Effects of wall effusivity on secondary droplet atomisation from single and multiple drop impact onto heated surfaces (original) (raw)
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Thermally induced secondary drop atomisation by single drop impact onto heated surfaces
The paper reports an experimental study of single millimetric water drop impacts onto a hot aluminium alloy (AlMg 3 ) surface. The main aim of this investigation was to evidence the effect of the heat transfer mechanism on the secondary atomisation, but the effect of impact velocity and surface characteristics on the impact outcomes were also studied.
Secondary atomisation of drop impactions onto heated surfaces
2006
The work addresses an experimental study aimed at characterizing the effect of the impact angle on secondary atomization, at different boiling regimes. The study considers impacts of water and isooctane droplets, onto a heated stainless steel surface, with known roughness. Sizing of the larger droplets, within the range of 40 m up to a few millimeters is carried out by post processing of the images recorded with two CCD cameras (Image Analysis Technique – IAT), while the diameter of small droplets, within the range of 5.5 m up to 250 m, is evaluated based on phase Doppler anemometry measurements. The PDA measurements are integrated in time up to the impact instant t0, before comparison with the IAT probability distribution. Finally, a scaling of the IAT p.d.f. and of the integrated PDA p.d.f. is performed by equating the count values where the two size ranges overlap, obtaining an extended p.d.f., which is used to evaluate secondary droplet size diameters, over a wide range of dropl...
Secondary atomisation produced by single drop vertical impacts onto heated surfaces
The paper reports an experimental analysis of the secondary atomisation produced by the impact of a single drop on a solid heated surface. Different wall temperatures were used to study different boiling regimes. The size of secondary drops produced by the impact was measured by two techniques, namely the phase Doppler anemometry (PDA) and the image analysis technique (IAT); this allowed to extend the measurable size range from 5.5 lm up to few mm. Two impacting walls with different surface roughness were used to show the effect of this parameter on different atomisation regimes. The liquid viscosity was also varied in a limited range by using water-glycerol mixtures. Image analysis allowed also to define the details of the morphology of drop spreading and break-up.
Secondary atomization of drop impactions onto heated surfaces
The work presented here addresses an experimental study aimed at characterizing the effect of the impact angle on secondary atomization, at different boiling regimes. The study considers impacts of water and isooctane droplets, onto a heated stainless steel surface, with known roughness. Sizing of the larger droplets, within the range of 40µm up to a few millimeters is carried out by post processing of the images recorded with two CCD cameras (Image Analysis Technique -IAT), while the diameter of small droplets, within the range of 5.5µm up to 250µm, is evaluated based on phase Doppler anemometry measurements. The PDA measurements are integrated in time up to the impact instant t 0 , before comparison with the IAT probability distribution. Finally, a scaling of the IAT p.d.f. and of the integrated PDA p.d.f. is performed by equating the count values where the two size ranges overlap, obtaining an extended p.d.f., which is used to evaluate secondary droplet size diameters, over a wide range of droplet diameters, from 5.5µm up to a few millimeters. The results show that the measured secondary droplet diameter is faintly influenced by large impact angles (α>45º) but becomes considerably smaller for small inclination angles (α<15º), as the larger droplets are swept away due to gravitational forces. The effect of the gravitational forces in decreasing the measured mean droplet diameter is less significant for isooctane drop impacts, as the mean secondary diameters are globally smaller, when compared to those obtained from water drop impacts. Differences in secondary droplet size diameters between isooctane and water droplets are attributed to the much smaller surface tension of isooctane.
Secondary atomization of drop impactions onto heated inclined surfaces
The work presented here addresses an experimental study aimed at characterizing the effect of the impact angle on secondary atomization, at different boiling regimes. The study considers impacts of water and isooctane droplets, onto a heated stainless steel surface, with known roughness. Sizing of the larger droplets, within the range of 40µm up to a few millimeters is carried out by post processing of the images recorded with two CCD cameras (Image Analysis Technique -IAT), while the diameter of small droplets, within the range of 5.5µm up to 250µm, is evaluated based on phase Doppler anemometry measurements. The PDA measurements are integrated in time up to the impact instant t 0 , before comparison with the IAT probability distribution. Finally, a scaling of the IAT p.d.f. and of the integrated PDA p.d.f. is performed by equating the count values where the two size ranges overlap, obtaining an extended p.d.f., which is used to evaluate secondary droplet size diameters, over a wide range of droplet diameters, from 5.5µm up to a few millimeters. The results show that the measured secondary droplet diameter is faintly influenced by large impact angles (α>45º) but becomes considerably smaller for small inclination angles (α<15º), as the larger droplets are swept away due to gravitational forces. The effect of the gravitational forces in decreasing the measured mean droplet diameter is less significant for isooctane drop impacts, as the mean secondary diameters are globally smaller, when compared to those obtained from water drop impacts. Differences in secondary droplet size diameters between isooctane and water droplets are attributed to the much smaller surface tension of isooctane.
