Study on atomization and combustion characteristics of -- Fang, Xin-xin; Shen, Chi-bing -- Acta Astronautica, 136, pages 369-379, 2017 jul -- Elsevier -- 10.1016_j.actaastro.2017.03

Acta Astronautica 136 (2017) 369–379
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Acta Astronautica
journal homepage: www.elsevier.com/locate/actaastro
Study on atomization and combustion characteristics of LOX/methane
pintle injectors
Xin-xin Fang a,b , Chi-bing Shen a,b, ⁎
MARK
a College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China
b Science and Technology on Scramjet Laboratory, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China
ARTICLE INFO
Keywords:
Liquid oxygen/methane rocket engine
Pintle injector
Atomization cone angle
Combustion characteristics
ABSTRACT
Influences of main structural parameters of the LOX/methane pintle injectors on atomization cone angles and
combustion performances were studied by experiments and numerical simulation respectively. In addition,
improvement was brought up to the structure of the pintle injectors and combustion flow fields of two different
pintle engines were obtained. The results indicate that, with increase of the gas-liquid mass flow ratio, the
atomization cone angle decreases. In the condition of the same gas-liquid mass flow ratio, as the thickness of the
LOX-injection gap grows bigger, the atomization cone angle becomes smaller. In the opposite, when the half
cone angle of the LOX-injection gap grows bigger, the atomization cone angle becomes bigger. Moreover, owing
to the viscous effects of the pintle tip, with increase of the ‘skip distance’, the atomization cone angle gets larger.
Two big recirculation zones in the combustor lead to combustion stability of the pintle engines. When the value
of the non-dimensional ‘skip distance’ is near 1, the combustion efficiency of the pintle engines is the highest.
Additionally, pintle engines with LOX injected in quadrangular slots can acquire better mixing efficiency of the
propellants and higher combustion efficiency as the gas methane can pass through the adjacent slots. However,
the annular-channel type of pintle injectors has an ‘enclosed’ area near the pintle tip which has a great negative
influence on the combustion efficiency.
1. Introduction
Toxic propellants are used frequently in modern space activities.
Through decades of continuing development, the toxic propellant
rocket engines can achieve high performance. However, the toxicology
and corrosiveness of the propellants lead to many problems, like high
costs and environmental pollution [1]. Non-toxic propellants have the
advantages of environment-friendly, high safety, good maintainability
and low cost, etc. With enhancement of people's awareness of
environmental protection and rapid development of astronautical
technology, using non-toxic propellants in space vehicles begins to be
an inexorable trend. Development of green propellants depends on two
characteristics: high performance and low cost [2]. Many researches
had been conducted to figure out the possibility of methane being used
in rocket engines. The results indicate that methane has the advantages
of safety, low cost, low corrosiveness [3], high cooling performance [4]
low carbon deposition [5]. Additionally, Masters A I et.al pointed out
that methane can overcome the weakness of low specific impulse of
kerosene and low density of hydrogen, but has both the advantages of
them [6]. Thus, rocket engines using liquid oxygen (LOX) and methane
in combination as propellants developed rapidly under that background
[7–10].
On the other hand, Goddard R H had raised the necessity of
controlling the thrust of rocket engines in early 20th century. There are
two basic ways to control the thrust of rocket engines, one of which is to
control the thrust itself, and the other is to control the duration of the
thrust. To control the thrust itself, the most important way is to control
the mass flow rate of the propellants. Although the mass flow rate
varies from different injection pressure-drop of the propellants, it is
difficult to achieve high thrust variation or high performance by this
manner, as the combustion efficiency can be significantly affected by
the injection pressure-drop. Therefore, one practicable way is to alter
the area of the injection channel of the injectors in order to maintain
constant injection pressure-drop. Thus, pintle injectors, which use a
removable component named ‘needle’ to alter the area of injection
channel in order to change the mass flow rate of both oxidizer and fuel,
were applied widely, especially in Apollo project [11,12]. Pintle
injectors, originating from experiments at the Caltech Jet Propulsion
Laboratory (JPL) in 1957, were once used to characterize the reaction
rates of candidate rocket propellants [13]. Then, after improvement of
⁎ Corresponding author at: College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China.
