Study of Liquid Breakup Process in Solid Rocket MotorsAuthor: Michael Stefik & Bryan Sinkovec, Co-Author: Yi Hsin Yen, Faculty Advisor: Professor Ryoichi S. AmanoComputational Fluid Dynamics and Wind Tunnel Lab, University of Wisconsin-Milwaukee, Milwaukee, WI, USA

AbstractThis project presents research which attempts to characterize the average droplet size of a two-phase flow of water and air. This two-phase flow simulates how liquid aluminum propellant will behave during combustion. During combustion, the aluminum is oxidized into alumina (Al 2O3). Under particular flow conditions this alumina can solidify and attach to the inside of the rocket nozzle, which can cause a decrease in performance and fuel efficiency. Computational fluid dynamics (CFD) simulation data is obtained for a cold state to analogously predict real phenomena in room conditions and how the simulated water and airflow will behave. Experimental data is gathered by taking high quality images of the two-phase air and water flow enclosed by a physical horizontal test chamber. The experimental and CFD data sets are processed using a MATLAB program to determine the diameter of each particle within the test chamber, and this information is used to calculate the average droplet size. Through this study we can elucidate a phenomena of real combustion product behavior.

MotivationAlumina (Al2O3) agglomerates on the chamber wall in a high-temperature airflow channel. It is beneficial to model airflow using CFD as shown the vertical chamber in Figure 1.

Simulation Conditions• The 1-m long channel in Figure 2 has a 0.2-mm diameter

inlet where a 30-mm thick layer of liquid flows.• A ramp with a height of 15 mm and an angle of 20o is

placed at a distance of 0.159 m from the inlet.• The inlet section creates flow separation of liquid.• The initial liquid velocity is kept at 0.67 m/s and the

initial air velocity is tested at 20 m/s, 30 m/s and 40 m/s.

Imaging Equipment

Experimental MATLAB Imaging

• MATLAB targets individual droplets and measures the diameter of each droplet.

• The image is “masked”, meaning the unnecessary parts of the image are blacked out, as in Figure 3.

• This allows the program to accurately calculate the average droplet diameter with minimal outliers.

AcknowledgmentsThe author would like to thank Dr. Ryoichi Amano and Yi-Hsin Yen for assisting in obtaining the computational models and data.

Weber Number• The Weber number describes the ratio between the

inertial force and surface tension of a liquid droplet.• As the velocity of the airflow is increased it can be

seen that the average droplet size decreases.• A higher Weber number indicates that the average

droplet size is smaller. As shown in Figure 4, the smallest average droplet size occurs at 40 m/s.

Figure 2. Flow chamber illustrating two-phase flow.

Figure 4. Chart showing the Weber number history.

Droplet Size and Particle Count• The average droplet size is shown to have a trend

that is directly related to the velocity of the air.• Figure 5 shows an increase in the number of

particles as the velocity of the air increases.• Table 2 shows he average droplet size for the

various test velocities: 1.34 mm at 20 m/s, 0.97 mm at 30 m/s, and 0.95 mm at 40 m/s.

Table 2. Particle size and number for each velocity.

Figure 5. Particle count history for each velocity.

Conclusions• The average droplet particle size decreases as the

Weber number, average number of particles, and airflow velocities increase.

• It can be concluded that as the particle size decreases, a higher rate of liquid breakup within a rocket nozzle is indicated. This gives insight into the expected behavior of particles within the propulsion chamber.

Figure 1. Vertical CFD simulations.

Figure 3. Image processing distribution.

A Photron UX-50 high-speed camera was used for high resolution images. Specifications can be seen in Table 1.

Table 1. Photron UX-50 high-speed camera specifications.