Topology and distinct features of flashing flow in an injector nozzle

The effect of thermodynamic non-equilibrium conditions (liquid superheat) on the two-phase flow field developing inside an axisymmetric, single-orifice nozzle is numerically investigated by means of different variations of a two-phase mixture model. A number of "hybrid" mass-transfer models that take into account both the effect of inertial forces (cavitation) and liquid superheat have been proposed and evaluated against widely used, pure-cavitation models, in order to pinpoint the flow conditions necessary for flash boiling to occur and to elucidate the distinct features of the phase and velocity fields that characterize flashing flows. The effect of the number of nucleation sites, required as an input by the models, on the developing two-phase flow has also been looked into. The numerical results have shown that incorporation of an additional term corresponding to liquid superheat into the mass-transfer rate leads to increased evaporation rate, compared to pure-cavitation models with liquid vaporization taking place within the entire nozzle cross section. The cavitation nucleation sites have been confirmed to act as the necessary flow perturbations required for flash boiling to occur. In addition, the developing velocity field has been found to be in close correlation to the mass-transfer rate imposed. It has been established that increased liquid evaporation leads to choked-flow conditions prevailing in a larger part of the nozzle and accompanied by a more significant expansion of the two-phase mixture downstream of the injector exit that results to increased jet cone angle. Finally, the results demonstrated that liquid cooling due to the increased mass-transfer rate is not significant within the nozzle and thus consider that a constant liquid temperature produces adequately accurate results with a decreased computational cost

Measurement and evaluation of near-field spray kinematics for nozzles with asymmetrical inlet geometries

In diesel engines, fuel injection parameters have a commanding effect on mixing andcombustion quality. This research aims to enhance the fundamental knowledge of fuelsprays and their primary break-up. In addition, this research provides statistical data tovalidate simulation models and improve the prediction accuracy in mixing and combustion.This thesis report is based on evaluating the behavior and velocity profiles of near-fieldsprays generated by different inlet geometries under a range of injection pressures. Thestudied nozzles include single-hole nozzles with on-axis and off-axis orifices and a two-holenozzle with angled orifices. We applied time-gated ballistic imaging to capture high-resolutionspray images at the near-field. These high-resolution images provide a clearliquid/gas interface, which enables tracking of the spray structures. Furthermore, thedisplacement of the spray interface in two consecutive images over a specific time frameyields spray kinematics in two dimensions.The results show how velocity measurements can describe spray development and evolution.Asymmetrical inlet geometries significantly affect near-field spray profile and targetingbecause the distribution of velocity magnitude on the two sides of the spray is notsymmetric. In addition to inlet geometry, internal flow characteristics play a significantrole in spray behavior. The outlook for this project mainly consists of the validationand development of simulation models. The obtained results provide an opportunityto correlate the near-field spray to the internal nozzle flow and study the effect ofasymmetrical inlets on the internal flow

Flow in valve covered orifice nozzles with cylindrical and tapered holes and link to cavitation erosion and engine exhaust emissions

Results from a research programme addressing the development, testing, and production of valve covered orifice (VCO) nozzles operating with current production Tier 3 offhighway diesel engines are reviewed. The common rail injectors operate at pressures exceeding 1300 bar and include pilot and main injection events. Although acceptable engine exhaust emissions can be obtained with conventional VCO nozzles, cavitation erosion may lead to mechanical failure of the nozzle. Redesigning the injector in terms of its durability against surface erosion has been obtained through use of a computational fluid dynamics (CFD) flow solver incorporating a two-phase cavitation model and flow visualization in enlarged transparent nozzle replicas. The model has provided evidence of the flow distribution under realistic pressure and needle lift opening scenarios while at the same time it has been calibrated to indicate the locations where the possibility of cavitation erosion may become significant. The experiments performed in enlarged transparent nozzle replicas have provided evidence of the string cavitation structures formed inside the different nozzle designs. Crosscorrelation with engine emission tests indicates that string cavitation may be associated with increased engine exhaust emissions. Proposed injector designs with geometric modification easily implemented in the production series have been proved to be erosion-free while at the same time have improved the engine exhaust emissions

