Air flow and pressure inside a pressure-swirl spray and their effects on spray development

Air flow and pressure inside a pressure-swirl spray for direct injection (DI) gasoline engines and their effects on spray development have been analyzed at different injector operating conditions. A simulation tool was utilized and the static air pressure at the centerline of the spray was measured to investigate the static pressure and flow structure inside the swirl spray. To investigate the effect of static air pressure on swirl spray development, a liquid film model was applied and the Mie-scattered images were captured. The simulation and experiment showed that recirculation vortex and air pressure drop inside the swirl spray were observable and the air pressure drop was greater at high injection pressure. At high fuel temperature, the air pressure at the nozzle exit showed higher value compared to the atmospheric pressure and then continuously decreased up to few millimeters distance from the nozzle exit. The pressure drop at high fuel temperatures was more than that of atmospheric temperature. This reduced air pressure was recovered to the atmospheric pressure at further downstream. The results from the liquid film model and macroscopic spray images showed that the air pressure started to affect the liquid film trajectory about 3 mm from the nozzle exit and this effect was sustained until the air pressure recovered to the atmospheric pressure. However, the entrained air motion and droplet size have more significant influence on the spray development after the most of the liquid sheet is broken-up and the spray loses its initial momentum.     

Drop size measurements for annular two-phase flow in a 20 mm diameter vertical tube

As part of a study on the effect of tube diameter on the mean drop size and liquid film flow rate in annular two-phase flow, data was obtained for the vertical upflow of an air-water system in a 20 mm internal diameter tube, held at a pressure of 1.5 bar and ambient temperature. This complements data taken in earlier experiments on 10 and 32 mm tubes. Increases in the superficial gas velocity caused reductions in the mean drop size whilst increasing the liquid mass flux in all but the lowest gas velocity case, caused the drop size to rise. Comparisons were made between the current drop size data and that from a 10 mm and 32 mm internal diameter tube, for similar conditions of temperature and pressure. The current drop size measurements, which fall between those from earlier work, confirm the dependence of drop size on tube diameter. The performance of several drop size correlations have been tested. Because the correlations do not account for the influence of tube diameter, they fail to predict the drop size data accurately. The influence of gas and liquid flow rate on the measured film flow rate show trends similar to those seen in data from the 10 mm and 32 mm diameter tubes. Models, to calculate the entrained liquid mass flux were tested; good predictions were given.     

Experimental investigation on splashing and nonlinear fingerlike instability of large water drops

The fluid physics of the splashing and spreading of a large-scale water drop is experimentally observed and investigated. New phenomena of drop impact that differ from the conventional Rayleigh–Taylor instability theory are reported. Our experimental data shows good agreement with previous work at low Weber number but the number of fingers or instabilities begins to deviate from the R–T equation of Allen at high Weber numbers. Also observed were multiple waves (or rings) on the spreading liquid surface induced from pressure bouncing (or pulsation) within the impacting liquid. The first ring is transformed into a radially ejecting spray whose initial speed is accelerated to a velocity of 4–5 times that of the impacting drop. This first ring is said to be “splashing,” and its structure is somewhat chaotic and turbulent, similar to a columnar liquid jet surrounded by neighboring gas jets at relatively high impact speed. At lower impact speeds, splashing occurs as a crown-shaped cylindrical sheet. A second spreading ring is observed that transforms into fingers in the circumferential direction during spreading. At higher Weber number, the spreading of a third ring follows that of the second. This third ring, induced by the pressure pulsation, overruns and has fewer fingers than the second, which is still in a transitional spreading stage. Several important relationships between the drop impact speed, the spray ejection speed of the first ring, and the number of fingers of the second and third rings are presented, based on data acquired during a set of drop impact experiments. Issues related to the traditional use of the R–T instability are also addressed.     

