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Thermal radiation from a converging shock implosion
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
2010 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 4, 046102- p.Article in journal (Refereed) Published
Abstract [en]

High energy concentration in gas is produced experimentally by focusing cylindrical shock waves in a specially constructed shock tube. The energy concentration is manifested by the formation of a hot gas core emitting light at the center of a test chamber at the instant of shock focus. Experimental and numerical investigations show that the shape of the shock wave close to the center of convergence has a large influence on the energy concentration level. Circular shocks are unstable and the resulting light emission varies greatly from run to run. Symmetry and stability of the converging shock are achieved by wing-shaped flow dividers mounted radially in the test chamber, forming the shock into a more stable polygonal shape. Photometric and spectroscopic analysis of the implosion light flash from a polygonal shock wave in argon is performed. A series of 60 ns time-resolved spectra spread over the 8 mu s light flash shows the emission variation over the flash duration. Blackbody fits of the spectroscopic data give a maximum measured gas temperature of 5800 K in the beginning of the light flash. Line emissions originating in transitions in neutral argon atoms from energy levels of up to 14.7 eV were also detected.

Place, publisher, year, edition, pages
2010. Vol. 22, no 4, 046102- p.
Keyword [en]
aerospace components, argon, explosions, flow instability, flow visualisation, heat radiation, numerical analysis, shock tubes, shock waves
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-27927DOI: 10.1063/1.3392769ISI: 000277242300036Scopus ID: 2-s2.0-77953340892OAI: oai:DiVA.org:kth-27927DiVA: diva2:384708
Note
QC 20110110Available from: 2011-01-10 Created: 2011-01-03 Last updated: 2017-12-11Bibliographically approved
In thesis
1. On dynamics and thermal radiation of imploding shock waves
Open this publication in new window or tab >>On dynamics and thermal radiation of imploding shock waves
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Converging cylindrical shock waves have been studied experimentally. Numericalcalculations based on the Euler equations and analytical comparisons basedon the approximate theory of geometrical shock dynamics have been made tocomplement the study.Shock waves with circular or polygonal shock front shapes have been createdand focused in a shock tube. With initial Mach numbers ranging from 2 to4, the shock fronts accelerate as they converge. The shocked gas at the centreof convergence attains temperatures high enough to emit radiation which isvisible to the human eye. The strength and duration of the light pulse due toshock implosion depends on the medium. In this study, shock waves convergingin air and argon have been studied. In the latter case, the implosion lightpulse has a duration of roughly 10 μs. This enables non-intrusive spectrometricmeasurements on the gas conditions.Circular shock waves are very sensitive to disturbances which deform theshock front, decreasing repeatability. Shocks consisting of plane sides makingup a symmetrical polygon have a more stable behaviour during focusing,which provides less run-to-run variance in light strength. The radiation fromthe gas at the implosion centre has been studied photometrically and spectrometrically.Polygonal shocks were used to provide better repeatability. Thefull visible spectrum of the light pulse created by a shock wave in argon hasbeen recorded, showing the gas behaving as a blackbody radiator with apparenttemperatures up to 6000 K. This value is interpreted as a modest estimation ofthe temperatures actually achieved at the centre as the light has been collectedfrom an area larger than the bright gas core.As apparent from experimental data real gas effects must be taken intoconsideration for calculations at the implosion focal point. Ideal gas numericaland analytical solutions show temperatures and pressures approaching infinity,which is clearly not physical. Real gas effects due to ionisation of theargon atoms have been considered in the numerical work and its effect on thetemperature has been calculated.The propagation of circular and polygonal have also been experimentallystudied and compared to the self-similar theory and geometrical shock dynamics,showing good agreement.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2010. vii, 73 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:03
Keyword
converging shock waves, polygonal shock waves, temperature measurments, argon, plasma creation, ionisation, spectrometry
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-12365 (URN)978-91-7415-610-2 (ISBN)
Presentation
2010-04-16, S40, KTH, Teknikringen 8D, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20110502Available from: 2010-04-12 Created: 2010-04-12 Last updated: 2011-05-02Bibliographically approved
2. Energy concentration by converging shock waves in gases
Open this publication in new window or tab >>Energy concentration by converging shock waves in gases
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Converging shock waves have been studied experimentally in a shock tube, and numerically using inviscid calculations and the theory of geometrical shock dynamics. The converging shock waves were created in a shocktube with two modular test sections designed to create cylindrical respectively spherical waves. In the spherical case the shock waves take the shape of spherical cap before propagating into a cone, while the cylindrical shocks converge in a fully circular cylindrical chamber.

The dynamics and symmetry of circular and polygonal cylindrical shock waves with initial Mach numbers ranging from 2 to 4 were studied. The shocked gas at the centre of convergence attains temperatures high enough to emit radiation which is visible to the human eye. The strength and duration of the light pulse due to shock implosion depends on the medium. In this study, shock waves converging in air, argon, nitrogen and propane have been studied. Circular shock waves are very sensitive to disturbances which deform the shock front, decreasing repeatability. Shocks consisting of plane sides making up a symmetrical polygon have a more stable behaviour during focusing, which provides less run-to-run variance in light strength. The radiation from the gas at the implosion centre has been studied photometrically and spectrometrically. The full visible spectrum of the light pulse created by a shock wave in argon has been recorded, showing the gas behaving as a blackbody radiator with apparent temperatures up to 6,000 K. This value is interpreted as a modest estimation of the temperatures actually achieved at the centre as the light has been collected from an area larger than the bright gas core. Circular shock waves attained higher temperatures but the run-to-run variation was significant. The propagation of circular and polygonal shocks was also studied using schlieren photography and compared to the self-similar theory and geometrical shock dynamics, showing good agreement.

Real gas effects must be taken into consideration for calculations at the implosion focal point. Ideal gas numerical and analytical solutions show temperatures and pressures approaching infinity, which is clearly not physical. Real gas effects due to ionisation of the argon atoms have been considered in the numerical work and its effect on the temperature has been calculated.

A second convergent test section was manufactured, designed to smoothly transform a plane shock wave into the shape of a spherical cap. After the convergent transformation the spherical shock propagates through a conical section, where it is aimed to retain the spherical shape and converge in the tip of the truncated cone, which has an end radius of 0.3 mm. Spherical implosion is more efficient than cylindrical and the target volume is much smaller than that in the cylindrical chamber. The new set-up does not suffer from large losses through reflections. Spectrometric and photometrical measurements of the implosion show significantly stronger radiation of longer duration. Preliminary results show measured apparent blackbody temperatures up to 27,000 K during implosion of shock waves of initial Mach number MS = 3.9.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. vii, 80 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:09
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-95186 (URN)978-91-7501-368-8 (ISBN)
Public defence
2012-05-25, D2, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note
QC 20120521Available from: 2012-05-21 Created: 2012-05-15 Last updated: 2012-05-21Bibliographically approved

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