@article{garcia_kim_vinod_sahoo_wax_kim_fang_narayanaswamy_wu_jiang_2024, title={Carbon nanofibers/liquid metal composites for high temperature laser ultrasound}, volume={138}, ISSN={["1874-9968"]}, url={https://doi.org/10.1016/j.ultras.2024.107245}, DOI={10.1016/j.ultras.2024.107245}, abstractNote={As the demand for clean energy becomes greater worldwide, there will also be an increasing demand for next generation nuclear power plants that incorporate advanced sensors and monitoring equipment. A major challenge posed by nuclear power plants is that, during normal operation, the reactor compartment is subjected to high operating temperatures and radiation flux. Diagnostic sensors monitoring such structures are also subject to temperatures reaching hundreds of degrees Celsius, which puts them at risk for heat degradation. In this work, the ability of carbon nanofibers to work in conjunction with a liquid metal as a photoacoustic transmitter was demonstrated at high temperatures. Fields metal, a Bi-In-Sn eutectic, and gallium are compared as acoustic mediums. Fields metal was shown experimentally to have superior performance over gallium and other reference cases. Under stimulation from a low fluence 6 ns pulse laser at 6 mJ/cm2 with 532 nm green light, the Fields metal transducer transmitted a 200 kHz longitudinal wave with amplitude >5.5 times that generated by a gallium transducer at 300 °C. Each high temperature test was conducted from a hot to cold progression, beginning as high as 300 °C, and then cooling down to 100 °C. Each test shows increasing signal amplitude of the liquid metal transducers as temperature decreases. Carbon nanofibers show a strong improvement over previously used candle-soot nanoparticles in both their ability to produce strong acoustic signals and absorb higher laser fluences up to 12 mJ/cm2.}, journal={ULTRASONICS}, author={Garcia, Nicholas and Kim, Howuk and Vinod, Kaushik and Sahoo, Abinash and Wax, Michael and Kim, Taeyang and Fang, Tiegang and Narayanaswamy, Venkat and Wu, Huaiyu and Jiang, Xiaoning}, year={2024}, month={Mar} }
 @article{chaudhury_sahoo_vinod_fisher_ekkad_narayanaswamy_fang_2024, title={Characteristics of Premixed Ammonia/Methane/Air Blends as an Alternative Fuel in a Swirl-Stabilized Gas Turbine Combustor Under Varying Pilot Percentage}, volume={146}, ISSN={["1528-8919"]}, DOI={10.1115/1.4065923}, abstractNote={Abstract Alternative low carbon fuel blends are a promising way towards clean energy transition in the transportation and power generation sectors. In this work, the objective was to study the combustion characteristics of one such low carbon fuel blend (premixed Ammonia, Methane and Air) in a swirl stabilized Gas Turbine Can Combustor under varying % of pilot fuel flow (= 8 % to 10 % of the main fuel flow rate) at atmospheric pressure conditions. Pure Methane was used as the pilot flame which helped in the ignition and stabilization of the main flame and was kept on throughout the experiment. Different volume % of Ammonia and Methane blends were analyzed (starting from 10 to 50 % Ammonia in the fuel blend and the rest being Methane) at Reynolds number of the incoming air ~ 50000, and at equivalence ratio = 0.6 and 0.7. Characteristics such as Combustor liner wall heat load and flame stability were studied using the Infrared Thermography technique and High-Speed flame imaging respectively. Additionally, both carbon and NOx emission trends were estimated for selected cases using the CONVERGE CFD software under steady state conditions incorporating the RANS RNG k-ε and SAGE modeling techniques. Among all cases, wall heat load was observed to be the least for the 50 % Ammonia-50 % Methane case and for cases under reduced pilot %. Also, under reduced pilot %, flames were mostly unstable wherein the manifestation of instabilities at equivalence ratio = 0.6 and 0.7 were markedly different from one another.}, number={11}, journal={JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME}, author={Chaudhury, Meghna Das and Sahoo, Abinash and Vinod, Kaushik Nonavinakere and Fisher, Wesley and Ekkad, Srinath V. and Narayanaswamy, Venkateswaran and Fang, Tiegang}, year={2024}, month={Nov} }