Evaluation of FFT-based cross-correlation algorithms for PIV in a periodic grooved channel

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

Atomized non-equilibrium two-phase flow in mesochannels: Momentum analysis

Two-phase pressure drop measurements are very difficult to make while the fluid is in non-equilibrium condition, i.e. while phase change is taking place. This is further complicated when an atomized liquid is introduced in the system at much higher velocity than other components such as liquid layer, vapor core, and entrained droplets. The purpose of this paper is to develop a model to predict the two-phase pressure characteristics in a mesochannel under various heat flux and liquid atomization conditions. This model includes the momentum effects of liquid droplets from entrainment and atomization. To verify the model, an in-house experimental setup consisting of a series of converging mesochannels, an atomization facility and a heat source was developed. The two-phase pressure of boiling PF5050 was measured along the wall of a mesochannel. The one-dimensional model shows good agreement with the experimental data. The effects of channel wall angle, droplet velocity and spray mass fraction on two-phase pressure characteristics are predicted. Numerical results show that an optimal spray cooling unit can be designed by optimizing channel wall angle and droplet velocity.     

A pneumatic droplet-on-demand generator

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

Droplet dynamics and size characterization of high-velocity airblast atomization

Airblast atomizers are especially useful and commonplace in liquid fuel combustion applications. However, the spray formation processes, the droplet dynamics and the final drop size distributions are still not sufficiently understood due to the coupled gas-liquid interactions and turbulence generation. Therefore, empirical and semi-empirical approaches are typically used to estimate the global spray parameters. To develop a physical understanding of the spray evolution, a plain-jet airblast atomizer was investigated in an atmospheric spray rig using the phase-Doppler technique. The simultaneous drop size and axial and radial velocity components were measured on radial traverses across the spray at various axial distances from the nozzle for a range of atomizing pressures. The droplet turbulent and mean kinetic energies were found to be proportional to the atomizing pressure. Hence, the scatter of the radial motion of the droplets increased with the atomizing pressure. A droplet stability analysis was performed to locate the regions characterized by ongoing secondary atomization. The volume-to-surface diameter, D₃₂, of the fully developed spray was compared with estimates provided by five published formulae. The role of liquid viscosity, hence the Ohnesorge number, was found to be negligible in the investigated regime. Three commonly used size distribution functions were fitted to the measured data to analyze their dependence on the atomizing pressure. The Gamma distribution function was found to give the best approximation to the atomization process.     

Experimental investigation of parameters effect on heat transfer of spray cooling

The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.     

Experimental and analytical investigation of liquid sheet breakup characteristics

The main objective of this research is to study analytically and experimentally the liquid sheet breakup of a flat fan jet nozzle resulting from pressure-swirling. In this study the effects of nozzle shape and spray pressure on the liquid sheet characteristics were investigated for four nozzles with different exit widths (1.0, 1.5, 2.0 and 2.5 mm). The length of liquid sheet breakup, liquid sheet velocity and the size of formed droplets were measured by a digital high speed camera. The breakup characteristics of plane liquid sheets in atmosphere are analytically investigated by means of linear and nonlinear hydrodynamic instability analyses. The liquid sheet breakup process was studied for initial sinuous and also varicose modes of disturbance. The results presented the effect of the nozzle width and the spray pressure on the breakup length and also on the size of the formed droplets. Comparing the experimental results with the theoretical ones for all the four types of nozzles, gives a good agreement with difference ranges from 4% to 12%. Also, the comparison between the obtained results and the results due to others shows a good agreement with difference ranged from 5% to 16%. Empirical correlations have been deduced describing the relation between the liquid sheet breakup characteristics and affecting parameters; liquid sheet Reynolds number, Weber number and the nozzle width.     

