Numerical and Experimental Analysis of Multiple Injection Diesel Sprays

A customized version of FIRE\sT code is chosen to simulate the spray injected into a controlled environment by means of a Common-Rail system driven via a Programmable Electronic Control Unit. Numerical results are compared to experimental data collected under various operating conditions, namely varying the injection pressure up to 120 MPa, and the gas backpressure between 0.1 and 5.0 MPa. Non-evaporating conditions are considered. The employed optically accessible test chamber allows to light the spray with a flash lamp or a pulsed laser sheet, generated on the second harmonic of a Nd-YAG laser (532 nm, 12 ns in duration). Images are collected at different instants of time after the start of injection by means of a CCD camera. A digital image processing software is used to evaluate the major characteristics of the spray, as the penetration length and the cone angle. The injection flow rate is properly measured on a rate-of-injection flow bench for different injection strategies. This allows to recover the instantaneous injection velocity, which is an input quantity for the numerical simulation. The spray dynamics is reproduced by accounting for the flow inside the nozzle as well as the successive occurrence of primary and secondary atomization. The competing effects of turbulence, cavitation and aerodynamic shears are considered as concurring to primary atomization. Secondary atomization is simulated by adding models previously used for blob injection. Turbulent dispersion and drops interaction are also considered. Different grids are tested, in order to verify how the cell size affects the numerical results.