 @article{ramachandran_sahoo_narayanaswamy_lyons_2024, title={Dynamics of regime transition of autoignitive jet flames from conventional to MILD combustion}, volume={19}, ISSN={["2666-352X"]}, DOI={10.1016/j.jaecs.2024.100277}, abstractNote={Turbulent combustion of jet flames in a hot diluted coflow of combustion products has been studied across a range of jet Reynolds numbers for propane, ethylene, and an ethylene-propane blend as fuels. The study revealed a transition from conventional autoignitive combustion to a regime of Moderate or Intense Low-oxygen Dilution (MILD) combustion, which is characterized by a nearly invisible flame. The flames studied are luminous at low jet velocities and become MILD at higher jet velocities. Planar Laser-Induced Fluorescence (PLIF) of formaldehyde (CH2O), a key intermediate species in hydrocarbon combustion, is combined with CH*-chemiluminescence imaging and Rayleigh scattering to investigate the phenomena. The transition to MILD combustion is found to accompany a broadening of the formaldehyde region, indicating a broader low-temperature reaction zone. As the transition is approached, the appearance of holes in the chemiluminescent front is also observed. Correspondence between the location and structure of these holes with regions of formaldehyde in the flame are illustrated. These regions are proposed to be signatures that precede the transition to a fully MILD flame. In other words, the transition to MILD combustion begins at a local level and the MILD region spreads to engulf the entire combusting mixture. Scalar gradients were imaged to further unravel the local turbulence/chemistry interactions that yield the MILD inception. The measured scalar gradients were found to be commensurate with scalar gradients obtained from chemical kinetic simulations at the stoichiometric mixture fractions, while being an order of magnitude larger at other locations; this suggests that the inception of MILD combustion occurs near the stoichiometric region.}, journal={APPLICATIONS IN ENERGY AND COMBUSTION SCIENCE}, author={Ramachandran, Aravind and Sahoo, Abinash and Narayanaswamy, Venkateswaran and Lyons, Kevin M.}, year={2024}, month={Sep} }
 @article{sahoo_narayanaswamy_lyons_2023, title={Quenching measurements of Kr 5p[3/2](2) <- <- 4p(61)S(0) electronic transition using absorption spectra}, volume={62}, ISSN={["2155-3165"]}, url={https://doi.org/10.1364/AO.475382}, DOI={10.1364/AO.475382}, abstractNote={Quenching rate is an important parameter to include in fluorescence
					measurements that are aimed at quantifying the thermochemical field of
					a reacting flow. Traditionally, the quenching measurements were
					obtained at low pressures using the direct measurements of quenching
					times followed by a linear scaling to the desired pressure. This
					approach, however, cannot account for the possible deviation from the
					linear pressure scaling at elevated pressures due to three and
					multi-body collisions. Furthermore, the best accuracy on the quenching
					rate is obtained with ultra-short pulse lasers that are typically not
					readily available. This study offsets these limitations by
					demonstrating a new approach for making direct quenching measurements
					at atmospheric conditions and using nanosecond lasers. The quenching
					measurements are demonstrated in a krypton-perturber system, and the 
      
