Multi-Scale Investigation of Nucleate Boiling Phenomena in Microgravity

The subject of the present thesis is the experimental investigation of nucleate boiling phenomena in microgravity on different length scales. Nucleate boiling is a highly efficient heat transfer mechanism and widely applied in industry. For the design of nucleate boiling processes the availability of precise and reliable calculation tools is mandatory to predict the performance of the process. Moreover, a precise prediction of the process limits, like the critical heat flux is of tremendous importance with regard to safety aspects since exceeding these limits may lead to the destruction of the system. Due to the highly dynamic nature of boiling processes and the large number of involved phenomena that are coupled in a complex way, a closed theoretical description is not yet achieved. The aim of the present thesis is to contribute to the fundamental understanding of nucleate boiling. The core of the experimental setup is a stainless steel heating foil with a thickness of 20 µm. A single, artificial nucleation site is manufactured electrolytically in the centre of the foil’s upper side. The heating foil is located in a cuboid boiling cell filled with the working fluid FC-72. The saturation conditions (pressure and temperature) of the boiling cell can be adjusted. The temperature distribution of the heating foil was measured via infrared thermography from the back side with high spatial (30 µm/pixel) and high temporal (1000fps) resolution. Bubble shapes and motion were recorded by a synchronized high speed camera. The experiments were conducted on parabolic flights. One of the reasons was the enhancement of the temporal and spatial resolution because the bubble detach later at a larger diameter in microgravity. Furthermore, effects could be observed that are normally masked by natural convection which vanishes in weightlessness. By using the complete spectrum of gravity levels of the parabolic flight (0g-1.6g) the implementation of gravity in common empirical correlation could be investigated. Before recording a measurement sequence, mostly, a continual boiling process was established at the artificial nucleation site already during normal or hypergravity phases, respectively. Alternatively, fully developed boiling was investigated during some of the experiment runs. The measurement sequence with a duration of 4 s was triggered during microgravity or during the transition into the microgravity phase. The influence of gravity on bubble departure diameter and bubble frequency was studied and compared to common empirical correlations. Even if the implementation of gravity in some of the correlations agrees with the measurements the absolute value of the predicted departure diameter or the bubble frequency largely deviates from the experimental data. A new correlation for the departure diameter was developed based on a force balance between surface tension as attaching and buoyancy as detaching force, similar to the approach in the well known Fritz’ correlation. This new correlation is solely based on theoretical assumption and includes no empirical “fitting” factor. Nevertheless, it shows a very good agreement to the experimental results. Additionally, a new correlation for the bubble frequency based on the correlation of Mikic and Rhosenow was proposed. The transient heating foil temperature distribution and the local heat flux to the fluid were investigated. The local heat flux was calculated by applying an unsteady energy balance at each pixel element of the heating foil. A characteristic ring shaped maximum in the heat flux distribution was observed that was caused by strong evaporation in the vicinity of the three phase contact line at the bubble foot. The ring surrounds a region of negligible heat flux. In this region a non-evaporating adsorbed film exists at the wall and only poor convective heat transfer to the vapor inside the bubble takes place. The heat flux outside the ring is governed by convective heat transfer to the liquid and practically not influenced by the observed boiling process. This convective heat flux to the liquid is in the range of the electrical power density of the foil. Additional to the heat flows in these three regions the latent heat flow was included in an evaluation of the various heat transfer paths during a bubble ebullition cycle. The comparison to numerical simulations revealed very good qualitative but also reasonably good quantitative agreement between experimental and numerical results. As an additional stimulus a high voltage electrode was placed in a distance of 5 mm above the heating foil. The washer shaped electrode was charged to up to 10 kV. The objective of this setup was to study the influence of the electric field on the boiling process regarding the evaluation of a possible replacement of buoyancy as the detaching force in weightlessness. After the activation of the electric field in microgravity the attached spherical vapor bubble was elongated, sucked into the electrode’s centre and, thereby, detached due to the dielectrophoretic force. A reliable fully developed boiling process could be established in microgravity in the presence of the electric field