Study on Boiling Heat Transfer in Narrow and Flat Space

Abstract

Boiling heat transfer experiments were performed by using ethanol as test fluid. A heat transfer surface was at the bottom wall of the flow channel. Two types of experiments were conducted; horizontal-rectangular-narrow flow channel experiments and horizontal-narrow-flat space experiments. Experiments were conducted at the pool condition and at 0.1 MPa. In the rectangular-narrow flow channel experiments, the width of the flow channel was varied in the range of 1.0 ∼ 2.0 mm and the height of the flow channel was varied in the range of 0.5 mm ∼ 8 mm. The length of the flow channel was 30 mm. The diameter of the heat transfer surface was the same as the width of the flow channel. Both ends of the flow channel were opened to wide space filled with liquid. In the experiments of the narrow-flat space experiments, the heat transfer surface was at the center of the flat space of 20 mm × 20 mm or 30 mm × 30 mm. The size of the heat transfer surface diameter was 3.0 mm. The space was varied in the range of 0.5 mm ∼ 8 mm. Pool-type experiments of no flat space were also conducted for comparison. The circumference of the narrow-flat space was opened to wide space filled with liquid. In the rectangular-narrow flow channel experiments, a large bubble periodically left from the heat transfer surface and moved to the both ends of the flow channel. As the heat flux was increased, the critical heat flux condition was finally reached. The critical heat flux decreased with a decrease in the flow channel height. When the flow channel height was large, the critical heat flux was close to the pool boiling value. In the experiments of the narrow-flat space experiments, when the space was wide, bubbles generated on the heat transfer surface left freely from the flat space. The heat transfer characteristics were close to those of the pool boiling. As the space became narrow, a large bubble sat on the heat transfer surface, which resulted in the critical heat flux condition. When the space was narrow, the initiation of the bubble generation on the heat transfer surface immediately resulted in the critical heat flux condition.