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18th
Annual CFD Symposium, August 10-11th
, 2016, Bangalore
Numerical Simulation of the Liquid Flow in a Pulsed Fuel Injection System
for Triggering Combustion Instabilities in a Developmental
Aero-Gas Turbine Afterburner
S. Chenthil Kumar*, G. Sriram*, Shambhoo*, H.S. Raghukumar*, M. Janaki Rami Reddy*, C. Rajashekar*1
,
K Ashirvadam#
, and J J Isaac
*
Summary
High performance aero-gas turbine afterburners generally encounter detrimental combustion instabilities in the
developmental phase. Experimental methods are conventionally resorted to arrive at solutions to mitigate these
instabilities as their complexity pose formidable difficulties to adopting analytical and computational approaches.
Unsteady and spatial unevenness in heat release lead to the onset of combustion instabilities. This is essentially due
to any associated poor fuel distribution and fuel spray characteristics.
This behavior of combustion instability onset pattern has been proposed to be exploited by deliberately
creating heat release unevenness and periodicity by controlled pulsed fuel injection into the Vee gutter flame holder.
The frequency and magnitude of the requisite fuel mass flow rate can be systematically varied to achieve this goal.
Towards this, a novel spinning ball valve actuator has been successfully developed for controlled secondary fuel
injection. The valve flow characteristics have been numerically simulated using FLUENT and complemented with
experimental studies.
Introduction
One of the most critical challenges faced in the development of a high performance aero-gas turbine afterburner is
the mitigation of combustion instabilities. These combustion instabilities are believed to originate due to the
Rayleigh coupling between the unsteady heat releases in the presence of transient fluid dynamics and combustor
acoustics. A thorough understanding of the physical processes responsible for generating these dangerous
combustion driven oscillations will help evolve methods to mitigate, if not, even eliminate them. The complex,
nature of combustion instability lies at the root of the difficulties in understanding and the strong non-linearities
deter, at present, the employment of analytical tools.
The range of frequencies of the related large-amplitude pressure oscillations that accompany the combustion
processes is very wide. The frequency of longitudinal modes, known as rumble or buzz, could be as low as 50-100
Hz, whereas the radial or tangential modes, known as screech, could be as high as 5000 Hz. The frequency spectra of
these strong oscillations are known to contain a number of harmonics too. (Ref.1).
Keywords: combustion instability; combustors; pulsed fuel injection; afterburner, unsteady heat release, screech
* Propulsion Division, CSIR-National Aerospace Laboratories, Bangalore 560017, India
#ABES, Gas Turbine Research Establishment, DRDO, Bangalore 560093, India
1
Corresponding author address: Propulsion Division, CSIR-NAL, Bangalore 560017, India
1
E-mail address: rajashekarc@nal.res.in

