Dual-Channel Waterjet Propulsion Module
Tasked with the research, design, development, and manufacturing of a small, versatile underwater propulsion module. The module utilizes a waterjet system and an innovative dual-channel nozzle design to generate thrust. To view the final copy of my thesis or defense presentation, click on the "Thesis" or "Defense" buttons located on the right.
Dual-Channel Nozzle Design
The dual-channel nozzle design served as the primary research focus. High-velocity fluid energized by the impeller exits the nozzle's inner channel and creates a low-pressure "suction" zone between the inner wall of the outer channel and the stream of fluid. This low-pressure zone draws fluid from the external environment through the outer channel's inlet. This additional fluid volume conjoins with the energized fluid exiting the inner channel, theoretically resulting in additional thrust gains due to an increased mass flow rate of fluid exiting the nozzle's outer channel outlet.
Multiple designs with varying channel overlap distances were simulated and physically tested with the final module prototype. The maximum overlap distance nozzle (30 mm) proved to be the best performer, producing 16.93 lbf of thrust. This represented an 8.25% increase in thrust relative to its single-channel nozzle counterpart.
Impeller Design
Multiple impellers were designed, simulated, manufactured, and tested throughout the duration of the project. The two impellers pictured on the right were considered for the final module prototype. The two blade impeller was chosen for the final prototype due to its excellent combination of performance and efficiency.
Powertrain
Multiple powertrain setups were tested throughout the duration of the project. In its final configuration, the impeller was driven by a 790 KV brushless motor, controlled by a 150 A waterproof ESC, and powered by a 3200 mAh LiPo battery. Both the ESC and brushless motor were capable of operating completely submerged in water, significantly improving the cooling performance of the components.
Final Propulsion Module Prototype Design
The Final Propulsion Module Prototype design features a "mesh" patterned intake, external ESC and brushless motor mount, waterproof battery and wiring compartment, and internal threads on each end to connect the waterproof cap and dual-channel nozzle. The module has a dry mass of 2310.30 g, a primary diameter of 104 mm, and is 423 mm long with the waterproof cap and 30 mm overlap dual-channel nozzle attached.
​
The mesh intake allows water from the external environment to be drawn into the waterjet channel from the negative pressure created by the rotating impeller. The mesh geometry also acts as a filter, keeping potentially hazardous particulates that could lead to critical failure from entering the waterjet channel.
​
Both the ESC and brushless motor are mounted externally to improve the cooling performance of the components. The ESC is located directly above the battery compartment to easily route the battery and motor wires into the compartment below via wiring holes. The brushless motor is mounted directly to the wall separating the waterjet channel from the waterproof battery compartment. The motor wires are routed through three holes on the bottom of the wall into the compartment.
All wire connections and the LiPo battery are located inside of the waterproof compartment. An aluminum sheet is secured to a window at the bottom of the waterproof compartment. The battery sits atop the aluminum sheet inside the compartment. The opposite side of the sheet is exposed to open water, acting as a heat sink and allowing the battery to be actively cooled by the water passing over the aluminum sheet. The compartment is sealed by a waterproof cap, featuring a silicone-filled seal channel that compresses against the circular lip on the chassis to create a water-tight connection.
The entire propulsion module chassis was 3D printed as a single component to eliminate the need for additional water-tight connections.
COG and Buoyancy Analysis
It was crucial to consider the propulsion module's center of gravity and buoyancy forces to maximize performance and usability. A CAD assembly was used in conjunction with an analysis methodology to identify these key module characteristics.
A horizontally centered, vertically low center of gravity was targeted to help maintain the module's intended orientation when submerged without flipping over. Neutral buoyancy was targeted to minimize the number of opposing forces acting on the module.
Lead ballast was used to manipulate the module's center of gravity and drive its buoyancy force delta as close to 0 as possible. Four rectangular pieces of ballast were mounted externally to each side of the battery compartment. A single circular piece of a ballast was secured to the inside of the waterproof cap.
Testing Apparatus
A static testing apparatus was designed and constructed to complete all of the physical testing required for this project. The apparatus consists of a wooden frame, lever arm, and force gauge that could be mounted atop a 110-gallon tub.
When power is supplied to the motor to rotate the impeller, a thrust force is created in the direction opposite of the force gauge. This thrust force creates a moment about the point of the lever arm connected to the horizontal rod on the frame, causing the top half of the rod to move in the direction of the thrust force. The electrical wire connecting the lever arm to the force gauge resists the motion of the lever arm, allowing the force gauge to output a thrust force reading.
​
A lever arm mount was designed onto the top of the propulsion module to secure the module to the lever arm for testing.
Conclusions
Five of the seven design specifications created at the start of the project were successfully achieved by the Final Propulsion Module Prototype (thrust, length, diameter, additive manufacturing, modular battery). The 30 mm overlap dual-channel nozzle demonstrated promising results, producing a maximum thrust output of 16.93 lbf. This represented an 8.25% improvement in thrust relative to it's single-channel nozzle counterpart.
The total cost of the module exceeded the initial target cost ($100) by $67.55, primarily due to the addition of the high-performance waterproof ESC and the purchase of a new motor. The 30 minute operation run-time was also not achieved as a significantly larger, and consequently more expensive, battery would have been required. Instead, the project focused on maximizing thrust output with short operational run-time (~3 minutes at maximum throttle).
URI's Senior Mechanical Engineering Capstone Team 28 has taken over this project and I am continuing to support their efforts in an advisement role. Currently, the team is testing new powertrain configurations and is making great progress developing and optimizing the performance of the dual-channel nozzle design.