Gobyzov O.A.   Ryabov M.N.   Lozhkin Y.A.   Markovich D.M.  

Application of optical techniques to study dynamics of submillimeter droplets in a supersonic flow with shock wave

Reporter: Gobyzov O.A.

Particle Image velocimetry (PIV), high-magnification imaging using background illumination (shadow photography) and background-oriented schlieren (BOS) techniques were applied to study dynamics of submillimeter droplets in supersonic flow with shock wave. In this paper, specific features of the flow and experimental set-up affecting measurement system configuration are described and some characteristics of implemented measurement system are considered.
Experiments were performed in small-scale 2D Laval nozzle operated in underexpanded flow regime (supersonic flow with shock wave in nozzle). Critical cross-section was 6.5×10 mm2, and Mach number up to 1.6. Polydisperse water droplets with approximate mass-average size of 25 μm were introduced into the flow. Droplets were generated by atomization of 0.8-mm-diameter water jet in the high-speed air flow upstream the critical section of the nozzle. In some experiments 1 μm droplets from aerosol generator based on modified Laskin nozzle were put into the flow to perform PIV velocity measurements. The aim of the work was to study dynamics of shape and velocity of submillimeter droplets in high-speed air flow with shock wave.
PIV, SP and BOS techniques were implemented on the basis of “POLIS” measurement system. Shadow photography was used to measure droplets since most of optical techniques analyzing light scattering pattern operate under the assumption that droplets are spherical, that could not be applied to the case. Due to small size and high velocity of droplets long-distance microscope (optical magnification ≈ 4:1) and pulsed Nd:YAG laser illumination of droplets were used. To produce short-duration incoherent lighting laser-excited fluorescent screen was used. BOS technique was employed simultaneously with SP to locate and control position of shock wave during experiment since PIV is not applicable to measure continuous phase velocity in the presence of dispersed phase in high number concentration. Small depth-of-field of microscope made it possible to use one background screen with irregular pattern for SP and BOS measurements. Beamsplitter mirror was used to direct both cameras to one plane of view.
Details of data processing algorithms are also considered including image filtering for PIV and SP, location of droplets and its shape analysis.
Experimental results demonstrate applicability of PIV/SP/BOS techniques combination to study high-speed particle-laden flows with shocks. In the current set-up droplets were found to have almost two times lower velocity than gas in supersonic area of the flow. Downstream from the shock velocity difference reduces, but droplets are still moving slower. Primary atomization is mostly finished upstream the critical section of the nozzle. In the area of maximum aerodynamic forces impact several breakup regimes of larger droplets were observed, including bag breakup and vibrational breakup regime [1],while droplets of 20-30 μm size exhibited shape oscillations only. It should be noted that Weber number estimation , according to [2, 3] and other works, predicts that droplets of mentioned size should also exhibit breakup.
The work was supported by Russian Foundation for Basic Research (grant number 14-08-31718).

1. Boiko V. M., Papyrin A. N., Poplavskii S. V. Dynamics of droplet breakup in shock waves// Journal of Applied Mechanics and Technical Physics. 1987. V. 28. I. 2. pp 263-269
2. Pilch M., Erdman C.A. Use of breakup time data and velocity history data to predict the maximum size of stable fragments for acceleration-induced breakup of liquid drop // Int. J. Multiphase Flow. 1987. V. 13. pp. 741–757
3. Wierzba A. Deformation and Breakup of liquid drops in a gas stream at nearly critical weber numbers // Exp. Fluids. 1990. Vol 9. pp 59-64

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