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A feedback or coupling mechanism is required to tune the processes that drive the combustion instability to the
natural resonant frequencies for acoustic disturbances. The damping or attenuation processes will attempt to balance
the driving processes so as to reach a steady amplitude for the combustion instability. Any unbalance of the driving
over the damping processes will lead to an uncontrolled growth of the dangerous combustion – driven oscillations
unless impeded by powerful acoustic dampers, like Helmholtz resonators. Fuel preparation, flame stabilization and
flame spreading are dominant coupling mechanisms in an afterburner (Ref.1)
In a conventional afterburner, flames are anchored at Vee-gutters, which also cause periodic vortex
shedding. Fuel is supplied in the form of fine two phase sprays from closely coupled fuel injectors located inside and
ahead of the Vee–gutters. Pressure and velocity fluctuations due to flow instabilities, caused by uneven combustion,
can easily affect the fuel injection, atomization and vaporization rates which in turn will cause even more uneven
heat release leading to further levels of flow and combustion instabilities. This undesirable interference in the fine
balance of fuel distribution and fuel spray characteristics will lead to unsteady and spatial unevenness in heat release
when coupled to combustor acoustics along with vortex dominated fields together with feedback. Of all these
driving/coupling processes, vortex shedding from bluff body flameholders, typical of afterburners, is known to be the
dominant driving mechanism for combustion instabilities. The development of coherent structures and the creation of
fine-scale turbulence due to the breakdown of such structures lead to periodic heat release. This, when in phase with
the duct pressure oscillations, can be a strong Rayleigh driver (Ref. 2).
Hence, by varying the afterburner fuel feed pressure and frequency, carefully controlled combustion
instabilities can be deliberately triggered. This will allow screech damping systems, typically based on Helmholtz
resonators, to be optimized under close control. A study of a novel, pulsed fuel injection system, incorporating a
spinning ball valve actuator to trigger, and hence investigate, tuned aero-gas turbine combustion instabilities is
presented.
Experimental
. Fig. 1 shows a schematic of an afterburner pulsed fuel injection system which incorporates a spinning ball
valve actuator. Based on sensed pressure oscillations in the afterburner duct, closed-loop fuel modulation will be
attempted to produce controlled fluctuating heat release to trigger the sought combustion instabilities. Fig. 2 shows
the test set-up. The liquid fuel is fed from a reservoir to the fuel injector through the rotating spindle of a specially
designed ball valve. The spindle has a through hole at right angles to the spindle axis. The spindle is driven by a
variable rpm DC motor. By careful selection of the shaft rpm, through hole diameter, number of holes and spindle
diameter, the fuel feed pressure to the fuel injector can be varied in pulsing frequency and magnitude, together with
the servo controlled throttle valve. In principle, by sensing the duct pressure, a control algorithm could be devised to
actuate the fuel injection pressure in frequency and magnitude. This, in turn, will lead to a periodic fuel flow rate,
which will lead to periodic heat release due to variation in the local fuel/air ratio and fuel spray characteristics. The
present study was restricted to investigate the flow characteristics of the spinning ball valve actuator, using water as a
simulated fuel. The experimental study was used to help complement the associated CFD study.
Computational
In order to help arrive at a design methodology, an unsteady numerical simulation of the liquid fluid flow
inside the spinning valve actuator was successfully carried out. The system was modeled and meshed in ICEM CFD
and the analysis was done with ANSYS FLUENT 14.0. FLUENT and Tecplot 360 were used for post processing.
The numerical simulation was carried out after a domain and grid independence study. A UDF (User Defined
Function) was written for rotating the spindle at the desired RPM.
Discussions
Fig 3 shows the quasi-steady velocity contours and streamlines of the liquid flow through the spinning ball
valve actuator for various spindle angles. As the spindle angle increases, constriction increases at the inlet and exit of
the spindle hole. The phenomenon of band flow, as observed by Ackeret (Ref.3), is clearly seen. Eventhough
separation occurs at the interfaces with associated formation of vortices, there is a band of rather regular flow

3. 3
twisting through the channel. This leads to the question whether this flow could be treated as that passing through an
equivalent sudden contraction followed by a sudden expansion. An analytical framework has been successfully
worked out (Ref.4)
The heat release characteristics surrounding the fuel injector, supplied fuel through the spinning ball valve
actuator, depend on the actual mass flow rate. The spray characteristics also depend inversely on the injected liquid
exit velocity and hence inversely with the mass flow. The heat released is thus adversely affected by poor spray
characteristics, consequent to low mass flow of the liquid jet (Ref. 5). This mass flow rate has been computed for
various spindle positions for quasi-steady conditions (Fig.4). It has been non-dimensionalised by a critical mass flow
rate (ṁCr) which is that which occurs when the spindle hole is in line with the inlet and exit actuator feed tubes. For
the present set-up, the diameter of the spindle hole, inlet and exit feed tubes were chosen to be 7mm and the spindle
diameter 22mm. The analytical (Ref.4) and computed values are seen to be near coincidental and the experimental
results (for the case of an inlet total pressure of 100kPa and outlet of 91kPa which were also the chosen cases for the
analytical study) are also closely matched. All the studies were carried out for spindle angles of 0 to 36° (the limiting
value before the spindle hole is totally blocked). The rest of the (ṁ/ṁCr) curve has been indicated by symmetry.
(ṁ/ṁCr) has a finite non zero computed value even for the blocked case due to leakage around the spindle. With
rotation (200 RPM), there is an apparent partial blockage to the liquid flow through the spinning valve actuator, for
the same applied pressure conditions although the flow features are essentially preserved, particularly the vortex
characteristics (Fig.5). Clearly, the valve design has to be improved upon to permit good through flow, even with
rotation. The pulsing frequency is expected to increase with spindle rpm in order to match and tune high frequency
combustion instabilities present in the afterburner duct.
Fig.6 shows the experimentally measured liquid mass flow rate through the spinning valve actuator with
rotation (200 RPM). The non-dimensional mass flow rate appears to be independent of the applied pressure
differential for eventual discharge to the ambient through a nozzle bank. The general desired pulsating flow
characteristics have been obtained. Further work needs to be done to achieve the deserved pulsating frequency
spectrum. Fig.7 shows the corresponding FFT. There is an initial peak at 6.836 Hz. Since the spindle hole will be
exposed twice per rotation, the expected initial frequency for 200 RPM should be 6.67 Hz which is close to that
observed. Interestingly, there are exact overtones detected at twice and four times the basic frequency. This is a
desirable characteristic as all combustion instabilities are found to have a dominant frequency along with multiple
overtones. Fig. 8 shows high speed time lapsed images of the cyclic variation of the injected pulsed liquid jet from
the injector bank, with rotation. The liquid jet was clearly seen to pulse with two dominant frequencies.
Concluding remarks
The development of a novel, pulsed fuel injection system, incorporating a spinning ball valve actuator, for
controlled triggered aero-gas turbine afterburner combustion instability studies has been successfully carried out. The
complex liquid flow through the spinning ball valve actuator has been numerically simulated together with
complementary experimental studies.

