Assessment of large-eddy simulation feasibility in modelling the unsteady diesel fuel injection and mixing in a highspeed direct-injection engine

Abstract

This paper aims to assess the feasibility of largeeddy simulation LES as applied to a real engine configuration. Two cases of LES were performed together with a reference Reynoldsaveraged NavierStokes RANS simulation to study their performances in the prediction of the fuel injection and mixing in a highspeed directinjection diesel engine. In addition, sensitivity of the LES results with respect to mesh resolution was investigated, as LES requires a higher mesh density and the filtering process depends on the mesh size. The LES meant that detailed information could be obtained on the fuel spray development and turbulent flow field that was not possible to capture using the RANS modelling approach. It was found that the LES captures flow structures that have a pronounced impact on the prediction of the fuel injection and mixing. Highresolution LES predicted higher liquid and vapour penetration into the cylinder, which is in better agreement with the experimental data obtained at Delphi Diesel Systems than is the RANS prediction. It was also shown that using a finer mesh in LES yields improvements in turbulent flow field prediction, at the same time reducing contribution of the modelled part of the turbulent kinetic energy. Overall, it was proved that LES is a superior alternative to the traditional RANS approach for turbulent twophase flows encountered in diesel fuel injections where flow unsteadiness is prominent. Improved predictions obtained from an advanced LES can subsequently enhance the understanding of the fuel injection process and assist the engine experimentation and design.

Physical modelling and advanced simulations of gas–liquid two-phase jet flows in atomization and sprays

“…One of the major bottlenecks for industrial applications of LES is still the high computational cost compared to RANS, mainly due to the need for significant finer mesh and higher order numerical schemes. A comparative RANS/ LES study of diesel fuel injection and mixing using the same numerical methods [121] showed that the mesh resolution needed for LES could be 20 times higher than that for RANS, meaning that the computational time could increase about one hundred times on a serial machine since the time step becomes smaller when the mesh is refined. Fortunately, as computer power increases constantly, LES will become more affordable and more attractive.…”

“…-Limited to academic problems employed. Although a specific case was mentioned in Section 3 for comparison between RANS and LES [121] based on the same numerical methods, it cannot be regarded as a generalization because the cost is dependent on multiple parameters. Table 3 shows the expected changes in computational costs when gasÀliquid two-phase flows are concerned.…”

Large Eddy Simulation of Diesel Engine In-cylinder Flow

“…The same mesh is used for RANS, DES and LES simulations. The mesh resolution varies in the range 0.52 to 0.79 mm, with an average value of about 0.71 mm, which is comparable to that in [7,11,12,18] and relatively finer than that in previous LES computations, as in [19,20,38]. At the walls, a mesh refinement is employed with two layer of cells of 0.3 mm thickness.…”

Large eddy simulation of fuel injection and mixing process in a diesel engine

“…Bini and Jones [18] presented a jet laden with liquid droplets of acetone with the large eddy model. Also, several works have been conducted for the case of liquid spray in diesel engines [19][20][21][22].…”