Several studies regarding the combustion characteristics of a pintle injector, which is one of techniques for throttling liquid rocket engines, have been conducted; however, the primary focus was on the performance instead of on the flame. To study the fundamental flame characteristics and provide a useful design guide for a pintle injector, gaseous methane and oxygen flames under the laminar condition were visualized by CH* chemiluminescence and Schlieren imaging techniques. The pintle injector used in this study had a fully opened annular injection area, and the combustion chamber was designed as a square cylinder with two windows to visualize the flames. The mass flow rates of methane and oxygen were controlled to change the injection condition, and the pintle opening distance was changed as a geometrical variation. The flame shapes could be classified globally into two types: a shear layer flame, located between two injected propellants and a tip-attached flame located near the pintle tip. The flame angles were measured from deconvoluted flame images, and measured angles were compared with spray angles from previous studies. In conclusion, the flame-anchoring locations and flame shapes were found to be critical for the injection momentum flow rate. Additionally, a new non-dimensional number, the pintle number, Kp, for the normalized angle is proposed, and the pintle number has a strong relation with the flame and spray angles. It is expected that the pintle number can be used to design a pintle injector.
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