Optimization of endwall cooling for high-pressure nozzle guide vanes

This thesis reports a combined experimental, numerical, and analytical work aimed to develop and optimize platform cooling for high-pressure nozzle guide vanes. The work is divided into a number of sections, each aimed to tackle different challenges in designing endwall cooling systems.
In the frst section, a fundamental work on the scaling of flm cooling is presented and discussed. This work aims to give insights into the use of the correct parameters when scaling flm effectiveness data. With the aid of extensive RANS and LES predictions, a number of non-dimensional groups are independently varied whilst keeping the remaining groups constant. This enabled the independent effect of each group to be studied. An important observation that departs from previous literature is that there is a strong region dependence of the sensitivity of the flm effectiveness to each of these parameters.
The second section involves an experimental investigation of the effect of the aerodynamic eld on the adiabatic lm eectiveness distribution on an engine-realistic endall geometry. The study is carried out in aerody- namic similarity of the flms between engine and laboratory conditions. It is shown that for relatively low values of momentum flux ratios (i.e. representative of the low portion of the engine range) jet lift-of is ob- served, with ineffcient use of coolant. This suggests that-for a given coolant mass flow rate-endwall flm cooling systems may be optimized by increasing internal pressure loss and therefore reducing the momentum flux ratio.
The third section focuses on the optimization of external flm cooling. The aim is to develop a rapid optimization routine based on the scalar tracking model and multi-objective genetic algorithms. The individual contribution of each flm cooling hole is evaluated in CFD by solving an independent passive-scalar equation, and flm effectiveness sensitivity to coolant mass fow rate is dealt with linear superposition techniques. The latter is used as a proxy for the CFD and it is computationally inexpensive. The approach can be therefore used in standard optimization techniques| such as genetic algorithms-to perform rapid system optimizations. An optimization routine involving the flm cooling of a flat plate with external flow acceleration is performed by minimizing the temperature difference from a target distribution whilst minimizing mixing loss. Limitations of the proposed strategy are also reported and discussed in detail.
The fourth section involves the design of a HP-NGV endwall cooling system based on the reverse-pass scheme. That is, a system in which the internal coolant flows substantially in the opposite direction to the main- stream flow. The HP-NGV platforms are designed and assessd by using bespoke conjugate heat transfer and aerodynamic models. The proposed designs are shown to outperform the baseline confguration by reducing the metal temperature peak and flattening the temperature distribution. Three designs are down-selected for manufacturing by laser-sintering using titanium alloy and tested at high speed conditions.
The final section includes the aero-thermal characterization of novel HP-NGV platform designs based on the reverse-pass scheme and manufactured by laser-sintering. Tests are performed at engine-matched Reynolds number, Mach number, Biot number, and coolant-to-mainstream pressure ratio, on a full-annulus test section. Detailed flm effectiveness distributions on vane platforms are measured by means of infrared thermography. Engine baseline and reverse-pass-based designs are compared in back-to-back experimental tests at engine-representative conditions over a range of coolant-to-mainstream pressure ratios.

Advisor: Povey, T