Acoustic variations

Thermal power plant components operated at high temperatures (>500°C) and pressures, such as superheater headers, steamline sections and Y-junctions, deserve great attention for both operation safety and plant availability concerns. In particular, during plant operation transients -startups, shutdowns or load transients - the above components may undergo high rates of temperature / pressure variations and, consequently, non-negligible time-dependent stresses which, in turn, may locally destabilize existing cracks and cause the release of acoustic emission. 

The results of over 1 year of continuous, on-line acoustic emission (AE) structural surveillance of high temperature / high pressure steam headers, gained on 2 M-scale 600MW supercritical multifuel ENEL power units in normal operation, are presented in the paper. The influence of background noise, the correlation between plant operating conditions (steady load, load variations, startup / shutdown transients) and AE activity and the diagnostic evaluation of recorded AE events are also discussed. 

Different plant operating conditions (steady load, load variations, startups / shutdowns) have been encountered during the monitoring period. Electrical load, steam pressure and steam temperature values vs time have been acquired and stored during the entire period. At the same time, the RMS values of the acoustical background noise were have been continuously checked and stored, thus providing a quick check of proper instrumentation condition and a correlation between variations of plant parameters and the acoustical behaviour of the components. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

Table 4-128 shows maximum dissolved gas concentrations in drilling muds at the bottom of the hole. Figure 4-264 shows the variation of the acoustic velocity for two water-base muds and two oil-base muds of 9 and 18 Ib/gal at pressures of 5,000 and 10,000 psi. 

Scanning acoustic microscopy (SAM) is a relatively new technique which broke through in the mid-seventies and was commercialized recently. The SAM uses sound to create visual images of variations in the mechanical properties of samples. The ability of acoustic waves to penetrate optically opaque materials makes it possible to provide surface or subsurface stmctural images nondestmctively, which might... 

The acoustic microscopy s primary application to date has been for failure analysis in the multibillion-dollar microelectronics industry. The technique is especially sensitive to variations in the elastic properties of semiconductor materials, such as air gaps. SAM enables nondestructive internal inspection of plastic integrated-circuit (IC) packages, and, more recently, it has provided a tool for characterizing packaging processes such as die attachment and encapsulation. Even as ICs continue to shrink, their die size becomes larger because of added functionality in fact, devices measuring as much as 1 cm across are now common. And as die sizes increase, cracks and delaminations become more likely at the various interfaces. 

The general experimental approach used in 2D correlation spectroscopy is based on the detection of dynamic variations of spectroscopic signals induced by an external perturbation (Figure 7.43). Various molecular-level excitations may be induced by electrical, thermal, magnetic, chemical, acoustic, or mechanical stimulations. The effect of perturbation-induced changes in the local molecular environment may be manifested by time-dependent fluctuations of various spectra representing the system. Such transient fluctuations of spectra are referred to as dynamic spectra of the system. Apart from time, other physical variables in a generalised 2D correlation analysis may be temperature, pressure, age, composition, or even concentration. 

Acoustic cavitation In this case, the pressure variations in the liquid are effected using the sound waves usually ultrasound (16 kHz to 100 MHz). The chemical changes taking place due to the cavitation induced by the passage of sound waves are commonly known as sonochemistry. 

Transient cavitation is generally due to gaseous or vapor filled cavities, which are believed to be produced at ultrasonic intensity greater than 10 W/cm2. Transient cavitation involves larger variation in the bubble sizes (maximum size reached by the cavity is few hundred times the initial size) over a time scale of few acoustic cycles. The life time of transient bubble is too small for any mass to flow by diffusion of the gas into or out of the bubble however evaporation and condensation of liquid within the cavity can take place freely. Hence, as there is no gas to act as cushion, the collapse is violent. Bubble dynamics analysis can be easily used to understand whether transient cavitation can occur for a particular set of operating conditions. A typical bubble dynamics profile for the case of transient cavitation has been given in Fig. 2.2. By assuming adiabatic collapse of bubble, the maximum temperature and pressure reached after the collapse can be estimated as follows [2]. 

Acoustic cavitation is as a result of the passage of ultrasound through the medium, while hydrodynamic cavitation occurs as the result of the velocity variation in the flow due to the changing geometry of the path of fluid flow. In spite of this difference in the mechanisms of generation of two types of cavitation, bubble behavior shows similar trends with the variation of parameters in both these types of cavitation. The two main aspects of bubble behavior in cavitation phenomena are ... 