E-mail address: cbshen@nudt.edu.cn (C.-b. Shen).
http://dx.doi.org/10.1016/j.actaastro.2017.03.025
Received 17 September 2016; Received in revised form 10 March 2017; Accepted 27 March 2017
Available online 31 March 2017
0094-5765/ © 2017 IAA. Published by Elsevier Ltd. All rights reserved.

X.-x. Fang, C.-b. Shen Acta Astronautica 136 (2017) 369–379
Nomenclature
π Pi
LOX Liquid Oxygen
m ĊH4 Mass flow rate of gaseous methane, kg/s
ṁLOX Mass flow rate of liquid oxygen, kg/s
L s Length of needle in combustor or ‘skip distance’, mm
D fo Outside diameter of methane channel, mm
D p Outside diameter of pintle tip, mm
Ls/
Dp Non-dimensional ‘skip distance’
D pi Inside diameter of needle, mm
h o Thickness of LOX-injection gap, mm
α o Half cone angle of LOX-injection gap, °
L c Cylindrical section length of combustor, mm
Diameter of combustor, mm
D c
D t Diameter of nozzle throat, mm
L* Characteristic length of combustor, m
N Number of quadrangular slots
δ o Circumferential size of quadrangular slots, mm
L o Axial size of quadrangular slots, mm
BF Blockage Factor, BF =( Nδo)/( πDp)
η Combustion efficiency
C* Characteristic velocity, m/s
C th Theoretical characteristic velocity, m/s
P cs , Stagnation pressure in the combustor, Pa
A t Throat area, mm 2
SMD Sauter Mean Diameter, μm
TMR Total Momentum Ratio
R-R distribution Rosin-Rammler distribution
the structure, pintle injectors have the characteristics of deep throttling,
fast response and ‘face shut off’ [14]. Over 60 different pintle
engines have been developed at least to the point of hot fire
characterization testing in TRW (Thompson-Ramo-Wooldridge Inc.)
and over 130 bipropellant engines using pintle injectors have flown
successfully [15]. In addition, pintle injectors were used in gel
propellants tactical missiles [16] and cryogenic propellants rocket
engines [17–23].
However, fundamental research about pintle injectors is little. The
conical liquid sheet of the pintle injectors is more stable if the inner
face of the injection channel is longer than the outer face [24].
Numerical simulation results about inner flow process of the pintle
injectors, which have been demonstrated by experiments [25], indicate
that manufacturing process tolerance has a great influence on flow
state of the conical liquid sheet in the exit of the injectors [26].
Moreover, effects of environmental pressure, gas-liquid momentum
ratio and Weber number on atomization characteristics of the pintle
injectors were studied experimentally [27–29]. The combustion flow
fields of the pintle injectors which are important to understand the
combustion characteristics of the pintle engines were rarely studied.
In the present study, the influences of main structural parameters
of the pintle injectors on atomization cone angles are studied experimentally,
and combustion flow fields inside the pintle engines are
obtained through numerical simulation. Finally, a comparison of
combustion flow fields of two different pintle engines is given to come
up with the improved structure of the pintle injectors. The results
provide a reference for the structural optimization of the pintle engines.
distribution of the pintle injectors, the Malvern measurement system is
used in the experiments. The SMD distribution results provide
references to the inflow parameters of the numerical simulation.
2.2. Pintle injector
Fig. 2(a) presents the pintle injector used in the experiments. The
pintle injector is mainly composed of installation fixture, pintle, needle
and base. The needle can be positioned along the axial direction. It is
achieved by replacing the gaskets which have different thickness. There
are two pressure sensors in the pintle injectors to measure the injection
pressure of the water and air respectively. The pintle and needle form
the injection channel of the water, while the base and needle form the
injection channel of the air. The physical configuration of the pintle
injector is shown in Fig. 2(b).
