Surface Roughness Effects on Thermally Stressed Aviation Fuel

Thermal instability in aviation fuels has been thoroughly explored over the last 50 years. The problem is complex, with coupling of fuel chemistry, heat transfer and fluid dynamics. Most efforts have been applied to the chemical kinetics of deposit formation and studying physical effects such as temperature, flow rate and Reynolds number in a multitude of small to large scale testing devices. However, much less attention has been paid to the effects of wall surface roughness. This is surprising - since for turbulent flow, wall roughness enhances momentum, heat and mass transfer by disrupting the quiescent viscous layer adjacent to the wall and interfering with structures of turbulence further into the boundary layer. Furthermore, a rough surface increases the wall surface area, presenting more active sites for heterogeneous catalytic reactions. Additive Layer Manufacturing (ALM) has been touted as ’game changing’ technology and is now being proposed as a method to create components for gas turbine engines. The technology results in near net shape parts with reduced weight, number of welds and material waste compared to conventional subtractive machining methods. However, the surface roughness of ALM components can be orders of magnitude greater than machined components and can be highly non-uniform. While reducing external surface roughness is trivial, typical methods of internal roughness reduction (ie. abrasive flow machining) may not be possible for small scale passages. This may result in internal fuel passageways with high relative roughness in components which are subject to high thermal loading - for example, injector feed arms which are exposed to compressor discharge air. The effect of wall roughness on deposition of thermally stressed aviation fuel was investigated in both laminar and turbulent flow regimes using small to medium scale test devices. Deposition over ALM components was tested in the laminar regime with a modified Jet Fuel Thermal Oxidation Tester (JFTOT) and in the turbulent regime with the Aviation Fuel Thermal Stability Test Unit (AFTSTU). The High Reynolds Number Thermal Stability Tester (HiReTS) was used to examine deposition in micro-scale tubes with very high relative roughness. As well as microscopy and 3D optical profilometry, momentum and heat transfer experiments were conducted to characterise the roughness as fully as possible. In the laminar regime, the effect of roughness was negligible. For turbulent flow, substantial differences in heat transfer and deposition rate were consistently observed for tubes with the highest relative roughness. The increase in deposition rate is thought to be related to the projection of roughness elements into regions of intense turbulent activity in the boundary layer. The turbulence structures, which are more energetic and have reduced anisotropy over rough walls, increase wall-normal transport - thereby replenishing the near wall region with deposit precursor and providing insoluble particles formed off the wall with inertia with which to deposit.