(274c) Room Temperature Bubble Point Tests On Porous Screens: Implications for Cryogenic Liquid Acquisition Devices

Gravity affects many fluidic processes, such as the positioning of the liquid and vapor phases within a propellant tank. In a standard 1-g environment the density of the individual fluid phases dictates the location of vapor and liquid phase; the heavier fluid (liquid) settles to the bottom while the lighter fluid (vapor) rises to the top.  However, in the reduced gravity of a space environment, surface tension becomes the controlling mechanism for this liquid/vapor separation. When transferring cryogenic propellant from a storage tank to an engine or depot receiver tank, it is necessary to transfer only a homogeneous liquid phase. Multiple liquid acquisition devices (LAD) may be required to ensure liquid phase sufficiently covers the tank outlet in varying thermal and gravitational environments experienced during a space mission. A popular type of LAD is a screen-channel LAD. The channel side that faces the tank wall is covered by a fine mesh screen, which creates micron sized pores that act as a barrier to vapor ingestion. Liquid is wicked across the screen during tank outflow, preventing the pores from drying out when in contact with vapor. The total pressure loss across the screen and in the LAD channel must be less than the bubble point to prevent vapor ingestion into the channel. While certain screen-channel LAD meshes have proven flight heritage with storable propellants, finer meshes may be required to safely deliver low surface tension cryogenic propellants to an engine.

We present experimental results for room temperature bubble point tests conducted at the Cedar Creek Road Cryogenic Complex, Cell 7 (CCL-7) at the NASA Glenn Research Center. The purpose of these tests was to investigate the performance of three different fine mesh screens in room temperature liquids to provide pretest predictions in cryogenic liquid nitrogen (LN₂) and hydrogen (LH₂) as part of NASA's microgravity LAD technology development program. Bench type tests based on the maximum bubble point method were conducted for a 325x2300, 450x2750, and 510x3600 mesh sample in pure room temperature liquid methanol, acetone, isopropyl alcohol, water, and mixtures of methanol and water to cover the intermediate to upper surface tension range.

A theoretical model for the bubble point pressure is derived from the Young-LaPlace equation for the pressure drop across a curved interface. Governing equations are reduced in complexity through a set of simplifying assumptions to permit direct comparison with the experimental data. Screen pore sizes are estimated from scanning electron microscopy (SEM) to make pretest predictions. Pore sizes based on SEM analysis are compared with historical data available in the literature for the 325x2300 and 450x2750 screens as well with data obtained from bubble point tests conducted in this work.

Experimental results show that bubble point pressure is proportional to the surface tension of the liquid. We show that there is excellent agreement between data and model for pure fluids when the data is corrected for non-zero contact angle measured on the screens using a modified Sessile Drop technique. For mixtures, bubble point pressure scales with the methanol - water mole fraction composition for the range of 0% to 50% methanol. However, data obtained at methanol mole fractions greater than 50% show significant deviation from the predicted values. Deviation from the model is likely attributable to small differences in mixture compositions within the micron sized pores.

SEM image analysis of the three meshes indicated that bubble point pressure would be a maximum for the finest mesh screen. The pore diameters based on SEM analysis and experimental data obtained here are in excellent agreement for the 325x2300 and 450x2750 meshes, but not for the finest 510x3600 mesh. Therefore the simplified model can be used to interpolate predictions for low surface tension cryogenic liquids only when pore diameters are based on room temperature bubble point tests and not SEM analysis as presently implemented.

Predictions indicate that bubble point pressures in excess of 750 Pascals are achievable in LH₂ with the 450x2750 mesh, which represents a 25% margin over the baseline 325x2300 screen. Results here have implications for future LH₂ LADs technology development, as higher bubble points imply the LAD can deliver higher flow rates to an engine or depot receiver tank in micro-gravity.