	5
	p
	
	  
	    
	      [
	    
	    
	      3
	      2
	    
	    
	      ]
	    
	  
	  2
	
	←←
	4
	
	  
	    p
	    6
	  
	
	
	  
	    
	    1
	  
	
	
	  
	    S
	    0
	  
	
      
     two-photon electronic transition is
					accessed. A theoretical construct is presented that relates the
					absorption spectral parameters and the integrated fluorescence signal
					to the quenching rate, referenced to a given species and conditions.
					Using this formulation, the relative quenching rates for different
					perturber species, namely, air, 
      
	
	  
	    
	      C
	      H
	    
	    4
	  
	
      
    , 
      
	
	  
	    
	      C
	    
	    2
	  
	
	
	  
	    
	      H
	    
	    4
	  
	
      
    , and 
      
	
	  
	    
	      C
	      O
	    
	    2
	  
	
      
    , are reported as measured at 1 atm
					and 300 K. As such, the present technique is limited to the
					measurement of the relative quenching rate, unlike the previous
					studies where absolute quenching rates are measured. Nonetheless, when
					the reference quenching rate is independently measured, the relative
					quenching rates can be converted to absolute values.}, number={6}, journal={APPLIED OPTICS}, publisher={Optica Publishing Group}, author={Sahoo, Abinash and Narayanaswamy, Venkateswaran and Lyons, Kevin M.}, year={2023}, month={Feb}, pages={110–117} }
 @article{sahoo_ramachandran_narayanaswamy_lyons_2022, title={Mixture fraction measurement in turbulent non-premixed MILD jet flame using Rayleigh scattering}, volume={61}, url={https://doi.org/10.1364/AO.444109}, DOI={10.1364/AO.444109}, abstractNote={Turbulent combustion of jet flames in a hot diluted coflow of
					combustion products is conducive to the transition from conventional
					flamelet combustion to a regime of moderate or intense low oxygen
					dilution (MILD) combustion, which is commonly characterized by a very
					low emission and noise. MILD combustion is also characterized by
					distributed combustion where the net heat release is positive across
					the entire combustion domain. The turbulence/chemistry interactions in
					this regime that determine the flame structure, local temperature, and
					species distribution critically depend on the mixture fraction and
					scalar dissipation fields. However, there are no experimental tools to
					measure the mixture fraction field in a distributed (MILD) combustion
					regime. The present work offsets this limitation by demonstrating a
					Rayleigh scattering-based approach to measure mixture fraction in a
					turbulent ethylene MILD combustion zone. 1D counterflow flame
					simulations enabled mapping the locally calibrated Rayleigh scattering
					fields to mixture fractions in the fuel-rich regions. This approach
					also shows very low sensitivity to the local temperature and
					composition. Overall, the results provide compelling evidence that the
					distributed heat release does not significantly impact the turbulent
					processes of the flow-field for the conditions examined. The
					measurement uncertainty from this approach and its extension to more
					complex fuels are also discussed. The present technique is limited to
					mildly turbulent, fully MILD/distributed flame with representative
					scalar dissipation rates.}, number={9}, journal={Applied Optics}, author={Sahoo, Abinash and Ramachandran, Aravind and Narayanaswamy, Venkateswaran and Lyons, Kevin M.}, year={2022}, month={Mar}, pages={2338–2351} }
 @article{sahoo_zelenak_narayanaswamy_2020, title={Pressure scaling of the collisional broadening parameters of Kr 4p(6)S(0)(1) ->-> 5p [3/2](2) transition}, volume={59}, ISSN={["2155-3165"]}, url={https://doi.org/10.1364/AO.394932}, DOI={10.1364/AO.394932}, abstractNote={Pressure scaling of collisional broadening parameters of krypton
					(absorber) 
      
	4
	
	  
	    p
	    6
	  
	
	
	  S
	  0
	  1
	
	→→
	5
	p
	
	  [
	  3
	  
	    /
	  
	  2
	  
	    ]
	    2
	  
	
      
     transition centered at 107.3 nm in
					the presence of nitrogen 
      
	
	  
	    
	      N
	    
	    2
	  
	
      
     (perturber) is investigated. The
					absorption spectrum in the vicinity of the transition is obtained from
					the two-photon excitation scan of krypton in the presence of the
					perturber at different prescribed pressures varying from a few torrs
					to 10 atm. The absorption spectra reveal noticeable asymmetry at
					atmospheric pressure, and the asymmetry becomes increasingly
					pronounced with pressure; however, the absorption spectra at
					sub-atmospheric pressures tested are symmetric. The absorption spectra
					are fitted with synthetic asymmetric Voigt profiles across all
					pressures, wherein the asymmetry parameter is varied to capture the
					asymmetry at different pressures. The collisional
					shift (
      
	
	  
	    δ
	    C
	  
	
      
    ), the symmetric equivalent
					collisional full width at half maximum (
      
	
	  
	    w
	    
	      C
	      ,
	      0
	    
	  
	
