Converting Compressor to an Alpha Stirling Engine

[tonyanderson]

Heater tubes 5mm ID 190mm long X 16
Cooler tubes 4.5mm 51mm long X 36
Regenerator 25mm long 94mm total volume 90% filled, 0.03mm thick s/s foil dimpled by 0.3mm

[staska]

Working fluid pressurization ? Piston rings one way ?

[staska]

And regenerator od id

[tonyanderson]

Working fluid air at +1 bar over atmospheric
Low tension piston rings parallel face
Regenerator 95mm OD, 68.5mm ID

[tonyanderson]

just to clarify the regenerator filling, that is 90% working fluid (10% foil)

[staska]

It shows some indicated power, but pv chart is very thin, all power will be wasted.

This is for 125 angle between expansion and cooling:



Red is force work area then buffer pressure is half mean cycle pressure :



At 960 rpm.

[staska]

Btw - my project was also born from similar china twin 65mm bore air compressor..

viewtopic.php?f=1&t=5223

[tonyanderson]

Many thanks Staska.
That is very interesting. I was considering reducing the bore of the cold cylinder to about 55mm. This would give a ratio of 1.3:1 which might improve the power? Changing the separation angle to 125 deg. would mean a lot more work. What do you think?

[staska]

You welcome. Do you have Senft book on mechanical efficiency of stirling engine ?

Changing angle from 90 to 120 or 125 degree is not as hard. For our baby power level - aluminum crankshaft is totally ok. I was able to make new one for my first attempt in one evening turning session. Do you have bearing on both ends or only on one end ?

Other option is alpha-gamma approach which do same angle twist with clever geometry (my way). But this is totally redesign..

Last one - better heat exchange.. I am getting fatter PV plots on 200 degree lover temps with slotted ones. (100 slots 0.6 x 6 mm 40 mm length )

Plus you have too long stroke / compared to re-generator length - shuttle heat loss.

110 cc cold side :

PS. I did start from same air compressor, but now i am on 30% of totally new design..

[tonyanderson]

No I have not seen the Senft book. Looks quite expensive at £75!
I have read Andy Ross's book and my design was based on Divid Kirk's engine.
I am still puzzled as to why the engine was more reluctant to run when pressurized.

[staska]

Give me you email, will send some exerts from book.

It will be better to run pressurized, due to lower total friction loss -ie 1 bar at 1000 rpm is close to 2 bar at 500 rpm, but latest one have twice lower piston ring friction + aerodynamic losses too.. I did it ran with your current geometry - its still stucks at ~ 120 watt of indicated power, just curve is a little fatter.

If you could install matlab - https://www.ohio.edu/mechanical/stirlin ... _flow.html i am running this. It is not the best possible solution (there is more expensive one) - but it is not "simple" too as mos spreadsheet based ones - it took into account hx geometry and many small things. Have you seen my thread ?

Plus - i do see you have used copper heater tubes ? Or they are nickel/copper ?

Ps. Kirk engine was able to run only with slotted heat exchangers..

[staska]

Did run more simulations, to find a configuration with same bore x stroke and cooler length.

slotted heater and cooler 6x1 slots for cooler x 100 , 5.5 id tube with 3.5 internal tube for heater x 37, both 51 mm length



result:



Drilled cooler 100 pcs of 2.5 mm holes 51 mm length :

