By considering the initial point of steam,the effects of tubes temperature on inlet temperature,heat flux,mass velocity,mass fraction of steam and so on were discussed.

The magnitude sequence of the factors that have effects on ONB (Onset of Nucle- ate Boiling) of nature circulation are confirmed as the heating power, inlet quality, liquid temperature, mass flow and pressure.

Two kinds of thermodynamics quality at onset of nucleate boiling with subcooled boiling were calculated for force circulation by using Bergles and Rohesenow method or Davis and Anderson method,and natural circulation by using Tsinghua University project group s empirical equations suggested in our natural circulation experiment at same condition.

Experimental investigation on the subcooled boiling heat transfer was carried out in a vertical up-flow double narrow annulus with 1.

Several models for calculating the initial points of subcooled boiling were introduced.

The anal-ysis shows:①Sub-cooled boiling,condensation and flashing play an important role on the flow instabil-ities in a natural circulation system,and have connection with lots of instabilities,which is different to the forced ci.

Characteristic study of highly subcooled boiling in the 5MW nuclear heating reactor;