Vortex ring-like structures in gasoline fuel sprays under cold-start conditions

A phenomenological study of vortex ring-like structures in gasoline fuel sprays is presented for two types of production fuel injectors: a low-pressure, port fuel injector (PFI) and a high-pressure atomizer that injects fuel directly into an engine combustion chamber (G-DI). High-speed photography and phase Doppler anemometry (PDA) were used to study the fuel sprays. In general, each spray was seen to comprise three distinct periods: an initial, unsteady phase; a quasi-steady injection phase; and an exponential trailing phase. For both injectors, vortex ring-like structures could be clearly traced in the tail of the sprays. The location of the region of maximal vorticity of the droplet and gas mixture was used to calculate the temporal evolution of the radial and axial components of the translational velocity of the vortex ring-like structures. The radial components of this velocity remained close to zero in both cases. The experimental results were used to evaluate the robustness of previously developed models of laminar and turbulent vortex rings. The normalized time, (t) over bar = t/t(init), and normalized axial velocity, (V) over bar (wx)((t) over bar) = V-wx((t) over bart(init))/V-wx(t(init)), were introduced, where t(init) is the time of initial observation of vortex ring-like structures. The time dependence of (V) over bar (wx) on (t) over bar was approximated as (V) over bar (wx)((t) over bar) = (t) over bar (-2.97) and (V) over bar (wx)((t) over bar) = (t) over bar (-1.14) for the PFI and G-DI sprays respectively. The G-DI spray compared favourably with the analytical vortex ring model, predicting (V) over bar (wx) = (t) over bar (-x), in the limit of long times, where alpha = 3/2 in the laminar case and alpha = 3/4 when the effects of turbulence are taken into account. The results for the PFI spray do not seem to be compatible with the predictions of the available theoretical models.