Transient phenomena

The observation of transient effects is closely connected to the feature of Doppler switching by sudden changes of the ion velocity. Applications include cascade-free lifetime and quantum beat measurements by observation of the free decay after a short excitation region. Following the first experiments by Andra/ the alternative to produce a short excitation pulse by crossing the ion and laser beams has been applied extensively (cf. Part B, Chapter 20 by H. J. Andra). 

Silverans et studied interference effects between ground (or metastable) and excited states in a setup that is analogous to Ramsey s separated microwave field method. In comparison with other optical experiments, the interference conditions are easy to establish with fast ion beams, and the observed fringes are consistent with a Fourier analysis of the evolution in time of the laser frequency probed by the ions.  

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Lahcy, R. T, Jr., 1988, Turbulence and Phase Distribution Phenomena, in Transient Phenomena in Multiphase Flow, ICHMT Int. Seminar, N. H. Afgan, Ed., Hemisphere, New York. (3)... 

These methods hardly take spatial distributions of velocity field and chemical species or transient phenomena into account, although most chemical reactors are operated in the turbulent regime and/or a multiphase flow mode. As a result, yield and selectivity of commercial chemical reactors often deviate from the values at their laboratory or pilot-scale prototypes. Scale-up of many chemical reactors, in particular the multiphase types, is still surrounded by a fame of mystery indeed. Another problem relates to the occurrence of thermal runaways due to hot spots as a result of poor local mixing effects. 

Response time. In the literature, response time is usually specified as the time taken for the electrode to reach > 90% of the output. Typical response times are around 30 sec. A fast response time is critical when one is measuring transient phenomena such as oxygen respiration rates in tissue or suspended cells and dynamic measurements of the volumetric mass transfer coefficient in bioreactors. 

When operating conditions were changed, transient phenomena were sometimes observed that first move in one direction and in the reverse direction on going to the final steady state. To study these transients and to design an improved control strategy for the unit, a dynamic response model was needed. With the inclusion of the fast coke in the model, it became possible to extend the steady-state model to obtain useful dynamic response results by the addition of time-dependent accumulation terms (Weekman et al., 1967). 

The reduction of obtainable light-pulse durations down to subpicosecond pulses (halfwidth about 10 sec) allows fast transient phenomena which were not accessible before to be studied in the interaction of light with matter. One example is the extension of spin echoe-techniques, well known in nuclear-magnetic-resonance spectroscopy, to the photon echoes in the optical region. 

Wilkinson et al. [64] worked in a range of solvents on a range of compounds using the same transient absorption experimental protocol. The compounds that they studied are shown in Scheme 8. While confirming the results of the previous authors [26] for the NOSH, wherever experimental similarity existed, these authors also found several other transient phenomena for other compounds. 

The transient phenomena of c-Sf that involved the isomerization and HT quenching of c-Sf using AN are shown in Scheme 12. According to Scheme 12, the linear Stern-Volmer plots of vs. [AN] afforded = 120 30 psec being one-half... 

Considering diabatic process is proposed for the isomerization of c-Sf to It has been proposed the analogous process in which c-... 

D ) is of particular interest to elucidate the transient phenomena depending on the excited state. Although it has been considered that is nonluminescent, we have... 

TNT , 50% fine 50% coarse (-6+8 mesh) of density 1.12 and chge diam 4.8 mm, initiated with "Knotted Primacord" had initial vel of 2070 at the distance from initiator of 5.0 cm, then at the distance of 7.4 cm the vel was 3190 and after this a stable regime was established with vel 5150m/sec. The three-regime transient phenomena are also illustrated by framing-camera results for cast TNT shown in Fig 3.13, p 55. Here... 

The transient phenomena occurred not only in the nonideal region but extended also well into the ideal detonation region in Tetryl, but disappeared in EDNA when deton became ideal. This can be observed in Fig 3.11 and 3.12 of Ref 52, p 53... 

Under the title "Recapitulation of Observed Transient Phenomena ,... 

Before closing our discussion of transient phenomena it should be remarked that the emission transients are sometimes nonexponential, due to electric field effects (Makram-Ebeid, 1980), nonuniform doping, and other causes. When this problem exists, the standard boxcar or lock-in techniques (Fig. 9c) will give spurious results (White et al., 1979). Thus, the transients themselves should always be examined before any data are taken. Methods of dealing with nonexponential behavior are discussed elsewhere (Kirchner et al., 1981). 

Great advances have occurred in the application of IR techniques to the study of transient phenomena, in the quant identification of trace contaminants and in the resolution of the spectra of mixts. The new techniques are known as Fourier Transform spectrometry. The following description will be necessarily brief but it is intended to highlight potential new areas of application in the study of rapid reaction phenomena ... 

More data are needed on which to base the models. In the field, longterm production tests are needed to eliminate transient phenomena for validation of the reservoir models. In the laboratory more data are needed for permeability, conductivity, hydrate kinetics, and for determining the transition between heat-, mass-, and kinetic-controlled dissociation. 

Other permafrost-associated hydrate wells should be drilled in 2007-2008, to move beyond the proof of concept, to a proof of production, eliminating the transient phenomena in the 2002 Mallik three-day tests. 

Because field tests are so expensive it is important to validate prediction models to obviate future tests, with a minimum of reliable data. However, the three-day Mallik 2002 test contained too many transient phenomena to be modeled accurately. Instead a new 3-6 month production test that eliminates transients should be carried out at a site such as Mallik 2002, to enable modeling of production via state-of-the-art work such as Moridis LBNL model for energy production from hydrates. Once the model is refined by production data, cost savings of several orders of magnitude can be realized, to reliably predict the outcome of future tests. 

Table 9.2 Quasi-steady versus constant versus dynamic solution domains. The darker middle boxes denote the timescale, r, for the given transient phenomena. Phenomena with timescales to left of the computational time step, At, can be considered quasi-steady. Phenomena with timescales to the right of the integration time, T, can be considered constant.

In this section we examine the primary transient phenomena that are of interest to SOFC analysis, and provide the fundamental model equations for each one. Examples for the use of these models are given in later sections. While the focus is on reduced-order models (lumped and one-dimensional), depending on the needs of the fuel cell designer, this may, or may not be justifiable. Each fuel cell model developer needs to ensure that the solution approach taken will provide the information needed for the problem at hand. For the goal of calculating overall cell performance, however, it is often that one-dimensional methods such as outlined below will be viable. 

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