A computational approach to predict external spray characteristics for flashing and cavitating nozzles (original) (raw)
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The Effect of Flashing on Characteristics of Sprays of Splash-Plate Nozzles
An experimental study of the effect of flashing on sprays of splash-plate nozzles was conducted. The investigation was focused on the effect of injection temperature, pressure and delivery system on flow regime inside the nozzle, breakup mechanism and droplet size. The droplet size and disintegration mechanism were found to be very dependent on injection temperature and the internal flow regime was highly affected by both the temperature and the geometry of the system. Flashing was observed to occur few degrees above the saturation temperature. The droplet size decreased dramatically at the point of onset of flashing in a narrow range of temperature. Further increase of temperature had less effect on droplet size.
Prediction of the Impact of Nozzle Geometry on Spray Characteristics
ACS Omega, 2021
In the present paper, the formation and development of cavitation inside the nozzle of an atomizer with different geometrical characteristics have been studied numerically. Different shapes of inlet nozzles and different nozzle-length-to-diameter ratios have been investigated. The developed model has been built as a three-dimensional (3D) one, where the turbulence is modeled considering large eddy simulation. The obtained computational results showed good agreement with the reported experimental results. It has been found that the occurrence of cavitation depends on the amount of energy needed to overcome the viscosity and friction between the liquid layers. The mass flowing through the nozzle decreases with increasing cavitation. The intensity of cavitation depends on the nozzle entrance shape. Sharp edges cause cavitation to occur early in the nozzle, followed by an inclined shape, and then the curved entrance. The dissipative energy in the cavitation and bubble collapse result in...
Spray Formation by Bi-Component Liquid Flashing: A Theoretical Approach
2002
Spray formation by bi-component liquid flashing through a special-design injector has been studied. The injector is composed o f an inlet orifice (i), an expansion chamber (m), and a discharge orifice (c). In principle, in this method, a given mixture comprising of two different types of liquids; a solvent (s) and a propellant (p), flows through the inlet orifice into the expansion chamber ( Fig 1), and leaves it t hrough the discharge orifice. The propellant, which h as a higher vapor pressure than the solvent, undergoes a rapid b oiling process (flashing process), in which many tiny bubbles are created, and then grows along the expansion chamber. In this process, the specific volume of the mixture increases and therefore it’s velocity increases. Nevertheless, due to the very low acceleration, no slip between the two phases inside the expansion is expected. Slip, however, is important inside the discharge orifice.
MODELLING OF CAVITATION FLOW IN A NOZZLE AND ITS EFFECT ON SPRAY DEVELOPMENT
Jets, 2006
The experimental observations and theoretical models of spray formation due to the effect of cavitation development in injection nozzles are reviewed. Particular attention is focused on the effects of cavitation disturbances on jet and spray break-up. Models of jet and spray break-up which take into account the stochastic character and non-equilibrium spectrum of product droplets are essential when modelling the primary and secondary stages of break-up. Single-fluid models of cavitation are shown to be robust, but contain empirical rate parameters which require adjustments for specific flows. This study addresses the liquid quality and viscous shear stress effects on cavitation flow. In order to account for the liquid quality effect on cavitation a model derived from bubble dynamics theory is developed. The model for the concentration of cavitation nuclei in a liquid is derived by assuming hydrodynamic similarity of cavitation flows. The model accounts for the variation in the number density of cavitation bubbles as a function of liquid tension in the cavitation region. This model was developed using an analogy with the effect of liquid superheat on the number of nuclei in models for nucleate boiling. The model contains a parameter, which describes the liquid quality. This can be adjusted using one set of measurements for a given liquid. The influence of viscous shear stress on the cavitation threshold in high-speed flows, such as those observed in the nozzles of a direct-injection diesel engine, has been clarified. In order to describe this effect on hydrodynamic cavitation in high-speed turbulent flows a model that takes into account the critical vapour pressure was developed. The model was adjusted to describe sub-cavitation and super-cavitation flows in real-scale models of diesel injectors. Nomenclature t C -constant in equation (12); D -diameter of nozzle; H -height of nozzle; ∞ l -hydrodynamic length scale of the flow; n -number density of cavitation bubbles; * n -parameter in equation (11); p 1 -pressure at the inlet of nozzle; p 2 -pressure at the outlet of nozzle; p v -vapour pressure; p min -minimum pressure in cavitation region; R -radius of cavitation bubble; ij S -strain rate tensor; u -velocity; W -width of nozzle, m; α -volume fraction of the vapour phase;
cavitation effects on spray characteristics in the near-nozzle field
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Influence of cavitation on near nozzle exit spray
Journal of Physics: Conference Series, 2015
The importance of cavitation inside multi-hole injectors for direct injection internal combustion (IC) engines has been addressed in many previous investigations. Still, the effect of cavitation on jet spray, its stability and liquid breakup and atomisation is not yet fully understood. The current experimental work aims to address some of these issues. It focuses on the initiation and development of different types of cavitation inside a 7x enlarged transparent model of a symmetric 6-hole spark ignition direct injection (SIDI) injector and quantifies the effect of cavitation on near-nozzle spray cone angle and stability utilising high speed Mie scattering visualisation. The regions studied include the full length of the nozzle and its exit jet spray where the primary breakup takes place.
