- Email: [email protected]
Laser-induced plasma ignition studies in a model scramjet engine
Combustion and Flame 160 (2013) 145–148
Contents lists available at SciVerse ScienceDirect
Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Laser-induced plasma ignition studies in a model scramjet engine Stefan Brieschenk a,⇑, Sean O’Byrne b,1, Harald Kleine b a b
Centre for Hypersonics, The University of Queensland, Brisbane, QLD 4072, Australia School of Engineering and Information Technology, The University of New South Wales, The Australian Defence Force Academy, Canberra, ACT 2600, Australia
a r t i c l e
i n f o
Article history: Received 16 July 2012 Received in revised form 26 August 2012 Accepted 27 August 2012 Available online 1 October 2012 Keywords: Hypersonic flow Supersonic combustion Planar laser-induced fluorescence imaging Plasma-assisted combustion Laser-induced ignition Laser-induced plasma
a b s t r a c t An experimental investigation of the behavior of laser-induced plasma (LIP) ignition for scramjet inlet injection is presented. The presented results demonstrate for the first time, that LIP can be used to promote the formation of hydroxyl in a hypersonic flow. The temporal evolution of the LIP-ignited region is monitored using the planar laser-induced fluorescence technique on the hydroxyl radical. This study is the first laser spark study in a hypersonic flow, shown to generate combustion products where they would not otherwise occur. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Providing ignition in supersonic flows using traditional flame holders relies on the principle of establishing stable, subsonic combustion zones that ignite the adjacent supersonic stream. Introducing injector struts or bluff bodies into the supersonic combustor flow for flame holding results in high entropy increases and can cause the engine to unstart [1]. Wall cavity flame holders minimize the introduced entropy rise but may only provide ignition above the cavity, rather than anchoring the flame at the cavity [2]. For this reason, cavity type flame holders work efficiently at sonic and supersonic speeds [3,4], but are less effective at hypersonic speeds. This study examines the applicability of providing ignition in a hypersonic flow using laser-induced plasma (LIP). In terms of electromagnetic bandwidth, excitation using lasers presents the possibility of a more controlled method of energy deposition than thermal excitation using discharge-based methods. LIP ignition has been demonstrated successfully in controlled, laboratory environments using relatively high gas pressures and well mixed, lowspeed flows [5,6]. This study aims to answer the question of whether or not a non-resonant LIP can provide ignition in a hypersonic, turbulent non-premixed flow with conditions comparable to
⇑ Corresponding author. E-mail addresses: [email protected] (S. Brieschenk), [email protected] (S. O’Byrne), [email protected] (H. Kleine). 1 Principle corresponding author.
those experienced by a scramjet-powered vehicle. Potential difficulties are: Spark ignition in turbulent flows has a stochastic nature and the probability of establishing a flame depends on the local strain rate [7]. The strain rates that are present in the interaction region between a sonic jet and a hypersonic crossflow are high and can exceed flame extinction strain rates. The fuel and air streams in a scramjet engine operating a high Mach number are poorly mixed, and engine efficiency is typically determined by the quality of fuel–air mixing. The residence time for atmospheric air ingested into a scramjet inlet and exiting from the engine nozzle is of the order of one millisecond and the injected fuel must mix with the air stream within tens of microseconds [8]. The flow in a scramjet-powered engine is of low density and high velocity. The gas density at an altitude of 30 km is roughly 1/70 the density at sea level, while the pressure is roughly 1/90 of the atmospheric pressure at sea level. The laser-absorption threshold is a function of gas pressure implying potential difficulties in applying LIP ignition at low pressures. Ignition delay and reaction times scale with gas pressure and the ignition delay time for a stoichiometric hydrogen-air mixture at a pressure of 1/90 atm and a temperature of 1500 K, which is twice the autoignition temperature at that pressure [9,10], is of the order of 1 ms [11]. This translates into an ignition delay length of the order of 2 m for a flow speed of 2000 m/s. This example calculation clearly demonstrates the great difficulties that exist for LIP ignition to be effective in such high-speed flows.
0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2012.08.011
146
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
Fig. 1. Schlieren image of flowfield on the compression ramp; flow features at their approximate locations are superimposed as a line drawing.
Fig. 2. Luminosity in the shear-layer LIP ignition experiment.
