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1. Introduction
Liquid fuel is directly injected into a supersonic flow in a scramjet engine normally; it must atomize fully and mix well in milliseconds [1–3]. A lot of experiments about liquid jet breakup in high-speed gas have been studied in recent years [4–7]. It is a significant phenomenon that the atomization of liquid jet in a gaseous crossflow had a wide application in fields such as combustion in liquid rocket engine, gas turbine engines, or scramjets [8–13].
Obviously, there are three important physical processes that control atomization characteristics, the deformation of liquid column, the formation of surface waves, and the primary breakup [14–17]. To study these physical processes, one of the most convenient ways is to change the geometry of liquid orifices. The breakup mechanisms of liquid jets in still air of different orifice geometries have been studied in the last three decades [18–22]. An interesting conclusion been found that the liquid jets injected from a noncircular orifice had a shorter breakup time than a round orifice in a same orifice area. Among the many noncircular orifices, the elliptical orifices have attracted plenty of attentions. Compared with the round liquid jets, the surface energy of elliptical liquid jets is smallest normally in a same orifice area. After the study of trajectories of elliptical jets in a gas flow, a theoretical model was developed in the works of [23]. In this model, as the aspect ratio increases, the penetration depth decreases. The spatial distribution of elliptical jets was studied using an empirical correlation experimentally in [24]. The deformation of the liquid column has some significant differences at different aspect ratios. A hybrid model was improved to apply a liquid jet in gas flow in [25]. The cross-section of the liquid column underwent a process from a circle to an ellipse, and the liquid mass is converted to discrete droplets at the same time.
Though many researchers referred to the improvement of spray efficiency through a process from a circle to an ellipse in elliptical jets, the elliptical liquid jet in a supersonic crossflow has not been studied so far. The speed of the crossflow is usually supersonic in a scramjet. There are many complex surface waves ahead of the liquid column; the flow field near the liquid column is very chaotic. The significance of this investigation focuses on the breakup processes of the elliptical jet, spatial distribution, and flow field characteristics presented in a supersonic crossflow which has not been fully investigated by previous studies.
Numerical research can obtain more flow field details than experimental research are applied in [26]. The detailed breakup process of a droplet in a supersonic flow has been studied numerically in [27–29]. The secondary atomization of a liquid jet has been predicted numerically in Ref. [30, 31]. Under the Euler-Lagrange framework, a two-phase simulation scheme was proposed in [32]. Under the Eulerian-Lagrangian framework, the liquid trailing phenomenon of a liquid jet in a supersonic crossflow was captured using large eddy simulation (LES) in [33, 34]. Using the CLSVOF method, the breakup process of a pulsed liquid jet in high-speed gas flow was studied in the works of [35].
This paper is organized as follows. The governing equations and numerical methods are described in Section 2. The breakup processes of the elliptical liquid jet in a supersonic crossflow are presented in Section 3.1; the spatial distribution of the elliptical liquid jet including the penetration depth, spreading angle, and breakup length is presented in Section 3.2. Then, Section 3.3 analyzes the flow field characteristics of the elliptical jet and round jet. Finally, a conclusion is presented in Section 4.
2. Governing Equations and Numerical Method
We adopted the CLSVOF method to track the liquid/gas interface. It absorbs the advantages of the LS method and the VOF method, which can capture the interface exactly and keep quality conservation for the two-phase flow in [36, 37]. The VOF is defined as the volume fraction of liquid in every cell. The LS
To study the effects of the elliptical orifice, five geometries of elliptical orifices have been run. And the gas conditions and liquid condition are given in Table 1. The simulation domain is divided into 240 blocks as shown in Figure 2(a), which is
Table 1
Supersonic crossflow conditions and liquid jet condition.
Supersonic crossflow | |||
Mach number | 2.0 | Gas velocity, | 517.2 |
Static pressure, | 17.2 | Gas density, | 0.3 |
Static temperature, | 167 | Viscosity coefficient, | |
Liquid jet | |||
Injection velocity, | 17.9 | Water density, | 1000 |
Water pressure, | 160.2 | Momentum ratio, | 4.0 |
Surface tension coefficient, | 0.072 | Viscosity coefficient, |
[figures omitted; refer to PDF]
Table 2
Geometries of injection nozzles (crossflow is from left to right).
Configuration | Orifice | AR | WeG | ||||
Case 1 | Elliptical | 0.4 | 0.1 | 0.25 | 0.0314 | 223 | |
Case 2 | Elliptical | 0.2828 | 0.1414 | 0.5 | 0.0314 | 223 | |
Case 3 | Circular | 0.2 | 0.2 | 1 | 0.0314 | 223 | |
Case 4 | Elliptical | 0.1414 | 0.2828 | 2 | 0.0314 | 223 | |
Case 5 | Elliptical | 0.1 | 0.4 | 4 | 0.0314 | 223 |
Accordingly, a similar method has been used to check the grid resolution in Ref [38]. We used three different resolutions of the grid to simulate the cases in Table 3. There is a subtle difference between Grid-A and Grid-F in Figure 3. However, Grid-C predicts a significant interface shape compared with Grid-A and Grid-F. At last, the current grid (Grid-A) is chosen to be applied for all the cases in our works.
