This document discusses the design of an optical diagnostic system using Schlieren photography to visualize flame propagation in a wave rotor combustor. It identifies that a Z-type Schlieren system is suitable due to its low cost and ease of setup. The research team has ordered all components for a Z-type system and built it. Initial tests using a candle flame were successful in capturing Schlieren images. Upcoming experiments will use the system to visualize ignition and flame propagation in a wave rotor ignition rig using different fuels.

1. Acknowledgement
Design of Optical Diagnostic System for Wave Rotor
Constant Volume Combustor
Students: Fatin Baharuddin, Zhen Wei Yong,
Kok Hwang Chow
Mentors: Dr. Mani Rajagopal, Dr. Razi Nalim, Mr. Kyong-Yup Paik
Indiana University-Purdue University Indianapolis
Abstract
Wave Rotor Constant Volume Combustors (WRCVC) are a
growing interest in gas turbine propulsion and power generation
applications. Experiments and numerical studies have been
conducted to study the ignition and flame propagation process for
different hydrocarbon fuels (methane, ethylene and propane).
Currently, hydrogen cannot be used for experiments because it
does not produce soot; consequently, the flame propagation
process cannot be visualized using high speed camera images.
Therefore, the objective of the present research is to design an
optical system to visualize the flame propagation process in a
WRC for many fuels including hydrogen. Among various optical
techniques available for flow visualization, Schlieren photography
is commonly utilized for flows with sharp density gradients. Based
on the configuration, there are different types of Schlieren
imaging system such as Z-type, parallel beam and single mirror
system. It was identified that Z-type Schlieren system meets all
our requirements; suitable for present study, low cost, easy to set-
up. Experiments will be conducted using different fuels to
visualize the flame propagation process inside the wave rotor
combustor.
Methods
Introduction
 It is identified from the present research that Z-type Schlieren
system is suitable for the wave rotor ignition rig experiments.
 Z-type Schlieren system (Fig. 3) consist of two concave
mirrors, light source, mirror holder and knife edge.
 The light source is carefully adjusted no more than 10 degree
respect to the horizontal line to avoid the image distortion.
 Light source is directed at mirror 1 and is located one focal
length from the mirror.
 The first mirror collimates the light, which produces parallel
rays of light in the test region. A second mirror then focuses
the collimated light towards the camera and filter.
 Knife edge is placed at a focal point to block half of the light.
Adjusting height of the knife edge will increase the contrast of
the image.
The recent energy price hike brings the urge to produce more
efficient power generation engines, which could result in lower
fuel consumption, higher efficiency, and less pollutants. Such
an approach is being developed at IUPUI with the participation
of global engine maker Rolls Royce, utilizing a alternate
thermodynamic cycle (Humphrey cycle) and a novel pulsed
combustor for gas turbine engines, called WRCVC. The
performance of a WRCVC is presented in Temperature-entropy
diagram in Fig. 1.
References
1. Amrita, M. (2013, June 18th). PRINCIPLES AND TECHNIQUES OF SCHLIEREN IMAGING SYSTEMS. Columbia University New York, NY
2. Denise, G. (2008, October 27th). The Mysterious Cough, Caught on Film. the New York Times. Retrieved from
http://www.nytimes.com/2008/10/28/science/28cough.html?_r=1&
3. Sean Michael, R. (2009, July 26). How-To: Take Schlieren photographs at home. Makezine. Retrieved from
http://makezine.com/2009/07/26/how-to-take-schlieren-photographs-a/
4. P. Akbari , M. R. N. (2009, February). Recent Developments in Wave Rotor Combustion Technology and Future Perspectives: A Progress
Review. Indiana University-Purdue University Indianapolis (IUPUI).
5. Prasanna , C., Abdullah , K., Manikanda , R., & Razi, N. (2013, May). Experimental Study of Transversing Hot-Jet Ignition of Lean
Hydrocarbon-Air Mixtures in a Constant-Volume Combustor. Indiana University-Purdue University Indianapolis (IUPUI).
6. Ozasa, T., Kozuka, K., and Fujikawa, T., "Schlieren Observations of In-Cylinder Phenomena Concerning a Direct-Injection Gasoline
Engine," SAE Technical Paper 982696, 1998, doi:10.4271/982696.
7. Salazar, V. and Kaiser, S., "Influence of the Flow Field on Flame Propagation in a Hydrogen-Fueled Internal Combustion Engine," SAE Int.
J. Engines 4(2):2376-2394, 2011, doi:10.4271/2011-24-0098..
Fig. 2 Experimental set-up of Wave Rotor Ignition Rig at IUPUI
Fig. 5 Schlieren images captured for candle flame
The wave rotor ignition rig (Fig. 2) established at IUPUI consists
of two combustion chambers (a rotating pre-chamber, and a
stationary main chamber), electrical and ignition systems, high
speed camera and data acquisition system. Experiments and
computer simulations of combustion in the rig have been
performed to understand the jet penetration and mixing, vortex
structures, ignition kernels, propagating turbulent flames,
pressure waves, and flame-wave interactions. However, much of
the modern knowledge about conventional combustion processes
have come from relatively recent optical and laser diagnostics,
such as laser-induced fluorescence and Raman spectroscopy. It
is expected that optical diagnostics will illuminate the design of
improved combustion systems to meet the challenges of
tomorrow’s energy conversion systems. The present study is
aimed to design an optical diagnostic system to visualize the
combustion process inside the wave rotor ignition rig. The high
speed camera is employed for the visualization of ignition and
flame propagation, but it does not capture the mixing and
turbulence processes. Moreover, the optical system technique will
be useful to visualize to ignition process for hydrogen fuel which
does not produce soot.
Current Status of
Research
From the present research, we have identified that Z-type
Schlieren system meets all our requirements; suitable for this
study, low cost, easy to set-up. However, such imaging in a
closed channel is challenging, and will require special mirrors
and optical techniques. All the components required to design Z-
type Schlieren system have been finalized, ordered and all the
components have been received. The optical diagnostic system
has been built (Fig. 4). Initially, Schlieren system has been tested
using a simple combustion light source such as candle flame.
The images captured using candle flame is shown in Fig. 5.
Further, experiments will be conducted to capture & visualize the
flame and ignition process in the wave rotor ignition rig.
Tturbine
Entropy
Temperature
2
3 b
4 b
1
4
3
1-2-3-4: Humphrey Cycle
(Constant-Volume Combustion)
with Pressure Gain
1-2-3b-4b: Brayton Cycle
Combustor
Compressor Turbine
1
2 3
4
Fig. 1 Performance of the Humphrey cycle over Brayton cycle
Fig. 3 Schematic of the proposed Z-type Schlieren System [1]
Fig. 4 Set-up of the Schlieren system designed at IUPUI
Mirror 1Light Source
Knife Edge
Mirror 2
Candle
Camera
We would like to acknowledge Multi-disciplinary Research Institute (MURI) for
sponsoring this research and also, Mr. Joe Huerkamp (MET Lab Technician) for his
support in designing the mirror holders.