Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
54

Optical Measurement and Numerical Simulation
Technologies for
Aircraft Engine Combustor Development

HIDEKI MORIAI*1

YOSHIAKI MIYAKE*2

Combustors in aircraft are required to be lightweight and compact in size, and also need tosatisfy many technical requirements for emission performance, combustion efficiency, relightcapability at high altitude, and combustion stability performance to comply with environmental
regulations that are made stricter every year. However, it is difficult to predict the combustion
phenomena that occur in a combustor, and thus overall combustor performance. Therefore, newcombustor development relies mainly on physical performance tests. We are proceeding with anindustry–government–academia collaboration to develop the measurement and analysis techniques
to examine the aerodynamics and combustion in a combustor, as well as possible heat-transfertechniques. This paper presents heat-transfer performance measurement and simulation techniquesthat will lead to a high-performance cooling structure, as well as laser measurement andsimulation techniques of the spray-combustion flow field that will lead to reduced NOx emissions.

|1. Introduction
Gas turbines continue to be used in aircraft engines. They consist of a compressor,combustor, and turbine. An environmentally-friendly engine has been required for many years.
Improvements to fuel consumption, which depend mainly on the compressor and turbine
performance, will lead to reduced CO2 emissions, and will be a decisive countermeasure against
global warming. The reduction of emissions, such as NOx and soot, which cause health hazards,
require improved kerosene-based liquid fuel combustor characteristics. The compressor and turbine
rotate very quickly, and the technical difficulty in improving their performance lies in thehigh-temperature gas flowing through their blades. However, the flow through these devices does
not include combustion phenomenon, and the numerical analysis is relatively straightforward
compared to that of a combustor. Also, in component testing, the scale can be minimized byutilizing a similarity method. The remaining scope of performance improvement for compressors
and turbines is quite limited for these reasons as well as high-speed rotation restrictions.
The phenomena in a combustor are unsteady and complicated including phase changes,mixing, and combustion reactions. It is therefore difficult to predict the phenomena and design new
combustors using numerical analysis. As a result, development tends to rely mainly on hardware
experiments. Similarity theory cannot be applied, so experiments using the actual pressure,temperature, and flow are necessary. Thus, the development of a combustor requires a long periodof time and is very expensive.
Improved numerical analysis technology is needed for combustor development, and the newexperimental and measuring techniques will be useful to ensure the validity of the new technology.
Therefore, an industry–government–academia collaboration is working to develop the followingtechnologies:(1) Visualization and numerical simulation of the heat transfer in a combustor cooling liner(2) Visualization measurement and numerical simulation of the spray-combustion flow field
This report provides an outline of these activities.
*1

Engineering Manager, Commercial Aero Engines Department, Guidance & Propulsion Division, Aerospace Systems*2

Deputy Chief Engineer, Guidance & Propulsion Division, Aerospace Systems

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)55|2. Visualization and numerical simulation of the heat transfer in acombustor cooling liner
Liner cooling is indispensable in a combustor. However, when much of the air is used forcooling, the amount available for combustion is reduced, which increases the combustion
temperature, resulting in more NOx emissions. Cooling with a small amount of air is therefore onemeans of reducing NOx. Two typical measures to cool the combustor liner are double-wall pin-fincooling and single-wall film cooling. In this report, we present a heat transfer measuring techniqueand a numerical analysis technique using Large-Eddy Simulation (LES) to develop the double-wall
pin-fin cooling (Figure 1). The research was performed in collaboration with Osaka University.

Figure 1

2.1

Double-wall combustor and its cooling liner panel

Cooling Structure

Figure 2 shows the double-wall cooling panel (liner) that was investigated in this study. In a
conventional double-wall cooling panel, vertical pins are installed in a honeycomb shape on theinner side of the combustor, as shown in Figure 2(a). The flow of cooling air through the space
applies forced cooling to the pins and inner wall. The difficulty is the characteristics of nickel alloy,
which is used as the panel material to provide resistance against high temperatures and oxidation.
The nickel alloy has low heat conductivity so fin efficiency is low. It is therefore desirable toincrease cooling flow to the surface of the inner wall (panel) more than cooling flow to the surfaceof the pins. One idea is to install protrusions between the pins on the inner panel face to increase
the flow turbulence. This idea was verified to increase the cooling capability in a study subsidized
by the Society of Japanese Aerospace Companies, and is shown in Figure 2(b). However, themechanism causing this improved performance was not verified enough.
Research and development concerning the detailed measurement of the heat transfer
coefficient distribution and LES numerical analyses were conducted as described in sections 2.2
and 2.3 below. The results obtained for the panel configurations shown in Figures 2(a) and (b)allow quantitative verification of the heat transfer coefficient distribution and flow conditions.
Based on this knowledge, a new cooling panel was designed with a slanted pin-fin and wavy
endwall, as shown in Figure 2(c). For vertical pins, the majority of the heat transfer is locatedlocally along a horseshoe eddy, which is generated in front of the pin by the boundary layerinteraction, and looks like a string hooked on a stake in a river. By introducing inclined pins, thestrength of the horseshoe eddy is reduced. However the flow is accelerated through the narrow
space between the pins and is deflected downward, acting like a jet colliding against the inner wallto increase the mean heat transfer coefficient. The inner wall face has waves, as shown in Figure2(c), to smooth the flow. The wavy shape is also thought to increase the collision angle and the heattransfer area slightly. As a secondary effect, the pin inclination causes a drastic reduction inpressure loss at the pin, which was verified in the numerical simulations and experiments.

