(303a) Bubble Point Pressures of Binary Mixtures Wetting Screens Against a Vapor: Implications for Low Gravity Cryogenic Liquid Acquisition

Bubble Point Pressures of Binary Mixtures Wetting Screens Against a Vapor: Implications for Low Gravity Cryogenic Liquid Acquisition

Jason Hartwig, Cryogenic Propulsion Engineer, NASA Glenn Research Center, Cleveland, OH  44135

J. Adin Mann Jr., Professor of Chemical Engineering, Case Western Reserve University, Cleveland OH, 44106, USA

Gravity affects many fluidic processes, such as the position 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 interface; the heavier fluid (liquid) settles to the bottom and 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 a transfer line en route to an engine or depot receiver tank, it is required to transfer vapor free liquid. Multiple liquid acquisition devices (LAD) may be required to ensure the liquid phase sufficiently covers the tank outlet across the widely varying thermal and gravitational environments experienced during a space mission. Of the available choices, the most popular, flexible, and robust type of LAD is the screen channel gallery arm. The channel side that faces the tank wall is covered by a fine mesh screen, which creates a system of micron sized pores that act as a barrier to vapor ingestion. The other channel sides are solid metal. The channels converge and connect at a common location within the tank to position liquid over the outlet. During tank outflow, liquid is wicked across the screen, preventing the pores from drying out when in contact with vapor. Here, the “maximum bubble point pressure” (the pressure that is signaled by bubble formation on the screen) defines the upper limit on the LAD system performance. The total pressure loss across the screen and in the LAD channel must be less than the bubble point pressure 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 liquid propellants to an engine.

Experimental results are presented here for the room temperature binary mixture 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 stainless steel fine mesh LAD screens in room temperature liquid mixtures which span the intermediate to high surface tension range (25 – 70 mN/m) to provide pretest predictions for cryogenic liquid 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 binary mixtures of methanol and water.

The simplified bubble point model from Hartwig and Mann (2013) for pure liquids is extended to predict bubble point pressures of binary liquid mixtures. A Langmuir adsorption isotherm is used to obtain a curve fit to mixture surface tension data in order to predict the bubble point pressure at any mixture mass fraction. The Gibbs equation using the convention that the excess volume and the excess of the solvent (water) are set to zero in order to satisfy the Gibbs phase rule that specifies the number of independent intensive variables allowed in the representation of the liquid/vapor (L/V) surface tension of the binary solution, . Contact angle between LAD porous screen and binary liquid is measured as a function of methanol mass fraction using the Sessile Drop technique. The solid/vapor and solid/liquid surface tensions are then estimated using the Young-LaPlace equation. From a Zisman plot (work of adhesion vs. liquid/vapor surface tension) of the binary mixture data, one can obtain the so-called “critical” Zisman surface tension for a given liquid/solid pair where contact angle approaches zero. The critical Zisman surface tension defines the point of total wettability at the surface, and for , contact angle is not expected to deviate from , and the surface can be assumed to be completely wetted.

Experimental results are as follows: Contact angle measurements from this work indicate that  for the porous LAD screen. Since the surface tension of all cryogenic liquids is less than this critical value, the contact angle can be assumed to be zero for all cryogenic propulsion systems employing stainless steel LAD screens. Bubble point tests conducted in methanol/water mixtures are in agreement with the previous pure fluid tests, in that the middle 450x2750 mesh outperforms both the coarsest 325x2300 and finest 510x3600 meshes. Excellent agreement between experimental data and the model predicted values is obtained for methanol mass fractions greater than 50%; for methanol mass fractions less than 50%, as the mixture moves toward higher concentrations of water, theory and model deviate proportionally. Discrepancies between model and data are due to differences in mixture compositions in the bulk liquid vs. the liquid at the liquid/vapor interface at the LAD screen. As the mixture tends toward higher water concentrations, the low surface tension of methanol and high contact angle of water dominate the breakdown sites within the porous screen, effectively lowering the bubble point from the expectation value. ,  surface tension data are all consistent with the Langmuir isotherm description of the thermodynamics of adsorption.

The two implications for cryogenic propulsion LAD design from this work are that

1) “Perfect” wettability can be assumed between porous LAD screen and all cryogenic liquids.

2) Predictions for LH₂ bubble point pressures based on experimental data here indicate that bubble point pressures in excess of 750 Pascals may be achievable with the 450x2750 mesh, which represents a significant improvement in performance over the baseline 325x2300 screen.

Results here have implications for all future LH₂ fueled missions, as higher bubble point pressures imply that the LAD can deliver higher flow rates to an engine or receiver fuel depot tank in the micro-gravity conditions of space.