Kyriopoulos, Olympia Natalia (2010)
Gravity effect on liquid film hydrodynamics and spray cooling.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Spray-wall interaction is a key element in a multitude of technologies. It occurs in numerous industrial applications such as in internal combustion engines, gas turbines, spray drying, spray coating and cooling. A large diversity of phenomena is associated with the flow in the liquid layer initiated by single drop impacts onto a wall and their interactions. Due to the complexity of the problem, the hydrodynamics of liquid films created by sprays and the associated heat transfer are not entirely understood. Current approaches, usually presented in the form of empirical correlations or developed as a simple superposition of single drop impacts, disregard completely the physics of spray wall interaction. No reliable model describing reliably the behavior of the liquid film or predicting the effectiveness of spray cooling has been developed to date. The main difficulty in spray research is the fact that most sprays are usually polydisperse. Their behavior is governed by a large number of parameters which cannot be varied and controlled independently. Thus, it is not easy to identify the main influencing parameters or their combinations which define the problem. This thesis is devoted to the dynamics of a liquid film produced by normal spray impact onto a heated target. It is aimed at a better understanding the hydrodynamics and heat transfer associated with spray impact onto a heated target and at providing a basis for the modeling of spray cooling. Further progress in this field of research is achieved by performing experiments for diverse spray parameters and under various gravity conditions. The experiments under microgravity conditions are performed during parabolic flights and on board a ballistic rocket. Additionally, experiments have been performed with varied gravity levels, between -20g to +20g, in a centrifuge. Spray propagation and spray-wall interaction are observed using high-speed visualization under various gravity levels and various volumetric rates. The characteristic spray parameters are then determined with the help of image processing. Complementary to this, the spray is characterized using the phase Doppler instrument in the laboratory. To study the liquid film hydrodynamics, a robust method is developed to determine a typical film thickness created by spray impact. In addition, the heat transfer mechanisms involved in the spray-wall interaction are outlined. Heat fluxes are measured for various spray impact parameters and target temperatures. The influence of gravity, film thickness and other parameters on spray-wall interaction and spray cooling are discussed. Based on the collected data, a sound basis for reliable modeling of spray cooling is provided and validated by comparison with other experimental data from the literature. Semi-empirical models are proposed for the secondary spray as well as for the characteristic film thickness. The values of the typical film thickness are then used for description of the spray cooling efficiency.

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