Abstract
Keywords - Diesel, Spray, Primary Breakup, Velocity, Back-Pressure
Abstract - The injection system of a modern direct injection Diesel engine plays an important role on the mixture preparation, and thus on power, fuel consumption and emissions, which are directly influenced by the atomisation of the spray in the engine cylinder. Detailed knowledge of the interaction between the liquid spray and the surrounding gas is necessary in order to understand the atomisation process, and is required by modern CFD codes to provide boundary conditions for the simulations and to validate them.
However, quantitative experimental data about the primary breakup of Diesel sprays is scarce. Due to the high density, high speed and small dimensions of these sprays, most standard measurement methods cannot be applied. Laser Correlation Velocimetry (LCV) is a technique capable of measuring time-resolved spray velocities in the dense primary breakup zone of Diesel sprays. The basic principle is to continuously illuminate a spray and to measure the intensity of the light scattered from two distinct points in the spray. These signals are modulated by spray structures passing these detection volumes. If the second focal point is located slightly downstream of the first, the signals will be very similar in appearance, and the time delay by which the first signal leads the second is then inversely proportional to velocity. Knowing the exact distance between the focal points allows the velocity to be calculated.
In previous implementations of the LCV method, spray speeds close to the nozzle exit were measured, but the fuel was not injected into a high pressure environment, which would have been necessary to approximate conditions found in a typical Diesel engine at the time of injection. It is not straightforward to apply LCV in a pressure vessel, because high aperture optics located close to the spray are required in order to achieve small detection volumes and thus good spatial resolution. For the investigations presented in this paper, a compact pressure vessel was built, using spray traps to inhibit deposition of fuel droplets onto the windows of the chamber. A low continuous gas flow in the vessel prevents fogging. The optical system used to create the detection volumes was modified to meet the demands of LCV measurements inside a pressure vessel. Time-resolved velocity measurements were performed at different positions in Common Rail sprays, and the results show significant differences depending on the pressure of the surrounding gas.