Experimental And Numerical Study On Spray Characteristics And Combustion Process Of Dimethyl Ether

Recently, as an ideal alternative fuel for diesel engine, dimethyl ether(DME) has been attracting much attention from many researchers. Because the properties of DME are large different from diesel fuel, the spray mixture and combustion processes of DME are investigated in detail using advanced laser technique and multi-dimensional simulation in this paper. The main contents and achievements of this research are as following.In order to improve accuracy of spray simulation, the grid dependency of O'Rourke droplet collision model is analyzed, and proposed a cross mesh droplet collision(CMC) algorithm which is a combination of cross mesh method and collision limit angle. In constant volume apparatus and D.I. diesel engine, the effects of mesh schemes, mesh resolutions and parcel numbers to the calculation results using modified algorithm are checked from spray structure, Sauter mean radius(SMR) and spray penetration, respectively. The results show that: CMC droplet model is less mesh-dependent and less sensitive to mesh resolution and parcel number in both polar mesh and Cartesian mesh. However, the predicted spray penetration using CMC droplet model is only slightly better than that of O'Rourke algorithm. The main reason is that the CMC method is designed to better resolve the radial component of the droplet collision, the spray penetration is a measure of axial motion.Through completely analyzing the papers about DME spray, found that flash boiling phenomena are easily occurred in DME spray. Based on the digital particle image velocimetry (PIV) system of TSI Company, a PIV test bench for DME spray investigation is set up. According to the spray images captured by CCD camera, the spray structure and developing process of DME flash boiling spray formed by a single-hole D.I. diesel injector are analyzed. The results show that: according to the axial direction, the spray structure of flash boiling spray can be classified into follows: spray initial core region; main body region of flash boiling spray; vortex ring formation region. Based on the process of vortex ring structure formation and development, the flash boiling spray development can be classified into four stages: vortex ring structure forming stage; vortex ring structure developing and distorting stage; vortex ring structure breaking up stage; full development stage of flash boiling spray. In addition, due to the effect of micro-explosion the spray macro-characteristics of DME flash boiling spray are large different from diesel fuel. The internal characteristics and forming mechanisms of DME conventional spray are investigated using the above PIV test bench. Similar with diesel fuel spray, there are non-homogeneous"branch-like structures"in DME spray. However, the fuel distribution characteristics of DME spray are large different from diesel fuel, and there are apparent high concentration fuel region and low concentration fuel region in DME spray body. Subsequently, the velocity and vorticity of DME spray are measured, and compared the dimensionless velocity distributions at each cross section with Abramovich distribution. It is found that the dimensionless velocity of DME spray is better consistent with the predicted value using Abramovich distribution in the middle of spray, but the predicted value using Abramovich function is less than the experimental results at the edge of spray. In order to improve the simulation accuracy of mixture and combustion process of DME spray using quasi- dimensional combustion model, a new velocimetry distribution function which is suitable for DME spray is proposed. In addition, the influence of spray impingement against the wall on DME spray is investigated.Based on a single cylinder diesel engine ZS195, the test bench of D.I. DME engine is set up through the modification of fuel system. In three different conditions, the combustion processes of diesel engine fuelled with DME are analyzed. It is found that the advance angle of oil supply of 27°CA BTDC is better for ZS195 diesel engine fuelled with DME, and investigated the effects of intake air temperature and compression ratio on the combustion and emission of DME engine. In addition, the test bench of DME HCCI engine is set up using the mixture formation method similar with M.P.I. gasoline engine. In three partly engine loads, the combustion process and emission characteristic of DME HCCI engine are investigated. In HCCI condition the DME combustion shows clear"two stages"heat release, and the detonation phenomena in combustion process impede the expansion of engine performance to medium or high load.In order to profoundly understand the ignition and combustion mechanics of D.I. DME engine, and clear the essential reason that there are large difference in combustion and emission characteristics between DME engine and diesel engine, this work analyzes the ignition and combustion processes of single cylinder diesel engine ZS-195 fuelled with DME using modified KIVA code. The results show that: The main reason that the ignition and combustion processes of D.I. DME engine are large different from diesel engine is that DME has better atomization or evaporation characteristics. In the influence of flow in cylinder, the combustible mixtures of DME easily accumulate at the wall of combustion chamber and downstream of vortex, and complete auto-ignition. Subsequently, the combustible mixtures are moved from the wall of combustion chamber to the axis of cylinder by the action of entrainment flow in cylinder, and the flames of combustion region are also expanded to the axis of cylinder and finally overflow the whole combustion chamber.