Flow characteristics of spray impingement in PFI injection systems

Abstract

Internal Combustion engines with multi-point, or sequential, fuel injection have a fuel injector for each cylinder, usually located so that they spray right at the intake valve. While in a fully warmed-up engine, fuel droplets vaporize before entering the combustion chamber, at cold start, a significant proportion of the injected gasoline deposits on the intake port and valve surfaces. The result is a deterioration of mixture homogeneity, decreased metering accuracy and increased hydrocarbon emissions. A better control of mixture preparation at cold start conditions can be achieved if the interaction between the gasoline spray and the time-varying liquid film which forms during the period of injection can be accurately predicted. The work reported here considers part of an experimental study performed with that objective in a purposed built experiment considering a simplified geometry.Droplet characteristics measured with a PDA system in the vicinity of the impact surface, prior and right after the impact, are used to infer about the transient spray-wall interaction mechanisms when a liquid film forms. Conservation laws allow to derive the fractions (α θ ) of mass, momentum and energy from the spray prior to impact that are transferred to/from the liquid film. Most of the inflowing mass deposits within the core of the spray and transfers momentum to the liquid film in the radial direction, influencing the velocity and direction of secondary droplets formed at impingement. The use of α θ , together with the mass flux ratio of outgoing to inflowing droplets in existing empirical correlations to predict the impingement outcome, is shown to improve prediction of flux ratios, either of number, momentum or energy. These fractions are suggested as a simple way of including the influence of the liquid film in spray/wall interaction and their modelling is the scope of future work. LFS SS ST LFS SS ST

Advances and challenges in explaining fuel spray impingement: How much of single droplet impact research is useful?

“…Although this has been relatively successful for continuous water sprays, Panão and Moreira [30], confirmed that it cannot accurately describe intermittent fuel sprays. Furthermore, a form of predicting the size and velocity distributions of secondary droplets was also developed but unfortunately it does not include any elements resulting from the previous analysis on interaction phenomena.…”

“…Therefore, most quantitative and qualitative results are obtained assuming that interaction effects are implicit. This approach was followed in Panão and Moreira [30], and it was possible to show a 'cross-talk' effect between the impacting spray momentum and the film radial spreading, further imparting momentum to secondary droplets in outward regions. This interaction result could only be observed when the analysis considered the spray as a whole.…”

“…However, simulation models incorporating the criteria previously described to identify the hydrodynamic regimes of impact do not always provide accurate results, mostly because of their inability to allow for the complex interactions which occur in very dense sprays. The efforts are then directed towards obtaining averaged values from the integration of the rate equation for the mass conservation, e g. Panão and Moreira [30]; in this way the mass of liquid deposited, m dep , can be related to the mass of droplets impacting onto the surface, m b , and the mass of secondary droplets, m a , as:…”

Heat transfer at multiple-intermittent impacts of a hollow cone spray

“…In dense sprays, however, the behaviour of single drop impacts is altered by multiple drop interactions, such as those occurring between droplets in the vicinity of each other [3][4][5] and between consecutive drop impacts [6]. Moreover, when the mass rate of liquid adhered to the surface overcomes the vaporization rate, a liquid film forms which spreads over the surface; crown development and thus secondary atomization is mainly influenced by local variations of the liquid film induced by drop impact; re-atomized droplets may emerge from liquid jets induced by pressure fluctuations generated by multiple drop impacts in the liquid film; interfacial activity in the vicinity of the wall caused by successive injections induce vertical oscillations, which may trigger a chain of ejection-and-re-impaction of droplets with a rate faster than the injection frequency [7,8].…”

A numerical analysis of drop impact on liquid film by using a level set method

“…Drop impact phenomena on a surface widely occurs in practical engineering fields, which include spray cooling [1], spray coating [2], jet printing [3] and fuel injection in an internal combustion engine [4], among others. Drop behavior differs upon impact on a dry or wet surface.…”