Experimental data and calculated results for the onset of nucleate boiling （ONB） in natural circula- tion systems show that for the same operating conditions, the equilibrium vapor quality for ONB in natural circulation is lower than for forced convections. The differences can be explained using second stir theory. The weak vortices and small stir energy in natural circulation systems result in nucleate boiling occurring earlier than in forced convection systems. In natural circulation systems high mass flow rates are accompa- nied by large kinetic energies and large stir energies, which enables changes in the directions of flow eddies and energy transport. The equilibrium vapor qualities at ONB are then higher at higher mass flow rates. The influence of other flow parameters on ONB can be evaluated by the relationships between these flow pa- rameters and the mass flow rate. The same values can lead to different results due to different eddy direc- tions. This indicates that the quantitative comparability in mathematics cannot be considered as only scien- tific standard. The second stir theory offers a new visual angle for researches on natural circulation.
Based on the experiment of onset of nucleate boiling(ONB)in natural circulation and the nonequilibrium thermodynamics dissipative theory,the mechanism of ONB in narrow rectangle channels of natural circulation is proposed.It points out that the onset of nucleate boiling is influenced by the degree of superheat and the special conditions of narrow channels.Under the conditions of both density difference in natural circulation and narrow rectangle channels,the prediction model of ONB in natural circulation of narrow channels based on fluctuating is established.The experimental results show that the present model can be used to predict the heat flux of ONB in narrow rectangle channels.Features of ONB in natural circulation narrow rectangle channels are as follows:heating power is the incentive of the happen of ONB;the higher the heating power is,the higher the degree of superheat is,and the earlier the ONB will appear.With the pressurizing,the appearance of ONB will be delayed.The higher the degree of supercooling is,the later the ONB appears.The ONB will happen easier when there are noncondensable gases and roughness in the channels.
