Nonintrusive Measurement Techniques

Most emphasis in recent times has been in the development of nonintrusive flow measurement techniques for measuring vector as well as scalar quantities in ihe flow. These techniques have been mostly optically based, but when fluid opaqueness prohibits access, other techniques are available. A quick overview of several of these nonintrusive measurement techniques is given for completeness in the next few sections. More extensive discussion on these techniques can be found in the references cited. 

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PIV is an optical whole-field, nonintrusive measurement technique to measure fluid velocity by recording the displacement of small tracer particles added to the fluid. PIV has been extensively reviewed in the past [5] and can be divided into pattern-matching velocimetry (PMV) and particle-tracking velocimetry (PTV). PIV offers more flow information per interrogation, and thus substantially, more work has been done in this field compared to LDV and ODT. 

Since many reactions that may benefit from the use of MAFBRs are performed at elevated temperatures, one of the important steps is to develop a nonintrusive measurement technique that allows insight in the fluidization behavior under reactive conditions. It can be anticipated that the fluidization behavior changes not only due to a different temperature but also due to the addition or removal of gas due to chemical reactions. 

Measuring Technique Quantity Measured Point/Whole Field Technique Instantaneous/ Time-Averaged Intrusive/ Nonintrusive Type of Flow Application... 

Optimization of internal engine combustion in respect of fuel efficiency and pollutant minimization requires detailed insight in the microscopic processes in which complex chemical kinetics is coupled with transport phenomena. Due to the development of various pulsed high power laser sources, experimental possibilities have expanded quite dramatically in recent years. Laser spectroscopic techniques allow nonintrusive measurements with high temporal, spectral and spatial resolution. New in situ detection techniques with high sensitivity allow the measurement of multidimensional temperature and species distributions required for the validation of reactive flow modeling calculations. The validated models are then used to And optimal conditions for the various combustion parameters in order to reduce pollutant formation and fuel consumption. 

Today we are convinced that there are numerous applications for this new analytical tool, especially when controls have to be performed on rare, high-cost, or infectious material for which a nonintrusive remote technique is compulsory or that repetitive measurements need to be done on the same sample over prolonged periods. 

Several methods have been proposed to nonintrusively measure the thicknesses of walls, corrosion profiles, and macrodefects [i.e., 49], Two methods at room temperature that require point contact with the cold face of the furnace are known. The first is impact-echo method, used in construction concretes and pavements (Sect. 1.4). The second method is the frequency-modulated continuous-wave (FM-CW) radar technique [50], which can produce wall thickness data in real time. 

To measure liquid density and viscosity, we must characterize a solid/liquid flow. In this section, we describe an in-line nonintrusive ultrasonic technique for determining liquid density and viscosity. 

Intrusive measurement techniques such as a Pitot static tube and hot-wire anemometer [24-26], and nonintrusive techniques such as laser Doppler velocimeter and particle image velocimetry (PIV) have been used to study the flow field. Goh, Kusadomi, and Gollahalli [13-15] mapped the velocity field in the flame using a Pitot static tube with a pressure transducer (Barocel). Details of the techniques and selection guidelines are presented in books on experimental aspects of fluid mechanics. Interested readers are referred to Holman [27], Goldstein [28], and Miller [29], to name a few. 

Optical techniques, i.e., laser Doppler velocimetry O DV), particle image velocimetry (PIV), and holographic PIV, have matured as successful nonintrusive velocity measurement techniques for large-scale applications. Panigrahi et al. [2] obtained the shear stress from PIV measurements by assuming the validity of law of the wall for turbulent flow. In recent years, the p-PIV has matured as a successful velocity measurement technique for MEMS applications [3]. 

Acoustic emission (nonintrusive). This technique involves measuring acoustic sound waves that are emitted during the growth of microscopic defects, such as stress corrosion cracks. The sensors can thus essentially be viewed as microphones which are strategically positioned on structures. The sound waves are generated from mechanical... 

Ultrasonic Doppler velocimetry is a nonintrusive technique that has been developed into a very useful technique for opaque liquid flows [3]. This technique provides good measurement of velocity new high-frequency techniques give a space resolution on the millimeter level, and even the large turbulent scales can be resolved. 

Particle size measurement is one of the essential requirements in almost all uses of colloids. However, our discussion in Section 1.5 makes it clear that this is no easy task, especially since even the definition of particle size is difficult in many cases. A number of techniques have been developed for measuring particle size and are well documented in specialized monographs (e.g., Allen 1990). Optical and electron microscopy described in the previous section can be used when ex situ measurements are possible or can be acceptable, but we also touch on a few nonintrusive methods such as static and dynamic light scattering (Chapter 5) and field-flow fractionation (see Vignette II Chapter 2) in other chapters. 

Measurement of the static structure factor using scattering techniques also provides us with a nonintrusive method to probe the structure of dispersions and the nature of interaction forces in colloids. Structural changes in colloids are particularly of interest in colloid-based techniques for fabrication of structural and special-purpose ceramics. 

Complex Index of Refraction of Soot. Soot refractive index has been measured by several researchers. The experimental techniques used can be broadly categorized as in situ and ex situ techniques. In the former, the measurements are performed nonintrusively in a flame environment. The necessary information is retrieved either from spectral transmission data or both the transmission and scattering information, as in Refs. 215-224. The ex situ measurements involve the reflection/transmission of incident spectral radiation on planar pellets of soot, and the optical properties are determined using the Fresnel relations [225]. An alternative ex situ technique was used by Janzen [226], who dispersed the soot particles in a KBr matrix and used transmission measurements to extract the required optical properties. 

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