Numerical Study On Properties Of Gasoline And Injection Characteristics Under Supercritical Conditions

In order to explore the more efficient and cleaner combustion process of the engine, some new type of combustion modes, such as HCCI, SCCI, LTC, are getting more and more attention. These new type of combustion modes will increase the average temperature and pressure in cylinder, which may close to or surpass the critical point of the fuel. Therefore, the research on supercritical/transonic combustion has become an unavoidable issue. For the GDI engine, making the gasoline injected into the cylinder as supercritical state has the potential to break through the limit of thermal efficiency caused by compress ratio. At the same time it can significantly reduce the particulate emissions and realize the clean combustion. Supercritical fluid has the low viscosity, high diffusion coefficient similar to the gas and high density similar to the liquid. And the mixing process does not experience droplets broken and evaporation. These factors will greatly promote the mixture of fuel and air, reduce over rich mixture in local, and reduce particulate emissions.The specificity of supercritical fluid physical properties causes the supercritical jet different from the subcritical jet. So research on supercritical injection mixture and combustion of gasoline cannot do without reliable physical parameters of gasoline. In this paper, a series of experiments were made in order to achieve the density, capacity and viscosity of gasoline under supercitical and subcritical conditions. The testing temperature range was 300 K to 640 k and the pressures were 4, 5and 6MPa. The test results showed that all the physical parameters were fluctuated dramatically when close to the critical point. But away from the critical point, the parameters change relatively gentle. In addition, the greater the pressure, the gentler the parameters fluctuation. And then, twelve gasoline surrogates for predicting gasoline properties under a wild temperature range, including critical temperature, critical pressure, density, heat capacity, viscosity and heat conductivity were evaluated. The results showed that the surrogate most close to the critical point of gasoline was Sur8. The most reliable prediction of density, viscosity, heat capacity and heat conductivity of gasoline were achieved by Sur8, Sur10, Sur2, and Sur10 respectively. Sur10 and Sur2 were the most reliable surrogates on predicting the overall parameters. The research on gasoline physical surrogates greatly broadens the research range of temperature and pressure, which has laid a foundation on the numerical simulation study of supercritical spray and combustion.In this paper, a two-dimensional supercritical jet numerical model was built based on FLUENT software. Simulated the process of supercritical aviation fuel Jet A injected into supercritical nitrogen conditions using this numerical model. According to comparing the simulation results with the experimental data, verified the reliability of the numerical model. And then simulated the process of supercritical gasoline injected into supercritical nitrogen, combined with the physical parameters calculation methods of supercritical gasoline. Mainly studied the influence of injection conditions(fuel temperature and fuel Reynolds number) and the injection environment(nitrogen temperature and pressure) on the spray characteristics. The results showed that when the fuel temperature rose, the jet length decreased but the spread angel nearly didn't change. With the increase of Reynolds number of the fuel, the jet length increased and the spread angle didn't change. when the nitrogen temperature rose jet length increased and spread angle decreased. With the increase of environment pressure, jet length and spread angle decreased.In addition, a comparative study between supercritical injection and variable density gas injection was made in order to figure out the similarities and differences between them. The results showed that empirical formula of variable density gas could accurately predict axis density distribution of supercritical gasoline injection,but they couldn't be used to predict the spread angel and mass fraction distribution.