Study On Subcooled Flow Boiling In Vertical Rectangular Narrow Channel

Plate type fuel element is often used in the reactor core to improve the power density of the reactor,so the flow channel of the reactor core is narrow rectangular channel.In the pressurized water reactor,saturation boiling is not allowed in the core to ensure the safe operation of the reactor,so the coolant requires a high degree of supercooling.However,a small amount of supercooled boiling occurs in the core during actual operation,so the study of the subcooled flow boiling in the narrow rectangular channel is of great significance to the design and operation of the reactor.In this paper,the phenomenon of subcooled boiling in narrow rectangular channel is studied experimentally and numerically,and a wall boiling model suitable for narrow rectangular channel is proposed.According to the experimental and numerical simulation results,the heat transfer characteristics of subcooled boiling in narrow rectangular channel are analyzed.Firstly,the phenomenon of subcooling boiling in a narrow rectangular channel is studied experimentally.The test section is a one-side heating narrow rectangular channel with a heating length of 550 mm.The experiments were conducted at the pressure range of 0.1 to 0.4MPa,with the inlet subcooled temperature range of 20 to 60 K,the velocity range of 0.347 to1.736m/s and the heat flux range of 227 to 306 k W/m~2.According to the results,the heat transfer characteristics at different conditions are analyzed,and the behaviors of the static bubbles and slidings bubbles observed in the experiments are analyzed.After that,according to the behaviors of the bubbles,we propose a modified wall boiling model considering the influence of sliding bubbles to the heat transfer based on the RPI wall boiling model.The modified model divides the heat flux into evaporation heat flux,single phase convection heat flux,stationary bubble quenching heat flux and sliding bubble quenching heat flux.We use the user defined functions to apply the modified model into the fluent code,and the simulation results are compared with experimental data ant RPI modle simulation results,which shows that the accuracy of the modified model simulation results is better than RPI model at all the work conditions.Finally,the heat transfer characteristics of subcooled boiling in narrow rectangular channel are analyzed according to the simulation results.The wall temperature distribution,the void fraction near the heating wall and the void fraction of the cross section are dissucssed.And we analyzed the variation of bubble parameters and the distribution of the heat flux along the axial height at different conditions.In this paper,according to the experimental phenomenon and theoretical analysis,we proposed a modified wall boiling model considering sliding bubbles based on the RPI wall boiling model,which is applicable to the simulation of the subcooled flow boiling in the rectangular channel.In addition,we did some discussions on the heat transfer characteristics according to the experimental and simulation results,which have a important significance for the design and operation of the nuclear power plants.