Abstract
• Flow patterns in helically coiled tubes (HCTs) are classified by visual observation. • Flow-regime map is proposed and mechanisms of flow transitions are presented. • Void fraction for the entire tube cross-section is measured by WMS technique. • New correlations of distribution parameter and drift velocity are proposed. Helically coiled tubes (HCTs) are widely used in industries owing to their compact structure and excellent heat and mass transfer efficiency. Due to the complexity of three-dimensional flow in HCTs, profound knowledge on the flow pattern characteristics is not well documented in existing literature. In this paper, we employ the wire-mesh sensor (WMS) and image tomography technique to measure a series of essential parameters (e.g. flow structures, phase fraction) to explore the flow pattern transition mechanism. According to our observation, bubbly, plug, slug, wavy and annular flow are recognized, and the mechanisms of flow pattern transitions are carefully discussed based on the analysis of the forces. In addition, the variation of void fraction in both temporal and spatial dimensions is discussed in detail. By introducing the drift flux analysis, new correlations of distribution parameter and drift velocity are proposed taking both HCTs structure and flow condition into account.
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