Controlled generation of vapor/liquid slug flows by local boiling in microgravity

Abstract

We design, construct, and test a device for controlled boiling by means of local vapor bubble formation in a cavity that injects vapor bubbles in a capillary crossflow. The outcome of the device is a regular slug flow of approximately monodisperse bubbles that can be used for a variety of applications. The setup is designed to be independent of gravity level and is tested succesfully both in normal gravity and in microgravity using drop tower experiments. The device is calibrated by a systematic study determining the size and frequency of bubble formation as a function of pressure and flow conditions. Results turn out to be virtually independent of gravity. Different regimes of operations are identified and described, and basic properties of the behavior of the bubbles inside the nucleation cavity are discussed. The slug flow downstream the crossflow is characterized and shows some differences depending on gravity. Finally, we illustrate the use of a controlled regular slug flow generated by our device by using it as an input of a serpentine heat exchanger. This emphasizes an important virtue of our method for practical applications, namely, the ability to separate the process of bubble formation from the actual heat exchange by phase transformation in the region of interest.