sinusoidal drive engine data summary:
comp clearence,swept vols 30.0, 142.0 [cm^3]
exp clearence,swept vols 30.0, 142.0 [cm^3]
expansion phase angle advance 90.0[degrees]
homogeneous bundle of smooth pipes
cooler data summary:
void volume(cc) 25.03
free flow area (cm^2) 4.91
wetted area (cm^2) 400.55
hydraulic diameter(mm) 2.50
cooler length (cm) 5.10
annular regenerator housing
wrapped foil matrix
unrolled foil length: 9.075(m)
foil thickness 0.030(mm)
hydraulic diam 0.671(mm)
total wetted area 0.466591(sq.m)
void volume 78.27(cc)
porosity 0.920
slots heat exchanger
heater data summary:
void volume(cc) 30.19
free flow area (cm^2) 5.92
wetted area (cm^2) 622.71
hydraulic diameter(mm) 1.94
heater length (cm) 5.10
gas type is air
operating parameters:
mean pressure (kPa): 200.000
cold sink temperature (K): 300.0
hot source temperature (K): 973.0
effective regenerator temperature (K): 572.0
operating frequency (herz): 36.0
pressure phase angle beta 17.1(degrees)
total mass of gas: 0.450(gm)
Pressure drop available work loss: 4.7[W]
Actual power from simple analysis: 329.5[W]
Actual heat power in from simple analysis: 1068.3[W]
Actual efficiency from simple analysis: 30.8[%]
quitting simulation...
>>

Close one results to simple slotted heat exchanger.

And drilled heater, same configuration:

Pressure drop available work loss: 4.7[W]
Actual power from simple analysis: 312.4[W]
Actual heat power in from simple analysis: 1053.4[W]
Actual efficiency from simple analysis: 29.7[%]

I was personally able to make drilled cooler from aluminium with similar configuration.

Anther simple one - current heater with insert to make gap 1mm and drilled cooler:

cooler data summary:
void volume(cc) 18.36
free flow area (cm^2) 3.60
wetted area (cm^2) 642.60
hydraulic diameter(mm) 1.14
cooler length (cm) 5.10
annular regenerator housing
wrapped foil matrix
unrolled foil length: 9.075(m)
foil thickness 0.030(mm)
hydraulic diam 0.671(mm)
total wetted area 0.466591(sq.m)
void volume 78.27(cc)
porosity 0.920
slots heat exchanger
heater data summary:
void volume(cc) 72.96
free flow area (cm^2) 3.84
wetted area (cm^2) 1018.40
hydraulic diameter(mm) 2.87
heater length (cm) 19.00
gas type is air
operating parameters:
mean pressure (kPa): 200.000
cold sink temperature (K): 300.0
hot source temperature (K): 973.0
effective regenerator temperature (K): 572.0
operating frequency (herz): 36.0
pressure phase angle beta 17.1(degrees)
total mass of gas: 0.466(gm)

Pressure drop available work loss: 15.7[W]
Actual power from simple analysis: 344.8[W]
Actual heat power in from simple analysis: 1074.3[W]
Actual efficiency from simple analysis: 32.1[%]

And last one - current heater with insert and cooler with insert to make 1 mm gap :

sinusoidal drive engine data summary:
comp clearence,swept vols 30.0, 142.0 [cm^3]
exp clearence,swept vols 30.0, 142.0 [cm^3]
expansion phase angle advance 90.0[degrees]
slots heat exchanger
cooler data summary:
void volume(cc) 25.70
free flow area (cm^2) 5.04
wetted area (cm^2) 532.44
hydraulic diameter(mm) 1.93
cooler length (cm) 5.10
annular regenerator housing
wrapped foil matrix
unrolled foil length: 9.075(m)
foil thickness 0.030(mm)
hydraulic diam 0.671(mm)
total wetted area 0.466591(sq.m)
void volume 78.27(cc)
porosity 0.920
slots heat exchanger
heater data summary:
void volume(cc) 72.96
free flow area (cm^2) 3.84
wetted area (cm^2) 1018.40
hydraulic diameter(mm) 2.87
heater length (cm) 19.00
gas type is air
operating parameters:
mean pressure (kPa): 200.000
cold sink temperature (K): 300.0
hot source temperature (K): 973.0
effective regenerator temperature (K): 572.0
operating frequency (herz): 36.0
pressure phase angle beta 17.1(degrees)
total mass of gas: 0.484(gm)

Pressure drop available work loss: 14.3[W]
Actual power from simple analysis: 327.3[W]
Actual heat power in from simple analysis: 1048.1[W]
Actual efficiency from simple analysis: 31.2[%]