The Laws And Mechanisms Of Porous Media On Liquid Fuels Atomization And Evaporation In Thermal Environments

Power systems using liquid fuels,such as gas turbines,internal combustion engines,aircraft engines and liquid rocket engines,have always had higher requirements for fuel combustion performance.Compared with gaseous fuel combustion,the spray performance of liquid fuels is an important factor in determining the combustion characteristics.Improvement of spray performance can significantly increase combustion efficiency and reduce pollutant emissions.Therefore,this thesis focuses on the scientific problem of"flow/atomization/heat transfer/evaporation behavior of n-heptane droplets during multi-effective treatment with porous media"to carry out experiments on the optimal treatment of n-heptane sprays with porous media as well as numerical simulations in three-dimensional pore scale,in order to investigate the influence of porous media treatment on the spray flow,atomization and heat transfer characteristics.The mechanism of porous media influence on spray flow,atomization and heat transfer characteristics were also detected.In this thesis,the effects of increasing atomizing air temperature,AAT（in the range of 300-450 K）and the gas-liquid mass ratio,ALR（0.9-3.3）on the n-heptane air-blast atomization process were investigated.The effects of AAT and ALR were obtained by analyzing the spray cone angle,droplet size and velocity characteristics,and droplet energy distribution（kinetic and surface energy）.The variation of spray cone angle,φwas found to divided into three stages,called increase,rapid decrease and stabilize with ALR increase,finallyφstabilized in the range of 25°-26°.ALR played a decisive role in droplet size and velocity,while the AAT effect on droplet size was more pronounced at low ALR,and the promotion for droplet velocity was enhanced with the ALR increase.The same conclusion was obtained in the analysis of droplet energy.The porous media effect with porous media parameters（length,pore size and porosity）on the flow characteristics of n-heptane fuel sprays were explored experimentally,and the patterns of spray morphology,droplet sizes and velocity distributions were investigated by changing operating parameters like ALR,AAT and thermal feedback.The porous media geometry parameters as well as thermal parameters on the spray characteristics were analyzed by calculating the We number of the liquid fuel re-atomization process within the porous media.The n-heptane spray treated with porous media showed cohesion effusion or sputtering effusion or good atomization of three morphological.The effusion could be avoided and got good atomization by increasing ALR/AAT or thermal feedback.The experiments obtained the range of operating parameters for good atomization.In the range of AAT=400-450 K,atomizing air and thermal feedback promoted the droplets heat transfer and evaporation in the porous media,and the Sauter mean diameter（SMD）decreased to 10-20μm equal to the untreated droplets size.The droplet volumetric root-mean-square velocity,v_vms was reduced by the porous media.The increasing AAT not only promoted droplet fragmentation and atomization,but also enhanced the micro droplets evaporation in the spray,while thermal feedback promoted the evaporation of droplets attached to the porous media skeleton.The porous media structural parameters,the ALR/AAT and the thermal feedback on the porous media temperature distribution,the spray outlet temperature and the n-heptane vapor concentration were further analyzed in experiments,and the volumetric heat transfer power of the porous media,P was calculated.A dimensionless criterion number,θ_T was obtained by using P and droplet SMD calculation in order to evaluate the n-heptane spraying performance improvement by the porous media treatment.The porous media center temperature T_c and edge temperature T_e increased rapidly after the AAT increased to 450 K.Increasing the porous media length and reducing the pore size can significantly increase the spray outlet temperature T_s,and the effect of increasing the thermal feedback with AAT was more obvious.The experimental volumetric heat transfer power,P can be up to 3.84 J/（cm³?s）,which results in a higher n-heptane vapor mass flow rate m_hg（3.18 g/min）.The criterion numberθ_T>1 indicates the spray performance was improved.AAT≥400 K was crucial to ensure thatθ_T>1,and thermal feedback with high ALR increasedθ_T to 7.On the other hand,higher thermal feedback compensated for the lack of heat input due to low AAT,and as the thermal feedback was 1%,it still maintained a high atomization performance even AAT decreased to 350 K.Combustion experiments were carried out on n-heptane spray obtained by porous media treatment in combination with the experimental results of n-heptane spray porous media treatment.The effects of each parameter on the combustion flame pattern,temperature distribution in the combustion chamber,combustion flue gas components,pollutant concentration and combustion efficiency were investigated.The n-heptane spray without porous media treatment was unable to ignite and stabilize combustion at ALR greater than 2,and the porous media widened the upper limit of stable combustion range to ALR=6.5.Increasing the porous media length and reducing the pore size can significantly reduce the spray flow rate,promote the n-heptane spray mixing with the secondary air,and increase the fuel combustion efficiency.However,the reduction of flame rigidity also leads to flame lifting and flame contact with the combustion chamber wall,resulting in flame wall quenching,leading to CO emission increase.The Weaire-Phelan foam structure was used to establish the porous media model,the gas-liquid-solid three-phase flow and heat transfer atomization process of the n-heptane spray treated by porous media was explored in numerical simulation.The influence of each parameter on spray flow characteristics and thermal characteristics,and the process inside the porous media were also analyzed.The turbulent motion in the porous media was enhanced to generate a large number of eddies and promote the conversion of turbulent kinetic energy into thermal energy.The heat storage capacity of the porous media promoted the droplets and the film evaporation in the central region of the flow field.AAT=450 K and thermal feedback,α=1%increased the n-heptane vapor mass flow rate by 38%and 144%,respectively.The coupled heat transfer/evaporation model showed that liquid film evaporation played an important role in spray evaporation,over more than 50%of the total evaporation.However,excessive heat input had a diminishing marginal effect on the droplet evaporation growth.