Spray Region Research Articles

Heat transfer in a high temperature region of spray cooling interacting with liquid film flow

We present experimental results on heat transfer distribution in the high temperature region of spray cooling interacting with subcooled liquid film flow. The results show that the flow field can be divided into the interacting and film flow regions by the heat transfer distribution. In the interacting region, the heat transfer coefficient can be correlated to the liquid-film-flow heat transfer by using a heat-transfer enhancement coefficient defined as the ratio of the droplet flow rate to liquid film velocity. In the wall region, it can be predicted from the equation obtained from a previous study, which is very similar to that of turbulent heat transfer of single-phase flow. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 236–248, 1997

An experimental study on swirling spray flame structure by air-blast nozzle

Detailed experimental study has been made of air blast kerosene spray flames with and without swirl in combustion air flow. Phase-Doppler detect technique is used to measure Sauter mean diameter, axial component mean and rms velocity, size-velocity correlation, and number density. These measurements are obtained for both nonreacting and reacting cases under several stable flame conditions. The results show that the introduction of swirl to the combustion air modifies the spatial distribution of droplet size, velocity, and number density, and thus alters the flame structure. However, due to the weak swirl intensity, the overall structure of swirling flames are essentially same as that of nonswirling flames. Physical model of structure of air blast atomized spray flames is projected to show that spray flames are composed of three distinct regions: the two-phase mixture region, the main reaction and the intermittent combustion region. Near the atomizer, two phase mixture of droplet and air is formed in the core region. This dense spray region is characterized by high droplet number density and the strong convective effect. There follows the main combustion region where the main flame penetrates within the spray boundary. Main reaction region of these flames are governed by internal group combustion mode. Finally there exists the intermittent combustion region where local group burning or isolated droplet burning occurs.

Effect of Ambient Gas Viscosity on the Structure of Diesel Fuel Spray.

This study deals with the effect of the viscosity of the ambient gas on the atomization process of a diesel spray. The liquid fuel is injected through a single-hole nozzle (ln/dn=0.75 mm/0.25 mm)with the injection differential pressure of 16.2 MPa inside a constant-volume vessel under a high-pressure field at room temperature. In practical combustion chambers of diesel engines, the viscosity of the ambient gas varies mainly with its temperature. In this experiment, various gases (CO2, N2, Ar, Ne, and Ar+Ne) were utilized for the ambient atmosphere in order to change the ambient gas viscosity. The vaporization of fuel drops was negligible. This study, revealed that the higher the viscosity, the more the spray spreads in the radial direction, thus the spray angle increased and tip penetration decreased. The droplets became smaller and were distributed in the inner region of the cold-state diesel spray under high ambient viscosity conditions. Further more, it was found that processes of ligament formation in the liquid jet and initial atomization were affected by kinematic viscosity ratio between the liquid fuel and ambient gas, and the spray dispersion and mixing process in the downstream region was affected only by the ambient gas kinematic viscosity.

Heat Transfer in High Tempetature Region of Spray Cooling Interacting with Liquid Film Flow.

We present experimental results on heat transfer distribution in the high temperature region of spray cooling interacting with subcooled liquid film flow. The results show that the flow field can be divided into the interacting and film flow regions by heat transfer distribution. In the interacting region, the heat transfer coefficient can be correlated to the liquid-film-flow heat transfer by using a heat-transfer enhancement coefficient defined as the ratio of the droplet flow rate to liquid film velocity. In the wall region, it can be predicted from the equation obtained from the previous study, which is very similar to that of turbulent heat transfer of single-phase flow.

Simultaneous 2-D Imaging of OH and Soot in an Unsteady Spray Flame.

The OH radicals and soot in an unsteady spray flame, which was achieved in a rapid compression machine, were visualized simultaneously by the laser-induced fluorescence and the laser-induced scattering techniques, The fuel mixture consisting of 90% paraffin hydrocarbon (reference fuel) and l0% polypropylene glycol was used to reduce the optical attenuation caused by dense soot cloud. The simultaneous images of the fluorescence from OH radicals and the scattering from soot show that the soot and OH radicals exist separately from each other in the leading portion of the spray flame, and neither soot nor OH radicals exist in the center part of the quasisteady region of spray during injection.

Heat Transfer Model of Spray Cooling Focusing on Liquid Sensible Heat.

In this report, a simple model to predict heat transfer distribution in the high temperature region of spray cooling is developed by focusing on the effect of sensible heat of droplets. In the model, it is assumed that the droplets rebound repeatedly on the surface and heat transfer upon droplet impact is proportional to the sensible heat which heats up the droplets to the saturation temperature. The calculated results are compared with existing experimental results and it is shown that the present model can predict the heat transfer distribution in the high temperature region of spray cooling.