The spray characteristics of a pressure-swirl injector with various exit plane tilts

The gasoline spray characteristics of a pressure-swirl injector were investigated with various exit plane tilts. The analysis focused on the correlation between tilt angle and flow angle. Mie-scattering technique and phase Doppler anemometry were employed to analyze the macroscopic spray development and droplet size distribution of the spray. An analytical method for mass flux estimation was applied to understand the velocity distribution at the nozzle exit. The results showed that the spray shape and velocity distribution of the spray were more asymmetrical at high tilt angles. In particular, an opened hollow cone spray was formed when the tilt angle is greater than the complementary flow angle. The pressure drop inside the spray, one of the crucial factors for the swirl spray collapse at various surrounding conditions, was attenuated in this opened hollow cone spray since the pressure inside the spray was assimilated to the surrounding air pressure. The spray collapse at high fuel temperature and back pressure conditions did not appear when the tilt angle is larger than the complementary flow angle due to the reduced pressure drop inside the spray. However, tilt angle should be optimized to fulfill the requirements of spray robustness and avoid the locally rich area. The droplet size of 70° tilted nozzle spray shows a value similar to that of the original swirl spray in the plane that includes nozzle axis and the major axis of exit surface ellipse (Major axis plane) while it shows an increased value in the plane that includes nozzle axis and the minor axis of exit surface ellipse (Minor axis plane).     

Drop trajectory predictions and their importance in the design of spray dryers

A knowledge of the trajectories of atomized droplets in both the nozzle zone (where the droplets are rapidly decelerating from their initial high velocity) and in the free-entrainment zone (where the droplets are conveyed by the drying gas) is required for the design of spray dryers, since it governs the evaporative capacity and thermal efficiency of the chamber, while affecting the moisture content and general quality of the product through the control of the drying time.

The trajectories of droplets in three-dimensional motion were determined theoretically in both zones. In the case of two-fluid pneumatic atomizers, the characteristics of the jet of atomizing fluid were found to be important in both the zones.

Predictions of droplet trajectories were tested in an experimental circular cocurrent spray-drying chamber with a conical bottom, in which the drying air was introduced tangentially near the top. Water was used as the feed material. A study was made of the effects of liquid feed rate and temperature, drying air flowrate and temperature, and of nozzle position on the thermal efficiency and evaporative capacity of the chamber. The results were interpreted in the light of the droplet trajectories predicted.     

EFFECT OF NOZZLE FAN ANGLE ON SPRAYS IN GAS-SOLID RISER FLOW

A three-dimensional simulation study is performed for investigating the hydrodynamic behaviors of a cross-flow liquid nitrogen spray injected into an air-fluidized catalytic cracking (FCC) riser of rectangular cross-section. Rectangular nozzles with a fixed aspect ratio but different fan angles are used for the spray feeding. While our numerical simulation reveals a generic three-phase flow structure with strong three-phase interactions under rapid vaporization of sprays, this paper tends to focus on the study of the effect of nozzle fan angle on the spray coverage as well as vapor flux distribution by spray vaporization inside the riser flow. The gas-solid (air-FCC) flow is simulated using the multi-fluid method while the evaporating sprays (liquid nitrogen) are calculated using the Lagrangian trajectory method, with a strong two-way coupling between the Eulerian gas-solid flow and the Lagrangian trajectories of spray. Our simulation shows that the spray coverage is basically dominated by the spray fan angle. The spray fan angle has a very minor effect on spray penetration. The spray vaporization flux per unit area of spray coverage is highly non-linearly distributed along the spray penetration. The convection of gas-solid flow in a riser leads to a significant downward deviation of vapor generated by droplet vaporization, causing a strong recirculating wake region in the immediate downstream area of the spray.     

