Subscriber access provided by Gothenburg University Library
Article
Experimental study of Diesel fuel droplet impact onto similar size polished spherical heated solid particle Hesamaldin Jadidbonab, Nicholas Mitroglou, Ioannis Karathanassis, and Manolis Gavaises Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01658 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Experimental study of Diesel fuel droplet impact onto similar size polished spherical heated solid particle Hesamaldin Jadidbonab*, Nicholas Mitroglou, Ioannis Karathanassis and Manolis Gavaises Department of Mechanical Engineering, City University of London, UK Abstract The head-to-head impact of Diesel fuel droplets onto a polished spherical brass target is experimentally investigated. High speed imaging has been employed to visualise the impact process for wall surface temperatures, Weber and Reynolds numbers in the range 140-340°C, 30-850 and 210-1135, respectively. The thermo-hydrodynamic outcome regimes occurring during for the aforementioned range of parameters has been mapped on the We-T diagram. Seven clearly distinguishable post-impact outcome regimes were identified, which are conventionally termed as coating, splash, rebound, breakup-rebound, splash-breakup-coating, break up-coating and splash-breakup-rebound. In addition, the effect of Weber number and surface temperature on wettability dynamics have been examined; the temporal variation of the dynamic contact angle, non-dimensional spreading diameter and liquid film thickness forming on the solid particle have been measured and reported. Keywords: droplet impact, Diesel fuel, spherical particle, head-on collision 1. Introduction The dynamics of liquid droplet impingement onto solid objects are of relevance to many industrial-process systems such as spray coating, ink-jet printing, fire suppression, internal combustion engines, spray cooling and fluid catalytic cracking (FCC) among others. For instance, in Diesel engines, the impingement of fuel spray onto the heated chamber walls affects the engine-out emissions and fuel economy. The importance of studying the individual droplet impact on solid surfaces under well-controlled conditions was highlighted in [1], as it can be used to describe the behavior of spray impact and to predict its outcome.. The works of [1–3] summarise the important numerical and experimental studies of droplet impact onto solid surfaces for isothermal and non-isothermal conditions. Several parameters, such as droplet velocity, diameter and angle of impact [4,5], liquid physical properties [6], surface conditions (roughness, wettability) [7,8], surface temperature [9] and ambient pressure [10], are of great importance for the deformation of droplets upon impact. In order to macroscopically characterise the outcome of the dropletsurface interaction, three non-dimensional numbers are often used namely, Reynolds (Re=ρV0D0/µ), Weber 2 (We=ρV0 D0/σ) and Ohnesorge (Oh= µ/√D0σρ) numbers. If the droplet impingement occurs onto flat solid surfaces with infinite extent, the post-impact outcome is widely falling into three major regimes [6]: deposition, splash and rebound. This classification becomes more complex for impingement on heated and rough surfaces as the collision dynamics are greatly affected by the heat transfer from the surface to the liquid [11]. Upon impacting on a heated surface with temperature much higher than the liquid boiling temperature, a thin layer of vapor forms between the solid surface and the droplet that is typically a few nanometers thick [9]. This layer may prevent the contact between liquid and solid which, in turn, decreases the heat transfer rate. If the pressure force exerted on the droplet from the vapor layer overcomes the droplet’s weight, then it levitates from the surface and may rebound; this is known as the Leidenfrost regime [12]. This regime can be influenced by several parameters like droplet size [13], impact velocity [14], surface roughness [15] and impact angle [16]. It can be also expected that an increase in the ambient pressure, thus the air density, increases the Leidenfrost temperature, due to change in liquid’s boiling point [17]. When the boiling temperature (TBP) of the liquid is lower than the wall-surface temperature (TW), thermally induced disintegration may occur; this is mostly due to vapor bubble formation and breakup at the liquid free surface of the spreading lamella. Besides break-up of the lamella itself has been reported, as well [18]. Several studies (selectively [19–22]) have been performed to characterise this disintegration process within different heat transfer regimes. Hydrodynamic impact outcomes can be illustrated on regime maps as a function of surface temperature and impact Weber number [23–25]. It should be noted that the outcomes can be also categorized with regards to heat transfer associated with the corresponding boiling modes [26] (film evaporation for TW70. In order to obtain the spatial variation of the droplet periphery shape, a spreading angle α (Figure 4b) was initially defined as:
=
1 . cos ‖ 2 . ‖
(2)
where, and are two extended vectors drawn from the particle center to the left and the right droplet-particle intersections. This angle determines the angular position of the droplet edge profile on the particle surface. An indicative plot of the droplet lamella variation with spreading angle is presented in Figure 4c. The spreading diameter was subsequently calculated, as the perimeter length spanning between the two droplet-particle intersection points as: () = . !
