Rate-of-momentum measurements of transient fuel sprays are valuable for improving current combustion and emission strategies. This data provides boundary conditions for engine computational fluid dynamic (CFD) simulations and provides insight into the transient mixing characteristics of the spray prior to and during combustion. Previous researchers have quantified the rate-of-momentum of transient sprays using the impingement technique, but uncertainties remain in relating the impingement force to the injected fuel momentum at the nozzle exit. Rate-of-momentum measurements are typically performed by directing a spray onto the face of a calibrated transducer in close proximity to the nozzle. The measured impingement force is then used to quantify the rate-of-momentum at the nozzle orifice exit with the aid of a simplified control volume analysis. However, under elevated back pressures, additional terms in the control volume analysis are no longer negligible. Other non-idealities, such as non-orthogonal droplet impingement outcomes and transient mass accumulation in the control volume, can also contribute to errors in the simplified analysis.
This paper investigates the impact of non-idealities in impingement-based rate-of-momentum measurements on the quantified fuel injection rate. In specific, we compare the measured rate-of-momentum under back pressure and atmospheric pressure using two different transducers to quantify uncertainties that can arise under back pressure conditions. Uncertainties associated with transient mass accumulation and non-orthogonal spray deflection are also investigated. We found that back pressure affected both the start and end of injection when compared to atmospheric pressure. Under back pressure, there was a lengthened apparent start-of-injection transient, which likely results from a low pressure toroidal vortex occurring at the head of the spray. In addition, there was a longer apparent closing transient, which is likely a result of residual pressure distribution after the end-of-injection. No evidence of transient mass accumulation was observed for the injectors used in this study. Lastly, the transient spray was observed to deflect non-orthogonally from the impact point on the transducer instead of remaining parallel to the transducer face after initial impact. This deflection of the spray leads to uncertainties when quantifying the rate-of-momentum, where the apparent rate-of-momentum could be larger than the actual value.