Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct |
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| Affiliation: | 1. The Institute for Space and Nuclear Power Studies, University of New Mexico, Albuquerque, NM, USA;2. Nuclear Engineering, University of New Mexico, Albuquerque, NM, USA;3. Mechanical Engineering, University of New Mexico, Albuquerque, NM, USA;4. Chemical and Biological Engineering, University of New Mexico, Albuquerque, NM, USA;1. Department of Chemistry, Aristotle University of Thessaloniki, University Box 116, 54124 Thessaloniki, Greece;2. Mechanical Engineering Department, Technical University of Serres, 62124 Serres, Greece;1. Boiling and Two-Phase Flow Laboratory (BTPFL), School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, USA;2. NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, USA;3. Vantage Partners, 3000 Aerospace Parkway, Brook Park, OH 44142, USA |
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| Abstract: | Experiments are conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated. |
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