The penetration of DME spray is shorter than that of diesel fuel when the same injection system is used because DME cannot be injected at high pressure like diesel fuel due to its high compressibility. Chris reported that supercritical gasoline spray has a wider spray angle and has increased penetration. Therefore, there is possibility that supercritical DME spray has increased penetration. However, there is no observation on the shape, development process, combustion of supercritical DME spray. In this study, the spray and combustion characteristics of supercritical DME in an optically accessible constant volume vessel were observed under turbo charged engine-like ambient condition. Parallel light shadow graph and diffused light shadow graph method were applied to image the liquid and vapor phase behavior. Moreover, the spray angle of supercritical spray was compared with that of subcritical one under the same ambient condition to confirm the influence of fuel temperature. The results show that there is no significant difference in spray shape between supercritical and subcritical spray under turbo charged engine-like ambient condition. Moreover, the spray angle of supercritical spray was bigger than that of subcritical one under relatively low pressure ambient conditions. These can be explained by p - v diagram. The reasons are as follows the difference in specific volume between liquid and vapor is small at ambient pressure near critical pressure. The ignition timing of the supercritical spray was slightly earlier than the subcritical spray.
Polyoxymethylene dimethyl ether (OME) with high cetane number and low emission of soot is promising for the fuel of compression ignition engines. However, the heating value of OME is much smaller than the diesel fuel, and it is necessary to optimize the injection system. In this study, using a multi-hole injector, characteristics for the spray and the combustion of OME were investigated under the condition corresponding to the engine operation. First, the OME and diesel sprays was observed under the non-combustion condition with O2 concentration 0%. The result showed that the penetration of the OME and diesel sprays was almost same. Next, the experiment under the combustion condition was carried out. The result showed that the luminous flame was not observed in the OME spray. This suggests that the soot was generated hardly. Compared with the diesel fuel, the ignition delay of OME was similar although the cetane number of OME is higher than that of diesel fuel.
Supercritical DME spray in an optically accessible constant volume vessel under four ambient conditions was observed by a high-speed camera. Parallel light shadow graph and diffused light shadow graph method were applied to image the liquid and vapor phase behavior separately. The spray angle of supercritical spray was compared with that of subcritical one under the same ambient condition to confirm the influence of the difference in fuel temperature. The results show that the spray angle of supercritical spray was bigger than that of subcritical one under relatively low pressure ambient conditions. This can be explained by p - v diagram. However, under high pressure ambient condition like turbo charged engine, there is no difference in spray angle between supercritical and subcritical. The reasons are as follows: (a) the difference in specific volume between liquid and vapor is small at ambient pressure near critical pressure; (b) high ambient temperature has a greater influence on spray angle than fuel temperature because the heat transfer from the ambient increases with increase in temperature difference between fuel and ambient.
To investigate the choked flow in DME (dimethyl ether) injector nozzle, numerical calculations of two-dimensional steady state multi-phase flow was carried out in the range of injection pressure 30-60 MPa. Numerical results were compared with experimental ones of a previous report to examine validity. The degree of void generation in the nozzle under various differential pressures were investigated using numerical simulations. The results suggested that choke flow occures due to the decrease in effective cross section and the increase in velocity.
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