Aero Engine Intercooling

Doctoral thesis, 2016

Intercooling has the potential to provide a shortcut to the next generation aero engines with higher bypass ratio (BPR), higher overall pressure ratio (OPR) and higher turbine inlet temperature (TIT) by lowering the high pressure compressor (HPC) delivery temperature. To be able to establish a systematic understanding of aero engine intercooling, the heat transfer and pressure loss characteristics of a given intercooler architecture need to be known in the parameter range anticipated for the engine optimization. A two-pass cross flow tubular heat exchanger for aero engine intercooling applications was hence developed by the means of computational fluid dynamics (CFD). Optimizations with this intercooler installed were performed by considering the intercooler design parameters and the engine design simutaneously. A geared variant was adopted to complement the use of intercooling as it could support high OPR engines better by allowing a lower position installtion of the HPC. For a flight mission, further optimization of the intercooled engines was acheived by controlling the amount of intercooling for different engine operating points in two ways. One is intercooler external flow control by a separate variable nozzle and another one is intercooler internal flow variable flow path. As the flight altitude strongly influences the working condition for an aero engine, considerable SFC benefit can be obtained by limiting intercooling at high altitude operation. Nevertheless, the precondition is to enable a higher OPR at the take-off operation by intercooling. Compared to a reference non-intercooled geared engine, an optimal intercooled geared engine with intercooling control shows a 4.9\% better mission fuel burn under the same engine technology level assumptions. However, the optimum is still 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 combined HPC. The axial–radial high pressure ratio configuration allows for an ultrahigh OPR. With an optimal top-of-climb (TOC) OPR of 140, the configuration provides a 5.3\% fuel burn benefit over the geared reference engine. Experimental validation of the intercooler design and the CFD design tool is also presented in this thesis. With the help of particle image velocimetry (PIV) and pressure measurements, flow topology inside the intercooler was visualized. Generally, by comparing the CFD results and the experimental data, the computational capability of porous media modeling predicting the flow distribution within the tubular heat transfer units was confirmed. The flow topology within the associated ducts was considered well-described by CFD.