(3)
This diameter and the film thickness at the north pole of the particle surface, non-dimensionalised by the pre-impact diameter D0, constitute the spreading β factor and the non-dimensional film thickness h*, quantities suitable for the characterization of the droplet shape deformation. The maximum uncertainty of measured spreading factor in each case was ±8µm. The contact angle was measured exactly at the intersection of the deformed droplet interface with the solid surface. The interface was approximated by a cubic polynomial and, subsequently, two vectors originating from the triple contact point and being tangent to the particle surface and the approximated interface, respectively, 2 were drawn, so as to define the contact angle. The coefficients of determination (R ) of the fitted curves were above
6
ACS Paragon Plus Environment
Page 7 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
99% for all test cases and the dynamic contact angle was computed with an accuracy of ±1.5°. The percentage of variation between 5 repetitions of each test case was less than 2%. 3.
Results and discussion 3.1. Classification of post–impact regimes: During the process of droplet impingement on a heated surface and for the operating conditions summarized in Table 1, seven distinctive outcome regimes were observed during the course of the present experimental investigation, termed as coating, rebound, splash, breakup-rebound, splash-breakup, breakup and splash-breakup-rebound. Figure 5 presents the characteristic images of these outcomes. The 3D reconstructed view (produced employing a dual-view optical arrangement) allows the demonstration of the impact symmetry (which is missing from the available literature). It has been shown, that in the case of asymmetric impact, the unbalance between gravitational and centrifugal forces on the droplet can significantly change the hydrodynamic and heat transfer processes arising during the droplet impact [36,48,49]. The 3D reconstruction also facilitates a better illustration of secondary droplets setting in on the particle surface at breakup regime. The impact process and the post-impact regime depends on the impact Weber number, the target surface temperature and droplet-to-particle size ratio.
t*= 0
t*= 0.26
t*= 0.78
t*= 1.82
t*= 2.86
t*= 1.82
t*= 2.86
0.2 mm (a) 2D-front view of coating regime
t*= 0
t*= 0.26
t*= 0.78
(a’) 3D-Isometric view of coating regime
t*= 0
t*= 0.26
t*= 0.78
(b) 2D-front view of splash regime
7
ACS Paragon Plus Environment
t*= 1.82
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
t*= 0
t*= 0.26
Page 8 of 23
t*= 1.82
t*= 0.78
(b’) 3D-Isometric view of coating regime Splash regime
t*= 0
t*= 2.7
t*= 5.4
t*= 8.1
t*= 10.8
t*= 8.1
t*= 10.8
(c) 2D-front view of rebound regime
t*= 0
t*= 2.7
t*= 5.4
(c’) 3D-Isometric view of rebound regime
8
ACS Paragon Plus Environment
Page 9 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
(d) 2D-front view of droplet oscillation during the droplet lift up at rebound regime
t*= 0
t*= 2.7
t*= 8.1
t*= 5.4
t*= 10.8
(e) 2D-front view of breakup-rebound regime
t*= 0
t*= 2.7
t*= 5.4
t*= 8.1
t*= 10.8
(e’) 3D-isometric view of breakup-rebound regime
t*= 0
t*= 5.4
t*= 2.7
t*= 8.1
t*= 10.8
(f) 2D-front view of splash-breakup regime
t*= 0
t*= 2.7
t*= 5.4
t*= 8.1
t*= 10.8
(g) 3D-isometric view of splash-breakup-rebound regime Figure 5: Snapshots taken at different (non-dimentional) time instances of the impact of a Diesel fuel droplet onto a heated particle (a),(a’) We=30/T=140°C (coating regime), (b),(b’) We=520/T=140°C (splash regime), (c),(c’) We=30/T=340°C (rebound regime), (d) oscillating droplet during the lift up at rebound regime, (e),(e’) We=70/T=340°C (breakup-rebound regime) and (f) We=530/T=320°C
9
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 23
(splash-breakup regime), (g) We=530/T=340°C (splash-breakup-rebound regime). The 3D-isometric views presented were reconstructed based on two 2D-side views. Due to droplet complex interface in (f) and (g) 3D views have not been produced.
