Figure: 1 — Sketch of a spray momentum measurement rig and flow control volume THEORETICAL BACKGROUND

The global spray momentum flux measurement in steady state conditions is a well known technique that is used to evaluate the influence of the nozzle design on the fuel flow and the spray evolution. The global spray momentum flux is measured with an indirect method based on the evaluation of the spray impact force onto an orthogonal flat surface. The target diameter is larger than the spray structure to avoid loss of jet portions and it is positioned at a fixed distance from the nozzle; during the impact the spray is orthogonally deviated by the plate in order to nullify any axial velocity component. The application of the momentum conservation equation (Eq. (1)) to the control volume sketched in Figure: 1 ensures that, when the steady state conditions are achieved, the measured impact force must equate the global spray momentum flux.

F = dt tf-Vp dV + — Ikpp(v’n)dS (1)

For a transient injection analysis the complete momentum conservation equation (Eq. (1)) must be applied to the fluid control volume: the inertia of the fluid inside the control volume and the possibly non-zero velocity components of the fluid abandoning the control volume itself could be significant terms that affect the coincidence between what is measured in terms of impact force time-history and the momentum flux time-history at the nozzle exit.

In previous papers, the Authors discussed the application of the impact force measurement as an indirect method to estimate the spray global momentum flux also in transient conditions [12,13].

In order to further increase the spray investigation capabilities, a proper insight of the momentum flux distribution inside the spray structure would be of great importance, which implies the ability to correctly detect the spray momentum for small flow-tubes inside the spray flow structure, what in the following will be named as "Local Momentum Flux". Clearly the spatial integration of the local momentum flux should equate the "Global Momentum Flux" intended as the momentum flux of the entire spray flow structure. Kampmann et al. [5] showed the possible information obtainable by a very small diameter momentum probe (target diameter d= 2mm) positioned at various distances from the nozzle exit and in different angular position with respect to the spray nominal axis. In [5], the research targets were mainly the comparison of the spray atomization levels and the spray cone angle detection with different nozzle configuration, both of which were assumed to be efficiently indicated by the local value of the spray impact force on the target.

In the present research activity, a newly developed device will be presented which allows the local spray momentum distribution measurement for each single jet emerging from the nozzle of a Diesel injection system.

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