Numerical Simulation Of Flow Boiling Of Elemental Low-carbon Hydrocarbons In A Top-blown Submerged Lance

A numerical simulation of liquid hydrocarbon fuel flow boiling in the Top Submerged Lance(TSL)was conducted in this study.The vertical lance is a critical component of the Top Submerged Lance(TSL)process,which consists of a stainless-steel outer pipe and a carbon-steel inner pipe for injecting oxygen-enriched air and hydrocarbon fuel respectively.The lance is vertically inserted into the molten bath from the top of the furnace body,and the outer pipe of the lance is surrounded by high-temperature off-gas and turbulent molten bath inside the furnace.When liquid hydrocarbon fuel is adopted,the heat of the high-temperature off-gas and the melt is transferred into the inner pipe through the outer pipe,flow boiling may occur within the inner pipe due to the temperature increasing of the liquid fuel.Typically,the lance has a length of several meters once nucleate boiling occurs in the inner pipe,there may be a sufficient distance before the lance tip to develop boiling crisis.The occurrence of boiling changes the flow characteristics of the hydrocarbon fuel within the lance and affects the blowing effect of air-fuel mixture,which finally infect the smelting process.The sudden rise in pipe wall temperature caused by the boiling crisis will also accelerate the wear of the lance.Predicting the two-phase flow characteristics when the liquid fuel inside the lance is heated and boiling,the temperature distribution and heat transfer characteristics of the pipe wall when the boiling crisis occurs,and the pressure drop changes in the pipe during boiling are critical to the operation safety and process control of the top blowing smelting process are of great significance to structural design of lance.At present,most of the studies on the flow boiling of organic working fluids are aiming at cryogenic fluids or refrigerants,studies on hydrocarbons with higher saturation temperature have barely appeared,and studies on vertical downward flow are rather rare.Thus,the data of flow boiling in the TSL is difficult to obtain through experiments,nor can it be calculated by empirical formulas.However,a large number of numerical studies on complex boiling conditions benefit from the combination of the Eulerian-Eulerian two-fluid model and the wall heat flux partition model,and the current numerical study is also carried out under this framework.The Eulerian-Eulerian two-fluid model and the wall heat flux partition model are inherently constitutive;however,when this framework is used to simulate flow boiling,various empirical and semi-empirical correlations have been included as closure models to solve the equations.In order to improve the accuracy of the simulation results,the current study firstly compared the accuracy and sensitivity of a various of closure models in the subcooled R113 flow boiling simulation.Then,the closure models performing well in the subcooled R113 flow boiling simulation were used to simulate the saturated flow boiling of n-heptane,and the simulation results were verified.At last,a numerical simulation of subcooled n-heptane flow boiling in a vertical pipe under external radiation was carried out,which is corresponding to the heating conditions of the TSL.In the closure models comparison part,the investigation results show that,when the k-?turbulence model is used for simulating the subcooled flow boiling of R113 in a macroscale channel,the most applicable wall function is the Non-Equilibrium wall function;the most applicable combination of interfacial forces is:Universal drag force model,Lopez de Bertodano turbulence diffusion force model,no wall lubrication force,Moraga lift force model;the most applicable interfacial area concentration model and bubble diameter correlation combination is Symmetric IAC model and Unal's bubble diameter correlation.The above closure models combination is also applicable for simulating the saturated flow boiling of n-heptane in a circular pipe.Comparing the simulation results with the prediction results of Chen's^[28]correlation,all of the data points have a deviation less than 20%.In the part of numerical simulation of n-heptane flow boiling in a vertical pipe under external radiation,the current study simulated the flow boiling of n-heptane in the TSL.The two-phase flow features at different inlet velocities and pipe wall emissivities were predicted.Heat transfer and pressure drop characteristics of flow boiling regime from single liquid phase flow to dispersed flow film boiling were analysed,as well as the distributions of void fraction,quality,temperature of each phase,velocity of each phase,non-equilibrium effect of two-phase flow,wall temperature and wall heat flux.