Heat transfer and instability characteristics of a loop thermosyphon with wide range of filling ratios

Abstract

A series of experiment using flow visualization technique and simultaneous measurement of pressure and temperature signals, are conducted to thoroughly investigate the heat transfer and instability characteristics in a loop thermosyphon with filling ratios ranging widely from 38% to 87% over a broad range of input heat flux from 35 to 395 W cm−2. The filling ratios are classified into three groups based on their distinctive heat transfer and instability behavior. The effects of filling ratio, input heat flux and inclination angle on the temperature nonuniformity, thermal resistance and instability features are examined in detail. The results show that, under the high filling ratios, the heat transfer capacity is seriously limited by the phase-change suppression and the consequent pressure soaring, while is advantageous in avoiding the evaporator dryout. Under the moderate filling ratios, the heat transfer capacity could be significantly higher, but the geyser boiling instability is obvious for a relatively large range of input heat flux, inducing the fluctuation of pressure and temperature. Under the low filling ratios, the heat transfer capacity is the highest; however, the transient local “Near-dryout” is easier to occur, causing large temperature overshoot and oscillation. The different variation trends of thermal resistance and temperature nonuniformity with filling ratios and input heat flux can generally be explained by the variation of the portions of latent and sensible heat transfer. Under the moderate and high filling ratios, the loop thermal resistance decreases somewhat with the decrease of inclination angle due to the elongation of the phase-change surfaces. A thermodynamic state diagram is drawn, indicating the measured average pressure - temperature of the working fluid at the evaporator for the three groups of filling ratios, with distinctly different instability issues, along with the saturation line to aid in the mechanism analysis and trouble diagnosis. A stability map in terms of the phase change number, Npch, and the filling ratio is presented, in which the boundaries dividing the different types of instability and the stable conditions are marked as a guidance for practical design and operation.