### Professor Nick Hills

### Biography

### Biography

Prof. Nick Hills is the Head of Department of Mechanical Engineering Sciences. He holds a Chair in Computational Engineering at the University of Surrey, and is also the Director of the Rolls-Royce University Technology Centre in Thermo-Fluid Systems. He previously held a Royal Academy of Engineering / Rolls-Royce Chair.

His research has focussed on CFD development, particularly for massively parallel simulations (including leading the UK Applied Aerodynamics Consortium with time on the national super-computing facilities), multi-physics models and their application to internal air systems for gas turbine engines. Research funding has been through EPSRC, InnovateUK, EU and direct industrial support.

### My publications

### Publications

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. Nozzle performance is characterised by the nozzle discharge coefficient (C

), nozzle velocity coefficient (?·) and cooling air delivery temperature. Two commonly used eddy viscosity models are employed for the study, the standard º-µ and Spalart-Allmaras models with wall functions. Both models give very similar results for C

and · and are in reasonable agreement with available experimental data. Effects of nozzle or vane number and sealing flow have been analysed. The cascade vanes perform slightly better than the aerodynamically designed drilled nozzles but the final design choice will depend on other component and manufacturing costs. An elementary model is presented to separate temperature losses due to the nozzle, stator drag and sealing flow. Copyright © 2011 by Rolls-Royce plc.

flow in a rotor/rotor cavity. The numerical tool used employs a

spectral element discretisation in two dimensions and a Fourier

expansion in the remaining direction, which is periodic and corresponds

to the azimuthal coordinate in cylindrical coordinates.

The spectral element approximation uses a Galerkin method to

discretise the governing equations, but employs high-order polynomials

within each element to obtain spectral accuracy. A

second-order, semi-implicit, stiffly stable algorithm is used for

the time discretisation. Numerical results obtained for the rotor/

stator cavity compare favourably with experimental results

for Reynolds numbers up to Re1 = 106 in terms of velocities and

Reynolds stresses. The buoyancy-driven flow is simulated using

the Boussinesq approximation. Predictions are compared with

previous computational and experimental results. Analysis of

the present results shows close correspondence to natural convection

in a gravitational field and consistency with experimentally

observed flow structures in a water-filled rotating annulus.

Predicted mean heat transfer levels are higher than the available

measurements for an air-filled rotating annulus, but in agreement

with correlations for natural convection under gravity.

SPECTRAL ELEMENT-FOURIER METHOD, Proceedings of ASME Turbo Expo 2016: ASME

flow in a rotor/stator cavity without heat transfer and buoyant

flow in a rotor/rotor cavity. The numerical tool used employs a

spectral element discretisation in two dimensions and a Fourier

expansion in the remaining direction, which is periodic and corresponds

to the azimuthal coordinate in cylindrical coordinates.

The spectral element approximation uses a Galerkin method to

discretise the governing equations, similarly to a finite element

method, but employs high-order polynomials within each element

to obtain spectral accuracy. A second-order, semi-implicit,

stiffly stable algorithm is used for the time discretisation, and

no subgrid modelling is included in the governing equations.

Numerical results obtained for the rotor/stator cavity compare

favourably with experimental results for Reynolds numbers up to

Re1 = 106

in terms of velocities and Reynolds stresses. For the

buoyancy-driven flow, the energy equation is coupled to the momentum

equations via the Boussinesq approximation, which has

been implemented in the code considering two different formulations.

Numerical predictions of the Nusselt number obtained

using the traditional Boussinesq approximation are considerably

higher than available experimental data. Much better agreement

is obtained when the extended Boussinesq approximation is em-ployed. It is concluded that the numerical method employed has

considerable potential for further investigations of rotating cavity

flows.

CFD solutions are shown to vary from a buoyancy driven regime to a forced convection regime, depending on the radial in?ow rate prescribed at the shroud. At a high radial in?ow rate, the computations show an excellent agreement with the measured temperatures through a transient rig condition. At a low radial in?ow rate, the cavity ?ow is destabilized by the thermal strati?cation. Good qualitative agreement with the measurements is shown, although a signi?cant over-prediction of disc temperatures is observed. This is associated with under prediction of the penetration of the axial through?ow into the cavity. The mismatch could be the result of strong sensitivity to the prescribed inlet conditions, in addition to possible shortcomings in the turbulence modeling.

accurately to predict the interaction of the mainstream flow with the secondary air system cooling

flows using computational fluid dynamics (CFD) with Reynolds-averaged Navier?Stokes

(RANS) turbulence models have proved difficult. In particular, published results from RANS

models have over-predicted the sealing effectiveness of the rim seal, although their use in this

context continues to be common. Previous studies have ascribed this discrepancy to the failure to

model flow structures with a scale greater than the one which can be captured in the small-sector

models typically used. This article presents results from a series of Large-Eddy Simulations (LES)

of a turbine stage including a rim seal and rim cavity for, it is believed by the authors, the first

time. The simulations were run at a rotational Reynolds number Re ¼ 2.2 106 and a main

annulus axial Reynolds number Rex ¼ 1.3 106 and with varying levels of coolant mass flow.