2.2

Measurement of Heat Transfer Distribution Using a Naphthalene Sublimation
Technique

The naphthalene sublimation technique, which utilizes the analogy between heat transfer andmass transfer in the near-wall region of a boundary layer, is one method of heat transfer
measurement. The problem is the accuracy and resolution. At Osaka University, a heat transfer
measurement method is now being developed by controlling a high-resolution traverser and a laser
distance meter with a PC, measuring many points at high speed, and measuring the sublimed

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
56

naphthalene with a high degree of accuracy. We are now developing a heat transfer measuring
technique for the panels shown in Figure 2. Based on the reasons described in section 2.3,
measurements were conducted under conditions where the pin heat conductivity was assumed to be0, which does not affect flow and pressure loss. Figure 3 presents the results: the inclined pins
were verified to cool the entire wall evenly compared to conventional vertical pins.

Figure 2 Construction of a double-wall cooling liner panel(the figure shows an enlarged model for measurement)

Figure 3

2.3

Naphthalene sublimation technique and the heat transfer measurement results

Coupled Heat Transfer Numerical Analysis with LES

The naphthalene sublimation technique is convenient, but measurements of the heat source
temperature distribution are difficult to obtain. In a real case, the heat conductivity of the pinmaterial is small compared to the amount of heat transfer, and significant temperature gradientappears at the pin. For these reasons, the measurements were performed by idealizing the flow
conditions, as described previously. Therefore, supplemental numerical analyses are required
before applying the results to a new design. Some aspects of heat transfer phenomena depend on
unsteady flow characteristics. LES analyses were applied to elucidate the phenomena clearly.
Some ideas were applied to manage the huge CPU resource required for LES. First, cyclicboundary conditions were applied in the flow direction in addition to the direction perpendicular to
the flow, which assumes a well-developed flow condition after the flow passed several pins.
Second, the LES flow calculations and temperature distribution calculations were separated toreduce the CPU time and to prevent numerical instabilities. Namely the cooling flow calculation
results and pin (metal) temperature analyses were later coupled together. The heat conductivity of
the cooling air applied to the latter was obtained by averaging the turbulent flow in the LEScalculations, and is referred to as the eddy-heat conductivity.
Figure 4 shows the calculated temperature distributions. Different pin heat conductivity
cases could be calculated from one flow LES result. Figure 4(b) shows the results obtained using apin heat conductivity that was ten times that of a real engine. The cylindrical surface of the slanted
pin at the center of the figure changed from red to green, illustrating the temperature rise to the top.
For a real engine case (a), the blue color over most of the pin surface shows that it had little coolingeffect.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)57

Figure 4 Coupled calculation of the temperature distribution obtained using LES|3. Visualization
andNumerical
Spray-Combustion Flow Field

Simulation

oftheThe flow in an aircraft engine combustor consists of complicated phenomena involving
spray, atomization, and turbulent flow mixing (including swirling flow and chemical reactions),
and reproduction by numerical simulation is very difficult. Currently, combustion flow simulation
tools are mainly used in limited areas to provide qualitative predictions of the steady averaged flow
field and qualitative effects of design parameter changes. Highly accurate simulation tools are not
yet available to provide comprehensive spray combustion flow predictions. To allow for efficientdevelopment of combustors, it is important to elucidate the spray-combustion phenomena in thecombustor. This report describes the results of combining experimental and numerical analyses toimprove our understanding of these phenomena. The research was performed in collaboration with
Osaka University, Kyoto University, and the Central Research Institute of the Electric Power
Industry.

3.1

Visualization of the Spray-Combustion Flow Field

This experiment involved a sub-scale combustor, corresponding to one sector of a combustor
with the similar fuel nozzle as a real engine. Optical measurement of the spray-combustion flow
formed in the combustor was obtained to investigate the spray flame behaviors and the structure of
the recirculation zone due to the particle diameter/velocity distribution. The sub-scale combustor
used in this research was equivalent to the primary combustion zone of a Rich-burn Quick-quench
Lean-burn (RQL) combustor, which is one type of standard configuration of low-NOx combustors.
Figure 5 presents the structure of the pre-film air-blast atomizer, the sub-scale combustor, and thelaser measurement situation using the Phased Doppler Anemometry (PDA) in this research.

