Transient Capillary Channel Flow Stability

Abstract

Capillary channel techniques with free liquid surfaces provide very reliable means for liquid management in space. However, capillary channel flow is subject to limitation due to liquid surface instabilities when a critical flow rate is reached. Steady flow rate limitation is a consequence of the choking effect and well understood. Critical steady flow rate computation with a one-dimensional model is related to a numerical singularity which occurs at critical flow. For transient flow the singularity does not occur. Therefore, a new transient stability model is defined. It is based on the steady model, a simplified transient momentum balance, the consideration of the capillary pressure of typical observed surface shapes, and on a simplified dynamic inside the channel. The balance and dynamic are defined by liquid and geometrical properties only and therefore significantly easier to compute than a transient differential equation system. In 2011, experiments were performed in cooperation with NASA on the International Space Station (ISS) to confirm the model for steady flow and validate the new transient model. A new phenomenon is discussed, the flexibility effect, which provides significant additional transient stability for channels of sufficient length. An undesired feedback effect, provoked by the reuse of the liquid in a circular loop of the experimental setup, and which influenced the measurements, is compensated by a semi-empirical model for a feedback ratio.