Tony,

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:

1 - is the inlet plane

2 - is the compressor inlet plane

3 - is the compressor stage discharge plane

4 - is the combustor discharge plane

41 - is the NGV throat

44 - is the turbine rotor throat

45 - is the exit plane of the turbine

5 - is just downstream of the turbine discharge

9 - is the discharge nozzle throat

******************** Compressor Performance ***********************

*********** Compressor Inlet **************

Inlet corrected flow, W2c = 2.33 lbm/s

Inlet physical flow, W2 = 2.33 lbm/s = 139.8 lbm/min

Compressor inlet geometric area, Ageo2 = 9.292 in^2

Compressor inlet total pressure, Pt2 = 14.696 psia

Compressor inlet static pressure, p2 = 12.512 psia

Compressor inlet total temperature, Tt2 = 59.0 deg F

Compressor inlet static temperature, T2 = 35.696 deg F

Compressor inlet constant pressure specific heat, Cp2 = 0.2393 Btu/lbm R

Compressor inlet total enthalpy, ht2 = 124.1 Btu/lbm

Compressor inlet effective area, A2 = 9.292 in^2

Compressor inlet Mach number, M2 = 0.485

Compressor inlet acoustic velocity a2 = 1091.1 ft/s

Compressor inlet mean axial velocity, Cm2 = 529.2 ft/s

*********** Compressor Discharge ***********

Compressor discharge geometric area, Ageo3 = 14.186 in^2

Compressor discharge total temperature, Tt3 = 390.67 deg F

Compressor discharge total pressure, Pt3 = 58.78 psia

Compressor discharge total enthalpy, ht3 = 208.0 Btu/lbm

Temperature rise through the compressor Tt3-Tt2 = 331.67 deg F

Compressor temperature ratio, Tt3/Tt2 = 1.639

Compressor specific work, WTc3 = 77.5 Btu/lbm

Power required by the compressor, PwrC = 276.36 hp

Compressor discharge plane Mach number, M3 = 0.095

******************** Combustor Performance ************************

Fuel to air ratio, FAR = 0.0161

Specified combustor temperature rise, delT34 = 1209 deg F

Physical fuel flow rate to achieve Tt4 target, Wf = 0.0374 lbm/s

Volumetric fuel flow rate, VFf = 20.266 gal/hr

Combustor annulus Mach number, M31 = 0.103

******************** Turbine Performance ***************************

Turbine pressure ratio, PRt = 0.552

Turbine expansion ratio, ERt = 1.81

************ NGV **************

Turbine inlet total temperature, Tt4 = 1600 deg F

Turbine inlet static temperature, T40 = 1588.9 deg F

Throat static temperature, T41 = 1321.7 deg F

Turbine inlet total pressure, Pt4 = 55.845 psia

Turbine inlet static pressure, p4 = 52.154 psia

Throat static pressure, p41 = 30.44 psia

Turbine inlet constant pressure specific heat, Cp4 = 0.2865 Btu/lbm R

Turbine inlet enthalpy, ht4 = 590.07 Btu/lbm

Engine temperature ratio, Tt4/Tt2 = 3.971

Physical flow at plane 4, W4 = 2.3674 lbm/s

Corrected flow at plane 4, Wc4 = 1.2415 lbm/s

Turbine critical flow area, A41crit = 3.6985 in^2 (slight discrepancy between ideal solver and gas property solver)

NGV Throat effective area, Ae41 = 3.692 in^2

NGV absolute discharge velocity, V41 = 1998.7 ft/s

NGV absolute discharge angle, alpha41 = 62.7 deg

NGV absolute discharge static temperature, T41 = 1321.7 deg F

Turbine stage inlet Mach number, M40 = 0.186

Throat Mach Number, M41 = 0.997

Throat acoustic speed, a41 = 2004.7 ft/s

Throat mean velocity, V41 = 1998.7 ft/s

Turbine NGV critical pressure ratio, Pt4/p41crit = 1.841

Turbine NGV actual pressure ratio, Pt4/p4 = 1.835

********* Turbine Rotor ***************

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

Turbine discharge effective area, A5 = 12.629 in^2

Turbine discharge critical flow area, A50crit = 6.246 in^2

Turbine exit Mach number, M5 = 0.305

Turbine matching work, WT4 = 83.78 Btu/lbm

Turbine ideal enthalpy drop, delhideal = 161.3 Btu/lbm

Turbine stage enthalpy drop, delht4ht5 = 83.8 Btu/lbm

************************* Exhaust Nozzle ******************************

Nozzle total pressure ratio, Pt5/Pt9 = 2.0957

Nozzle static to total pressure ratio, Pt0/P5 = 0.4772

Nozzle driving pressure difference, P5-P9 = 14.29 psia

Nozzle discharge Mach number, M9 = 1.01

******************************* Performance ***************************

Compressor Power, PwrC = 276.4 hp

Turbine Power, PwrT = 280.6 hp

Turbine Ideal Power, PwrTideal = 540.1 hp

Mechanical power absorbed, PwrMech = 4.209 hp

Compressor spec work, SpecWC = 77.5 Btu/lbm

Turbine spec work, SpecWT = 83.8 Btu/lbm

********* Densities ***************

Compressor inlet density, = 0.0682 lbm/in^3

Compressor outlet density, = 0.1858 lbm/in^3

Combustor outler density, = 0.0732 lbm/in^3

NGV Throat density, = 0.0461 lbm/in^3

Turbine outlet density, = 0.0437 lbm/in^3

Nozzle throat density, = 0.029 lbm/in^3

********* Pressure Ratios ***************

Compressor pressure ratio, PRc = 4.0

Combustor pressure ratio, PRcomb = 0.95

Turbine pressure ratio, PRt = 0.552

Nozzle pressure ratio, PRn = 0.477

Pressure ratio Product Check = 1.0

********* Enthalpies ***************

Compressor inlet enthalpy = 124.129 Btu/lbm

Combustor inlet enthalpy = 207.98 Btu/lbm

Turbine inlet enthalpy = 590.067 Btu/lbm

Nozzle inlet enthalpy = 506.284 Btu/lbm

********* Station Properties ***************

Compressor inlet, Plane 20 Tt2 = 59.0 deg F, Ts2 = 35.7 deg F, Pt2 = 14.696 psia Ps2 = 12.512 psia

Compressor outlet, Plane 31 Tt3 = 390.7 deg F, Ts3 = 389.2 deg F, Pt3 = 58.784 psia Ps3 = 58.417 psia

Combustor outlet, Plane 40 Tt4 = 1600 deg F, Ts4 = 1588.9 deg F, Pt4 = 55.845 psia Ps4 = 52.154 psia

Turbine NGV oulet,Plane 41 Tt41 = 1600 deg F, Ts41 = 1321.7 deg F, Pt41 = 55.845 psia Ps41 = 30.435 psia

Turbine outlet, Plane 45 Tt5 = 1326.8 deg F, Ts5 = 1300.2 deg F, Pt5 = 30.799 psia Ps5 = 28.985 psia

Nozzle outlet, Plane 9 Tt9 = 1326.8 deg F, Ts9 = 1072.9 deg F, Pt9 = 25.832 psia Ps9 = 14.696 psia

********* Turbine Areas ***************

Turbine NGV discharge, Plane 41 Area = 3.692 in^2 Throat Mach Number = 0.997

Turbine discharge relative, Plane 44 Area = 6.051 in^2 Throat Mach Number = 0.859

Good luck

Chris