alt.energy.renewable

"Bruce Richmond" wrote in message
news:67078de7-e284-4e32-ae0f-7fc3099f3643@p25g2000hsf.googlegroups.com...
On Apr 6, 7:14 pm, Morris Dovey wrote:

>I saw your web page about building the fluidyne engine a while back
>and after reading the mention that it was a Stirling engine didn't
>give it any more thought to the details of how it works. I Know how
>Stirling engines work so that was a I figured I needed to know. But
>after looking again I realize the principals may be the same but the
>details are different.
>
>The diagrams shown are close to the standard ones but are still a bit
>confusing to most that see them the first time. The reason I say
>close is that line 2-3 on the p-v chart should be shorter than line
>4-1. With them the same length you have the same heat going out as
>coming in and no energy going out as work.
>
>http://www.cse.iitk.ac.in/~amit/courses/371/abhishe/main2.html
>
>Your explaination of the rise in air pressure pushing the water down
>on the hot side doesn't agree with the diagram either. While it takes
>pressure to push the water down, the chart shows the pressure
>decreasing from 1-2. Actually the chart shows the pressure rise from
>4-1 with the volume held constant. The volume is then allowed to
>expand from 1-2 while more heat is added to hold the temp constant,
>despite the drop in pressure.
>
>At this point it should be obvious that the idelized charts do not
>match up perfectly with what is happening in a real engine. In the
>real engine the water is constantly moving up and down. There is no
>time when the volume stays constant as shown in the chart.
>
>You mention the regenerator when going from 2-3, but a regenerator is
>not a required part of a Stirling engine.

According to "Engineering Thermodynamics with Applications" (Burghardt), "A
regenerator is the key element in both cycles [Stirling and Ericsson]; heat
must be stored in the regenerator during one part of the cycle and reused in
another part."

Ideally, the regenerator heats the gas all the way up to the hot-side
temperature and the heat source only inputs enough to maintain the gas
temperature during the expansion. Similarly, the regenerator cools the gas
all the way to the cold side temperature and the heat sink only rejects the
heat of compressing the gas. Of course unless you have an 'infinite
regenerator', this can't happen in the real world and there is additional
heat added /rejected by the source/sink.

> It does improve efficency
>and is worth explaining how it does so, but it need not be included in
>the basic operation explaination.
>
>You also wrote, "at maximum engine volume". Normally a Sterling
>engine is a closed system, so the volume of the engine as a whole does
>not change. In the fluidyne engine you have built the right hand tube
>is open to the atmosphere, so I suppose you could say it changes
>volume as the fluid moves up and down in the vented tube.
>
>One of the ways to increase efficiency in a Sterling engine is to
>increase the pressure of the working fluid. The higher density of the
>gas allows faster heat transfer. Have you tried having the water level
>in the open tube higher than in the other two? That would raise the
>pressure of the gas trapped in the closed loop.
>
>Another tweak is to use heleium rather than air for a gas. The
>smaller molocules again allow better heat transfer. Not sure if
>heleium would stay put, trapped by the water, or would be absorbed
>into it. Might be worth a try.
>
>For 3>4 you write, "The cooled air is compressed in the cold head, and
>heat Qc is sunk to the cold head at constant temperature Tc.
>Consequently, the engine volume decreases, while the engine pressure
>increases." Again, the lower temp contributes to decreasing the
>volume, but in doing so it would decrease the pressure, not increase
>it. The increased pressure comes from mechanical means.

3-4 is isothermal compression. As the piston compresses the gas, heat is
removed at the same time. The result is that the gas temperature stays
constant (ideally) and the pressure rises. His statements are correct,
"...the engine volume decreases, while the engine pressure increases."


>
>
>Have you tried closing off the open pipe? I was just thinking of ways
>to increase the pressure in the system. Raising the water level in
>that pipe will raise the pressure, but it could be raised much more if
>it was closed off. Might need to add an air chamber to allow the
>fluid to continue moving fairly freely in the tube.
>

There are several 'odd' thing about the fluidyne versus other Stirling
cycles. The water column on the far right that is open at the top is the
'output' of the engine. It rises and falls as the gas pressure rises and
falls within the loop. As this water rises, you would call it the 'power
stroke' and as it falls, the compression stroke where the potential energy
of the column does work on the gas to compress it.

Although his video can't show the water column in the 'hot' side, if you
could see inside the heater, you'd find the water level in it does not
rise/fall as much as the outlet tube. But it does rise/fall more than the
water in the cold side. And it isn't in 'phase' with the cold side.

The 'displacer' is the water in the bottom of the loop. With proper
phaseing, the rising water in the hot side shifts the gas to the cold side
and just as the water reaches TDC, the water in the cold side is compressing
the gas again. Then as the water in the cold side reaches its peak, the
water in the hot side is half-way down its 'stroke' and expanding the gas.
This 90 degree phase shift is an important part of the cycle.

The water in the outlet tube has to be 'tuned' for maximum output. For
example, with more water in the outlet, when the gas expands in the hot
side, instead of pushing the outlet tube water up, it pushes more water into
the cold side. So at first it would seem 'the more the merrier'. But with
most of the water leaving the hot side going into the cold side and not the
outlet tube, you don't get very much movement of the outlet water and you
just shuffle a lot of heat from the hot side to the cold side.

Similarly, if you have very little water in the outlet tube, then most of
the water from the hot side goes there and very little moves over to the
cold side so very little gas gets displaced from side to side.

Playing around and 'tuning' the height of the outlet column isn't so much
about raising the overall pressure in the gas as it is adjusting the phase
shift between the hot-side water column and the cold-side.

daestrom