| Summary: | 碩士 === 國立清華大學 === 核子工程與科學研究所 === 104 === In this study, the two-phase flow and heat transfer in a bubble pump, which is used in a diffusion absorption refrigeration system, with parallel diverging channels is investigated to improve the efficiency. Based on the findings of our previous studies, a diverging channel may enhance the transport of bubble and significantly stabilize the two-phase flow. The experiment was operated at atmosphere pressure and using ethanol solution as the working fluid. To investigate their effects on the bubble pump efficiency, the heating power, ethanol concentration in the solution and inlet flow rate were varied under two kinds of driving modes: constant flow rate using high performance liquid chromatography pump, and constant pressure head with constant height liquid reservoir. In addition, a theoretical model based on the homogeneous two-phase flow for the bubble pump was developed to compare the model prediction with the experiment results.
The results show that when inlet flow rate was 10ml/min and the heating power was gradually increased to 130W, the energy efficiency increases as the heating power increases. The mass efficiency shows the same increasing tendency as of the energy one. Under high power conditions, the constant velocity mode demonstrates a higher mass efficiency than that driven by a constant liquid height. For the gravity-driven mode with constant liquid height, the inlet flow rate increases with increasing the heating power until a maximum value, and then the inlet flow rate decreases as the heating power increases. The efficiency for the bubble pump is better at the reservoir height of 20 cm than that for the reservoir height of 40 cm. At the reservoir height of 20 cm, the efficiency increases with increasing the heating power until the highest value, and then the efficiency decreases as the heating power increases. The efficiency at the reservoir height of 40 cm increases with increasing the heating power monotonically. The efficiency for the ethanol solution with mole fraction of 20% is better than that for 30% under constant inlet velocity. The inlet mass flow rate versus heating power predicted by the theoretical model is similar to the experiment observation under constant pressure head. The theoretical analysis also shows that the mass efficiency at the reservoir height of 20cm is superior to 40cm.
Based on the experiment results of this study, the optimal condition for the present bubble pump with diverging channels is using the 20% ethanol solution at the reservoir height of 20 cm and heated at 120W. The flow pattern under such a condition is churn flow or annular flow.
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