One-dimensional mathematical model for selecting plasma spray process parameters

A simple, unified, one-dimensional model has been developed to relate the effects of plasma spray parameters on the temperature and velocity of the plasma and particles and on the void content in the coating. The torch, spray, and substrate regions in a plasma spray process were first modeled independently and then coupled so that the plasma and particle characteristics calculated in one region served as inputs for the subsequent region. Comparison of the model predictions with experimental data showed reasonable agreement. Deviations from the measured data were attributable to the simplifying assumptions used in modeling the different regions of the process. A parametric analysis of the unified one- dimensional model showed that, despite its simplicity, the model is well suited for optimizing process parameters in terms of particle type and size to obtain high- integrity coatings.

The role of dynamic surface tension in air assist atomization

Aqueous surfactant solutions were atomized using an air assist atomizer, and the effects of dynamic surface tension on atomization were studied. The solutions differ from one another in their dynamic behavior, but all have the same equilibrium surface tension. The constant equilibrium surface tension was achieved by choosing surfactant concentrations above the critical micelle concentration (CMC). The dynamic change of surface tension in aqueous Tergitol NP-10 solutions of 1.5, 3, 6, 12, and 60 mM was measured using the oscillating jet technique. While atomizing the liquid jet, it was found that frequencies and amplitudes of the flapping motion of the liquid jet increased with surfactant concentration showing that surface tension decreases in the region between the nozzle exit and breakup. The length of the intact liquid jet was found to be insensitive to surfactant concentration, suggesting that, due to sudden increase of surface area, surface tension is near its initial value in the breakup region. Measurements in the spray region show a strong influence of the dynamic change of surface tension on drop size. Finally, using an analysis based on a critical Weber number, the actual time average of surface tension was estimated at an axial location of 1.5× the intact length downstream from the nozzle exit, and an overall profile of surface tension as a function of axial location was proposed.

Drift Predictions in the Near Nozzle Region of a Flat Fan Spray

Previous work concerned with modelling spray drift from agricultural flat fan nozzles has used random-walk approaches to predict individual droplet trajectories in the region close to the nozzle and further downwind. Work reported here has extended this approach such that droplets are tracked in three dimensions in the near nozzle region and then a two-dimensional random-walk is used to predict trajectories further downwind. Model predictions were validated against wind tunnel drift experiments. Measurements in a wind tunnel with a 110° flat fan nozzle operating with a flow rate of 0·61 min -1 at a pressure of 3·0 bar, gave volumes of airborne spray 2·0 m downwind of the nozzle of 5·2% of the nozzle output with the spray fan oriented at right angles to the direction of airflow at a speed of 2-0 m s -1 and 0·6% when the spray fan was aligned with the direction of the airflow at a speed of 2·0 m s -1. The model gave drift percentages of 7·5% and 2·8% for the same conditions, and also over-predicted drift for all remaining angles of nozzle orientation. These results suggest that further work is required to: (1) account for the air interactions associated with spray formation processes from a flat fan nozzle; (2) examine the interactions between wind speed and the forward motion of the sprayer.

Modeling on atomization and vaporization process in flash boiling spray

This paper presents the model analysis on atomization and vaporization processes in a flash boiling spray based on experimental results obtained from an injection system in the suction manifold of a gasoline engine. Two kinds of liquid fuel, n-Pentane and n-Hexane, are injected into quiescent gaseous atmosphere at room-temperature with low pressure through a pintle type injector. Fuel spray is observed, by taking photographs, with variation of the ambient back pressure. Then, in the flash boiling spray region where the back pressure was below the saturated vapor pressure of fuel, the bubble nucleation process due to flash boiling was modeled by both experimental results and the nucleation rate equation as a parameter of the pressure difference between back pressure and vapor pressure. Further, the fuel vaporization process was assessed by considering growth calculation of cavitation bubbles and fuel evaporation from the film surface due to heat transfer at the gas-liquid interface. Accordingly, we could estimate quantitatively the transient changes in the bubble growth and the vapor mass fraction inside the spray for each back pressure condition.

Focused-image holography as a dense-spray diagnostic

The denser regions of sprays need to be probed for us to understand further the basic phenomena of the breakup of a bulk liquid into droplets and the subsequent drop dynamics. The instruments currently available to spray diagnosticians, the Malvern spray analyzer and the phase/Doppler particle analyzer, cannot be used in the denser regions of the spray and in regions where ligaments or nonspherical droplets exist. Holography, as applied in the past, has permitted the interrogation of nonspherical drops but in general has been applied to droplet-dominated dilute regions of sprays. Using a focused-image holographic system, with the advantages of an imaging lens and side lighting, we can probe highly complex regions of sprays, such as those that include bubble explosions and complex ligament formation at the nozzle exit. In this paper we present components of a focused-image holographic system for spray analysis, advantages and limitations of that system, and how the longitudinal magnification varies as a funct on of the length of the object along the optical path.