In the experiments, the total momentum ratio (TMR) is the same
between the simulants and the real propellants. The TMR is defined by
Eq. (1) in which ρ represents density, v represents velocity and A
represents the area of injection channel. The subscript index “g” and “l”
2. Experimental facilities
2.1. Experimental platform
The atomization experimental platform used in this research is
shown in Fig. 1. The nitrogen resource works as pressurization bottle.
Due to bad atomization characteristics of liquid/liquid pintle injectors,
the gaseous methane rather than liquid methane was chosen as fuel,
mainly because the gaseous methane can improve the atomization
characteristics of the pintle injectors. A combination of water and air is
selected as simulant of the LOX and gaseous methane respectively. The
water and air are marked as simulant No. 1 and No. 2.
The pressure of the simulants is controlled by a gas pressure
regulator which is installed in the gas distribution board. The injected
water is collected by collecting system and pumped out by the air draft
system. The measurement system is independent from the experimental
apparatus. A Tamron high speed camera with a lens of Nikon
80–200 mm is used to obtain atomization images in the experiments.
The frame frequency in this research is 10,000 f/sand the exposure
time is 1/40,000 s. To measure the Sauter Mean Diameter (SMD)
Fig. 1. Schematic configuration for experimental platform.
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2.3. Image processing method
To obtain the atomization cone angles, the atomization images
should be processed following the process shown in Fig. 3. Fig. 3(a) is a
background image, in which x and y represent the axial direction and
radial direction respectively. An original atomization image is shown in
Fig. 3(b), in which the partial image in red box whose height is 30 mm
is extracted. After that, the partial image is converted to grayscale
image firstly, and then to binary image. The boundary between black
and white is atomization gas-liquid boundary as shown in Fig. 3(c). To
obtain the atomization cone angles, the atomization gas-liquid boundary
was fitted by ordinary least squares techniques as shown in
Fig. 3(d), in which the red curve represents the atomization gas-liquid
boundary, the yellow lines represent the fitting curve and the blue lines
represent the fitting error. The angle between the two yellow lines is
defined as the atomization cone angle. The scale between the atomization
images and the physical dimensions in Fig. 3 is 76.5 mm per 648
pixels.
To reduce errors during image processing, atomization cone angles
of 1000 images were obtained firstly, and then an average was
computed. Fig. 4 shows average value of the atomization cone angles
for different number of images processed. With increase of images
processed, the average value fluctuates. But when the number of the
images is larger than 300, the atomization cone angles tend to be
stabilizing. Thus, it is reasonable to suppose the stable value as the
atomization cone angle in certain operating condition. In this paper,
1000 images were selected to obtain the atomization cone angles.
3. Numerical simulation set-up
3.1. Numerical simulation conditions
represent gaseous and liquid simulants respectively. In our experiments,
the mass flow rate of the simulants is smaller than that of the
real propellants. The values of TMR for different h o are given in Table 1.
TMR =( ρ v A )/( ρ v A )
Fig. 2. Pintle injector.
g g 2 g l l 2 l
(1)
During to the limits of the experimental platform, the effects of
ambient pressure on the atomization characteristics of the pintle
injectors are not taken into account yet. In addition, the influences of
the dissimilar of fluid properties between the simulants and the real
propellants on the experimental results were supposed keep the same
for each condition. So the conclusion of the experiments is reasonable.
Numerical simulation on LOX/methane pintle engines was conducted.
The influences of the main structural parameters of the pintle
injector and combustor on combustion performance were studied. The
numerical simulation is performed on the ANSYS Fluent platform. A
second-order, double-precision solver was utilized to conduct the
simulations.
The schematic configuration of the pintle engines is shown in Fig. 5.
Because the structure is axisymmetric, the numerical simulation was
conducted in two-dimensional mainly. The fuel and oxidizer are
gaseous methane and LOX respectively. The numerical simulation
conditions are listed in Table 2. The throat diameter and the contraction
ratio of the pintle engines are 47.17 mm and 4.49 respectively. The
boundary condition and the mesh of a close-up view of the injection
area in the two-dimensional numerical simulation are shown in Fig. 6.
