J.M. Pastor

Contribution to the application of two-colour imaging to diesel combustion

The two-colour method (2C) is a well-known methodology for the estimation of flame temperature and the soot-related KL factor. A 2C imaging system has been built with a single charge-coupled device (CCD) camera for visualization of the diesel flame in a single-cylinder 2-stroke engine with optical accesses. The work presented here focuses on methodological aspects. In that sense, the influence of calibration uncertainties on the measured temperature and KL factor has been analysed. Besides, a theoretical study is presented that tries to link the true flame temperature and soot distributions with those derived from the 2C images. Finally, an experimental study has been carried out in order to show the influence of injection pressure, air density and temperature on the 2C-derived parameters. Comparison with the expected results has shown the limitations of this methodology for diesel flame analysis.

The role of detailed chemical kinetics on CFD diesel spray ignition and combustion modelling

Spray ignition and flame stabilisation in the frame of diesel-like combustion conditions combine fundamental and complex physical and chemical processes. In this work, a numerical investigation has been performed to evaluate the potential of integrating detailed chemistry into CFD calculations, in order to improve predictions and gain more insight in involved processes. This work has been carried out using the capabilities of OpenFOAM^(C)code, which provides an opensource framework for 3D-CFD simulations, including an ODE solver for solving chemical kinetics. As a general methodology, this study is based on simulating free n-heptane sprays injected into a constant volume vessel, corresponding to the conditions of the experimental database provided by Sandia National Laboratories. Calculations results have been compared to experiments, evaluating the effect of a wide range of ambient conditions on spray ignition and combustion characteristics. Specifically, this research checks the performance of some relevant n-heptane oxidation mechanisms found in the literature, with different degree of complexity, for modelling the chemical history of the fuel. The results of this investigation show the relative influence of chemical mechanism on spray/flame structure in terms of ignition delay and also ignition and flame stabilisation sites. The comprehensive mechanism performs generally better than more simplified chemistry models. However, its accuracy is also compromised for modelling advanced diesel-like combustion concepts based on injecting the spray into a low oxygen concentration environment.

Analysis of calibration techniques for laser-induced incandescence measurements in flames

In order to obtain quantitative soot concentration results in flames using laser-induced incandescence (LII), different correction and calibration procedures are needed. In this work, two different calibration methodologies were applied to a laminar diffusion flame in order to obtain the most suitable calibration procedure for sooting conditions. First, a series of spatially resolved light extinction measurements were implemented and correlated to the laser-induced incandescence measurements under the same experimental conditions. Second, one single-point light extinction and the same laser-induced incandescence measurement were used to calculate a calibration constant for LII measurements by implementing an iterative method. Some correction routines have been performed to the data due to the non-homogeneities in the laser sheet, attenuation of the laser and attenuation of the incandescence signal itself due to the soot between the measurement plane and the detection system. Finally, an analysis of its applicability in real flames is detailed.

Experimental facility and methodology for systematic studies of cold startability in direct injection Diesel engines

Cold start at low temperatures in current direct injection (DI) Diesel engines is a problem which has not yet been properly solved and it becomes particularly critical with the current trend to reduce the engine compression ratio. Although it is clear that there are some key factors whose control leads to a proper cold start process, their individual relevance and relationships are not clearly understood. Thus, efforts on optimization of the cold start process are mainly based on a trial-and-error procedure in climatic chambers at low ambient temperature, with serious limitations in terms of measurement reliability during such a transient process, low repeatability and experimental cost. This paper presents a novel approach for an experimental facility capable of simulating real engine cold start, at room temperature and under well-controlled low speed and low temperature conditions. It is based on an optical single cylinder engine adapted to reproduce in-cylinder conditions representative of those of a real engine during start at cold ambient temperatures (of the order of −20 °C). Such conditions must be realistic, controlled and repeatable in order to perform systematic studies in the borderline between ignition success and misfiring. An analysis methodology, combining optical techniques and heat release analysis of individual cycles, has been applied.

Evaluation of combustion models based on tabulated chemistry and presumed probability density function approach for diesel spray simulation

Two combustion models of different complexity have been implemented in a RANS solver in the CFD platform OpenFOAM®. Both models rely on the flame prolongation of ILDM (FPI) method, which allows the use of detailed chemistry mechanisms at relatively low computational costs. The homogeneous auto-ignition (HAI) model, directly using the chemical data from the FPI tabulation, does not take into account subgrid turbulence-chemistry interaction. Therefore, the second, more advanced model combines the FPI method with a presumed conditional moment approach. This auto-ignition-presumed conditional moment (AI-PCM) model accounts for the fluctuations of the mixture fraction and the progress variable caused by the turbulent flow. Both models have been evaluated by means of a parametric study of a single diesel spray at varying initial temperatures and oxygen concentration levels. The results obtained with the CFD models have been compared with experimental data from the engine combustion network (ECN). The comparison of the two models demonstrates the important role of the subgrid turbulence-chemistry interaction on the accuracy of the auto-ignition process and the diesel flame structure, as indicated by the agreement of the AI-PCM predictions with the measured data.

Computational and Experimental Investigation of Interfacial Area in Near-Field Diesel Spray Simulation

Authors acknowledge that part of this work was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483-R) project.