Near-Horizontal Two-Phase Flow Patterns of Nitrogen and Hydrogen at Low Mass and Heat Flux
N T. Van Dresar, James D. Siegwarth
An experimental apparatus was constructed and operated at the National Institute of Standards and Technology (NIST) to obtain data on the two-phase (liquid and vapor) flow behavior of cryogenic fluids under conditions of low mass and heat flux . The range of flow rates (0.2 - 20 g/s for nitrogen; 0.02 to 0.02 g/s for hydrogen), heat flux (0.07W/cm2), and flow-path size (8.7- mm I.d.) were representative of thermodynamic vent systems planned for the pressure control of spacecraft propellent tanks in low gravity. The apparatus was operated in normal gravity with a 1.5 upflow configuration. Numerous tests were conducted with liquid nitrogen and a lesser number with liquid hydrogen. Viewports in the apparatus permitted the visual observation and videotape recording of the two-phase flow patterns.The two-phase flow pattern is a key parameter in understanding and predicting two-phase flow - having proper knowledge of the flow pattern is a critical first step in predicting two-phase flow behavior. In the experiments, a variety of flow patterns were observed and recorded on videotape. The data were used to construct flow pattern maps for each test fluid. Results are presented using the traditional format of a two-dimensional flow pattern map with the liquid and gas superficial velocities used as coordinates. Computer codes to predict flow patterns were developed from theoretical and/or empirical models reported in the literature. The vast majority of the published modeling work has been validated by a database that is largely limited to air-water test fluids. Experimental observations were compared with the predicted flow patterns from the computer codes to determine the robustness of the models when applied to cryogenic fluids whose properties can differ significantly from air and water. Cryogens also readily evaporate and condense whereas phase change is negligible in air-water flows .while phase change is negligible in air-water flows. This work is a small step in the process of validating models for cryogenic two-phase flow because it is limited to a single angle of inclination, heat flux, and tube diameter. In general, the agreement between the cryogenic experimental results and the analytical predictive methods is reasonably good. Identified in the flow pattern maps are small regions where the models are deficient as a result of phase-change phenomena not accounted for in the analytical models. Certain regions of the maps were beyond the range of the experimental data and could not be completely validated.Areas that could benefit from further work include modeling of the transition from separated flow, collection of additional data in the bubble and annular flow regimes, and collection of experimental data at other inclination angles, tube diameters, and higher heat fluxes.