An adaptive optical system is developed to correct the wavefront of laser radiation distorted by a turbulent air flow. The use of a field-programmable gate array as the main control element makes it possible to achieve a system bandwidth of 2 kHz. The results of experiments on dynamic correction of the phase of a laser beam distorted by a flow of heated air are presented and analysed.
We report the results of the analysis of experiments on dynamic phase correction of laser radiation distorted by a turbulent airflow in an adaptive optical system (AOS) with a Shack–Hartmann wavefront sensor (WFS). A field-programmable gate array is used as the main AOS control element, which provides a closed loop AOS bandwidth of up to 2000 Hz. The WFS is used to estimate the characteristic bandwidth of the generated turbulence ν
turb. Changes in the spatial and temporal spectra of the laser radiation phase, as well as the quality of the beam during its dynamic phase correction at various AOS frequencies, are analysed. It is shown experimentally and by calculation that to ensure a high efficiency of the wavefront correction, the AOS frequency should be at least 20 times higher than ν
turb.
Translated from Élektricheskie stantsii, No. 12, December 2007, pp. 22 -25. Coils of 12Kh18N12T steel become magnetized during operation because the surface layer of the metal at the scale boundary is depleted in chromium and enriched in ferromagnetic nickel. A large-scale study of the relationship between the thickness of the altered layer and the operating temperature and individual service life is underway at Mosénergo's TÉTs-26 heating and electric power plant. Ferritometry and selective replacement have made it possible to reduce the amount of strain-induced and embrittlement damage and to extend the service life of steam superheaters. This could not be done by traditional methods, in particular ultrasonic monitoring of the wall thickness. The data obtained here can serve as a basis for creating a new technique for monitoring thermal inhomogeneity in steam superheaters made of austenitic steels. The advantages of ferritometric monitoring include the following: special preparation (cleaning) of coils is not required, the measurements are convenient, and segments with the highest temperature can be detected.The existing technique for evaluating the individual service life of steam superheater coils made of austenitic 12Kh18N12T steel is based on calculating the equivalent operating temperature of the outer surface based on the amount of the ó-phase in the metallic structure, the stress in the metal owing to internal pressure, and the time to failure corresponding to its high-temperature strength [1]. These data are obtained by laboratory study of a limited number of samples, as a rule 3 -6, which have been selected for excision based on ultrasonic measurements of the residual wall thickness immediately in the boiler. The choice of segments of the coil for excision of representative samples is often not formal, but is a fundamental operation. The validity of the service life estimate depends on making the correct choice.Nonuniform thinning of the wall and weakening of the metal in coils along the length, width, and height of the gas duct is a characteristic feature of all heating surfaces. The intensity of this process depends on the thermal inhomogeneity and the kind of fuel being burnt. The rate of overall corrosion of the metal under isothermal conditions is 5.5 times greater when a boiler is run with high sulfur fuel oil than with natural gas. Information on the thermal inhomogeneity of steam superheaters based on thickness measurements is not reliable, especially in the early stage of operation and when a boiler is run on natural gas 92 -97% of the time. A majority of the boilers at Mosénergo are currently running under just these conditions. The absence of new high speed methods for nondestructive monitoring of thermal inhomogeneity based on the state of the metal makes it impossible to determine the service life and amount of repair work reliably, to correct the temperature regime as a preventive measure, to extend the service life, and to reduce the number of shutdowns owing steam superheater damage....
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