Influence of spiky twisted tape insert on thermal fluid performances of tubular air-water bubbly flow

An experimental study that comparatively examined the two-phase flow structures, pressure drops and wall-to-fluid heat transfer properties between the plain tube and the enhanced tube with the spiky twisted-tape insert (swirl tube), was performed to disclose their differential thermal-fluid performances with air-water flows. On-line and post-processed high-speed digital images of air-water two-phase phenomena in plain and swirl tubes were detected to ensure the bubbly flow pattern in plain tube and to visualize their characteristic interfacial structures. Superficial liquid Reynolds number (Re_L) and air-to-water mass flow ratio (AW), which were respectively controlled in the ranges of 5000-15,000 and 0.0004-0.01, were selected as the controlling parameters of heat transfer performances. The dispersed rising air bubbles in the plain tube and the centrifugal-force induced coherent spiral stream of coalesced bubbles in the swirl-tube core considerably modify the pressure-drop and heat-transfer performances from the single-phase conditions. Selected results of pressure-drop and heat-transfer measurements, flow images and tube-averaged void fractions detected from the plain and swirl tubes with air-water two-phase flows were cross-referenced to illustrate the mechanisms responsible for the modified thermal-fluid performance due to the spiky twisted-tape insert. Empirical heat transfer correlations which evaluate the Nusselt numbers over the developed flow regions of the plain and swirl tubes with air-water two-phase flows were generated for industrial applications.