ISSN Online: 2377-424X
ISBN Print: 978-1-56700-421-2
International Heat Transfer Conference 15
Visualization of Ammonia Boiling Flow Phenomena Inside Narrow Flat Plates
Abstract
Ocean thermal energy conversion (OTEC) systems generate electricity using thermal energy from the ocean.
The OTEC uses a plate heat exchanger as evaporator and condenser. Ammonia or ammonia/water mixture is
used as the working fluid because the temperature difference between the surface and deep seawater is very
small, necessitating a working fluid with low boiling point. To improve the power efficiency of OTEC,
improving the heat transfer performance of the plate evaporator is essential. However, the boiling phenomena
of ammonia on a plate evaporator have scarcely been clarified. The author previously investigated the
measurement of the heat transfer coefficient of boiling ammonia on a flat plate evaporator; however, the
characteristics of boiling ammonia flowing inside the evaporator have not yet been clarified. In the present
study, time-variant flow patterns of ammonia were observed using a high-speed video camera at three locations
above a flat plate evaporator. The visualization was obtained at a range of mass fluxes (7.5-15 kg/m2s), heat
fluxes (15-25 kW/m2), and pressures (0.7-0.9 MPa). A void fraction was calculated from the video images
using our digital image processing software. The results show that the void fraction exhibits time variation,
and the relationship between vapor quality and time-averaged local void fraction was clarified. A proposed
correlation based on a drift flux model can predict the experimental data of local void fraction with an error
less than ±20%.