Experiments in Fluids, 2009
The work presented here addresses the study of the parameters governing the secondary atomization mechanisms occurring at the impact of individual droplets onto heated targets, within the various boiling regimes, in order to determine correlations describing the secondary droplet characteristics. The analysis is based on the experimental data obtained by performing a qualitative and quantitative description of the size and velocity of the secondary droplets generated at the impact of droplets of different liquids, at variable conditions, (including different impinging angles), onto surfaces with dissimilar topographical characteristics. Following the work previously presented, this characterization is performed making use of simultaneous image analysis and phase Doppler measurements, as the combination of both techniques allows evaluating extended size distributions from 5.5µm up to a few millimetres and to cover the full range of secondary droplet sizes which occur at all heat transfer regimes. Therefore, particular emphasis is given to the relations concerning secondary droplet sizes, which account for droplet diameters evaluated from the extended size distributions, to take the most out of this methodology. Final correlations are proposed to describe representative extended time averaged diameters within the nucleate and the film boiling regimes.
Journal of heat transfer, 2015
We studied experimentally the behavior o f a single cold water droplet that falls onto a hot horizontal flat solid surface in the film boiling region. We found that when the droplet hits the surface (for the first time), three different regimes may occur. These regimes depend on the ratio o f fluid's inertia and its surface tension (the Weber number, indicated as We) and on the surface temperature. For relatively low We numbers or surface tem peratures, the droplet completely bounces hack from the surface and no breakup occurs. For intermediate We numbers or surface temperatures, the spreading stage is faster and the droplet undergoes spreading and partial recession before it breaks up into bouncing small secondary droplets that leap inward and successively coalesce above the surface to form a single droplet once again. For high We numbers or surface temperatures, the spreading velocity is higher, the contact area with the surface is greater, and the liquid film thickness is smaller. Thus, during the expansion o f the spreading stage, the droplet breaks up into bouncing small secondary droplets that uninterruptedly leap outward and travel independently. We also present the limiting conditions differentiating between the different behaviors found. This work shows droplet film boiling behaviors that are essen tially different than droplet levitation on top o f a thin vapor layer, as mainly assumed in theoretical models. We also observed that when a droplet hits the surface fo r the second, consecutive time (and on), the droplet behaves somewhat differently due to its preheating, very low impact velocity, different shape, spin, orientation, and the surface temperature. At the second impact on the surface (and on), the droplet can continue its bounce in a unique and different manner than in the first impact or it can explode violently to small secondary droplets. Both are unique and differentiating mainly by the droplet's shape and orientation at the exact moment o f impact on the surface. Additionally, a rare and unique view o f droplet-droplet collision during film boiling is presented. This type o f col lision behaves in a different way than other droplet-droplet collisions and compared to the adiabatic case o f droplet-droplet collision on a nonlieated surface. The behaviors found, presented, and discussed in this study change our view o f the droplet-surface and droplet-droplet interactions that occur in spray cooling in the film boiling region.
Secondary atomization of water and isooctane drops impinging on tilted heated surfaces
Experiments in Fluids, 2007
The present paper reports an experimental study aimed at characterizing the effects of heat transfer on the secondary atomization, which occurs during droplet impact on hot surfaces at conditions reproducing those occurring at fuel injection in internal combustion engines. The experiments consider single isooctane and water droplets impacting at different angles on a stainless steel surface with known roughness and encompass a range of Weber numbers from 240 to 600 and heat transfer regimes from the film-vaporization up to the Leidenfrost regime. The mechanisms of secondary breakup are inferred from the temporal evolution of the morphology of the impact imaged with a CCD camera, together with instantaneous measurements of droplet size and velocity. The combination of a technique for image processing with a phase Doppler instrument allows evaluating extended size distributions from 5.5 lm up to a few millimetres and to cover the full range of secondary droplet sizes observed at all heat transfer regimes and impaction angles. Temporal evolution of the size and velocity distributions are then determined. The experiments are reported at impact conditions at which disintegration does not occur at ambient temperature. So, any alteration observed in droplet impact behavior is thermally induced. The analysis is relevant for port fuel injection systems, where droplets injected to impact on the back surface of the valves, behave differently depending on fuel properties, particularly when the use of alcohols is considered, even as an additive to gasoline.
The combined effects of surface topography and heat transfer on droplet/wall interaction mechanisms
This paper addresses an experimental study aimed at characterizing the combined effects of heat transfer and surface topography on the dynamic behaviour of fuel droplets. The experiments make use of a simplified flow configuration consisting of individual droplets impacting onto flat hot surfaces with different topographies, roughness and temperatures. The analysis considers the onset of splash, which involves the identification and characterization of different disintegration mechanisms, performed in terms of crown morphology, temporal evolution and secondary droplet characteristics, for different boiling regimes. Generally, the results show that increasing liquid viscosity generates less efficient secondary atomization since secondary droplets are larger and in a small number. In this context the surface topography has an important role since it promotes the occurrence of fast disintegration mechanism ("prompt" splash), at lower Weber numbers. However this favourable effect is less evident in nucleate and in film boiling regimes, as the increase of surface roughness seems to enhance bubble boiling but also enhances diameters of secondary droplets.