Approaches for Detailed Investigations on Transient Flow and Spray Characteristics during High Pressure Fuel Injection

High pressure injection systems have essential roles in realizing highly controllable fuel injections in internal combustion engines. The primary atomization processes in the near field of the spray, and even inside the injector, determine the subsequent spray development with a considerable impact on the combustion and pollutant formation. Therefore, the processes should be understood as much as possible; for instance, to develop mathematical and numerical models. However, the experimental difficulties are extremely high, especially near the injector nozzle or inside the nozzle, due to the very small geometrical scales, the highly concentrated optical dense spray processes and the high speed and drastic transient nature of the spray. In this study, several unique and partly recently developed techniques are applied for detailed measurements on the flow inside the nozzle and the spray development very near the nozzle. As far as possible, the same three-hole injector for high pressure diesel injection is used to utilize and compare different measurement approaches. In a comprehensive section, the approach is taken to discuss the measurement results in comparison. It is possible to combine the observations within and outside the injector and to discuss the entire spray development processes for high pressure diesel sprays. This allows one to confirm theories and to provide detailed and, in parts, even quantitative data for the validation of numerical models

Thermal effects influence on the Diesel injector performance through a combined 1D modelling and experimental approach

[EN] The injection system is one of the topics that has been paid most attention to by researchers in the field of direct injection diesel engines, due to its key role on fuel atomization, vaporization and air-fuel mixing process, which directly affect fuel consumption, noise irradiation and pollutant emissions. The increasing injection pressures in modern engines have propitiated the need of studying phenomena such as cavitation, compressible flow or the effect of changes in the fuel properties along the process, whose relative importance was lower in early stages of the reciprocating engines development. The small dimensions of the injector ducts, the high velocities achieved through them and the transient nature of the process hinder the direct observation of these facts. Computational tools have then provided invaluable help in the field. The objective of the present thesis is to analyse the influence of the thermal effects on the performance of a diesel injector. To this end, the fuel temperature variation through the injector restrictions must be estimated. The influence of these changes on the fuel thermophyisical properties relevant for the injection system also needs to be assessed, due to its impact on injector dynamics and the injection rate shape. In order to give answer to the previous objectives, both experimental and computational techniques have been employed. A dimensional and a hydraulic experimental characterization of a solenoid-actuated Bosch CRI 2.20 injector has been carried out, including rate of injection measurements at a wide range of operating conditions, with special attention to the fuel temperature control. A 1D computational model of the injector has been implemented in order to confirm and further extend the findings from the experiments. Local variations of fuel temperature and pressure are considered by the model thanks to the assumption of adiabatic flow, for which the experimental characterization of the fuel properties at high pressure also had to be performed. The limits of the validity of this assumption have been carefully assessed in the study. Results show a significant influence of the fuel temperature at the injector inlet on injection rate and duration, attributed to the effect of the variation of the fuel properties and to the fact that the injector remains in ballistic operation for most of its real operating conditions. Fuel temperature changes along the injector control orifices are able to importantly modify its dynamic behaviour. In addition, if the fuel at the injector inlet is at room temperature or above, the temperature at the nozzle outlet has not been proved to importantly change once steady-state conditions are achieved. However, a significant heating may take place for fuel temperatures at the injector inlet typical of cold-start conditions.[ES] El sistema de inyección es uno de los elementos que más interés ha despertado en la investigación en el campo de los motores diésel de inyección directa, debido a su papel clave en la atomización y vaporización del combustible así como en el proceso de mezcla, que afectan directamente al consumo y la generación de ruido y emisiones contaminantes. Las crecientes presiones de inyección en motores modernos han propiciado la necesidad de estudiar fenómenos como la cavitación, flujo compresible o el efecto de los cambios de las propiedades del combustible a lo largo del proceso, cuya importancia relativa era menor en etapas tempranas del desarrollo de los motores alternativos. Las pequeñas dimensiones de los conductos del inyector, las altas velocidades a través de los mismos y la naturaleza transitoria del proceso dificultan la observación directa en estas cuestiones. Por ello, las herramientas computacionales han proporcionado una ayuda inestimable en el campo. El