This part of STORM is concerned with understanding the mechanical and adhesive properties of the ices which can form on the fan, the inlet guide vanes and sometimes on other surfaces at the front end or an aerospace gas turbine engine. Much is known about how ice forms and what range of properties it can have on non-rotating structures, but we have significant forces to consider which affect both the forming of the ice and the subsequent loads it experiences in this application. It is a quite a variable material and predicting the point at which ice on a spinning body will break off, and how large that piece might be, turns out to be quite demanding task.
Ice is in one sense a single material but like many materials, the way it behaves can vary greatly. Just as polystyrene can be made into a foam and used to make huge blocks even a child can lift, or break, it can be used to make cd cases or even cutlery. The ice we expect in an engine may be a smooth layer, a series of spiny lumps, or a combination of these. The density may be near that of solid ice or considerably less. The grain size can vary over two orders of magnitude. Changes in fan speed produce not only changes in loading and blade shape but also produce changes in temperature, adding in thermal stresses.
Why does this matter? The heat needed to keep a gas turbine engine fan above freezing point in cloud conditions is too great for effective continuous thermal protection to be practicable and cost effective on current engines. Delivering that heat would also add considerable weight and design limitations to the engine system. Coatings may be used to help the ice stick less well, but, with what we know currently, ice will form and grow until it reaches a thickness where the loads acting on it cause it to crack and be flung outward. The matter in hand is how much will form before it stats to break off. Impact damage caused by the detached ice and the out of balance forces generated need to be careful considered during the development of a new engine. By knowing more about how much the ice sticks to different surfaces and how strong the ice itself is, we can explore the potential of new coatings and blade designs using small scale lab experiments and analysis, greatly reducing the reliance on full scale engine tests.
Having a low stick (ice-phobic) surface may help us control the ice shedding process but we need to be sure that the surface will retain some level of its useful properties. To explore the issues around this, we are looking into the erosion and perhaps to other aspects of the weathering and ageing of the materials which may be useful to us.