Because of the significance of engineering applications and arisen problems in the small inclination angle evaporator or horizontal evaporator of Separate Type Heat Pipe (STHP), an experimental investigation on the working fluid flow characteristics and the flow instability in the evaporator was presented. Flow patterns were observed and analyzed by visualization experiment. It was found that bubble flow, slug flow, wave foam flow occurred orderly in the evaporator with a small inclination angle when the heat flux increased, but the annular flow was not found, which appeared in a vertical evaporator. Two types of flow instabilities occurred, they were the flow pattern transformation instability and the density wave instability. The effects of pressure, mass velocity, inlet subcooling, heat flux, exit throttle on the flow instability were determined in the modelling experiment. The thresholds of the flow pattern transformation instability and that of the density wave instability were obtained. Nondimensional correlations for predicting the flow instability in STHP were given by means of one-dimension single-fluid model. The results may be used for the engineering design of large scale heat exchangers of STHP with a small inclination angle in evaporator.
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