The capillary channel flow experiments on the International Space Station: experiment set-up and first results

This paper describes the experiments on flow rate limitation in open capillary channel flow that were performed on board the International Space Station in 2011. Free surfaces (gas-liquid interfaces) of open capillary channels balance the pressure difference between the flow of the liquid in the channel and the ambient gas by changing their curvature in accordance with the Young-Laplace equation. A critical flow rate of the liquid in the channel is exceeded when the curvature of the free surface is no longer able to balance the pressure difference and, consequently, the free surface collapses and gas is ingested into the liquid. This phenomenon was observed using the set-up described herein and critical flow rates are presented for steady flow over a range of channel lengths in three different cross-sectional geometries (parallel plates, groove, and wedge). All channel shapes displayed decreasing critical flow rates for increasing channel lengths. Bubble ingestion frequencies and bubble volumes are presented for gas ingestion at supercritical flow rates in the groove channel and in the wedge channel. At flow rates above the critical flow rate, bubble ingestion frequency appears to depend on the flow rate in a linear fashion, while bubble volume remains more or less constant. The performed experiments yield vast data sets on flow rate limitation in capillary channel flow in microgravity and can be utilised to validate numerical and analytical methods.