Gabriela Bracho

Effect of fuel properties on diesel spray development in extreme cold conditions

Abstract This paper presents an experimental and theoretical study with the purpose of quantifying and predicting the influence of fuel properties in extreme cold conditions on macroscopic diesel spray behaviour. The parameters studied were the fuel density and viscosity at different temperatures. Previous correlations available in the literature were analysed and extended in order to introduce the viscous term. A specific test rig that simulates the real engine in-cylinder air pressure and density was used. The fuel was injected from a convergent nozzle with eight holes at two injection pressures of 30MPa and 120MPa, two chamber densities of 23kg/m3 and 56kg/m3, and two temperatures of 255K (winter) and 298K (reference). In order to achieve cold conditions, the facility was introduced into a specialized climatic room with optical access, where the temperature and humidity were carefully controlled. The experimental results enabled the theoretical analysis that related the viscosity effect with the spray-tip penetration to be validated, obtaining high accuracy. The main conclusion of the paper is that fuel properties change significantly with the temperature in cold conditions, which affects the spray development, producing a shorter penetration in the transition stage. This phenomenon can be explained by a decrease in the fluid discharge coefficient in the outlet section of the hole as a result of the change in the fluid properties with the temperature.

The potential of Large Eddy Simulation (LES) code for the modeling of flow in diesel injectors

This work deals with numerical simulations of the internal flow in diesel injectors, evaluating the skills of the Large Eddy Simulation (LES) code for capturing the turbulent patterns present in the flow. In order to evaluate the potential of the LES code, the results of the simulations have been compared to standard numerical RANS method results, and simultaneously validated with experimental values and DNS data. The experiments were made using special diesel injector geometry, reaching a high injection pressure condition and so ensuring a turbulent regime. In general, results show good agreement with the experimental values and DNS data. It was found that LES is capable of reproducing the turbulent structures in diesel nozzles, and as expected is more accurate than RANS, mainly in the boundary layer.

Large Eddy Simulation for high pressure flows: Model extension for compressible liquids

The present study gives a general outline for the fluid-dynamical calculation of flows at high pressure conditions. The main idea is to present a mathematical description of high pressure processes in liquids at compressible conditions, quantifying the effect of density variations on the flow pattern due to those pressure variations. The improved mathematical approach is coupled to a Large Eddy Simulation (LES) solver. The main code was developed by OpenSource Ltd. for OpenFOAM, and the authors have introduced the additional expressions in order to calculate particular variables. For validating the code improvement, the LES solver is applied to a modern common-rail nozzle injector used in diesel engines. Results have been compared against other calculations that assumed constant properties and simultaneously validated with experimental data.

On the rate of injection modeling applied to direct injection compression ignition engines

Modern engine design has challenging requirements toward maximum power output, fuel consumption and emissions. For engine combustion development programs, the injection system has to be able to operate reliable under a variety of operating conditions. Today’s legislations for quieter and cleaner engines require multiple-injection strategies, where it is important to understand the behavior of the system and to measure the effect of one injection on subsequent injections. This study presents a methodology for zero-dimensional modeling of the mass flow rate and the rail pressure of a common rail system, constructed from a set of experimental measurements in engine-like operating conditions, for single- and multiple-injection strategies. The model is based on mathematical expressions and correlations that can simulate the mass flow rate obtained with the Bosch tube experiment, focusing on the shape and the injected mass, using few inputs: rail pressure, back pressure, energizing time and so on. The model target is to satisfy two conditions: lowest computational cost and to reproduce the realistic injected quantity. Also, the influence of the rail pressure level on the start of injection is determined, especially for multiple-injection strategies on the rate shape and injected mass. Good accuracy was obtained in the simulations. The results showed that the model error is within the 5%, which corresponds at the same time to the natural error of the injector and to the accuracy of the measures which had been done. The benefits of the model are that simulations can be performed quickly and easily for any operation points, and, on the other hand, that the model can be used in real-time on the engine test bench for mass estimations when doing additional experiments or calibration activities.

Vapor phase penetration measurements with both single and double-pass Schlieren for the same injection event

Schlieren imaging has been adopted as a standard optical technique for the analysis of diesel sprays under enginelike conditions. A single-pass Schlieren arrangement is typically used for the study of single-orifice nozzles, asvessels with multiple optical accesses regularly allow line of sight visualization. Contrarily, for multi-spray nozzles,measurements are commonly performed through a single optical access, in which case a double-pass arrangementis employed. As a consequence, the light beams pass through the test section twice, increasing the optical sensitivityof the Schlieren setup. However, the impact this has on the macroscopic spray characteristics is still unclear.The scope of this study is to analyze the differences in vapor phase penetration for the same injection event,through high-speed imaging, for both single and double-pass Schlieren configurations. Experiments were carriedout with a three hole nozzle with a nominal orifice diameter of 90 μm, named Spray B from the Engine CombustionNetwork, using commercially available diesel fuel and in non-reactive conditions. The impact of different injectionpressures and chamber densities on the spray captured by each setup was assessed. On the results, vapor phasepenetration followed the expected trend found in the literature, as it increases with increasing injection pressure anddecreasing chamber density. Comparing the optical setups, vapor phase penetration obtained with the double-passarrangement was marginally higher. The deviation was observed throughout all tested conditions. Although thediscrepancy was approximately constant for different injection pressures and chamber temperature, it increasedwith increasing density. These results highlight the importance of a proper understanding regarding the limitationsof optical diagnostics, in particular for results used in calibration of computational models. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4884