Dewen Yuan

Bubble Sliding Process on Subcooling Flow Boiling in Vertical Rectangular Narrow Channel

In present work, a visualized investigation was conducted to understand the phenomenon of bubble sliding on vertical heating wall with upward subcooled flow boiling in narrow rectangle channel with gaps of 2.5 mm. Experimental results show that the characteristics of bubble slipping are significantly influenced by mainstream velocity and temperature. With increasing mainstream velocity, bubble sliding distance and speed are all increased. In present working conditions, the bubbles sliding distance is of 3–10 times of the bubble departure diameter. Based on the energy balance on bubble sliding process, the impact of bubble sliding on the characteristics of heat transfer was analyzed as well. Considering the different mechanisms effect on heat transfer, such as micro-layer evaporation, transient conduction and single-phase forced convection between fluid and wall, the ratio of every mechanism was calculated and compared. Analysis results show that the bubble with sliding movement have significant heat transfer enhancement than that of the stationary bubbles. With the proposed concept of disturbance intensity of sliding bubble, the heat flux expressions caused by disturbance of sliding bubble or stationary have been proposed.Copyright

Experimental Investigation of Pressure Drop of Flow Boiling at Vertical Narrow Rectangular Channel

Narrow channel heat transfer element has been extensive adopted in engineering applications, especially at electronics technology, this kind of elements often be used to construct compact heat exchanger. Pressure drop of flow boiling at vertical channel with gaps of 1.7, 2.2 and 3.6 mm was experimentally investigated in this paper. The variation of the two-phase frictional multiplier vs. heat flux at various operating conditions was gotten experimentally, possible mechanism of the two-phase frictional multiplier trends of narrow channel were analyzed. Experimental results revealed that the two-phase frictional multiplier increased at lower flow rate and heat flux, as well as higher vapor quality, and dropped at wider flow gap. The multiplier can not be estimated by commonly used method for ordinary gap, thus a modified model of pressure drop for narrow channel was proposed considering the size effects of channel. The error of the predicted two-phase frictional multiplier is within ±15.4% compared with experimental results.Copyright

Experimental Investigation of Saturated Flow Boiling Heat Transfer of Vertical Narrow Rectangular Channel

Narrow rectangular channel has been widely used at evaporator of cryogenic engineering and electronics cooling. At present paper, saturated flow boiling heat transfer at vertical narrow rectangular channel with gaps of 1.0, 1.5 and 2.5 was experimentally investigated, the heat transfer characteristics at various operating conditions were tested and discussed, and the correlation of saturated boiling heat transfer coefficient was regressed used Least Square Method according to experimental results. It shows that heat transfer coefficient decreases at wider gaps and higher inlet pressure, and it increases at higher flow rate, heat flux and increased vapor quality. Compared with traditional correlation for ordinary channel, the discrepancy is significant. Considering the size effects, a new correlation was proposed to estimate the heat transfer coefficient. The error of the regressed correlation is within ±12%.Copyright

Departure and Lift-Off Point of Bubbles at Heating Wall in Vertical Channel

This paper proposed a theoretical model involving in conditions of bubble departure point and lift-off point, the principle and estimating method of individual forces have been proposed as well. The forces acted on growing bubble are, namely, surface tension force Fs , growth force of bubbles Fdu , shear lift force FsL , drag force of flow Fqs , hydrodynamic pressure force Fh , contact pressure force Fcp , buoyancy force Fb and bubble acceleration inertia force d(mvc )/dt as well. Horizontal and vertical momentum equations are presented according to forces balance of bubble, meanwhile, the estimating method of lean angle of bubble before departure also has been discussed.Copyright