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Acknowledgments
This work has been carried out under the sponsorship of GATET (Gas Turbine Enabling Technologies)
scheme of DRDO, Ministry of Defence, Government of India. The authors thank the Director, CSIR-NAL for
permitting the work to be carried out at CSIR-NAL. The authors also thank Mr Kiruthigaraj D for his technical
support in the instrumentation of the test setup.
References:
1. Zukoski, E.E, “Afterburners’ in “ Aerothermodynamics of aircraft engine components”, Oates, G.C (Ed),
AIAA Education Series, 1985
2. Schadow, K.C and Gutmark E, "Combustion stability related to vortex shedding in dump combustors and
their passive control" , Progress in Energy and Combustion Science, 1992 vol.18, pp 117-132
3. Ackert J, "Aspects of Internal Flow",’ Fluid Mechanics of Internal Flow’, Proc. Sym. Fluid Mechanics
of Internal Flow , G M Research Labs, Warren, Michigan,1965, G Sovran(Ed), Elsevier Publishing, 1967
4. S. Chenthil Kumar, G. Sriram, Shambhoo, H.S. Raghukumar, M. Janaki Rami Reddy, C. Rajashekar, K
Ashirvadam, and J J Isaac, Fluid Flow characteristics in a spinning ball valve actuator, Project Document,
CSIR- National Aerospace Laboratories (To be published)
5. Lefebvre A H., Gas turbine combustion, Hemisphere Pub. Corp. Washington , 1983

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Fig 1. Schematic – Developmental afterburner pulsed fuel injection system for triggering controlled combustion
instabilities

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Fig. 2 : Pulsed fuel injection system

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Fig.3 Quasi-steady velocity contours and streamlines of the liquid flow through the spinning ball valve actuator for
variations in the spindle angle
Fig 4 Variation, with spindle angle, of the quasi-steady liquid mass flow rate through the spinning ball valve
actuator; Inlet total pressure 100 kPa (abs); outlet static pressure 91 kPa (abs)

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Fig 5: Time lapsed velocity contours and streamlines of the liquid flow through the spinning ball valve actuator (200
RPM, Inlet total pressure 100 kPa (abs); outlet static pressure 91 kPa (abs))

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Fig. 6 Variation, with time, of the liquid mass flow through the spinning ball valve actuator at 200 RPM and
discharge to the ambient through an injector bank
Fig. 7 FFT of the mass flow variation characteristics of the liquid flow through the spinning ball valve actuator with
discharge to the ambient through an injector bank; 200 RPM

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Fig. 8 High speed
time lapsed images of
cyclic variation in
injected pulsed liquid
jet characteristics :
discharge to the
ambient through an
injector bank of liquid
supplied from the
spinning ball valve
actuator : 200 RPM
Injector bank
6 port holes, Dia. = 0.5 mm
Disintegrating
pulsed jet array
Injector bank
Inlet