X.-x. Fang, C.-b. Shen Acta Astronautica 136 (2017) 369–379because the LOX is injected as conical sheet and as a consequence itforms an ‘enclosed’ area near the pintle tip which leads to bad mixing ofthe propellants. Thus, improved structure of the pintle injectors isbrought up [41] (see in Fig. 20). Fig. 20(b) is an enlarged schematicconfiguration of the new pintle tip. The LOX injection channel ischanged from the circularity (see in Fig. 5) to the quadrangular slots(see in Fig. 20). The most important advantage of the slots is that thegas methane can pass through between the adjacent slots, so mixing ofthe propellants is much more efficient.The combustion field of the pintle engines with different LOXinjectionchannels 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) (scheme1) and quadrangular slots in Fig. 21(b) (scheme 2). There are 16quadrangular slots on the pintle tip in scheme 2. The circumferentialand axial sizes of the quadrangular slots are 2 mm and 2.95 mmrespectively. Thus, BF of the scheme 2 with quadrangular slots is 0.5.Other sizes of the pintle injectors and combustor are same as conditionNo. 1 in Table 2.Fig. 22 shows the temperature field of the two different pintleengines. The temperature of the gas near the combustor wall is lower inscheme 1, while the high-temperature area is larger in scheme 2. Thecooling effect of the low-temperature methane on the wall of thecombustor is better for scheme 1. However, the mixing efficiency of thepropellants and the combustion efficiency (0.981) are better for scheme2. So in consideration of the combustion efficiency of the pintleengines, scheme 2 is better than scheme 1. In addition, the combustorpressure is higher for scheme 2.For the pintle engines with quadrangular slots, the temperaturefield is more uniform in the last half than the first half of thecombustor. The mole fraction distribution of the gaseous methane isshown in Fig. 23. It can be seen that for scheme 1 the methanedistributes mainly near the combustor wall, while the methane ismainly in the center of the combustor for scheme 2. So the mixingefficiency of scheme 2 is much higher than scheme 1. All in all,considering the mixing efficiency and combustion efficiency of thepintle engines, LOX injected through the quadrangular slots is betterthan the circularity.6. ConclusionsIn this paper, atomization characteristics of the pintle injectorswere studied experimentally. With increase of the gas-liquid mass flowratio, the atomization cone angle decreases. In the condition of thesame gas-liquid mass flow ratio, as h o grows bigger, the atomizationcone angle becomes smaller. In the opposite, when the half cone angleof the LOX-injection gap grows bigger, the atomization cone anglebecomes bigger. Owing to the viscous effects of the pintle tip, withincrease of Ls/Dp, the atomization cone angle becomes larger. The SMDalong 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 pintleinjectors and combustor on the combustion performances of the LOX/methane pintle engines were studied by numerical simulation. Two bigrecirculation zones exist in the combustor, which lead to combustionstability of the pintle engines. When L s is too small, there exist two lowtemperature zones in the combustor, and that will lead to decrease ofthe combustion efficiency. When L s is too big, the effective characteristiclength of the combustor reduces. In consideration of the combustionefficiency, Ls/Dp should be chosen around 1.0. Additionally, thecombustion efficiency decreases along with increase of h o , and thedecrease of the size of the recirculation zones is responsible for that.Finally, improvement was brought up to the structure of the pintleinjectors and a comparison of the combustion flow fields of the twodifferent pintle engines is given by three-dimensional numericalsimulation. Pintle engines with LOX injected in the quadrangular slotscan acquire better mixing efficiency of propellants and higher combustionefficiency as the gas methane can pass through the adjacent slotswhile the annular-channel type has an ‘enclosed’ area near the pintletip which has a negative influence on the combustion efficiency.AcknowledgementsThe authors would like to express their gratitude for the financialsupport provided by the Fund of Innovation, Graduate School of NUDT(No. S150105). The authors are also grateful to the reviewers for theirextremely constructive comments.References[1] D. Haeseler, A. Götz, A. 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