Flow regime effects on non-cavitating injection nozzles over spray behavior
This paper deals with the influence of flow regime (laminar, transition or turbulent) on the internal flow behavior, and how it affects the spray development in diesel nozzles. In particular, the research described here aims at studying and quantifying the internal flow regime effects on the spray behavior. With this purpose, internal flow results, based on mass flow rate and momentum flux measurements performed on three different tapered nozzles and which helped to determine the flow regime, has been taken into account as a point of departure for the spray behavior analysis. Thus, in this work, spray macroscopic visualization tests have been performed and analyzed which clearly revealed a change in the behavior of the angle and penetration of the spray related to the change of the flow nature. Moreover, with all the experimental data available, it has been possible to relate macroscopic parameters of the spray with those describing the internal flow (momentum and effective velocity) or the geometry of the nozzle (length or diameter) through correlations.
Characterization of Splash-Plate Atomizers Using Numerical Simulations
Atomization and Sprays, 2007
A computational model has been developed that can be used for the spray characterization of splash-plate atomizers. The computer model, called BLSpray, can accurately simulate the impingement of a liquid jet on the surface of a splashplate nozzle, as well as the formation of the liquid film and subsequent droplets. To validate the model, simulation results were compared to experimental measurements for the film thickness and velocity distributions in a typical splash-plate nozzle. Close agreement between numerical results and measurements validated the model and its underlying assumptions. Correlations were developed between liquid film characteristics at the nozzle exit and the spray mean drop sizes. This was done by running many different numerical simulations on a typical splashplate nozzle using the developed computer code. The correlations were obtained by performing a close inspection of the numerical results in order to extract all information regarding the liquid film and spray. The results of the developed code were combined with the correlations to get the spray drop size distribution in a more practical approach, with less computational time and effort. This capability, along with the program module developed for analyzing the output data, has turned the developed code into an efficient and practical tool in the design and characterization of splash-plate nozzles. The developed computer model can be used to predict the behavior of a flow into a nozzle at different operating conditions, and also as a tool in the design of new nozzles. This paper presents mathematical formulations, results of model validation, and the spray drop size distribution for a typical ALSTOM splash-plate nozzle. Also, the effect of some of the main parameters on the spray pattern, such as nozzle diameter, nozzle angle, and nozzle velocity, are investigated.
The effect of string cavitation in various transparent Diesel injector nozzles on near nozzle spray dispersion angle is examined. Additional PDA measurements on spray characteristics produced from real-size transparent nozzle tips are presented. Highspeed imaging has provided qualitative information on the existence of geometric and string cavitation, simultaneously with the temporal variation of the spray angle. Additional use of commercial and in-house developed CFD models has provided complimentary information on the local flow field. Results show that there is strong connection between string cavitation structures and spray instabilities. Moreover, elimination of string cavitation results in a stable spray shape that is only controlled by the extent of geometric-induced cavitation pockets. Finally, PDA measurements on real-size transparent nozzle tips have confirmed that such nozzles reproduce successfully the sprays generated by production metal nozzles.
Towards a unified treatment of fully flashing sprays
2014
This paper presents a systematic study on flashing atomisation, which includes both standards and retrograde fluids. A novel data reduction method is proposed in terms of the controlling parameters for (bubble) nucleation. The analysis indicates that bubble nucleation is the rate-controlling process for both the transition to fully flashing and for the spray lateral spreading. Specifically, the onset condition coincides with the surmount of the energy barrier to nucleation. The spray lateral spreading, instead, is directly linked to the population of bubble clusters: the larger the population the wider the spray angle. Theoretical aspects of bubble nucleation theory are also reviewed. An interesting conclusion of the analysis is that the experimental trends observed in fully flashing jets are compliant with recent advances in nucleation theory. At very high initial superheat, a complex shock wave structure appears around the flashing jets. The novel aspect of this work is that such shock-systems are observed consistently in both standard and retrograde substances. This similarity indirectly confirms that, far from the critical temperature, the phase transition mechanism is the same for all substances, independently from their degree of retrogradicity.