A New Technique to Determine Accommodation Coefficients of Cryogenic Propellants

The control of propellant boil-off is essential in long-term space missions. However, a clear understanding of cryogenic propellant phase change and the values of accommodation coefficients are lacking. To that effect, a new method to determine accommodation coefficients using a combination of neutron imaging, thin film evaporation modeling and CFD modeling has been established. Phase change experiments were conducted in the BT-2 Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) by introducing cryogenic vapor (H2 and CH4) at a set pressure into Al6061 and SS316L test cells placed inside a 70mm cryostat. Condensation is achieved by lowering the cryostat temperature below the saturation condition and vice versa for evaporation. Neutron imaging is used to visualize the liquid-vapor interface inside metallic containers due to the difference in attenuation between the cryogen and the metal. Phase change tests are conducted using liquid hydrogen and methane at a range of saturation points between 80 - 230 kPa and corresponding phase change rates were determined. The contact resistances and other transient heat transfer properties of the cryostat setup is determined from the combination of a CFD thermal transport model and a “dry” thermal cycling test. The calibrated CFD model then allows for the determination of the inner wall temperature profile. Results from neutron imaging and the thermal model serve as boundary conditions to a multiscale evaporation model. A macroscale 2D FEA model is used to compute evaporation flux in the bulk meniscus while a thin film evaporation model is used to account for enhanced evaporation near the contact line. Using a combination of neutron imaging, CFD thermal model and a multiscale evaporation model, there is a possibility to extract the accommodation coefficient while accounting for the curvature, disjoining pressure and a variable interface temperature. The accommodation coefficient of H2 decreases from 0.65±0.12 at 88 kPa to 0.22±0.1 at 226 kPa and is independent of container material/geometry. The error is dominated by the uncertainty in the temperature measurements (±0.25K

AN ASSESSMENT OF THE VALIDITY OF THE KINETIC MODEL FOR LIQUID-VAPOR PHASE CHANGE BY EXAMINING CRYOGENIC PROPELLANTS

Evaporation is ubiquitous in nature and occurs even in a microgravity space envi- ronment. Long term space missions require storage of cryogenic propellents and an accurate prediction of phase change rates. Kinetic theory has been used to model and predict evaporation rates for over a century but the reported values of accommodation coefficients are highly inconsistent and no accurate data is available for cryogens. The proposed study involves a combined experimental and computational approach to ex- tract the accommodation coefficients. Neutron imaging is used as the visualization technique due to the difference in attenuation between the cryogen and the metallic container. Phase change tests are conducted using liquid hydrogen and methane at a range of saturation points between 15 psia and 30 psia. In order to account for the thermal gradient in the wall at the interface, a CFD thermal model is employed. Results from neutron imaging and the thermal model serve as boundary conditions to a transition film kinetic model. Using a combination of neutron imaging, CFD thermal model and kinetic model, there is a possibility to extract the accommodation coefficient while accounting for the curvature, disjoining pressure, nanoscale interac- tions and a variable wall temperature at the interface. An accommodation coefficient of 0.5705 ± 0.0001 is obtained for liquid hydrogen evaporating from a 10mm Al6061 cylinder at 21K using a constant wall temperature of 21.00005

Drifting mass accommodation coefficients: in situ measurements from a steady state molecular dynamics setup

