Modelling of Gasoline Spray Impingement

This paper is concerned with aspects of gasoline spray droplet impingement simulation, for which an improved model has been developed for predicting the outcomes of spray droplets impacting on a wall. The model is assessed by simulating experiments on oblique spray impingement in a wind tunnel. This is done with the aid of a specially-devised procedure for deducing the specification of initial droplet sizes and velocities at the gasoline injector from downstream measurements. The calculated wall spray characteristics show favourable agreement with the measurements. INTRODUCTION Spray impingement on walls occurs in a wide range of situations. These include various types of reciprocating engines, gas turbines, spray cooling systems, ink-jet printing and many other applications in agriculture and medicine. In small high speed direct-injection Diesel engines, the fuel is directly injected into the compact chamber at a high speed and strikes the walls due to short free spray path. In direct injection spark ignition engines, the fuel droplets may also impact onto the combustion chamber walls for similar reasons. In port fuel injection gasoline engines, it is often the case that a large portion of the spray impinges on the walls of the port or the intake valve(s). In the aforementioned applications the hydrodynamic, and in some cases thermal, characteristics of the post-impingement droplets are important issues in injection system design. Of particular interest in the engine context are : (i) the total fuel mass, momentum and energy deposited as and to wall films which, if not evaporated completely during combustion, are responsible for contributing to enhanced levels of unburnt hydrocarbon emissions [1, 2]; and (ii) the fuel vapour distribution in the near-wall region. The latter is closely related to the flame quenching phenomenon [3] which occurs when the flame closely approaches the cool walls of the chamber. The unburnt fuel may also contribute to emissions. A basic physical picture of the impingement process can be constructed by considering a single impinging droplet (see Figure 1) which, following its impact on a solid surface, first undergoes deformation and spreads out at a certain velocity under the impingement-induced pressure gradients. This spreading flow either remains stable or becomes unstable, leading to different impingement regimes (eg. stick, spread, rebound, and splash) as observed experimentally [4– 7]. The outcome is, as described in detail by Bai [8], determined by a number of parameters characterising the impingement conditions. These include droplet diameter dI , temperature Td, velocity VI , incidence angle θI wall temperature Tw as well as fluid properties such as viscosity μ, density ρ and surface tension σ. Also important are the surface roughness rs and, if present, pre-existing wall film thickness δ0 and gas boundary layer characteristics in the near-wall region. These quantities may be combined to yield a number of dimensionless parameters [8]. The most important are: (i) droplet Weber number We = ρV 2 INdI/σ, (where VIN is droplet incident normal impact velocity); and (ii) droplet Laplace number La = ρσdI/μ .