Structure of a spray flame stabilized on a production engine combustor swirl CUP

The near-filed structure of a turbulent flame stabilized on a production gas-turbine engine combustorfuel injector/air swirler system (i.e., swirl cup) has been studied using phase-Doppler anemometry and coheren anti-Stokes Raman spectroscopy (CARS) at atmospheric pressure. A Jet-A fuel spray from a simplex atomizer impinges onto a coaxially located liquid filming surface between counterswirling air streams for secondary atomization using high shear forces. An inverted conical flame with a cone angle of ≈90 o was formed in the inner shear layer of the low-temperature droplet-laden hollow-cone air jet, which divides high-temperature internal and external recirculation zones. Small droplets disappeared in the inner and downstream spray regions by evaporation along the droplet trajectories at higher temperatures and longer residence times, theraby increasing arithmetic and Sauter mean diameters. No fuel droplets were found in the internal recirculation zone. Despite frequent laser-induced breakdown in the dense spray region, CARS thermometry successfully determined the gas temperature. The temperature histograms show a wide distribution inside the flame zone, indicating enhanced stirring of cold air, vaporized fuel, and hot combustion products. On the other hand, the fate of fuel droplets, which did not participate in the combustion process directly in the near field, must play an important role in determining the combustion efficiency and pollutant emission levels.

The structure of the dense core region in transient diesel sprays

Intermittent and highly transient dense diesel sprays were investigated using two-dimensional laserlight scattering and transmission techniques to decipher the internal spray structure. The experiments were conducted at maximum injection pressures ranging from 19 to 32 MPa using an electronic diesel injector with a single-hole nozzle. The experiments involved the imaging of the unperturbed structure of the dense core region of a full-cone intermittent diesel spray on photographic film at high resolution and the simultaneous measurement of laser sheet transmission along the centerline of the spray by an array consisting of 2048 diodes. At all injection pressures, line-of-sight laser sheet transmission measurements showed that the dense core region is fragmented very near to the nozzle exit, about 25–30 nozzle diameters downstream, and perhaps much closer. Further downstream from this location, the transmission measurements and simultaneous 90° scattering images revealed that the structure has an intermittent appearance with pockets of dense spray separated by relatively void regions. Two-dimensional images displayed a highly atomized spray structure beyond 50 nozzle diameters downstream, with no indication of an intact liquid core for the range of injection pressures studied. After eliminating the secondary scattering present by applying a low-pass filter and subtracting the filtered image from the original image, it was demonstrated for enlarged views of the spray that the scattering is from randomly spaced point sources, which were interpreted as droplets, rather than liquid columns, ligaments, or large blobs of fuel. It seems that pressure-atomized and fully pulsed round liquid sprays have a significantly different near-field structure than their steady counterparts. These findings, along with the recent work of others, are inconsistent with the existence of a long (on the order of 100 nozzle diameters or more) intact liquid core in unsteady diesel sprays.

Numerical Simulation of Entrainment of an Unsteady 2-D Turbulent Jet.

The mixing process in an unsteady two-dimensional jet issued into quiescent surroundings at a high Reynolds number was simulated numerically by a discrete vortex method. Elements of injected fluid and surrounding fluid were marked by different colors, and their motion was visualized. Observation of fluid motion inside and outside the jet provided new insights on behavior inherent to the transient jet : 1) the elements of fluid injected at the initial stage of injection are distributed in the peripheral region of spray, and 2) in the spray tip region, the surrounding fluid is only slightly entrained into a jet because the surrounding fluid is pushed outward by a pair of large vortices.

Spray Cooling with Formation of Liquid Film. Distribution of Heat Transfer Coefficient in High-Temperature Region.

This report presents experimental results on the distribution of the heat-transfer coefficient along a large flat plate in the high-temperature region of spray cooling. The results show that the heat-transfer characteristics of spray cooling in the high-temperature region can be divided into the impinging and the wall regions. In the impinging region, with increasing distance from the stagnation point of the spray flow, the local heat-transfer coefficient decreases rapidly with the decrease of the local droplet-flow rate and the minimum temperature of the high-temperature region increases due to the propagation of the wetting front. However, in the wall region, the local heat-transfer coefficient approaches a constant value dependent on the total droplet-flow rate, and the minimum temperature is independent of the distance from the stagnation point.