In the coating regime (see Figure 5a), the entire mass of the droplet is deposited on the particle upon impact and the droplet spreads on the surface. The combined effect of gravity and initial kinetic energy force the droplet lamella to stretch until its maximum diameter, which is detected at approximately t*=2. A receding phase follows until the droplet relaxes on the particle surface. This regime has been observed for Weber numbers below the splash threshold and when the heat transfer is not adequate to induce breakup or rebound. Splash (see Figure 5b) occurs when the droplet inertia becomes significantly larger than the viscous and surface tension forces. As the droplet approaches the particle surface, a liquid film spreads radially outwards. The droplet becomes unstable, deforms and smaller droplets are detached from the moving edge of the spreading lamella, while the remaining part of the droplet sticks on the solid particle. Impact characterized by higher values of the Weber number results in a much larger droplet contact area and a thinner liquid film over the surface at t*=0.26 could be detected, compared to the one in coating regime. However, this thin film, at the lower half of the droplet, remains only for a short period on the particle surface. Afterwards, it detaches from the particle surface through a flapping motion evident at t*=0.78. Due to the high radial velocity of this liquid sheet, which is larger than the capillary retraction of the liquid rim, small drops are detached and ejected radially at t*=1.82. This regime is very similar to corona splashing that occurs often during droplet impact on flat surfaces [50]. In rebound (Figure 5c), the droplet impacts onto the surface, spreads, recoils and finally levitates or rebounds from the surface. The static contact angle of the spherical particle and the Diesel fuel is around 30°, thus, the surface can be considered as hydrophilic for impact under cold condition. For low impact Weber-number values, upon the first touchdown of the droplet on the heated particle, a vapor layer forms due to the high heat transfer rate and the intense evaporation at the solid-liquid interface, as shown for t*=2.7. At this time, the impact momentum forces the droplet to deform to a disk-like shape. When the inertial force pushes the droplet towards the particle surface, a relatively large pressure gradient builds up and generates a vapor flow in the spreading direction. The specific vapor flow acts as an opposing force (upward thrust), hence creates strong retraction (t*=5.4), lifts up the droplet (t*=8.1), until it completely rebounds from the particle surface (t*=10.8). The shape deformation continues even after the droplet rebounds (t*>10.8). As depicted in Figure 5d, upon the departure from the particle surface, the droplet undergoes a strong fluctuating expansion and contraction along its main axis, while the inertial force is counteracted by the surface tension force, which tends to minimize the interfacial area. In the case that surface tension does not suffice so as to maintain the droplet shape, breakup occurs at the location of minimum droplet thickness; a similar behavior has been reported in [38] for water-glycerol mixture (35 wt.%) at TP=~274°C and We=20.6; however, the rebounding droplet was much more elongated in the rebound direction while most of the droplet mass accumulated in the leading part of the droplet followed by a narrow connecting ligament. During the spreading and recoiling phases of the rebound regime there is almost no direct contact between the particle surface and the droplet [51]. As it can be seen in Figure 5e, if the impact Weber number and/or particle temperature are high enough, the droplet spreads widely over the particle periphery; then, the lamella thickness decreases significantly at the wetted area until the maximum spreading diameter is reached (t*=2.7). Irregular interface deformation is more intense due to nucleate boiling. Neck areas (locations where the liquid film raptures) start to appear leading to the formation of small drops over the dry surface. The remaining droplet mass forms a hollow circular ring with a thick rim (t*=5.4). This ring also undergoes strong deformation and finally breaks up into three droplets (t*=10.8). Similarly to the We=30 case, these three droplets are finally lifted from the surface and rebound in different directions (t*=16.3), in the so-called breakuprebound regime. However if the surface temperature is lower than the Leidenfrost point, the droplets remain on the surface (breakup). At this condition, the droplet impact process shows an asymmetric behavior. At the highest values of impact-Weber number and particle temperature (see Figure 5f and e), the droplet splashes at t*=2.7 with the mechanism discussed for the conditions of Figure 5b. Increasing the Weber number, enhances the prevalence of inertial forces during the early stages of the impact. Also, surface tension becomes less pronounced above the Leidenfrost temperature. After splashing, the remaining mass of a droplet spreads tangentially and deforms into a liquid sheet of highly complex and perturbed topology over the particle surface. The sheet then breaks up into
10
ACS Paragon Plus Environment
Page 11 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
several small droplets (t*=8.1). Depending on the temperature, the satellite droplets are possible to either stick to the particle (Figure 5e) or rebound from it (Figure 5f), as is shown in detail in Figure 6.
