# Professor John Chew

### Biography

### Research interests

- Fluid Dynamics, Heat Transfer and Aero-elasticity
- Methods for and Application of Computational and Theoretical Fluid Dynamics and Heat Transfer.
- Turbo-machinery internal flow systems and seals

### Departmental duties

Professor of Mechanical Engineering

Editor of the Journal of Mechanical Engineering Science (Proc. IMechE Part C).

Technical Programme Chair for ASME Turbo Expo 2016.

Member of the Aerodynamics National Technical Committee.

Member of the Engineering and Physical Sciences Research Council College Peer Review College.

### Current Research Programmes

- Large Eddy Simulation (LES) methods for flow and heat transfer prediction in rotating disc cavities
- Unsteady Simulation of Rim Sealing in Turbine Cavities
- Pre-Swirl Systems for Cooling Air Delivery
- Fluid-Solid Aero-Thermo-Mechanical Coupling for Industrial Flows
- Brush Seal Modelling
- Modelling Techniques for Buoyancy-affected Flow in Turbomachinery Disc Cavities
- Thermal Contact Resistance

### My publications

### Publications

tends to be ingested into the turbine disc cavities. This leads to

overheating which will reduce the disc?s life time or lead to

serious damage. Often, to overcome this problem, some air is

extracted from the compressor to cool the rotor discs. This

also helps seal the rim seals and to protect the disc from the

hot annulus gas. However, this will deteriorate the overall

efficiency. A detailed knowledge of the flow interaction between

the main gas path and the disc cavities is necessary in

order to optimise thermal effectiveness against overall efficiency

due to losses of the cooling air from the main gas path.

The aim of this study is to provide better understanding of

the flow in a turbine stator-well, and evaluate the use of different

CFD methods for this complex, 3-dimensional unsteady

flow. This study presents CFD results for a 2-stage turbine.

The stator-well cavity for the second row of stationary vanes

is included in the calculation and results for both turbine performance

and stator-well sealing efficiency are presented.

INTERNAL AIR SYSTEMS APPLICATIONS, Fluid Machinery and Fluid Mechanics 2009 pp. 399-404 Springer Berlin Heidelberg

for aero-engine component temperature predictions. This paper presents a review of the latest progress in this aspect with

emphasis on internal air system applications. The thermal coupling methods discussed include the traditional finite element

analysis (FEA), the conjugate heat transfer, FEA/CFD coupling procedure and other thermal coupling techniques. Special

attention is made to identify the merits and disadvantages between the various methodologies. Discussion is further extended

on the steady and transient thermal coupling applications.

Some good qualitative and quantitative agreement with measurements has been found, but significant discrepancies and uncertainties remain. Overall the results of this first attempt to analyse the bearing chamber flow are considered very encouraging.

domain with periodic boundary conditions imposed in the third direction. The numerical results are compared with solutions available in the literature and with numerical results obtained using a commercial software that employs a low-order finite volume method. Good agreement with previous work is obtained for the value of the Rayleigh number investigated, Ra = 2E9, which is greater than the critical value of Ra where transition to an unsteady, chaotic state is known to occur. The results are presented in terms of the time-averaged flow structure, Reynolds stresses and modal energies. Although the time-averaged velocity and temperature fields obtained with a commercial finite volume code are in general good agreement with the results obtained with the spectral element code, it does not give accurate predictions of second-order statistics.

^{-0-8}w and Re

^{Æ}are the nondimensional flow rate and rotational Reynolds number, respectively. The measured pressures are in good agreement with the predicted values, and the pressure drop across the cavity can be significantly less than that associated with solid-body rotation. The flow rate produced by the pressure drop across the cavity is not unique: there are up to three possible values of flow rate for any given value of pressure drop.

Our study revealed a number of lessons for pool operators, designers and policy makers: disinfection reaches the majority of a full scale pool in approximately 16 minutes operating at the maximum permissible inlet velocity of 0.5m/s. This suggests that where a pool is designed to have 15 paired inlets it is capable of distributing disinfectant throughout the water body within an acceptable time frame.