Ethanol–CO2 two-phase flow in diverging and converging microchannels

The present study investigates experimentally two-phase flow patterns and pressure drop of ethanol and CO₂ in a converging or diverging rectangular microchannel. The two-phase flow pattern visualization is made possible using a high speed video camera. The increased superficial gas velocity due to the acceleration effect and the large pressure drop in a converging channel may result in the elongation of bubbles in slug flow, while the decreased superficial velocity owing to the deceleration effect and the possible pressure rise in the diverging channel may cause shortening of bubbles in slug flow significantly. For both types of channel, the collision and merger of two consecutive bubbles may take place and result in necking of bubbles. Two-phase flow pressure drop in the converging microchannel increases approximately linearly with the increasing liquid or gas flow rate with the frictional pressure drop being the major contributor to the channel pressure drop. In the diverging microchannel, the deceleration effect results in the pressure rise and counteracts the frictional pressure drop. Consequently, for low liquid flow rates the channel pressure drop increases only slightly with the gas flow rate while it is low and a reversed trend appears while it is high. For high liquid flow rates the effect of increasing gas flow rate on channel pressure drop is much more significant; a more significant reverse trend of the effect of gas flow rate is present in the region of high gas flow rates. The two-phase frictional multiplier in the converging or diverging microchannel is quite insensitive to the liquid flow rate and can be fitted very well within ±15% based on the Lockhart–Martinelli equation with a modified Chisholm parameter for the diverging microchannel and together with a modified coefficient for the X⁻² term for the converging microchannel.     

Dynamic characteristics of pulsed supersonic fuel sprays

This paper describes the dynamic characteristics of pulsed, supersonic liquid fuel sprays or jets injected into ambient air. Simple, single hole nozzles were employed with the nozzle sac geometries being varied. Different fuel types, diesel fuel, bio-diesel, kerosene, and gasoline were used to determine the effects of fuel properties on the spray characteristics. A vertical two-stage light gas gun was employed as a projectile launcher to provide a high velocity impact to produce the liquid jet. The injection pressure was around 0.88–1.24 GPa in all cases. The pulsed, supersonic fuel sprays were visualized by using a high-speed video camera and shadowgraph method. The spray tip penetration and velocity attenuation and other characteristics were examined and are described here. An instantaneous spray tip velocity of 1,542 m/s (Mach number 4.52) was obtained. However, this spray tip velocity can be sustained for only a very short period (a few microseconds). It then attenuates very quickly. The phenomenon of multiple high frequency spray pulses generated by a single shot impact and the changed in the angle of the shock structure during the spray flight, which had already been observed in previous studies, is again noted. Multiple shock waves from the conical nozzle spray were also clearly captured.      

Large eddy simulation of high pressure spray with the focus on injection pressure

This paper presents a detailed numerical analysis of diesel engine spray structure induced by the Engine Combustion Network (ECN) Spray A at different injection pressures. The non-reacting simulations are performed using OpenFOAM where an Eulerian–Lagrangian model is adopted in the large eddy simulation (LES) framework. Effects of the LES mesh resolution as well as the spray model parameters are investigated with the focus on their impact on spray structure as the injection pressure varies. The predicted liquid and vapour penetration lengths agree well with the measurements at different injection pressures. The mixture fraction is well captured for the injection pressure of 100 and 150 MPa while a slight deviation from the measurements is observed for the injection pressure of 50 MPa near the nozzle. The parametric analysis confirms that the LES mesh resolution has significant effects on the results. A coarser mesh leads to higher liquid and vapour penetration lengths where the deviation from the measurements is larger, resulting in the highest error at the lowest injection pressure. As the mesh size increases, the droplet size distribution becomes narrower, its pick moves to the smaller droplet size and the probability of droplets with higher temperature increases. On the other hand, with increasing the mesh size, the carrier gas velocity decays slower and its radial dispersion decreases. It is found that the droplet characteristics are more affected by the mesh resolution when the injection pressure is the lowest while the opposite is true for the carrier phase. The number of Lagrangian particles also affects the droplet characteristics and the fuel-air mixing but their effects are not as significant as the mesh size. The results become less sensitive to the number of Lagrangian particles as the pressure injection decreases. Finally, the importance of the initial droplet size distribution is investigated, confirming its impact is marginal, particularly on the liquid length. It is observed that the initial droplet size is only important at very close to the nozzle and its impact on the spray structure becomes quickly insignificant due to the high rates of breakup and evaporation. This trend is consistent at different injection pressures.     