There are currently no experimental data available in the literature regarding the effect of laser energy deposition on ignition and combustion in a hypersonic, turbulent non-premixed flow. Laser ignition has the potential to fill a gap in the methods of igniting a hypersonic flow that other methods cannot adequately address. This warrants a fundamental study to investigate whether or not a LIP can generate combustion radicals in a hypersonic flow where autoignition does not occur. 2. Experimental arrangement, results and discussion The LIP ignition experiments presented in this paper were conducted in the T-ADFA free piston shock tunnel using a flow condition with a total specific enthalpy of 2.7 MJ/kg and a freestream velocity of 2075 m/s. The scramjet model features a rectangular duct with a 9° compression ramp, followed by a constant-area
combustor. Hydrogen is injected through four 2-mm-diameter holes located on the compression ramp of the model, which are distributed across the compression ramp, 120 mm downstream of the leading edge. The LIP is formed in the shear layers 1.7 mm downstream of the injectors. The laser accesses the flow through a 3-mm-diameter open port located mid-way between the two inner injector holes. The LIP is formed by a Q-switched ruby laser, focussed using a 100-mm-focal-length lens. Pulse energies of 750 mJ are used in this experiment. A significant portion of the laser energy is transmitted through the focal region, and is unable to contribute in the plasma formation process. A series of experiments in a gas cell have shown that 54% of the laser energy is absorbed by the LIP in the scramjet experiment The flowfield on the compression ramp is depicted in Fig. 1. The underexpanded hydrogen fuel jet expands into the crossflow forming a barrel shock (7) which separates the upstream
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
147
laser initiation, plasma formation is initiated at the focal point of the laser-focussing lens, located 8 mm above the ramp surface. During the laser pulse, the plasma quickly grows towards the laser source within the laser beam. This results in an initial plasma bubble which is typically ellipsoidal in shape, measuring about 6.5 mm in height and 2 mm in diameter (Fig. 1). Plasma is luminous due to plasma recombination resulting in continuum radiation, and the evolution of the LIP luminosity is shown in Fig. 2. The presented images are obtained at 1 Mfps with 500 ns integration time for each image. Several sequences were acquired during the experimental campaign. With all sequences being identical, it is concluded that despite the turbulence in the flow mid-way between the two injectors, the random elements involved in the LIP formation process and the stochastic character of spark-type ignition [7], the phenomenon is repeatable. A shock wave is generated by the LIP which travels ahead of the luminosity cloud. The luminosity breaks up into two separate regions, traveling along the shear layer behind the injector bow shock. The size of the luminosity cloud is approximately 10 mm in the horizontal direction at 1 ls delay. At 1 ls delay, the luminosity from the spark and shock wave spans up to 3 mm upstream and 8 mm downstream from the focal waist, corresponding to a bulk motion of around 2000 ms 1 which is equal to the flow speed on the compression ramp upstream of the injection ports. The OH PLIF technique is employed to yield combustiondiagnostic data for delay times >12 ls. For shorter delay times, the strong self-luminosity restricts the use of the OH PLIF technique since plasma continuum radiation cannot be completely spectrally filtered from the fluorescence signal. Secondary experiments, where no laser sheet was introduced into the test section were conducted to ensure that the signal recorded with the PLIF system is indeed OH fluorescence and not luminosity arising due to plasma recombination or radiative de-excitation. The evolution of the OH signal is shown in Fig. 3. For each delay time, several images were recorded in different shock tunnel runs. The LIP generates OH which is recorded downstream of the injectors. The flame stretches from the upper to the lower boundaries of the fuel jet, which in this setup is the entire combustor height. The OH signal decays with distance, but measurable concentrations exist in the flow from the point of plasma formation to the combustor entrance. Using the observed flame width at a delay time of 17 ls and calculated post-shock velocity on the compression ramp, the period between individual laser pulses required to form a continuous stream of OH is 10 ls, translating to laser pulse frequency requirements of the order of 100 kHz. 3. Conclusions These experiments have demonstrated, for the first time, that a LIP can be used to promote hydroxyl formation in a hypersonic flow. The LIP generates hydroxyl downstream of the fuel injectors with the same vertical extension as the fuel jet. The laser pulse repetition frequency required to form a continuous stream of hydroxyl is estimated to be 100 kHz. Acknowledgments
Fig. 3. Fluorescence of the OH radical in the LIP ignition experiment. One image is obtained in each experimental run with different delay timings.
boundary layer (2, 3) creating recirculation zones in the front of the jet (4). A recirculation zone (8) forms in the wake of the jet, with a weak recompression shock (9) forming further downstream. Upon
The authors gratefully acknowledge the mechanical workshop at UNSW Canberra as well as the outstanding technical and scramjet design support received from Paul Walsh and Gianfranco Foppoli. References [1] S. O’Byrne, M. Doolan, S.R. Olsen, A.F.P. Houwing, Journal of Propulsion and Power 16 (2000) 808–814.
