The performance and stability of liquid rocket engines is determined to a large degree by atomization, mixing, and combustion process. Control over these processes is exerted through the design of injector. The functions of the rocket injector are similar to those of a fuel injector in an internal combustion engine. The injector has to introduce the flow to the combustion chamber, and atomize and mix the propellants.
The quality of the injector is usually checked by performing cold tests with inert simulant liquids instead of reactive propellant liquids. In this study, water is used to inert simulant liquids.
This paper presents the single and twin spray characteristics of F-O-O-F impinging injectors for a liquid rocket engine. The PDPA(Phase Doppler Particle Analyzer) is employed to measure droplet size data. Droplet mean diameter(SMD) and size distribution are used to investigate the effect of the momentum ratio and the pressure drop variation. The main results of this study are that SMD of droplets is small in general, as the momentum ratio of oxidizer/fuel increase, which the total flowrate is fixed. As the momentum ratio of oxidizer/fuel increase from MR=1.19 to MR=6.48 in a cross section which is 100mm away from the injector plane, SMD decreases. Droplet size distributions of the impinging spray flow-field coincide well with both Rosin-Rammler and Upper-limit distribution function.
The interaction position observed from the image of the dual injector is 25mm from the injector surface in case of No. 1 injector having a distance(L) of 20.8mm between the two F-O-O-F individual injectors, In the case of the No. 4 injector having a distance(L) of 62.4 mm, interaction occurs at 70mm away from the injector surface. The location of interaction occurs as the mixing ratio of fuel and oxidizer increases.
The velocity of the droplet at the position where the interaction between the two individual sprays occurred slightly increased the axial velocity by the collision of the droplet. In addition, the turbulence intensity in the interaction area is increased due to the influence of the non-uniform velocity due to the collision of droplets.
The arithmetic average diameter(D10) of droplets in the interaction area was small because the droplets with large momentum collided and separated into small droplets, and the SMD in the interaction area also became smaller. This is because large droplets are divided into small droplets and small droplets that have a small momentum are coalesced together and the droplet size distribution becomes uniform. The standard deviation of the droplet(Dsdev) is closely related to the SMD. The smaller the standard deviation of the droplet is, the smaller the SMD becomes. Therefore, since the Dsdev in the interaction area has a small value, the SMD in the interaction area has a small value
As a result of investigating the behaviors of collided droplets using Weber number(We) defined in this study, the critical weber number which is the boundary of the separation and coalescence is We=40 when assuming that the size and velocity of the colliding droplet are the same. It is assumed that the size and velocity of the colliding droplet are different from each other, the critical weber number of the boundary between coalescence and separation is We=35.
This experimental result can be conveniently used to the preliminary design stage of the F-O-O-F impinging injectors for a liquid rocket engine.