Chihiro INOUE, Dr.Eng.
Project Associate Professor
Rocket and Spacecraft Modeling Lab.
Department of Aeronautics and Astronautics
University of Tokyo
In liquid propulsion systems, thrust performance and combustion stability are profoundly dependent on atomization and mixing process of propellants.
Under sub-critical pressure conditions of propellants, for instance in upper stage rocket engines and in attitude control engines of satellites, spray combustion takes place. Here, atomization is important as the initial process to determine the following spray characteristics. Therefore, it is indispensable to achieve desirable atomization characteristics at all the operating conditions. Under supercritical pressure conditions, however, turbulent diffusion combustion occurs without atomization mainly in 1st stage booster engines.
Though, in the future, combustion pressure at rated operation will be designed to supercritical pressure in booster engines and in prospective reusable ones, off design operation will be also required for throttling thrust and aborting flight. In such off design operations, it is conceivable that the combustion pressure falls below the propellant’s critical pressure. Therefore atomization is an inevitably essential phenomenon not only today’s engines but also oncoming ones.
【Impinging Type Injector】
←： Two liquid jets impinge to create the liquid sheet, and then they breaks up into tiny droplets.
Several types of injectors have been proposed and used corresponding to combinations of propellants. Impinging jet injectors are the preferred type especially for rocket engines and for attitude control engines that use storable propellants or liquid hydrocarbons.
In our study, atomization of impinging type injector is mainly investigated. Numerical annd experimental studies have been carried out to clarify the detailed flow field and to propose control techniques of atomization.
【Impinging Jets and Single Jet】
←： left：two-jet impingement right：single jet injection
Atomization is enhanced by jet impingement. Single jet is not atomized.
←： black lines : stacked image during 20ms, white sheet : instantaneous image
Each black line corresponds to a trajectory of each droplet.
The outer edge of the spray consists of droplets fragmented close to the impingement point.
【Atomization Enhancement by Microjet Injection】
←original atomization phenomenon (left: w/o microjet) is strongly enhanced by small amount of microjet injection(right: with microjet).
Impinging atomization is able to produce fine drops at a rated operation. In contrast, the atomization characteristics deteriorate under off design conditions when injection velocity comes to be slower.
For improving atomization characteristics at off design conditions, an effective technique is verified utilizing small amount of gas (microjet) injection. The microjet is supplied from a pressurized reservoir and is injected from the center of the liquid nozzles toward the impingement point in like/unlike doublet injectors.
To clarify the flow field and the enhancement mechanism, experimental visualizations, drop size measurements and corresponding numerical analyses are carried out.
It is elucidated that Sauter Mean Diameter (SMD) becomes one-tenth of the original SMD by the microjet injection with the amount of only 1% of liquid mass flow rate. The dominant non-dimensional numbers are found to be the relative Weber number and the ratio of the dynamic pressure (microjet/liquid jet) at the impingement point. The optimized atomization efficiency is achieved when the dynamic pressure ratio is approximately two.
←： numerical results of impinging atomizations considering the interactions between two injectors. (1-day calculation by a stand-alone computer)
Atomization process is simulated from continuous liquid jets, sheet to dispersed droplets by coupling method of an Eulrelian approach and a Lagrangian approach. The method has been developed and validated by comparing the results with corresponding experimental results.
By using the Eurelian-Lagrangian hybrid method, the series of atomization can be simulated by a stand-alone computer within a short period of time.
Gallery of Sparkling Fireworks (YouTube)
About 400 years ago(Edo-era), sparkling fireworks began in Japan.
Sparkling fireworks are composed of a black powder, which is a mixture of potassium nitrate, carbon and sulfur, simply wrapped in a twisted paper. The fireworks scatter beautiful streaks of light with soothing sounds. The formation of these streaks is attributed to sequential atomization and luminescence of droplets ejected from the fireball driven by a series of chemical reactions.The physics behind the beauty of sparkling fireworks, however, has not been clarified yet. So, we do not know how and why they are beautiful.
In our study, atomization process in sparkling fireworks is elucidated by using a high-speed video camera. A scenario of droplets generation is found as follows: a liquid thread extends from the bottom of the bursting fireball, and fragments into droplets. Thus the droplets originate from inside the fireball rather than from its surface.
←： self-luminous pictures of the main fireball.
The spherical fireball at t=+0ms bursts at the right bottom at t=+0.4ms.
The hole is enlarged by the surface tension on the rim, which is similar to the case of a bursting soap bubble.
The thread begins to extend from the interior of the fireball at t=+2.0 to +2.4ms.
At t=+2.8ms, the droplet breaks from the thread.
A bubble bursting on a liquid surface is subjected to a similar atomization process.