Mark Weislogel

Computing Existence and Stability of Capillary Surfaces Using Surface Evolver

In response to the growing need for design specific solutions predicting interface existence and stability in an ever-increasing number of low-gravity fluids systems applications, K. A. Brakkes surface evolver (SE) algorithm, developed to compute complex interfacial static equilibria, is demonstrated to successfully compute critical contact angles and the onset of the Rayleigh-Taylor instability. SE is first benchmarked against limited available analytical solutions and then extended to solve the specific problem of stability in rectangular containers where the effect of discontinuous boundaries and wetting play a dramatic role. In this new light SE is shown to serve the purposes of an equilibrium interface solver, critical contact angle solver, and a capillary stability solver. Concerning the latter and in contrast to other numerical dynamic stability schemes, the significantly increased time efficiency and accuracy of the approach for capillary stability problems of significant geometric complexity argue for the use of SE as a valuable tool for spacecraft systems design

Capillary Flow in Containers of Polygonal Section

An improved understanding of the large-length-scale capillary flows arising in a low-gravity environment is critical to that engineering community concerned with the design and analysis of spacecraft fluids management systems. Because a significant portion of liquid behavior in spacecraft is capillary dominated, it is natural to consider designs that best exploit the spontaneous character of such flows. In the present work a recently verified asymptotic analysis is extended to approximate spontaneous capillary flows in a large class of cylindrical containers of irregular polygonal section experiencing a step reduction in gravitational acceleration. Drop tower tests are conducted using partially filled irregular triangular containers for comparison with the theoretical predictions. The degree to which the experimental data agree with the theory is a testament to the robustness of the basic analytical assumption of predominantly parallel flow. As a result, the closed-form analytical expressions presented serve as simple, accurate tools for predicting bulk flow characteristics essential to practical low-g system design and analysis

Capillary Rewetting of Vaned Containers: Spacecraft Tank Rewetting Following Thrust Resettling

Recent investigations have successfully demonstrated closed-form analytical solutions of spontaneous capillary flows in idealized cylindrical containers with interior corners. In this work, the theory is extended and applied to complex containers modeling spacecraft fuel tanks employing propellant-management devices (PMDs) consisting of networks of vanes. The specific problem investigated is one of spontaneous rewetting of a typical partially filled liquid-fuel/cryogen tank with PMD after thrust resettling. The transients of this flow impact the logistics of orbital maneuvers and potentially tank thermal control. The general procedure to compute the initial condition for the closed-form transient flows is first outlined and then solved for several complex cylindrical tanks exhibiting symmetry. The utility and limitations of the technique as a design tool are discussed in a summary, which also highlights comparisons with NASA flight data of a model propellant tank with PMD

Damped Oscillations of a Liquid/Gas Surface upon Step Reduction in Gravity

The reorientation and settling of a liquid/gas interfacein a right circular cylinder upon step reduction in gravity is investigated numerically. A modie ed dynamic contact angle boundary condition is implemented where the hysteresis parameter ° is employed as an adjustable parameter representing the degree of relative slip at the contact line. The effect of the boundary condition on the axial interface oscillation is studied and the dependence thereon of the resonant frequency and settling time is given. Results are shown regarding the behavior of the dynamic contact angle and the penetration of the surface oscillations into the bulk liquid. The numerical results aretuned via comparisons with existing experimentaldata, and a method forestimating theappropriateboundary condition providing good agreement is suggested. An accompanying scale analysis depicts ° as a function of the Ohnesorge number, the cylinder radius, and the static contact angle. These independent parameters are measures of the system damping, characteristic surface dee ection, and effective stiffness of the interface. Applications of the results may be made to spacecraft e uids management and the use of spacecraft and/or drop towers for e uids experimentation.

Experimental investigation of condensation heat transfer in small arrays of PCM-filled spheres

Abstract Experimental investigations on the condensation heat and mass transfer between flowing steam and a small regularly packed bed of encapsulated spheres filled with PCM (phase-change material) are performed. The objectives of the research are to obtain transient transport characteristics of the dual-latent heat thermal storage system during the charging process. A special device is used to instrument the inside of the sphere. This device retards the sinking of the unmelted solid PCM toward the bottom of the spherical shell, thus reducing the strong dependence of the heat flux on tangential angle location. Instantaneous heat transfer rates are obtained for spheres of three different sizes (with single sphere and small packed bed arrangements). Tests performed to note Reynolds number and Stefan number dependencies of the two phase transfer and energy storage are also carried out. The device worked well, particularly for the larger spheres (6.35 and 7.62 cm diameter), which allowed the normalized thermal energy stored to be correlated by a single dimensionless time scale for all of the tests performed.

