Oscillating flow behavior of a natural circulation loop using minichannels at atmospheric pressure

Natural circulation loops at the macroscale have been widely applied in the passive cooling of nuclear power plant. However, little has been done on the miniaturized natural circulation loop for electronic cooling. The present study is to develop a miniaturized natural circulation loop consisting of an evaporator, a condenser, a riser (vapor line) and a downcomer (liquid line). Heat is dissipated from the heated chip to the evaporator, and transferred to the condenser by the air natural convection. The working fluid is selected as methanol. It is demonstrated that the system can dissipate the heating power up to 80 W with the temperatures of a simulated heated chip less than 73 °C. With the heating power varying from 10 to 80 W, the loop operates from the oscillating liquid flow to the periodic liquid/two-phase alternate flow. The thermal oscillatings of the simulated heating chip are always random. However, the inlet/outlet fluid temperatures and pressures display periodic oscillating behavior. A single full cycle is identified by the parameter traces and the simple flow visualizations by the naked eye to have three stages: liquid flow stage, sensible heat receiving stage, boiling two-phase discharging stage. These have clear switch points. The oscillating time period can be as long as 57 s at the heating power of 30 W, and is sharply decreased with increasing heating power. It is also shown that the mean wall temperatures only slightly increase with the increasing heating power, providing the better performance of the present natural circulation loop using minichannels at atmospheric pressure.