How to improuve efficiency of homemade turbines?

[merlin]

Single radial compressor + single radial or axial turbine

[racket]

Either one , as theres a lot more to "efficiency" than just a choice of turb wheel type

[merlin]

Nov 26, 2020 15:49:05 GMT -5 racket said:

Either one , as theres a lot more to "efficiency" than just a choice of turb wheel type

Could you give more details pls?

how would you do it?

[racket]

What sort of "efficiency" improvements are you trying to achieve ??

[merlin]

Nov 26, 2020 15:58:47 GMT -5 racket said:

What sort of "efficiency" improvements are you trying to achieve ??

I'm actually confused, i know that to get a proper kerosen burn the ideal fuel/air mixture is 1/15.

on paper the kerosen consemption chart of turbines should be linear with the power they produce, but its not the case, some turbines produce more hp per liter of kerozen then others. While the air viscosity does impact the efficency of small turbines it still doesn't explain the difference between turbines of the same size.

Is it all about the temperature at the turbine wheel or is there something else?

[racket]

Theres a multitude of reasons why there are differences , but remember we only "burn" ~25-30% of the air , the remainder is required to cool the combustion gases so as to produce temperatures that the turbine wheel can survive with.

We can use turbine inlet temps up around 900 deg C , even less with older turbos or those with unknown wheel material , whereas the "state of the art" high pressure ratio full sized engines can use maybe 400 deg C hotter , those high pressure ratios also have high expansion ratios so more of that temperature can be extracted as power output .

The early fighter jet engines with radial compressors and pressure ratios and temperatures similar to what we use, as well as similar specific fuel burn rates and similar thrusts per pound of air per second provide a relative comparison .

Specifications:
Nene
Type: Centrifugal compressor turbojet
Length: 96.8 in (2,458.7 mm)
Diameter: 49.5 in (1,257.3 mm)
Dry weight: 1,600 lb (725.7 kg)
Compressor: 1-stage double-sided centrifugal compressor
Combustors: 9 x can combustion chambers
Turbine: Single-stage axial
Fuel type: Kerosene (R.D.E.F./F/KER)
Oil system: pressure feed, dry sump with scavenge, cooling and filtration, oil grade 70 S.U. secs (13 cs) (D.T.D 44D) at 38 °C (100 °F)
Maximum thrust: 5,000 lbf (22.24 kN) at 12,300 rpm at sea level for take=off
Specific fuel consumption: 1.06 lb/lbf/hr (108.04 kg/kN/hr)
Thrust-to-weight ratio: 3.226 lbf/lb (0.0315 kN/kg)
Military, static: 5,000 lbf (22.24 kN) at 12,300 rpm at sea level
Max. cruising, static: 4,360 lbf (19.39 kN) at 12,000 rpm at sea level
Cruising, static: 3,620 lbf (16.10 kN) at 11,500 rpm at sea level
Idling, static: 120 lbf (0.53 kN) at 2,500 rpm at sea level

Theres no point us using those very high turb temps that the full sized jet engines use because with our limited expansion ratios we'd have very poor fuel efficiency because of the very high exhaust temperatures exiting the jet nozzle , for every pressure ratio/expansion ratio theres an ideal turbine inlet temperature .................its a very complicated business juggling conflicting requirements .

When you say there are "differences between turbines of the same size" what difference are you talking about, thrust , thrust for case diameter, SFC , thrust/lb/sec ?

Cheers
John

[merlin]

Thanks Racket, i'm still learing about the turbine witchcraft, i'm greatful for your wizard wisdome
I was missing the overall pressure ratio from the equation

[racket]

Yep , the Pressure Ratio , or more importantly the Expansion Ratio has a large bearing on fuel efficiency, exactly the same as with a piston engine, basic thermodynamics .

For a pure thrust engine ( no fan), theres not much point going past a 3.5 - 4.5 :1 Pressure Ratio as we soon end up "on the wrong side of the curve" with regards thrust output , only useful to go to higher pressure ratios in supersonic applications where the higher exhaust velocity can be utilised .

A SFC of a bit over 1 lb of fuel/hour / lb of thrust is about as good as can be achieved with the components ( comp/turb) used.

[merlin]

Whats the relationship between compressor maximum air flow and the combustor volume?

[racket]

Combustor volume has more to do with "combustion intensity" than maximum airflow .

Combustion intensity is the amount of fuel that can be burnt per hour , per cubic foot of space , per atmosphere of combustor pressure .

So as the pressure goes up the volume can be reduced for the same burn rate .

Cross sectional area of the combustor has more of a relationship with mass flow rate , though again the pressure will influence that , we need "slow" airspeeds for combustion , so a long skinny combustor might not work even though it has the volume .

LOL............10,000 words could easily be written on this subject :-)

[merlin]

Nov 28, 2020 16:48:50 GMT -5 racket said:

Combustor volume has more to do with "combustion intensity" than maximum airflow .

Combustion intensity is the amount of fuel that can be burnt per hour , per cubic foot of space , per atmosphere of combustor pressure .

So as the pressure goes up the volume can be reduced for the same burn rate .

Cross sectional area of the combustor has more of a relationship with mass flow rate , though again the pressure will influence that , we need "slow" airspeeds for combustion , so a long skinny combustor might not work even though it has the volume .

LOL............10,000 words could easily be written on this subject :-)

Its a fascinating subject tho

[racket]

Yep , its impossible to know everything , the more you know , the more you know you know nothing .

[merlin]

i'v been searching about heat recuperator, that would greatly improuve the efficiency, but i'm affraid it would also put the turbine in extrem temperatures.

[finiteparts]

Recuperators do not increase the thermal loading on the turbine. They only move some of the waste heat from the exhaust back into the cycle upstream of the combustor...thus the reduction in needed fuel burn to heat the flow up to the required turbine inlet temperature. The work done back on the automotive turbines is a good resource to understand the challenges and benefits to using recuperators (or regenerators as they are called when they rotating the exhaust stream).

If you go to NASA's Technical Report Server (TRS) and search for "automotive gas turbine", you will be rewarded with days of wonderful reading material.

- Chris

[merlin]

Heat exhangers seem hard to design considering the poor thermal conductivity of stainless steel