First and Second Law Analysis of Intercooled Turbofan Engine

Abstract

Although the benefits of intercooling for aero engine applications have been realized and discussed in many publications, quantitative details are still relatively limited. In order to strengthen the understanding of aero engine intercooling, detailed performance data on optimized intercooled turbofan engines are provided. Analysis is conducted using an exergy breakdown, i.e. quantifying the losses into a common currency by applying a combined use of the first and second law of thermodynamics. Optimal intercooled geared turbofan engines for a long range mission are established with CFD based two-pass cross flow tubular intercooler correlations. By means of a separate variable nozzle, the amount of intercooler coolant air can be optimized to different flight conditions. Exergy analysis is used to assess how irreversibility is varying over the flight mission, allowing for a more clear explanation and interpretation of the benefits. The optimal intercooled geared turbofan engine provides a 4.5% fuel burn benefit over a non-intercooled geared reference engine. The optimum is constrained by the last stage compressor blade height. To further explore the potential of intercooling the constraint limiting the axial compressor last stage blade height is relaxed by introducing an axial radial high pressure compressor. The axial-radial high pressure ratio configuration allows for an ultra-high overall pressure ratio. With an optimal top-of-climb overall pressure ratio of 140, the configuration provides a 5.3% fuel burn benefit over the geared reference engine. The irreversibilities of the intercooler are broken down into its components to analyze the difference between the ultra-high overall pressure ratio axial-radial configuration and the purely axial configuration. An intercooler conceptual design method is used to predict pressure loss heat transfer and weight for the different overall pressure ratios. Exergy analysis combined with results from the intercooler and engine conceptual design are used to support the conclusion that the optimal pressure ratio split exponent stays relatively independent of the overall engine pressure ratio.

Ultra Low Emission Technology Innovations for Mid-Century Aircraft Turbine Engines

“…Consider for example the use of intercooling. On a long range mission intercooling could provide around 5% fuel burn reduction [23]. This may be rendered insufficient to reach the SRIA 2050 targets and hence be considered as an unsuitable technology to attack the core exhaust loss.…”

An Outlook for Radical Aero Engine Intercooler Concepts

“…Equations (2) and 3above are now applied to illustrate some characteristics of intercooling thermodynamics when integrated into aero engines. The arguments are established using data from the optimal geared intercooled engine presented in [7]. This engine is an intercooled turbofan with a take-off fan pressure ratio (FPR) of 1.45, a take-off bypass ratio of 17.1 (BPR), operating in cruise at 35000 feet, ISA conditions and a flight Mach number of 0.81 is considered.…”

“…A fixed amount of transferred heat is assumed based on a core temperature drop of 58 K in cruise, referring to the internal flow side of the intercooler. The stagnation pressure loss is computed in [7].…”

“…The original concept applied a downstream mixer [6]. Later publications have discussed the use of a variable intercooler exhaust nozzle [5,7].…”

“…In the past, intercooler concepts, like the one illustrated in Figure 1 [7] have rejected heat by creating additional drag in the bypass duct. The nacelle heat rejection system studied herein (a Type 1 concept) evaluates the potential for increased efficiency by using drag surfaces already available in the aircraft.…”