Study on Performance Improvement of Evaporator used in Binary Cycle for Geothermal Sources
-Boiling Heat Transfer in Horizontal Tube Bundle-
Purpose
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A kettle reboiler is widely used as an evaporator in geothermal binary power generation system.
In the kettle reboiler, horizontal tube bundle is installed in liquid pool of the working fluid, and boiling is occurred by heating with hot water flowing in the tubes. Boiling heat transfer would be enhanced in the upper part of the tube bundle by the agitation effect of rising bubbles from the lower part of the tube bundle.
When the vapor bubbles becomes smaller and the distribution becomes homogenous, the boiling heat transfer enhancement effect by the bubble agitation would become stronger.
In this study, boiling experiments are conducted for a horizontal tube bundle to evaluate the heat transfer characteristics, especially the bubble agitation effect.
In addition, boiling heat transfer enhanced tubes are partially inserted at the lower part of the tube bundle to form a homogenous bubbly flows with small bubbles, and the improvement effect in boiling heat transfer is evaluated.
Research content
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Boiling experiments are conducted in horizontal tube bundle with 3 columns and 8 rows using HFC-245fa as the working fluid. The heat transfer coefficients of the center tube at each row are measured, and the boiling behaviors in the tube bundle are observed. An example of measured heat transfer coefficients for smooth tube bundle with staggered arrangement are plotted in the figure. The broken line shows the Cooper’s correlation equation for pool boiling heat transfer around single horizontal tube. It can be seen that the heat transfer of the upper tubes of the tube bundle were enhanced by the bubble agitation effect. The effect became larger to the upper tube and with decreasing heat flux. On the other hand, when the 1st row tubes were changed to thermal spray coated tubes, the bubble agitation effect was obviously increased for the 2nd and 3rd row tubes just at the downstream of the thermal spray coated tubes.
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