THE ANALYSIS ON THE BREAKUP AND MIXING OF THE IMPINGING JETS

• Main Authors: Wei-Cheng Chen, 陳威丞
• Other Authors: Tony Yuan
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/44825489412643869143

博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 95 === The impingement of liquid jets is generally used for the breakup and mixing of the liquid propellants in the rocket injector design. While impinging, the jets become aerodynamic and hydrodynamic unstable thus disintegrating. The factors, such as momentum flux of the jets and the surface tension of the liquids, have a significant effect on the impinging spray. In this study, the effect of the momentum flux and surface tension on the impinging spray was experimentally investigated and a conceptual mixing mechanism has been established. For the design of a 5-lbf rocket, the cold-flow and hot-fire observations of the impingements of NTO/MMH system were also conducted. The spray phenomena of impinging jets were studied by a simple image technique as well as PLIF technique coupled with statistical analysis. From the PLIF observation, the mass probability distributions of the impinging spray were constructed. Thus, the uniformity and mixing efficiency of the impinging spray can be determined. For NTO/MMH impingements, the distributions of local mixture ratios and flame temperatures were deduced, and the characteristic exhaust velocities were estimated. The results demonstrated that the momentum flux as well as the fluid’s surface tensions crucially affected the spray pattern and mixing of the impinging jets. A characteristic momentum flux was observed which can be used to justify an effective breakup and mixing condition. Above the characteristic momentum flux, the spray uniformities were almost invariant while the droplets distributed in a progressively smaller area. Smaller surface tension of liquids leads to more uniform distribution with smaller characteristic momentum flux for their easier to be disintegrated. The mixing of the like-doublet impinging jets was mainly depended on the mutual penetration of the jets. However, the mixing phenomena can be differentiated into two types: merged liquid mixing at momentum flux lower than the fully developed condition, and droplet penetration mixing at momentum flux higher than fully developed condition. It also showed that, at the characteristic momentum flux, the optimum mixing efficiency occurred as the penetration was improved. Above the characteristic value, the penetration slightly increased and decreased the mixing of the jets. For lower surface tension liquid, higher penetration was shown, and it resulted in the poor mixing comparing with the higher surface tension liquid. Normalization of the characteristic momentum flux to eliminate the surface tension difference, the above described characteristic conditions appeared at Weber number at ~90,000. In the cold-flow study of the NTO/MMH system, triplet impingement showed its superior properties in symmetry and uniformity of the spray than that of the doublet impingement. Triplet impinging mixing was less sensitive to momentum flux ratio’s varying, however, for the operation of a 5-lbf NTO/MMH rocket, the optimum mixing efficiency and average characteristic exhaust velocity occurred at O/F�l1.2, corresponding to the unity momentum flux ratio, with doublet impingement. Comparison of the hot-fire and the prediction from the cold-flow observation of NTO/MMH impinging combustion, the results demonstrated that the predicted planar temperature distributions adequately described the shape and location of flame zones in the hot-fire observations. In view of the flame images, an induction distance after jets’ impingement always exists before reaching the intensive reaction zone, while the length of the induction distance was controlled by the MMH local evaporation rate. The analysis also verified that the impinging combustion of MMH and NTO followed the conventional spray combustion pattern. A maximum length of ��80 mm of the intensive reaction zone was shown, while diffusion-type combustion occurred at high O/F conditions.