Abstract:
The runback ice phenomenon is well-known for anti-icing or de-icing systems
when the system is not evaporating 100% of the water impinging the surface.
The water runs back to the point where the added heat no longer raises
the surface temperature above freezing. The water freezes behind this limit.
No runback ice is tolerated for some flight configurations, but not for all.
Then for o.-design cases, some runback ice may grow on the wings surface.
However, data from full-scale realistic runback ice is not very well-known by
aircraft manufacturers and they are not sure what thickness is allowed before
the e.ect of the ice on the flow becomes too adverse.
To better understand full-scale high-fidelity runback ice growth and how it
can be simulated with simplistic shapes, test campaigns and CFD studies
were undertaken. First of all, tests in the Cranfield icing tunnel were performed.
In this work, full-scale runback ice shapes were grown on a model
with a full-scale leading edge equiped with an electrical heating system. An
innovative moulding and casting technique has been introduced which allowed
the production of 3D planarised full-scale realistic runback ice castings. In
parallel to the icing tunnel tests, a mass and energy balance has been computed
on Excel. This energy and mass balance can predict the heat and
mass fluxes involved in the runback ice accretion mechanism. Following this,
aerodynamic tests of the ice castings were lead in one of the low speed wind
tunnels at Cranfield University. The aerodynamics of simplistic shapes such
as geometrical shapes or ballotini layers were also studied. The e.ects of the
ice castings on the flow were compared to the e.ects of the simplistic shapes.
The tests were done on a flat surface and not on an airfoil due to technical
complications. The boundary layer displacement thickness was the parameter
used to quantify the e.ect of the shapes on the flow. 2D CFD simulations
were performed as a support to the testing but as well to compare with the
experimental data. The CFD simulations were for steady or unsteady flow.
It has been possible to grow full-scale ice shapes in a relatively small icing
tunnel. The shapes have been successfuly moulded and cast using silicone
and plaster mixed with polymer. A catalogue of runback ice shapes for different
liquid water content, heat inputs and positions along the chord has
been recorded. Following the wind tunnel tests, it has been possible to find a
relationship between the real ice and the simplistic shapes. Thin runback ice
shapes (4 mm) has a similar e.ect on the flow as a layer of 1 mm ballotini.
It was found that thicker ice shapes, of the order of 1 cm, is equivalent to a
rectangle with rounded corner, associated with 1mm ballotini. The triangle
shape which is usually used to simulate runback ice by the aircraft manufacturers,
was found to be the most aerodynamically penalising simplisitc shape
that has been investigated in this PhD project. It was found that rounded
corners greatly improve the representativeness of the simplistic shapes, such
as triangle or rectangle.