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

X.-x. Fang, C.-b. Shen Acta Astronautica 136 (2017) 369–379
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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