NUMERICALLY DETERMINING ACOUSTIC MINOR LOSSES IN CRYOGENIC HYDROGEN USING REAL FLUID PROPERTIES

Kian Conroy

Acoustic oscillations in cryogenic systems fall into one of two categories: naturally occurring, such as self-excited Taconis oscillations, or intentionally induced, as is the case of thermoacoustic cryocoolers. An accurate analysis of these systems requires a confident way to model minor losses at junctions between segments. Present modeling of minor losses in acoustic systems tends to be based on correlations established for steady flow, which may not fully apply when used in oscillatory flows. Additionally, fluids under cryogenic conditions may have significantly different properties than those near ambient conditions, particularly near the saturation curve, where some cryogenic hydrogen systems operate. High-fidelity numerical simulations using computational fluid dynamics provide a detailed and relatively rapid means to obtain parametric results. System properties of interest include pressure and temperature of the fluid, acoustic wave phase, and geometric parameters. This research aims to produce useful correlations for acoustic minor losses in systems with cryogenic fluids.The specific setups considered are an abrupt expansion at a junction between two pipes, which can be more broadly applied to other situations, such as expansion out of a stack, and minor losses at a pipe exit, which are important to modeling the behavior of effects like Taconis oscillations. Correlations are established for both of these systems with cryogenic hydrogen, demonstrating strong inverse dependence on Reynolds number and a lesser dependence on other properties, like temperature. Frequency is observed to have little effect on minor losses in the explored range. Steady flow correlations available for common fluids are found to be inaccurate even in the asymptotic region, and variously overpredict or underpredict, depending on geometry. Two further applications of this line of research are examined. Geometric modifications to a pulse tube section in a thermoacoustic cryocooler are examined for their effects on losses, and how they depend on flow velocity amplitude. One particular modification to the tube wall is found to significantly reduce losses, decreasing them by more than half. Additionally, phase change and multiphase flow effects are explored in the open pipe configuration modified from a prior study. Phase change in a saturated liquid is found to appreciably reduce minor losses, while phase change in a saturated mixture with 50% vapor has a negligible effect.

NUMERICALLY DETERMINING ACOUSTIC MINOR LOSSES IN CRYOGENIC HYDROGEN USING REAL FLUID PROPERTIES

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