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International Heat Transfer Conference 16

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

EFFECT OF SURFACE STRUCTURE ON VOID FRACTION OF SUBCOOLED FORCED BOILING IN RECTANGULAR NARROW CHANNEL

Hitoshi Asano
Kobe University, Department of Mechanical Engineering 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

Tomohiko Nakamura
Department of Mechanical Engineering, Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe 657-8501, Japan

DOI: 10.1615/IHTC16.bae.023156
pages 1349-1356


KEY WORDS: Boiling and evaporation, Two-phase/Multiphase flow, Heat transfer enhancement, Void fraction, CHF, Subcooled forced boiling

Abstract

This study deals with subcooled boiling heat transfer characteristics in a horizontal narrow channel with the width of 20 mm and the height of 4 mm. The effect of heating surface structure, smooth and thermal spray coating, was evaluated. In our previous report in the last IHTC, it was shown that the critical heat flux due to boiling transition was higher for the thermal spray coating, although the surface produced higher boiling heat transfer coefficient. Higher boiling heat transfer coefficient should be caused by increasing vapor generation rate on the heat transfer surface. Larger vapor generation rate might lead to higher void fraction and easy transition to film boiling. In order to clarify the void fraction characteristics in subcooled boiling flow with large subcooling degree, two types of capacitance sensor were developed and applied for the measurement of the cross-sectional average void fraction and local void fraction near the heating surface, respectively. As the results, the local void fraction near the heating surface was slightly higher than the cross-sectional average void fraction. The effect of the surface structure on void fraction was a little. Void fraction at the boiling transition was higher for the thermal spray coating due to the higher critical het flux, and became lower with decreasing the inlet subcooling degree for each surface. Moreover, it was confirmed from the local void fraction fluctuations that boiling heat transfer enhancement by the thermal spray coating was caused by surface boiling with tiny bubbles under the low heat flux condition.

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