148
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
[2] S. O’Byrne, I. Stotz, A.J. Neely, R. Boyce, N.R. Mudford, A.F.P. Houwing, OH PLIF imaging of supersonic combustion using cavity injection, in: 13th AIAA/CIRA International Space Planes and Hypersonics Systems and Technologies Conference, 2005. [3] C.G. Rodriguez, CFD analysis of the CIAM/NASA scramjet, in: 38th AIAA/ASME/ SAE/ASEE Joint Propulsion Conference and Exhibit, Indianapolis, USA, 2002. [4] A. Ben-Yakar, R.K. Hanson, Journal of Propulsion and Power 17 (2001) 869– 877. [5] S. Barbosa, P. Scouflaire, S. Ducruix, G. Gaborel, Comparisons between spark plug and laser ignition in a gas turbine combustor, in: 3rd European Combustion Meeting, Chania, Crete, 2007.
[6] J.X. Ma, D.R. Alexander, D.E. Poulain, Combustion and Flame 112 (1998) 492– 506. [7] E. Mastorakos, Progress in Energy and Combustion Science 35 (2009) 57–97. [8] J.P. Drummond, S.D. Glenn, A.D. Cutler, Chapter 6: fuel–air mixing and combustion in scramjets, in: Technologies for Propelled Hypersonic Flight, NATO-RTO-AVT, National Technical Information Service, Virginia, USA, 2006. [9] U. Maas, J. Warnatz, Combustion and Flame 74 (1988) 53–69. [10] C.J. Sung, J.G. Li, G. Yu, C.K. Law, AIAA Journal 37 (1999) 208–214. [11] M.B. Colket, L.J. Spadaccini, Journal of Propulsion and Power 17 (2001) 315– 323.
Contents lists available at SciVerse ScienceDirect
Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Laser-induced plasma ignition studies in a model scramjet engine Stefan Brieschenk a,⇑, Sean O’Byrne b,1, Harald Kleine b a b
Centre for Hypersonics, The University of Queensland, Brisbane, QLD 4072, Australia School of Engineering and Information Technology, The University of New South Wales, The Australian Defence Force Academy, Canberra, ACT 2600, Australia
a r t i c l e
i n f o
Article history: Received 16 July 2012 Received in revised form 26 August 2012 Accepted 27 August 2012 Available online 1 October 2012 Keywords: Hypersonic flow Supersonic combustion Planar laser-induced fluorescence imaging Plasma-assisted combustion Laser-induced ignition Laser-induced plasma
a b s t r a c t An experimental investigation of the behavior of laser-induced plasma (LIP) ignition for scramjet inlet injection is presented. The presented results demonstrate for the first time, that LIP can be used to promote the formation of hydroxyl in a hypersonic flow. The temporal evolution of the LIP-ignited region is monitored using the planar laser-induced fluorescence technique on the hydroxyl radical. This study is the first laser spark study in a hypersonic flow, shown to generate combustion products where they would not otherwise occur. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Providing ignition in supersonic flows using traditional flame holders relies on the principle of establishing stable, subsonic combustion zones that ignite the adjacent supersonic stream. Introducing injector struts or bluff bodies into the supersonic combustor flow for flame holding results in high entropy increases and can cause the engine to unstart [1]. Wall cavity flame holders minimize the introduced entropy rise but may only provide ignition above the cavity, rather than anchoring the flame at the cavity [2]. For this reason, cavity type flame holders work efficiently at sonic and supersonic speeds [3,4], but are less effective at hypersonic speeds. This study examines the applicability of providing ignition in a hypersonic flow using laser-induced plasma (LIP). In terms of electromagnetic bandwidth, excitation using lasers presents the possibility of a more controlled method of energy deposition than thermal excitation using discharge-based methods. LIP ignition has been demonstrated successfully in controlled, laboratory environments using relatively high gas pressures and well mixed, lowspeed flows [5,6]. This study aims to answer the question of whether or not a non-resonant LIP can provide ignition in a hypersonic, turbulent non-premixed flow with conditions comparable to
⇑ Corresponding author. E-mail addresses: [email protected] (S. Brieschenk), [email protected] (S. O’Byrne), [email protected] (H. Kleine). 1 Principle corresponding author.
