Synopsis Cryogenic slush fluids such as slush hydrogen and slush nitrogen are solid-liquid, two-phase fluids containing solid particles in a liquid. As a functional thermal fluid, there are high expectations for use of slush fluids in various applications such as fuels for spacecraft engines, clean-energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. Experimental flow tests were performed using slush nitrogen to elucidate pressuredrop characteristics of converging-diverging (C-D) pipes and corrugated pipes. The results of pressure-drop tests for two different types of C-D pipes, long-throated and short-throated, both with an inner diameter of 15 mm, confirmed a pressure-drop reduction for the short-throated pipe at a flow velocity of 1.5 m/s or greater, and a maximum pressure-drop reduction of 40-50% compared to liquid nitrogen. In the case of two different types of corrugated pipes with an inner diameter of either 12 mm or 15 mm, a pressure-drop reduction was confirmed at a flow velocity of 2 m/s or greater, and reached a maximum value of 37% at 30 wt.% compared to liquid nitrogen. The greater the solid fractions, the smaller the pipe friction factor became, and the pipe friction factor at the same solid fraction showed a constant value regardless of the Reynolds number. From the observation of the solid particles' behavior using a high-speed video camera and the PIV method, the pressure-drop reduction mechanisms for both C-D and corrugated pipes were demonstrated.
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