try, and this technique has been applied successfully in a number of studies on measurements in gaseous flames. In these studies, the gas flow was seeded with micron or submicron particles, and the velocity of these particles was taken to be representative of the velocity of the local gas flow. For the study reported here, a laser anemometer was adapted for the special problem of measurements in a spray flame which initially contains a polydisperse cloud of droplets up to 300 /un in diameter. Droplets and carbon particles are present, and seeded particles are added to the annular air flow. For the particles larger than 1 /un, significant differences exist between velocities of particles and surrounding gas. A complete description of the velocity field requires simultaneous measurement of velocity and size of individual particles. This has not yet been achieved, and, for this study, the velocity of all particles passing through the measurement control volume of the laser anemometer are reported. 

Figure 1. Laser anemometer for measuring velocity in an air blast spray...

Figure 2. Mean velocity on the axis of spray flame as measured by laser anemometer. Fuel and air flow rates varied with the ratio constant at 0.3.

Figure 5.4 Laser-doppler anemometer profiles of mean velocity in and above a wind tunnel model study of flow over an isolated sinusoidal two-dimensional ridge covered with a tall canopy. Profiles are located upwind (x = -2L, dot dash line) on the hilltop (solid line) and downwind (x = +2L, dotted line). The hill parameters were H/L = 0.1 hc/H = 1.0. Full details of the experiment can be obtained from the author (Finnigan and Hughes, pers. comm.)...

Fig. 1.5. A typical velocity signal recorded by a laser Doppler anemometer (LDA) in a liquid flow at NTNU. (v) dentotes the time average of the velocity over the time period displayed. The work was performed as a part of the strategic university program (SUP) entitled CFD Applied to Reactor ProcEss Technology (CARPET) .

Fig. 2. Schematic side view of the wind-wave tank. 1 blower, 2 honeycomb, 3 wave flap, 4 anemometer, 5 rain generator, 6 radar absorber plates, 7 wire, 8 laser, 9 laser optics, 10 X band antennae, 11 beach and overflow baffle, 12 diffusor, 13 slick deployment.

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