Study on Performance Improvement of Evaporator used in Binary Cycle for Geothermal Sources
-Void Fraction and Heat Transfer Characteristics around Tubes in Two-phase Flows across Horizontal Tube Bundles-
Purposes
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In geothermal binary power plant, kettle reboilers are used as an evaporator. Knowledge of the two-phase flow characteristics across horizontal tube bundles is important.
Hence, clarification of the relation between bubbles motion and heat transfer is important for optimizing the heat exchanger.
However, the two-phase flow characteristics are affected by the geometries such as the tube alignment and the pitch-to-diameter ratio in the horizontal tube bundles.
This study has been conducted with the objective of clarifying the local characteristics of void-fraction distribution and heat transfer around tubes in two-phase flows under adiabatic conditions with different geometries of the tube bundles.
In particular, focus are placed on the flow regime of bubbly and intermittent flows that mainly appear in the upstream region of the heat exchangers.
In this study, two-dimensional void-fraction distributions are measured using X-ray radiography, and the heat-transfer coefficients around each tube obtained using a platinum wire placed on a measurement tube.
Subsequently, the results obtained for in-line and staggered tube bundles at different pitch-to-diameter ratios are compared.
Research content
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Fig. 1 represents the results of two-dimensional void-fraction distributions are shown.
In bubbly flow, many bubbles traveled along the vertical tube gaps, and the void fraction became higher than that in the other region in bubbly flow. Some bubbles passed through the bottom of the tubes.
As a result, the void fraction around the lower section of each tube became higher than that around the upper section of the tubes.
In the intermittent flow regime, large bubbles intermittently passed through the tubes. As a result, the void fraction around the maximum gap and the vertical minimum gap increased.
The maximum void fraction around the maximum gap and the horizontal minimum gap were almost the same. In contrast, the void fraction around the vertical minimum gap was slightly lower than that around the maximum and horizontal minimum gaps.
Large bubbles intermittently passed between the tube gaps, and occupied the maximum and vertical minimum gaps. Consequently, a slightly lower void fraction was observed at the horizontal minimum gap.
Fig. 2 depicts the heat-transfer coefficient around a The vertical axis indicates the local heat-transfer coefficients normalized by the average heat-transfer coefficient over the tube in single-phase flow for the in-line tube bundle.
The value 0° represents the bottom of the tube, and the counterclockwise rotation is defined as positive degree.
In bubbly flow, heat-transfer coefficient between ± 90 and 180° is higher than that in single-phase flow. With increased gas flow rate, the local heat transfer increases over the tube.
Under the intermittent flow condition, distribution of the heat-transfer coefficient is confirmed to flatten over the tube, in comparison to that in single-phase flow.
Because the void fraction around the tube is almost uniformly distributed, enhancement of the heat transfer occurred by bubbles motion all over the tube under the condition.
Publications
H. Murakawa, Y. Miyoshi, K. Araki, K. Sugimoto, H. Asano, S. Makimoto
Effect of bubble motion on local heat transfer around a tube across horizontal in-line and staggered tube bundles in bubbly and intermittent flows, Mechanical Engineering Journal 9(4), 22-00069 (2022). https://doi.org/10.1299/mej.22-00069
H. Murakawa, M. Baba, T. Miyazaki, K. Sugimoto, H. Asano, D. Ito
Local void fraction and heat transfer characteristics around tubes in two-phase flows across horizontal in-line and staggered tube bundles,
Nuclear Engineering and Design 344 (2018) pp.66-74.
https://doi.org/10.1016/j.nucengdes.2018.05.005
T. Miyazaki, M. Baba, H. Murakawa, H. Asano, K. Sugimoto, D. Ito
Two-phase flow behavior and heat transfer characteristics in kettle reboiler,
Proc. of the ASME 2017 Power Conference Joint With ICOPE-17, #POWER-ICOPE2017-3293
(2017). https://doi.org/10.1115/POWER-ICOPE2017-3293