The Mechanism Research Of Heat Transfer To Flow In Supercritical Water Natural Circulation

Supercritical Water-Cooled Reactor(SCWR)is the only water-cooled reactor in the fourth generation of advanced reactors.It has many advantages such as economy and continuity.It is designed and built from the current pressurized water reactor,boiling water reactor and supercritical thermal power unit.The supercritical water is selected as the cooling medium,and the core outlet pressure is 25 MPa and the temperature is 500 0C.The study of the effect of natural circulation heat transfer on the flow of supercritical water has theoretical significance for mitigating the core accident and the design of the core residual heat removal system of the fourth generation supercritical water cooled reactor.First of all,a supercritical water natural circulation flow heat transfer experiment was carried out.The effect of the natural circulation system of supercritical water on its natural circulation flow heat transfer characteristics under different system pressure conditions was studied.The results show that the fluctuation range of the subcritical natural circulation flow is relatively small compared with the supercritical natural circulation flow.O n the basis of the experimental data signal of the supercritical water natural circulation flow,the signal denoising analysis of the experimental flow signal is carried out by selecting various wavelet basis functions.Secondly,the characteristics of the variation of thermal conductivity,dynamic viscosity,constant pressure specific heat and expansion coefficient of supercritical water during the transition from pseudo-liquid to pseudo-vapor region are analyzed.The calculation formula for calculating the pseudo-critical expansion coefficient of supercritical water is obtained by fitting.The classification decision model of supercritical water in critical region is summarized.Based on this analysis,the three-zone analysis and determination model of supercritical water is improved,and the new supercritical water in the critical region is obtained.And the new supercritical water partition boundary calculation relation is obtained by this fitting.Thirdly,based on the experimental data,a prediction model of supercritical water heat transfer coefficient and natural circulation flow based on BP neural network is established.Through the prediction model,the influence of changes in parameters such as pipe diameter,flow,power and pressure on the heat transfer coefficient of supercritical water is predicted and analyzed.A three-dimensional distribution cloud of natural circulation flow instability onset points under different pipe diameters,height differences and inlet temperatures is obtained.The application of the calculation formula of the heat transfer deterioration point and the error distribution are analyzed.The predictive calculation formula for the heat transfer deterioration point power is optimized and improved.The newly established calculation model takes into account the influence of flow rate,pipe diameter,pressure and inlet temperature,and has a small average relative deviation and average absolute deviation.And then,using numerical analysis software,the flow and heat transfer of the supercritical water natural circulation under the condition of uniform distribution and uneven distribution of heating power were simulated and analyzed.When the heating power distribution characteristics are uniform heating,power increasing,power decrementing and Sin type distribution,the characteristics of the different heat transfer conditions affecting the natural circulation flow rate under different heating power distribution conditions are analyzed.At last,the simulation of the movement and deposition of the fine particles in the supercritical water is carried out.The effects of heating power,inlet temperature,inlet velocity,average particle diameter,and asymmetry of heating between different pipes on the fine particles in supercritical water were studied.The results show that with the increase of inlet temperature,the peak concentration of particulate matter gradually approaches the inlet section of the pipe;as the inlet flow velocity increases,the peak of the distribution of particulate matter along the pipe gradually moves toward the outlet of the pipe.The heating effect has a certain inhibitory effect on the concentration reduction and deposition of the particulate matter.Finally,on the basis of all the mentioned,the results calculated by different research methods are compared and analyzed.The transition characteristics of flow stability in supercritical natural circulation system after sudden pressure change are summarized.It is concluded that under supercritical pressure,when the pipe is affected by the flow,it will in turn further change the length of the different stages of thermal acceleration drive and thermal resistance drive.And the characteristic mechanism of different stages of movement and deposition process of fine partic les in supercritical water after being heated was obtained and analyzed.