those experienced by a scramjet-powered vehicle. Potential difficulties are: Spark ignition in turbulent flows has a stochastic nature and the probability of establishing a flame depends on the local strain rate [7]. The strain rates that are present in the interaction region between a sonic jet and a hypersonic crossflow are high and can exceed flame extinction strain rates. The fuel and air streams in a scramjet engine operating a high Mach number are poorly mixed, and engine efficiency is typically determined by the quality of fuel–air mixing. The residence time for atmospheric air ingested into a scramjet inlet and exiting from the engine nozzle is of the order of one millisecond and the injected fuel must mix with the air stream within tens of microseconds [8]. The flow in a scramjet-powered engine is of low density and high velocity. The gas density at an altitude of 30 km is roughly 1/70 the density at sea level, while the pressure is roughly 1/90 of the atmospheric pressure at sea level. The laser-absorption threshold is a function of gas pressure implying potential difficulties in applying LIP ignition at low pressures. Ignition delay and reaction times scale with gas pressure and the ignition delay time for a stoichiometric hydrogen-air mixture at a pressure of 1/90 atm and a temperature of 1500 K, which is twice the autoignition temperature at that pressure [9,10], is of the order of 1 ms [11]. This translates into an ignition delay length of the order of 2 m for a flow speed of 2000 m/s. This example calculation clearly demonstrates the great difficulties that exist for LIP ignition to be effective in such high-speed flows.
0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2012.08.011
146
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
Fig. 1. Schlieren image of flowfield on the compression ramp; flow features at their approximate locations are superimposed as a line drawing.
Fig. 2. Luminosity in the shear-layer LIP ignition experiment.
There are currently no experimental data available in the literature regarding the effect of laser energy deposition on ignition and combustion in a hypersonic, turbulent non-premixed flow. Laser ignition has the potential to fill a gap in the methods of igniting a hypersonic flow that other methods cannot adequately address. This warrants a fundamental study to investigate whether or not a LIP can generate combustion radicals in a hypersonic flow where autoignition does not occur. 2. Experimental arrangement, results and discussion The LIP ignition experiments presented in this paper were conducted in the T-ADFA free piston shock tunnel using a flow condition with a total specific enthalpy of 2.7 MJ/kg and a freestream velocity of 2075 m/s. The scramjet model features a rectangular duct with a 9° compression ramp, followed by a constant-area
combustor. Hydrogen is injected through four 2-mm-diameter holes located on the compression ramp of the model, which are distributed across the compression ramp, 120 mm downstream of the leading edge. The LIP is formed in the shear layers 1.7 mm downstream of the injectors. The laser accesses the flow through a 3-mm-diameter open port located mid-way between the two inner injector holes. The LIP is formed by a Q-switched ruby laser, focussed using a 100-mm-focal-length lens. Pulse energies of 750 mJ are used in this experiment. A significant portion of the laser energy is transmitted through the focal region, and is unable to contribute in the plasma formation process. A series of experiments in a gas cell have shown that 54% of the laser energy is absorbed by the LIP in the scramjet experiment The flowfield on the compression ramp is depicted in Fig. 1. The underexpanded hydrogen fuel jet expands into the crossflow forming a barrel shock (7) which separates the upstream
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
147
