Assessment of multi-fuel powered turbofans: Performance, dimensions and emissions

This paper investigates the impact of adopting different hydrogen fuel fractions on the turbofan’s design, performance, dimensions and emissions. Performance-wise, the thermal efficiency of the cycle increases by adding more hydrogen fractions into the blended fuel. By switching the fuel from 100% Jet-A to 100% H 2 at the design point, one can expect an increase up to 1% in thermal efficiency throughout a wide range of varying design parameters of specific thrust, overall pressure ratio, jet velocity ratio and pressure ratio split considering turbomachine component size correction. When fixing the design point and varying the fuel blends at critical off-design points, such as take-off, a key observation is that the combustor exit temperature could decrease as much as 70 K with an increasing hydrogen fraction. Dimension-wise, designing the turbofan with hydrogen-richer blended fuel could reduce the engine dry weight (core +fan) by up to 7%, which is mainly driven by the more powerful core with more hydrogen. The extent of benefits mentioned above is directly proportional to the hydrogen energy fraction in the blended fuel. From a first analysis of emissions using kinetics reactions with empirical residence time and primary zone fuel air ratio correction, the results show that the introduction of hydrogen has the potential of lowering NO x emissions even with a remarkably high NO x index. As expected, more hydrogen fractions in the fuel lower CO 2 emissions. The relatively flat CO 2 emissions index curves in the equivalence ratio range for typical turbofan operations suggest that constants can be applied for estimating the CO 2 emissions for all fuel blends. The indices are approximately 3.16, 2.38, 1.61, and 0.83 (in kg/kg fuel) for 100% Jet-A, 75%/25% Jet-A/H 2 , 50%/50% Jet-A/H 2 and 25%/75% Jet-A/H 2 (in mass-based fraction) fuel blends, which are simply proportional to the Jet-A content in the fuel. Nevertheless, the prerequisite for granting the benefit of using the multi-fuel concept is a combustion system which could deliver the same performance as the combustor of modern turbofans.