However, the study also showed that the exchange rate of water is not uniform across the pool tank and that there is potential for areas of the pool tank to retain contaminated water for significant periods of time. ?Dead spots? exist at either end of the pool where pathogens could remain. This is particularly significant if there is a faecal release into the pool by bathers infected with Cryptosporidium parvum, increasing the potential for waterborne disease transmission.

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.

heat recovery applications and have been subject to significant research over the last two decades. The most common

geometrical design uses a scroll profile generated by the involute of a circle with a constant wall thickness. A major

disadvantage of this approach is that the increase of the geometric expansion ratio is constrained, since it is accompanied

with a large increase in the scroll profile length and is associated with a decreased efficiency. In this paper, the

published literature related to scroll expander geometry is reviewed. Investigations regarding the influence of varying

scroll geometrical parameters on the performance of scroll expanders with a constant wall thickness are first examined.

The use of variable wall thicknesses and their effects on the performance are then considered. Finally, the impact of

scroll expander geometries using unconventional scroll profiles and scroll tip shape variations on the performance is

discussed and summarised. The major conclusion to be drawn from this review is that scroll expanders with variable

wall thickness scrolls should be further designed and developed. It is possible to increase the geometric expansion ratio

without increasing the length of the scroll profiles. CFD simulations are a promising tool to illustrate and understand

the non-uniform and asymmetric inner flow and temperature fields. The related benefits could lead to scroll devices

with variable wall thickness not only improving the performance of organic Rankine cycle (ORC) systems but also

opening a broad new field of applications such as refrigeration cycles and other power cycles where a high pressure

ratio is preferred.

placed on the underlying flow physics and modelling capability. Rim seal flows play a

crucial role in controlling engine disc temperatures but represent a loss from the main

engine power cycle and are associated with spoiling losses in the turbine. Elementary

models that rely on empirical validation and are currently used in design do not

account for some of the known flow mechanisms, and prediction of sealing

performance with computational fluid dynamics (CFD) has proved challenging. CFD

and experimental studies have indicated important unsteady flow effects that explain

some of the differences identified in comparing predicted and measure sealing

effectiveness. This review reveals some consistency of investigations across a range

of configurations, with inertial waves in the rotating flow apparently interacting with

other flow mechanisms which include vane, blade and seal flow interactions, disc

pumping and cavity flows, shear layer and other instabilities, and turbulent mixing.

turbine internal air systems, and are particularly challenging to

model due to the inherently unsteadiness of these flows. While

the global features of such flows are well documented, detailed

analyses of the unsteady structure and turbulent quantities have

not been reported. In this work we use a high-order numerical

method to perform large-eddy simulation (LES) of buoyancyinduced

flow in a sealed rotating cavity with either adiabatic or

heated disks. New insight is given into long-standing questions

regarding the flow characteristics and nature of the boundary

layers. The analyses focus on showing time-averaged quantities,

including temperature and velocity fluctuations, as well as on

the effect of the centrifugal Rayleigh number on the flow structure.

Using velocity and temperature data collected over several

revolutions of the system, the shroud and disk boundary layers

are analysed in detail. The instantaneous flow structure contains

pairs of large, counter-rotating convection rolls, and it is shown

that unsteady laminar Ekman boundary layers near the disks are

driven by the interior flow structure. The shroud thermal boundary

layer scales as approximately Ra?1/3, in agreement with observations for natural convection under gravity.

(RANS) models of turbulence are generally used for predicting

the performance of gas turbine pre-swirl systems, these models

are known to have shortcomings. Motivated by the need to

predict mixing where cooled-cooling air (CCA) and un-cooled

cooling air (UCA) streams are introduced in pre-swirl systems,

computational fluid dynamics (CFD) modelling capability is reassessed.