laser initiation, plasma formation is initiated at the focal point of the laser-focussing lens, located 8 mm above the ramp surface. During the laser pulse, the plasma quickly grows towards the laser source within the laser beam. This results in an initial plasma bubble which is typically ellipsoidal in shape, measuring about 6.5 mm in height and 2 mm in diameter (Fig. 1). Plasma is luminous due to plasma recombination resulting in continuum radiation, and the evolution of the LIP luminosity is shown in Fig. 2. The presented images are obtained at 1 Mfps with 500 ns integration time for each image. Several sequences were acquired during the experimental campaign. With all sequences being identical, it is concluded that despite the turbulence in the flow mid-way between the two injectors, the random elements involved in the LIP formation process and the stochastic character of spark-type ignition [7], the phenomenon is repeatable. A shock wave is generated by the LIP which travels ahead of the luminosity cloud. The luminosity breaks up into two separate regions, traveling along the shear layer behind the injector bow shock. The size of the luminosity cloud is approximately 10 mm in the horizontal direction at 1 ls delay. At 1 ls delay, the luminosity from the spark and shock wave spans up to 3 mm upstream and 8 mm downstream from the focal waist, corresponding to a bulk motion of around 2000 ms 1 which is equal to the flow speed on the compression ramp upstream of the injection ports. The OH PLIF technique is employed to yield combustiondiagnostic data for delay times >12 ls. For shorter delay times, the strong self-luminosity restricts the use of the OH PLIF technique since plasma continuum radiation cannot be completely spectrally filtered from the fluorescence signal. Secondary experiments, where no laser sheet was introduced into the test section were conducted to ensure that the signal recorded with the PLIF system is indeed OH fluorescence and not luminosity arising due to plasma recombination or radiative de-excitation. The evolution of the OH signal is shown in Fig. 3. For each delay time, several images were recorded in different shock tunnel runs. The LIP generates OH which is recorded downstream of the injectors. The flame stretches from the upper to the lower boundaries of the fuel jet, which in this setup is the entire combustor height. The OH signal decays with distance, but measurable concentrations exist in the flow from the point of plasma formation to the combustor entrance. Using the observed flame width at a delay time of 17 ls and calculated post-shock velocity on the compression ramp, the period between individual laser pulses required to form a continuous stream of OH is 10 ls, translating to laser pulse frequency requirements of the order of 100 kHz. 3. Conclusions These experiments have demonstrated, for the first time, that a LIP can be used to promote hydroxyl formation in a hypersonic flow. The LIP generates hydroxyl downstream of the fuel injectors with the same vertical extension as the fuel jet. The laser pulse repetition frequency required to form a continuous stream of hydroxyl is estimated to be 100 kHz. Acknowledgments
Fig. 3. Fluorescence of the OH radical in the LIP ignition experiment. One image is obtained in each experimental run with different delay timings.
boundary layer (2, 3) creating recirculation zones in the front of the jet (4). A recirculation zone (8) forms in the wake of the jet, with a weak recompression shock (9) forming further downstream. Upon
The authors gratefully acknowledge the mechanical workshop at UNSW Canberra as well as the outstanding technical and scramjet design support received from Paul Walsh and Gianfranco Foppoli. References [1] S. O’Byrne, M. Doolan, S.R. Olsen, A.F.P. Houwing, Journal of Propulsion and Power 16 (2000) 808–814.
148
S. Brieschenk et al. / Combustion and Flame 160 (2013) 145–148
[2] S. O’Byrne, I. Stotz, A.J. Neely, R. Boyce, N.R. Mudford, A.F.P. Houwing, OH PLIF imaging of supersonic combustion using cavity injection, in: 13th AIAA/CIRA International Space Planes and Hypersonics Systems and Technologies Conference, 2005. [3] C.G. Rodriguez, CFD analysis of the CIAM/NASA scramjet, in: 38th AIAA/ASME/ SAE/ASEE Joint Propulsion Conference and Exhibit, Indianapolis, USA, 2002. [4] A. Ben-Yakar, R.K. Hanson, Journal of Propulsion and Power 17 (2001) 869– 877. [5] S. Barbosa, P. Scouflaire, S. Ducruix, G. Gaborel, Comparisons between spark plug and laser ignition in a gas turbine combustor, in: 3rd European Combustion Meeting, Chania, Crete, 2007.
[6] J.X. Ma, D.R. Alexander, D.E. Poulain, Combustion and Flame 112 (1998) 492– 506. [7] E. Mastorakos, Progress in Energy and Combustion Science 35 (2009) 57–97. [8] J.P. Drummond, S.D. Glenn, A.D. Cutler, Chapter 6: fuel–air mixing and combustion in scramjets, in: Technologies for Propelled Hypersonic Flight, NATO-RTO-AVT, National Technical Information Service, Virginia, USA, 2006. [9] U. Maas, J. Warnatz, Combustion and Flame 74 (1988) 53–69. [10] C.J. Sung, J.G. Li, G. Yu, C.K. Law, AIAA Journal 37 (1999) 208–214. [11] M.B. Colket, L.J. Spadaccini, Journal of Propulsion and Power 17 (2001) 315– 323.