The heat transfer was not taken into account in the numerical
simulation. And a “No Slip” boundary condition is used on the wall
in the numerical simulations. Demonstration of the grids could be
found in Section 3.2.
The Rosin-Rammler (R-R) distribution was used in the Discret
Phase Model (DPM). The R-R distribution is a cumulative mass
fraction distribution:
f ( d) = exp[−( d/ d) n
]
(2)
In Eq. (2) f ( d) represents the cumulative mass fraction of particles
Table 1
Gas-liquid momentum ratio for different h o.
h o (mm)
0.06 3.02
0.08 4.03
0.10 5.03
0.12 6.04
Momentum ratio
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Fig. 3. Acquisition process of atomization cone angles.
Table 2
Numerical simulation conditions of pintle engines.
No. ṁLOX m ĊH 4 D p Ls/
Dp h o D fo Dc / DP L c L*
(kg/s) (kg/s) (mm) (mm) (mm) (mm) (m)
1 2.056 0.793 30 1.00 0.10 31.2 3.3 220 1.0
2 2.056 0.793 30 0.75 0.10 31.2 3.3 220 1.0
3 2.056 0.793 30 0.25 0.10 31.2 3.3 220 1.0
4 2.056 0.793 30 1.50 0.10 31.2 3.3 220 1.0
5 2.056 0.793 30 1.00 0.08 31.2 3.3 220 1.0
6 2.056 0.793 30 1.00 0.06 31.2 3.3 220 1.0
7 2.056 0.793 30 1.00 0.12 31.2 3.3 220 1.0
8 2.056 0.793 30 1.00 0.10 31.2 3.3 180 0.8
9 2.056 0.793 30 1.00 0.10 31.2 3.3 260 1.2
10 2.056 0.793 30 1.00 0.10 31.2 3.3 300 1.4
Fig. 4. Atomization cone angles for different number of pictures processed.
Fig. 5. Schematic configuration for pintle engines.
whose diameters are bigger than d. In addition, d and n represent the
mean diameter and spread parameter respectively. The mean diameter
is discussed in Section 4.
The eddy-dissipation model was used as the chemical reaction
model, as the combustion process in rockets is a non-premixed
diffusion process. The model and its applicability in the numerical
simulation of rockets had been demonstrated by other researchers
[30,31].
In the present simulations, a single-step global reaction (Eq. ()) is
used as other researchers [32,33]. It is reasonable as the global reaction
allows the coupling of pressure and unsteady heat release to be
captured and reduces the amount of computation [34].
CH4 +2O2 → CO2 +2H2 O
(3)
Researches have demonstrated that the non-equilibrium evaporation
model is more accurate than the quasi-equilibrium one, especially
for the droplet with small radius [35–37]. So the non-equilibrium
evaporation model is adopted in the present simulation. The details
about the model can be found in reference [38].
The standard k–ε turbulence model is applied for modeling the
turbulence. In order to obtain a high-quality solution of the flow in the
turbulent boundary layer, the standard wall functions are used for the
near-wall treatment [32].
3.2. Mesh study
The grid size has a great influence on the simulation results,
especially the mesh near the wall. The first grid height is 0.05 mm in
the boundary and the standard grid has a total of 147,196 cells. In
addition, the y plus near the wall is larger than 10 by checking the
numerical results to make sure the validation of the standard wall
functions. There are both 25 cells for the inlet of gaseous methane and
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Fig. 6. Boundary condition and mesh of numerical simulation.
Table 3
Pressure and temperature of monitor points of different grids in pre-simulation.
Monitor points a b c d
Grids
Parameters
72,538 Pressure (MPa) 2.930 2.923 2.908 2.898
Relative deviation (%) 1.52 1.39 0.972 1.68
Temperature (×10 3 K) 2.938 2.886 2.863 2.815
Relative deviation (%) 16.17 15.07 12.50 10.31
147,196 Pressure (MPa) 2.895 2.893 2.876 2.863
Relative deviation (%) – – – –
Temperature (×10 3 K) 2.489 2.532 2.561 2.535
Relative deviation (%) – – – –
218,550 Pressure (MPa) 2.886 2.883 2.880 2.850
Relative deviation (%) −0.31 −0.35 0.14 −0.46
Temperature (×10 3 K) 2.529 2.508 2.545 2.552
Relative deviation (%) 1.58 −0.96 −0.63 0.67
Fig. 8. Atomization cone angles for different α o.