An initial study focuses on a normal jet in crossflow

(JICF), illustrating some of the shortcomings of a popular

Reynolds-averaged Navier-Stokes (RANS) model and the possible

advantages and disadvantages of wall-modeled large eddy

simulation (WMLES). Pre-swirl system studies focus on a previously

investigated low radius feed, direct flow configuration for

which measurements and CFD solutions are available, and extend

this to consider a mixed feed system. The velocities in the

near-jet region of the pre-swirl chamber were predicted differently

by unsteady RANS (URANS) and WMLES, however no significant

differences were observed closer to the receiver holes.

Comparing to measurements shows little overall difference between

the models, with similar results to earlier studies. Although

no measurements are available for a mixed feed system,

comparison of predictions from the two models indicates significant

sensitivity and uncertainty involved in these predictions.

Reynolds-averaged Navier-Stokes (URANS) calculations

of a turbine rim seal configuration previously investigated experimentally.

The configuration does not include any vanes, blades

or external flows, but investigates inherent unsteady flow features

and limitations of CFD modelling identified in engine representative

studies. Compared to RANS and URANS CFD models, a

sector LES model showed closer agreement with mean pressure

measurements. LES models also showed agreement with measured

pressure frequency spectra, but discrepancies were found

between the LES and experiment in the speed and the circumferential

lobe number of the unsteady flow structures. Sensitivity of

predictions to modelling assumptions and differences with experimental

data are investigated through CFD calculations considering

sector size, interaction between the rim cavity and the inner

cavity, outer annulus boundary conditions, and the coolant mass

flow. Significant sensitivity to external flow conditions, which

could contribute to differences with measurements, is shown, although

some discrepancies remain. Further detailed analysis of

the CFD solutions is given illustrating the complex flow physics.

Possible improvement of a steady RANS model using a priori

analysis of LES was investigated, but showed a rather small improvement

in mean pressure prediction.

turbine internal air systems, and are particularly challenging to

model due to the inherently unsteadiness of these flows. While

the global features of such flows are well documented, detailed

analyses of the unsteady structure and turbulent quantities have

not been reported. In this work we use a high-order numerical

method to perform large-eddy simulation (LES) of buoyancyinduced

flow in a sealed rotating cavity with either adiabatic or

heated disks. New insight is given into long-standing questions

regarding the flow characteristics and nature of the boundary

layers. The analyses focus on showing time-averaged quantities,

including temperature and velocity fluctuations, as well as on

the effect of the centrifugal Rayleigh number on the flow structure.

Using velocity and temperature data collected over several

revolutions of the system, the shroud and disk boundary layers

are analysed in detail. The instantaneous flow structure contains

pairs of large, counter-rotating convection rolls, and it is shown

that unsteady laminar Ekman boundary layers near the disks are

driven by the interior flow structure. The shroud thermal boundary

layer scales as approximately Ra?1/3

, in agreement with observations

for natural convection under gravity.

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.

in the lower part of the disc and over-predicted in the outer region by both URANS models, whereas the LES provides a much better agreement with the measurements. The behaviour results primarily from a different flow structure in the source region, which, in the LES, is found to be considerably more extended and show localized buoyancy phenomena that the URANS models investigated do not capture.

bristle pack and is consistent with anecdotal reports of seal behavior. The critical swirl velocity was reduced when the downstream pressure level was raised, keeping the same upstream total to downstream static pressure difference. This is caused by the increased dynamic head associated with the inlet swirl. Inclusion of a front plate in the seal design does not offer the intended protection to the bristle pack in highly

swirling environments. This is associated with highly swirling flow impinging on the bristle tips. Fitting of roughness elements on the upstream face of the front plate could improve stability by reducing swirl of the flow impacting on the bristles. Increasing the bristle diameter and bristle

stiffness does not necessarily prevent slip at higher inlet swirl velocities, but reduces the magnitude of slip of the upstream bristles.

swirl ratio predicted by different sub-grid scale models tested is in good agreement with the measurements, although a slight overprediction is observed at lower radii. This has been demonstrated to be caused by an excessive numerical dissipation. Adopting a stable, less dissipative I-LES solution, the swirl ratio matches the data almost perfectly. In the next activity, the prediction from a Large-Eddy Simulation conducted for a rotating cavity with a radial inflow introduced from the shroud and heated on one wall have been compared with experimental data available from the literature, and with those obtained using two URANS eddy-viscosity models. The LES solution has shown a very good agreement especially in the outer part of the cavity, capturing buoyancy effects. The results of two URANS models are considerably worse than the LES.

