Instability characteristics and heat transfer of a separator assisted dual-evaporators loop thermosyphon under heating power maldistribution condition

Abstract

Thermosyphon loop with multiple evaporators has been widely used in the data centers thanks to its compact structure and local hot spot avoiding. However, under the heating power maldistribution condition, on account of the separation and convergence of flow steams from different evaporators, the instability of the system can be exacerbated which leads to premature failure and damage of servers. Thus, a separator assisted loop thermosyphon with dual evaporators had been proposed to improve its heat transfer performances and stability. An experimental study of the instability characteristics and heat transfer of the system are carried out under different filling ratios, cooling water temperatures and heating powers in this paper. To fully understand the interaction mechanism of two evaporators behind the oscillation, the visualization of flow patterns in the liquid tube and two-phase tube are observed as well. Results showed that during the oscillatory operation, since the evaporator2 (high heat load) circuit will circulate first, the variation of fluid temperature at evaporator1(low heat load) outlet lags others. Besides, when the inlet water temperature increases, the operating states of the system transfer from oscillatory operation to stable operation. When the filling ratio varies from 50 %-70 %, instability occurs in the system, the oscillation period increases from 115 s to 911 s and amplitude increases from 2.5 °C to 11.1 °C with an increase of the filling ratio. It should be noted that in our new system, it still can operate stably even when the heating power difference between two evaporators are as large as 350 W. In addition, alternatively oscillation in the two evaporators, maximum heat transfer coefficient of evaporator2 and minimum heat transfer coefficient of evaporator1 can be found when the heat load varies due to the mass competition between two circuits. According to above results, it is expected that the system could be beneficial to the application in the data center cooling.