Thermal boundary effects on a GT liner structure

GT combustor liners are subjected to mechanical and thermal loads that damage the structure and reduce their operational life. Among those, the thermo-acoustic instabilities develop, generating pressure oscillations because of the interaction between heat release, acoustic waves and structure vibrations. The vibratory behaviour of the structure is the result of these phenomena and undergoes repeated reversals of the main deformation mechanisms as a function of the operating load of the engine. Monitoring and evaluating the operational load history and the life consumption rate of combustor components is essential to sustain a reliable risk-based maintenance in the GT combustion hardware. The non-linear material behaviour can activate possible interactions causing coupled damage mechanisms and become a life threatening mode of failure. A methodology for modelling both the dynamic and static behaviour of a GT cannular combustion chamber by utilizing a combined fluid-structure approach is presented in this study. Together with the calculation of the heat fluxes through the liner, the effects of the modifications at the thermal boundary conditions were used to investigate the modifications in the liner structural properties and the stresses development at different GT loads. The monitored pressure oscillations during operations has been investigated by performing both acoustic and structural dynamics. A correlation with the observed failure has been proposed by investigating stress relaxation phenomena’s, creep and plastic effects for base load and part load operations

Calculations of the Exclusive Processes 2H(e,e'p)n, 3He(e,e'p)2H and 3He(e,e'p)(pn) within a Generalized Glauber Approach

The exclusive processes 2H(e,e'p)n, 3He(e,e'p)2H and 3He(e,e'p)(pn), have been analyzed using realistic few-body wave functions and treating the final state interaction (FSI) within a Generalized Eikonal Approximation (GEA), based upon the direct calculation of the Feynman diagrams describing the rescattering of the struck nucleon with the nucleons of the A-1 system. The approach represents an improvement of the conventional Glauber approach (GA), in that it allows one to take into account the effects of the nuclear excitation of the $A-1$ system on the rescattering of the struck nucleon. Using realistic three-body wave functions corresponding to the AV18 interaction, the results of our parameter free calculations are compared with available experimental data. It is found that in some kinematical conditions FSI effects represent small corrections, whereas in other kinematics conditions they are very large and absolutely necessary to provide a satisfactory agreement between theoretical calculations and experimental data. It is shown that in the kinematics of the experimental data which have been considered, covering the region of missing momentum and energy with p_m < 0.6 GeV/c and E_m < 100 MeV in the perpendicular kinematics, the GA and GEA predictions differ only by less than 3-4 %.Comment: Typos detected and removed while Proof reading. Physical Review C. in Pres

Three-body analysis of incoherent η\eta-photoproduction on the deuteron in the near threshold region

A three-body calculation of the reaction $\gamma d\to\eta np$ in the energy region from threshold up to 30 MeV above has been performed. The primary goal of this study is to assess the importance of the three-body aspects in the hadronic sector of this reaction. Results are presented for the $\eta$-meson spectrum as well as for the total cross section. The three-body results differ significantly from those predicted by a simple rescattering model in which only first-order $\eta N$- and $NN$-interactions in the final state are considered. The major features of the experimental data are well reproduced although right at threshold the rather large total cross section could not be explained.Comment: 6 pages revtex including 3 figure

Quasifree pion photoproduction on the deuteron in the Δ\Delta region

Photo production of pions on the deuteron is studied in the spectator nucleon model. The Born terms of the elementary production amplitude are determined in pseudovector $\pi$N coupling and supplied with a form factor. The $\Delta$ resonance is considered both in the $s$ and the $u$ channel. The parameters of the $\Delta$ resonance and the cutoff of the form factors are fixed on the leading photoproduction multipoles. Results for total and differential cross sections are compared with experimental data. Particular attention is paid to the role of Pauli correlations of the final state nucleons in the quasifree case. The results are compared with those for pion photoproduction on the nucleon.Comment: 17 pages LateX2e including 5 postscript figure

Dehydration-Induced Loss of Corrosion Protection Properties in Chromate Conversion Coatings on Aluminum Alloy 2024-T3

It is well known that chromate conversion coatings (CCCs) exhibit a prompt loss in corrosion resistance when exposed to moderately elevated temperatures (60-100°C). They also suffer a gradual loss in corrosion resistance due to ambient temperature exposure. To better understand the origins of losses in corrosion resistance, CCCs were formed on 99.99% Al, 1100 Al [Al-1.0(Fe, Si, Cu)], and 2024-T3 (Al-4.4Cu-1.5Mg-0.6Mn), exposed to elevated and ambient temperature exposures for various lengths of time, and characterized using several different methods. The Cr(VI):total Cr ratio in CCCs was estimated by analysis of data derived from X-ray absorption spectroscopy. Corrosion resistance was measured by electrochemical impedance spectroscopy. The evolution of shrinkage cracking was examined by environmental scanning electron microscopy. Cr(VI) leaching experiments were also carried out to characterize the effect of thermal exposure on Cr(VI) release. Thermal analysis and X-ray diffraction were conducted to characterize the temperature-dependent changes in CCCs. Results show that CCCs degrade in several distinct steps over the temperature range of 20 to 500°C. Our findings support the longstanding notion that dehydration is the root cause for losses in corrosion resistance due to aging at ambient and low temperatures (<150°C). Extended X-ray fine structure measurements indicate a shortening in Cr(III)-Cr(III) nearest neighbor distances upon dehydration. This is interpreted as a consolidation in the Cr(OH)_3 backbone of the CCC which leads to shrinkage cracking and immobilization of Cr(VI). This finding is important because it links older observations of the effects of aging and heating on CCCs to newer interpretations of CCC formation based on inorganic polymerization. Overall, these results reveal CCCs to be highly dynamic coatings with corrosion resistance properties that vary considerably in both the short term and long term