A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is many discrepancies in the usage of phase-change models and associated coefficients. First, a brief review of the kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, (Formula presented.)) and its inconsistent definitions are discussed. The discussion focuses on the departure from equilibrium; represented as a macroscopic “drift” velocity. Then, a continuous flow, phase change driven molecular-dynamics setup is used to investigate steady-state condensation at a flat liquid-vapor interface of argon at various phase-change rates and temperatures to elucidate the effect of equilibrium departure. MAC is computed directly from the kinetic theory-based Hertz–Knudsen (H-K) and Schrage (exact and approximate) expressions without the need for a priori physical definitions, ad-hoc particle injection/removal, or particle counting. MAC values determined from the approximate and exact Schrage expressions ((Formula presented.) and (Formula presented.)) are between 0.8 and 0.9, while MAC values from the H-K expression ((Formula presented.)) are above unity for all cases tested. (Formula presented.) yield value closest to the results from transition state theory [J Chem Phys, 118, 1392–1399 (2003)]. The departure from equilibrium does not affect the value of (Formula presented.) but causes (Formula presented.) to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a nonuniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density is observed when compared with the corresponding bulk values. When the deviation from bulk values are taken into account, all values of MAC including (Formula presented.) show a small yet noticeable difference that is both temperature and phase-change rate dependent

Multiscale approach to model steady meniscus evaporation in a wetting fluid

Evaporation along a curved liquid vapor interface, such as that of a wetting meniscus, is a classic multiscale problem of vital significance to many fields of science and engineering. However, a complete description of the local evaporative flux over all length scales, especially without arbitrary tuning of boundary conditions, is lacking. A multiscale method to model evaporation from steady meniscus is described such that a need for tuning of boundary conditions and additional assumptions are alleviated. A meniscus submodel is used to compute evaporation flux in the bulk meniscus while a transition film submodel is used to account for enhanced evaporation near the contact line. A unique coupling between the meniscus and transition film submodels ensures smooth continuity of both film and mass flux profiles along the meniscus. The local mass flux is then integrated over the interfacial area to investigate the contribution from the different regions on the surface. The model is evaluated with data from cryoneutron phase-change tests conducted previously at National Institute for Standards and Technology (NIST) [K. Bellur et al., Cryogenics 74, 131 (2016)]. It is found that the peak mass flux in the transition region is two orders of magnitude greater than the flux at the apex. Despite the enhanced evaporation in the thin film region, it was found that 78–95% of the evaporation occurs in the bulk meniscus due to the large area. The bulk meniscus contribution increases with increase in vapor pressure and Bond number but decreases with an increase in thermal conductivity of the substrate. Using a nonuniform temperature boundary suggests that there is a possibility that the adsorbed film may have a nonzero mass flux

Modeling liquid–vapor phase change experiments: Cryogenic hydrogen and methane

Mass accommodation coefficients are essential inputs to kinetic models of liquid–vapor phase change, yet after nearly 100 years there remains significant discrepancy in reported values. These discrepancies have been attributed to a wall material or geometric dependency resulting in the need for an empirical correction factors. The lack of experimental results for cryogenic fluids poses a serious impediment to modeling/predicting propellant behavior for long term space missions. Using a combination of neutron imaging experiments and multi-scale modeling, mass accommodation coefficients for liquid hydrogen and methane are determined. When the local variation in thermophysical properties are accounted for, the experimentally derived accommodation coefficients for hydrogen are invariant to container size, material and evaporation rate. The discrepancy in prior measurements of the accommodation coefficient for other fluids can be alleviated by a multi-scale analysis that incorporates local variation in thermophysical properties. The values of accommodation coefficients for hydrogen and methane are consistent with generalized transition state theory. This suggests that a mass accommodation coefficient is a solely a function of the liquid–vapor density ratio, making it a fluid-independent property easily determined without the need for empirical correction factors as reported in previous investigations

Determining solid-fluid interface temperature distribution during phase change of cryogenic propellants using transient thermal modeling