Passive oscillatory heat transport systems

An underdeveloped class of oscillatory passive heat transport cycles are discussed that have the potential to transport significantly higher heat loads than current heat pipes. Prototype cycles employing inferior working fluids have demonstrated transport of higher heat loads over significantly greater distances than similarly sized heat pipes (including CPLs and LHPs) employing ammonia. Most of the proposed cycles do not require capillary forces to circulate the working fluid. They are also relatively insensitive to gravity and might best be compared to thermal systems using mechanical pumps. The history of development to date of such cycles is presented in relation to other approaches under consideration for various semi/passive thermal control applications. Specific operational characteristics of a select loop are presented. The obvious pros and cons of these systems are discussed as well as potential applications—particularly as regards electronics cooling.

Capillary Flow in Cylindrical Containers with Rounded Interior Corners

Interior corners are common constructs employed in ∞uid systems aboard spacecraft to passively control large length scale capillary-dominated liquid inventories, i.e. fuel/cryogen storage tanks. Due to either design or fabrication, an ideally sharp corner may not be achieved, but rather a somewhat rounded one. In this work, low-gravity drop tower tests are performed to benchmark recent analytical results predicting capillary driven ∞ows along interior corners possessing any degree of roundedness. The analysis provides closed-form solutions that predict meniscus tip location and ∞ow rate as a function of system geometry and ∞uid properties and is easily verifled by low-g test. In this paper, a summary of the analytical solutions is followed by drop tower experimental results for the problem of sudden imbibition in containers possessing interior corners of varying roundedness. Digitized data such as meniscus tip location and transient free surface proflle are presented. It is shown that the predictions compare well with experiment, the former of which may be employed for the immediate and time-e‐cient design and analysis of current and advanced low-g ∞uids management systems.

Corner Radius Effects on Capillary Instability in Tank Geometries

Propellant management devices exploit wetting properties of liquid rocket propellants to control propellant positioning in spaceflight. One fundamental element of common propellant management devices is a corner. Corners formed by bending have finite corner radius. Corners formed between a vane and a tank wall often has small gaps. The effects of corner radius and a vane-wall gap on re-wetting of corners are examined in the context of the ConcusFinn condition for existence of a finite-height form of the interface. Modeling of geometries involving finite-sized vane bends and gaps is performed with the Surface Evolver code. Results indicate that vane bends have wetting properties similar to analytical solutions for rounded-rectangles and that vane-wall gaps may have a strong tendency not to re-wet after some maneuvers.

Development of a Solid Chlorate Backup Oxygen Delivery System for the International Space Station

The International Space Station (ISS) Program requires that there always be a 45 calendar day contingency supply of breathing oxygen. In the early assembly stages, there is only one flight system, the Russian Solid Fuel Oxygen Generator (SFOG), that can meet that requirement. To better ensure the contingency oxygen supply, the Crew and Thermal Systems Division was directed to develop a flight hardware system that can meet all contingency oxygen requirements for ISS. Such a system, called the Backup Oxygen Candle System (BOCS), has been built and tested. The BOCS consists of 33 chlorate candles, a thermal containment apparatus, support equipment and packaging. The thermal containment apparatus utilizes the O2 produced by the candle as the motive stream in an ejector to passively cool the candle during operation.

Stability of a capillary surface in a rectangular container

The linearized governing equations for an ideal fluid are solved numerically for the stability of free capillary surfaces in rectangular containers against unfavorable disturbances (accelerations, i.e. Rayleigh-Taylor instability). The preliminary results are expressed graphically in terms of a critical Bond number as a function of system contact angle. A critical wetting phenomena in the corners is shown to significantly alter the region of stability for such containers when contrast to simpler geometries such as the circular cylinder or the infinite rectangular slot. Such computational results provide additional constraints for the design of fluids systems for space-based applications.