Laser-Based Techniques

12.3.10.1Introduction

In addition to their widespread use in research and development in fluid dy­namics, laser-based techniques58-60 are also suited to experiments in industrial ventilation. The use of these advanced experimental methods is reasonable when their advantages in comparison with traditional measurement techniques counterbalance the significantly higher expenses of instrumentation.

Laser-based techniques are characterized as follows:

Not a solid-state measurement device but an “optical probe” of high — intensity laser light is introduced into the fluid under investigation, avoiding disturbance on the flow field.

With the use of appropriate transmission optics, high focusing of the laser light is carried out and the extension of the optical probe is considerably reduced. Accordingly, laser-based techniques offer the possibility of measurements of high spatial resolution.

The optical flow measurement techniques are based on detection of light scattered by microscopic objects moving with the fluid. Since the light- scattering objects trace the flow, the scattered light provides relevant information on the flow properties. The flow information can be quickly obtained, according to the fast propagation of light, the applied fast-response optical detectors, and advanced electronic data acquisition and signal processing techniques. Hence, measurements of high temporal resolution and high data rate can be carried out.

Depending on the structure of the optical probe, components of vector quantities (velocity field, displacement field) and their signs can be distinguished in measurements, ensuring directional sensitivity.

Since the calibration factors for these techniques are known and relatively stable (depending on constant optical and geometrical parameters), the calibration process for laser-based measurements is simple or can be ignored.

In certain cases, the optical probe can be used at a large distance from the experimental equipment. Hence, spatial zones can be probed that are normally accessible with difficulty or inaccessible for traditional measurement devices. This arrangement provides a means for remote measurements.

The use of laser-based techniques is ideal for practicing engineers in the field of industrial ventilation for the following on-site measurements and labo­ratory applications:

Determination of high-resolution, accurate flow data providing a basis for boundary conditions used in CFD, for the verification of CFD results, and for improvement in CFD codes

Fast, accurate, and convenient on-site calibration of traditional measurement devices

Comprehensive experiment-based preparation for the selection of special air-handling system elements

Convenient methods of studying flow characteristics related to siru i iral elements that are accessible with difficulty for traditional me tsur u :nt devices

For the investigation of airflow, air-particle and air-droplet systems (dusty and humid air), and the related technical equipment, laser-based techniques fulfill the following experimental demands:

Flow visualization

Velocity and turbulence measurements

Particle and droplet size distribution measurements

Particle and droplet concentration measurements

Simulation of transport and measurement of concentration distribution of air pollutants

Vibration and acoustic measurements

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.

12.3.10.2Laser Doppler Anemometry (LDA)

Even though the LDA principle is based on the optical Doppler effect, its lifelike interferometric interpretation is presented here.

In the intersection space (optical probe) of two coherent laser beams, pla­nar interference fringes are formed. These are normal to the plane of beams, parallel to the beam bisector, and of a known uniform distance.

If the detected frequency of the flashing light scattered by a microscopic ob­ject when crossing the fringes is multiplied by the fringe distance, the velocity component of the scattering object normal to the beam bisector and parallel to the laser beam plane is determined.

The light-scattering objects must track the flow accurately, ensuring that their velocity represents the fluid velocity with a high accuracy. The light-scatter­ing objects are either in the flow7 as a natural impurity such as dust, or are artifi­cially introduced into the airflow at an optimum concentration (“seeding”).

The complete LDA system includes the appropriate transmission and detec­tion optoelectronics, traverse mechanisms, computer-controlled signai process­ing, and a data acquisition and evaluation system. The LDA equipment is a powerful too] for the measurement of flow velocity and velocity fluctuation, as well as the local concentration of particles or droplets transported in the airflow.

In industrial ventilation applications, LDA experiments fulfill the follow­ing needs:

The highly resolved velocity profile can be mapped in the vicinity of solid boundaries such as the walls of a room and in the entire enclosure, providing relevant data for CFD boundary conditions. These data form a basis for verification of CFD results and for improvement of CFD codes.

Very low air velocity can be measured; it is limited, however, by the sedimentation of the light-scattering objects.

Full use can be made for the on-site and off-site calibration of traditional anemometers such as thermal probes and propeller anemometers.

Velocity measurements in flow regions where other devices fail to operate suitably—boundary layers, stagnating air zones—are typical applications.

The local dust concentration in the airflow can be measured.

When the use of LDA is required in industrial ventilation, a single-compo­nent LDA system measuring one velocity component at a time is sufficient and provides a method of reasonable cost.

The layout recommended is a small-size, mobile, compact LDA probe that integrates the transmitting and back-scattering detection section and com­prises a diode laser and fiber optics. Such LDA equipment is portable, together with a laptop data acquisition and evaluation computer. It is easy to install and reconfigure for measuring different velocity components in field experi­ments. The internal characteristics of the rugged LDA probe are unaffected by normal vibration, mechanical shock, and temperature change. Hence, no cali­bration or adjustment is required. To protect the LDA equipment from a hu­mid or aggressive environment, special sealing is required.