Fig. 7. Atomization cone angles for different gas-liquid mass flow ratio.
Fig. 9. Atomization cone angles for different Ls/
Dp.
liquid oxygen. In order to demonstrate the grid convergence, presimulation
was conducted. Two grids which have smaller and larger
quantity of cells were used to compare with the standard grid. The two
grids have a total of 72,538 and 218,550 cells respectively. Four points,
named a, b, c and d (see in Fig. 6) were set to compare their pressure
and temperature measured in the three grids. The positions of the four
points (a, b, c and d) are (0.03 m, 0), (0.03 m, 0.03 m), (0.12 m, 0) and
(0.12 m, 0.03 m) respectively. The results are shown in Table 3. We can
see that, compared with the results of the standard grid, the relative
deviation of the larger quantity grid is negligible, while that of the
smaller quantity grid is large. So the standard grid is suitable for the
present simulation.
4. Experimental results
In the experiments, the mass flow of the gaseous simulant were kept
unchanged, while the mass flow of the liquid simulant changed
according to different gas-liquid mass flow ratio. Fig. 7 shows the
variation curve of the atomization cone angles along with the gas-liquid
mass flow ratio in the condition of different h o (see in Fig. 5). With
increase of the gas-liquid mass flow ratio, the atomization cone angles
decrease, and the trend becomes flat. In addition, as h o grows bigger,
the atomization cone angles become smaller under the condition of
same gas-liquid mass flow ratio. It is because when h o becomes bigger,
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Fig. 10. SMD and spread parameter along axial direction.
the injected liquid sheet slows down and the gas-liquid momentum
ratio (see in Table 1) becomes higher.
Fig. 8 shows the atomization cone angles for different α o (see in
Fig. 5). In the condition of same gas-liquid mass flow ratio, the bigger
α o is, the larger the atomization cone angle becomes. It is because with
the increase of α o , the radial momentum of injected liquid sheet
becomes greater.
Fig. 9 shows the variation curves of the atomization cone angles for
different Ls/
Dp (see in Fig. 5). In the condition of same gas-liquid mass
flow ratio, the atomization cone angles become larger along with
increase of Ls/
Dp, although the difference between the case of 0.25
and 0.50 is not significant. It is because the larger Ls/
Dp is, the longer
the gas propellant travels before colliding with the liquid sheet. And in
the effect of the viscous force of the needle's outer wall, velocity of the
gas propellant decreases. Thus, the gas propellant becomes slower and
the gas-liquid momentum ratio becomes lower in the case of high
Ls/
Dp.
The SMD and spread parameters of R-R distribution of the particle
diameters in six different positions were measured. The positions
locate at x=2 cm, 4 cm, 6 cm, 7 cm, 8 cm, 10 cm, and y=0 for all the
points (the coordinate axes can be seen in Fig. 3). The results can be
seen in Fig. 10. It can be seen that the SMD along the central
atomization zone of the pintle injectors is about 70( ± 10) μm. The
spread parameters are in the range of 1.5 and 1.8. The relation between
the average diameter and SMD is given by Lefebvre [39]. The SMD and
spread parameter are chosen as 70 µm and 1.65 respectively. As a
result, the average diameter is 50.7 µm. So in the numerical simulation
of the pintle engines the mean diameter of the R-R distribution was
chosen 50.7 µm.
Fig. 12. Combustor pressure for different L s .
5. Numerical simulation results
5.1. Influence of Ls/
Dp on combustion performance
To study influences of Ls/
Dp on combustion performance of the
pintle engines, different Ls/
Dp like 0.25, 0.50, 1.00 and 1.50 are
selected. As D p =30 mm (see in Table 2), L s are 7.5 mm, 15 mm,
30 mm and 45 mm respectively. The minimize mesh size near the wall
and the inlet of the propellants keep unchanged and the construction
method of the grid refinement is the same for different pintle lengths.