objetivo de la presente tesis es analizar la influencia de los efectos térmicos en el funcionamiento de un inyector diésel. Para tal fin, se debe estimar la variación de la temperatura del combustible a lo largo de las restricciones internas del inyector. La influencia de estos cambios en las propiedades termofísicas del combustible más relevantes en el sistema de inyección también debe ser evaluada, debido a su impacto en la dinámica del inyector y en la forma de la tasa de inyección. Para dar respuesta a estos objetivos, se han utilizado técnicas experimentales y computacionales. Se ha llevado a cabo una caracterización dimensional e hidráulica de un inyector Bosch CRI 2.20 actuado mediante solenoide, incluyendo medidas de tasa de inyección en un amplio rango de condiciones de operación, para lo que se ha prestado especial atención al control de la temperatura del combustible. Se ha implementado un modelo 1D del inyector para confirmar y extender las observaciones extra\'idas de los experimentos. El modelo considera variaciones locales de presión y temperatura del combustible gracias a la hipótesis de flujo adiabático, para lo cual también se ha tenido que llevar a cabo una caracterización experimental de las propiedades del combustible a alta presión. Los límites de la validez de esta hipótesis se han analizado cuidadosamente en el estudio. Los resultados muestran una influencia significativa de la temperatura del combustible a la entrada del inyector en la tasa y duración de inyección, atribuida al efecto de la variación de las propiedades del combustible y al hecho de que el inyector permanece en operación balística para la mayoría de sus condiciones de funcionamiento. Los cambios en temperatura del combustible a lo largo de los orificios de control del inyector son capaces de modificar su dinámica considerablemente. Además, si el combustible a la entrada del inyector se encuentra a temperatura ambiente o por encima, se ha observado que la temperatura a la salida de la tobera no varía de manera importante una vez se alcanzan condiciones estacionarias. No obstante, un calentamiento significativo puede tener lugar para temperaturas de entrada típicas de las condiciones de arranque en frío.[CA] El sistema d'injecció és un dels elements que més interés ha despertat en la investigació en el camp dels motors dièsel d'injecció directa, degut al seu paper clau en l'atomització i vaporització del combustible, així com en el procés de mescla, que afecten directament el consum i la generació de soroll i emissions contaminants. Les creixents pressions d'injecció en motors moderns han propiciat la necessitat d'estudiar fenòmens com la cavitació, flux compressible o l'efecte dels canvis de les propietats del combustible al llarg del procés, la importància relativa dels quals era menor en les primeres etapes del desenvolupament dels motors alternatius. Les menudes dimensions dels conductes de l'injector, les altes velocitats a través dels mateixos i la natura transitòria del procés dificulten l'observació directa en estes qüestions. Per això, les ferramentes computacionals han proporcionat una ajuda inestimable en el camp. L'objectiu de la present tesi és analitzar la influència dels efectes tèrmics en el funcionament d'un injector dièsel. Per a tal fi, es deu estimar la variació de la temperatura del combustible al llarg de les restriccions internes de l'injector. La influència d'estos canvis en les propietats termofísiques del combustible més relevants en el sistema d'injecció també ha de ser avaluada, degut al seu impacte en la dinàmica de l'injector i en la forma de la tasa d'injecció. Per tal de donar resposta a estos objectius, s'han utilitzat tècniques experimentals i computacionals. S'ha dut a terme una caracterització dimensional i hidràulica d'un injector Bosch CRI 2.20 actuat mitjançant solenoide, incloent mesures de tasa d'injecció en un ampli rang de condicions d'operació, per al que s'ha prestat especial atenció al control de la temperatura del combustible. S'ha implementat un model 1D de l'injector per tal de confirmar i estendre les observacions extretes dels experiments. El model considera variacions locals de pressió i temperatura del combustible gràcies a la hipòtesi de flux adiabàtic, per la qual cosa també s'ha hagut de dur a terme una caracterització experimental de les propietats del combustible a alta pressió. Els límits de la validesa d'esta hipòtesi s'han analitzat acuradament en l'estudi. Els resultats mostren una influència significativa de la temperatura del combustible a l'entrada de l'injector en la tasa i duració d'injecció, atribuïda a l'efecte de la variació de les propietats del combustible i al fet que l'injector roman en operació balística per a la majoria de les seues condicions de funcionament. Els canvis en temperatura del combustible al llarg dels orificis de control de l'injector són capaços de modificar la seua dinàmica considerablement. A més, si el combustible a l'entrada de l'injector es troba a temperatura ambient o per damunt, s'ha observat que la temperatura a l'eixida de la tobera no varia de manera important una vegada s'han assolit condicions estacionàries. No obstant això, un escalfament significatiu pot tenir lloc per a temperatures d'entrada típiques de les condicions d'arrancada en fred.Carreres Talens, M. (2016). Thermal effects influence on the Diesel injector performance through a combined 1D modelling and experimental approach [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/73066TESI

