Showing papers in "Atomization and Sprays in 1999"

Simple description of the combustion structures in the stabilization stage of a spray jet flame

Abstract: The structure of a two-phase flame has been investigated within its stabilization region in the nearfield of a spray jet. The spray is produced by a coaxial air-blast injector fed with liquid methanol. We focus on a specific structure of the two-phase flame, observed experimentally where the flame presents a double structure with a predominant diffusion character for each reaction zone. The analysis is justified from experimental results of phase Doppler anemometry and planar laser-induced fluorescence of OH. The dynamics of the spray obtained from phase Doppler velocimetry are studied in terms of size classes defined from the Stokes number. The size classification shows that, where the flame stabilizes, the spray is composed of two fluids, one with high inertia (high Stokes number), the other characterized by a low Stokes number. The structure of the two-phase flame is analyzed in the low-inertia part of the spray. The emphasis is put on a regime where tau(ch) < tau(vap) < tau(mix); a double structure may develop in what we called the "vaporization regime," since droplets carl cross reaction zones. Such a double structure has been predicted by Continillo and Sirignano [25] and by Greenberg and Sarig [26, 27] through numerical modeling of a hue-phase counterflow flame. The present article gives an experimental confirmation of a real occurrence of such a flame structure in turbulent spray jets and proposes a simplified description in the low-inertia part of the spray and for the flame sheet approximation.

Transient 3D analysis of a DI gasoline injector spray

Abstract: A cycle-resolved, phase Doppler anemometry (PDA) methodology is appraised for the collection and analysis of data from a gasoline fuel injector. A high-pressure swirl injector is utilized, spraying unleaded gasoline into air at ambient pressure and temperature. Results are presented in terms of Sauter mean diameter (SMD) of droplets, three components of velocity, and semiquantitative mass flow rate for the entire flow field. Spray development is analyzed using time increments of between 0.25 ms and 1 ms over the first 12 ms after injection. High-speed photography confirms the validity of some of the global trends identified, including head vortex development, spray penetration, and needle bounce. PDA measurements indicate that larger droplets are produced in the early stages of the injection. These populate the head and periphery of the spray cone, which becomes essentially hollow for a period between 0.75 and 2 ms. Smaller droplets in the center of the cone attain velocities in excess of 50 m/s, while those on the edge are entrained by the recirculating head vortex. During the early injection period, the majority of the liquid mass resides within the "head" and an annular section of the spray, which indicates the hollow cone design. After 3 ms, the spray becomes more homogenous, with little mass flow rate variation across the cone identifiable after 4.5 ms. The data are finally compared with a standard time-averaged correlation usually utilized for this type of injector. This emphasizes the need for continued effort on transient predictive spray modeling in future direct-injection (DI) gasoline investigations.

The effect of manifold cross-flow on the discharge coefficient of sharp-edged orifices

