Quantifying volumetric distribution of fuel mass in a transient high pressure reacting spray via the application of rainbow schlieren deflectometry
The fuel atomization, evaporation and subsequent mixing process with ambient air during fuel injection in a direct injection internal combustion (IC) engine are crucial to subsequent combustion and engine performance. Optimization of the fuel injection mechanism has been continuously explored with the advancement of the IC engine. This is especially true regarding direct injection involved with diesel engines where there is very minimal time for the fuel and compressed air to mix. With accumulative precedence being placed on increasing fuel efficiency and decreasing toxic emissions such as nitric oxides and carbon monoxides, research has been focused on ensuring the combustion processes comply with government or otherwise issued regulations. In order to investigate the intricate physical phenomena following the start of injection (SOI) that dictate how eventual combustion occurs, local quantitative characterization of the spray is necessary. In this thesis, the Rainbow Schlieren Deflectometry (RSD) technique is employed to provide local quantitative measurements during the transient fuel-air mixing process by examining a reacting n-heptane fuel spray injected into ambient air at 28 bar and 825 K. An optically accessible constant pressure flow vessel (CPFV) and common rail diesel injector allows for the capturing of high-speed, instantaneous RSD images and subsequent ensemble averaging of the data for repeated injections in immediate succession. From this, the average mixture temperature and equivalence ratio distribution downstream of the liquid length are determined for the entire radial span of the spray, along with the liquid and vapor boundaries. The two-dimensional spray data is then converted into a spray volume and the low temperature heat release (LTHR) region is identified. Next, the fuel mass distribution is obtained in this LTHR region with a corresponding mixture temperature and equivalence ratio. The adiabatic flame temperatures in the LTHR region are calculated by a chemical equilibrium solver via ANSYS Chemkin. Experimental results reveal local equivalence ratio, temperature, and fuel mass distributions for the entire spray volume downstream of the liquid length, along with quantification of the conditions of the mixture in the region that ultimately exhibits low temperature heat release prior to and during the main ignition event.