Experimental Investigation of Shock-induced Droplet Break-up and Numerical Simulation of Collapsing Clouds of Vapor Bubbles

Collapsed and partially
rebounded vapor bubbles
and vapor pattern located at
a solid surface (bottom). The
colors indicate the shock-wave
intensity due to prior collapse
processes.

Motivation and Objectives

1) Shock-induced droplet break-up
The break-up of liquid droplets and fluid ligaments in a gaseous ambience is a key element of atomization processes. In combustion engines, the quality of the spray inside the combustion chamber has a large impact on the combustion efficiency and also on size and composition of particles in the exhaust gas. Furthermore, droplet break-up can play an important role in the production of metal powders as used for additive manufacturing. In this case, liquid metal atomization needs to be controlled in order to optimize the quality of the resulting powder. Our objective is to gain insight into break-up mechanisms by investigation of Newtonian and non-Newtonian liquid drops exposed to shock waves generated by a shock tube.

2) Collapsing clouds of vapor bubbles
It is well known that the collapse of vapor bubbles in a pressurized liquid can lead to intense pressure waves with amplitudes of several GPa. The formation of those bubbles can be on purpose, such as in biomedical applications and food engineering, or inevitable, such as in control valves of injection systems [1], rocket engines and in the vicinity of ship propellers. Since the release of potential energy during the collapse of a bubble can be highly focused, it may be used to destroy cancer cells. On the other hand, if clusters of bubbles collapse close to a material surface, severe damage of mechanical devices can be a consequence [2]. Our objectives are to develop and improve numerical techniques for prediction of vapor bubble collapses and to improve understanding of bubble-bubble interaction in collapsing vapor bubble clouds. Furthermore, experimental investigations are performed by exposing bubbles trapped in gelatin to shock waves generated by a shock tube [3-4].

Approach to Solution
We develop and improve mathematical models and highly efficient numerical approaches for simulation of compressible multi-phase flows, especially physically consistent LES (large eddy simulation) codes. The codes are capable of high performance computations on supercomputers, such as SuperMUC at the Leibniz-Rechenzentrum München. The figure above shows collapsed and partially rebounded bubbles, together with a vapor pattern located at a solid surface. The colors indicate shock waves due to prior collapse processes. In this investigation [5], the effects of bubble interaction on intensification of material loads were characterized. It was possible to demonstrate that rebounding vapor patterns can be as erosive as the primary collapse of a bubble cloud. The shock tube at the institute was recently equipped with a droplet generator in order to investigate shock-induced droplet break-up processes. State-of-the art high speed cameras/sensors allow for high-quality data acquisition. The following figure shows two time series of break-up processes. In both cases, the bubble is hit by a planar shock wave from left.

Our research is funded by the European Union (project ‘CaFE’ and project ‘UCOM’), the European Space Agency, the German Research Foundation (DFG), and by partners from the automotive industry.

Shock-induced droplet break-up: bag-stamen type (left) and catastrophic type(right) for single water droplets (d=1mm) with Weber numbers of 33 and 1310, respectively.

 

Key Results

  • Trummler, T.; Rahn, D.; Schmidt, S.J.; Adams, N.A.: LES of cavitating flow in a step nozzle with injection into gas. Atomization and Sprays, Vol. 28, Issue 10, 2018, pp 931-955
  • Gorkh, P.; Schmidt, S. J.; Adams, N. A.: Numerical investigation of cavitation-regimes in a convergingdiverging nozzle. International Symposium on Cavitation, 2018
  • Wang, Z.; Hopfes, T.; Giglmaier, M.; Adams, N.A.: Influence of non-Newtonian gelatinous fluids on bubble collapse dynamics. International Conference on Experimental Fluid Mechanics, 2018
  • Hopfes, T.; Wang, Z.; Giglmaier, M.; Adams, N.: Experimental study on the effects of phase change during a bubble collapse. International Symposium on Cavitation, 2018
  • Ogloblina, D.; Schmidt, S. J.; Adams, N.A.: Numerical simulation of collapsing vapor bubble clusters close to a rigid wall. International Symposium on Cavitation, 2018