Abstract: The objective of this study is to determine the effect of manifold cross-flow on the discharge coefficient and cavitation characteristics of sharp-edged orifices over a wide range of flow-rates, back-pressures and cross-flow velocities. The orifice geometries studied cover a range of orifice diameters, length to diameter ratios and orifice angles characteristic of impinging element. liquid rocket injectors. Experimental results for an orifice angle of 900 with respect to the manifold are presented here, Along with the experimental effort, an analytical model is being developed. The model predicts the discharge coefficient for a sharp edged orifice over a wide range of flow regimes including cavitating and non-cavitating flow, and for a wide range of orifice geometries. The analytical model generally shows good agreement with the experimental data over the range of conditions studied here. The model also closely follows the experimental data for cavitating flow except when the orifice length to diameter ratio is small, in which case the model over-predicts the discharge coefficient. INTRODUCTION In many atomization applications, such as rocket injectors, the manifolding used to deliver liquid to an orifice can introduce a component of velocity normal to the axis of the orifice flow. Previous studies have indicated that manifold cross-flow can affect the orifice discharge coefficient and cavitation characteristics [1,21. The net result is a variation in the discharge coefficient and thus mass flowrate by as much as fRffy-pereent depending on flow conditions and orifice geometry. Such a profound change in flowrate of an individual orifice can have a detrimental effect on spray characteristics, including droplet size, spray angle and cone angle. Any significant change in the expected spray characteristics could lead to decreased injector performance and damage to a combustion chamber due to increased heat flux near the wall. DISTRIBUTION STATEMENT A \"To whom correspondence should be addressed. Approved for Public Release Distribution Unlimited Background Ver' little work has been done in the past with respect to manifold cross flow effects on discharge coefficient. Northrup 11] studied the effects of cross-velocity on sharp-edged orifices at pressure-drops up to 100 psi and atmospheric back pressure with cross-velocities up to 20 ft/s. He found that the discharge coefficient decreased as cross-velocity increased. He concluded that cross-velocity should be held to a minimum to avoid degradation in spray quality. Nurick [2) also studied the effects of manifold cross-flow on sharp-edged orifices of various shapeat a chamber back-pressure of 0.69 Mpa and a pressure drop of 0.14 Mpa using water. He found that increasing the cross-velocity component from 0.3 m/s to 5.5 m/s decreased the discharge coefficient by about 5-10%. He also found that the increased cross-flow created visible disturbances, described as a brushy appearance, in the jets emanating from the orifices. Andrews and Sabersky (3] experimentally investigated the effect of cross-flow velocity on the discharge coefficient of a circumferential slot formed between two sections of circular pipe. The working fluid was water and the maximum cross-velocity studied was 6 m/s. Over the range of slot Reynolds numbers studied by the authors, the discharge coefficient was found to vary by as much as 50r% as cross-velocity was varied. The discharge coefficient was found to decrease as the manifold to slot velocity ratio was increased beyond a certain value. They concluded that the fraction of available dynamic head in the approaching cross-flow that was converted into pressuredrop across the slot was much less than one, and decreased with increasing cross-flow to slot velocity ratio. Rhode and coworkers (41 experimentally measured the discharge coefficient of a round hole drilled in the side of a circular pipe using air as the working fluid. They showed as much as an eight-fold decrease in discharge coefficient for high manifold Mach number and low pressure-drop configurations. They found that the discharge coefficient correlated with the velocity head ratio, which was defined as the ratio of orifice total pressure-drop to manifold cross-flow momentum, l/2pUz. Discharge coefficient was found to increase with increasing velocity head ratio. Most of the work that has been described has been limited to relatively low injection pressures and low backpressures. Also, most of the previous studies have been limited to a single flow regime, ie. either cavitating or noncavitating flow. Most of the existing data has been presented with very little explanation as to why the discharge coefficient is affected by the manifold cross-flow. This makes it very difficult to extrapolate data that has been

Modeling the effects of gas density on the drop trajectory and breakup size of high-speed liquid drops

Abstract: An experimental and numerical study has been conducted of drop trajectory and breakup mechanisms for liquid drops injected into high velocity gas flows with various chamber gas pressures at room temperature. In the experimental study, air-assisted liquid drop atomization processes were investigated using photographic techniques under well-controlled experimental conditions. A monodisperse stream of drops from a vibrating-orifice drop generator was injected into a transverse high velocity gas stream. The back pressures and gas velocities were adjusted independently to control the drop Weber numbers. The Weber numbers used in the experiments were 72, 148, 270, 532. High-magnification photographs and conventional spray field photography revealed the microscopic breakup mechanisms and the parent drop trajectory in the high velocity flow field, respectively. Drop sizes were measured using a Phase/Doppler particle analyzer. The experimental results were used to test and assess spray models in the KIVA3 code. The breakup model considered Kelvin-Helmholtz (K-H) instability mechanisms to account for secondary drop breakup. The computations show good agreement with experimental results of parent drop trajectories and for the spatial drop size distributions which result from secondary breakup at high gas densities. At low gas densities, it is concluded that the use of K-H model to predict drop breakup may not be justified.

High-Energy-Density Fuel Blending Strategies and Drop Dispersion for Fuel Cost Reduction and Soot Propensity Control

Abstract: High-energy-density (HED) liquid fuels have high soot propensity and are expensive. The idea of mitigating these characteristics by adding a less expensive, low soot propensity liquid fuel to the HED is tested through numerical simulations. The model represents an axisymmetric, polydisperse, dense cluster of binary-fuel (solvent/solute) spherical drops embedded into a vortex. Since soot propensity depends on the partial density of the evaporated fuel, this partial density is compared for uncharged and electrostatically charged drops; charging is used here as an effective way to increase dispersion and reduce sooting propensity. Results from the simulations show that while the solvent soot propensity indeed decreases with drop charging, contrary to simplistic expectations, addition of HED as a solute increases sooting propensity of the solute with increased drop dispersion. This is due to the additional dispersion maintaining the slip velocity at the drop surface and preferentially evaporating the solute. These counterintuitive but correct physical effects are independent of the initial solvent/solute mass ratio, and the soot propensity decreases with decreasing solute volatility. Based on these results, blending strategies are suggested for minimizing sooting propensity and decreasing fuel costs.