Characteristics of Gas-centered Swirl Coaxial Injector for Staged Combustion Cycle Engines

Abstract

Experiment about characteristics of gas-centered swirl coaxial injector for staged combustion cycle engine was investigated. A 9-tonf class staged combustion cycle engine for space launch vehicle was developed based on its basic research. Gas-centered swirl coaxial injector was used for this experiment and it was designed by hydraulic of swirl injector method through ideal and viscous liquid theory and some references. In liquid rocket engine, role of injector is not only supplying spray but also suppressing combustion instability. For this similar experiment, spray characteristics of injector and dynamic characteristics were understood. Spray photographs were captured by synchronizing stroboscope and digital camera and sometimes high speed camera and Xenon lamp were used. To measure the liquid film, liquid film measurement system was adopted by using electrical conductance method. For excitation, an acoustic speaker was used to supply its frequency to gas. Gas velocity and effect of excitation was measured though hot-were anemometry. The liquid fuel and gaseous oxidizer are simulated by water and air, respectively. To know the spray characteristic of gas-centered swirl coaxial injector, liquid film thickness and spray angle were measured. It was found by changing momentum flux ratio, recess ratio and gap thickness which are important variables in coaxial injector. Liquid film thickness and spray angle were changed by following the velocity of gas and liquid. When the velocity of liquid was slow and gas was fast, liquid film thickness and spray angle were decreased. On the other hand, when the velocity of liquid was fast, liquid film thickness and spray angle were maintained steady even though the velocity of gas was fast. Spray characteristics are well expressed by showing velocity of gas and liquid in momentum flux ratio. It was found out that overall spray characteristics was reduced by increase in momentum flux ratio. These characteristics were changed when momentum flux ratio was low and recess ratio was increased, and also spray angle was highly decreased. Moreover, spray characteristics was varied depended on the change in gap thickness, and when gap thickness went thicker, it did not fully fill but certain liquid film thickness was maintained. When gap thickness is thin, which also means when liquid film thickness is thin, it went thicker though entrain effect of liquid which comes from gas. From these results, not only momentum flux ratio and recess ratio, which are well known as major parameter of coaxial injector, but also gap thickness could be used as major parameter in design. To find out the dynamics characteristics of gas-centered swirl coaxial injector, excitation was supplied to gas supply line through an acoustic speaker. Together with spray characteristics experiment, momentum flux ratio, recess ratio and gap thickness were varied. the ratio of gas velocity to velocity perturbation was marked as input value, and the ratio of liquid film thickness that was measured in injector exit to liquid film thickness perturbation ratio was marked as output value. Ratio of input and output value were expressed in injector transfer function. Gain was changed based on input frequency, and higher gain was occurred in 2 specific frequencies. High gain occurred in low frequency was judged as resonance effect between the input frequency and the frequency of gas-liquid shear instability. However, high gain occurred in high frequency was judged as resonance effect between the input frequency and the resonance frequency according to length of gas supply line. The frequency with high gain has spray pattern carrying its frequency so it has similar spray pattern with self-pulsation. Gain was increased by following the increase in momentum flux ratio. Gain was generally increased because mixing space of gas and liquid was increased when recess was longer. Also, when gap thickness got thinner, perturbation of liquid film relatively occurred bigger so gain went increased.