The droplet group microexplosions in water-in-oil emulsion sprays and their effects on diesel engine combustion

To clarify the combustion mechanism of water-in-diesel fuel emulsion sprays and to evaluate the possible benefit of emulsions in practical usage, combustion bomb experiments, dynamic engine tests, and computer simulation were carried out, and some useful conclusions have been reached. The droplet group (lump-fashioned) microexplosions in water-in-diesel fuel emulsion sprays on an eddysizescale during the atomization, evaporation, and combustion processes in a high-pressure, high-temperature bomb were observed with a multipulsed, off-axis, image-plane, ruby laser holocamera and a highspeed camera. The explosions eject droplet fragments from the spray region to several millimeters away, improving the fuel-air mixing process and speeding up the flame propagation. A no-water layer formed by a Hill vortex was also observed in emulsion droplets. The ambient temperature has the most important influence on the occurrence and violence of the microexplosion. Road-load-simulation engine tests were carried out on a dynamic engine test bed. The experimentalresults show that emulsion fuels have no significant influence on fuel consumption and reduce engine torque if no adjustment is made for the injection system, but that smoke emission is much improved when emulsion fuel is used. The combustion characteristics and the rate of heat release are also analyzed to reveal the difference between emulsion and diesel fuel. The relationships between the optimum water percentages and fuel consumption under various operating conditions were analyzed by numerical combustion modeling.

Modelling of Atomization and Vaporization Process in Flash Boiling Spray. 2nd Report, Model Analysis on Atomization and Vaporization Process.

This paper presents the model analysis of atomization and vaporization process in a flash boiling spray based on the experimental results. Two kinds of liquid fuel, n-Pentane and n-Hexane, are injected into quiescent gaseous atmosphere at room temperature with low pressure through a pintle-type injector. Then, in flash boiling spray region where the back pressure is below the saturated vapor pressure of fuel, the bubble nucleation process due to flash boiling is modeled by the nucleation rate equation. Furthermore, fuel vaporization process is assessed by considering bubble growth calculations of vapor cavitation phenomena and fuel evaporation due to heat transfer process. Accordingly, we could estimate quantitatively the transient changes in bubble diameter and vapor mass fraction inside the spray for each back pressure condition.

Speed measurements in the developing region of an electrohydrodynamic spray using laser diagnostics

Experiments were conducted to assess the ability of a particle-counter-sizer velocimeter (PCSV) to measure droplet speeds in the developing region of an electrohydrodynamic spray. The working fluid, ethanol, was pumped through a stainless steel capillary tube at a constant mass flow rate. The capillary tube was held at various positive potential voltages with respect to an externally grounded funnel located below the tube. Data were acquired at two axial stations, directly under the capillary tip, in the developing region of the subject spray. The results indicated that the PCSV, with an experimental modification to the speed reduction process, performed well in this region. Speeds were measured at the near station within an average uncertainty of 2.2% and a corresponding maximum uncertainty of 3.0%; speeds at the far station were measured within an average uncertainty of 4.6% and a corresponding maximum uncertainty of 6.0%.

Continuous- and dispersed-phase structure of dense nonevaporating pressure-atomized sprays

The structure and breakup properties of the dense-spray region of nonevaporating pressure-atomized sprays were studied. The multiphase mixing layer near the injector exit was emphasized, considering large-scale (9.5mm injector diameter) water jets injected vertically downward in still room air. Phase-discriminating laser velocimetry and double-pulse holography were used to measure phase velocities and drop-size properties for both nonturbulent and turbulent jet exit conditions. Present test conditions involved two types of primary breakup: 1) aerodynamic breakup for nonturbulent jets where properties could be correlated using earlier aerodynamic breakup theories; and 2) turbulent breakup for turbulent jets where drop properties could be related to liquid turbulence properties. Both mechanisms yielded Weber numbers exceeding secondary drop breakup limits near the liquid surface. Significant effects of separated flow were observed for present test conditions; however, scaling analysis suggests reduced effects of separated flow at higher injector velocities and ambient pressures—largely due to finer atomization.

Structure of the near-injector region of nonevaporating pressure-atomized sprays

The dense-spray region of pressure-atom ized nonevaporating sprays was studied, emphasizing the properties of the multiphase mixing layer that surrounds the liquid core during atomization breakup. The dispersed-phase properties of a large-scale (9.5-mm injector diameter) water jet injected vertically downward in still air were measured using single- and double-pulse holography for both fully developed and slug flow jet exit conditions. The inner portion of the mixing layer contained large irregularly shaped liquid elements and drops, and the proportion of spherical drops increased and drop sizes decreased with increasing radial distance. For present test conditions, the liquid core and the large liquid elements cause mean liquid volume fractions to be high near the axis; however, the gas-containing region was relatively dilute at each instant. Additionally, the velocities of large drops were generally much larger than small drops and predictions based on the locally homogeneous flow approximation, providing direct evidence of significant separated-flow effects in the flow. Finally, the degree of flow development at the jet exit had a substantial effect on the structure of the mixing layer, with increased turbulence levels increasing the number and size of large irregular liquid elements through distortion of the surface of the liquid core—enhancing rates of removal of liquid from the core.