As society gets more aware of atmospheric pollution and the negative effects of the transport sector on the climate, new concepts are being innovated to mitigate these effects. One of these concepts is the Auxiliary Power and Propulsion Unit (APPU) project, in which the APU of the A321neo is replaced by a hydrogen powered turboshaft system that includes a propulsor and boundary-layer-ingestion (BLI) to alleviate the main engines, causing less kerosene to be burnt whereby less CO2 and soot are emitted.
As in most aircraft, certain parts of the engine system are in need of cooling, most notably the HPT blades. Concurrently, the onboard cryogenic hydrogen needs to be heated up before entering the combustor to avoid high thermal stresses. The cryogenic hydrogen can be used to cool these parts, while at the same time reaching a more appropriate temperature before entering the combustor. Alleviating conventional cooling methods by making use of the cryogenic hydrogen increases the thermal efficiency of the engine system. Moreover, increasing the temperature of the fuel increases the LHV, which increases the thermal efficiency of the engine system even further.
Three different cases are analysed: firstly, the cryogenic hydrogen is used to cool the bleed air, by which less bleed air is required, increasing the core flow, which increases the thermal efficiency of the cycle. Secondly, using the same principle, the TIT is increased, while the amount of (now cooled) bleed air stays constant to the baseline. In this case the thermal efficiency increases due to the higher LHV of the fuel. Thirdly, the cryogenic hydrogen is used to intercool the core flow between the booster and the HPC, after which the pressure ratio is increased accordingly.
It is found that intercooling and increasing the pressure ratio of both compressors to 8, increases the thermal efficiency with 3.6% with respect to the baseline. Performance complications due to increasing the OPR to 64 have not been analysed into great detail and might make this result hard to achieve in reality. The next best option is increasing the TIT 1840K, which increases the efficiency by 0.7%. Cooling and reducing the bleed flow however, results into a thermal efficiency increase of 0.2%.
It is concluded that the largest contributor to the thermal efficiency is the LHV of the hydrogen. This means that any process in which the hydrogen extracts the most amount of heat is most advantageous, efficiency-wise. Furthermore, it is concluded that adding intercooling and subsequently increasing the pressure ratio increases the thermal efficiency more than cooling and subsequently reducing the bleed flow.