Quasifree photoabsorption on neutron-proton pairs in 3He

Three-body photodisintegration of 3He is calculated in the photon energy range 200 - 400 MeV assuming quasifree absorption on np pairs both in initial quasideuteron and singlet configurations. The model includes the normal nucleonic current, explicit meson exchange currents and the Delta(1232)-isobar excitation. The total cross section is increased by a factor of about 1.5 compared with free deuteron photodisintegration. Well below and above the Delta region also some spin observables differ significantly from the ones of free deuteron disintegration due to the more compressed wave function of the correlated np pairs in 3He compared to the deuteron. The initial singlet state causes a significant change in the analyzing power Ay. These differences could presumably be seen at the conjugate angles where two-body effects are maximized and where photoreactions could complement similar pion absorption experiments. Figures by fax or post from [email protected]: 23 pages, report MKPH-T-94-10/HU-TFT-94-1

VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey

We report the discovery of the highly peculiar, radio-loud quasar VPMS J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present spectroscopic, imaging and photometric observations. The unusual spectrum shows a strong depression of the continuum over a wide wavelength range in the blue part without the typical structures of broad absorption line (BAL) troughs. The image of the quasar is unresolved and there is no evidence for a foreground object on the line of sight. The broad-band spectral energy distribution is not consistent with obvious dust reddening with the standard SMC extinction curve. The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can be caused by dust reddening only if the reddening curve is steeper then the SMC curve in the ultraviolet and is very flat at longer wavelengths. Alternatively, the dominant spectral features can be explained by low-ionization BALs forming unusually wide, overlapping absorption troughs.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter

Image segmentation using local-base pyramids

This paper describes an approach to image segmentation. ne methods intended for images where local topographie irregularities contrast with their noisy backgrounds . Since the method bas to bel localized and adoptive, the description of each object uses a local-base pyramid, called "local pyramid" . It simulates the human vision in its attention focusing . Using such a tool, each object is described independently and with respect to the image texture .Dans cet article, nous abordons un problème de segmentation d'images, caractérisées par des irrégularités topographiques qui n'émergent que localement d'un fond bruité. Dans ce cas, la méthode de segmentation ne peut être que localisée et adaptative. Pour cela, la description d'un objet se fait par l'utilisation d'une pyramide à base locale - appelée «pyramide locale» - simulant la focalisation de l'œil sur les zones sensibles de l'image. Ainsi, chaque objet est analysé indépendamment et en fonction de la texture intrinsèque de l'imag

Formation of Chromate Conversion Coatings on Aluminum and Its Alloys

In situ X-ray adsorption near-edge structure (XANES) has been used to investigate the formation of chromate conversion coatings on pure Al, commercial Al alloys (AA 1100, AA2024, and AA7075), and a series of binary Al–Cu alloys. The method employed cells designed to determine the growth of the total chromium [Cr(total)] and hexavalent chromate [Cr(VI)] in the chromate conversion coating (CCC) as a function of exposure time to a chromate solution. Three sets of data were obtained, where (i) the Al was exposed to only a limited amount of solution; (ii) the chromate solution was excluded after short periods of repeated exposures to the solution; and (iii) the Al was exposed continuously to the chromate solution. All the results showed a very rapid initial growth within the first seconds, followed by a continued increase in thickness for exposures up to 1 h. Measurements with Al–Cu binary alloys demonstrated that the difference observed in AA2024 and AA1100 may not be due to Cu alloying. The proportion of Cr(VI) in the coatings becomes approximately constant after 180 s of exposure for all the specimens examined, even though the coatings continued to grow

Pion emission in 2H, 12C, 27Al, gamma pi+ reactions at threshold

The first data from MAX-lab in Lund, Sweden on pion production in photonuclear reactions at threshold energies, is presented. The decrease of the total yield of pi+ in gamma + 12C, 27Al reactions below 200 MeV as well as differential, dsigma/dOmega, cross sections follow essentially predictions from an intranuclear cascade model with an attractive potential for pion-nucleus interaction in its simplest form. Double differential, d2sigma/dOmegadT, cross sections at 176 MeV show, however, deviations from the model, which call for refinements of nuclear and Coulomb potentials and possibly also for coherent pion production mechanisms.Comment: 19 pages, 7 figure