Figure 5
Structure of the atomizer (left), sub-scale combustor (middle), and PDA
measurements (right)

Figure 6 shows typical PDA measurement results. Positions with enough droplets for themeasurements are shown by (○), and positions without enough droplets are shown by (●). With the
exception of the upstream area in the vicinity of the wall, measurement results were good. Figure 6indicates droplet velocities in the opposite direction to main flow direction. The rate of negative
velocity was high around the upstream center axis area and in the vicinity of the wall. This strong
re-circulating flow seemed to sustain the flame. The amount of re-circulating droplets in this part of
the flow ranged from 20% to a maximum of 50%. Furtherly, acquisition of detailed measurement
flow field data was carried out to elucidate the physical phenomena in a spray-combustion field
clearly, and the results were utilized to verify the simulations (described below).

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
58

Figure 6

3.2

PDA measurement points and measurement results of reverse-flow droplets

Numerical Simulation of Spray Combustion

Currently, the major tool used to predict a combusting flow field is Reynolds-averaged
Navier–Stokes (RANS) simulations. However, LES is beginning to attract attention because it
requires fewer adjustment parameters and can precisely simulate unsteady turbulent flow.
Therefore, we used a three-dimensional LES with the flamelet/progress-variable approach in theturbulent combustion model to simulate the spray-combustion flow field in the sub-scale combustor
described previously. Using this approach drastically reduces the CPU time required to determine
the chemical species and temperatures after the reactions, by using a predetermined chemical
database that includes 181 chemical species and 1195 reactions. The position, velocity,temperature, and mass change of the liquid droplets accompanying vaporization can be solved in a
Lagrangian manner.
Figure 7 shows the simulation results of the instantaneous value distribution for eachparameter that characterizes the combustion field.

Figure 7

Instantaneous distribution of the flow field parameters (simulation)

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)59

The simulations used the FrontFlow-red code extended by Kyoto University, the Central
Research Institute of the Electric Power Industry, and Numerical Flow Designing (known as
FrontFlow/red-KCN). In the figure, T, U, and VMAG indicate the temperature (K), mainstream
direction velocity (m/s), and absolute velocity magnitude (m/s), respectively, while Z and C
indicate the fuel mass fraction and progress-variable, which is sum of the mass fractions of CO2and H2O. YF, YO2, YH2O, YCO, and YNO indicate the mass fraction of the gas phase fuel,oxygen, water vapor, carbon monoxide, and nitric monoxide. The high concentration part of theatomized fuel was limited to a small area just downstream of the fuel atomizer, and the combustion
reaction gradually progressed further downstream, consuming O2 and producing H2O. The vicinityof the upper wall showed a high temperature, but the O2 concentration was close to zero, and areverse flow component was observed. The high temperature of this area was maintained mainly by
the gas recirculation after combustion.
Figure 8 compares the experimental and LES results for the mainstream direction flow
velocity of the droplets, as well as the droplet diameter distribution, at a position 12 mm
downstream from the atomizer. The tendency indicated in the two sets of results was approximately
the same, although the velocity near the wall and the droplet diameter near the central axis showedsome discrepancies. The experiment revealed that Sauter mean diameter (SMD) tended to increasenear the vicinity of the central axis. The poor LES results are likely due to an insufficient numberof droplets (number of parcels) used to follow the droplet movement and a scarcity oflarge-diameter droplets near the axis. It will be necessary to improve the accuracy of the LES using
measures such as increasing the number of parcels. However, the predicted flow behavior wasverified to be in accordance with the flow visualization PDA measurements, implying the validityof using an LES analysis. We will continue to improve the accuracy of LES, including verificationof the emissions prediction such as NOx and soot.

Figure 8 Comparison between the experimental and simulation results(left: velocity distribution of the main flow; right: droplet diameter distribution)|4. Conclusions
It is difficult to predict the combustion phenomena in a combustor, and consequently its
performance, when developing an aircraft engine. Therefore, development work must depend
mainly on costly hardware-based experiments. Improvements to the measurement and simulation
technologies for the heat transfer performance of combustor cooling liners and spray-combustion
fields, which are the base technologies used in combustor development, are currently in progress.
These improvements are now being applied to develop high-performance cooling liners and lowNOx combustors.
Acknowledgements
The research results presented in the second section were obtained from collaborative
research with Prof Kenichiro Takeishi, Associate Prof Seiji Komiyama and Assistant Prof YutakaOda, and others at Osaka University. The experimental results in the third section were obtained incollaboration with Prof. Fumiteru Akamatsu and Assistant Prof. Jun Hayashi, while simulations
were obtained in collaboration with Prof. Satoru Komori, Associated Prof. Ryoichi Kurose at
Kyoto University, Mr. Hiroaki Watanabe at the Central Research Institute of the Electric PowerMitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)60

Industry, and others. This research is part of the ‘Research and Development of a Combustor for an
Environmentally Compatible Small Aero Engine’ project promoted by the New Energy andIndustrial Technology Development Organization (NEDO), and the ‘Study on the Cooling
Efficiency Improvement of a Combustor Liner Wall’ project sponsored by the Society of Japanese
Aerospace Companies. We wish to extend our deep appreciation for their support.

Reference1.

Ryoichi Kurose, Turbulent multiphase combustion, Journal of the Combustion Society of Japan, Vol. 52,
pp. 302-310 (2010)