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 m 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
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