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
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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