It is a hot research field to study the Critical Heat Flux (CHF) occurrence mechanism in boiling heat transfer. Although lots of researchers have studied on it, no unified conclusion has been achieved up to now. The proposed CHF occurrence mechanism is also not widely accepted. Because the void fraction close to the heating surface is larger when the heat flux approaches CHF, it is difficult to make visual observations of the boiling heat transfer on the heating surface. Usually the CHF mechanism is based on certain assumptions, and then confirmed by testing. Therefore, it is meaningful to study the occurrence of CHF by experimental methods.

Based on the system design of the test system for CHF in a rectangular channel, an experimental facility was set up. The main test section consists of a rectangular flow channel with the copper heating surface downwards mounted into one of the channel walls. The fluid is deionized water. Fluid subcooling is 15 K. The entire test section is mounted on a rotating arm which can be set at different inclination angles from 0° (horizontal) to 90° (vertical). By adjusting the loop, the natural circulation and forced circulation test conditions can be achieved from the experimental facility. Through test research, visual observations are acquired about the bubble growth characteristics in the rectangular channel in the range of 0 ° to 90 °, and the test data about the CHF is also attained.

It is found that under the condition of forced circulation and natural circulation, the CHF values is increased effectively with the improved mass flow rate and the increased inclination angles of the test section. Through visual observation, it is found that in forced circulation conditions, CHF occurs first at the entrance location of the test surface, rather than the maximal heat flux location of the test surface (test surface center) or the maximal local void fraction of the test facility (exit location of the test facility). In the natural circulation conditions, CHF occurs first at the exit location of the test facility. This phenomenon may imply that the mechanisms of CHF are different in forced circulation and natural circulation. In forced circulation, the flow plays a main role. In natural circulation, the local void fraction plays a main role.

There are some differences in the experimental phenomena compared with the traditional CHF theory, like the bubble crowding theory and the micro fluid layer theory. Through the experiment research, the complexity of the flow boiling heat transfer was found. The flow and boiling heat transfer affect each other. It is can help us to research the CHF theory in flow boiling heat transfer.

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