The Characteristics Of Onset Of Nucleate Boiling Under Under Flow Rate Fluctuation

The onset of nucleate boiling model and bubble behavior in the deionized water were studied under the condition of flow rate fluctuation.The characteristics of subcooled boiling under different inlet water temperature,heat flux,mass flow rate,flow pulsation period and amplitude were studied by constructing a heat transfer and visualization experiment device.The research includes data processing method under flow rate fluctuation condition,determination method of the onset of nucleate boiling under steady and unsteady state,main factors and predicting model of the onset of nucleate boiling,single bubble behavior under the flow rare fluctuation condition.The range of experimental parameters:the system pressure is 0.1MPa,the mass flow rate is 98.7~294.1kg/(m~2s),the inlet subcooling degree is 10℃~50℃,and the experimental heat flux is 0~200kW/m~2.The flow fluctuation period is 10s~30s,and the fluctuation amplitude is0.1,0.2,0.3 respectively.According to the simple harmonic motion,the differential equations of wall temperature distribution in the tube with unsteady boundary condition and internal heat source are established Compared with the concentrated parameter method considering the wall heat storage,the two methods to calculate the inner wall temperature are consistent.The method of judging the onset of nucleate boiling point of steady state boiling is obtained by analyzing thermal parameters.It is shown that the instantaneous thermal parameters and the time-averaged thermal parameters can determine the onset of the nucleate boiling point.The pressure drop and the outlet water temperature can only determine the onset of nucleate boiling point at the exit.The wall temperature can determine the onset of nucleate boiling point for the local position in the experimental section.The position of the onset of nucleate boiling point of subcooled boiling moves to the inlet section as the heating flux increases,and the superheat of the onset of nucleate boiling point increases as the inlet subcooling and mass flow rate increases.Base on the Jens-Lottes model,a new correlation for predicting the onset of nucleate boiling superheat is developed,and the error of new prediction relation is within 15%.The method for determining the onset of nucleate boiling point of the unsteady boiling is obtained.Instantaneous thermal parameters can determine the starting point of the boiling.Onset of nucleate boiling point can also be determined by the point where time-averaged wall temperature deviates from that of steady state under high inlet undercooling condition.The larger the pulsation period and lower pulsation amplitude is,the higher the wall superheat is at the onset of nucleate boiling point.The main reason for this change is that the flow fluctuation causes the change of heat transfer in the boundary layer near the wall.By analyzing the visual data obtained from high-speed camera,it was found that the growth and condensation characteristics of the bubbles at low inlet subcooling and high inlet subcooling were different,which is mainly attributed to the different growth and condensation rate of the bubble at different degree of undercooling.Under the condition of flow fluctuation,the maximum diameter of the bubble is affected by the flow fluctuation.The amplitude and period of flow fluctuation increase,and the maximum steam diameter of bubble increases,mainly due to the change of superheat degree of superheated liquid layer.