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 (ReL) 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.
|Number of pages||14|
|Journal||International Journal of Thermal Sciences|
|Publication status||Published - 2009 Dec 1|
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics