uncorrected proof

Fuel xxx (2018) xxx-xxx

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Fuel

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Spray morphology transformation of propane, n-hexane and iso-octane under flash-boiling conditions Yanfei Lia⁠ , Hengjie Guoa⁠ , Zhifu Zhoub⁠ , Zhou Zhanga⁠ , Xiao Maa⁠ , Longfei Chenc⁠ ,⁠ ⁎⁠ a b c

State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China School of Energy and Power Engineering, Beihang University, Beijing 100191, China

ABSTRACT

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ARTICLE INFO Keywords: Spray Flash boiling Gasoline direct injection Collapse Nucleation

Spray collapse has been widely observed under both flash-boiling and non-flash-boiling conditions when multi-hole gasoline direct injection (GDI) injectors with compact nozzle configurations were used. The main objective of this study is to further understand the collapse mechanisms by using liquids with appreciably different properties (propane, n-hexane and iso-octane). The tests were carried out in a constant volume vessel with ambient pressures ranging from 0.6 bar to 11.0 bar and fuel temperatures from 30 °C to 110 °C. Flashing propane sprays presented a non-collapse feature under elevated-ambient-pressures but flash-boiling conditions. By close-up examination of flashing propane sprays over a wide range of liquid temperatures and ambient pressures, it was found that there should be an ambient pressure threshold between 1.0 and 3.0 bar between the collapsed and the non-collapse sprays for propane. The spray collapse occurred as the ambient pressure is below the threshold. The non-collapse feature for flashing propane sprays under the ambient pressures beyond the threshold was attributed to the prohibition of nucleation and bubble growth under elevated ambient pressures.

Nomenclature GDI JIC CIC ASOI

Gasoline Direct Injection Jet induced Collapse Condensation induced Collapse After Start of Injection

1. Introduction

Flash boiling is a rapid phase change once a liquid is abruptly exposed to an environment with the ambient pressure lower than its local saturation vapor pressure. It has been widely found and used in the fields, such as desalination [1,2], ice slurry production [3], spray cooling [4–6] and so on, while

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several comprehensive reviews have also been made [7–9] to summarize the available experimental and numerical findings. In the past years, flash boiling has been viewed as a promising way to enhance atomization and the potential in improving fuel economy and emissions of gasoline direct injection (GDI) engines. Thus, intensive studies have been carried out experimentally and numerically from the fundamental nucleation and bubble growth to external spray behaviors under the thermal conditions of GDI engines [10–20]. Zhang et al. [13,14] studied the effect of bubble formation inside a slit nozzle on the external breakup process of a superheated liquid jet and claimed that bubble number density was the main driving force for the breakup of a superheated jet. Serras-Pereira et al. [21] found that the in-nozzle flow regime is highly sensitive to the fuel temperature and reported that primary breakup and atomization quality could be enhanced by increasing superheated degree. Zeng et al. [11] examined the effect of fuel properties on

Corresponding authors. Email addresses: [email protected] (Y. Li); [email protected] (L. Chen)

https://doi.org/10.1016/j.fuel.2018.08.160 Received 19 February 2018; Received in revised form 19 August 2018; Accepted 31 August 2018 Available online xxx 0016-2361/ © 2018.


Fig. 1. Experimental setup.

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Table 2 Test conditions.

Table 1 Main properties of injection liquid.

Liquid density (@300 K) Viscosity (@300 K) Surface tension (@300 K) Vapor pressure (@300 K) Latent heat (@300 K) Specific heat (@300 K)

Unit

Hexane

Propane

Isooctane

kg/m3⁠

654.63

490.32

688.89

mN*s/m2⁠ mN/m

0.291 17.78

0.097 6.8

0.456 18.16

kPa

22.03

1000.33

7.18

kJ/mol kJ/kg*K

369.73 2.24

331.84 2.68

299.8 2.04

Propane, n-hexane, iso-octane

Injection pressure/MPa Injection temperature/°C Ambient pressure/bar

10.0 30/40/50/60/70/80/90/100/110 0.6/0.8/1.0/3.0/5.0/7.0/9.0/11.0

ing a multi-hole GDI injector. It was reported that Pa⁠ mb/Ps⁠ at is a fundamental parameter for spray angle and that the experimental spray angle can be used to obtain the saturation pressure curve for liquid. Wang et al. [19,23] reported that flash boiling can considerably improve the atomization quality during the end of injection stage and impingement with split injection strategy. Guo et al. [24] also characterized the external flashing spray and found that nucleation rate and thermal energy were the key parameters influencing the jet expansion. Macroscopic spray behaviors and atomization characteristics of flash-boiling sprays in optically accessible GDI engines were also investigated [10,12,15]. However, the collapse of multi-jet flash-boiling sprays [11,15–17,19,20,25] would negatively influence the designed jet trajectories and fuel distribution. Furthermore, the longer penetration length as a result of the spray collapse could lead to wall impingement and lubricant dilution, which were believed as one of the sources for super-knock and engine damage [26]. With the increasingly stringent emission regulations, wall impingement also becomes one of the main concerns on soot emissions [27–29]. The increase in superheated degree would enhance the collapse in a certain range of conditions [11,30,31]. The collapse was owing to the occurrences of jet overlap and low-pressure zone surrounded by the high-speed jets, but no consensus on the origin of the low-pressure zone have been achieved. Some researchers believe that the low-pressure zone was induced by the surrounding high-speed jets [30,32], which limited the gas exchange between the low-pressure zone and ambience. Recently, Li and Guo et al. [16,25] reported that the above-mentioned jet-induced low-pressure zone could not fully explain the collapse under flash boiling condi

Fig. 2. Footprint of the tested injector.

Item

Fuel

the spray structure at varied superheated degrees and found that a good self-similarity could be achieved for comparable superheated degrees for different liquids. Li et al. [22] found that the change in ambient pressure could significantly influence the spray morphology. Araneo et al. [17] investigated the effects of fuel distillation curve on the flash boiling process us

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Fig. 3. Display of image processing.

Fig. 4. Temporal evolution of n-hexane spray under different conditions.

tions using the proof by contradiction. They further proposed that the collapse was the consequence of vapor condensation based on a series of previous studies showing the liquid temperature at the nozzle exit instantly dropped to the ones below the local saturation temperature from the superheated state [33–36]. Furthermore, their latest study [37] demonstrated that the counteraction of radial momentum of vapor due to expansion could lead to the significant rise in static pressure and fulfill one of the necessary conditions for the vapor condensation. Besides, Li and Guo et al. [16,25,37] termed the collapse due to jet-induced low-pressure zone as Jet Induced Collapse (JIC) and termed the collapse under flash-boiling conditions as Condensation Induced Collapse (CIC). More details of the basic patterns of JIC and CIC can be described in Section 3.1. In spite of the intensive investigations on multi-jet flash boiling sprays, the mostly used liquids were gasoline and gasoline-like fuels, e.g. n-hexane, methanol, ethanol and iso-octane, which showed certain similarity in terms of spray morphology against Rp⁠ (pressure ratio of saturation pressure over ambient pressure) under certain conditions [11,17,31,38] and Zeng et al. [11] reported that the multi-jet flash-boiling sprays completely collapsed at Rp⁠ > 3.3. Recently, using propane as the injected substance, Lacey et al. [39] found that the flashing propane collapses at Rp⁠ much larger than 3.3, indicating the less generality of Rp⁠ in correlating spray morphology under flash boiling conditions for liquefied gas (i.e. propane). They further also proposed a new criteria for the collapse taken into account the geometrical parameters and liquid thermodynamic properties. The present study is inspired by one case with ambient pressure of 1000 kPa in the study by Lacey et al. [39], which

showed that the flashing propane sprays did not collapse. This finding may support the deduction in our previous study [16] that disruptive evaporation and thus massive vapor could eliminate the generation of jet-induced low-pressure zone if the jet-induced low-pressure zone intended to formulate. (i.e. JIC cannot fully explain the collapse under flash boiling conditions.) Therefore, the first purpose in this study is to double-check whether the non-collapse feature of flashing propane sprays would still be observed or not when the nozzle configuration is changed. Then, what’s more important, the second one is to conduct the flashing propane spray tests with wider operating conditions to further understand the non-collapse process of flashing propane sprays, which is finally conducive to understanding the collapse behaviors of gasoline-like liquids. Herein, flashing propane sprays were studied in comparison with flashing sprays of iso-octane and n-hexane using a five-hole GDI injector. The ambient pressure ranged from 0.6 bar to 11.0 bar and the fuel temperature ranged from 30 °C to 110 °C. In contrast to the results by Lacey et al. [39], similar phenomena were found given the similar boundary conditions, but more interesting and worth noting results were delivered and a new understanding on the non-collapse feature of flashing propane sprays was proposed. 2. Experimental setup Fig. 1 shows the schematic of the experiment set-up. The experiment was carried out in a constant volume vessel. The injector was mounted at the top of the vessel, and two quartz windows were mounted oppositely. The fuel temperature was realized by the heaters around the injector, and monitored by

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Fig. 5. Effect of ambient pressure on spray morphology (Tf⁠ uel = 30 °C, imaging timing: 1.6 ms ASOI).

the thermocouple near the injector. The sub-atmospheric pressures were achieved by an Agilent HS452 centrifugal pump and the elevated ambient pressures were realized by high-pressure gas bottle. Two Omega pressure gauges (DPG409-025HG and DPG409-002BG) were used to monitor sub-atmospheric and elevated pressures, respectively. A high-speed camera (Photron SA X2) was utilized to capture the spray development of liquid phase using backlit method. The camera speed was set as 12,500 frame/s and the spatial resolution was 38 μm/pixel. A five-hole gasoline direct injector with the hole diameter of 0.18 mm was used, and the spray footprint is shown in Fig. 2. Propane, n-hexane and iso-octane were used. Propane has very high saturation pressure, n-hexane is frequently used to represent the light component of gasoline [17], and iso-octane is the surrogate fuel for gasoline [39]. The liquid properties have been listed in Table 1. The data of saturation vapor pressure at different liquid temperature is obtained from NIST chemistry WebBook.

2.2. Image process and data processing Herein, the main parameter extracted from the images is spray width, rather than penetration length and cone angle, which have been widely used and studied regardless of diesel sprays or gasoline sprays. The present study mainly focused on the spray collapse and the spray width against the axial distance can well quantitatively describe the collapse intensity using the image captured at the timing when the spray is fully developed. Similar method was also used in previous studies [16,25]. Fig. 3 gives the image processing procedure to extract spray width using in-house MATLAB codes. A typical spray image is shown in Fig. 3(a). In Fig. 3(b), the background was subtracted and the contrast was enhanced. In Fig. 3(c), the intensified image was binarized with the threshold values based on Ostu’s method. Due to the same image background, the variation of threshold values for different conditions is less than 2.6%. In Fig. 3(d), the edge of the spray was obtained from the binarized image. Finally, the spray width at different axial distance was obtained by measuring the distance from the injector axis to the right side of the spray because the right jet was the target jet. There are two main uncertainties associated with experimental results. One is the shot-to-shot variations due to the inherent feature of transient fuel spray and the other is the spray

2.1. Test conditions

The test conditions are listed in Table 2. The ambient pressure ranged from 0.6 to 11.0 bar, while the injection temperature varied from 30 to 110 °C. Note that the maximum liquid temperature for propane was 90 °C because the propane spray would become trans-critical if the temperature is higher (the critical temperature for propane is 96.74 °C).

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Fig. 6. Effect of ambient pressure on spray morphology (Tf⁠ uel = 90 °C).

edge determination. The turbulent and irregular edge is relatively diluted and this uncertainty may be 1–2 pixels, which correspond to physical length of 0.038–0.076 mm. However, the two uncertainties could be significantly reduced by the data averaging. To guarantee the accuracy of the results, six repeated tests were conducted under each experiment condition and the averaged spray widths are used. 3. Results

injection (ASOI) and the sprays have fully developed. For the sprays with no or weak jet-to-jet interaction, the spray width is generally linear with the axial distance, as shown in the reference case. Comparing with the spray width evolution of the reference case, the spray width in the non-flash-boiling case is almost superimposed in the initial 3 mm, then becomes smaller till 10 mm and becomes larger after 10 mm. With respect to the spray width evolution the flash boiling case, it shows a reverse trend that it is larger near the nozzle and becomes narrower than the spray width of the reference case. More details on the inner reason for the difference can be found in Refs. [16,25,37].

3.1. Typical spray patterns of JIC and CIC

3.2. General spray morphology at varied Pa⁠ mb and Tf⁠ uel

In the Introduction section, two collapse mechanisms, JIC and CIC, have been mentioned to explain the collapse under non-flash-boiling and flash-boiling conditions, respectively. Herein, for the better description in the following sections, the typical features of JIC and CIC are elucidated in Fig. 4, taken n-hexane sprays for instance. The top row of Fig. 4a indicates the spray under Pa⁠ mb of 1.0 bar and Tf⁠ uel of 30 °C, termed as the reference condition. The middle row indicates the non-flash-boiling spray under Pa⁠ mb of 11.0 bar and Tf⁠ uel of 30 °C, where JIC occurs. The bottom row indicates the flash-boiling spray under Pa⁠ mb of 0.6 bar and Tf⁠ uel of 90 °C, where CIC occurs. The main difference between JIC and CIC in terms of spray morphology can be found in Fig. 4b, where the spatial-resolved spray width against axial distance from the injector tip is plotted. The selected timing for the images is 1.6 ms after start of

After the brief introduction of JIC and CIC, the spray morphologies of propane, n-hexane and iso-octane under varied ambient pressures and liquid temperatures are to be examined. Figs. 5 and 6 present the effect of ambient pressure on the spray morphology at the liquid temperatures of 30 °C and 90 °C, respectively. Only the images taken at 1.6 ms ASOI are given. The spray width against axial distance is also plotted for each condition and fuel. The ambient pressure ranges from 0.6 bar to 11.0 bar and the pressure ratio Rp⁠ is marked in each spray image. At the liquid temperature of 30 °C, both n-hexane and iso-octane sprays are non-flash-boiling as Rp⁠ is less than 1. The

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Fig. 7. Propane sprays at different Tf⁠ uel.

Fig. 8. Spray morphology with similar Rp⁠ .

spray width within the axial distance less than 8 mm decreases with the increase in Pa⁠ mb, indicating the stronger JIC. The increase in ambient pressure would further prohibit the gas exchange between the zone surrounded by the jets and the zone outside the jets, increasing the pressure difference between the internal and external sides of the jets [16]. For propane sprays, both CIC and JIC can be found as Pa⁠ mb varies. Under Pa⁠ mb of 11.0 bar, where propane is sub-cooled, typical JIC can be observed. As Pa⁠ mb decreases to 7.0 bar, the liquid becomes superheated, but there is no large difference in spray with from that under 11.0 bar within the axial distance of 9 mm. As Rp becomes larger with the decrease in Pa⁠ mb to 1.0 bar, typical CIC becomes more obvious can be observed, i.e., the near-field spray width becomes larger and the spray width beyond 5 mm is much narrower, as shown in Fig. 5a. However, as Pa⁠ mb decreases to 0.6 bar, larger spray width than

Fig. 9. Flashing propane spray with similar Rp⁠ but combinations with different Pa⁠ mb and Tf⁠ uel.

that at 1.0 bar can be observed. Similar phenomenon was also observed by other researchers [25,30,31] when the ambient pressure was below certain values. Li et al. [25] attributed it to the complicated role of Pa⁠ mb in determining spray morphology under flash boiling conditions. Due to the larger resistance for fuel dispersion under relatively larger Pa⁠ mb, local vapor concentration became higher, leading to the enhanced condensation intensity and the stronger collapse, as the cases of Pa⁠ mb from 7.0 bar to 1.0 bar. However, relatively small Pa⁠ mb, in

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ary conditions on morphology. Herein, the images for the propane sprays under varied Tf⁠ uel are given in Fig. 7. Three zones (A, B, and C) are divided based on the morphological features in Fig. 7. In Zone A, i.e. under Pa⁠ mb of 0.6 bar and 1.0 bar, the liquid spray becomes narrower and shorter with the increase in Tf⁠ uel, attributed to the stronger intensity of CIC and evaporation. In Zone B, for the cases at 30 °C, the liquid transits from non-superheated to superheated and JIC gradually transforms to CIC as Pa⁠ mb decreases from 11.0 bar to 3.0 bar. The details in the morphology evolution and the mechanism have been fully discussed in Fig. 5. In Zone C, i.e. at the conditions of Tf⁠ uel higher than 30 °C and Pa⁠ mb larger than 1.0 bar, the occurrence of non-collapse flashing sprays can be clearly observed. Among these non-collapse cases, stronger evaporation could occur with the increase Tf⁠ uel and resultantly the individual jets become thinner and shorter. Furthermore, it should be noted that the non-collapse flashing propane sprays under elevated Pa⁠ mb did not exhibit the typical feature of JIC, as observed in the elevated-ambient-pressure and non-flash-boiling cases of n-hexane and iso-octane (e.g. results in Fig. 4). This is believed to be the result of the production of massive vapor, which could counteract the tendency of jet-induced low-pressure zone. Besides, from a global perspective of Fig. 7, it seems that there is a Pa⁠ mb threshold between 1.0 bar and 3.0 bar. Below the threshold, the sprays collapse while discernible jets can be observed beyond the threshold. Furthermore, under the ambient pressures beyond the threshold, the change in Tf⁠ uel and Pa⁠ mb mainly affects the spray liquid area, but not the trajectories of superheated jets. Thus, it can be deduced that Rp⁠ has less generality to judge whether multi-jet flash-boiling propane sprays collapse or not and the role of Pa⁠ mb in this process should be illustrated. Some preliminary discussion will be given in Section 4. Herein, a close-up view on the generality of Rp⁠ on spray morphology under close Rp⁠ are given. In Fig. 8, a set of images for the three liquids under similar Rp⁠ were given and the selected Rp⁠ are around 2.3 ∼ 2.4. Clearly, n-hexane and iso-octane have the similar collapsed structure while individual jets can be observed for propane sprays. Furthermore, Fig. 9 gives two groups of images with similar Rp⁠ for flashing propane sprays. Rp⁠ is around 10.5 in Group 1 and around 3.7 in Group 2. Clearly, in the cases with relatively low Pa⁠ mb and Tf⁠ uel, the collapsed spray can be observed in both Group 1 and 2. Achieving the similar Rp⁠ by simultaneously increasing Pa⁠ mb and Tf⁠ uel, discernable individual jets rather than collapsed jets can be found, as shown in the cases of relatively high Pa⁠ mb and Tf⁠ uel for the two groups. Thus, Rp⁠ lacks the generality to

Fig. 10. Nucleation rate at different Pa⁠ mb and Tf⁠ uel (Propane).

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spite of the increased superheated level, could cause stronger vapor dispersion, lower vapor concentration and finally relatively weaker collapse or wider spray, as indicated in Fig. 5. At the liquid temperature of 90 °C, as shown in Fig. 6, the pressure ratio, Rp⁠ , for all the liquids become higher. For n-hexane sprays, the obvious CIC can be clearly observed at Pa⁠ mb of 0.6 bar and 1.0 bar, where the n-hexane sprays are in superheated states. With the further increases in Pa⁠ mb, as well as Rp⁠ , the collapse mode transits to CIC from JIC. For iso-octane sprays, in spite that the iso-octane spray is superheated under Pa⁠ mb of 0.6 bar, it has similar morphology with the sub-cooled case under Pa⁠ mb of 1.0 bar. No typical feature of CIC can be observed for the case of 0.6 bar and this can be attributed to the low Rp⁠ . Generally, the iso-octane spray width within the axial distance of 9 mm becomes smaller with the increase in Pa⁠ mb, indicating the occurrence of JIC. However, with respect to the flashing propane sprays under 90 °C, the most worthnoting is the morphology as Pa⁠ mb further increases beyond 3.0 bar, where discernable jets rather than the collapsed spray can be observed. 3.3. Close-up view on flashing propane sprays

Actually, the spray evolution of n-hexane and iso-octane against different Pa⁠ mb and Tf⁠ uel are consistent with the previous studies using gasoline or gasoline-like liquids. However, the non-collapse feature of flashing propane sprays in Fig. 6 necessitate a more detailed understanding of the effects of bound

Fig. A1. Nucleation rate of n-hexane (Left) and iso-octane (Right) at different Pa⁠ mb and Tf⁠ uel.

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be applied to all liquids, even though proper generality for Rp⁠ can be established under certain conditions [11]. More in-depth analysis are needed to comprehensively understand the collapse mechanism under flash boiling conditions.

the temperature range of Zone III, as shown in Fig. 7. Therefore, further analysis on bubble growth is needed in order to understand the non-collapse feature. The metastable liquid has a strong tendency to achieve an equilibrium state. Once stable nuclei are generated, the excessive energy from the metastable liquid will be released via flashing evaporation, leading to the bubble growth. One of the key stages in the bubble growth can be described using Eq. (3) [41]:

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4. Discussion for the non-collapse flashing propane sprays In order to further understanding the non-collapse feature of flashing propane sprays under elevated ambient pressures and the less generality of Rp⁠ in correlating collapse intensity under flash boiling conditions, in the following, some qualitative analysis will be conducted to examine the flash boiling from the two fundamental process, i.e. nucleation and bubble growth.

(3)

where pv⁠ is the saturation pressure, p∞ ⁠ is the ambient pressure, and ρ is the liquid density. Physically, the bubbles need to overcome the resistance by ambient pressure in order to keep growing. In fact, ambient pressure does not only play a role of resistance, but also suppress the evaporation rate and heat transfer in the interface between bubble and liquid, which are the basic driving forces for the bubble growth. Given the same pressure difference between bubble pressure and ambient pressure, more rapid growth rate can be achieved with a relatively low ambient pressure because more obvious temperature drop in the interface occurred [42]. The role of Pa⁠ mb in suppressing evaporation rate and heat transfer can also be well supported by the two non-collapse cases with relatively high and Pa⁠ mb cases in Fig. 9. More study are needed to quantify the effect of ambient pressure on bubble growth in the future. By the above analysis on nucleation and bubble growth, it can be found that nucleation and bubble growth can be suppressed under relatively larger Pa⁠ mb and this is believed to be the main reason for the non-collapse feature of flashing propane sprays. The flashing evaporation for propane would mainly occur externally and proceed in the way of surface boiling to release its excessive energy, rather than the boiling inside the nozzle once Pa⁠ mb is beyond the threshold value. However, more in-depth investigations using advanced diagnostics techniques and theoretical modeling are necessary to unveil and quantitate the detailed difference in the future.

4.1. Nucleation and bubble growth The classical nucleation theory can be well used to analyze the nucleation process. Generally, the rate of nucleation J can be expressed as [40]: (1)

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where σ is the surface tension, m is the mass of liquid molecular, ΔG*⁠ is the free energy barrier, and kB⁠ is the Boltzmann constant. The free energy barrier for homogeneous nucleation can be expressed as: (2)

In Eq. (2), Δμ = kB⁠ ·Ti⁠ nj·ln(Rp⁠ ) is the chemical potential difference between the liquid and vapor phases and vm ⁠ = m/(ρl⁠ ·NA ⁠ ) is the specific volume of liquid state, where ρl⁠ is the liquid density and NA⁠ is the Avogadro constant. Fig. 10 gives the calculated nucleation rates of propane under superheated conditions over a range of Tf⁠ uel and Pa⁠ mb and the nucleation rate for n-hexane and iso-octane are in Appendix. Note that the liquid temperature range for propane in Fig. 10 is larger than the test range. As Tf⁠ uel varies, unimodal curves can be observed and this is reasonable because there are two competitive factors (liquid temperature and surface tension) influencing nucleation rate. On one hand, the increase in liquid temperature could increase Rp⁠ and enhance nucleation; on the other hand, the decrease in surface tension caused by the increased liquid temperature could be of help for the formation of nucleus, but over decrease in surface tension could suppress nucleation. Intuitionally, as the liquid temperature increases towards critical point, the surface tension will gradually become zero and no nuclei would appear. In Fig. 10, the conditions in Zone I is out of the test range in the test range and herein no more discussion will be made. In Zone II, clearly, the nucleation rate only slightly decreases as Pa⁠ mb increase from 0.05 bar to 1.0 bar, but significantly decreases as increases from 1.0 bar to 11.0 bar. The demarcation point at 1.0 bar coincides well with the one in separating collapsed sprays to non-collapse sprays, as shown in Fig. 7. However, the much reduced nucleation rate at Pa⁠ mb larger than 1.0 bar may only partly account for the non-collapse feature of flashing propane sprays because the nucleation rates under different Pa⁠ mb are quite close in Zone III, where both collapse and non-collapse features can be observed when Pa⁠ mb varies under

5. Conclusions

The spray morphologies of propane, n-hexane and iso-octane were carefully examined and the spray collapse was observed under both non-flash-boiling and flash-boiling conditions. The main conclusions can be withdrawn as follows: 1) For the flashing propane sprays, there should be an ambient pressure threshold between 1.0 bar and 3.0 bar. Below the threshold value, the multi-jet flash-boiling sprays collapsed; beyond the threshold value, the sprays presented a non-collapse feature and each jet penetrated along its original trajectory; 2) Suppression of nucleation rate and bubble growth under elevated ambient pressures inside the nozzle account for the non-collapse feature of flashing propane sprays under the ambient pressures beyond the threshold value. 3) The non-collapse feature of flashing propane sprays under elevated ambient pressures also demonstrated that the massive production of vapor via surface boiling could counteract the low-pressure tendency induced by the high-speed jets.

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Acknowledgment

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The research was supported by National Key R&D Program of China under Grant of 2018YFB0106000.

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Appendix See Fig. A1 References

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