Effect of Bubble flow on Heat Transfer Characteristics in Gas-Liquid Two-Phase Flow in a Horizontal Tube Bundle

Kettle type heat exchanger with horizontal tube bundle have been widely used as an evaporator in geothermal binary power plants. It is known that bubbles movement agitates the liquid phase, resulting in increased heat transfer in the nucleate boiling region of two-phase flows. Hence, clarification of the relation between bubbles movement and heat transfer is important for optimizing the heat exchanger. This study was conducted with the objective of clarifying the bubbles movement and heat transfer around tubes in two-phase flows under adiabatic conditions. In this study, air-water two-phase flow experiments were carried out using a vertical duct test sections with inner dimensions of 90 × 90 mm2. The tube bundle was staggered containing four columns and eight rows. The tube diameter was 18 mm and the pitch was 22.5 mm. The working fluids were air and water, and flow observation and measurements of local heat transfer around a tube were conducted. Fluctuation of heat transfer was caused due to large bubbles movement. It was found that the heat transfer at top of the tube was more influenced by movement of the large bubbles. Furthermore, the instantaneous heat transfer in intermittent flow was higher than that in bubbly flow. Therefore, it is considered that the flow fluctuation caused by large bubbles plays an important role to enhance the heat transfer.