Development of Methodology for Spray Impingement Simulation

A computer model was used to study the impingement of sprays on walls. The spray model accounts for the effects of drop breakup, drop collision and coalescence, and the effect of drops on the gas turbulence. A new submodel was developed to describe the spray/wall interaction process. Predictions of the effect of engine swirl, ambient gas pressure (density), wall inclination angle and the distance from the nozzle to the wall, were in good qualitative agreement with the experiments

Modeling Engine Spray/Wall 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 .

Modeling of gasoline spray impingement

Mathematical Modelling of Wall Films Formed by Impinging Sprays

Dynamics of droplet impact on solid surface with different roughness

The purpose of this paper is to clarify the mechanisms of unburnt hydrocarbon (HC) emissions from a small direct - injection (DI) diesel engine. HC emission levels of small DI diesel engines are considerably higher than those of corresponding indirect - injection (IDI) diesel engines, even when sacless injection nozzles that are effective in reducing HC emissions are installed on them. In this study, analytical engine tests were performed to evaluate the relative significance of various potential sources of HC emissions from a small DI diesel engine equipped with sacless type injectors. The results suggest the major sources of HC emissions to be 1) over-lean mixture formation beyond the lean limit of combustion during the ignition delay period, 2) spray impingement on the combustion chamber wall, the phenomenon of which may be closely related to the spray induction length, 3) spray droplets of relative larger size due to poor spray atomization in the final injection stage, and 4) flame quenching or incomplete combustion in the narrow clearance between the piston top and the cylinder head.

Kunihiko Sugihara

Papers

Sources of Hydrocarbon Emissions from a Small Direct Injection Diesel Engine