The EU Horizon 2020 ULTIMATE project aims to mitigate one of the greatest loss sources in modern turbofans: the heat in core exhaust gases. The introduction of a closed-circuit recuperated bottoming cycle, using supercritical CO2 (S-CO2) as a working fluid that is heated by the exhaust gases, has been shown to be a feasible option.
Involute spiral heat exchangers have been studied for intercoolers and cooled cooling air systems. However, placing them in the core exhaust and using S-CO2 implies significant mechanical design challenges from the elevated temperatures and high internal pressures. The studied scenario considers a heat exchanger of 60% effectiveness, with an internal fluid pressure of 32 MPa. The primary objective is to minimise the total mass of the heat exchanger tubes.
The work is focused on studying elliptical tubes and an alternative multi-arc cross-section design with internal webs that is more structurally efficient. A parametric analysis of the proposed geometry has been conducted to capture the influence of each of the geometric variables on the resulting stresses.
The alloy Ti-6Al-4V is selected as the tube material and the results show that for a 400 MPa maximum allowable stress, a chord of 10 mm, a chord to thickness ratio of eight and a mi nimum wall thickness of 0.2 mm, the minimum tube weight is 20.5 g/m.