Simulation Research On Conditions Of LNG Flash Boiling Spray And Heat And Mass Transfer Mechanism

Natural gas is more environmentally friendly and more energy-efficient and securer than coal,oil and other energy.Therefore,the development of natural gas engines is of profound significance in terms of its rich global reserves and good combustion performance.Aiming at the natural gas engine have many shortcomings,the industry put forward the concept of LNG(liquefied natural gas)liquid injection.When the LNG is sprayed into the cylinder under certain conditions,it will lead to severe flash boiling phenomena.The quality of fuel atomization can be improved by flash boiling spray,which is very beneficial to the formation of air-fuel mixture in the cylinder,so it is very important to study the flash boiling spray of LNG.In this paper,the conditions of LNG flash boiling spray and heat and mass transfer mechanism were researched.The main contents and conclusions of this dissertation are as follow:(1)The conditions of LNG flash boiling spray is researched.First of all,the flow process of LNG inside the nozzle was analyzed,and the calculated results were taken as initial and boundary conditions of LNG spray calculation.On this basis,a numerical simulation model of pure liquid LNG flash boiling spray was created to investigate the development of the spray process,and the conditions of LNG liquid injection occur flash boiling phenomenon were determined.All results obtained by the numerical model of LNG flash boiling spray were consistent with the exact solutions given in the literatures.The results show that,when the ambient pressure is in the range of [0.1,4.59]MPa,flash boiling phenomenon occurs,as long as the ambient pressure is lower than the saturation vapor pressure of the liquid LNG;when the ambient pressure is more than 4.59 MPa,no matter how the temperature of liquid LNG changes,flash boiling phenomenon never occurs.It is also found that the shape of LNG flash boiling spray remains incompletely symmetrical cone,and the droplet diameter decreases rapidly in the process of development.A vortex structure in spray tip is formed,and then the structure is broken and disappeared.(2)The influence of different parameters on the LNG flash boiling spray characteristics is researched.The temperature of LNG?the injection pressure and the ambient pressure are studied by the method of AVL-FIRE software.Finally the diesel and LNG spray process is analyzed.The results show that,with the increase of the injection pressure,the spray penetration and spray angle is increased and the Sauter mean diameter(SMD)is decreased distinctly.With the increase of the injection pressure,the spray penetration and spray angle is increased and the SMD is decreased distinctly.Ensure that keeping the other conditions,rising LNG temperature and reducing the ambient pressure,the flash boiling phenomenon will be more acute and the atomization quality will be improved.Under the same initial injection conditions,LNG flash boiling spray cone angle is greater than the routine diesel spray,the spray penetration and SMD is less than the routine diesel spray.LNG atomization quality is much better than diesel spray,which is very beneficial to the formation of air-fuel mixture in the cylinder.(3)Under the condition of flash boiling LNG bubble growth is researched.A computational program is written to simulate the process of LNG bubble growth under flash boiling condition.The computational program is used to simulate the influences of temperature disturb,ambient pressure and LNG temperature on the LNG bubble growth is carried out.The results show that according to the curve of time varying interface acceleration,the process of LNG bubble growth can be divided into three domains: surface tension controlled growth,transition domain and heat transfer controlled growth.The bubble growth behaves differently in the three domains.Temperature disturbance values only have an impact on the initial stage of LNG bubble growth process,and which have no influence to the later bubble growth process and the final results of convergence.The influence of ambient pressures on LNG bubble growth process and LNG temperature on LNG bubble growth process is the opposite.All of them can describe as the influence of superheated degree on LNG bubble growth process.And when the LNG temperature is greater,the ambient pressures is smaller,the superheated degree is greater.