3D printed Liquid Piston Stirling Engine Cont'd: Minutiae research
More work on v4 of the 3dPLPSE tonight.
First, I tried (spoiler:failed) to resolve the brown muck dissolving in the water. Wrapping as much as possible of the resistor body and leads in kapton tape did not reduce the muck. A series of protracted runs totaling over an hour revealed the culprit. There is a significant amount of corrosion on one of the stainless steel bolts that provides the power feedthrough. It's electrolysis from the input power jumping between the terminals. That's not an easy fix, and I'll need to think about it.
Second, I noticed a problem where the water level in the output column would rise as hot air accumulated in the displacer "bubble". This has a couple of negative impacts. First, the additional mass of the water above the average level in the hot and cold columns reduces the output power. Second, at the extreme, the bubble displaces all of the water from around the heater and the engine would melt. The book suggests using a hypodermic needle to allow the displacer bubble to vent at a slow enough rate not to interfere with operation. I tried a few different approaches here. I tried notching the gasket and body, and that worked for a while, but stopped. Second, I tried drilling a 0.90mm (tiny!) hole in the top of the housing. This worked for a while too, but again it stopped. Drilling a third 1.2mm hole vented the pressure successfully, but then the engine stopped and wouldn't resume running until I covered the hole with my thumb. I believe the underlying issue here is that water vapor is condensing in the small hole and it's sufficient to block the airflow. This problem remains unresolved.
Next up, something exciting, I tried my first run with a "regenerator". This was a small bundle of copper wires packed loosely into the displacer bubble space. I don't have conclusive data, but this appeared to work. With it in place I was able to turn the input power down to about 6 watts and have it continue operating stably. That's several watts lower than I was able to run previously. I'm uncertain if that's because it's regenerating or if it's working as a heat sink for the cool end. More experiments required. I observed that at high input power some water was splashing into the regenerator area. I'll adjust this in the next revision to make it higher and angled so water will drain out.
After the last long run I observed the plastic nearest the heater and power feedthroughs was too hot to touch comfortably and had visibly distorted. Noted.
I've ordered temperature and pressure sensors from Amazon. They should be here Saturday.
I reread chapter 3 of the book that deals with calculating operating frequency, tuning column parameters, and power output. That answered several questions and created several new questions. More to come on that later.
I attempted to fabricate a couple of check valves so the engine could be coupled to a water supply and pump (tiny amounts of) water, but my usual sources were dry. I'll head up to the good hardware store tomorrow and see if I can move the ball there.
I think I'll be making a v6 engine, slightly larger than 4, this weekend. The primary design goals for it will be a.) don't catch my house on fire. b.) practice instrumenting a working engine c.) confirm the regenerator is actually "working". Part of me wants to make some areas of the housing (e.g. near the heater block) with a water jacket and liquid cooled, but I don't want to lose momentum going down that bunny trail. On the other hand, I know that will be required to make an engine with useful power output, so I might as well get my feet wet. We'll see.