Considering the possible contamination of the LDA front optics in field experiments as well as the reduced signal quality due to flaring noise from dif­fuse surfaces such as building surfaces, a sophisticated signal processor is used to clear up noisy signals. The LDA software controls the measurement, per­forms the signal processing, and presents the data in a comprehensive manner.

Some general quantitative characteristics (orders of magnitudes) of LDA systems are: velocity measurement range 1 mm s-1-100 m s_1; relative mea­surement uncertainty 0.1-1%; rate of accepted data 0.1-10 kHz; size of the optical probe 10 n-m-l mm for each dimension; measuring distance 0.1-1 m.

The LDA equipment must be installed so that the velocity component to be mapped is parallel to the beam plane, and normal to the beam bisector.

If the light-scattering objects originally present in the airflow are unsuit­able for LDA measurements due to insufficient concentration or incorrect esti­mated flow-tracking capability, the air must be seeded with oil smoke, tobacco smoke, or titanium dioxide tracer particles or droplets. A simple smoke candle is generally suitable for seeding, even if the enclosure is large and the air path is not closed as in several cases of industrial ventilation.

Attention must be paid to the specific technical problems posed by mea­suring flow in industrial ventilating systems, such as high turbulence level and long time-variation of mean velocity. The LDA measurement conditions (sta­tistically sufficient number of LDA data, suitably long duration of LDA mea­surements for recognition of long-term phenomena) must be carefully selected for an appropriate treatment of these problems.

12.3.10.3Phase Doppler Anemometry (PDA)

Single-component PDA equipment is similar to LDA, but two detectors (not one) are installed with different detection angles. By means of simulta­neous processing of signals supplied by the two detectors, information on the velocity and on the size of the scattering objects can be acquired. Therefore, ve­locity distribution, size distribution, and number density (local concentration)

Distribution on the multitude of spherical scattering objects in the submicrome­ter to larger particulate matter range can be measured. Air-partide and air — droplet systems can be simultaneously mapped. Such experiments assist in the preparation or design of special dust or liquid separation equipment by field measurements of filter performance claims.

12.3.10.4Laser Sheet (LS) Techniques

A thin laser “light sheet” of high intensity is produced with the use of ap­propriate optics, by means of which an entire “slice” of the fluid flow is illu­minated (probed). If a sufficient concentration of light-scattering material such as oil smoke is ensured, the LS technique is a powerful tool for flow visu­alization. Beyond such qualitative studies, a quantitative, tomographic charac­terization of the flow field can be carried out by recording and digital processing of the scattered image.

Particle image velocimetry (PIV) supplies two-dimensional velocity data simultaneously in a planar flow section illuminated by the laser sheet. Another quantitative application of the LS method is the simulation of transport and measurement on the concentration distribution of gaseous or dusty air pollut­ants. If the dusty pollutant is present in the airflow at a high concentration, it appears as a “natural” multitude of light-scattering objects. Otherwise the flow must be seeded, simulating the dusty or gaseous pollutants bem — ts n ported. The seeding particles or droplets must be capable of tracing the turbu­lent flow with high accuracy. Based on the analogy of turbulent mixing of gases, the seeding particle or droplet distribution corresponds to the gaseous pollutants in the actual application. The grey-level distribution of the scattered image is determined and corrected by means of an image processing method, and corresponds to the pollutant concentration distribution.

12.3.10.5Laser Optical Vibration and Acoustic Measurements

This aspect is not included here, but is related to optical flow’ diagnostics. It is based again on the principle of the optical Doppler effect. Multifunctional equipment is available for noncontact measurements of flow-induced vibra­tion on surfaces of structural elements, for acoustic measurements, and for calibration of accelerometers and vibration transducers.

As the measuring distance for such equipment can be up to about 50 m, this technique is advantageous in cases when conventional acoustic or Vibro­meter devices are difficult or even impossible to use, such as high-frequency vi­bration measurements on the walls of high-level air ducts.

Nomenclature

A area

C material heat capacity

C capacity, coefficient of discharge, concentration

D diameter (throat)

D diameter (duct)

Dx

Confidence limits for the quantity X

Dy

Confidence limits for the quantity Y

E

Velocity approach factor

E,

Inertial error

E{a)

Cumulative distribution

Fix)

Probability density

G

Acceleration due to gravity

H

Heat transfer coefficient, height

I

Electrical current

I

Length

L

Relative length

M

Experimental mean, mass

N

Sample size

N(/x, A)

Normal distribution

P

Pressure

P

Probability

LL m

Mass flow rate

<7,

Volume flow rate

R

Radius

R

Resistance, radius

Re

Reynolds number

S2

Experimental variance

Experimental standard deviation of the varial

T

Random variable, time

T

Temperature

T(t, V)

^-distribution

U

Variable

U(a)

A value of the N(0, 1) distribution

U

Voltage

V

Velocity

V

Volume

W

Humidity ratio

X

Value of the random variable (quantity) X

X

Random variable (quantity)

Y

A quantity derived from measurement results

A

Risk level, angle, flow coefficient

P

Diameter ratio

E

Expansion factor

<P

Relative humidity

K

Coefficient

V

True mean

V

Number of degrees of freedom

P

Density

Rr2

True variance

T

Time constant

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