Study On Boiling Characteristics Of N-heptane Flow In Submerged Top-blowing Lanc

The Top Submerged Lance(TSL)is the core equipment of the oxygen-enriched topblowing molten pool smelting process,which is responsible for blowing oxygen-enriched air and supplementary fuel into the melt.Since the TSL is always in an environment surrounded by high temperature melt or gas in the melting furnace,when liquid fuel is used,it may cause a flow boiling in the tube.Under the condition of immersion injection,due to the mushroom head structure easily formed in the TSL head and the existence of the multi-channel nozzle structure in the TSL head,the flow area of the spray gun head will be reduced,resulting in a pressure drop in the spray gun head.When heated liquid fuel flows through the spray gun head,flash boiling is easily triggered by the pressure drop.The appearance of flow boiling in the spray gun tube and flash boiling at the nozzle will change the flow state of the liquid fuel in the tube,thereby affecting the injection effect of the spray gun.Therefore,accurate prediction of the flow state of liquid fuel in the tube,the temperature distribution on the spray gun wall and the flash boiling characteristics at the nozzle is of great significance for improving the service life and injection effect of the TSL,and has guiding significance for the safe operation of the submerged top blown smelting process and the design of the lance structure.Firstly,according to the heat transfer characteristics of the TSL,the flow process of n-heptane liquid fuel in the TSL is numerically simulated based on the Euler multiphasefluid model combined with the improved RPI(Rensselaer Polytechnic Institute)wall heat flux partition model.The effect of the flow rate and temperature of n-heptane at the inlet of the spray gun,the pressure and radius at the outlet of the spray gun on the flow boiling in the tube were mainly studied.The distribution law of gas content,pressure,temperature,flow rate,wall temperature and heat flux density,heat transfer efficiency and mass transfer rate in the tube is analysed.The results show that when the inlet flow rate increases from0.6 m/s to 1.0 m/s,the inlet temperature decreases from 313.15 K to 293.15 K,the outlet pressure increases from 0.1 MPa to 0.4 MPa,and the nozzle radius increases from 5mm to7.5 mm,the development of flow boiling will be delayed.The ONB(Onset of Nucleate Boiling)in a 5 mm radius pipeline will be delayed by 1260 mm,750 mm,and 2390 mm,respectively,and the starting point of core state boiling in a 7.5 mm radius pipeline will be delayed by 1900 mm,570 mm,and 2000 mm,respectively.At the same time,the changes in the above conditions result in a decrease in the outlet void fraction,a decrease in the upward trend of wall temperature,a decrease in wall heat flux,and a decrease in heat transfer efficiency.The average temperature and average velocity of the gas-liquid twophase section in the tube also decrease.Then,based on the structural characteristics of the multi-channel nozzle,the flash boiling of liquid fuel n-heptane at the multi-channel nozzle caused by pressure drop was numerically simulated using the Euler two-fluid model.The effects of n-heptane liquid fuel temperature,pipeline inlet pressure,nozzle outlet ambient pressure,and nozzle geometry on flash boiling characteristics were studied.The results showed that an increase in pipeline inlet pressure from 0.2 MPa to 0.5 MPa,a decrease in fuel temperature from420 K to 340 K,and a decrease in nozzle radius from 3.5 mm to 2.5 mm all increased the severity of flash boiling at the nozzle inlet.The gas content of the nozzle increased by0.026,0.049,and 0.042,respectively.However,changes in environmental pressure had little effect on the severity of flash boiling at the nozzle inlet;reducing the inlet pressure of the pipeline from 0.5 MPa to 0.2 MPa,reducing the environmental pressure from 0.1MPa to 0.04 MPa,increasing the inlet temperature from 340 K to 420 K,and reducing the nozzle radius from 3.5 mm to 2.5 mm all increase the severity of flash boiling at the nozzle outlet.The gas content at the nozzle outlet increases by 0.124,0.119,0.656,and 0.037 respectively.At the same time,in order to ensure that the models used in this study can accurately predict the occurrence of flow and flash boiling of n-heptane liquid fuel in the tube,the validity of the mathematical models used in this study was verified using relevant experimental data,and the results showed that the models used can effectively predict flow and flash boiling phenomena.