Dendritic cooling for nozzle guide vanes
Keywords: Heat transfer co-efficient
External cross flow
Nusselt numbers
External blade surface
Nozzle guide vanes
Dendritic cooling systems
Dendritic cooling hole geometries
Gas turbine cooling
Film cooling effectiveness

This thesis describes the development and characterisation of an entirely new form of gas turbine cooling scheme, in which passages repeatedly branch within the wall of the cooled component. This type of cooling is known as dendritic cooling. Such geometries offer particular performance improvements compared to traditional cooling holes in film-cooling schemes when the external cross flow is low and conventional films have a tendency to lift off the surface. In these regions, improved internal cooling efficiency is achieved while the coolant film is ejected at a low momentum ratio resulting in reduced aerodynamic losses between the film and hot gases and a more effective surface film. By varying the number of branches, it is possible to optimise the internal convective cooling at a particular location whilst maintaining a low overall coolant flow rate and high pressure margin. This makes the system robust against ingestion of hot gases. The low flow rates through the exit holes, resulting from the internal branching structure, allows quasi-transpiration cooling at the external blade surface and thus improved film-cooling effectiveness.
Many potential cooling arrangements have been investigated in the current study, using a mixture of experimental testing and analytical modelling. The research strategy is multi-stranded: a simple analytical model of the metal temperature response to an applied cooling scheme is required to allow the full design space to be explored; in contrast detailed heat transfer coefficient and film-cooling effectiveness distributions are needed as inputs to stressing and lifing models; finally manufacturability of the components must be assessed. In both the former cases large scale, low temperature, tests of models of the cooling systems were used to obtain detailed data. These are suitably averaged to populate the component model. To assess manufacturability metal test components at real engine scale have been manufactured using two manufacturing techniques. Working with a colleague in the Department of Engineering Science, a relatively conservative scheme has been tested at engine scale to confirm improvements in performance. Two main campaigns of experimental heat transfer testing are reported, characterising the cooling systems’ behaviour both within the component wall (internal) and over the external surface. The experiments are conducted in the presence of a representative external crossflow. Local Nusselt number distributions are reported on the internal surfaces of dendritic systems with 1, 2 and 3 internal flow branches at a range of engine representative Reynolds numbers and exit momentum ratios. These data are correlated to the Reynolds number of the flow at the entrance to each dendritic branch, and are shown to be extremely insensitive to the exit momentum ratio. The heat transfer coefficient-area product of the dendritic schemes is shown to be superior to conventional straight cooling passages. The pressure loss associated with these designs is also characterised, and correlated. Interestingly the trend of these results is against initial expectations, but may be explained by the capping of the holes exiting into the mainstream flow.
The film-cooling effectiveness and heat transfer coefficient performance on the external component surface is characterised for two idealised arrays of dendritic systems, using transient and steady-state tests. The film-effectiveness up to 80 hole diameters downstream of the point of injection was determined and correlated. Using the well known superposition model for film-cooling, it is possible to correlate the results such that they are applicable to arbitrarily spaced rows of holes over the component surface. The use of dendritic cooling hole geometries is shown to be better than plain circular holes. A dramatic improvement in film-cooling performance is, however, seen when the hole exits are contoured to form a contiguous spanwise slot at exit, this being achieved in the deliberate absence of flow acceleration in the hole. The benefit is achieved over the full range of blowing ratios tested. These local data allow component lifing to be undertaken.
The implementation of such cooling arrangements is only achievable using new manufacturing processes. At the outset of this research the best candidate process was virtual pattern casting. Casting trials were conducted of thermodynamically feasible cooling geometries specified for a Nozzle Guide Vane (NGV) leading edge. The partial success of these trials allowed the proposal of simple design rules for dendritic cooling systems. Subsequently a direct metal laser deposition technique was used to construct engine scale models. While this clearly cannot be used in the formation of a single crystal component, it offers advantage elsewhere in the engine. The preliminary design flow and heat transfer model is developed based on the well known standard blade model (Holland and Thake, 1980) including their modification to account for film-cooling. This allows candidate cooling geometries to be compared to other arrangements and the component metal temperature (chordwise averaged temperature distribution) to be obtained. Data are expressed as non-dimenstional coolant flow vs metal effectiveness, though more detailed temperature distributions are also presented. This piecewise isothermal wall model allows the effects of varying geometric parameters to be assessed. The model shows that the dendritic schemes are extremely promising, with both a substantial reduction in non-dimensional cooling flow and a significant increase in blade metal effectiveness at engine take off conditions. The performance improvement has been confirmed by a co-worker at the Osney Laboratory who measured the metal effectiveness distribution of metal NGVs.
Experimental results on the internal cooling performance have been submitted and accepted for presentation at the 2011 IGTI conference and a patent application has been submitted, covering novel aspects of the dendritic cooling designs. Further papers are in preparation.