Energy concentration by converging shock waves in gases

Sammanfattning: 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.