Two-dimensional viscoelastic flows: experimentation and modeling

Extensive experimental data on the birefringence in converging and diverging flows of a polymeric melt have been obtained. The birefringence and pressure drop measurements were carried out in working cells of planar geometry having different contraction angles and contraction ratios. For investigation of diverging or abrupt expansion flow, the direction of flow in the cells was reversed. The theoretical predictions are based upon the Leonov constitutive equation and a finite element scheme with streamwise integration.In contrast to Newtonian and second-order fluids, viscoelastic fluids at high shear rates show significant differences in pressure drop and birefringence (i.e. stresses) in converging and diverging flows. For a constant flow rate, the pressure drop is higher and the birefringence smaller in diverging flows than in converging flows. This difference increases with increasing flow rate. Further, for the same contraction ratio but different contraction angles, the birefringence maximum increases considerably with contraction angle. In addition, an increase in contraction ratio has the same effect.The viscoelastic constitutive equation of Leonov has been shown to describe all the above viscoelastic effects observed in the experiments. In general, a reasonable agreement between theory and experiment has been obtained, which shows the usefulness of the Leonov model in describing actual flows.     

Visualization of high-speed gas jets and their airblast sprays of cross-injected liquid

A planar and instantaneous visualization study of high-speed gas jets and their airblast sprays was performed to qualitatively examine the different atomization performances of different gas nozzles. For the visualization of high-speed gas jets (with no liquid injected), Nd:YAG pulsed laser sheets imaged the clustered vapor molecules in the Rayleigh range (d?λ), condensed from the natural humidity during the isentropic gas expansion through a nozzle. This method visualized both underexpanded sonic gas jets from a converging nozzle (SN-Type) and overexpanded supersonic gas jets from a converging-diverging nozzle (CD-Type). When liquid is cross-injected, the same laser sheet images the spray droplets of relatively large sizes (d?λ). The present visualization results show that the SN-Type nozzle develops a wider spray than the CD-Type nozzle, quite probably because the SN-Type nozzle has a wider gas jet (in the absence of liquid) than the CD-Type. Also, the wider spray of the SN-Type nozzle lowers the probability of droplet coalescence and generates finer sprays compared to the CD-Type nozzle. These visualization results qualitatively agree with the previous quantitative finding of the different atomization characteristics of the two types of nozzles (Park et al. 1996).     

Imaging and PDA analysis of a GDI spray in the near-nozzle region

This paper describes a combined LDA, PDA and imaging analysis of the pressure swirl spray in the near-nozzle region of a GDI injector. This innovative approach in the use of multiple, complementary diagnostics facilitates the interpretation of a complex spray flow field.The LDA and PDA data were ensemble-averaged into time bins to produce comprehensive time-history and spatial profiles of liquid velocity, droplet velocity and size and the sample number. They indicated times at which the spray exhibited seven different characteristics. These were identified as: (a) pre-swirl spray, (b) spray cone develops, (c) spray cone relaxes, (d) maximum velocity in spray cone, (e) fully developed steady state, (f) spray cone collapses and (g) the spray detaches from the nozzle. The most effective method to present the spatial and temporal development of the spray was to superimpose the velocity vector and drop size field plots onto the spray images.This article is part of the special issue 11^th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisboa, Portugal, July 2002, January 2004, Vol. 36, Issue no. 1     

Spray Penetration in a Turbulent Flow

Analytical expressions for mass concentration of liquid fuel in a spray are derived taking into account the effects of gas turbulence, and assuming that the influence of droplets on gas is small (intitial stage of spray development). Beyond a certain distance the spray is expected to be fully dispersed. This distance is identified with the maximum spray penetration. Then the influence of turbulence on the spray stopping distance is discussed and the rms spray penetration is computed from a trajectory (Lagrangian) approach. Finally, the problem of spray penetration is investigated in a homogeneous two-phase flow regime taking into account the dispersion of spray away from its axis. It is predicted that for realistic values of spray parameters the spray penetration at large distances from the nozzle is expected to be proportional to t ^2/3 (in the case when this dispersion is not taken into account this distance is proportional to t ^1/2). The t ^2/3 law is supported by experimental observations for a high pressure injector. This revised version was published online in July 2006 with corrections to the Cover Date.