Abstr:Compression of vortex in cavity
Underlying Flow Regime 4-13
A compression of a vortical turbulent flow inside a cavity is a typical unsteady flow, which can be found in a combustion chamber of reciprocating engine. It has given rise to interesting flow phenomena such as vortex breakdown, which are not yet fully understood.
The motion of air within the cylinder of internal combustion engines has been identified to affect both engine performance and emissions. The in-cylinder flow control is achieved through the design of inlet ports, the cylinder head and the piston crown geometry. A combination of the aforementioned geometric parameters results in both large-scale air motion like tumble or swirl and small-scale turbulence. At the end of compression, this large-scale vortex motion breaks up and generates high turbulent fluctuations. The understanding of the dynamics of these compressed flow structures is fundamental to optimise combustion and emissions.
In the present analysis, we were especially interested in the compression of a tumble like vortex, which leads to higher levels of turbulence at the end of compression, when the tumbling motion breakdown occurs. This underlying flow regime is of special relevance for the application challenge TA2-05.
The tumble-like flow in combustion engine was described for the first time at the beginning of the 80s, Gosman A.D., 1985 & Arcoumanis C., 1987. It is a result of the interaction of the jets from the valves and the chamber walls. The flow is reflected at first by the wall of the cylinder then by the piston to generate a vortical structure with high angular moment, Khalighi B., 1990. At the end of the intake stroke, a clear large-scale vortex occupies the whole volume of the chamber. This coherent structure, which persists after the filling is terminated, is subjected to compression forces exercised by the ascendant piston.
As the compression progresses, the vortex is confined in a smaller and smaller volume and, according to the angular moment conservation principle, its rate of rotation increases. However, when the volumetric ratio reaches a limit value, the vortical structure becomes unstable. Not having sufficient space to remain stable, it undergoes a complex disintegration by releasing the kinetic energy that it had stored. As the axial swirl is not destroyed in the same way, it tends to lead to the generation of less turbulence. Through LDA measurements, Arcoumanis C. et al., 1990 found that a pure tumbling motion is able to produce a 42% enhancement in turbulence intensity, while a 24% enhancement in turbulence can be achieved under the combined tumbling/swirling vortical flow particularly generated in their experiments.
The reasons for the disruption of the vortex are not yet fully understood. This process is difficult to analyse because of the presence of other physical phenomena, such as the cyclic variation that can interfere with the mechanisms of the vortex breakdown or hide them, Marc D., 1998.
More and more experimental test benches with real IC engine chambers have been reported recently, Baby X., 1997. Even for the most advanced experimental set-ups, modifications of the real geometry are still required because of the difficulties to access the whole flow field to obtain precise and resolved measurements. In addition, the IC engine cylinders geometry involves moving parts and complex boundary conditions that make the flow field difficult to mesh and numerically difficult to solve. The most detailed numerical modelling simulations up to now have been carried out with test cases with simplified geometries.
Among these experimental set-ups, the "Motor-Driven central-Fixed-Valve Internal Combustion Engine" supported a large set of experimental analyses, Morse et al., 1979. It has a fixed valve with a valve gap of 4 mm in radial direction, and opening at 30 degrees from the axis. The piston is driven as a simple harmonic motion at a rate of 200 RPM, which leads to the mean piston velocity of 40 cm/s, and the Reynolds number based on the mean piston velocity and the bore diameter, 7.5 cm, is around 2000. More details could be found in Verzicco R. et al., 2000. This test case was used for several CFD validations, Haworth D.C et al., 2000, Celik I. et al., 1999 and Sone K., 2001.
The experimental device that is used in the test-case for the Underlying flow regime is a compression engine with simplified geometry with suitable optical access available at IMFT (Institut de Mécanique des Fluides de Toulouse). It was specially designed to study the vortex behaviour during intake and compression. It is well-adapted as a test case to validate this underlying flow regime, Borée J., 1997-2000, Reize J., 2000., Devesa et al 2004, Moreau et al. 2004.
This set-up consists of a rectangular chamber where the intake flow enters the chamber through an opening adjacent to one edge of the chamber and facing the piston. The experiment was not designed to reproduce all the complexity of in-cylinder flow but to focus on the compression of the tumbling vortex behaviour Borée J. 2002. The reasons for choosing this test case are:
- the configuration provides clear picture of the evolution the compressed vortex and its disruption
- widely tested, this experimental bench has an established experimental data-base ( CD Rom available)
- it is a practical case for experimental characterization and CFD validation because of its simple geometry with a full optical access and well defined boundary conditions
Contributors: Afif Ahmed - RENAULT