高压燃气射流宏观特性的可视化研究
首发时间:2017-05-18
摘要:掌握燃气的高压脉冲喷射特性是实现天然气发动机燃烧系统优化设计的前提。本文基于纹影法对不同孔径的单孔喷嘴在多种气源压力和环境压力条件下的高压燃气射流宏观结构特性进行研究。结果表明,即使在很小的压力差条件下,喷孔出口处仍然会形成不充分膨胀气体射流,而气体在出口处的膨胀、加速过程和诱导激波是区别于液体喷雾的主要特征。环境压力变化对气源与环境压力比的影响明显,其对射流贯穿距及锥角的影响要大于气源压力和孔径带来的影响。虽然甲烷和氮气的密度差别较大,但在相同条件下贯穿距结果无明显区别,壅塞现象对高压燃气喷射具有重要影响。由此,本文初步提出了针对高压燃气脉冲喷射的射流贯穿距经验公式。喷孔出口处的膨胀过程使燃气射流的近场锥角要明显大于远场锥角,同时使气体速度达到了超声速,但对射流前锋面速度无明显影响,在T=1ms之后,前锋面速度便降低到了50m/s以下。在低背压条件下,燃气射流表现出较强的空间拓展和分布能力,但随着背压增加,气体射流的面积和体积明显减小。
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Visualization research on the macroscopic characteristics of high-pressure gas jet
Abstract:The injection characteristics of high-pressure gas jet should be researched deeply, in order to optimize the combustion system of natural gas engines. The macroscopic structures of gas jet injected through different single-hole nozzles are analyzed under different origin pressure and ambient pressure conditions, by using schlieren imaging system in this paper. Experimental results show that an under-expanded supersonic gas jet is observed on the downstream of nozzle exit. The expansion and acceleration of the gas, as well as induced shock wave, are the important characteristics which are different from the liquid fuel spray. The variation of ambient pressure changes the ratio of origin pressure and ambient pressure significantly. It leads to more obvious effect to jet tip penetration and jet angle than that of origin pressure and orifice diameter. The density of methane is smaller than nitrogen, But which does not cause a significant difference in the jet tip penetration result. Choking phenomenon at the nozzle exit has important influence on the high-pressure gas fuel injection. On this basis, the empirical formula of high-pressure gas jet tip penetration is proposed preliminarily. In addi-tion, the expansion process occurring at orifice exit leads that the near-field angle becomes bigger than the far-field angle. Meanwhile, it also makes the gas speed up to the supersonic. But the acceleration does not promote the front-edge velocity of CH4. In fact, the front-edge velocity reduces to below 50 m/s after the time point of T=1ms. Gas jet shows significant spatial expansion and distribution capacity under atmospheric pressure. But the area and volume of gas jet decreased obviously with the increase of ambient pressure.
Keywords: natural gas engine gas injection macroscopic characteristics choking schlieren imaging
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