Transition from non-uniform to smooth 4He flow in micro-orifices: The impact of thickness on pressure drop and entropy production

Micro-orifices are commonly used as throttling elements in 4He Joule–Thomson refrigerators, but the influence of micro-orifice thickness on 4He flow behavior remains poorly understood. This study investigated the pressure drop and entropy production characteristics of choked 4He flow in micro-orifices with thicknesses ranging from 50 to 500 μm, using computational fluid dynamics (CFD). The results reveal two distinct flow regimes based on the orifice thicknesses. For orifices of 50 and 150 μm, the flow undergoes rapid depressurization over short distances, driven by shock and expansion waves, leading to complex flow patterns at the micro-orifice outlet. In contrast, orifices with greater thicknesses allow the flow to stabilize over longer distances, establishing a smoothly decreasing axial pressure profile. The length of this smooth, pressure-declining region increases with orifice thickness, while the non-uniform flow segment decreases. For a 500 μm thick orifice, the non-uniform segment is reduced to less than 20%, although it still contributes significantly to entropy production (47%). This study highlights the critical role of micro-orifice thickness in governing 4He flow behavior and entropy production, offering essential insights for the design and optimization of cryogenic micro-orifices.