Experimental and analytical study on vapor phase and liquid penetration for a high pressure diesel injector

[EN] In this study, a macroscopic characterization has been performed on a solenoid diesel injector (2200 bar-8 hole nozzle) under various non-reacting but evaporative conditions. For vapor penetration a two pass Schlieren visualization set up was selected. A high speed camera was used to record high speed images of the injection event to analyze the transient evolution of the vapor phase of the spray. The transient liquid penetration of the spray has been measured via MIE-Scattering imaging technique using a high speed camera as well. Unsteady RANS based CFD Simulations have been performed to simulate the experimental conditions and correlation results are presented. Built-in models from commercial code StarCD have been used to model spray formation which includes submodels for turbulence, nozzle flow, break-up and fuel properties. A novel CAE process using an automation and optimization tool has been used to achieve robust model settings, and the final model prediction are compared with the experimental observation for the injector characterization with respect to the non-reacting spray penetration with change in ambient and injection conditions. The model correlates well with the sensitivities for temperature and injection pressures qualitatively however improvements required to capture the density effects mainly related to the mesh orientation, fixed time step size where further analysis required.This research has been partially funded by FEDER and Spanish Ministerio de Economia y Competitividad through project TRA2015-67679-c2-1-R. Additionally Jhoan Sebastian Giraldo had a grant FPI-SUB 2 from Universitat Politecnica de Valencia.Payri, R.; Giraldo-Valderrama, JS.; Ayyapureddi, S.; Versey, Z. (2018). Experimental and analytical study on vapor phase and liquid penetration for a high pressure diesel injector. Applied Thermal Engineering. 137:721-728. https://doi.org/10.1016/j.applthermaleng.2018.03.097S72172813

Effect of liquid break-up model selection on simulated diesel spray and combustion characteristics

Accurate modelling for spray vapour fields is critical to enable adequate predictions of spray ignition and combustion characteristics of non-premixed reacting diesel sprays. Spray vapour characteristics are in turn controlled by liquid atomization and the KH-RT liquid jet break-up model is regularly used to predict this: with the KH model used for predicting primary break-up given its definition as a surface wave growth model, and the RT model used for predicting secondary break-up due to it being a drag based, stripping model. This paper investigates how the alteration of the switching position of the KH and RT sub-models within the KH-RT model impacts the resulting vapour field and ignition characteristics. The combustion prediction is handled by the implementation of a 54 species, 269 reaction skeletal mechanism utilising a Well Stirred Reactor model within the Star-CD CFD code. Following on from the derivation and implementation of an Ohnesorge based switch between the KH and RT sub-models, this model is now tested in igniting cases for an n-dodecane fuelled single holed injection representing the ECN “Spray A” condition, and is compared to the baseline Reitz-Diwakar model. Differences in flame behaviour, particularly within the temperature distribution, are seen and directly traced from the effect of liquid break-up position and model selection, through atomised droplet size distribution and mixture fraction distribution. Different criteria for judging the ignition delays and lift-off-lengths are compared, with all methods predicting very similar results for both models. The KH and RT sub-models are also tested against each other, with heavy instabilities seen when the RT model is solely applied to the spray. This correlates with the instabilities shown in the vapour fields, suggesting the enabling of the RT model near-nozzle is to be avoided

Manifold resolution study of the FGM method for an igniting diesel spray

Spray H (baseline n-heptane) cases of Engine Combustion Network (ECN) are investigated with the Flamelet Generated Manifold (FGM) method. In the FGM method, all thermo-chemical properties are stored as a function of controlling variables, here the mixture fraction and the progress variable. Two approaches are used to construct the FGM tables, igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR). To capture ignition delay times in a reliable way, a sensitivity study is performed to determine the optimal FGM table properties. It is observed that using quadratic discretization in progress variable space, i.e. more points in early stages of chemistry, is essential to predict accurate ignition timings. Once quadratic spacing is applied, the ignition delay time results show good correlation with those of experiments. This is observed with both ICDF- and HR-based FGM tables. The similarity between ignition delay results of two approaches indicates that the effect of mixing on chemistry is low and the diffusion process plays a minor role at elevated ambient conditions such as in conventional diesel engines. Finally, the effect of including the mixture fraction variance is analyzed and the impact is found mainly on the lift-off length. The ignition delay is hardly affecte

