Study on Performance Improvement of Evaporator used in Binary Cycle for Geothermal Sources
-Pool Boiling Heat Transfer Enhancement by Thermal Spray Coating-
Purpose
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In geothermal binary power generation, a kettle-type reboilers are used for the evaporator.
In order to improve the power generation efficiency, it is necessary to reduce the temperature difference between the heat source temperature and the evaporation temperature, and to increase the evaporation pressure.
If there is a delay in the onset of boiling, vapor bubbles grow large, then liquid entrainment into vapor flow leading to erosion in the steam turbine. Therefore, prompt boiling, homogeneous flow field, and promotion of boiling heat transfer are required.
The purpose of this study is to reduce the wall superheat at the onset of boiling by thermal spraying on the heat transfer surface, to enhance boiling heat transfer by increasing the nucleation site density, and to form a homogeneous boiling flow field with small bubbles. On the other hand, the boiling heat transfer coefficient is evaluated for HFO refrigerant with low GWP performance with HFO refrigerant to replace the HFC refrigerant with high GWP.
Research content
The target is pool boiling around a horizontal circular tube. Several types of thermal spray coatings are applied on the superhard stainless steel tube used in a real binary cycle.
Pool boiling experiments of HFC-245fa and HFO-1233zd (E) are conducted to evaluate the effect of refrigerant physical properties on boiling heat transfer performance, especially a hysteresis effect due to the delay in the onset of boiling.
The right figure shows the boiling behavior around a thermal spray coated tube, and it can be seen that many small bubbles are uniformly formed from the heat transfer surface.
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