This article deals with the conceptual approach to optimize efficiency and power of a highly loaded gas turbine flow path design. Different problems were analyzed during low pressure turbine (LPT) preliminary design of high-bypass aircraft jet engine.

PROBLEM STATEMENT

At present, turbomachines’ elements design using integrated software is developing intensively [1, 2, 3, etc.] Simplified models of computation (1D/2D) [1, 4] as well as precise full-dimensional ones [5] may be used in such systems. A comparison of aerodynamic computational results using carefully selected 1D/2D models and CFD analysis with experimental data [3, 4] is evidence of simplified calculations adequate accuracy for preliminary design.

More detail review of the integrated software for turbomachines design you can find in [5].

Gas turbine engine qualitative characteristics are determined by the concepts taken into account on early phases of engine components design [1].

Design specification requires a mandatory parameters values list and the main requirements were high efficiency value (~94 %), weight minimization, strict axial and radial dimension constraints and outlet flow angle deviation from axial direction restriction to a value less than 20 deg. Complex turbine flow path outline shape in meridional plane is the result of these facts. Several low pressure turbine designs with 6 and 7 stages were examined to ensure turbine reliability. Flow path designs in meridional plane with given axial and radial constraints is presented on Fig 1.

The LPT was designed with the help of a multidisciplinary turbomachinery design and optimization suite. It allowed for designing various turbine flow paths while meeting specification requirements.

Turbomachinery design and optimization suite gave the designer a possibility to use different tools to solve flow path design and analysis tasks. Optimization tasks formulations are very flexible. Software service functions provide convenient interface with data project, strong reliability and quick response during the design process.

Tasks used during the turbine scheme definition and their descriptions are listed below:
- Multistage preliminary flow path design is based on finding the solution of 1D problem in inverse formulation. Quasi-random search methodology is used to find optimal solution.
- Preliminary chords and relative pitch values estimation provides minimum total losses taking into account structural and modal constraints.

Initial data for the first design are reported in the Table 1.

Table 1. Initial design data

1	Design point
1.1	LPT Inlet Total Temperature	1144.3 [K]
1.2	LPT Inlet Total Pressure	323.64 [kPa]
1.3	HPT Exit Swirl (from axial direction)	20 [deg]
1.4	LPT Rotational Speed	2020 [rpm]
1.5	LPT Inlet Mass Flow	39.39 [kg/s]
1.6	LPT T-T Pressure Ratio	7.26 [-]
1.7	DH/T (cpDT/Tin)	424.7 [J/kgK]
1.8	Fuel to air ratio	0.0165
1.9	Max Tip Speed (LPT last Blade)	200 [m/s]
1.10	HPT Exit Inner Diameter	0.750 [m]
1.11	HPT Exit Outer Diameter	0.875 [m]
1.12	LPT Inlet Hub Diameter (recommended)	0.957[m]
1.13	LPT Inlet Tip Diameter (recommended)	1.097[m]
1.11	LPT Efficiency required, min	94.0% [-]
2	Geometry constraints
2.1	Exit LPT Max Swirl Angle (from axial direction)	25 [deg]
2.2	Max LPT Length (duct included)
2.3	LPT Outlet Tip Diameter, max	1.866[m]
2.4	TE edge diameter, min	0.7[mm]