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.