For mi = m2, the expression reduces to that obtained for a monoatomic chain (eq. 8.18). When q approaches zero, the amplitudes of the two types of atom become equal and the two types of atom vibrate in phase, as depicted in the upper part of Figure 8.10. Two neighbouring atoms vibrate together without an appreciable variation in their interatomic distance. The waves are termed acoustic vibrations, acoustic vibrational modes or acoustic phonons. When q is increased, the unit cell, which consists of one atom of each type, becomes increasingly deformed. At < max the heavier atoms vibrate in phase while the lighter atoms are stationary. 

The model immunoassay is the enzyme-linked immunosorbent assay (ELISA) in which a non-specific capture antibody is bound to a surface, such as a multi-well plate or small tube [13]. In the basic form of ELISA, a second antibody tagged with an enzyme interacts specifically with the analyte. The enzyme assay produces a colored product that is read with a spectrophotometer. There are many variations on the basic immunoassay format that serve to increase sensitivity, specificity, linear range, and speed. Many commercial instruments have been developed to take advantage of various technologies for reporter molecules. The immunoassay may be coupled to an electronic sensor and transducer, such as a surface acoustical wave (SAW) sensor. Electrochemiluminescence (ECL) is a method in which the detector antibody is tagged with a ruthenium-containing chelate [13-15]. When the tag is... 

Moreover further developments will be restricted to linear acoustic. Therefore, space variations are small enough to approximate p(r) p0. Regarding the velocity field v as the gradient of the velocity potential v = — V we can write... 

The acoustic impedance Z is defined as the ratio of a variation of acoustic pressure to the induced velocity of matter ... 

The strain-stress relationship ay = Cykfiki is valid when strain is instantaneously provoked by stress, i.e., no dephasing process occurs. In fact, it was observed that the magnitude of acoustic waves decreased as a function of the travelling distance. Introducing a damping term proportional to the time course of strain variations can solve this discrepancy ... 

It has been pointed out by numerical experiments that pulsating bubbles subject to acoustic waves can exhibit chaotic behavior [51]. A second-order model for the pulsating bubbles which is governed by slow variations in amplitude was analyzed in [51]. The ehect of parameters such as amplitude and frequency of the external wave was found to induce chaotic behavior. 

This section of the programme calculates and displays the variation in the applied acoustic pressure, the resonance pressure (or frequency) and the bubble dimensions with time. The GCOL statements in lines 1200 and 1220 refer to colour (e.g. GCOL0,l is red and GGOL0,3 is white). 

To explain his observed variations of polymerisation rate with time, reaction volume and acoustic intensity, Kruus adopted the following reaction mechanism in which he regarded cavitation bubbles as a reactant and represented their concentration as [C]. 

An effective and environmentally friendly acoustic barrier consists of a framework of woven, freshly cut willow, filled with soil, into which the willow stems root. Variations include a soil-filled framework of dried willow, in which ground-cover plants are established. Dense, compressed reed walls designed to reduce noise are also available. 

The acoustic spectra were recorded simultaneously as other process experiments, in themselves not related to acoustic chemometrics, were carried out. This resulted in many days with stable conditions in the reactor, and no particular variations in the acoustic signals. Therefore there were only a limited number of days (hours) which displayed significant variation in process parameters, which are necessary for successful multivariate analysis and calibration. 

The motion of atoms in the lattice can be depicted as a wave propagation (phonon). By dispersion we mean the variation in the wave frequency as reciprocal space is traversed. The propagation of sound waves is similar to the translation of all atoms of the unit cell in the same direction hence the set of translational modes is commonly defined as an acoustic branch. The remaining vibrational modes are defined as optical branches, because they are capable of interaction with light (see McMillan, 1985, and Tossell and Vaughan, 1992, for more exhaustive explanations). 

It is possible that microbubble shell may be shattered during the interaction with an ultrasound pulse. Indeed, drastic variation of microbubble size, up to several-fold in less than a microsecond, has been reported [33], with linear speeds of the wall motion of microbubble approaching hundreds of meters per second in certain conditions. At these rates, it is easy to shatter the materials that would otherwise flow under slow deformation conditions. In some cases (e.g., lipid monolayer shells, which are held together solely by the hydrophobic interaction of the adjacent molecules), after such shattering the re-formation of the shell maybe possible in other cases - e.g., with a solid crosslinked polymer or a denatured protein shells - the detached iceberg-like pieces of the microbubble shell coat would probably not re-form and anneal, and the acoustic response of microbubbles to the subsequent ultrasound pulses would be different [34]. 

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