Since LES is currently limited for application in industry by the high computational demand, Reduced Order Methods (ROM) that use data from LES have been considered in order to construct a model which could result in a computationally efficient method for design purposes. The POD-Galerkin procedure has been validated for the relative simple turbulent shear flow of the plane Couette flow. Then, the low Mach number turbulent flow in a rotor-stator cavity has been modelled. Overall, it is possible to claim that the models studied reasonably well predict the turbulence phenomenon for the rotor-stator flow (LES statistics and experimental measurements have been used as a benchmark).

*ß*?

*T*) and rotational Reynolds number

for a given Rayleigh number, are investigated with the full

compressible model. The mean centrifugal and radial

Coriolis forces are analysed. Heat transfer predictions from

the Boussinesq and compressible models agree to within

10%, for

*ß*?

*T*d 0.2.

cavity with a radial inflow introduced from the shroud. The dimensionless mass flow rate of the

radial inflow is C

_{w}= 3500 and the rotational Reynolds number, based on the cavity outer radius, is equal to

*Re*= 1.2 x 10v. The time averaged local Nusselt number on the heated wall is compared

_{¸}with the experimental data available from the literature, and with those derived from the solution

of two Unsteady Reynolds Averaged Navier-Stokes (URANS) eddy viscosity models, namely the

Spalart-Allmaras and the

*k - É SST*model. It is shown that the Nusselt number is under-predicted

in the lower part of the disc and over-predicted in the outer region by both URANS models, whereas

the LES provides a much better agreement with the measurements. The behaviour results primarily

from a different flow structure in the source region, which, in the LES, is found to be considerably

more extended and show localized buoyancy phenomena that the URANS models investigated do

not capture.

Rayleigh number

*Ra*in the range 10w to 10y. DNS for an incompressible model with

the Boussinesq approximation is compared with LES for a compressible gas flow model.

The compressible solver's solutions show the shroud Nusselt number scales with

*Ra*

^{0.286},

in close agreement with the corrected experimental correlation and the

*Ra*

^{2/7}scaling for

gravitational heat convection between horizontal plates, but differs from the

*N u*?

*Ra*

^{1/3}

scaling given by the incompressible solver. The shroud thermal boundary layer thickness,

based on the root mean square of the temperature

fluctuation, can be estimated with

»* = 0:5

*N u*

^{-1}Velocities scale approximately with

&

*a?²?T*. Disc laminar Ekman layer

behaviour is confirmed up to

*Ra*= 109. An Ekman layer scrubbing effect, associated with

the viscous energy dissipation, is considered to be mainly responsible for the difference in

*N u*between the two solvers at

*Ra*= 10

^{9}, in spite of rather small Eckert number. The

analysis of the turbulent kinetic energy budget shows a dominant constant buoyancy production in the core. The use of the incompressible formulation for the considered problem

is restricted by the applicable range of the Boussinesq approximation characterised by

the buoyancy parameter

*²?T*and neglect of viscous heating and compressibility effects

characterised by the Eckert number

*Ec =*

&

=(.

&

^{2}r^{2}_{m}=(

*C*)_{p}?Tthe geophysics, oceanographic and atmospheric research communities. Inertial waves, which are

driven by restoring forces, for example the Coriolis force, have been detected in the research fields

mentioned above. This paper investigates inertial waves in turbine rim seal flows in turbomachinery.

