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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

Visualization of Ammonia Boiling Flow Phenomena Inside Narrow Flat Plates

Hirofumi Arima
Institute of Ocean Energy, Saga University, 1-48, Hirao, Kubara-aza, Yamashiro-machi, Imari-shi, Saga 849-4256, Japan

Fumiya Mishima
Graduate school, Division of Science and Engineering, Saga University, 1-Honjo, Saga-shi, 840-8502, Japan

Kohei Koyama
Institute of Ocean Energy, Saga University, 1-48, Hirao, Kubara-aza, Yamashiro-machi, Imari-shi, Saga 849-4256, Japan

Y. Ikegami
Institute of Ocean Energy, Saga University, Saga, Japan

DOI: 10.1615/IHTC15.tpf.008971
pages 8421-8429


KEY WORDS: Two-phase/Multiphase flow, Renewable energy, Visualization, Ammonia, Boiling heat transfer, Void fraction

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%.

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