Combustion Chamber Walls

Overview

These are projects I am working on in a research lab, Braun's Engineering Factory for Advanced Supersonic Technologies (BEFAST). BEFAST is currently developing a baby Rotating Detonation Engine (RDE) for testing applications. The RDE will allow for different teams to test many individual projects on the same egnine and test stand.

This semester I've taken on a project to research, design, manufacture, and test a new combustion chamber walls for the RDE. What will be unique about my chambers is that they will be focused on two aspects we haven't yet gathering data for.

The first new chamber will include a series of thermocouples that will be placed in the wall to gather temperature data. The reason we haven't implemented this on the original chamber is due to the extreme temperature inside the flow. With temperatures reaching 3000-4000K, almost any ordinary thermocouples are destroyed before providing any data. So the challenge is to find accurate ways to record the data over a short test duration.

The second new chamber will be a quartz design that will allow us to see the flow and detonation waves in the chamber. Having optical access to the chamber flow should help confirm simulations and provide a nice visual to show what we are achieving in the chamber.

Research

The harsh conditions inside a Rotating Detonation Engine (RDE) chamber make thermocouple survival extremely challenging. In terms of mechanical stress, the detonation waves in RDEs travel at Mach 5-7 (approximately 1,700-2,400 m/s), and these waves create intense shock fronts with sudden pressure jumps of 15-30 times the initial pressure. The rapid acceleration of flow behind these shock fronts creates extreme shear forces that can damage anything extruding into the chamber flow. Then you consider the thermal impact, the detonation waves create temperatures of 3000-4000K, which is too hot for conventional thermocouples.

So the challenge is to find a affordable and accurate way to use thermocouples to gather temperature data. I had two ideas to solve this problem. The first was to have the thermocouple port drilled fully through the wall, but having the thermocouple tip just a short distance away from the inner wall. This could allow the thermocouple to read the temperature of the ccombustion, without experiencing the full shearing force of chamber flow. The second idea, which I plan to attempt if the thermocouple fails due to temperatue, is to not bore the thermocouple port all the way through the wall. By not having the thermocouple tip fully exposed to the chamber flow, the instumentation would be protected. This may be a good option as we could use a lower quality thermocouple, as it would not experience such extreme conditions. The downside to this is that the data would be less accurate and our results would be based on heat transfer calculations, and would naturally not be as accurate as direct temperature data.

Design Process

First Draft

The idea behind this first iteration was a single wall that incorporated the quartz window and the thermocouple port. Thus saving cost and time by not having to manufacture two separate pieces. While this design is solid and would function as intended, it might not be the best option for our Baby RDE. I realized a few things after I had finished this model. Firstly, the quartz window would only have a rough area of 1 square inch. That is a very small area to see the flow, and I don't think it would be worth the effort to manufacture this design. Secondly, the cost of a finely machined quartz window is very expensive. I noticed as size increased we actually saw a decrease in price due to less complexity.

So I decided this design will be used fro out full scale RDE that we plan to design and build in the future. THis would have a larger window which could actually be used for good optical data.

Quartz Combustion Chamber

The current plan is to create two combustion chambers for different tests. The first will be a full quartz design, designed specifically for optical tests. It consists of a quartz insert that is going to be cheaper and easier to manufacture. The insert is bolted into place by our top plate and base plate and connects to the injector.

Combustion Chamber with Thermocouples

The second chamber will be a simple design with thermocouple instrumentation that will be used to gather the temperature data. I am still working through that researc for the design so I don't have any images yet. It should be a simple design process fortunately.

Future Plans

I've still got a lot of exciting work to do on this project, I plan to have the chamber walls manufactured and hopefully tested before summer. Feel free to check back soon for updates!

Skills Developed