Hisashi Sakurai

Study on Boiling Heat Transfer in Narrow and Flat Space

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.Copyright

Critical Heat Flux by High Velocity Liquid Flow in Narrow Rectangular Channel

Experiments of critical heat flux of extremely thin-fast plate jet film sub-cooled flow were conducted. The extremely thin-fast film-type jet of sub-cooled water was erupted into a stagnant pool. The heat transfer is augmented by the fast jet flow on the heat transfer surface. Vapor generated on the surface is easily taken away from the surface by the fast jet flow and leaves upward from the surface. The static head of water in the pool depress down the fast film-type jet flow on to the heat transfer surface and may collapse the vapor film that is formed between the heat transfer surface and the fast film flow. All these combine to have the possibility to improve the critical heat flux. In the experiments, the liquid sub-cooling was in the range of 30 ∼ 70 K. The thickness of the jet film was 0.2 mm and 0.5 mm. The width of the jet film was 2 mm. The velocity of the erupting jet film was 5.0 ∼ 32 m/s. The heat transfer surface was 2.0 × 2.0 mm heated electrically. The heat transfer surface was placed on the bottom of the pool. The fast-thin film jet was erupted on the bottom of the pool parallel to the heat transfer surface. Bubble behavior generated on the heat transfer surface was recorded by a high speed video camera at 10,000 frames/s. The highest critical heat flux obtained up to now is 3.2 × 107 W/m2 . The analytical model of the critical heat flux for the present flow system will be presented.Copyright

Study on Critical Heat Flux of High Velocity Liquid Flow

Critical heat flux experiments of subcooled, thin, and high-velocity water flow were performed. The test flow channel was rectangular. The width of the flow channel was 2 mm and the height was 0.5 mm or 0.2 mm. The heat transfer surface was 2 mm × 2 mm. At the low heat flux, tinny bubbles were formed at the downstream part of the test heating surface. As the heat flux was increased, the bubble diameter increased and the coalescence of bubbles occurred. Then, the coalesced bubbles grew larger to cover the whole area of the heat transfer surface. Finally, the dried area appeared at the downstream end of the heat transfer surface to cause the critical heat flux condition. The critical heat flux was considerably higher than that of the subcooled flow boiling for the usual-size pipe as well as those of the saturated and the subcooled pool nucleate boiling. As the flow rate was increased, the period between the onset of boiling and the critical heat flux occurrence became narrow. The critical heat flux in the present experiments where the heat transfer surface was located at the just downstream of the flat channel outlet was considerably larger than those in the previous experiments where the heat transfer surface was located at the outlet end of the flat channel or the upstream of the outlet. By producing a fast jet along the surface and providing enough space for generated bubbles to leave from the surface, the critical heat flux was considerably augmented. Critical heat fluxes obtained in the present experiments were in in-between of the correlations for the flowing-upward film flow and for the flowing-downward film flow. The increasing trend for the flow rate was similar to that of the correlations.Copyright

Study on Flow Mechanism in Loop-Type Micro Heat Pipe

A simple design micro-heat pipe was proposed. It was composed of 20.0×20.0 mm square flow circuit which had two adjacent narrow-sides and two adjacent wide-sides. A heating spot was at the narrow side and a cooling spot was at the wide side. Working fluid was ethanol. The flow circuit was placed horizontally. Bubbles generated at the heating spot migrated toward the wide side, the bubbles coalesced there to form a large bubble, and then the large bubble moved to the cooling spot. Finally, the large bubble was condensed at the cooling spot. This cycle repeated continuously. As a result of it, heat transport from the heating spot to the cooling spot was produced in the micro heat pipe. Temperature property and flow state in channel was confirmed. An analysis of a flow mechanism was performed by solving a simple flow equation based on the flow resistance. It was proved that one-way circulation flow could be formed in the flow circuit. Predicted flow velocities were close to measured velocities.

Spot Cooling of Local-High Heat Load by High-Velocity Thin Liquid Flow

Spot cooling of local-high heat load by high-velocity thin liquid flow was examined experimentally. Steady state experiments were conducted using a copper thin-film and rectangular sub-millimeter-channels. The width of the test channel was 2 mm. The heights of the test channel were 0.5 and 0.2 mm. The width and length of a test heater was 2 mm and 2 mm, respectively. The test liquid was degassed pure water. The liquid velocities were 1.5, 5, 10 and 15 m/s. The liquid subcooling was 20 K. Location of the heater in the test channel also was an experimental parameter: the positions of the heater from the exit of the test channel were 30 mm (middle) and 0 mm (exit). Experimental results showed that the maximum heat flux (CHF or cooling limit) during experiment with the heater at exit of the test channel was similar to that with the heater at middle of the test channel: the maximum heat flux was independent of the position of heater in the test channel. The maximum heat flux occurred when bubbles coalesced together or a dry patch appeared on the heater. The coalescence bubble covered over the heater was observed at CHF in condition of low liquid velocity. For condition of high liquid velocity, a dry patch appeared on the heater, and then the dry region extended over the heater to come around the CHF. The maximum heat flux (critical heat flux) was about 8 MW/m2 in a range of present experiments. The CHF for the present sub-millimeter channel was similar to that for conventional channel. Furthermore, models were proposed using heat transfer around a coalesced bubble and at a dry patch on a heater.Copyright

Study on Heat Transfer and Flow Mechanism in Loop-Type Micro Heat Pipe

A simple design micro-heat pipe was proposed. It was composed of 20.0 × 20.0 mm square flow circuit which had two adjacent narrow-sides (1.0×1.0 mm or 0.5×1.0 mm) and two adjacent wide-sides (5.0×1.0 mm, 5.0×0.5 mm, 3.0×1.0 mm or 2.5 × 1.0 mm). A heating spot was at the narrow side and a cooling spot was at the wide side. Working fluid was ethanol. The flow circuit was placed horizontally. It was confirmed experimentally that the circulation flow was formed in the micro flow circuit without any pump and the gravity effect. It was also analytically proved by solving a simple flow equation that the net circulation flow could be formed when the flow circuit; the flow resistance, was asymmetric. The boiling phenomena of the narrow space of the present scale (~ 1 mm) could be expressed with the existing correlation. Further examination is necessary for the narrow space condensation phenomena. It was proved that the micro heat pipe proposed is advantageous in the high heat road condition and in the light weight condition.Copyright

Orientation-Free Micro Heat Pipe

A simple design micro-heat pipe was proposed. It was composed of a 20.0 × 20.0 mm square flow circuit which had two adjacent narrow-sides (1.0 × 1.0 mm2 or 0.5 × 1.0 mm2 ) and two adjacent wide-sides (5.0 × 1.0 mm2 or 2.5 × 1.0 mm2 ). A heating spot was at the narrow side and a cooling spot was at the wide side. Working fluid was ethanol. The flow circuit was placed horizontally. Bubbles generated at the heating spot migrated toward the wide side, the bubbles coalesced there to form a large bubble, and then the large bubble moved to the cooling spot. Finally, the large bubble was condensed at the cooling spot. This cycle repeated continuously. As a result of it, heat transport from the heating spot to the cooling spot was produced in the micro heat pipe even if it was arranged horizontally. It was confirmed that this simple device works as the heat pipe. An analysis of a flow mechanism was performed by solving a simple flow equation based on the flow resistance. It was proved that one-way circulation flow could be formed in the flow circuit. Predicted flow velocities were close to measured velocities. The heat transport performance of the proposed micro heat pipe was much better than the heat conduction of a stainless steel plate.Copyright