Fig. 11 shows streamline for different L s . It can be seen that there are
two big recirculation zones in the combustor.
The combustion efficiency is defined by Eq. (4) below [40]. The η
represents the combustion efficiency, while C* and C th represents the
characteristic velocity of numerical simulation and theoretical value.
The P cs , represents the stagnation pressure in the combustor while the
A t represents the throat area.
C
η = *
Cth
Pcs
, At
C* =
m ̇ CH4 + m ̇ LOX
(4)
The combustion efficiency when Ls/
Dp is 0.25, 0.5, 1.0 and 1.5 is
0.884, 0.907, 0.958 and 0.927 respectively (see in Fig. 12). Heister S D
points out that the pintle engines have demonstrated high combustion
efficiencies in the 96–99% range for most of the engines which have
been developed for flight programs [15,41]. The low combustion
efficiency of the pintle engines in the present study is due to the bad
mixing efficiency of the propellants. So the structure of the pintle
Fig. 11. Streamline for different L s .
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Fig. 13. Temperature fields for different L s .
Fig. 14. Contours of mass fraction of O 2 and particle traces of LOX drops when Ls is
7.5 mm.
engines needs to be improved. Actually, the gaseous methane has two
aspects of influences on the injected LOX drops. Firstly, the acceleration
effect of the gaseous methane on the injected LOX drops is higher
when Ls/
Dp is smaller, so the LOX drops is faster in this case. As a
result, the residence time of the LOX drops is shorter and this is the
main reason why the combustion efficiency is lower when Ls/
Dp is
smaller. Secondly, the gaseous methane makes the injected LOX drops
in the condition of smaller Ls/
Dp break into smaller drops more quickly,
and this has a positive influence on the combustion efficiency. Thus, the
variation of the combustion efficiency along with Ls/
Dp is a tradeoff
between that two factors. In addition, when Ls/
Dp is too big, the
effective characteristic length of the combustor in which the flame
exists is smaller as there is no flame near the pintle tip (see in Fig. 13).
Thus, the combustion efficiency when Ls/
Dp is bigger than 1.0
decreases.
The combustor pressure (area average total pressure) has a similar
varying trend as the combustion efficiency for different Ls/
Dp (see in
Fig. 12). The combustor pressure increases firstly and then decreases
along with enlarging of L s . In considering of the combustion efficiency,
L s should be chosen 30 mm. In other words, when Ls/
Dp is around 1.0,
the pintle engines could acquire the best combustion efficiency. This
similar conclusion was got by Heister S D who analyzed the liquid/
liquid propellants pintle engines [41]. He pointed out that the typical
value of the “skip distance” (Ls/
Dp) is around 1.0.
The temperature fields for different L s are shown in Fig. 13. There
are two low temperature zones (big temperature gradient) in the
combustor marked as A and B in Fig. 13. Contours of the mass fraction
of O 2 and particle traces of LOX drops when Ls is 7.5 mm is shown in
Fig. 14. We can see that there exists much O 2 near the pintle tip and in
the center of the combustor which are the positions of zone A and B in
Fig. 13. From the particle traces in Fig. 14 we can see that there are
some LOX drops rebound from the wall and move toward the center of
the combustor. And the evaporation of LOX drops and mass fraction of
O 2 is much high in the region A. In addition, there are some particles
reach the center of the combustor in the position of region B. Thus it is
the evaporation of LOX drops which causes the low temperature in
region A and B in the combustor. In addition, the gas temperature near
the first half of the combustor is low, which is because there is no flame
exists in this region, and it provides protection to the wall of the
combustor.
5.2. Influence of h o on combustion performance
Different values of h o were selected like 0.06 mm, 0.08 mm,
0.10 mm and 0.12 mm to study influences of h o on combustion
performance of the pintle engines. Different h o leads to different gasliquid
momentum ratio (see in Table 1).