COMPUTATIONAL STUDY OF INTERNAL FLOW, NEAR NOZZLE AND EXTERNAL SPRAY OF A GDI INJECTOR UNDER FLASH-BOILING CONDITIONS

The early and late portions of transient fuel injection have proven to be a rich areaof research, especially since the end of injection can cause a disproportionate amountof emissions in direct injection internal combustion engines. While simulating theinternal flow of fuel injectors, valve opening and closing events are the perennialchallenges. A typical adaptive-mesh CFD simulation is extremely computationallyexpensive, as the small gap between the needle valve and the seat requires verysmall cells to be resolved properly. Capturing complete closure usually involves atopological change in the computational domain. Furthermore, Internal CombustionEngines(ICE) operating with Gasoline Direct Injection(GDI) principle are susceptibleto flash boiling due to the volatile nature of the fuel.The presented work simulates a gasoline direct injector operating under cavitatingconditions by employing a more gradual and easily implemented model of closure that avoids spurious water-hammer effects. The results show cavitation at low valve lift forboth flash boiling and non-flash boiling conditions. Further, this study reveals post-closure dynamics that result in dribble, which is expected to contribute to unburnthydrocarbon emissions. Flashing versus non-flashing conditions are shown to causedifferent sac and nozzle behavior after needle closure. In particular, a slowly boilingsac causes spurious injection behavior.Furthermore, a qualitative analysis of the injector tip-wetting phenomena underboth flash-boiling and non-flashing conditions are conducted and different wettingmechanisms are identified. The jet expansion mechanism is observed to dominate thewetting process during the main injection period, whereas the sac conditions drive thepost-closure wetting phenomena. Additionally, the effect of flash-boiling conditionson the near-nozzle spray during the quasi-steady period of the injection cycle is explored. The exploration captured hole-to-hole variations in the rate of injection (ROI), rate of momentum (ROM), and hydraulic coefficients of injection. Moreover, it also indicates influences of the in-nozzle variations on the near-nozzle spray behaviors.Finally, a novel plume-based coupling approach is developed to couple the Eulerian near nozzle simulations with the Lagrangian spray simulations under both non-flashing and flash-boiling conditions. Predictions from the novel coupling approachare validated with the experimental observations. This coupling approach requiresrunning an Eulerian primary atomization model, i.e., the Σ −Y model, to initializethe Lagrangian parcels for the secondary atomization process. Hence, this couplingapproach does not depend upon the linearized instability models to simulate the dense spray region