Jörg Klatte

Stability limits of unsteady open capillary channel flow

This paper is concerned with steady and unsteady flow rate limitations in open capillary channels under low-gravity conditions. Capillary channels are widely used in Space technology for liquid transportation and positioning, e.g. in fuel tanks and life support systems. The channel observed in this work consists of two parallel plates bounded by free liquid surfaces along the open sides. The capillary forces of the free surfaces prevent leaking of the liquid and gas ingestion into the flow. In the case of steady stable flow the capillary pressure balances the differential pressure between the liquid and the surrounding constant-pressure gas phase. Increasing the flow rate in small steps causes a decrease of the liquid pressure. A maximum steady flow rate is achieved when the flow rate exceeds a certain limit leading to a collapse of the free surfaces due to the choking effect. In the case of unsteady flow additional dynamic effects take place due to flow rate transition and liquid acceleration. The maximum flow rate is smaller than in the case of steady flow. On the other hand, the choking effect does not necessarily cause surface collapse and stable temporarily choked flow is possible under certain circumstances. To determine the limiting volumetric flow rate and stable flow dynamic properties, a new stability theory for both steady and unsteady flow is introduced. Subcritical and supercritical (choked) flow regimes are defined. Stability criteria are formulated for each flow type. The steady (subcritical) criterion corresponds to the speed index defined by the limiting longitudinal small-amplitude wave speed, similar to the Mach number. The unsteady (supercritical) criterion for choked flow is defined by a new characteristic number, the dynamic index. It is based on pressure balances and reaches unity at the stability limit. The unsteady model based on the Bernoulli equation and the mass balance equation is solved numerically for perfectly wetting incompressible liquids. The unsteady model and the stability theory are verified by comparison to results of a sounding rocket experiment (TEXUS 41) on capillary channel flows launched in December 2005 from ESRANGE in north Sweden. For a clear overview of subcritical, supercritical, and unstable flow, parametric studies and stability diagrams are shown and compared to experimental observations.

The Capillary Flow Experiments Aboard ISS: Moving Contact Line Experiments and Numerical Analysis

This paper serves as a first presentation of quantitative data reduced from the Capillary Flow Contact Line Experiments recently completed aboard the International Space Station during Expeditions 9-16, 8/2004-11/2007. The simple fluid interface experiments probe the uncertain impact of the boundary condition at the contact line—the region where liquid, gas, and solid meet. This region controls perhaps the most significant static and dynamic characteristics of the large length scale capillary phenomena critical to most multiphase fluids management systems aboard spacecraft. Differences in fluid behavior of nearly identical static interfaces to nearly identical perturbations are attributed primarily to differences in fluid physics in the vicinity of the contact line. Free and pinned contact lines, large and small contact angles, and linear and nonlinear perturbations are tested for a variety of perturbation types (i.e. axial, slosh, and other modes) to right circular cylinders. The video and digitized datasets are to be made publicly available for model benchmarking. In parallel with the experimental effort, blind numerical predictions of the dynamic interface response to the experimentally applied input perturbations are offered as a demonstration of current capabilities to predict such phenomena. The agreement and lack of agreement between the experiments and numerics is our best guide to improve and/or verify current analytical methods to predict such phenomena critical to spacecraft fluid systems design.

The Capillary Flow Experiments (CFE-2) on ISS: Status

Eleven handheld experiment units have been designed and fabricated for a second round of capillary fluidics experiments aboard the International Space Station (CFE-2). Four of the test containers were launched to ISS on STS 131 (April 5, 2007) and experiments are currently underway to investigate certain capillary flows in complex geometries including passive migration and phase separation in weakly three-dimensional containers and critical geometric wetting phenomena with disparate capillary length scales. Progress is reported for at least ten operations conducted to date. Parallel data reduction and numerical modeling show favorable agreement with the recently developed theory, which may be applied with greater confidence to spacecraft fluid system design. In other tests, highly repeatable critical wetting phenomena are pursued with nearly indistinguishable changes in geometry (i.e., vane angle rotations < 0.5o).

Dynamic Fluid Interface Experiments Aboard the International Space Station: Model Benchmarking Dataset

DOI: 10.2514/1.47343 This paper introduces a video database reduced from the handheld capillary flow contact line experiments completed aboard the International Space Station during expeditions 9–16, August 2004–November 2007. The simple fluid interface experiments quantify the uncertain impact of the boundary condition at the contact line: the region where liquid, gas, and solid meet. This region controls many significant static and dynamic characteristics of the large length scale capillary phenomena critical to multiphase fluids management systems aboard spacecraft. Differences in fluid behavior of nearly identical static interfaces to nearly identical perturbations are attributed primarily to differences in fluid physics in the vicinity of the contact line. Free and pinned contact lines, large and small contact angles, and linear and nonlinear perturbations are tested for several manually imparted perturbation types(i.e.,axial,slosh,andothermodes)torightcircularcylinders.Thevideoandsampledigitizeddatasetsaremade publiclyavailableformodelbenchmarking.Asademonstrationoftheutilityofthedatabase,andinparallelwiththe experimental effort, blind numerical predictions of the dynamic interface response to the experimentally applied inputperturbationsareofferedasanexampleofcurrentcapabilitiestopredictsuchphenomena.Theagreementand lack of agreement between the experiments and numerics is a guide to improve or verify current analytical methods to predict such phenomena critical to practical spacecraft fluid systems design.

Analysis of LN2 Filling, Draining, Stratification and Sloshing Experiments

The accurate prediction of the mechanical and thermodynamic behavior of cryogenic liquids in tanks of liquid propelled rocket stages is critical for the design of the tank layout, the pressurization system, and the control algorithm. To analyse the behavior of cryogenic liquids in tanks before and during flight, a set of LN2 experiments with a cryogenic tank with a volume of 711 l was performed. In the first test campaign, first mode sloshing and damping was investigated at different fill levels. In the second test campaign, the stratifica-tion of the liquid and wall temperature behavior during LN2 tank fillingof the unchilled tank, during chilled tank draining, and at a constant fill level was investigated.In the third test campaign, the pressure evolution during first mode sloshing was investigated at different helium concentration levels. For each test campaign, a representative case is presentedand analysed in the current paper. Furthermore, numerical predicitions using the commercial flow solver Flow-3Dand an in-house tool to determine the tank pressure evolution are com-pared with the experiments, showing good agreement.