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Completed TRL 2 (started at 2, targeting 4)
Project Objective
Develop a piezo-driven actuator to induce cavitation in a feed line to reduce pressure pulsation communication between turbopump outlet and injector manifold inlet on highly throttleable and rotating detonating liquid rocket engines.
Project Description
The primary motivation of the innovation is to assist in maintaining combustion stability in highly throttleable liquid rocket engines and rotating detonation rocket engine systems for lander applications. Reducing pressure oscillations/pulsations in the feedlines assists in maintaining combustion stability throughout the main chamber operating pressure range. This main chamber operating pressure range can also be extended to lower pressure or throttle setpoints with an increase in throttle operating resolution.
The current state of the art utilizes cavitating venturis to create cavitation within the fluid that reduces pressure oscillation waves traveling back upstream. The innovation consists of replacing existing cavitating venturis with a piezo-driven actuator that induces cavitation in the flow by means of high-frequency vibration. Where cavitating venturis are designed around a specified fluid flow rate, the piezo-actuated cavitating inducer can initiate cavitation at a wide range of flow rates and is also controllable by utilizing tunable feedback inputs from pressure measurements upstream and/or downstream of the device.
The goal of the project is to design a device with the piezo-driven actuators vibrating a probe that is submerged in a feedline. The device will be tested first with water as the flowing fluid followed by testing with Liquid nitrogen. A few different probe designs will also be tested for their effect in creating various levels of cavitation.
Project Results and Conclusions
The research for this project consists of first analyzing the force output versus frequency capabilities of various commercial-off-the-shelf piezo-driven actuators and their overall effect of converting electrical input energy into thermal energy to initiate cavitation within a flowing fluid. To maximize energy transfer into the fluid, the aim is to have the piezo-driven actuator operating at both its highest-frequency capability and highest-force output concurrently. This leads to sizing the piezo-driven actuator for the device with a lower electrical capacitance value when compared to sizing based on only highest frequency attainable or highest-force output.
In the device itself, the probe will oscillate axially as the probe’s tip surface imparts momentum into the fluid thus creating a region of rapidly compressing and expanding flowing liquid. This rapid compression and expansion allow cavitation bubbles to form. Various probes will be tested where the surface area of the tip is varied as well as surface roughness features along the length of the probes to study their effects on the level of cavitation produced.
At the time of writing, a CAD model has been produced, and manufacturing drawings are in development. The piezo-driven actuators and their electrical driving equipment have been purchased and are expected to arrive at the beginning of the 2nd quarter of 2025. Test plans will be written once all purchased equipment has arrived and hardware is manufactured. Testing of the device is anticipated to be performed at the Marshall Space Flight Center’s Component Development Area towards the end of the 2nd quarter of 2025. Results are expected to be published soon after. Further testing with integrating the device into the feedline of an engine assembly will be pursued in the future.
Once all planned tests are completed and the device demonstrates successful operation in reducing pressure oscillations within a feedline, a few benefits for liquid rocket engines can be anticipated. The reduction in pressure oscillations can lead to more stable combustion in the main chamber from a more stable propellant flow rate. This will also enable lower deep throttling capabilities at higher resolutions as well. Another benefit would be the reduction in the pressure needed at the inlet to the injector thus allowing a trade of the maximum system pressure to be either reduced or allowing that portion of the injector inlet pressure to be transferred to the main combustion chamber for increased performance.
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