Numerical study of a novel needle valve as the double inlet for 4 K pulse tube refrigerators

When the temperature falls below 4.2 K, entering the helium temperature regime, macroscopic quantum effects such as superconductivity, superfluidity, and the Josephson effect become observable. The two-stage low-frequency Gifford McMahon (GM) type pulse tube cryocooler is a key technical solution for achieving this temperature range. However, it faces critical challenges in phase shifting, specifically in the precise tuning of the phase relationship between pressure oscillations and controlling the direct current (DC) flow. Conventional needle valves—which rely on a single conical needle tip for flow regulation—are typically used as first-stage flow controllers. However, their limited modulation capability leads to suboptimal cooling performance. To address this, we propose an innovative long-stem needle valve design featuring dual adjustable segments: a truncated cone and a cylindrical section. This design significantly improves flow control precision. We evaluated the design's effectiveness through computational fluid dynamics (CFD) simulations, analyzing internal flow behavior under varying cone heights and taper angles. For each configuration, we calculated forward and reverse flow coefficients alongside the proportion of DC flow. The results demonstrate a clear correlation between the front taper's geometric parameters and DC flow magnitude. Notably, a front taper height of 0.5 mm combined with a 15° taper angle proved highly effective in suppressing DC flow. By combining velocity contour plots and streamline diagrams, we qualitatively analyzed flow patterns, enabling systematic elucidation of DC flow generation mechanisms. This paper also reviews existing research on DC flow in cryogenic systems.