Manifold resolution study of the FGM method for an igniting diesel spray

Spray H (baseline n-heptane) cases of Engine Combustion Network (ECN) are investigated with the Flamelet Generated Manifold (FGM) method. In the FGM method, all thermo-chemical properties are stored as a function of controlling variables, here the mixture fraction and the progress variable. Two approaches are used to construct the FGM tables, igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR). To capture ignition delay times in a reliable way, a sensitivity study is performed to determine the optimal FGM table properties. It is observed that using quadratic discretization in progress variable space, i.e. more points in early stages of chemistry, is essential to predict accurate ignition timings. Once quadratic spacing is applied, the ignition delay time results show good correlation with those of experiments. This is observed with both ICDF- and HR-based FGM tables. The similarity between ignition delay results of two approaches indicates that the effect of mixing on chemistry is low and the diffusion process plays a minor role at elevated ambient conditions such as in conventional diesel engines. Finally, the effect of including the mixture fraction variance is analyzed and the impact is found mainly on the lift-off length. The ignition delay is hardly affecte

Application of the FGM method to Spray A conditions of the ECN database

Modeling turbulent diesel spray combustion which combines complex flow and transport phenomena with combustion event including a vast amount of species and reactions is a major challenge. The Flamelet Generated Manifold (FGM) method is a promising technique to model reacting flows using tabulated chemistry approach. The method is adopted for diesel spray combustion by tabulating chemistry as a function of the mixture fraction (Z) and a reaction progress variable ( Y ). In previous work, the method has been successfully applied to simulate Spray H cases as defined by the engine combustion network (ECN). Two different tabulation approaches (ignit- ing counterflow diffusion flames (ICDF) and homogeneous reactors (HR)) were investigated and compared to the available experimental data of the ECN. In this paper, the FGM method is applied to simulate Spray A conditions of the ECN. First, the sensitivity of the spray sub-models (atomization and breakup models) is studied for the non-reacting case of the Spray A setup. Later, the FGM approach is applied on the reacting case for FGM’s generated with two different n-Dodecane reaction mechanisms, using two tabulation approaches, and with and without inclusion of a turbulent closure (PDF approach based on variance of Z). The 3D-RANS (Reynolds Averaged Navier-Stokes) simulations are performed with the commercial CFD code STAR-CD. The combustion results are analyzed by comparing the simulated and measured ignition delays and lift-off lengths. One mechanism results in ignition for all simulations, whereas the other mechanism does not. It was found that this can be attributed to the different sensitivity of the mechanisms to the strain rate. In general, HR tabulation predicts shorter ignition delay and lift-off length (LOL) than the ICDF in line with the observations from previous work. The atomization model does not show major effect on ignition delay however it affects the LOL significantly in both tabulation approaches. Inclusion of the turbulent closure does not affect ignition delay or LOL predictions. In general compared to the experiments, the ICDFs slightly over predict whereas the HRs systematically under-predicts

Modeling fuel spray auto-ignition using the FGM approach : effect of tabulation method

The Flamelet Generated Manifold (FGM) method is a promising technique in engine combustion modeling to include tabulated chemistry. Different methodologies can be used for the generation of the manifold. Two approaches, based on igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR) are implemented and compared with Engine Combustion Network (ECN) experimental database for the baseline n-heptane case. Before analyzing the combustion results, the spray model is optimized after performing a sensitivity study with respect to turbulence models, cell sizes and time steps. The standard High Reynolds ( Re ) k-e model leads to the best match of all turbulence models with the experimental data. For the convergence of the mixture fraction field an appropriate cell size is found to be smaller than that for an adequate spray penetration length which appears to be less influenced by the cell size. With the optimized settings, auto-ignition and flame lift-off length are analyzed. In general, both techniques capture the qualitative trend of experimental results. However, typically, the HR tabulation method predicts shorter ignition delay and LOL results than the ICDF method. In a quantitative sense, the ICDF and HR methods give better results in LOL and auto-ignition predictions, respectively