Control of boil-off of cryogenic propellants is a continuing technical challenge for long duration space missions. Predicting phase change rates of cryogenic liquids requires an accurate estimation of solid-fluid interface temperature distributions in regions where a contact line or a thin liquid film exists. This paper described a methodology to predict inner wall temperature gradients with and without evaporation using discrete temperature measurements on the outer wall of a container. Phase change experiments with liquid hydrogen and methane in cylindrical test cells of various materials and sizes were conducted at the Neutron Imaging Facility at the National Institute of Standards and Technology. Two types of tests were conducted. The first type of testing involved thermal cycling of an evacuated cell (dry) and the second involved controlled phase change with cryogenic liquids (wet). During both types of tests, temperatures were measured using Si-diode sensors mounted on the exterior surface of the test cells. Heat is transferred to the test cell by conduction through a helium exchange gas and through the cryostat sample holder. Thermal conduction through the sample holder is shown to be the dominant mode with the rate of heat transfer limited by six independent contact resistances. An iterative methodology is employed to determine contact resistances between the various components of the cryostat stick insert, test cell and lid using the dry test data. After the contact resistances are established, inner wall temperature distributions during wet tests are calculated

Rapidly responsive smart adhesive-coated micropillars utilizing catechol–boronate complexation chemistry

Smart adhesive hydrogels containing 10 mol% each of dopamine methacrylamide (DMA) and 3-acrylamido phenylboronic acid (APBA) were polymerized in situ onto polydimethylsiloxane (PMDS) micropillars with different aspect ratios (AR = 0.4, 1 and 2). Using Johnson–Kendall–Roberts (JKR) contact mechanics tests, the adhesive-coated pillars demonstrated strong wet adhesion at pH 3 (Wadh = 420 mJ m−2) and can be repeatedly deactivated and reactivated by changing the pH value (pH 9 and 3, respectively). When compared to the bulk adhesive hydrogel of the same composition, the adhesive-coated pillars exhibited a significantly faster rate of transition (1 min) between strong and weak adhesion. This was attributed to an increased surface area to volume ratio of the adhesive hydrogel-coated pillars, which permitted rapid diffusion of ions into the adhesive matrix to form or break the catechol–boronate complex

Surface plasmon resonance imaging: An inexpensive tool to study the water transport in thin film PFSA ionomers

The kinetics of water transport in confined thin film Perfluorinated sulfonic-acid (PFSA) ionomers is of vital importance in various applications such as a proton-exchange membrane or catalyst layers in polymer-electrolyte fuel cells. Advanced imaging techniques such as Neutron reflectivity, grazing-incidence x-ray scattering, and atomic force microscopy have been used for studying interfacial water transport in thin film ionomers. The instruments mentioned are considered high-end, expensive, super-resolution microscopes. The need for an expensive microscopic apparatus restricts many laboratories in developing countries from conducting experiments in the field of interfacial sciences such as visualization and in-situ measurement of water transport in thin film PFSA ionomers due to financial constraints, limited infrastructure, and lack of high-end technical support. Following the notion of portable and low-cost technologies, which is a vision of many researchers, we investigated the application of high-speed surface plasmon resonance imaging (SPRi) in the visualization of diffusion transport phenomena of water in thin film (7-250 nm) ionomers (Nafion and 3M). The preliminary results show that the water uptake in the thin film ionomers locally decreases the refractive index of the material, whose changes can be tracked by SPRi. This can provide the 3D map of diffusion during water transports through the thin film ionomer

Optical properties and swelling of thin film perfluorinated sulfonic-acid ionomer

Perfluorinated sulfonic-acid (PFSA) ionomers are members of a class of ion-conductive polymers that are commonly used in both membranes and catalyst layers in polymer-electrolyte fuel cells. Transport properties of thin film ionomers and their deviation from bulk properties are still unclear. Label free, high-speed Surface Plasmon Resonance imaging (SPRi) is proposed to simultaneously measure refractive index and thickness of a 30 nm thick Nafion film at relative humidity of 3%, 36%, and 93% . The result shows the thickness of the thin film increases with increasing the relative humidity. The refractive index of the thin Nafion film decreases from 1.310±0.003 to 1.228±0.003 when the relative humidity increases from 3% to 93%. The drop in optical properties of 30 nm thick Nafion film, indicates the void part of hydrated Nafion film is filled with a mixture of water and water vapor