No abstract

This work evaluates the performance of the Σ-Y Eulerian atomization model at reproducing the internal structure of a diesel spray in the nearfield. In the study, three different computational domains have been used in order to perform 3D and 2D coupled simulations, where the internal nozzle flow and external spray are modeled in one continuous domain, and 2D decoupled simulations, where only the external spray is modeled. While the 3D simulation did the best job of capturing the dense zone of the spray, the 2D simulations also performed well, with the coupled 2D simulation slightly outperforming the decoupled simulation. The similarity in results between the coupled and the decoupled simulation show that internal and external flow calculations can be performed independently. In addition, the use of spatially averaged nozzle outlet conditions, in the case of an axisymmetric (single-hole) convergent nozzle, leads to a slightly worse near-field spray predictions but to an accurate far-field ones. Finally, a novel constraint on turbulent driven mixing multiphase flows is introduced which prevents the slip velocity from exceeding the magnitude of the turbulent fluctuations through a realizable Schmidt number. This constraint increased model stability, allowing for a 4x increase in Courant number.

A comparison between the Σ-Y atomization model and a classical DDM approach has been carried out for diesel spray computational fluid dynamics (CFD) simulations. The Σ-Y model, originally proposed by Vallet and Borghi, is based on a Eulerian representation of the spray atomization and dispersion by means of a single-fluid variable density turbulent flow. The locally homogeneous flow approach has been applied to develop a spray vaporization model based on state relationships. A finite-volume solver for model equations has been created using the OpenFOAM CFD open-source C++ library. In the case of the Lagrangian-discrete droplet method (DDM) approach, the original dieselFoam solver of OpenFOAM is used. Model predictions have been compared to experimental measurements of free diesel sprays under vaporizing conditions from the database of the Engine Combustion Network (ECN). Accurate predictions of liquid and vapor spray penetration, as well as mixture fraction, can be achieved for the nominal condition with both models, although DDM simulations tend to be less accurate. Additionally, the near nozzle flow structure of the Spray A condition of ECN is also studied with both models. The conclusion is a more accurate prediction of the near-field internal structure of the spray in the case of the Eulerian model, due to both a higher mesh resolution and a more adequate modeling approach. Consequently, results shown in this work put in evidence the benefits of using a Eulerian model to predict qualitatively and accurately the diesel spray behavior under different ambient conditions and injection pressures.

ElsevierMolina Alcaide, SA.; García Martínez, A.; Pastor Enguídanos, JM.; Belarte Mañes, E.; Balloul, I. (2015). Operating range extension of RCCI combustion concept from low to full load in a heavy-duty engine. Applied Energy. 143:211-227. doi:10.1016/j.apenergy.2015.01.035. overlimit function is used to select the best engine settings for each operating point. 23Finally, engine emissions and performance results from that RCCI operation are 24 compared with conventional Diesel combustion (CDC). 25Results suggest that double injection strategies should be used for RCCI operation from 26 low to mid load. However, from high to full load operation, single injection strategies 27 should be used, mainly to avoid excessive in-cylinder pressure gradients. In addition, it 28 is confirmed the suitability of RCCI combustion to overcome the soot-NOx trade-off 29 characteristic of CDC, from 6 to 24 bar of BMEP, while improving fuel consumption. 30

Simulating liquid spray first and second atomization is not an easy task. Many models have been developed over the past years, but Eulerian ones have proved their better performance for the dense zone of the spray. In this work a new compressible Eulerian model is used to compute the internal flow together with the spray. Up to five two-equation turbulence models have been tested and its influence is remarkable in terms of spray behavior, but also greatly affects the mass flow rate and the momentum flux. At the end, SST k − ω model proves to be best than the others. Additionally, different types of inlet boundary conditions have been also tested and analyzed. Results when compared with previously obtained experimental data show that the commonly used for external flow time-varying velocity boundary condition gives also good performance for the internal flow.

Elsevier Naud, B.; Novella Rosa, R.; Pastor Enguídanos, JM.; Winklinger, JF. (2015). RANS modelling of a lifted H2/N2 flame using an unsteady flamelet progress variable approach with presumed PDF. Combustion and Flame. 162 (4) AbstractAn unsteady flamelet / progress variable (UFPV) approach is used to model a lifted H 2 /N 2 flame in a RANS framework together with presumed PDF. We solve the unsteady flamelets both in physical space and in mixture fraction space. We show that in the former case, the scalar dissipation rate profile strongly varies in time (while it is assumed to be fixed in time in the latter). However, this does not result in significant qualitative differences in the corresponding flamelet libraries. The progress variable is carefully defined, including both the main combustion product (H 2 O) and a key radical species in ignition process (HO 2 ). The presumed-PDF model is proposed in terms of the non-normalised progress variable, without assuming its statistical independence with mixture fraction. We introduce a modelled transport equation for the mean progress variable which is consistent with the basic underlying UFPV assumption, derived from the Lagrangian flamelet model. The influence of different model parameters on the results for the mean temperature and mean species mass fractions and their fluctuations is discussed. Good results are obtained for the conditions of the considered lifted flame where detailed experimental data is available. However, at low coflow temperature the modelled flame lift-off height is shorter than expected.

This work investigates the effects of the piston bowl geometry on RCCI heat transfer and combustion losses and its repercussion on the engine efficiency. For this purpose, three piston geometries with compression ratio 14.4:1 have been studied and compared by means of computational modeling. In addition, the engine operating conditions proposed at low, medium and high load were also validated experimentally in a heavy-duty single-cylinder engine adapted for dual fuel operation. The engine speed was kept constant at 1200 rev/min during the research. Results suggest that heat flux through the piston surface represent the major portion of the heat transfer energy. Thus, the comparison of the three geometries demonstrates that reduced piston surface area and reduced charge motion, are the key factors to improve the gross indicated efficiency over the different engine loads. Moreover, it is found that a shallow piston geometry with a smooth transition from the center to the squish region, with a 16% reduced surface area, strongly improves the gross work at low load. However, this gain diminishes due to increased combustion losses as engine load increases. Finally, an intermediate geometry was confirmed as the best balanced piston geometry for RCCI operation over the three different loads.

A mis padres, Josep y Empar, porque han estado siempre presentes en todas las dificultades que enfrenté. Os debo mucho porque sois ejemplos de carácter, esfuerzo, cariño y dedicación. A mi hermano Carles que es una gran persona, que merece mucho la pena, y que es un compañero de infatigables aventuras. A mi mujer Maria José que ha estado a mi lado todo este tiempo, ayudándome durante el doctorado, con su dedicación, cariño y paciencia. Gracias por tu optimismo. Mi hijo Pau que ha sido una fuente de inspiración, de diversión y una bocanada de aire fresco.