Heat transfer enhancement & stabilisation to transcritical hydrocarbon fuels within additively manufactured turbulated microtubes

• Roughness from additive manufacture affects heat transfer to transcritical fluid. • Microtube turbulators can increase the nu in supercritical n-decane by up to 6500 %. • Microtube turbulators can produce thermal performance factors of > 10. • Turbulators significantly reduce flow pulsation in transcritical heat exchangers. • Turbulators reduce temperature oscillation in transcritical heat exchangers. Heat exchangers (HEXs) using supercritical hydrocarbon fuel as a regenerative coolant have been proposed to manage the extreme aerothermal heating experienced by the external surfaces of scramjet aircraft. Addit. Manuf. (AM) can improve scramjet HEX efficiency by easing both the fabrication of conformal HEX designs and the inclusion of internal heat transfer enhancement (HTE) features. To determine the potential of AM to improve HEX efficiency for hydrocarbon scramjet which become supercritical during heating, a heat transfer dataset using liquid decane as the fuel/coolant proxy was measured for three AMd microtube designs and compared to that obtained for a conventionally manufactured (CMd) tube. Two of the AMd designs included HTE features (helical ribs & ring ribs) that are characterised here for HTE performance the first time, with the third design omitting turbulators but featuring increased wall roughness (relative to the CMd microtube). The influence of wall roughness & HTE features on steady state Nusselt number, friction factor, and overall thermal performance factor (TPF) under two heat loads are characterised alongside the inhibition of transcritical oscillatory behaviour commonly experienced for smooth tube designs. HTE turbulator features significantly increased heat transfer (Nu increased by 66x relative to CMd tube leading to peak TPF of 12) and pressure & temperature oscillations reduced from 580 % & 12 % of the CMd tube's mean to 16 % & 1 %, respectively. The datasets presented here can thus underpin the design of high-performance scramjet HEXs, leading to significantly reduced skin & structural temperatures and thus reduced cost & complexity.