Design Condition and Sizing

Parametric cycle analysis yielded thrust specific performance approximations for a selected cycle, but these figures constituted a rubber engine with no definite size. Scaling was required to fit the engine to desired design specifications. Dividing the

desired thrust (668 N) by the cycle’s specific thrust (587 _kg/sN ) produced an air mass

parameters at each station, including station areas. Since the combustor would be

wrapped around the exit of a centrifugal compressor, the compressor exit area (A3)

was critical in the design of the Disk-Oriented Engine. AEDsys estimated A3 =

0.0025m2_{, but this was for an assumed axial compressor. While cycle analysis would}

present the same areas for axial and centrifugal compressors, AEDsys reported the effective flow area required to achieve such a pressure rise. In an axial compression system, the effective and actual areas are identical as the fluid has been turned axial by the last stator row. The same would be true if a centrifugal compressor forced air only in the radial direction at Station 3, but the Disk-Oriented Engine maintained

the bulk swirl imparted by the compressor. The required A3 was the effective area,

adjusted by the trigonometric relation of the circumferential inlet velocity, as shown

in Equation 19, where α2 is the compressor exit tangency angle as shown in Figure

44. A3 = A3,ef f sin(α2) = 0.0025m 2 sin(22.87◦₎ = 0.0064m 2 (19)

A compressor to fit this 0.0064 m2 _{exit area had two major degrees of freedom}

in the design, impeller diameter and tip height, and these would be required to set the combustor inlet. Research into commercially available turbochargers for relative sizing discovered a turbocharger compression system capable of operating at the mass flow rate and CPR required for the Disk-Oriented Engine selected cycle. The Garret Motion INC. GTX5544R GEN II turbocharger operates on the compressor map shown in Figure 46 [55]. The design point for the Disk-Oriented Engine cycle is shown, adjusting for the corrected mass flow rate, and this point is within the stable operation islands of the compressor and would not likely stall. The GTX5544R GEN II operates with an outer diameter of 14.4 cm, and the compressor would operate at an efficiency of 70% if used on the selected cycle with a speed between 65500 and 69000 RPM. In

order to move the design cycle closer to the compressor optimum operability line, the compressor would have to be scaled up in application with the Disk-Oriented Engine.

CPR=4.0

Stall Line

Figure 46. Compressor map of Garrett GTX5544R GEN II turbocharger, adapted from [55].

The present research assumed that the compressor diameter would have to be scaled 12% to a diameter of 16.1 cm, with the diameter of Station 3 measuring 20.8 cm. This measurement includes a stator section that added an additional 30% to the 16.1 cm disk diameter to increase the static pressure of the fluid expelled by the

impeller. Since the target A3 was 0.0064 m2, this diameter allowed for a tip height

of 0.97 cm. These dimensions set the inlet of the computational domain as a 0.97 cm opening about a 20.8 cm diameter. Without performing extensive turbine design analysis, the sizing of the combustion exit was less refined. A nominal exit angle was chosen based on the calculations in Section 2.3.2 to maintain the engine bulk swirl, but the area was reduced to a 2.54 cm opening at the same 20.8 cm diameter,

accelerating the exhaust products to near sonic Mach numbers. This dimension will be updated as needed in a later phase of the Disk-Oriented Engine design, once the turbomachinery is designed.

It is important to note the assumptions made in the cycle performance analysis that resulted in the parameters used in the present study. Compressor polytropic efficiency was assumed to be 76%, as that was the highest efficiency island shown in the compressor map of a similarly sized compressor. The turbine was assumed to operate on a similar polytropic efficiency, rounded to 80% as there was little information on the efficiency of radial-in flow turbines operating in this class. The turbine of the GTX5544R GEN II claims to operate at a maximum efficiency of 74%, but a custom-designed RIT was assumed to operate with slightly higher efficiency. AEDsys allowed for cooling air to be bled off the compressor to cool the turbine, and a modest 4% secondary air of the 1.138 kg/s was assumed sufficient to force fuel across the bearings for lubrication purposes and to extract bearing heat as a phase change. This secondary air exchange is similar to that used by JetCat engines, discussed in depth by Bohan [26]. Pressure loss across the combustor for initial estimates was assumed to be 5%, based upon modern axial combustion systems [1], and all other AEDsys settings were left default. The design condition cycle results are included in Appendix B, for engine operation at SLS, full-power conditions. It is important to note that

the AEDsys program estimated the mass flow rate of fuel ( ˙mf uel) at design condition

to be 0.0265kg/s, while equivalence ratio analysis based on previous experimentation (Section 3.3.3) estimated a required fuel flow rate of 0.0227kg/s, a difference of 15%. For comparison to previous research, the fuel flow rate determined by equivalence ratio was used for design condition analysis. The AEDsys estimated 100% conditions were also run for comparison with off-design settings, with results presented in Section 4.3.