Fig. 15 shows the streamline for different h o . The size of the
recirculation zone D varies little, while the size of the recirculation
zone C decreases along with increase of h o . The bigger h o is, the smaller
the velocity of the injected LOX becomes and the bigger gas-liquid
momentum ratio becomes. As a result, the size of the recirculation zone
C decreases. Both the recirculation zone C and D have positive
influences on the combustion stability of the pintle engines. For
recirculation zone D, its effects are little, as the recirculating flow is
formed by unreacted propellants [42]. But the low-temperature
recirculating flow has a cooling effect on the front part of the combustor
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Fig. 15. Streamline for different h o.
brought into the center of the combustor. Finally, the high-temperature
combustion products lead to more rapid spray mixing.
Fig. 16 shows the combustor pressure for different h o . The
combustor pressure decreases and the trend becomes flat along with
increase of h o , and decrease of the size of the recirculation zone C is
responsible for that. Thus, when designing pintle injectors, the velocity
of LOX could not be chosen too small or the gas-liquid momentum
ratio could not be chosen too big.
Fig. 17 shows the temperature field for different h o . The gas
temperature near the pintle tip decreases along with increase of h o .
When h o is 0.06 mm there is a strong temperature gradient along the
axis of the combustor. The reasons are the same as that in Section 5.1.
The recirculation zones C in Fig. 15 has great influences on the
combustion stability of the pintle engines as discussed before. The size
of the recirculation zone C is bigger and more high-temperature gas is
entrained into it when h o is small.
Fig. 16. Combustor pressure for different h o.
(see in Fig. 13). As for recirculation zone C, it has great positive
influences on the combustion stability of the pintle engines. There are
three reasons for this [42]. Firstly, it provides a continuous heat source
for ignition purpose. Secondly, it enables the combustion zone to be
5.3. Influence of L* on combustion performance
Four different parameters were selected, which are 0.8 m, 1.0 m,
1.2 m and 1.4 m to study influences of L*. The diameter of the
combustor keeps constant, while the lengths are 180 mm, 220 mm,
260 mm and 300 mm respectively (see in Table 2). For different L*, the
size of the grids keeps unchanged. Thus, bigger L* has greater amount
Fig. 17. Temperature field for different h o.
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Fig. 18. Streamline for different L*.
Fig. 21. Schematic configuration for two different pintle engines in three-dimensional
numerical simulation.
Fig. 19. Combustor pressure for different L*.
Fig. 22. Temperature (K) field for two different pintle engines.
Fig. 20. Schematic configuration for improved pintle engines.
of cells. Fig. 18 shows the streamline for different L*. The size of the
two recirculation zones changes little.
Fig. 19 shows the combustor pressure for different L*. It can be seen
that the larger L* is, the higher the combustion pressure of the pintle
engines is. But increase of the combustor pressure is little while L* is
larger than 1.2 m. It has been analyzed in Section 5.1 that the bigger
Ls/
Dp is, the smaller the effective characteristic length of the combustor
is. When the characteristic length is small, the time for mixing and
reaction is less. The biggest particle residence time is 2.33 ms, 3.05 ms,
3.27 ms and 3.36 ms when the characteristic lengths are 0.8 m, 1.0 m,
1.2 m and 1.4 m respectively. Bigger particle residence time means the
Fig. 23. Mole fraction distribution of gaseous methane.
oxygen can react with the methane more sufficiently. Thus bigger
characteristic length means higher combustion efficiency and combustor
pressure (see in Fig. 19). On the other hand, larger values of L*
mean heavier pintle engines. So the value of L* is a tradeoff between the
combustion efficiency, weight and cooling of the wall of the pinle
engines.
