The turb scroll choke throat should be at the end of the "tongue' close to the wheel where the gases enter it .
If you measure the area at that point and divde by the distance from the "Flow area centre" to the shaft centre , it should give your A/R .
OK, I was able to do that and it was helpful to understand your description. Previously I was under the impression that A/R meant scroll inlet area / wheel radius. Which is very different from choke area at the tongue / the distance to shaft centerline.
The 98 mm might be OK for bleed , but probably not for thrust .................somehow remember doing the calcs somewhere for someone .
With bleed the temps will be low initially and as the bleed was instigated the temps will need to rise but the mass flow through the turb stage will drop , sorta keeping things in "balance"
May I suggest you do some calcs using the turbine flow map with T I Ts of between 500C and 1,000 C at your desired 4:1 PR
When I purchased my TV84 with its 3.5"- 89mm inducered comp , the turb stage had an undersized 97/86 mm turb wheel, and a 1.08 A/R scroll which was soon changed to a 1.39 A/R , the engine was equiped with a 73 mm jet nozzle and the engine operated OK up to ~3:1 PR but then went into surge with a thrust of ~30 kgs - 66 lbs. indicating a mass flow of ~1.4 lbs/sec .
The surge knocked off turb blade tip bits at both inducer and exducer from contact with the housing shroud.
Eventually a change to a 110/96 mm turb wheel was made and no more problems encountered .
Post by finiteparts on Feb 25, 2019 0:14:38 GMT -5
I measured up my GT55 turbine, which looks to be the same size and I would guess, the same casting as yours...I got a throat area (Plane 44) of:
A44 = 6.051 inches
I am still working on my cycle program, but I thought that I would plug the numbers in to see what I come up with... I am still checking the solver for the gas properties (enthalpy, entropy, Cp, gamma, etc.), but so far the checks that I have done have lined up well to the NASA published properties.
This program uses a compressible flow solver to find the local gas properties and then standard relations to solve the momentum/mass/torque exchanges between each station. I assumed a mechanical efficiency of 98.5% and rotor speed of 76 krpm. I am still working on the radial turbine module, but I did have it to the point where I was able to solve for the turbine throat plane relative gas properties, so I just added a mass continuity check to solve for the relative Mach number at the turbine throat and with the 6.05 inch throat area I am not seeing any choking. I have a vane passage filled with RTV to make a mold of the passage so that I can cut it up and measure it more accurately.
Like I said, I am still working through this program and it is not guaranteed to be correct...just thought I would show my first pass at running the GT55 turbine through it.
Here is the output that I get...by the way...I am using the SAE AS755F standard station numbering scheme...that means that:
Turbine discharge total pressure, Pt5 = 30.8 psia Turbine discharge static pressure, p5 = 28.99 psia Turbine discharge total temperature, Tt5 = 1326.75 deg F Turbine discharge total enthalpy, ht5 = 506.28 Btu/lbm Turbine total temperature drop, Tt4-Tt5 = 273.2 deg F Turbine static temperature drop, T4-T5 = 123.0 deg F Turbine NGV static temperature drop, T4-T41 = 267.2 deg F Turbine rotor static temperature drop, T41-T5 = -144.2 deg F Turbine Total to total Expansion ratio, Pt4/Pt5 = 1.813 Turbine Total to static Expansion ratio, Pt4/p5 = 1.927 Turbine temperature ratio, Tt4Tt5 = 1.153
Post by finiteparts on Feb 25, 2019 0:26:24 GMT -5
Also, I used the RMS blade discharge plane to do these calculations...this the need to fill the turbine passage with silicon and then mark and cut the resulting plug to be able to take accurate measurements of the RMS blade angle. Not to mention, if you use the tip angle to represent the entire flow turning, you will over predict the torque.
Post by finiteparts on Feb 25, 2019 0:41:38 GMT -5
One more additional point....notice that the static temperature of the gas entering the turbine is only around 1321 F...I didn't include the turbine inducer total relative temperature, but it is around 1310 F. What is often misunderstood is that the 1600F inlet temperature is not what the metal of the turbine wheel sees...it is exposed to this lower temperature gas temperature by virtue of the gas being in a rotating frame of reference. The NGV sees the full effect of the 1600F gas, but the gas properties in the turbine wheel are more complex due to flow being onboarded to a rotating reference frame.
This is what really drives the complexity of the design model to capture accurately the gas flow at the inducer, throat and exducer planes of the turbine wheel...you can't just assume the conditions that you calculate from the total pressure or total temperature changes across the rotor...they only apply in the stationary reference plane upstream and downstream of the turbine, not in the passages fo the turbine itself.