Comparison is made with previously published experimental data and with unsteady RANS simulations.

The LES models are shown to be in closer agreement with the experimental sealing

effectiveness than the unsteady RANS simulations. The result indicates that the previous failure

to predict rim seal effectiveness was due to turbulence model limitations in the turbine rim seal

flow. Consideration is given to the flow structure in this region.

K

be complex and attempts accurately to predict the interaction of

the mainstream flow with the secondary air system cooling flows

using CFD with RANS turbulence models have proved difficult.

In particular, published results from RANS models have overpredicted

the sealing effectiveness of the rim seal, although their

use in this context continues to be common. Previous authors

have ascribed this discrepancy to the failure to model flow structures

with a scale greater than can be captured in the small sector

models typically used. This paper presents results from a series

of Large-Eddy Simulations (LES) of a turbine stage including a

rim seal and rim cavity for, it is believed by the authors, the first

time. The simulations were run at a rotational Reynolds number

Re¸ = 2.2 × 106 and a main annulus axial Reynolds number

Rex = 1.3 × 106 and with varying levels of coolant mass

flow. Comparison is made with previously published experimental

data and with unsteady RANS simulations. The LES models

are shown to be in closer agreement with the experimental sealing

effectiveness than the unsteady RANS simulations. The result

indicates that the previous failure to predict rim seal effectiveness

was due to turbulence model limitations in the turbine rim seal

flow. Consideration is given to the flow structure in this region.

20%?38% reduction of the number of iterations needed for convergence compared to Runge-Kutta coefficients recommended in the literature for comparable schemes and with the same computational cost per iteration.

Initially, the research focussed on modelling of a rotor-stator disc cavity. Steady CFD validations for a plane disc and for a disc with protrusion were carried out and a simplified body force model was developed for including the 3D effects of rotating and stationary bolts into the axisymmetric CFD models. The simplified rotor bolt model was verified and validated by comparing the results with Sussex Windage rig test data and 3D CFD data. The simplified stator bolt model was verified using 3D CFD results. The simplified rotor bolt model was found to predict the drag and windage heat transfer with reasonable accuracy compared to 3D sector CFD results. However, 3D sector CFD under-predicts the high core flow swirl and the adiabatic disc surface temperature inboard of the bolt, compared to experimental data.

In the second part of the study, unsteady Reynolds averaged Navier-Stokes (URANS) calculations of the rotating bolts cases were performed in order to better understand the flow physics. Although the rotor-stator cavity with bolts is geometrically steady in the rotating frame of reference, it was found that the rotor bolts generate unsteadiness which creates time-dependent rotating flow features within the cavity. A systematic parametric study is presented giving insight into the influence of the bolt number and the cavity geometric parameters on the time dependent flow within the cavity. The URANS calculations were extended to a high pressure turbine (HPT) rear cavity to show possible unsteady effects due to rotating bolts in an engine case.

Following this, the body force model was adapted to model the rotating hole velocity changes and flow through honeycomb liners. The honeycomb and hole models were verified by comparing the results with available experimental data and 3D CFD calculations.

In the final part of the study, coupled FE-CFD calculations for a preliminary design whole engine thermo-mechanical (WETM) model for a transient square cycle was performed including the effects of non-axisymmetric features. Six cavities around the HPT disc were modelled using CFD. The coupled approach provides more realistic physical convective heat transfer boundary conditions than the traditional approach. The unvalidated baseline thermo-mechanical model results were verified using the high fidelity coupled FE-CFD solution. It was demonstrated that the FE-CFD coupled calculations with axisymmetric modelling of 3D features can be achieved in a few days time scale suitable for preliminary engine design. The simplified CFD based methods described in this thesis could reduce the computational time of transient coupled FE-CFD calculations several orders of magnitude and may provide results as accurate as 2DFE-3DCFD coupled calculations.