I don't have a whole lot of dead volume but you're absolutely right about the size of the ports needed to transfer the working fluid, they don't need to be that big.
Since my last post, I decided to ignore the temperature of the regenerator and cylinder and just heat up, expecting the orings to melt, etc.
Turns out it got to a point where it almost started (10 or so revolutions) before it locked up. I'm still trying to figure out why, since both the cylinder and the piston were made of stainless steel, which should rule out differential temperature expansion.
After letting it cool off, I disassembled and found nothing abnormal on the hot cylinder. Strangely the cold piston/cylinder is the one that locked up. Both are made of aluminum and without active lubrication, oil eventually disappears.
Quick Update on the new design:
I settled on a new design and started making parts already: crankcase, cold cylinder and piston. I am going back to the idea of o-rings on the cold cylinder/piston since there is no high temperature requirements. I designed in a way that I should be able to fine tune the o-ring size to find the sweet spot when between compression and friction. The big change with this new design is an active lubrication system. I lengthened the piston (no change to diameter or travel) to allow the mid line of the stroke to always be located between the top and bottom of the piston regardless of the crank angle. This allows me to locate 2 holes on each side of the cylinder to circulate compressor oil through a V-groove channel. A small electric pump will circulate the oil, either continuously or on a timer/as needed. Since there are 2 holes (in and out) I can technically run it continuously without having to worry about oil pressure building up and making its way through the orings. As is I am hopeful that the film of oil will remain capped by the upper and lower orings.
MicrosoftTeams-image (4).png
The biggest change of the design is the Gamma conversion. Cooler and Heater are now on the same cylinder. The regenerator is annular.
Cooler is liquid cooled copper with internal fins, similarly to what the cooler of the Sunpower engine looks like:
https://youtu.be/adLZIDxM8tQ?t=158
The displacer itself is mounted on a 8 mm shaft connected to yoke articulated to the connecting rod pin. So there is no piston/cylinder seal to worry about in the displacer. I am taking care of the displacer shaft seal using a PTFE dynamic seal. I have ordered the seal and would like to get a feel for them with an 8mm shaft before I start machining more parts.
As for the regenerator, I've gathered enough CFD results to be comfortable with its size and geometry. Which brings me to:
Quick Update on the regenerator study:
While I'm technically still running simulations, I have learned a lot of things by doing these alternating pulses.
I've done a number of steady states analysis to evaluate the boundary conditions static pressures needed to achieve the volumetric flow rates matching that 192 cc transfer. As it turns out the mass flow rate going from cold to hot versus those going from hot to cold are not equal. Maybe it sounds absurd as you would think mass is conserved, and it is. However, the transfer is done by displacement of a given physical volume. And because the temperatures on either side of the regenerator are different, the densities are also different and consequently the mass flow rates are different as well.
It took me a bit a time to figure this one out, as it wasn't that intuitive. Aside from this being pretty critical to getting good analysis, I discovered something pretty interesting: the heat transfer between fluid to regenerator is increasingly different than from regenerator to fluid.
As you know I did all my analysis with Helium pressurized to 10 bar.
When the difference in densities between 300K and 600K at 10 bar are actually significant. I put this graph together using ideal gas law.
Helium-Density.png
Those densities differences make the cold blow to be much "stronger" cooling than that the heating caused by the hot blow.
The consequences from this I am not too sure yet.
I am running a simulation currently, which I started with an ideal temperature gradient across the regenerator (by starting with that ideal gradient it greatly reduces the time it takes the system to reach steady state); what I am seeing is the hot end of the regenerator is not able to hold something close to the 600K from the hot blow. The other end of the regenerator is basically at 300K. In other words, the cold blows are gradually winning over the hot blows. Based on the mass of the regenerator I expect I will each steady state after roughly 12-15s of physical time, I am currently at ~3 seconds.
This graphs shows the mass flow rates differences between the cold and hot blows.
mass-flow-rates.png
And this graph here shows you, how starting from an ideal gradient (hot end of regenerator starts at 600K, cold end starts at 293.2K) the cold end remains pretty much unchanged, while the hot end is gradually dropping.
The second observation (likely for the same reason) is how the heater temperature is also dropping much faster on cold blows than the cooler temperature is climbing on hot blows.
cold-blows-vs-hot-blows.png