The knowledge of interfacial heat and mass transfer is important for environmental and technical applications, especially nowadays for numerical simulations of two phase problems. However, the data available up to now are inconsistent, because most experiments performed on earth suffer under buoyancy and convection, and thus the boundary conditions at the evaluation could not clearly be defined. Therefore, we seized the opportunity to investigate interfacial heat and mass transfer in microgravity environment. In these experiments the growth and collapse in the overall superheated and subcooled bubles, respectively, liquid or free vapor bubbles were observed at various liquid temperature and pressure states and over periods of from a few seconds up to 300 seconds. It was for the first time that such very long periods of bubble growth could be observed. The experimental set-up allowed the control of the liquid supersaturation before the bubbles were initiated by a short heat pulse at a miniaturized heater. Therefore it was possible to perform a systematic parametric study. The measured curves for vapor bubble growth are in good agreement with our numerical simulation. Based on this model the kinetic coefficients for the evaporation and condensation according to Hertz-Knudsen have been derived from the experimental data.

Pool boiling experiments were conducted in the European Space Agency (ESA) multi-user facility, the bubble, drop, particle unit (BDPU) in the microgravity environment of space. A part of the study involved the heating of a small sphere immersed in R-123 to the onset of nucleate boiling. An analysis of the nucleation process is presented, based on a prior work for so-called quasi-homogeneous nucleation with a flat heater surface in microgravity. Reasonably good qualitative agreement exists between the analysis and measurements.

Detailed heat transfer distributions on the endwall and along the vane/blade surface are essential for component mechanical integrity and life predictions. Due to secondary flows, high gradients in heat transfer are present at the endwall and at the vane or blade surface itself where the passage vortex influences the mainstream flow. This paper documents the benchmarking of three turbulence models: 1) k-ε realizable with wall functions, 2) k-ε realizable with two layer model, and 3) the V2F model for endwall and surface heat transfer and flowfield predictions. Benchmark experimental data from a scaled-up low speed rig for both a stator and rotor geometry are used for comparisons of heat transfer and flowfield. While the k-ε realizable turbulence models give a good prediction of the secondary flow pattern, the heat transfer at the endwall and at the surface is not well predicted due to the inadequate modeling of near wall turbulence. The V2F model gives better agreement with the experiments on the endwall and vane midspan heat transfer is also well predicted, although transition occurs too far upstream on the suction surface. The results from this study represent the feasibility of CFD utilization as a predictive tool for local heat transfer distributions on a vane/blade endwall.

Experiments for the investigation of the interfacial heat and mass transfer have been performed in microgravity using the German drop tower and have been operated onboard the IML2 (1994) and LMS (1996) Space-Lab-Missions in the BDPU (Bubble, Drop and Particle Unit), ESA's multi-user facility for fluid physics experiments.During these experiment the growth and collapse of free vapor bubbles were observed over very long periods up to 300 seconds for the first time. The bubble growth and condensation rates in R11 and R113 were determined at various liquid temperature levels. The experimental set-up allowed the control of the liquid supersaturation before the bubbles were initiated at a miniaturized heater. Therefore it was possible to perform a systematic parametric study of the governing factors like fluid temperature and supersaturation of the liquid.The measured curves for vapor bubble growth are in good agreement with our numerical simulation. The numerical model for the heat conduction controlled bubble growth is based on the conservation equations formulated for the interior and exterior of the bubble and coupled by the well known Hertz-Knudsen equation at the interface. Based on this numerical model the kinetic coefficients for the evaporation and condensation have been derived from the experiment data.

Boiling heat transfer on a miniature heater has been studied under microgravity conditions during the IML 2 Space Shuttle Mission in 1994. These experiments are simulations for the application of the direct cooling of small electronic devices by boiling heat transfer in space. This becomes very important due to high thermal loads of modern electronic components. The results of this investigations are: Even at microgravity the heat transfer coefficients are very high and are even higher compared with other heater geometry. A remarkable influence of the gravity on the nucleate boiling heat transfer could not be observed, only in the transition and film boiling region a reduction up to 50 % was found. Several boiling modes have been observed during the experimental runs depending on the subcooling of the liquid, the liquid state, and the overall heat flux. Surface tension, wetting behavior, coalescence processes, the momentum of bubble formation, and thermocapillary convection play the most important role in boiling. The general statement can be made: boiling can be applied for cooling processes in microgravity.

The boiling heat transfer on a miniature heater has been measured under microgravity conditions during the IML 2 mission and under earth gravity after the mission in 1994. These experiments are simulations for the direct cooling of small electronic devices by boiling heat transfer, which becomes very important due to high thermal loads of modern electronic components.

Experiments for the investigation of the evaporation and condensation kinetics at the phase interface of single vapor bubbles have been performed under microgravity conditions. The bubble growth rates for bubbles growing in a homogeneous supersaturated liquid could be determined from films obtained during the experiments. A numerical model for the calculation of the bubble growth controlled by heat conduction has been developed. The calculated growth rates are in good agreement with the measured one. Finally the evaporation and condensation coefficients for the refrigerant R11 are determined.