| As the problems of energy shortage and environmental pollution are becoming more and more serious today,the ways to increase energy efficiency and to reduce pollution have become key factors concerning the economic growth of our country and the well-being of the people.In energy-intensive industries such as cement and steel,measures to utilize waste heat more efficiently is worth studying.The introduction of flash evaporation into the waste heat recovery and power generation system is an effective way to further improve the waste heat utilization efficiency and to reduce the heat pollution.The efficiency of waste heat utilization is closely related to the mode of flash evaporation,the level of parameters,and the efficiency of flashing.Therefore,it is important to investigate the mechanism and characteristics of flash evaporation.This dissertation makes a comprehensive research on high-temperature and high-pressure spray flash evaporation of water,which plays a significant role in waste heat recovery.Theoretical analysis and numerical simulation is done,experiments of a broad parameter range are carried out,and basic laws and characteristics of high-temperature and high-pressure spray flash evaporation process are obtained.On the basis of the above-mentioned analysis,a modified nozzle structure is proposed.A mathematical model for spray flash evaporation is built using the Euler-Lagrange approach.The flow and heat transfer of the vapor is solved in the Eulerian reference frame while the motion and flash evaporation of the droplets in the spray is tracked in the Lagrangian reference frame.Due to the inadequacy of classical evaporation model in predicting flash evaporation,a new mathematical model for superheated droplet flash evaporation is built considering the particularity of the flashing process.This model describes the changing of droplet mass,diameter and temperature during the flash evaporation process.The mathematical model for spray flash evaporation is solved numerically.A user-defined function is used to load the flashing sub-model into the CFD software solver.Three-dimensional full-scale numerical simulation of spray flash evaporation process is achieved.The development procedure and steady-state characteristics of the spray and flow field is obtained though calculation.The results show that at the beginning the spray is highly superheated and evaporates violently,resulting in high vapor velocity around the nozzle exit and a quick reduction of droplets’ temperature and diameter.As the spray develops,evaporation slows down,the influence of droplets’ initial momentum decreases and thus the radial spreading of the spray reduces.After both the spray and the flow field come to a steady state,the velocity of the droplets which are close to the centerline is higher,while that of the droplets at the edge of the spray is lower.Symmetrical large-scale vortices appear in the vapor flow field.The sensitivity of the results to various parameters is analyzed.The influence of spray cone angle,initial droplet diameter,injecting direction,initial velocity and evaporation pressure on the spray flash evaporation characteristics is investigated.The results show that a larger spray cone angle increases the spray’s coverage and enhances flash evaporation by a small extent.The decrease in droplet diameter reduces the radial spreading of the spray,but greatly accelerates evaporation,so that the velocity of the vapor flow near the nozzle exit is also increased.Upward injection is not recommended because the inner space of the flash chamber is not fully utilized and there’s possibility for small droplets to be carried out before flashing completes.The decrease in initial velocity extends the droplets’ residence time inside the chamber and raises the extent of flashing.The increase of evaporation pressure corresponds to a decrease in superheat,leading to a decrease in the violence of flashing.The radial expansion of the spray reduces due to the increase of drag force.On the basis of the research of flash evaporation of uniform sprays,a model for non-uniform spray flash evaporation is developed.Numerical investigation on flash evaporation process of a spray with a Rosin-Rammler size distribution is conducted.Smaller droplets are found to concentrate near the centerline of the spray,while larger droplets cover a wider range radially.Axially,the temperature of the spray is high at the top and low at the bottom.Radially,the temperature is low around the centerline and high on the outer part.The change in droplets’velocity is more pronounced than that of a uniform spray,with smaller droplets changing the most.The vapor velocity and turbulent intensity near the nozzle exit increases significantly.Two vortices which are different in scale and direction appear in the vapor flow field.Experiments on spray flash evaporation are conducted based on a large-scale experimental system,of which the heat source comes from industrial waste heat.In consideration of the low parameter level in previous experiments in literature,our experiments raise the level of parameters to a higher extent.The initial water temperature is raised to above 100℃.The degree of superheat is extended to 30-46℃.The pressure inside the flash chamber is maintained positive.Our experiments greatly expand the range of experimental study on flashing.The results indicate that the vapor mass flow rate increases with the increase of initial temperature and decreases with the increase of pressure.Downward injection outperforms upward injection for higher vapor generation rate and less water carried out by vapor.The nozzle size effects flash evaporation by two competing mechanisms:the fineness of spray and the residence time of it.Appropriate nozzles should be selected based on the space inside the flash chamber and the characteristic of the nozzle.The degree of superheat is the driving force of flashing.Empirical equations between flash efficiency and superheat are proposed based on experimental data.The dimensionless Jakob number(Ja for short)is chosen as the characteristic number for the flashing process.The evolution of flash efficiency versus Ja under different experimental conditions is investigated.Multiple regression analysis is done on dimensionless data and empirical equations between flash efficiency versus Ja and dimensionless pressure are proposed.After a comprehensive analysis on the flash evaporation of spray,investigation extends to the interior of the nozzle,considering the possibility of phase change in this area and its influence on spray features and spray flash evaporation.By combining multiphase models with a pressure-driven phase change model,numerical simulation of flow and flashing inside the nozzle is done.The nozzle,used in the experiments,has two S-shaped vanes.Based on the simulation results,an improved nozzle structure is proposed.Another pair of S-shaped vanes is added and two ends of the vanes are set vertical with the inner wall of the nozzle.Simulation on this new structure has proven its advantages in enhancing flashing,reducing fouling and enlarging passage area over the original one. |
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