Yonghua Huang

3D visualization of two-phase flow in the micro-tube by a simple but effective method

The present study provides a simple but effective method for 3D visualization of the two-phase flow in the micro-tube. An isosceles right-angle prism combined with a mirror located 45° bevel to the prism is employed to synchronously obtain the front and side views of the flow patterns with a single camera, where the locations of the prism and the micro-tube for clear imaging should satisfy a fixed relationship which is specified in the present study. The optical design is proven successfully by the tough visualization work at the cryogenic temperature range. The image deformation due to the refraction and geometrical configuration of the test section is quantitatively investigated. It is calculated that the image is enlarged by about 20% in inner diameter compared to the real object, which is validated by the experimental results. Meanwhile, the image deformation by adding a rectangular optical correction box outside the circular tube is comparatively investigated. It is calculated that the image is reduced by about 20% in inner diameter with a rectangular optical correction box compared to the real object. The 3D re-construction process based on the two views is conducted through three steps, which shows that the 3D visualization method can easily be applied for two-phase flow research in micro-scale channels and improves the measurement accuracy of some important parameters of the two-phase flow such as void fraction, spatial distribution of bubbles, etc.

Thermal conductivity of helium-3 between 3 mK and 300 K

Measurements on thermal conductivity of fluid helium-3 in the literature are abundant in the liquid phase, while almost no data are available in the gas phase except some at pressures not higher than 0.1 MPa. A quantum version of the principle of corresponding states is used to predict the thermal conductivity of gaseous helium-3 by using those of helium-4, hydrogen, neon and argon. An empirical equation is developed for the thermal conductivity of both the liquid and gas phases at temperatures from 3 mK to 300 K and pressures up to 20 MPa. Similar to other cryogenic fluids like nitrogen, a sharp peak of the thermal conductivity curve is observed at low pressures.

Ice formation modes during flow freezing in a small cylindrical channel

Abstract Freezing of water flowing through a small channel can be used as a nonintrusive flow control mechanism for microfluidic devices. However, such ice valves have longer response times compared to conventional microvalves. To control and reduce the response time, it is crucial to understand the factors that affect the flow freezing process inside the channel. This study investigates freezing in pressure-driven water flow through a glass channel of 500 μm inner diameter using measurements of external channel wall temperature and flow rate synchronized with high-speed visualization. The effect of flow rate on the freezing process is investigated in terms of the external wall temperature, the growth duration of different ice modes, and the channel closing time. Freezing initiates as a thin layer of ice dendrites that grows along the inner wall and partially blocks the channel, followed by the formation and inward growth of a solid annular ice layer that leads to complete flow blockage and ultimate channel closure. A simplified analytical model is developed to determine the factors that govern the annular ice growth, and hence the channel closing time. For a given channel, the model predicts that the annular ice growth is driven purely by conduction due to the temperature difference between the outer channel wall and the equilibrium ice-water interface. The flow rate affects the initial temperature difference, and thereby has an indirect effect on the annular ice growth. Higher flow rates require a lower wall temperature to initiate ice nucleation and result in faster annular ice growth (and shorter closing times) than at lower flow rates. This study provides new insights into the freezing process in small channels and identifies the key factors governing the channel closing time at these small length scales commonly encountered in microfluidic ice valve applications.

Flow and Heat Transfer Characteristics of Supercritical Helium for Magnet Cooling

Supercritical Helium (SHe) is frequently used for the large-scale magnet cooling in many applications. In the present study, the flow and heat transfer characteristics of SHe in the mini-tubes of about 1.0-2.0 mm in diameters are numerically investigated to provide the useful information for the design of the cooling configuration of Cable-in-Conduits Conductors (CICCs) and the related. And a simplified equivalent model is also proposed to estimate the flow and heat transfer characteristics of SHe in the complicated configuration of CICCs.