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Based on the standard k-epsilon model, a gas flow calculation model in a curved capillary is established, and the flow process of helium working medium in a curved capillary with gradual curvature changes is numerically simulated. Compared with other methods for studying micro-scale gas flow, this simulation obtains the gas density distribution in the curved capillary more conveniently, and has the same variation trend as the experimental measurement of the plasma electron density distribution, and can predict the gas flow distribution in the tube more accurately. The situation provides a theoretical basis for the structure design of the discharge capillary experiment. Based on this model, the gas flow process in the capillary of the one-sided direct flushing, double-sided hedging and "straight + curved" cascade acceleration structures are numerically simulated. The results and conclusions are summarized as follows.(1) Compared with the single-sided straight-bent capillary structure, the gas density fluctuation between the left and right gas inlets of the double-sided hedging-bend capillary is smaller, the gas flow is more stable, and a relatively stable plasma density channel can be generated.(2) In the double-sided hedged curved capillary, a relatively uniform gas density distribution is formed between the two inlets of the capillary under the same inflation back pressure; further research results show that different lengths can be obtained by controlling the position of the gas inlet A more uniform plasma density distribution.(3) In the "straight + curved" cascaded accelerating capillary structure, the diameter of the electron injection channel will affect the gas density distribution in the bend. When the diameter of the electron injection channel is small, the absolute pressure in the capillary is low. The larger pressure difference between them will lead to a higher gas flow rate in the elbow, which will increase the fluctuation of the gas density in the elbow; the final research shows that the diameter of the electron injection channel of 100 µm and 150 µm is more suitable for the application in the "direct +bend" cascade acceleration capillary structure design.In summary, the calculation model of gas flow in the curved capillary constructed in this paper can accurately predict the gas flow distribution in the tube. The double-sided hedged curved capillary can generate a relatively stable plasma density channel, and the electron injection channel diameter of 100 µm and 150 µm is more suitable for application in the "straight + curved" cascade accelerating capillary structure design. The research results obtained are expected to provide theoretical guidance and technical support for the laser wake cascade acceleration experiment based on the curved capillary with gradually changing curvature.

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Microchannel heat sinks have attracted much attention due to its superiority of removing high heat flux with a very small size, which recently have been applied in the electronic cooling. The flow boiling of deionized water in the ultrahigh-aspect-ratio copper microchannels was experimentally investigated in this work. The heat sink consists of 20 rectangular straight microchannels (5000μm × 200μm) with the hydraulic diameter of 385 μm and the aspect ratio of 25. The experiment was conducted with the heat flux range of 39.7–1368 kW/m2 and the mass flow range of 21.3–41.6 kg/(m2·s) (the inlet temperature of 30°C). The flow patterns in the ultrahigh-aspect-ratio microchannels were captured using the visualization technique and four flow regimes (bubbly flow, slug flow, churn flow and annual flow) were clearly observed. The nucleate boiling sites increase significantly in the depth direction on the large-area side wall at bubbly flow. An obvious delay for flow patterns transformation in the upper regions compared with the lower region of microchannel was found. The bubble sliding was observed with a large sliding distance compared to the conventional low-aspect-ratio microchannels, which leads to an enhanced the heat transfer. It shows that the higher effective heat flux and the lower pressure drop were achieved in ultrahighaspect-ratio microchannel due to its special flow patterns in the channel and large surface area in the limited volume.