VISUALIZATION OF POOL BOILING ON ENHANCED SURFACES

This work reports experiments to visualize nucleate boiling on an enha nced tubular surface having sub-surface tunnels and surface pores. The finned copper tube had 1575 fins/m (40 fins/in.) and 0.8 mm fin heigh t. The fins are covered by a thin foil sheet having 0.23 min pores at 1.5 mm pore pitch along each interfin region. Data are provided for tw o foil cover sheets, one copper and the other a transparent plastic. T he uniqueness of this work is that the visualization method allowed di rect observation of the boiling process in the subsurface tunnels. Use of a high speed camera with 30 x magnification allowed detailed obser vation of the evaporation process ill the tunnels and of the vapor bub bles emerging from the pores. The experiments were conducted for satur ated and subcooled boiling in the horizontal and vertical orientations . For the vertical tube, the saturated boiling experiments showed that all of the tunnels were vapor filled except For liquid menisci in the corners. This was also true for the horizontal tube at high heat flux . For the horizontal tube at low heal flux, portions of the tunnel len gth was liquid filled. A large portion (70-90%) of the region was vapo r filled except for liquid menisci in the corners, and the remaining p art of the region had oscillating menisci. These experiments provide c onclusive proof that the heat transfer mechanism in the subsurface tun nels is evaporation on the menisci in the corners. (C) 1998 Elsevier S cience Inc. All rights reserved.