These are associated with the large-scale unsteady

ow structures having distinct frequencies, unrelated

to the main annulus blading, identified in many experimental and numerical studies. These

unsteady flow structures have been shown in some cases to reduce sealing effectiveness and are

difficult to predict with conventional steady Reynolds-averaged Navier-Stokes (RANS) approaches.

Improved understanding of the underlying

ow mechanisms and how these could be controlled is

needed to improve the efficiency and stability of gas turbines. This study presents large-eddy simulations

for three rim seal configurations { chute, axial and radial rim seals { representative of

those used in gas turbines. Evidence of inertial waves is shown in the axial and chute seals, with

characteristic wave frequencies limited within the threshold for inertial waves given by classic linear

theory (i.e. jf=frelj 2), and instantaneous

flow fields showing helical characteristics. The radial

seal, which limits the radial

fluid motion with the seal geometry, restricts the Coriolis force and suppresses the inertial wave.

URANS and zonal-hybrid URANS-LES methodologies. The prediction is compared with experimental data, obtained via engine testing. The level of seal clearance and coolant

mass-?ow inside the secondary air system cavities of the turbine has a range of uncertainty during testing.

In the ?rst part of this work a small-sector URANS computation with the boundary conditions re?ecting the best understanding of the engine during testing is carried out

and a miss-match with experiments found. As a result, a sensitivity analysis with the seal clearance and coolant mass-?ow is undertaken, in which it is found that the inges

tion prediction is largely a?ected by these two parameters. To assess the e?ect of the computation of turbulence, a zonal-hybrid URANS-LES computation is carried out at

the condition with the best understanding of the test and it is found that the ingestion prediction improves compared to a URANS computation. The similar ?ow structures obtained between the two approaches suggests that the improvement is due to more accurate turbulent mixing being predicted by the zonal-hybrid URANS-LES computation.

Finally, some ingestion models are tuned to the CFD data originating from the sensitivity analysis. An optimization study with the resulting sealing e?ciency characteristic is

then carried out and a condition that minimizes the error to the experimental data found. A new small-sector URANS computation at this condition is unable to match the

experimental data, with a greater miss-match found in the more radially inwards region of the cavity. Due to the better agreement of the CFD solution to the ingestion models

in the part of the cavity closer to the main annulus than in the more radially inwards part of the cavity, it is suggested that predicting the ?ow structure in this region is key to the ingestion prediction. Considering the improvement seen with the zonal-hybrid methodology it is argued that a similar optimization study using this methodology would have improved the alignment to the experimental condition.

various nozzle configurations. The research also included comparison with experimental CCA/UCA pre-swirl rig results and a good agreement between the computational and experimental results was obtained using different turbulence modeling approaches.

An engine-scale model of this rig was simulated using engine-representative boundary conditions. The mixing characteristics of the engine model was found to be similar to those of the test rig. Cover-plate type pre-swirl systems were found to be more efficient in mixing the CCA and UCA flows compared with the direct-transfer systems. An alternating CCA-UCA pre-swirl nozzle configuration (nozzles placed at the same radial location) was also found to promote mixing of the two streams due to the relative motion of the preswirl nozzles on the stator and the receiver holes on the rotor disc. The prospects of

preferentially feeding the turbine blades using the colder cooling air stream were also investigated. The studied two-row feed CCA/UCA nozzle arrangements were found to

be suitable for this purpose with the mixing predictions showing sensitivity to turbulence modeling. A parametric study including various two-row feed pre-swirl system

configurations is presented.

The CFD simulations were performed using the HYDRA and ANSYS FLUENT CFD solvers. The solutions given by both CFD solvers show an overall good agreement in predictions of mean flow field, and small differences for the prediction of instantaneous flow field. Computational investigations of the unsteady flows inside alternating-feed CCA/UCA pre-swirl systems were also performed using computational models with different nozzle arrangements and non-nozzle passing frequency unsteadiness was observed in these investigations