Table 3
Minimum mesh size under three kinds of meshes.
Grid-A | Grid-C | Grid-F |
0.045 | 0.09 | 0.03 |
3. Results
3.1. Breakup Processes of Elliptical Liquid Jet in Supersonic Crossflow
Figure 4 shows the evolution of liquid jet breakup and the contours of the nondimensionalized pressure (
[figures omitted; refer to PDF]
[figures omitted; refer to PDF]
Figure 6 shows the cross-section of the jet at different locations, i.e.,
3.2. Spatial Distribution of Elliptical Jets
To obtain the penetration depth of the elliptical liquid jet clearly and accurately, we extracted 22 transient spray images at
The spreading angle is the next significant spray characteristics. The spreading angle is defined in the upper right corner in Figure 10. Under the two-dimensional top view, the liquid drops from the center of the spray hole to the outermost edges of the left and right sides are straight lines, and the clamped angle is the spreading angle [43]. Figure 10 shows the spread angles at five different aspect ratios. The round jet has the widest spread angle which is 50.2°. Through the analysis of Figure 6, we can better understand the reason for the large spray spreading angle of the round jet. Apparently, the surface breakup of the round jet occurs later, so it maintains a bigger windward area. The further the droplets splash, the wider the spreading angle is. Compared with the round jet, the droplets of elliptical jets move away through the axial direction fragment from the sides.
[figure omitted; refer to PDF]Another index of atomization quality is the breakup length which is divided into the breakup length in the
3.3. Flow Field Characteristics of Elliptical Jet and Round Jet
Figure 12 presents a difference of the instantaneous vortex structures at three different AR. The column and surface breakup are observed at three AR at the same time. Compared with the round jet, there are more vortex structures in different scales near the location of surface breakup from the top left corner of Figure 12. The length of surface breakup in the
The dispersion of liquid droplets is determined by the large-scale vortices in the wake region which affects the mixing process of the spray deeply in the supersonic air flow in [35]. Figure 13 presents the vorticity distributions in slice
Figure 14 shows the two-dimensional streamlines at different slices, i.e.,
[figures omitted; refer to PDF]
A three-dimensional liquid jet structure in space is presented in Figure 15. After the supersonic flow is obstructed by the liquid column, a separation zone is forming located ahead of the column represented by the green circle. The gas flows into the separation zone and forms a pair of horseshoe vortex represented by the purple circle. The red transparent region represents the bow shock. Furthermore, the green squares indicate the column breakup position and surface breakup position, respectively. It can be seen that two sections are taken in turn along the direction of the gas flow which is represented by dark blue, and two pairs of reverse vortex pairs are indicated by blue and orange circles. The vortex pairs become larger and larger gradually with the increases of the distance of gas flow.
[figure omitted; refer to PDF]4. Conclusions
In the current study, the primary breakup of a liquid jet of different orifices has been evaluated numerically. The aspect ratios are set to be 0.25, 0.5, 1, 2, and 4. The spatial distribution including the changes of the jet’s cross-section, liquid penetration depth, spreading angle, and breakup length has been compared at different aspect ratios. The flow field characteristics of the elliptical jet and round jet have been analyzed through the simulations. We have come to the following conclusions:
(1)
Under the condition of supersonic crossflow, the surface breakup dominated by the R-T instability occurs earlier than that of the round liquid jet. The main reason is that the initial segment of the liquid column has different lengths in both the flow direction and spanwise direction. This result breaks the sequence and dominant degree of the K-H instability with the R-T instability in the round orifice indirectly and promotes the occurrence of the surface breakup, so the surface breakup of the elliptical orifice jet occurs earlier than the round liquid jet. As a result, elliptical orifices can shorten the time of the surface breakup effectively
(2)
In the liquid jet spatial distribution, the jet has a higher penetration depth at a larger aspect ratio normally. The penetration depth of the elliptical orifice with
(3)
The changing trends of the breakup length of the jet in both directions are basically similar. The CBLx (
(4)
After analyzing the flow field characteristics of the liquid jet, it is found that the horseshoe vortex structure in front of the jet increases with the increase of the aspect ratio and becomes further away from the liquid column. The width of the wake area of the elliptical orifice (
Acknowledgments
This work was supported by National Natural Science Foundation of China (grant numbers 11872375 and 11902351).
Glossary
Nomenclature
AR:Orifice cross-section aspect ratio (
CBLx:Breakup length in the
CBLy:Breakup length in the
CLSVOF:Coupled level set and volume of fluid
LS:Level set
Ma:Mach number
VOF:Volume of fluid
WeG:Relative gas-liquid Weber number
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Abstract
The study of elliptical liquid jets in supersonic flow in a Mach 2.0 is performed numerically. The primary breakup process of the elliptical liquid jet is simulated for a Weber number 223, liquid/gas flux momentum 4.0. The aspect ratios of elliptical geometries are set to be 0.25, 0.5, 1, 2, and 5. The results show a remarkable difference in liquid jet disintegration morphology at different aspect ratios. Under supersonic crossflow conditions, the elliptical liquid jet shows more breakup characteristics than the round liquid jet. As the aspect ratio grows, the penetration depth decreases. The elliptical liquid jet with
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