Date of Award

December 2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ryoichi S Prof. Amano

Committee Members

Deyang Prof. Qu, Arsenio Andrew Prof. Pacheco, John R Prof. Reisel, Yongjin Prof. Sung

Keywords

Alumina agglomerates, Breakup, Eulerian-Lagrangian Approach, Mechanical Erosion, Multi-Phase Flow, Solid rocket motor Nozzle

Abstract

The solid rocket motor (SRM) is considered one of the essential engines that facilitatesaerospace research; thus, investigating the propellant burning process is vital. One of the challenges facing its growth is the oxidization of the aluminum into aluminum oxide at the exhaust’s high pressures and temperatures. The oxidized aluminum forms agglomerates, impinge on the exit nozzle walls, causing severe damage (erosion) to the nozzle material. Thus, the present work attempts to investigate and reduce this erosion. Two different approaches are followed in the current work, the first one aims to better understand the aluminum oxide agglomerates break-up mechanism and the factors affecting it experimentally (subsonic condition due to the safety purposes limitations), while the other establishes a numerical model to predict the nozzle mechanical erosion within the rocket’s combustion chamber severe conditions.

The breakup process and some factors affecting it are investigated in three sections. Twophaseair-water flow experimental set-up is used, as a substitute for liquid aluminum agglomerates and exhaust combustion gases, in the three sections. The first section’s experimental results show that increasing the exhaust air velocity enhances the droplet's break-up tendency to reduce the average diameter and increase droplet numbers per the testing channel volume. Numerical models were constructed and validated using the experimental results. The percentage error in the droplets’ average diameter and the number is between 6–15% and 8-18%. Furthermore, the effect of reducing the liquid surface tension was studied. The results showed that it facilitates water bodies’ separation from the interface surface, because of the reduced bounding forces between surface’s molecules, which enhances the break-up process (0.5-17% increase in the droplets’ average diameter and 4-100% increase in its number) and reduce the droplets impact on the nozzle walls, hence reduce the SRM nozzle erosion problem.

While the second section investigated the breakup process at different water flow rates andconstant air velocity, where the results were used to validate a numerical model. The results revealed an excellent acceptance between the numerical, the experimental data (6-19%), and the effect of increasing the water flow rate on the break-up mechanism. The validated numerical model was further used to study the airflow acceleration impact on the break-up process. It was found that applying acceleration to the airflow subjects the water surface to rapid and sudden changes in the relative velocity between the gas and liquid, thus separating more water fragments from the primary liquid. In other words, it enhances the break-up process by reducing the average diameter with a range from 6.5% to 9% compared to the no-acceleration case and increasing the average droplets’ number [8.5-17%].

Finally, the third section investigated the submerged nozzle configuration on the breakupprocess under different air and water flow rates, in addition compared between the submerged nozzle and the external one. It was found that having a submerged nozzle enhances the droplets breakup in the nozzle convergent section due to the existence of the recirculation zones. However, the separated droplets will have higher velocity to hit the walls with, hence a supersonic model simulates the actual conditions within the rocket is essential to decisively conclude the submerged nozzle effect on the nozzle mechanical erosion.

Erosion prediction of the solid propellent nozzle is vital for its design process. Thus, thesecond approach employing a multi-phase numerical model is established based on the Eulerian- Lagrangian approach to model the aluminum particles burning inside the combustion chamber, in addition to simulating the mechanical erosion of the nozzle. The numerical model is validated against numerical and experimental results from the literature. Then the validated model will be further used to investigate the SRM nozzle erosion at different boundary conditions, nozzle configurations, particles, and propellant properties. First the model was used to simulate the agglomerates' break-up, in addition to predicting the mechanical erosion for aluminum particles with lower surface tension. The results showed that applying the Reitz-Diwakar breakup model reduces the erosion rate by 6.2% - 24% depending on the injected droplets. In addition, it was found that a decrease in the erosion rate by 1% to 4.5% can be achieved by reducing the aluminum additive's surface tension by 15%.

Then, an investigation of the effect of increasing the propellant aluminum content anddifferent particles’ injection velocity on the nozzle mechanical erosion was conducted and the results showed that having higher aluminum content increases the nozzle erosion by 4-10% compared to the 15% case. Furthermore, the aluminum particles will not fully burn within the combustion chamber and will participate in the nozzle erosion. In the end, having particles with higher initial velocity at the burning surface increases the nozzle mechanical erosion, despite of the incident mass flux decline.

Finally, the submerged nozzle configuration effect on the mechanical erosion was studiedat seven particle diameters and was compared against the external nozzle results. And it was concluded that comparing the external nozzle and submerged nozzle configurations in terms of the predicted mechanical erosion, the external nozzle will perform better than the submerged one as lower mechanical erosion exists in its different sections.

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