5.4. Improvement of pintle structure
From the two-dimensional numerical simulation results, the combustion
efficiency of the pintle engines is low (around 0.96). It is
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X.-x. Fang, C.-b. Shen Acta Astronautica 136 (2017) 369–379
because the LOX is injected as conical sheet and as a consequence it
forms an ‘enclosed’ area near the pintle tip which leads to bad mixing of
the propellants. Thus, improved structure of the pintle injectors is
brought up [41] (see in Fig. 20). Fig. 20(b) is an enlarged schematic
configuration of the new pintle tip. The LOX injection channel is
changed from the circularity (see in Fig. 5) to the quadrangular slots
(see in Fig. 20). The most important advantage of the slots is that the
gas methane can pass through between the adjacent slots, so mixing of
the propellants is much more efficient.
The combustion field of the pintle engines with different LOXinjection
channels was studied by three-dimensional numerical simulation.
The methane is injected through the outer annular channel,
while the LOX is injected through the circularity in Fig. 21(a) (scheme
1) and quadrangular slots in Fig. 21(b) (scheme 2). There are 16
quadrangular slots on the pintle tip in scheme 2. The circumferential
and axial sizes of the quadrangular slots are 2 mm and 2.95 mm
respectively. Thus, BF of the scheme 2 with quadrangular slots is 0.5.
Other sizes of the pintle injectors and combustor are same as condition
No. 1 in Table 2.
Fig. 22 shows the temperature field of the two different pintle
engines. The temperature of the gas near the combustor wall is lower in
scheme 1, while the high-temperature area is larger in scheme 2. The
cooling effect of the low-temperature methane on the wall of the
combustor is better for scheme 1. However, the mixing efficiency of the
propellants and the combustion efficiency (0.981) are better for scheme
2. So in consideration of the combustion efficiency of the pintle
engines, scheme 2 is better than scheme 1. In addition, the combustor
pressure is higher for scheme 2.
For the pintle engines with quadrangular slots, the temperature
field is more uniform in the last half than the first half of the
combustor. The mole fraction distribution of the gaseous methane is
shown in Fig. 23. It can be seen that for scheme 1 the methane
distributes mainly near the combustor wall, while the methane is
mainly in the center of the combustor for scheme 2. So the mixing
efficiency of scheme 2 is much higher than scheme 1. All in all,
considering the mixing efficiency and combustion efficiency of the
pintle engines, LOX injected through the quadrangular slots is better
than the circularity.
6. Conclusions
In this paper, atomization characteristics of the pintle injectors
were studied experimentally. With increase of the gas-liquid mass flow
ratio, the atomization cone angle decreases. In the condition of the
same gas-liquid mass flow ratio, as h o grows bigger, the atomization
cone angle becomes smaller. In the opposite, when the half cone angle
of the LOX-injection gap grows bigger, the atomization cone angle
becomes bigger. Owing to the viscous effects of the pintle tip, with
increase of Ls/
Dp, the atomization cone angle becomes larger. The SMD
along the central atomization zone of the pintle injectors is about 70( ±
10) μm. And the spread parameters are in the range of 1.5 and 1.8.
Then, the influences of the main structural parameters of the pintle
injectors and combustor on the combustion performances of the LOX/
methane pintle engines were studied by numerical simulation. Two big
recirculation zones exist in the combustor, which lead to combustion
stability of the pintle engines. When L s is too small, there exist two low
temperature zones in the combustor, and that will lead to decrease of
the combustion efficiency. When L s is too big, the effective characteristic
length of the combustor reduces. In consideration of the combustion
efficiency, Ls/
Dp should be chosen around 1.0. Additionally, the
combustion efficiency decreases along with increase of h o , and the
decrease of the size of the recirculation zones is responsible for that.
Finally, improvement was brought up to the structure of the pintle
injectors and a comparison of the combustion flow fields of the two
different pintle engines is given by three-dimensional numerical
simulation. Pintle engines with LOX injected in the quadrangular slots
can acquire better mixing efficiency of propellants and higher combustion
efficiency as the gas methane can pass through the adjacent slots
while the annular-channel type has an ‘enclosed’ area near the pintle
tip which has a negative influence on the combustion efficiency.
Acknowledgements
The authors would like to express their gratitude for the financial
support provided by the Fund of Innovation, Graduate School of NUDT
(No. S150105). The authors are also grateful to the reviewers for their
extremely constructive comments.
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