. Figure 6: We-T regime diagrams of Diesel fuel droplet impact on a heated brass particle and Characteristic images of the outcome regimes
Figure 6 in particular, presents a map of the droplet-impact outcomes possible to set in for the We-T range examined. The respective characteristic droplet-shape images for each regime are also presented in Figure 6. In order to highlight the differences in reference to the flow phenomena arising during the impact of either single or multiplecomponent fuel droplets on a heated particle, a number of experimental studies referring to the regime mapping of the post-impact processes of single-component fuel droplets have been summarized in Table 2. As shown in Figure 6, the coating regime in the present study is detected for temperatures below the Leidenfrost point and relatively low Weber numbers (We50 for DTP=0.31. It has also been shown that increasing DTP significantly enhances the breakup mode for water, however this effect is less profound for isopropyl [38]. In this study, the Leidenfrost temperature was specified to be around 320°C for low Weber numbers and increased by ~20°C for We >50; this is in good agreement with the previously reported temperature (T=357°C), above which the Diesel fuel droplet impacting on a flat hot aluminum alloy surface starts to levitate on its own vapor layer [52]. The deviation of the reported values is attributed to the different composition of the Diesel sample, droplet deposition method, thermo-physical properties and roughness of the solid surface [53]. The dependency of the Leidenfrost temperature (TL) to the Weber number (We) can be illustrated by comparing the vapour pressure, which increases with particle temperature and the inertial energy of the impacting droplet, which is proportional to We. In order for the droplet impact outcome to remain above the Leidenfrost temperature with increasing We, higher vapour pressure and, thus, higher target-particle temperature is required. The dependency of the Leidenfrost point to the Weber number is generally limited to 40°C for We>20 [54,55]. However, in [56], a much stronger dependency has been reported (180°C). This could be attributed to the polished surface employed for the experiment as low surface roughness affects nucleation and thus heat transfer rate and consequently the Leidenfrost temperature. For Diesel-fuel droplet with Weber-number values above 320, a
11
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 23
transition to the splash regime was observed for the entire temperature range examined. It is already shown by several studies (see selectively [34,50,57]) that the splashing threshold for different single-component liquids depends mostly on viscosity and the DTP. The splashing threshold for a water droplet has been reported to be ~210 with DTP=0.287 [34], while it increases to 240 with DTP=0.4 for heptane [57]. In contrast to the Leidenfrost temperature, the transition border to the splashing regime is independent of the surface temperature and it is only a function of Weber number. In the present study, a particle temperature of 320°C was required, in order to induce breakup of the droplet, for the minimum value of the Weber number (We=70) examined. Nevertheless, the minimum temperature value for breakup gradually reduced, as the impact Weber number increased from 70 to around 850 (see Figure 6). Similar behavior has been manifested in [38] for water, where an increase in the particle temperature from 250°C to 350°C, has shifted the breakup We to higher values by ~6, 4 and 3 orders of magnitude for DTP=0.228, 0.249 and 0.287, respectively. This can be attributed to the effect of the particle temperature on heat transfer rate above and below the Leidenfrost point. Increase of the particle temperature for conditions below TL enhances heat transfer and thus, promotes breakup. On the contrary, for wall temperature above TL, the vapour layer thickness increases which hinders heat transfer [58]. Reference
[37]
[38]
System • • • • • • • • • • • • •
Findings
Water (8
