Effects of helical ridges on internal flow over a large range of flow conditions

Internally enhanced tubes with helical ridges have been widely used in industry applications to enhance heat transfer. Research work to understand the thermal and fluid characteristics of single-phase flow in tubes with helical ridges dates back to several decades ago. However, there is not a reliable tool to estimate the heat transfer enhancement and associated penalty of friction loss. Industry typically relies on expensive and time-consuming experimental work to obtain the tube performance. In this research, experiments were conducted to study the effects of integral helical ridges on the thermal and flow characteristics of internal flow through a cylindrical tube. The flow conditions cover a large range of Reynolds number from laminar to turbulent regions and Prandtl number 3.9 to 109. The test section is a 2.5-meter-long copper tube with an internal diameter of 16.54 mm wherein the inside surface has 48 integral helical ridges, each at 45° helix angle. The test fluid is a solution of water propylene glycol (PG) with PG at four different concentrations: 0%, 10%, 30%, and 50%. The measurements show that thermal and flow characteristics are significantly different from those reported in the literature for flow in commercial pipes with roughness. A comparison against the Moody chart and Churchill correlation, indicates that the transition region covers a wider range of Reynolds number and extends into a much larger Reynolds number. It is the author s belief that the departure of thermal and flow characteristics is attributed to a physical mechanism that is absent in the commercial roughness geometries. The helical ridges provide a channeling effect to direct flow down the stream, which suppresses the bulk fluid transition to fully-developed turbulent region. The heat transfer enhancements and the penalty of pressure drop measured in this work are further quantified and compared with correlations available in the literature.