Investigation of diesel liquid spray penetration fluctuations under vaporizing conditions

Diesel combustion and emissions formation is largely spray and mixing controlled and hence understanding spray parameters, specifically vaporization, is key to determine the impact of fuel injector operation and nozzle design on combustion and emissions. In this study, an eight-hole common rail piezoelectric injector was tested in an optically accessible constant volume combustion vessel at charge gas conditions typical of full load boosted engine operation. Liquid penetration of the eight sprays was determined via processing of images acquired from Mie back scattering under vaporizing conditions by injecting into a charge gas at elevated temperature with 0% oxygen. Conditions investigated included a charge temperature sweep of 800 to 1300 K and injection pressure sweep of 1034 to 2000 bar at a constant charge density of 34.8 kg/m3. Under these conditions there are noticeable fluctuations in liquid phase penetration once the steady state liquid length has been established, on the order of 10% of the mean liquid length. These fluctuations in penetration were seen in each plume with increases and decreases in liquid length around a mean value, with the mean quasi-steady liquid lengths ranging from 9 to 18 mm, dependent upon test conditions. An analysis of the fluctuations is undertaken including characterizing frequency content of the liquid length and fuel pressure fluctuations, along with a parametric sensitivity study using a 1-D liquid-length model (for the baseline test condition of 2000 bar injection pressure, 1100 K bulk gas temperature, 0% oxygen, 34.8 kg/m3 density environment, with a fuel temperature of 363 K). Hypotheses are presented to explain the causes of these fluctuations which include injection pressure oscillations due to fuel injection, cavitation and injector internal flow observed in injector coefficients, temperature gradients of the ambient charge gas, turbulence, or slugs of fuel detaching from the leading edge of the spray. Results dictate that the likely causes of liquid length fluctuations are a combination of detaching fuel slugs, turbulence, charge-gas temperature variations, and nozzle flow variations