Based on the experiments carried out over the past decade at microgravity conditions, an overview of our current knowledge of bubbly and slug flows is presented. The transition from bubble to slug flow, the void fraction and the pressure drop are discussed from the data collected in the literature. The transition from bubble to slug flow may be predicted by introducing a critical void fraction that depends on the fluid properties and the pipe diameter: however, the role of coalescence which controls this transition is not clearly understood. The void fraction may be accurately calculated using a drift-flux model: it is shown from local measurements that the drift of the gas with respect to the mixture is due to the non uniform radial distribution of void fraction. The pressuredrop happens to be controlled by the liquid flow for bubbly flow whereas for slug flow the experimental results show that pressure drops is larger than expected. From this study, the guidelines for future research in microgr...

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Bubble and slug flow at microgravity conditions: state of knowledge and open questions

Slug to annular flow transition of microgravity two-phase flow

Gas-liquid pipe flow under microgravity conditions: Influence of tube diameter on flow patterns and pressure drops

The effect of gravity on nucleate pool boiling is considered. The static and dynamic forces acting on a growing vapor bubble on a heating surface are evaluated and how their interaction causes the bubble to detach from the surface is presented. By using a force balance, an estimate of the bubble departure radius is calculated and compared with experimental measurements from literature under micro-gravity conditions. In addition, five well known theoretical correlations are used to predict the bubble departure radius and are compared for several gravitational accelerations.

Dynamics of bubble departure in micro-gravity

In the present treatment of the condensation heat transfer process in a microgravity environment, two mechanisms for condensate removal are analyzed in light of two problems: (1) film condensation on a flat, porous plate, with condensate being removed by wall suction; and (2) the analytical prediction of the heat transfer coefficient of condensing annular flows, where the condensate film is driven by vapor shear. Both suction and vapor shear can effectively drain the condensate, ensuring continuous operation in microgravity.

Condensation heat transfer in a microgravity environment

: The research effort described in this report was conducted to begin an analysis of the phase change and flow processes of interest for spacecraft thermal transport loops. Pertinent literature was examined to identify the limits of existing theory and experimental results. It was found that data from low gravity force experiments are very limited, and much of this data is not adequately explained by existing analytical methods. The basic process of bubble growth in a liquid was analyzed, and good correlation was obtained with previous experimental data of other investigators. An expanded explanation of the forces causing bubble separation from a heated surface indicates conditions under which pool bubbling may be possible in a weightless environment. Standard equations for pressure gradients and heat transfer coefficients in boiling and condensing systems were assembled as a starting point for further analysis. Criteria for predicting flow pattern boundaries were also reviewed, and implications for low effective gravity environments are discussed. Finally, initial concepts for two-phase heat transfer loops on a spacecraft are briefly discussed with indications of the need foe continued analytical and experimental work.

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R. E. Eastman

Papers

Two-Phase Fluid Thermal Transport for Spacecraft