      
    ), and the asymmetry parameter (
      
	a
      
    ) are determined from the synthetic
					fits at various pressures. All the parameters are observed to vary
					linearly with pressure over the entire range of the pressure values
					tested. The mechanisms that cause the asymmetry in the absorption
					spectra are also discussed.}, number={26}, journal={APPLIED OPTICS}, publisher={The Optical Society}, author={Sahoo, Abinash and Zelenak, Dominic and Narayanaswamy, Venkateswaran}, year={2020}, month={Sep}, pages={7760–7769} }
 @article{sahoo_zelenak_narayanaswamy_2020, title={Temperature dependence of collisional broadening and shift for the Kr 4p6S01→5p[3/2]2 electronic transition}, volume={59}, url={https://doi.org/10.1364/AO.380102}, DOI={10.1364/AO.380102}, abstractNote={Temperature scaling of collisional broadening parameters for krypton
					(absorber) 
      
	4
	
	  
	    p
	    6
	  
	
	
	  S
	  0
	  1
	
	→
	5
	p
	
	  [
	  3
	  
	    /
	  
	  2
	  
	    ]
	    2
	  
	
      
     electronic transition centered at
					107.3 nm in the presence of major combustion species (perturber) is
					investigated. The absorption spectrum in the vicinity of the
					transition is obtained from the fluorescence due to the two-photon
					excitation scan of krypton. Krypton was added in small amounts to
					major combustion species such as 
      
	
	  
	    
	      C
	      H
	    
	    4
	  
	
      
    , 
      
	
	  
	    
	      C
	      O
	    
	    2
	  
	
      
    , 
      
	
	  
	    
	      N
	    
	    2
	  
	
      
    , and air, which then heated to
					elevated temperatures when flowed through a set of heated coils. In a
					separate experimental campaign, laminar premixed flat flame product
					mixtures of methane combustion were employed to extend the
					investigations to higher temperature ranges relevant to combustion.
					Collisional full width half maximum (FWHM) (
      
	
	  
	    w
	    C
	  
	
      
    ) and shift (
      
	
	  
	    δ
	    C
	  
	
      
    ) were computed from the absorption
					spectrum by synthetically fitting Voigt profiles to the excitation
					scans, and their corresponding temperature scaling was determined by
					fitting power-law temperature dependencies to the 
      
	
	  
	    w
	    C
	  
	
      
     and 
      
	
	  
	    δ
	    C
	  
	
      
     data for each perturber species. The
					temperature exponents of 
      
	
	  
	    w
	    C
	  
	
      
     and 
      
	
	  
	    δ
	    C
	  
	
      
     for all considered combustion species
					(perturbers) were 
      
	−
	
	  0.73
	
      
     and 
      
	−
	
	  0.6
	
      
    , respectively. Whereas the
					temperature exponents of 
      
	
	  
	    w
	    C
	  
	
      
     are closer to the value (
      
	−
	
	  0.7
	
      
    ) predicted by the dispersive
					interaction collision theory, the corresponding exponents of 
      
	
	  
	    δ
	    C
	  
	
      
     are in between the dispersive
					interaction theory and the kinetic theory of hard-sphere collisions.
					Comparison with existing literature on broadening parameters of NO,
					OH, and CO laser-induced fluorescence spectra reveal interesting
					contributions from non-dispersive interactions on the temperature
					exponent.}, number={5}, journal={Applied Optics}, author={Sahoo, Abinash and Zelenak, Dominic and Narayanaswamy, Venkateswaran}, year={2020}, month={Feb}, pages={1438–1446} }
 @article{sahoo_narayanaswamy_2019, title={Two-dimensional temperature field imaging in laminar sooting flames using a two-line Kr PLIF approach}, volume={125}, ISSN={["1432-0649"]}, url={https://doi.org/10.1007/s00340-019-7280-2}, DOI={10.1007/s00340-019-7280-2}, number={9}, journal={APPLIED PHYSICS B-LASERS AND OPTICS}, publisher={Springer Science and Business Media LLC}, author={Sahoo, Abinash and Narayanaswamy, Venkateswaran}, year={2019}, month={Sep} }